where no external cause of disease can be discovered. It was formerly
the custom to group such cases of "Internal Causes" by themselves, but
apart from the fact that many of these mysterious diseases have
subsequently been shown to be due to the action of external agencies,
the whole question of internal causes resolves itself into one of
relations between the plant and its surroundings, and it becomes evident
that no inherited or internal disease can be regarded as explained until
we know the external causes which have so modified the structure and
working of the living cells as to make them abnormal in their reactions
to other parts of the plant. "Internal causes" of disease, therefore, is
a phrase expressing our ignorance, but somewhat more emphatically than
usual. If this is clearly understood there seems no reason against its
employment for the time being in the artificial scheme of classification
we require. With regard to external causes due to the non-living
environment, excess or deficiency of materials in the soil, water, or
atmosphere plays an important part, and--since we may neglect purely
aquatic plants--it is customary to speak of diseases due to unsuitable
soils or to injurious atmospheric influences. For instance, any
deficiency in the supplies of the necessary mineral salts (compounds of
calcium, magnesium, potassium with sulphuric, nitric and phosphoric
acids, etc.) leads to pathological changes, as also does the lack of the
necessary traces of iron. But it is equally true that the presence of
such ingredients in excess or in combinations unsuited to the plants
also leads to disaster, as also does the presence of minerals or other
compounds which poison the root-hairs--_e.g._ products of decomposition,
soluble salts of copper and other poisons. That these matters are bound
up with the whole question of manuring and of proper soil-analyses will
be evident.
Another essential factor is the nature and quantity of organic materials
in the soil, whether leaf-mould and decomposing vegetable remains,
stable manures, or other animal matters, all of which affect different
species very differently, and produce very different results in
different soils. It is necessary to apprehend in this connection what
has been stated above: that soil is not a mere dead structureless
medium, and that the root-hairs of ordinary plants cannot deal with
large quantities of putrefying organic matter: that a good soil must
abound in useful bacteria and fungi to render such substances
available--and in very various ways--and that it must be open and
aerated, of proper temperature and suitably supplied with water, and so
forth, or disaster will result. Here, again, then we are brought into
close contact with all that is known of fermentation, nitrification, and
the various biological changes going on in soil, and the application of
such knowledge to the practice of manuring and tillage in all its forms.
In view of the above remarks, the danger of "over-feeding," in this
sense, has a real meaning for horticulturists, though it must not be
forgotten that no substance is really a food until it is assimilable
into the protoplasm: manures, etc., are food-materials, not food. The
futility of mere chemical analyses to prove what a plant requires is now
well known, and it is only on the basis of long and carefully conducted
experiments that we can ever discover what a particular plant in a
particular soil, situation, and climate requires for healthy
development. Again, the quantity of water in soil may be too great or
too small for given species, and this either on the average for the
year, or during critical periods only; and it is obviously important
whether the excess or deficiency is due to improper supplies of water,
the depth or shallowness of the soil, its retentive powers, or the
nature of the sub-soil and so on, again bringing the whole matter into
connection with our understanding of the physical constitution and
structure of soils, and the nature of soil-drainage.
For instance, a common way of killing ferns is to keep the roots and
soil wet and the air and fronds dry, whereas the natural habitats
provide for wet and shaded fronds and well-drained soil.
It may be noted here that in most cases where gardeners speak of plants
being killed under the "drip" of trees--_e.g._ Beech, the injury is due,
not to the effects of water but to the shade: the loss of light is so
great that the shaded plants die of inanition because their leaves are
not able to provide sufficient carbohydrates.
Closely bound up with this is the question of the gases in soils. Apart
from the disastrous effects of poisons--_e.g._ coal gas escaping from
pipes under pavements in towns, etc., diseased conditions often result
from deficiency of oxygen at the root-hairs, due to imperfect aeration
of soils, brought about by stagnant water, excess of animal matter, and
so forth.
Unsuitable constitution of the atmosphere is also a fruitful source of
disease, though its effects are commoner in closed stoves and
greenhouses than in the open. Nevertheless the continual exhalation of
sulphurous fumes, chlorine, and other poisonous gases in the
neighbourhood of manufacturing centres or of large smoky towns,
volcanoes, etc., play their part in injuring plants; and excessive
moisture in the form of mist, rain, etc., is also important. All these
matters bring us at once into the region of physiology, and only an
intelligent appreciation of what is known about the action of the
atmosphere on the soil and the plant will save the peasantry of a
country from a hopeless mysticism but little removed from that of the
Middle Ages, when blights and other evils were vaguely referred to the
river-mists, thunder clouds, and easterly winds.
If we summarise the above as the material factors of the environment, we
may classify another set of external non-living causes of disease as the
non-material factors. Such are principally the following:
The space at the disposal of plants greatly affects their welfare. The
crowding of roots in the soil and of foliage in the air, resulting in
the loss of light to the leaves, involves deficiency of all the
materials referred to above--minerals, organic materials, gases, and
water--and no better illustration of the intense struggle for existence
among these apparently passive and motionless beings, plants, can be
given than an over-crowded seedbed or plantation. If left to themselves
such over-stocked areas exhibit to the keen eye of the trained observer
all the phases of starvation, weakness, wounding, rot, and, so to speak,
brutal dominance of the stronger over the weaker which it is the object
of cultivation to prevent. Here, then, we are brought face to face with
the true significance of thinning and weeding out, pruning, and similar
processes.
Unsuitable temperature is one of the commonest of all sources of
disease, for every plant is adapted to certain ranges of temperature,
and best adapted to a given optimum somewhere between the maximum and
minimum temperature for each function. Consequently any serious
departure from the mean may bring about physiological disturbances of
the nature of disease, and this in very various ways, as exemplified by
the results of frost, sun-scorching, drought, hail-storms, forest fires,
and so forth.
As a predisposing factor to disease abnormal temperature effects play a
great part. Many wound-fungi gain their entrance through frost-cracks,
bruises due to hailstones, or into tissues chilled below the normal.
No less remarkable are the diseases primarily due to insufficient or
improper exposure to light, which affects the chlorophyll-apparatus and
the process of carbon-assimilation and through these the whole
well-being of the plant. Every plant is adapted to certain ranges of
light intensity, and most cultivators know how impossible it is to grow
shade plants in fully exposed situations, and how easily plants which
live in open sunny situations are "drawn" and killed by shade. It is
equally important to have the right kind of light, as disastrous
experiences with greenhouses glazed with glass which cut off certain
rays of light have taught. Here, again, it is important to notice that
the optimum intensity or quality of light may differ for different
functions and organs of the plant, as is shown by many adaptations on
the part of species growing in natural situations--_e.g._ bud
protection, orientation of leaves, etc.--and it may be taken as a rule
that etiolated plants are peculiarly susceptible to other diseases.
As regards other factors of the inorganic environment, disasters which
come within the scope of our subject may be brought about by many
agencies, the mechanical effects of snow and hail, wind, avalanches,
etc., the effects of lightning, and so forth, being a few of them.
NOTES TO CHAPTER XI.
For other detailed classifications of the causes of disease
the reader is referred to the works of Sorauer and of Frank
referred to in the last chapter. Also Kirchner, _Pflanzen
Krankheiten_, Stuttgart, 1890.
Of more historical importance are the older classifications of
Berkeley, _Gardeners' Chronicle_, 1854, and Re, _Gardeners'
Chronicle_, 1849-50. These latter are interesting as showing
the very different views held by the earlier workers, and
comparison of these with the modern views helps to mark the
progress of physiology during the half century which has
intervened.
CHAPTER XII.
CAUSES OF DISEASE. THE LIVING ENVIRONMENT.
_Causes due to animals--Vertebrata--Wounds, etc.--Invertebrata
--Insects, etc.--Plants as causes of disease--Phanerogams,
weeds, etc.--Cryptogams, fungi--Epidemics, etc._
Passing now to those causes of disease which are connected with the
living environment, we may obviously divide them into two groups of
agents, animals and plants.
Among animals, the various vertebrata, including man, are especially
responsible for the larger kinds of wounds and wholesale destructive
processes due to breakage, stripping of leaves and bark, cutting and
biting, and so forth. Cattle, rabbits, rats and mice, squirrels and
birds of various kinds stand out prominently as enemies to trees and
other plants, to which they do immense injury in various ways by their
horns, teeth, claws, and beaks; and the damage which an ignorant
gardener or forester can do with his ill-guided footsteps, axe, spade,
and knife can only be appreciated by one who knows the habits of plants.
It is among the invertebrata, however, especially insects and worms,
that the most striking agents of disease in plants are to be found, for,
with the exception of certain rodents--and we may logically include also
human invasions--vertebrate animals do not often appear in such numbers
as to bring about the epidemics and scourges only too commonly caused by
insect pests.
Insects injure plants in very various ways. Some, such as locusts,
simply devour all before them; others, _e.g._ caterpillars, destroy the
leaves and bring about all the phenomena of defoliation. Others, again,
eat the buds--_e.g._ _Grapholitha_; or the roots--_e.g._ wire-worms, and
so maim the plant that its foliage and assimilation suffer, or its roots
become too scanty to supply the transpiration current. Many aphides,
etc., puncture the leaves, suck out the sap, and produce deformations
and arrest of leaf-surface, as well as actual loss of substance, and
when numerous such insects induce all the evils of defoliation. Others,
such as the leaf-miners, tunnel into the leaves, with similar results on
a smaller scale.
It must be remembered that a single complete defoliation of a herbaceous
annual, or even of a tuberous plant like the potato, so incapacitates
the assimilatory machinery of the plant, that no stores can be put aside
for the seeds, tubers, etc., of another year, or at most so little that
only feeble plants come up.
In the case of a tree the case is different, and since most large trees
in full foliage have far more assimilatory surface than is actually
necessary for immediate needs, a considerable tax can be paid to
parasites or predatory insects before the stores suffer perceptibly.
Still, it should be recognised that the injury tells in time, especially
in seed years.
Many larvae of beetles, moths, etc., bore into the bark and as far as
the cambium or even into the wood or pith of trees, the local damage
inducing general injuries in proportion to the number of insects at
work: moreover, the wounds afford points of entrance for fungi and other
pests.
Galls and similar excrescences result from the hypertrophy of young
living tissues pierced by the ovipositors of various insects, and
irritated by the injected fluid and the presence of the eggs and larvae
left behind. They may occur on the buds, leaves, stems, or roots, as
shown by various species of _Cynips_ on oak, _Phylloxera_ on vines,
etc., in all cases the local damage being relatively small, but the
general injury to assimilatory, absorptive, and other functions is great
in proportion to the number of points attacked.
Many grubs--larvae of flies, beetles, etc.--bore into the sheaths or
internodes of grasses, or the pith of twigs, or into buds, fruits, and
other organs of plants, and do harm corresponding to the kind and amount
of tissues injured.
Various species of so-called eelworms--Nematodes--also cause gall-like
swellings on young roots, or they invade the grains of cereals.
Finally, various slugs and snails cause much injury by devouring young
leaves and buds and diminishing the assimilatory area.
Plants as agents of disease or injury fall naturally into the two main
categories of flowering plants (Phanerogams) and Cryptogams, among which
the fungi are the especially important pests.
Beginning with weeds, we find a large class of injurious agents. Weeds
damage the plants we value by crowding them out in the struggle for
existence, as already stated, and when the weed-action is simply due to
superfluous plants of the same species, we speak of overcrowding. But it
must not be overlooked that the competition between crowded plants of
the same species--where every individual is acting as a weed to the
others--may be more dangerous than between plants and weeds belonging to
other species and genera, because in the former case they are struggling
for the same minerals and other necessary food-materials: a matter of
importance in connection with the rotation of crops.
The question of allowing grass to grow at the foot of fruit trees, as in
orchards, is a good case in point. Such grass may increase the damp and
shade, thus favouring fungi at one season, and dry up the moisture of
the soil to the injury of the fine superficial roots at another, as well
as exhaust the soil, owing to the competition of the roots for salts
and other materials. On the other hand, the checking of surface roots by
competition with the grass has been claimed as advantageous. In this
connection probably the whole question of the composition of the turf
arises, as well as that of possible cropping for hay, and manuring.
As regards any particular weed, the cultivator should learn all he can
respecting its duration, seeding capacity, method of dissemination, the
depth and spread of its root-system, and any other particulars which
enable him to judge when and how to attack it. It is only necessary to
see the victory of such drought-resisting weeds as _Hieracium
pilosella_, Plantains, _Hypochaeris_, on lawns to realise how weeds may
win in the struggle for existence with the finer grasses.
Many so-called weeds are, however, partially parasitic, with their roots
on the roots of others--_e.g._ _Rhinanthus_, _Thesium_, etc., and much
damage is done to meadow grasses and herbage by the exhaustive tax which
these semi-parasites impose.
This is carried still further in the case of such root-parasites as
_Orobanche_, where the host-plant is burdened with the whole support of
the pest, because the latter, having no chlorophyll, is entirely
dependent on the former for all its food.
Even ordinary climbing plants may injure others by shading them, either
by scrambling over their branches--_e.g._ Bramble, or twisting their
tendrils round the twigs--_e.g._ Bryony, or twining round them--_e.g._
Woodbine, _Convolvulus_, etc. The principal direct injury is in these
cases owing to the loss of light suffered by the shaded foliage, but
the weed-action is often increased by the competition of their
roots--_e.g._ briars; and in the case of woody climbers the gradually
increased pressure of the woody-coils round the thickening stems
compresses the cambium and cortex of the support and induces strictures
and abnormalities which may be fatal in course of time.
Epiphytes, or plants which support themselves wholly on the trunks,
branches, or leaves of other plants, also injure the latter more
especially by shading their foliage--_e.g._ tropical Figs, Orchids,
Aroids, etc.; and similar damage is done by our own Ivy, the main roots
of which are in the soil, but the numerous adventitious roots of which
cling to the bark.
When the climber or epiphyte is also parasitic, as in the case of the
Dodder, _Loranthus_, Mistletoe, etc., the direct loss of substance
stolen from the host by the parasite comes in to supplement any effect
of shading that the latter may bring about if it is a leafy plant.
Of Cryptogams, apart from a few epiphytic ferns, and the intense
weed-action of certain Equisetums, the rhizomes and roots of which are
as troublesome as those of twitch and other phanerogamic weeds, it is
especially the fungi which act as agents of disease, and which, as we
now know, are _par excellence_ the causes of epidemics.
The action of fungi may be local or general; and restricted, slow and
insidious, or virulent and rapidly destructive.
Examples of local action are furnished by _Schinzia_, which forms
gall-like swellings on the roots of rushes; _Gymnosporangium_, which
induces excrescences on the stems of junipers, and numerous leaf-fungi
(_Puccinia_, _AEcidium_, _Septoria_, etc.), which cause yellow, brown, or
black spots on leaves, as well as by _Ustilago_, which attacks the
anthers or the ovary of various plants, and so forth. In such cases the
injury done by a few centres of infection is very slight, but prolonged
action may bring into play secondary effects such as the gradual
destruction of the cambium round a branch, when, of course, the effect
of ringing results; or if the fungus becomes epidemic and myriads of
leaf-spots are formed, the destruction of foliar tissue, gradual taxing
of the assimilatory cells, etc., may end in rapid defoliation, and
renewed attacks soon exhaust the plants and lead to sterility and death,
as often occurs with Uredineae--_e.g._ the coffee leaf-disease.
It is highly probable that such fungi are particularly exacting owing to
their exhausting demands for compounds of potassium, phosphoric acid,
and other bodies.
Examples of virulent and rampant general action are afforded by finger
and toe in turnips, etc., where the roots are invaded by
_Plasmodiophora_, which induces hypertrophy and rotting of the roots;
and by the damping off of seedlings, where the fungus _Pythium_ rapidly
invades all parts of the seedlings and reduces them to a water-logged,
putrefying mass; or the potato-disease, which is due to the rapid
spread of _Phytophthora_ in the leaves and throughout the plant, which
it blackens and rots in a few days.
Many fungi not in themselves very virulent or aggressive do enormous
harm owing to the secondary effects they induce. Some of the
tree-killing hymenomycetes, such as _Agaricus melleus_, for instance,
penetrate the wood of a pine at the collar, and the result of the large
flow of resin which results is to so block up the water passages that
the tree dies off above with all the symptoms of drought. Similarly, the
_Peziza_ causing the larch disease, having obtained access to the stem
about a foot or so above the ground, will gradually kill the cambium
further and further round the stem, and so girdle the tree as
effectually as if we had cut out the new wood all round. In all such
cases--and the same applies to the leaf-diseases referred to above--the
fungus may be compared to an army which is not strong enough to invade
the whole territory, but which, by striking at the lines of
communication, cuts off the supplies of water, food, etc., and so brings
the struggle to an end. Indeed we might compare the cases of fungi which
attack the root and collar, and so strike at and cut off the water
supply, to a compact army which at once cuts off the enemy from his
narrow base; whereas the innumerable units which bring about an epidemic
attack on the leaves, and so surround the enemy and cut off his food
supplies all round, is rather like a much larger army which cannot get
in beyond the natural barriers of the tissues, and so puts a _cordon_
all round the territory and seizes the multitudes of food-stuffs at the
frontiers. The end result is similar in both cases, but the methods of
warfare differ.
Many fungi, however, though they make their presence noticeable by
conspicuous signs, cannot be said to do much damage to the individual
plant attacked. The extraordinary malformations induced by parasites
like _Exoascus_, which live in the ends of twigs of trees and stimulate
the buds to put out dense tufts of shoots, again densely
branched--Witches' brooms--are a case in point. Also the curious
distortions of nettle stems swollen and curved by _AEcidium_, of maize
stems and leaves attacked by _Ustilago_, and of the inflorescences of
_Capsella_ by _Cystopus_, etc., are not individually very destructive;
it is the cumulative effects of numerous attacks, or of large epidemics,
which tell in the end.
Some very curious effects are due to fungi such as _AEcidium elatinum_,
which, living in the cortex of firs, stimulate buds to put out shoots
with erect habit, and with leaves which are radially disposed, annually
cast, and differently shaped from the normal--characters quite foreign
to the species of fir in its natural condition.
Equally strange are the shoots of _Euphorbia_ infested with the aecidia
of _Uromyces_, those of bilberries affected with _Calyptospora_, etc. In
all these cases we must assume a condition of toleration, so to speak,
on the part of the host, which adapts itself to the altered
circumstances by marked adaptations in its tissue developments, mode of
growth and so forth.
This toleration is perhaps most marked in the case of those cereals
which, though infected by the minute mycelium of _Ustilago_ while still
a seedling, nevertheless go on growing as apparently healthy green
plants indistinguishable from the rest, although the fine hyphae of the
parasite are in the tissues and keeping pace with the growth of the
shoots just behind the growing points. As the grains of the cereal begin
to form and swell, however, the hyphae suddenly assume the part of a
dominant aggressor, consume the endosperm of the enlarging seed, and
replace the contents of the grain with the well-known black spores known
as Smut.
NOTES TO CHAPTER XII.
The reader will find a summary of such fungi as are here
concerned in Massee, _A Text-Book of Plant Diseases_, 1899, or
Prillieux, _Maladies des Plantes Agricoles_.
For further details the student should consult the works of
Frank and Sorauer referred to in the notes to Chapter IX., and
Tubeuf, _The Diseases of Plants_, Engl. ed. 1897, pp. 104-539.
For experiments on the effects of grass on orchard trees, see
_Report of the Woburn Experimental Fruit Farm_, 1900, p. 160.
For the further study of weeds, the interesting bulletins of
the Kansas State Agricultural College, 1895-1898, will show
the reader what may be done in the matter of classifying them
according to their biological peculiarities.
In regard to insects, the reader will find the following list
embraces the subject: Somerville, _Farm and Garden Insects_,
1897; Theobald, _Insect Life_, 1896; Ormerod, _Manual of
Injurious Insects_, 1890, and _Handbook of Insects Injurious
to Orchards, etc._, 1898.
The admirable series of publications of the U.S. Department of
Agriculture under the editorship of Riley and Howard, and
entitled _Insect Life_, 1888-1895, also abounds in
information.
Further, Taschenberg's _Praktische Insektenkunde_, 1879-1880,
and Judeich and Nietsche, _Lehrbuch der Mitteleurop. Forst.
Insektenkunde_, 1889.
For an elementary introduction to the study of fungus
diseases, see Marshall Ward, _Diseases of Plants_, Soc. for
Promoting Christian Knowledge, London.
CHAPTER XIII.
NATURE OF DISEASE.
_General and local disease--General death owing to cutting-off
supplies, etc.--Disease of organs--Tissue-diseases, e.g.
timber--Root-diseases--Leaf-diseases, etc.--Diseases of
Respiratory, Assimilatory, and other organs--Physiological and
Parasitic diseases--Pathology of the cell--Cuts--Cork--Callus
--Irritation--Stimulation by protoplasm--Hypertrophy._
On going more deeply into the nature of those changes in plants which we
term pathological or diseased, it seems evident that we must at the
outset distinguish between various cases. A plant may be diseased as a
whole because all or practically all its tissues are in a morbid or
pathological condition, such as occurs when some fungus invades all the
parts or organs--_e.g._ seedlings when completely infested by _Pythium_,
or a unicellular Alga when invaded by a minute parasite; or it may die
throughout, because some organ with functions essential to its life is
seriously affected--_e.g._ the roots are rotten and cannot absorb water
with dissolved minerals and pass it up to the shoot, or all the leaves
are infested with a parasite and cannot supply the rest of the plant
with organic food materials, in consequence of which parts not directly
affected by any malady become starved, dried-up, or poisoned or
otherwise injured by the results or products of disease elsewhere.
In a large number of cases, however, the disease is purely local, and
never extends into the rest of the organs or tissues--_e.g._ when an
insect pierces a leaf at some minute point with its proboscis or its
ovipositor, killing a few cells and irritating those around so that they
grow and divide more rapidly than the rest of the leaf tissues and
produce a swollen hump of tissue, or gall; or when a knife-cut wounds
the cambium, which forthwith begins to cover up the dead cells with a
similarly rapid growth of cells, the callus. Numerous minute spots due
to fungi on leaves, cortex, etc., are further cases in point, the
mycelium never extending far from the centre of infection.
Many attempts have been made to classify diseases on a basis which
assumes the essential distinction of the above cases, and we read of
diseases of the various organs--root-diseases, stem-diseases,
leaf-diseases, and so forth; or of the various tissues--timber-diseases,
diseases of the cambium, of the bark, of the parenchyma, and so on.
Furthermore, attempts have been made to speak of general functional
disease, of diseases of the respiratory organs, of the absorptive
organs, and so forth, as opposed to local lesions.
Critical examination, however, shows that no such distinctions can be
consistently maintained, partly because the organs and functions of
plants are not so sharply marked off as they are in animals, the
diseases of which have suggested the above classification, and partly
because all disease originates in the cells and tissues, and it is a
matter of detail only that in some cases--_e.g._ severe freezing or
drought of seedlings, or when some ingredient is wanting in the
soil--the diseased condition affects practically every cell alike from
the first, while in others it spreads more or less rapidly from some one
spot.
Even the distinction into physiological diseases _versus_ parasitic
diseases cannot be maintained from the standpoint of the nature of the
disease itself. All disease is physiological in so far as it consists in
disturbance of normal physiological function, for pathology is merely
abnormal physiology, no matter how it is brought about. This is not
saying that no importance is to be attached to the mode in which disease
is incurred or induced: it is merely insisting on the truth that the
disease itself consists in the living cell-substance--the
protoplasm--not working normally as it does in health, and this, whether
want of water, minerals, or organic food be the cause, or whether the
presence of some poison or mechanical irritant be the disturbing agent,
as also whether such want or irritation be due to some defect in soil
or air, or to the ravages of a fungus or an insect.
This being understood I need not dwell on the common fallacy of
confounding the fungus, insect, soil or other agent with the disease
itself, or of making the same blunder in confusing symptoms with
maladies. In this sense, wheat rust is not a disease: it is a symptom
which betrays the presence of a disease-inducing fungus, the Rust
fungus. Similarly, chlorosis is not a disease: it is a symptom of
imperfect chlorophyll action, and the best proof of the truth of both
statements is that in both cases the fundamental disease-action is the
starvation of the cell-protoplasm of carbohydrates and other essential
food matters--in the one case because the fungus steals the
carbohydrates as fast as the leaves can make them, in the second because
the leaf is unable to make them.
The foundation of a knowledge of disease in plants therefore centres in
the understanding of the pathology of living cells.
If a suitable mass of living cells is neatly cut with a sharp razor the
first perceptible change is one of colour: the white "flesh" of a potato
or an apple, for instance, turns brown as the air enters the cut cells,
and the microscope shows that this browning affects cell-walls and
contents alike. The cut cells also die forthwith; and the oxygen of the
air combining with some of their constituents forms the brown colouring
matter which soaks into the cell-walls. The uninjured cells below them
grow longer, pushing up the dead debris, and divide across by walls
parallel to the plane of the wound, and so form series of tabular cells
with thin walls, which also soon turn brown and die, the cell-walls
meanwhile undergoing changes which convert them into cork. The living
cells deeper down are now shut off from the outer world by a skin, of
several layers, of cork-cells, which prevent the further free access of
air or moisture. During the period of active cell-division which
initiates the cork, the temperature of the growing cells rises: a sort
of fever (wound-fever) is induced, evidently owing to the active
respiration of the growing cells.
This healing by cork occurs in any tissue of living cells exposed by a
cut--leaf-tissue, young stem or root, fruit, cambium, etc.; and the same
applies to any other kind of cutting or tearing injury--such as a prick
with a needle or the proboscis of an insect, a stripping, or even a
bruise.
Such healing is prepared for and carried out very thoroughly in the case
of falling leaves and cast branches, the plane of separation being
covered by a cicatrix of cork.
If the cell-tissue under the wound is actually growing at the time,
however, a further process is observed when the wound-cork has been
formed. The uninjured cells below go on growing outwards more vigorously
than ever, the pressure of the overlying tissues taken off by the cut
having been removed, and, lifting up the cork-layer as they do so, they
rapidly divide into a juicy mass of thin-walled cells which is of a
cushion-like nature and is termed a _Callus_. This callus is at first a
homogeneous tissue of cells which are all alike capable of growing and
dividing, but in course of time it undergoes changes in different parts
which result in the formation of tracheids, vessels, fibres and other
tissue-elements, and even organs, just as the embryonic tissues of the
growing points, cambium, etc., of the healthy plant give origin to new
growths. Such wound-wood, however, is apt to differ considerably in the
arrangement, constitution and hardness of its parts as compared with
normal wood, and its peculiar density and cross-graining are often
conspicuous.
If instead of a simple tissue, the cut or other wound lays bare a
complex mass such as wood, the resultant changes are essentially the
same to start with. The living cells bordering the wound form cork, and
then those deeper down grow out and form a callus. The exposure of the
wood however, entails alterations in its non-living elements also. The
lignified walls of tracheids, fibres, etc., turn brown to a considerable
depth, and this browning seems to be--like all such discolorations in
wounds--due to oxidation changes in the tannins and other bodies
present: the process is probably similar to what occurs in humification
and in the conversion of sap-wood into heart-wood in trees. Such wood is
not merely dead, but it is also incapable of conveying water in the
lumina of its elements, which slowly fill with similarly dark-coloured,
impervious masses of materials termed "wound-gum," the nature of which
is obscure, but which slowly undergoes further changes into resin-like
substances.
The exposure of wood by a wound results also in another mode of stopping
up the vessels and so hindering the access of air, loss of water, etc.,
for the living cells of the medullary rays and wood-parenchyma grow into
the lumina of the larger vessels through the pits, forming _thyloses_,
again a phenomenon met with in heart-wood. In Conifers the stoppage of
the lumina is increased by deposition of resin, which also soaks into
the cell-walls and the wounded wood becomes semi-translucent owing to
the infiltration.
Every living cell in an active condition is irritable, and one of the
commonest physiological reactions of growing tissues is that of
responding to the touch of a resistant body, as is vividly shown by the
movements of the Sensitive plant, _Dionaea_, etc., and by those of
tendrils, growing root tips, etc., on careful observation. We have
reason for stating that if a minute insect, too feeble to pierce the
cuticle, cling on to one side of the dome-shaped growing point of any
shoot, the irritation of contact of its claws, hairs, etc., would at
once cause the protoplasm of the delicate cells to respond by some
abnormal behaviour; and, as matter of experiment, Darwin showed long ago
that if a minute piece of glass or other hard body is kept in contact
with one side of the tip of a root, the growth on the side in contact is
interfered with. Moreover we know from experiments on heliotropism,
thermotropism, etc., that even intangible stimuli such as rays of light,
etc., impinging unsymmetrically on these delicate cells cause
alterations in their behaviour--_e.g._ arrest or acceleration of growth.
Perhaps the most remarkable class of stimulations, however, is that due
to the presence of the entire protoplasmic body of one organism in the
cell of another, each living its own life for the time being, but the
protoplasm of the host cell showing clearly, by its abnormal behaviour,
that the presence of the foreign protoplasm is affecting its physiology.
A simple example is afforded by Zopfs' _Pleotrachelus_, the amoeboid
protoplasmic body of which lives in the hypha of _Pilobolus_, causing it
to swell up like an inflated bladder, in which the parasite then forms
its sporangia. The _Pleotrachelus_ does not kill the _Pilobolus_, but
that its protoplasm alters the metabolic physiology of the latter is
shown by the hypertrophy of the cells, and by the curious fact that it
stimulates the _Pilobolus_ to form its sexual conjugating cells,
otherwise rare, an indication of very far-reaching interference with the
life-actions of the host.
An equally remarkable example is that of _Plasmodiophora_, the amoeboid
naked protoplasm of which lives and creeps about in the protoplasm of a
cell of the root of a turnip, to which it gains access through the
root-hairs. It does not kill the cell, but stimulates its protoplasm to
increased activity and growth and division, itself dividing also and
passing new amoebae into each new daughter-cell of the host. Here the
processes of stimulation, hypertrophy and further division are repeated,
until hundreds or thousands of the turnip root-cells are infected. The
externally visible result is the formation of distorted swellings on the
root (Finger and Toe), most of the cells of which are abnormally large
and filled with amoeboid _Plasmodiophora_ protoplasm, which finally
devours the turnip-protoplasm and itself passes over into spores. Here
we have most convincing proof of the stimulation of protoplasm by other
protoplasm in direct contact with it; and that the metabolism of the
host-cells is profoundly altered is shown not only by the abnormal
growth of the cells, but also by the starvation of the rest of the
turnip plant as the _Plasmodiophora_ gets the upper hand. We have here,
in fact, a local intracellular parasitic disease, gradually invading
large tracts of tissue and eventually inducing general disease resulting
in death--a state of affairs reminding us of cancer in animals.
Irritation and hypertrophy of cells, however, may be induced by
parasites which never bring their protoplasm into direct contact with
that of the host. Many Chytridiaceae penetrate the cells of plants, and
grow inside them as short tubes, vesicles, etc., the protoplasm of which
is separated by their own cell-walls from that of the host-cell;
nevertheless hypertrophy and abnormal cell-divisions and secretions are
induced, and the effect even extends to neighbouring cells--_e.g._
_Synchytrium_--showing that some influence is exerted through cells
themselves not directly affected. This latter point need not surprise us
now we know that the cells of plant-tissues are connected by fine
protoplasmic strands passing through the separating cell-walls.
But the invading plant need not actually enter the cells, and may still
stimulate them through both its own and their own cell-walls to abnormal
growth. This is well shown by the intercellular mycelium of _Exoacus_
and _Exobasidium_, and the latter affords an excellent illustration of
the far-reaching effects of hyphae on the cells (of _Vaccinium_) into
which they do not penetrate. Not only are the cells stimulated to grow
larger and divide oftener than normally, thus producing large gall-like
swellings, but the chlorophyll disappears, the cell sap changes colour
to red, the numerous compound crystals normally found in the tissues
diminish in number and are different in shape, large quantities of
starch are stored up, and even the vascular bundles are altered in
character. All these changes indicate very profound alterations in the
physiological working of the protoplasm of the cells of the host, and
yet the fungus has done its work through both its own cell-walls and
those of the host.
Even harmless endophytic algae in the intercellular spaces of plants may
stimulate the cells in their immediate neighbourhood to increased
growth, _e.g._ _Anabaena_ in the roots of Cycads.
NOTES TO CHAPTER XIII.
With reference to cork-healing and wound-fever the student may
consult Shattock "On the Reparative processes which occur in
Vegetable Tissues," _Journal of the Linnean Society_, 1882,
Vol. XIX., p. 1; and Shattock "On the Fall of Branchlets in
the Aspen," _Journal of Botany_, 1883, Vol. XXI., p. 306. Also
Richards, "The Respiration of Wounded Plants," _Annals of
Botany_, Vol. X., 1896, p. 531; and "The Evolution of Heat by
Wounded Plants," _Ann. of Bot._, Vol. XI., 1897, p. 29.
For details and figures respecting callus, see Sorauer,
_Physiol. of Plants_, p. 175.
In respect to the irritable movements referred to see Darwin,
_The Power of Movements in Plants_, 1880, chapter III. The
recent work of Nawaschin, _Beobachtungen ueber den feineren
Bau u. Umwandlungen von Plasmodiophora_, Flora, Vol. LXXXVI.,
1899, p. 404, should be read for details and literature
concerning "Finger and Toe."
CHAPTER XIV.
NATURE OF DISEASE (_Continued_).
_Actions of poisons in small doses--Results of killing a few
cells--Malformation--Enzymes--Secretions and excretions--
Acids, poisons, etc.--Chemotactic phenomena--Parasitism--
Epiphytes and endophytes--Symbiosis--Galls._
Physiological research has shown that the respiratory activity of cells
may be increased by small doses of poisons, and even that growth may be
accelerated by them--_e.g._ chloroform, ether--and, still more
remarkable, that fermentative activity may be enhanced by minute doses
of such powerful mineral poisons as mercuric chloride, iodine salts,
etc., and that the cells may be gradually accustomed to larger doses
without injury. Unfertilised eggs of insects have been started into
growth by treatment with acids and those of frogs with mercury salts,
and the germination of beans quickened by various poisonous alkaloids.
In other words, graduated doses of poison may alter the physiological
activity of living cells, inducing pathological phenomena, while larger
doses kill them.
Now we know at least one parasitic fungus which poisons the cells of its
host, and kills them, with similar symptoms to those resulting from
excessive doses of the above-named toxic agents. _Botrytis_ hyphae,
living in the cell-walls of plants, but not entering the cells, excretes
a poison which kills the protoplasm, and the fungus then feeds on the
debris. Numerous other fungi form powerful poisons, but we do not know
whether or how they employ them--_e.g._ Ergot.
It is obvious that if all the young cells of a root-tip or of the apex
of a shoot, or those of a young leaf, are growing and dividing
regularly, the killing of one or a few cells at one point on the side of
the organ must result in irregularities--in malformation--of the adult
organ. This has been proved experimentally by destroying a few cells
with a needle. It can also be done by planting a minute mycelium of
_Botrytis_ laterally on a young organ--_e.g._ a very young lily-bud. The
fungus adheres to the surface, kills a few epidermis cells, and forms a
foxy-red spot, which becomes concave as the dead cells lose water and
dry. Since the rest of the bud goes on growing, however, while this dead
point remains stationary, the latter gradually becomes the centre of a
concavity, the growing tissues having grown round it: the bud is
deformed. Numerous cases of malformed organs are explained in this way;
a minute insect has bitten or pierced the young tissue, or a fungus has
killed a minute area, or a drop of acid condensed from fumes in the air
is the lethal agent, and so forth. And even on a much larger scale we
see the same kinds of agents at work. Wherever a patch of cells is
killed whilst those around go on growing, there must result some
deformation of the resulting organ, since had the injury been withheld
the number and sizes of the cells now fixed in death would have
increased and covered a larger area: they now serve to pull over to
their side the still living and growing cells. The same results follow
on any lateral wound: the killed spot of tissue serves as a point round
which the continued growth of other parts of the organ turns. Hence the
malformation is in these cases a secondary effect, and not, as in simple
hypertrophy, a direct effect of the action of the cells involved in the
injury.
There is another class of bodies secreted by fungi, however, which act
directly on cells, viz. enzymes--that is, soluble bodies which are able
to dissolve cellulose (_cytases_), starch (_diastases_), proteids
(proteolytic enzymes), and other substances, by peculiar alterations in
their constitution. It is by means of its _cytase_ that _Botrytis_
hyphae pierce the cellulose walls of plants, and no doubt in all cases
where fungi pierce cell-walls it is by the solvent action of such a
cytase, and similarly when haustoria penetrate into the cells. It is
also by means of these starch-dissolving enzymes (diastases) and
proteolytic enzymes, etc., that the hyphae inside the cells are enabled
to make use of the starch, proteids, etc., they find there.
All living cells form materials, resulting from the activity of the
protoplasm, which we may compare with the refuse or by-products formed
in any great manufacturing industry: these by-products have to be got
rid of if they are injurious or noisome (_excretions_), and if
not--_i.e._ if they are capable of further use (_secretions_)--they have
to be stored away till required. Some of the most prominent of these
bodies excreted by fungi are, as we have seen, poisonous acids, such as
oxalic acid, enzymes, and organic poisons, such as those in ergot. But
similar enzymes, acids, poisons, etc., to those found in fungi are also
found in the cells of other plants and animals; for only by means of
their solvent actions can processes like digestion and assimilation of
the starchy and other materials into the body-substance be accomplished,
and we have seen that it is a general property of living cells to form
acids, and other excretions and secretions.
Now we know very little about what may happen when an organism--say a
fungus--secreting especially one kind of enzyme or poison or other
active substance, comes into intimate contact with another--say a
leaf-cell--which secretes predominantly others, but what we do know
points to the certainty that various complications will occur.
For instance, if certain bacteria which prefer an alkaline medium, and
yeasts which prefer an acid environment are mixed in a saccharine
solution, it depends on the reaction of the liquid which organism gains
the upper hand: if the liquid is acid the yeast may dominate the
bacteria; if alkaline it may be suppressed by them.
That a parasite may be prevented from successfully attacking a
particular plant is shown by the failure of _Cuscuta_ to establish its
haustoria in poisonous plants such as _Euphorbia_, _Aloe_, etc., and it
has been pointed out that poisonous secretions in the cells of the plant
protect them against the penetration of fungi. This cannot be taken as
meaning that any poison protects against any parasite, however, for
_Euphorbia_ is itself subject to attacks of Uredineae, and _Pangium
edule_, which contains prussic acid and is extremely poisonous to most
animals, is eaten with avidity by several insects, while nematode worms
can live in its tissues. This is no more remarkable, however, than the
fact that _Fontaria_, a myriapod, secretes prussic acid in its own
tissues, or than that certain glands of the stomach secrete free
hydrochloric acid, and _Dolium_ forms sulphuric acid in its glands.
There is yet a further point to notice here. It has been proved that
certain substances formed in plant-cells, not necessarily nutritive,
attract the hyphae of parasitic fungi or repel them, according to the
kind and degree of concentration. So clear has this proof been made that
it was possible in experiments conducted apart from a host plant, to
make the hyphae on one side of an artificial membrane--_e.g._
collodion--penetrate it by placing one of these attractive
(_chemotropic_) substances in suitable proportions on the other side.
The hyphae dissolved holes in the membrane by means of enzymes and
plunged into the attractive substance on the other side.
The foregoing sketch gives us a glimpse into the causes at work in
parasitism.
Suppose a fungus on the outside of the epidermis of a young organ--say a
leaf. It may be unable to penetrate into the plant, and finding no
suitable food outside it dies: or it may be satisfied with the traces of
organic matter on the epidermis and then lives the life of a saprophyte.
Or it may be able to establish a hold-fast on the tender epidermal
surface, but without entering the cells, and irritate the developing
organ by contact stimulation, inducing slight abnormalities; if in its
further, purely superficial growth such an epiphyte covers large areas
of the leaf, and especially if the hyphae are dark coloured--_e.g._
_Dematium_ and other "Sooty Moulds"--injury may be done to the leaf
owing to the shading action which deprives the chlorophyll below of its
full supply of solar energy. Some epiphytes, however, are able to fix
their hyphae to the epidermis by sending minute peg-like projections
into the cuticle--_Trichosphaeria_, _Herpotrichia_--while others send
haustoria right through the outer epidermal walls--_e.g._
_Erysiphe_--and thus supplement mere contact-irritation and shading by
actual absorption from the external cells. Here the fungus is a
parasitic epiphyte.
A stage further is attained in those fungi which enter the stomata and
live in the intercellular spaces--_e.g._ many Uredineae and
_Phytophthora_--and many such intercellular endophytes increase their
attack on the cells by piercing their walls with minute (_Cystopus_) or
large and branched (_Peronospora_) haustoria, or even eventually pierce
the cells and traverse them bodily (_Pythium_). In all these cases it is
clear that conflicts must occur between poison and antidote, acid and
alkali, attractive and repellent substances, enzyme and enzyme, etc., as
was hinted at above; and the same must take place when the parasite is
endophytic and intracellular from the first, as in Chytridiaceae, etc.,
the zoospores of which pierce the outer cell-walls and forthwith grow
into the cells. There are also fungi which, while able to pierce the
outer cell-walls, and grow forward in the thickness of the wall itself,
cannot enter the living cells themselves--_e.g._ _Botrytis_. In the
example mentioned, the fungus excretes a poison, oxalic acid, which
soaks into and kills the cells next its point of attack: into these dead
cells it then extends, and, invigorated by feeding on them, extends into
other cell-walls and excretes more poison, and so on.
On the basis of the foregoing it seems possible to sketch a general view
of the nature of parasitism. In order that a fungus may enter the cells
it must be able to overcome not only the resistance of the cell-walls,
but that of the living protoplasm also: if it cannot do the latter it
must remain outside, as a mere epiphyte, or at most an intercellular
endophyte. If it can do neither it must either content itself with a
saprophytic existence or fail, so far as that particular host-plant is
concerned. Its inability to enter may be due to there being no
chemotropic attraction, or to its incapacity to dissolve the cell-walls,
or to the existence in the cell of some antagonistic substance which
neutralises its acid secretions, destroys its enzymes or poisons, or is
even directly poisonous to it.
Moreover when once inside it does not follow that it can kill the cell.
The protoplasm of the latter may have been unable to prevent the fungus
enemy from breaking through its first line of defence--the cell-wall,
but it may be quite capable of maintaining the fight at close quarters,
and we see signs of the progress of the struggle in hypertrophy,
accumulation of stores, and other changes in the invaded cells and their
contents.
Finally, the invested or invaded cell may so adapt itself to the demands
of the invader that a sort of arrangement is arrived at by which life in
common--_Symbiosis_--is established, each organism doing something for
the other and each taking something from the other. In this latter case,
which is often realised--_e.g._ lichens, leguminous plants and the
organisms in their root-nodules, mycorrhiza, etc.--we leave the domain
of disease, which supervenes indeed if the other symbiont is lacking.
Some interesting facts bearing on the matters here under discussion,
have been obtained from the study of _Galls_, the curious outgrowths
found on many plants and due to the action of insects.
A typical gall exhibits three distinct and characteristic layers of
tissue surrounding the hollow chamber in which the larva of the insect
lies, viz., an outer layer of soft cells forming a parenchyma covered
with an epidermis, and frequently also with a layer of cork; an inner
stratum consisting of very thin-walled delicate cells filled with
protoplasmic and reserve food-materials on which the larva feeds; and
between the two a more or less definite layer of thick-walled
sclerenchyma cells which serve as a protection against accidents to the
larva as the outer layer shrivels or rots, or if it is exposed to the
attack of marauders. This layer may be absent from galls which have a
short life only. Vascular bundles run into the outer layer from the
leaf-veins or the stele of the shoot, etc. Such galls abound in tannin,
and are frequently of use in the arts on this account: they also contain
starch, and proteid substances and crystals of calcium oxalate. When the
larva has consumed the stores of food material and reached the adult
stage it eats its way out and escapes.
The growth of such a gall is preceded by the laying of an egg on or in
the embryonic tissue of a leaf, stem, or other young part, and it is
interesting to note that only organs in the meristematic stage can form
galls, and that it is by no means necessary that the tissues should be
wounded. Moreover, the egg as such is incapable of stimulating the plant
tissues, but when it hatches, the resulting larva, beginning to feed on
the cells, irritates the tissues and rapid growth and cell-division
occur, as in the case of other wounds or of fungus attacks. The actual
wound made by the ovipositor heals up at once. It is evident from
numerous recent researches that these true galls are not due to any
poisonous or irritating liquid injected by the parent, but that the
stimulus to the tissue formation is similar to that exerted by a wound.
The young gall is in fact a callus enclosing the living larva, and it is
the continued irritation of the latter which keeps up the stimulation.
The final shape and constitution of the gall depend on mutual
reactions--not as yet explained in detail--between the species of plant
and the species of gall-insect concerned, as may readily be seen from
the extraordinary variations in size, shape, colouring, hairiness and
other structural peculiarities of the galls on one species of, for
instance, the common oak. From what we have learnt about fungus
parasites, however, there can be little doubt that reactions between the
cells and the larva of the insect occur, resembling those which take
place between the cells and the hyphae of the fungus, and this is borne
out by the study of other hypertrophies due to animals; _e.g._ Nematode
worms in roots, and the remarkable galls--the simplest known--on
_Vaucheria_, caused by the entrance into this alga of a species of
_Notommata_, which induces a different gall on each of the various
species of its host plants.
It must be concluded that the formation of the _Vaucheria_ gall is
induced by the mechanical irritation which the Rotifer causes in the
protoplasm. These galls are comparable to the hypertrophies in
_Pilobolus_ caused by the presence of _Pleotrachelus_.
Attempts to induce the development of galls artificially by injecting
formic, acetic and other vegetable acids, poisons and other substances
into the tissues have, however, failed, and even the substances
contained in the insect or gall itself only produced negative results.
Nothing further was obtained than slight callus formations in some
cases. Nor have experimenters succeeded in obtaining more than slight
distortions by fixing insects on the growing leaves in such positions
that they could scratch the epidermis.
We must therefore conclude that very complex interactions between the
plant and insect are here concerned, among which may be the infiltration
of some liquid from larva to plant--many of these gall larvae are
strongly scented, and Kustenmacher says that fluids excreted by the
larva are absorbed by the gall-tissue apparently as nutriment. This
would point to the symbiotic character of galls and their guests.
NOTES TO CHAPTER XIV.
With regard to the action of poisons in small doses see
further Johannsen, _Das Aether-Verfahren beim Fruhtreiben_,
Jena, 1900, and, for _Botrytis_, see Marshall Ward, "A Lily
Disease," _Annals of Botany_, Vol. II., 1889, p. 388.
The subject of enzymes has been exhaustively treated by Green,
_The Soluble Ferments and Fermentations_, Cambridge, 1899, to
which the reader is referred for literature. I have taken the
statements regarding _Fontaria_ and _Dolium_ from Kassowitz,
_Allgemeine Biologie_, p. 182. The two most important works on
chemotactic phenomena are Pfeffer, "Uber Chemotaktische
Bewegungen," etc., _Unters. aus dem Bot. Inst. zu Tubingen_,
B. II., p. 582, and Miyoshi, "Die Durchbohrung von Membranen
durch Pilzfaden," _Pringsh. Jahrb. f. Wiss. Bot._, B. XXVIII.,
1895, p. 269, and from these the further literature can be
traced. As regards the nature of parasitism see Marshall Ward,
"On Some Relations between Host and Parasite," etc., being the
Croonian Lecture delivered before the Royal Society, _Proc.
Roy. Soc._, Vol. 47, p. 393. On Symbiosis, see Marshall Ward,
"Symbiosis," _Annals of Botany_, 1899, Vol. XIII., p. 549,
where the literature is collected. For a general account of
galls the reader may consult Kerner, _The Natural History of
Plants_, Eng. ed., 1895, Vol. II., pp. 527-554, and Adler,
_Alternating Generations, A Biological Study of Oak Galls_,
etc., 1894.
CHAPTER XV.
SPREADING OF DISEASE AND EPIDEMICS.
_Dissemination of fungi by the aid of snails, rabbits, bees,
and insects--Man--Distribution in soil, on clothes, through
the post, etc.--Worms, wind--Puffing of spores--Creeping of
mycelia--Lurking parasites--Spread of insects and other
animals--Losses due to epidemics._
The dissemination of plant diseases is a subject which has been far too
much neglected, but our knowledge of it is slowly increasing. The spores
of fungi such as Rusts and Erysipheae are often carried from plant to
plant by snails; those of root-destroying and tree-killing Polyporei by
rabbits, rats, and other mammals which rub their fur against the
hymenophores. Bees have been shown to carry the spores of _Sclerotinia_
and infect the stigmas of Bilberries, etc., with them; and flies convey
the conidia of Ergot from grain to grain. Insects, indeed, of all kinds
are great disseminators of disease--as witness also the part played by
mosquitoes in transferring the malaria parasite to man--and beetles,
bees, flies, etc., of all sorts probably play more active parts in this
work than has yet been proved, since they not only carry spores attached
like pollen to their hairy bodies, but in many cases in their alimentary
canal, to be spread later in the dung.
The part played by man in conveying fungi from plant to plant counts for
much. Not only do gardeners and farm labourers carry spores on their
boots and clothes as they pass from infected to non-infected areas, but
carted soil and manure are frequently infested with spores of Smuts,
_Fusarium_, _Polyporus_, and the sclerotia or rhizomorphs of
_Sclerotinia_, _Agaricus melleus_, _Dematophora_, etc. Man also sends
diseases through the post, and by rail and ship, by spores or mycelia
attached to seedlings, bulbs, fruits, flowers, etc., as shown in several
cases of potato, vine, hollyhock, lily, and hyacinth diseases. Every
time a carpenter saws a piece of fresh timber with the saw which has
been used previously for cutting wood attacked with dry rot, he risks
infecting it with the fungus. Similarly in pruning: every cut with a
knife which the gardener has used on infected branches may infect the
tree.
Cuttings made with a soil-contaminated knife and stuck into ordinary
soil in dirty boxes covered with equally dirty glass, present every
chance for infection by soil organisms; bacteria and fungi obtain access
to the vessels, and derive plenty of food from the juices, and the
wonder is not that so many cuttings "damp off," but that any are raised
at all under ordinary conditions.
That worms bring buried spores to the surface can hardly be doubted
after Pasteur's experiments with Anthrax, and the principle of Darwin's
discoveries of the important bearing of the habits of earthworms on this
subject, and that the soil attached to the feet of ducks and other birds
teems with small seeds, applies to fungi also. Wind is also responsible
for distributing fungus-spores over wide areas, as may be easily proved
by fixing a glass slide smeared with glycerine in the course of a breeze
passing over an infected area.
But although the fungi are, generally speaking, passive in regard to
their distribution, such is by no means always the case. Apart from the
fact that some forms attract insects by means of honey dew (Ergot), or
by sweet odours (Spermogonia, _Sclerotinia_), the zoospores of
_Pythium_, _Phytophthora_, etc., are motile, and although they cannot
move far in the films of water in which they travel, nevertheless in a
wet potato field, with the wind flapping the leaves one against the
other, some dissemination of importance must be actively brought about,
and similarly with the amoebae of _Plasmodiophora_ in the soil.
The shooting of ascospores into the air by certain species of _Peziza_,
from the discs of which the spores may be seen to puff out in clouds,
affords further evidence that fungi cannot be regarded as entirely
passive in respect to distribution of their spores. But when we come to
certain of the soil fungi--_e.g._ _Agaricus melleus_, _Dematophora_,
etc.--the active creeping forward by growth in the soil of their
rhizomorphs and mycelial strands afford examples of active spreading of
considerable importance in the vineyard and forest, since they pass from
root to root and from tree to tree and may infect the entire area in
course of time.
Not the least significant mode of dissemination is that by which what I
have termed "lurking parasites" are spread: such are fungi which attach
themselves to the seeds, fruits, tubers, etc., of other plants and so
obtain all the advantages of being carried and sown with the
latter--_e.g._ Ustilagineae and Uredineae which adhere to grain,
_Verticillium_, _Nectria_, etc., in potatoes and other plants.
The spread of diseases due to animals, especially insects, is of course
more active, in consequence of the motility of the distributing agents.
This is most marked in the winged species, of which locusts, beetles,
moths and butterflies, flies and wasps furnish well-known examples; and
is not inconsiderable in the case of wingless and merely creeping
species. It is noteworthy that many forms wingless in the parasitic
stage are winged at certain periods, _e.g._ the females of _Phylloxera_.
That man also spreads insect pests is well known and acted upon, as
witness the phylloxera laws--which, however, it is to be feared too
often only illustrate once more the adage concerning the shutting of the
stable door after the horse has gone.
It would be tedious to attempt anything like a complete account of the
estimates of loss in different countries, due to the ravages of insects
and fungi, but the following examples should surely serve to convince
anyone of the magnitude of these losses and of the economic importance
of the whole question, and the reader may be referred to the special
literature for further details.
The coffee leaf-disease of Ceylon, due to the fungus _Hemileia_, is
estimated to have cost that Colony considerably over L1,000,000 per
annum for several years. One estimate puts the loss in ten years at from
L12,000,000 to L15,000,000. The hop-aphis is estimated to have cost Kent
L2,700,000 in the year 1882. In 1874 the Agricultural Commissioner of
the United States estimated the annual loss, due to the ravages of
insects on cotton alone, to amount to L5,000,000; and in 1882 the annual
loss to the United States due to insects, calculated for all kinds of
agricultural produce, was put at the appalling figure of from
L40,000,000 to L60,000,000 sterling. In India, the annual loss due to
wheat-rust alone has recently been estimated at 4,000,000 to 20,000,000
rupees, and one insect alone is said to have cost the cotton planters a
quarter of the crop--valued at seven crores of rupees--in bad years.
Similarly, in Australia the annual loss from wheat-rust has been put at
from L2,000,000 to L3,000,000. In 1891 the loss in Prussia alone from
grain-rusts was officially estimated at over L20,000,000 sterling. Need
more be said? Even allowing for considerable exaggerations in such
estimates it is clear that the damage to crops in any country soon
amounts to sums which even at low rates of interest would easily yield
incomes capable of supporting the best equipped laboratories and staffs
for investigations directed to the explanation of the phenomena in
detail, the sole basis on which intelligent preventive and therapeutic
measures can be based. But it is far from likely that the estimates are
exaggerated. The planting and agricultural communities are as a rule
opposed to the publication of statistics--or at least have been so in
various countries and at different times--and if we knew the damage done
to all crops even in our own Empire, the results would probably astonish
us far more than the above figures have done.
NOTES TO CHAPTER XV.
On the dissemination of fungi, the reader will find Fulton,
"Dispersal of the Spores of Fungi by the Agency of Insects,"
_Ann. Bot._, Vol. III., 1889, p. 207, and Sturgis, "On Some
Aspects of Vegetable Pathology and the Conditions which
Influence the Dissemination of Plant Diseases," _Botanical
Gazette_, Vol. XXV., 1898, p. 187, both useful papers. Further
information will be found in Zopf, _Die Pilze_, Breslau, 1890,
pp. 79-95 and 228, and Wagner, "Ueber die Verbreitung der
Pilze durch Schnecken," in _Zeitschr. f. Pflanzen Krankh._,
1896, p. 144. The estimates as to losses due to epidemics are
taken from Watt, _Agricultural Ledger_, Calcutta, 1895, p. 71;
Balfour, _The Agricultural Pests of India_, London, 1887,
pp. 13-15; Eriksson and Henning, _Die Getreideroste_; the
publications of the U.S. Department of Agriculture, _The
Kew Bulletin_, and elsewhere. The reader will find further
examples in Massee, _Text-Book of Plant Diseases_, 1899, pp.
47-51. Both these subjects are well worth further attention,
and I know of no complete account of them.
CHAPTER XVI.
THE FACTORS OF AN EPIDEMIC.
_Illustrations afforded by the potato disease--The larch
disease--The phylloxera of the vine._
When we come to enquire into what circumstances bring about those severe
and apparently sudden attacks on our crops, orchards, gardens, and
forests by hosts of some particular parasite, bringing about all the
dreaded features of an epidemic disease, we soon discover the existence
of a series of complex problems of intertwined relationships between one
organism and another, and between both and the non-living environment,
which fully justify the caution already given against concluding that
any cause of disease can be a single agent working alone.
The statement of prophecy that a particular insect or fungus need not be
feared, because it is found to do so little harm in particular cases or
districts examined, will thus be seen to be a dangerous one: any pest
may become epidemic if the conditions favour it!
In 1844 and 1845 the potato disease assumed an epidemic character so
appalling in its effects that it is no exaggeration to say that it
constituted a national disaster in several countries. It was stated at
the time that this disease had been known for some time in Belgium, in
Canada and the United States, in Ireland, in the Isle of Thanet, and in
other parts of the world. Similar, but less devastating epidemics have
occurred in various years since. It was generally noticed during such
epidemics that the plants themselves were full of foliage, surcharged
with moisture, and of a luxuriant green colour promising abundant
crops. The now well-known spots, at first pale and then brown and
fringed with a whitish mould-like growth--the conidiophores of the
_Phytophthora_--were observed during the dull cloudy and wet weather,
cooler than usual, when the atmosphere was saturated for days together,
in July and August. The actual amount of rain does not appear to have
been excessive, but most observers seem to agree that dull weather with
moist air had succeeded a warm forcing period of growth. So rapidly did
the disease run its course that in a few days nearly all the plants were
a rotting blackened mass in the fields, and the potatoes dug up
afterwards were either already rotten or soon became so in the stores.
Further experience has confirmed this, and we now know that the epidemic
is very apt to appear in any region where potatoes are grown on a large
scale, in dull moist weather, especially in fields exposed to mists,
heavy dews, etc., about July and August, when the foliage is full and
turgid. Similarly on heavy wet soils, unless the season is remarkably
open and dry; but also on dry light soils in rainy seasons. So evident
was this that many believed that the mists and dew brought the
disease--harking back to the superstitions of earlier days. We must
remember that prior to 1860 the life-history of _Phytophthora_ was not
known. Since De Bary's proof of the germination of the zoospores and of
the infection of the leaves, the course of the hyphae in them and in the
haulms, the origin of the conidia, etc., and the confirmation by
numerous competent observers of the true fungus nature of this disease,
we are now in a position to understand the principal factors of the
various epidemics of potato disease.
It is not merely that the potato-fields afford plenty of food for the
fungus, and that the dull weather causes the tissues to be surcharged
with moisture, owing to diminished transpiration, but the mists and
dew--to say nothing of actual rain and the flapping of wet
leaves--favour the germination and spread of the zoospores throughout
the field. Whether the dull light also favours the accumulation of
sugars in the tissues, and the partial etiolation of the latter implies
less resistance to the entering hyphae, may be passed over here, but in
any case it is clear that we have several factors of the non-living
environment here favouring the parasite and not improving the chances
of the host, even if they do not directly disfavour it.
As another instance I will take the Larch-disease, which is due to the
ravages of a Peziza (_Dasyscypha Willkommii_) the hyphae of which obtain
access by wounds to the sieve-tubes and cambium of the stem, and
gradually kill them over a larger and larger area and so ring the tree,
with the symptoms of canker described below.
Now the Larch fungus is also to be found on trees in their Alpine home,
but there it does very little damage and never becomes epidemic except
in certain sheltered regions near lakes and in other damp situations.
How then are we to explain the extensive ravages of the Larch disease
over the whole of Europe during the latter half of this century? The
extensive planting, providing large supplies for the fungus, does not
suffice to explain it, because there are large areas of pure Larch in
the Alps which do not suffer.
In its mountain home the Larch loses its leaves in September and remains
quiescent through the intensely cold winter, until May. Then come the
short spring and rapid passage to summer, and the Larch buds open with
remarkable celerity when they do begin--_i.e._ when the roots are
thoroughly awakened to activity. Hence the tender period of young
foliage is reduced to a minimum, and any agencies which can only injure
the young leaves and shoots in the tender stage must do their work in a
few days, or the opportunity is gone, and the tree passes forthwith
into its summer state.
In the plains, on the contrary, the Larch begins to open at varying
dates from March to May, and during the tardy spring encounters all
kinds of vicissitudes in the way of frosts and cold winds following on
warm days which have started the root-action--for we must bear in mind
that the roots are more easily awakened after our warmer winters than is
safe for the tree.
It amounts to this, therefore, that in the plains the long continued
period of foliation allows insects, frost, winds, etc., some six weeks
or two months in which to injure the slowly sprouting tender shoots,
whereas in the mountain heights they have only a fortnight or so in
which to do such damage. That the lower altitude and longer summer are
not in themselves inimical to Larch is proved by the splendid growths
made by the trees first planted a century ago. Then came the epidemic of
Larch-disease: the fungus, which is merely endemic--_i.e._ obtains a
livelihood here and there on odd trees, or groups of trees in warmer or
damper nooks--in the Alps, was favoured by the more numerous points of
attack afforded to its spores by injuries due to insects--_Coleophora_,
_Chermes_, etc.--and frost wounds, as well as by the longer periods of
moist dull weather, and the longer season of foliation. Moreover, as
time went on almost every consignment of young Larch-trees sent abroad
was already infected. Here again, then, we find the factors of an
epidemic consisting in events which favour the reproduction and spread
of a fungus more than they do the well-being of the host.
As a third illustration I will take the case of an insect epidemic. In
1863 a disease was observed on vines in the South of France which
frightened the growers as they realised its destructive effects: the
roots decayed and the leaves turned yellow and died before the grapes
ripened, and such vines threw out fewer and feebler shoots the following
year, and often none at all afterwards. In 1865 the disease was
evidently becoming epidemic near Bordeaux, and in 1868 it was shown to
be due to an insect, _Phylloxera_, the female of which lays its eggs on
the roots, where they hatch. The louse-like offspring sticks its
proboscis into the tissues as far as the central cylinder. The irritated
pericycle and cortex then grow and form nodules of soft juicy
root-tissue at which the insect continues to suck. Rapid reproduction
results in the majority of the young rootlets being thus attacked, and
since they cannot form their normal periderm and harden off properly
they rot, and admit fungi and other evils, in consequence of which the
vine suffers also in the parts above ground.
Evidence that the general damage is due to the diminished root-action is
found in the peculiarly dry poor wood formed in the "canes" of diseased
plants.
By 1877 the epidemic had spread to the northern limits of the French
vineyards, and by 1888 half the vines in the country were attacked, and
the yield of wine reduced from half a million hectolitres to 50,000
only. Meanwhile the disease had spread to Italy, Germany, Madeira,
Portugal, and even to the Cape, though not in epidemic form as in the
Bordeaux centre whence it spread.
Now it appears that _Phylloxera_ has long been in the habit of doing
damage to vines in America, where, however, it attacks the leaves, on
which it makes pocket-like galls, rather than the roots. Moreover, there
are species and varieties of American vines which, even when planted in
Europe, do not suffer at all from this insect at the roots, either
because the rootlets do not push out at the same season as those of the
European form, or because they form wood more rapidly and completely, or
secrete resinous and other matters distasteful to the insect in greater
quantity and are thus capable of healing the wounds, or in some other
way they do not respond to the attack or suit the insect. In any case
the attack on the leaf rather than the root seems to be the exception in
European vineyards and the rule in American species, and we appear to be
face to face with a problem of specific predisposition to this
particular malady. That the resistant properties of the vines of
America--not all, only particular species and varieties are thus
"immune"--can be utilised has been proved by European growers; and not
only so, for Millardet and others have shown that the European vine
grafted on to these resistant stocks suffer less than when on their own
roots. It has also been shown that hybrids can be obtained which are
resistant.
But the most curious point of all is that _Phylloxera_ was itself a
native of America, and came thence to Europe. It had played its part
with certain fungi in ruining all the attempts to introduce the European
vine into America many years ago. A recent authority on the evolution of
American fruits writes as follows:
"All the most amenable types of grapes had long since perished in the
struggle for existence, and the types which now persist are necessarily
those which are, from their very make-up or constitution, almost immune
from injury, or are least liable to attack . . . the _Phylloxera_ finds
tough rations on the hard, cord-like roots of any of our eastern species
of grapes. But an unnaturalised and unsophisticated foreigner, being
unused to the enemy and undefended, falls a ready victim; or if the
enemy is transported to a foreign country the same thing occurs."
Further proof that it is in the "constitution" of the European vine that
the want of resistance to _Phylloxera_ resides, is furnished by the fact
that in California and the Pacific states the European vine was
introduced with more success, but is now suffering badly because
_Phylloxera_ has spread there also. It must not be overlooked, however,
that we are as yet very ignorant of all that is implied in the word
"constitution" as used above.
If we enquire further why the _Phylloxera_ epidemic was so much worse
in the Southern vineyards than in the more Northern ones of Germany, the
opinion seems to prevail that the warmer climates favour the insect.
Further, it appears that, in Italy, the vines in loose open soil,
provided it is equally rich in mineral food-materials and offers no
disadvantages as regards drainage, suffer less than those in closer
soils, the reasons alleged being that the young roots can push out more
rapidly and widely, and so obtain holdfasts with greater distances
between them.
NOTES TO CHAPTER XVI.
The student may obtain further information on the history of
the Potato disease by consulting the following: Berkeley,
"Observations, Botanical and Physiological, on the Potato
Murrain," _Journal of the Horticultural Society_, Vol. I.,
1846, p. 9; De Bary, _Die Gegenwaertig herrschende Kartoffel
Krankheit_, etc., Leipzic, 1861; and the pages of the
_Gardeners' Chronicle_ from 1860-1900.
For the Larch disease he should consult Hartig, _Unters. aus
der Foist. Botanischen Inst. Muenchen_, B. I., 1880; and
Willkomm, _Microscop. Feinde des Waldes_, B. II., 1868.
For _Phylloxera_ the literature is chiefly in the _Comptes
Rendus_ and other French publications since 1875, and in the
Reports of the U.S. Dept. of Agriculture.
For a summary of the facts concerning the life-histories of
the parasites referred to above, see Frank, _Krankheiten der
Pflanzen_, and Marshall Ward, _Diseases of Plants_, p. 59, and
_Timber and Some of its Diseases_, London, 1889, chapter X.
Also Marshall Ward, "On some Relations between Host and
Parasite in certain epidemic Diseases of Plants," _Proc. Roy.
Soc._, Vol. XLVII., 1890, pp. 393-443; and "Illustrations of
the Structure and Life-history of Phytophthora infestans,"
_Quart. Journ. Microsc. Soc._, Vol. XXVII., 1887, p. 413; also
Marshall Ward, "Researches on the Life-history of Hemileia
vastratrix," _Journ. Linn. Soc._, Vol. XIX., 1882, p. 299; and
"On the Morphology of Hemileia vastatrix," _Quart. Journ.
Microsc. Soc._, 1881, Vol. XXI., p. 1.
CHAPTER XVII.
REMEDIAL MEASURES.
_Preventible diseases--The principles of therapeutics--Powders
and their application--Spraying with liquids--Nature of
chemicals employed--Employment of epidemics and natural
checks--The struggle for existence._
It may be said that in no connection is the proverb "Prevention is
better than cure" more applicable than with this subject, and
undoubtedly the best utilitarian argument that can be used in favour of
a thorough study of the causes of disease is that only by understanding
these causes is there any hope of avoiding the exposure of crops, garden
plants, forest trees, etc., to the attacks of preventible diseases.
Moreover, only an intelligent appreciation of the causes of a disease
will enable the cultivator to take steps to mitigate their effects when
once the damage has begun its course. Every cultivator learns by
experience or by precept that there are some things he must avoid in
dealing with certain plants, or otherwise they will not succeed; in
other words they will succumb to diseased conditions and die. It is
partly owing to the want of systematisation of this knowledge, and its
extension in other directions, that such extraordinary blunders are made
in ignorant practice, and trees for instance are planted in low-lying
frost beds which would succeed in slightly higher situations, or seeds
subject to damping-off are sown in beds rife with the spores of
_Peronospora_ or _Pythium_, and so forth.
Many diseases, however, are not preventible in the present state of our
knowledge, or prevailing conditions are such that the risk must be run
of endemic diseases gradually becoming epidemic, and thus the natural
desire for some means of checking the ravages of some pest or another
has led to innumerable trials to minimise the effects by prophylactic
measures. The procedure almost invariably followed where parasites are
concerned, consists in either dusting the plants with some chemical in
the form of a powder, or spraying it with a liquid, or occasionally in
enveloping the plant in some gas, in each case poisonous to the insect-
or fungus-pest. The principal rules to be observed are: (1) the poison
employed must be sufficiently strong or concentrated to kill the
parasite, but not sufficiently powerful to injure the host; (2) it must
be applied at the right period, as suggested by a knowledge of the
life-history of the fungus or insect in question.
Obviously it is of no use to apply such topical remedies to a parasite
while it is spending the greater part of its life inside the tissues of
the host. Further, questions of expense of the materials employed and of
the labour of applying them help to limit the adoption of such measures.
Among the various kinds of powders employed, finely divided sulphur, or
a mixture of sulphur and lime, have been used with success in some
cases--_e.g._ against Hop mildew and other epiphytic Erysipheae, and
against red spider, aphides, etc., the gaseous sulphur dioxide evolved
being the efficacious agent. In other cases pyrethrum or tobacco powder,
wood ashes, etc., have been employed against insects. Such powders are
applied by hand or by means of bellows, and are very easily manipulated
in most cases, though, like all such applications, the dangers of
concentration at particular spots owing to uneven distribution, or of
dilution and washing off by rain, have to be incurred.
Far more numerous are the various liquids which have been employed for
washing, spraying, or steeping the affected parts of diseased plants.
Water alone, or aqueous decoctions or emulsions of various
kinds--_e.g._, quassia, tobacco, soap, or aloes, have been widely
employed against insects such as green fly, red spider, etc. In
greenhouses, where the leaves can be washed by hand or thoroughly
syringed, and the concentration and time of action thoroughly
controlled, such liquids are often serviceable, but great practical
difficulties are apt to interfere with their use in the open field.
The principal liquids employed against fungi have been copper sulphate
and other metallic compounds (Bordeaux mixture, Eau Celeste, etc.),
various compounds of arsenic (_e.g._ "Paris green"), potassium sulphite,
permanganate, etc., and emulsions of carbolic acid, petroleum, and such
like antiseptics, for the exact composition of which the special
treatises must be consulted. Some of these, especially Bordeaux mixture,
have been experimented with on a very large scale, especially in
America, and various forms of spraying machines have been introduced for
dealing with large areas.
It is clear that these spraying operations are more particularly adapted
to field crops such as Turnips, Hops, Vines, Potatoes, and to garden and
greenhouse plants than to woods and plantations; as a rule they cannot
be applied to forest trees--though they have been used in orchards--or
to roots, seeds, and other parts in the soil, and many special forms of
treatment have been devised for particular cases of these kinds.
One of the oldest of these is the steeping of grain in solutions of
copper, or in hot water, just before sowing, and the practical
eradication of Bunt and, partially, of Smut is due to this practice,
which has lately been adapted to potatoes, the principle being that the
parasitic germs shall be killed while still adhering to the outside of
the seeds, tubers, etc., before germination. "Finger and Toe" due to
_Plasmodiophora_ has been successfully dealt with by the application of
lime, but we do not know whether the effect is owing to indirect actions
in the soil, to direct actions on the plasmodia, or to the increased
production of root-hairs induced by liming.
_Phylloxera_ has been treated by plunging into the soil near the roots
small blocks of some slowly-soluble medium, such as gelatine,
impregnated with carbon-bisulphide, the volatile fumes of which kill the
insect, and even more drastic remedies have been tried along similar
lines. In America orchard trees infested with insects or fungi have been
covered one by one with light tents, and the vapours of prussic acid,
burning sulphur, and other poisons allowed to act inside the tent. In
all such cases it must be remembered that uncontrolled ignorance of the
properties of poisons on the part of the operator may lead to disaster,
and the same applies to the much easier treatment of greenhouses, and
cases where poisoned food is laid about for insects or vermin.
Attempts, not altogether unsuccessful on the small scale, have also been
made to introduce epidemic diseases among rats, mice, and locusts and
other insects, by inoculating some of them with parasitic bacteria or
fungi (_Empusa_, _Isaria_, etc.), and then allowing them to run loose in
the hope that they will communicate the disease to their fellows. The
introduction of lady-birds into districts infested with Coccideae and
similar pests which they devour, is also recorded as successful, as also
the importation of birds into forests plagued with caterpillars. It must
not be over-looked, however, that man's interference with the existing
balance of events in the natural struggle for existence is occasionally
disastrous, as witness the results of importing rabbits into Australia,
goats into the Canary Islands, and sparrows in various countries.
Darwin's well-known illustration of the inter-relations between clover,
bees, field-mice, and cats (_Orig. of Species_, 6th ed., 1876, p. 57),
which shows the astounding probability of the dependence of such a plant
on the number of cats in the neighbourhood, well illustrates the
situation.
Mere mention must be made of other special treatments.
Caterpillars and larger animals are often picked by hand or their
natural enemies--_e.g._ birds, are encouraged in forests. Locusts are
caught in nets, trenches, etc., and buried. Woodlice, slugs, etc., are
often trapped by laying attractive food such as carrots and overhauling
the traps daily: similarly with earwigs. Rings of tar round tree stems
have been employed to prevent caterpillars creeping up them.
American Blight has been treated by rapidly flaming the stems. Syringing
with hot water has also been employed for vines affected with mildew,
mealy bug, etc.
With regard to the alleged immunity from devouring insects of certain
poisonous plants, it has been pointed out that _Pangium edule_, which
abounds in prussic acid, is infested with a grub, and ivy is
occasionally eaten by caterpillars.
Another point as regards insect pests is the well-known destructive
effect of a cold, wet spring on the young larvae. The use of cyanide of
potassium requires especial care, but has been described as easily
carried out with success in greenhouses.
It seems probable that lady-birds, the larvae of wasp-flies and
lace-wings, and ichneumon-flies as well as wrens can keep down aphides.
For an example of the treatment of a complex case of "chlorosis" with
mineral manures, the reader may consult the _Gardeners' Chronicle_, 1899
(July), p. 405. Many similar cases have been recorded, but it should not
be overlooked that very complex inter-relations are here involved.
Charlock has been successfully dealt with by applying 5 lbs. of copper
sulphate in 25 gallons of water to each acre of land while the weeds are
young.
In all these cases the guiding idea is derived from accurate knowledge
of the habits of the insect, fungus, or pest concerned, and obviously
the procedure must be timed accordingly. It is a particular case of the
struggle for existence, where man steps in as a third and (so to speak)
unexpected living agent.
It is clear from our study of the factors of an epidemic that one of the
primary conditions which favour the spread of any disease is provided
by growing any crop continuously in "pure culture" over large areas.
This is sufficiently exemplified by the disastrous spread of such
diseases as Wheat-rust, Larch-disease, Potato-disease, Phylloxera,
Hop-disease, Sugar-cane disease, Coffee-leaf disease, and numerous other
maladies which have now become historic in agricultural, planting, and
forest annals. Providing the favourite food-supply in large quantities
is not the only factor of an epidemic, but it is a most important one in
that it not only facilitates the growth and reproduction of a pest, but
affords it every opportunity of spreading rapidly and widely.
Moreover, Nature herself shows us that such pests are kept in check in
her domain by the struggle for existence entailed by innumerable
barriers and competitors. As matter of experience also it is found that
rotation of crops, planting forests of mixed species, and breaking up
large areas of cultivation into plantations, fields, etc., of different
species afford natural and often efficient checks to the ravages of
fungus and insect pests. Over and over again it has been found that a
fungus or an insect which is merely endemic so long as it is isolated in
the forest, where its host is separated from other plants of the same
species by other plants which it cannot attack, becomes epidemic when
let loose on the continuous acres so beloved of the planter. And the
same reasoning applies to the success of such pests on open areas from
which the birds or other enemies of the pest have been driven. True, we
cannot always trace the tangled skein of inter-relationships between one
organism and another in Nature: the recognition of the principle of
natural selection and the struggle for existence is too recent, and our
studies of natural history as yet too imperfect to lay all the factors
clear, but no observant and thoughtful man can avoid the truth of the
general principle here laid down. The history of all great planting
enterprises teaches us that he who undertakes to cultivate any plant
continuously in open culture over large areas must run the risk of
epidemics.
NOTES TO CHAPTER XVII.
The principal literature, now very voluminous, on this subject
is contained in the publications of the U.S. Department of
Agriculture from 1890 onwards. See especially _Bulletins_,
Nos. 3, 6, and 9; _Farmers' Bulletin_, No. 91, 1899; and _The
Journal of Mycology_ during the same period. See also Lodeman,
_The Spraying of Plants_, London, 1896. A summary of the
principal processes will be found in Massee, _Text-Book of
Plant Diseases_, pp. 31-47.
With regard to the history of the subject, which still needs
writing, the reader should not overlook Roberts, "On the
Therapeutical Action of Sulphur," _St. George's Hospital
Reports_, date unknown, but subsequent to the following:
Berkeley, _Introduction to Cryptogamic Botany_, 1857, p. 277,
with references. These are, I believe, with the references to
steeping of wheat in De Bary, _Unters. ueber d. Brandpilze_,
Berlin, 1853, among the first attempts to utilise such
remedies.
Further facts will be found in the pages of the _Gardeners'
Chronicle_, especially since 1890, and in _Zeitsch. f.
Pflanzen-krankheiten_ since 1891.
CHAPTER XVIII.
VARIATION AND DISEASE.
_Predisposition and immunity--Pathological conditions
vary--Hardy varieties--"Disease-proof" varieties--Disease
dodging--Thick skins--Indian wheats, etc. Cell-contents
vary--Citrus, Cinchona, Almonds, etc. Double ideals in
selection--Cultivation of pest and host-plant--Variations of
fungi--Bacteria--Specialised races--Difficulties--Experiment
only will solve the problems._
The numerous and often expensive failures in the application of any
prophylactic treatment, have proved an acute stimulus to the research
for other ways of combating the ravages of plant diseases. It is a
matter of every-day experience that particular varieties of cultivated
plants may suffer less from a given disease than others in the same
district; also that one and the same species may suffer badly in one
country and not in another--_e.g._ the Larch in the lowlands of Europe
as contrasted with the same tree in its Alpine home, and the various
species of American Vines in Europe.
These matters, in the hands of astute observers, are turning the
attention of cultivators and experts to new aspects of the question of
plant diseases, namely, the possible existence of immunity, and the
breeding of disease-proof varieties; and the existence on the part of
the host plant of predispositions to disease which may depend on some
factors in the plant or in the environment over which it is possible to
exercise control, or which, if known, can be avoided.
The matter is complicated by the recent demonstration of the fact that
parasites also vary and can adapt themselves to altered conditions, as
is shown by the history of the coffee-leaf disease (_Hemileia_) in
Ceylon, and by Eriksson's results with Wheat-rusts (_Puccinia_) and
various experiments with _Coleosporium_ and other Uredineae; but there
are good grounds for concluding that hybridisation, grafting, and
selection of varieties may do much towards the establishment of races
which will resist particular diseases, as shown by Millardet's
experiments with Vines, and the results obtained by Cobb and others with
Wheat.
The great difficulty with so-called "disease-proof varieties" is to test
them under similar conditions in different countries, and for a
sufficient period of time. A particular race of Wheat may behave very
differently in Norfolk, Devonshire, and Northumberland, and the recent
introduction of the purely experimental method in this connection is a
marked advance. However rough the experiments may of necessity have to
be, it is only by such means that data can be gradually accumulated.
Having now obtained some insight into the factors concerned in disease,
let us enquire further into the bearing of variation on these. It is
evident that pathological conditions may vary; indeed they are
themselves symptoms of variation, as we have seen. The history of all
our cultivated plants shows abundantly that many of the variations
obtained by breeding in our gardens, orchards, fields, etc., involve
differences of response on the part of the plant to the very agencies
which induce disease. Every year the florists' catalogues offer new
"hardy" varieties; but a hardy variety is simply, for our present
purpose, one which succumbs less readily to frost, cutting winds, cold
damp weather, and so forth. If anyone doubts that hardy varieties have
been gradually bred by selection, I refer him to the evidence collected
by De Candolle, Darwin, Wallace, Bailey and others. When we come to
enquire into the causes of "hardiness," however, difficulties at once
beset us. The adaptation may express itself in a difference in the time
of flowering or leafing, the exigencies of the season being "dodged," as
it were, in a manner which was impossible with the original stock, as
appears to have occurred with Peaches in America; or it may be expressed
in deeper rooting, as is said to be the case in some Apples, or in the
acquirement of a more deciduous habit, or in actually increased
resistance to low temperatures. In such cases we cannot trace what
alterations have occurred in the cells and tissues concerned, though we
may be sure that some changes do occur.
No experienced cultivator doubts that some varieties of Potato, Wheat,
Vine, Chrysanthemum, etc., suffer more from epidemic diseases than
others, and our yearly catalogues furnish us with plenty of promises of
"disease-proof" varieties. Here also we may imagine several ways in
which a particular variety may resist or escape the epidemic attacks of
fungi which in the same neighbourhood decimate other varieties. If we
could breed a variety of the Larch which opened its buds later than the
ordinary form in our northern plains, the probability of its escaping
the Larch-disease would be increased in proportion to the shortness of
the period of tender foliation described on p. 153. It has been claimed
for certain varieties of Wheat that increased thickness of the cuticle
and fewer stomata per square unit of surface have diminished the risk of
infection by Rust fungi, and for certain varieties of Potato, that the
thicker periderm of the tuber protects them against fungi in the soil.
That certain thick-skinned Apples, Tomatoes, and Plums pack and store
better than those with a more tender epidermis seems proved--that is to
say, they suffer less from fungi which gain access through bruises and
other wounds; but it cannot be said that any convincing proof is yet to
hand explaining in detail why some races of wheat resist Rust, or why
the roots of American Vines suffer less from _Phylloxera_ than others.
One of the most extraordinary cases known to me in this connection is
the unconscious selection on the part of native Indian cultivators,
perfectly ignorant of the principles involved, of spring and autumn
forms of Rice, Wheat, Castor Oil, Sugar Cane, Cotton, and other crops.
"It has been estimated that Bengal alone possesses as many as 10,000
recognisable forms of rice." Now there is not the slightest ground for
doubt that these have been unconsciously bred from the semi-aquatic
native species during the many centuries of Indian agriculture, and
nevertheless they have, among other peculiar races, some hill-breeds
which they cultivate on dry soils and without direct inundation. That is
to say, they possess tropical and temperate races differing far more
than our spring and summer wheats.
Something has been gained, then, if we can show that there is nothing
absurd or hopeless in the search for disease-proof or resistant races,
and I think this can be done. We must not forget that the ideal usually
set before himself by a breeder of plants has hitherto been almost
exclusively some standard of size, form, colouring, and so forth, of the
flower, or of taste and texture of the fruit, tuber, etc., though
experiments with _Cinchona_, with brewery yeasts, and other plants
remind us that variations in other directions have been attended to
also.
Now it is obvious that in breeding sour limes and sweet oranges the
cultivator is selecting, and intensifying by selection, very different
metabolic processes in the cell: he can test the results of these, and
so the selection proceeds.
The question is, Could he select at the same time those variations in
cell activity which express themselves in properties of the flower,
fruit, foliage, etc., he desires, as well as such variations as aid the
cells in repelling fungi, insects, or exigencies of the non-living
environment?
That more or less disease-proof varieties could be selected if that
object alone were kept in view can hardly be doubted; plenty of examples
exist already which show that the necessary variations to work upon
exist in just those secretions of protoplasm, etc., which we have seen
are concerned in repelling or attracting parasites.
The Sweet Almond has lost the power of producing amygdalin and prussic
acid in its cells; Cinchona plants vary immensely in the quantity of
quinine formed, and in European hot-houses may even form none at all;
some varieties of Maize have sugar and dextrine instead of starch in
their endosperms, or coloured instead of clear sap in the aleurone
layer, and recent researches prove that they can transmit these
peculiarities to hybrid offspring; non-poisonous bacteria have
frequently been got from poisonous species simply by cultivation under
special conditions, and pigmented forms can be bred into non-pigmented
races.
But we see that the difficulty of selection is increased in the case
postulated above, because two ideals are to be worked up to, and they
may conceivably be incompatible. Not necessarily so, however, for
breeders have solved such problems before in obtaining early _and_ heavy
cropping races of potatoes, wheat, etc., sweet _and_ large grapes,
strawberries, etc., hardy _and_ brilliant flowers, and so forth.
There is, however, another aspect of this question of variability in
organisms in this connection to be considered. Ever since cultivation
began man has probably been cultivating not only the crops he desires,
but also the pests which infest them, and if variation of his chosen
plants occurs--and no one will deny that--surely variation of the fungi
and insects which live on them also takes place. That this is so can be
demonstrated, though, since it is not part of my theme to go into the
question of peculiarities of species and races of parasites, the subject
must here be passed over with a few remarks only.
Recent researches have shown not only that fungi vary immensely in form
and morphological characters according to the amount and kind of
food-materials put at their disposal, thus bringing the whole question
of polymorphism into the domain of experimental physiology, but that
their capacities for infection, spore formation, etc., are also capable
of variation and are dependent on the quality and quantity of food
supplies, water, as well as on the temperature, illumination, and other
factors of the environment. This is true of parasites as well as of
saprophytes. _Botrytis_ forms conidia only in darkness and in moist air.
Klebahn found that a _Puccinia_ growing on _Digraphis_ infected
_Polygonatum_ readily and completely, _Convallaria_ imperfectly, whereas
if sown on _Majanthemum_ it only just infected the plant and then
remained sterile, while it refused to infect _Paris_ at all. Magnus has
shown that _Peronospora parasitica_ can only infect meristematic
tissues, and that when it co-exists with _Cystopus_ on _Capsella_, as is
usually the case, it enters the latter plant by infecting the gall-like
pustules of hypertrophied tissue induced by that parasite. Numerous
parasitic fungi can only penetrate particular parts of plants. For
instance, the _Ustilago_ of wheat can only infect the young seedling,
and grows for weeks as a barren mycelium, only becoming a dominant
fungus in the endosperm. Numerous other examples could be given, but
these suffice to show some of the ways in which the nature of the food
substratum supplied by the host affects the fungus. It is obvious that
if the nature of this food changes, the fungus is also affected, and no
doubt this is the principal reason why Rust-fungi, for instance, vary so
much in their vigour and reproductive power on different wheats and
grasses, though the other factors of the environment must also be of
influence on them as well as on the hosts.
But--and this is the second point--modern research is also showing that
the various species of Rust-fungi have split up into different varieties
or specialised races, according to the particular host plants they
inhabit. For instance there are special varieties or races of the
particular species known as _Puccinia graminis_, the wheat rust, each of
which grows well on various kinds of grain and grasses but refuses to
infect others. Thus, the variety which infects Wheat refuses to infect
Barley or Oats, while that variety which grows on Rye will not take on
Wheat and so forth. Now it is important to notice that these specialised
races are indistinguishable one from another by their visible
microscopic characters: they are all botanically of the species
_Puccinia graminis_ which forms its aecida on the Barberry. We must
therefore conclude that we have here the same phenomenon as that met
with in culture-races of bacteria which, having been fed for several
generations on media rich in proteids, refuse to grow on media rich in
carbohydrates, or when attenuated races are developed by culture under
special conditions.
Now since such physiological races as I have described are by no means
confined to _Puccinia_ but are also known in _Melampsora_,
_Gymnosporangium_ and other fungi, we must conclude from this and from
what we know of variation in plants and animals generally, that
variation and adaptation are common among parasites, insects as well as
fungi.
These considerations will serve to show moreover that the question of
breeding disease-proof varieties of our cultivated plants is complicated
by the danger of our breeding at the same time adapted races of their
pests. It appears at first sight extremely improbable that we should
escape the danger by breeding from those specimens of our plants which
have best survived a fungus epidemic. Still, it must not be forgotten
that "hardy varieties," and races adapted to other exigencies of the
non-living environment, have been bred by selection--and nevertheless
this variable non-living environment is always with us. The matter is
therefore simply and solely one of experiment, and the retort that a
disease-resisting variety of any particular plant has not yet been
raised is no more valid than the objection that a true blue primrose has
not yet been obtained: whether the same remark can be made with regard
to any hope of a _disease-proof_ plant may be another matter, but in any
case it must be made more cautiously in the light of our present
experience.
NOTES TO CHAPTER XVIII.
The reader will find more on this subject in Bailey's
_Survival of the Unlike_ and the literature quoted in the
notes to Chapter VIII.
For varieties of Indian Wheats, etc., see Watt, _Agricultural
Ledger_, Calcutta, 1895.
For a discussion on so-called "Disease-proof Wheats" consult
Eriksson & Henning, _Die Getreideroste_.
Magnus' paper is in the _Berichte der Deutschen bot.
Gesellsch._, 1894, p. 39.
Concerning physiological races and adapted varieties of
_Puccinia_, etc., see Eriksson, "A General View of the
Principal Results of Swedish Research into Grain Rust,"
_Botanical Gazette_, vol. 25, 1898, p. 26.
For an account of Wheat-rust see Marshall Ward, "Illustrations
of the Structure and Life-history of _Puccinia graminis_,
etc.," _Ann. of Bot._, 1888, Vol. II., p. 215.
CHAPTER XIX.
SYMPTOMS OF DISEASE.
_Discolorations--Pallor--Etiolation--Laying of Wheat--
Chlorosis--Yellowing--Albinism--Variegation--Uprooting,
Exposure and Wilting of seedlings._
Everybody knows in a general way when the geraniums in the window pots
are drooping from want of water, or when the young Wheat is sickly, or
the Pear-trees "blighted," and we have now to see how far we can
systematise the knowledge that has been gained in course of time
regarding the signs which sick plants exhibit.
_Pallor._--Under this heading, which includes all cases where the normal
healthy green colour is replaced by a general sickly yellow or pale hue,
ultimately resulting in death of the parts if not arrested, we have
several totally distinct diseases of the chlorophyll apparatus, each
recognised by the co-existence of other subordinate symptoms. The
principal varieties of pallor usually met with are the following:
_Etiolation_ is due to insufficient intensity of light, the pale sickly
yellow organs being unusually watery and deficient in vascular tissue,
the internodes abnormally long and thin, and the leaves generally
reduced in size, or, in some plants also "drawn."
Forced Endive, Rhubarb, Asparagus, and earthed Celery afford examples of
etiolation purposely induced. The want of light causes the true
chlorophyll colouring matter to remain in abeyance, and consequently the
plant as a whole suffers from carbohydrate starvation.
_Laying_ of Wheat and other cereals is a particular case of etiolation.
The seeds having been sown too thickly, the bases of the haulms, owing
to the etiolation and consequent lack of carbohydrates, suffer from want
of stiffening tissues, and the top-heavy plants fall over.
_False etiolation_ depends on a similar abeyance of the chlorophyll, but
in this case due to too low a temperature. It is often seen in Wheat and
other monocotyledons when the young leaves unfold in cold weather in
spring. The symptoms of "drawing" and tenderness are however absent.
Pallor due to too intense illumination must be kept sharply distinct
from etiolation, the pale green or yellow hue being here due to the
destruction of the chlorophyll by insolation, and the accessory symptoms
of "drawing" are wanting.
_Chlorosis_ is a form of pallor where the chlorophyll grains themselves
are fully developed, but their green pigment remains in abeyance owing
to a deficiency of iron in the soil, and can often be cured by adding
traces of a ferrous salt. The distinction between _Icterus_, where the
organs are only yellow, and _Chlorosis_ proper, where they are nearly
white cannot always be maintained. In the typical case only those organs
whose cells are still young can become green on adding iron.
_Yellowing_ or _False Chlorosis_ may be experimentally induced by too
much carbon-dioxide in the atmosphere. It also often ensues when the
roots of plants in the open are waterlogged, owing to the stagnant water
not only driving air from the root-hairs but accumulating dissolved
substances which poison the plant. Trees frequently thus suffer from
"wet feet" when their roots have penetrated down to a sodden impervious
subsoil.
_Yellowing_ accompanied by _Wilting_ is a predominant symptom in most
cases where transpiration is more active than root-absorption beyond a
certain limit, as is well known in cases of prolonged drought. It may
also be caused in evergreens by the foliage transpiring actively in
bright January weather, for instance, while the ground is frozen and the
chilled root-hairs cannot absorb.
In other cases similar appearances are traceable to insects devouring
the roots, _e.g._ wireworms, and the malady is sometimes enhanced by
their accumulations so fouling the wet soil that the roots die off,
owing to want of oxygen and to the excess of carbon-dioxide and
poisonous matters.
Yellowing may also result from the presence of poisonous or acid gases
in the atmosphere or soil, such as chlorine, hydrochloric acid,
sulphurous acid, etc., in the neighbourhood of chemical works, or from
the escape of coal-gas in streets, etc., points of importance in
connection with the use of fungicides and insecticides.
Yellowness is the prevailing symptom in many cases of fungus attack of
the roots or collar of the plant, the resulting stoppage of
transpiration being also sometimes supplemented by rotting of the roots,
and the consequent deprival of oxygen and accumulation of foul gases. In
other cases Fungi, and even Bacteria, have been found to have made their
way into the principal vessels, the lumina of which they stop up, thus
reducing the transpiration current.
Certain insects may also induce a general yellowing and wilting of
plants by entering or destroying the tissues concerned in the
transpiration--_e.g._ _Oniscus_, the Frit Fly, and _Cecidomya_, the
Hessian Fly, which attack young winter wheat within the sheaths and
cause the plants to turn yellow and wilt.
_Albinism_ and _Variegation_ are apparently due to causes totally
different from any yet mentioned. Church's analyses have shown that
albino leaves contain more water and less organic matter than green ones
of the same plants, but not necessarily less ash constituents. The
composition of the ash points to there being more potash and less lime
in the white organs than in the green ones, and, speaking generally, the
former are related to the latter much as young leaves are related to
mature ones.
The whole matter is complicated by the behaviour of certain _variegated_
plants--_e.g._ Ribbon grass, _Calla_, _Abutilon_, which are usually
regarded as partial albinos.
Meyen showed long ago that such variegated plants, if grafted on green
ones, may induce the development of variegated leaves on both scion and
stock, and Morren and others have not only confirmed this but have also
shown that variegation may be inherited through the seed. Nevertheless
some care has to be taken with many of these variegations lest rich
soil, bright light, and other favourable treatment favour the
restitution of the green colour. These facts may be interpreted in
various ways. Some disturbance of physiological functions of the roots,
due to unfavourable conditions of soil, may be the cause; but Beijerinck
has lately published some results which show that some of these albino
diseases can be induced by inoculating normal plants with the juice of
spotted ones even though such juice has been filtered through porcelain,
and concludes that a "_contagium fluidum vivum_" of the nature of a
transmissible enzyme is the agent which disturbs the physiology of the
infected cells.
Koning, while confirming these results in the main, refers them to a
micro-organism so small that it traverses the porcelain filter.
_Upheaval of seedlings._--This is a common form of injury, resulting in
death by drought and exposure, especially in seedling pines, wheat,
etc., in soils exposed to alternate freezing and thawing during spring
when there is no snow to protect the plants. The soil freezes during the
night, and during the thaw next day water accumulates just below the
surface. The freezing is then repeated, and, partly owing to the
expansion of the forming ice and partly to the mechanical effect of the
ice-crystals in the interstices, the surface of the soil is lifted and
draws the roots with it. During the succeeding thaw the soil particles
fall away from the lifted root-fibres, and frequent repetition of these
processes results in such complete exposure of the roots to the full sun
that the plantlet falls over and wilts.
_Exposure of roots_ is also sometimes effected by winds displacing sandy
soils liable to shifting in dry weather, and the resulting wilting of
the plants thus exposed at their roots may be supplemented by damage due
to the repeated impact of the wind-driven sharp grains of sand, which
act like a sand-blast and erode the tissues.
In many of the cases given above the principal result is the weakening
or destruction of the chlorophyll action. This means a loss of
carbohydrates--sugars, starches, etc.--and in so far a starvation of the
plant. The injurious effects are quantitative and cumulative: if large
areas of foliage are concerned, or if the effect lasts a long time, the
plant suffers from loss of food, and may die. In those cases where the
effect is due to the cutting off of supplies at the roots, and where the
yellowing is a secondary symptom, the disease is more general in
character, and recovery is often impossible, because the loss of water
cannot be compensated, and the results may be further complicated by the
gradual penetration of poisonous matter into the cells. It is frequently
necessary, though sometimes very difficult, to decide which is the
primary and which secondary (or tertiary, etc.) symptoms in the order of
their importance, and the diagnosis may be complicated by a number of
accessory factors which it is impossible to treat generally.
NOTES TO CHAPTER XIX.
The principal cases here described are dealt with in works on
plant physiology, and in the works of Sorauer and Frank
already referred to.
As regards damage due to uprooting of seedlings by frost, see
Fisher, "Forest Protection" (Engl. ed. of Hess' _Forstchutz_),
in Schlich's _Manual of Forestry_, Vol. IV., 1895, pp.
439-442.
On Albinism, see Church, "A Chemical Study of Vegetable
Albinism," _Journ. Chem. Soc._, 1879, 1880, 1886.
Beijerinck's results are contained in his paper, "Ueber ein
Contagium vivum fluidum," etc. (with English abstract), in
_Verhandl. d. Kon. Akad. v. Wetensch, te Amsterdam_, 1898.
Koning's paper is in _Zeitschr. f. Pflanzenkrank._, Vol. IX.,
1899, p. 65. See also _Nature_, Oct. 11, 1900, p. 576.
CHAPTER XX.
SYMPTOMS OF DISEASE (_Continued_).
_Spotted leaves--The colours of spots--White, yellow, brown,
and black spots on leaves--Parti-coloured spots--The browning,
etc., of leaves._
_Discoloured spots_ or patches on the herbaceous parts of plants,
especially leaves, furnish the prominent symptoms in a large class of
diseases, due to many different causes, and although we cannot maintain
this group of symptoms sharply apart from the last, as seen from the
considerations on _albinism_, it is often well marked and of great
diagnostic value. By far the greater number of spot-diseases are due to
fungi, but this is by no means always the case. The most generally
useful method of subdividing the classes, though artificial like all
such classifications, will be according to the colour of the spots or
flecks, which, moreover, are usually found on the leaves. It is
necessary to note, however, that various conditions may modify the
colour of spots on leaves. Many fungi, for instance, induce different
coloured spots according to the age of the leaf or other organ attacked,
or according to the species of host, the weather, etc. Moreover the
spots due to these parasites are frequently yellow when young and some
other colour, especially brown or black, when older.
_Scale_ is the name given to the characteristic shield-like insects
(_Mytilaspis_, _Aspidiotus_, etc.) which attach themselves to branches
of Apples, Pears, Oranges, Camellias, and numerous other plants, and
suck the juices. It is the female insect which has the body broadened
out into the "scale," under which the young are brought up. Enormous
damage has been done by some forms--_e.g._ the San Jose scale in the
United States.
The superficial resemblances of the patches of eggs of some Lepidoptera
to Aecidia and other fungi may be noted in passing--_e.g._ _Bombyx
neustria_ on Apple twigs, _Aporia Crataegi_.
_White_ or _greyish spots_ are the common symptom marking the presence
of many Peronosporeae and Erysipheae in or on leaves, _e.g._
_Peronospora Trifoliorum_, _P. parasitica_ on Crucifers, etc., and
_Sphaerotheca_ on Hops; also _Septoria piricola_, _Cystopus_, _Entyloma
Ranunculi_, etc.
White spots are also caused by insects such as _Tetranychus_ (red
spider) on Clover and other plants.
_Yellow_, or _Orange-coloured Spots_. In cases where these occur on
leaves, and in the case of grasses, etc., on the leaf sheaths as well,
they commonly indicate the presence of Uredineae, and sections under
the microscope will show the mycelium in the tissues beneath. Species of
_Uromyces_, _Puccinia_, etc., in the Uredo state have the spots powdery
with spores; _Aecidia_ show the characteristic "cluster cups," and so
forth. These spots are often slightly pustular, and in some cases
markedly so.
Other fungi also induce yellow spots on leaves--_e.g._ _Phyllosticta_ on
Beans, _Exoascus_ on Poplars, _Clasterosporium_ on Apricot leaves,
_Synchytrium Succisae_ on _Centaurea_, etc.
Yellow spots are also a frequent symptom of the presence of Aphides, of
Red Spider, etc. Thus the minute golden yellow spots sometimes crowded
on Oak leaves are due to _Phylloxera_ punctures.
Yellow patches are formed on the large leaves of _Arisarum_ by a species
of parasitic Alga, _Phyllosiphon_, which lives in the mesophyll. Many
tropical leaves are spotted yellow by epiphytic Algae--_e.g._
_Cephaleuros_.
It must be noticed that many fungi produce yellow spots or flecks in the
earlier stages, which turn brown or black as the fructifications appear,
_e.g._ _Dilophia graminis_, _Rhytisma acerinum_.
The yellow-spotted leaves of _Farfugium grande_ (_Senecio Kaempferi_)
are so like those of _Petasites_ attacked with _Aecidium_ in its early
stages, that an expert might be deceived until the microscopic analysis
was completed.
_Red spots_, varying from rusty or foxy red to bright crimson, are the
symptomatic accompaniment of several fungi, the former often
characterising the teleutospore or aecidium stage of Uredineae--_e.g._
_Aecidium Grossulariae_--the latter sometimes indicating the presence of
Chytridiaceae.
Red spots are also caused by _Gloeosporium Fragariae_ on Strawberry
leaves, _Polystigma rubrum_ on Plums.
Crimson spots on Apple and Pear leaves are also due to _Phytoptus_: they
turn brown later.
_Brown spots_ or flecks, varying in hue from dull slaty brown to deep
red browns, are a common symptom of Fungus and Insect diseases, the
colour often indicating the death of the tissues, rather than any
special peculiarity of the action of the parasite. Good examples are
furnished by the Potato-disease, and by _Peronospora viticola_,
_Sphaerella vitis_ and other disease-fungi of the Grape Vine. The
teleutospore stage of many Uredineae also occurs in deep brown spots.
Black spots and flecks are exceedingly common symptoms of the presence
of fungi, _e.g._ _Fusicladium_ on Apples and Pears, and the pycnidial
and ascus stages of many Ascomycetes--_e.g._ _Phyllachora graminis_. The
teleutospore stages of species of _Puccinia_, _Phragmidium_, etc., are
also so deep in colour as to appear almost black.
_Scab_ on Pears is due to the presence of _Fusicladium_, which indurates
the outer skin of the fruit causing it to crack under pressure from
within, and to dry up, the deep brown to black patches of fungus
persisting on the dead surface.
Black spots on grasses and sedges are caused by Ustilagineae, and are
commonest in the grain, the soot-like powdery spores (Smut) being very
characteristic. _Ustilago longissima_ induces black streaks on the
leaves. Many of these fungi cause distortions or pustules on leaves and
other organs.
Brown and black leaf spots are frequently furnished with concentric
contours arranged round a paler or other coloured central point--_e.g._
_Cercospora_ on Beans, _Ascochyta_ on Peas.
Brown spots with bright red margins are formed in young Beans by
_Gloeosporium_.
Species of _Fumago_, _Herpotrichia_, etc., may cover the entire surface
of the leaf with sooty patches, or even weave the leaves together as if
with black spider-webs.
_Mal nero_ of the Vine is a particular case of black spotting and
streaking of the leaves for which no satisfactory explanation is as yet
to hand. As with Chestnuts, Walnuts, and other plants containing much
tannin, the dark spots appear to be due to this substance, but whether
the predisposing cause is a lack of some ingredients in the soil, or
some temperature reaction, or fungi at the roots, is as yet unknown. The
most recent explanation puts the disease down to the action of bacteria,
but the results obtained by different workers lead to uncertainty.
The "dying back" of leaves, especially of grasses, from the tip, is
usually accompanied by a succession of colours--yellow, red, brown, to
black--and is a common symptom of parching from summer drought; and
spots of similar colours, frequently commencing at the margins of
leaves, are characteristic symptoms of the injurious action of acid
gases in the air.
Brown and blackish spots on Pears are caused by a species of _Thrips_.
In many cases the minute spots of Rust-fungi on one and the same leaf
are bright orange yellow (_uredo_), deep brown, or almost purple-black
(_teleutospores_), foxy-red brown (older uredospores), or dead slaty
black where the old teleutospores have died off--_e.g._ _Uromyces Fabae_
on Beans, _U. Pisi_ on Peas, etc.
_Parti-coloured leaves._--The leaves sometimes start shrivelling with
red edges, while yellow, red, and finally brown and black blotches
appear on the lamina, from no known cause--_e.g._ Vines. In other cases
similar mimicry of the autumnal colouring of leaves results from the
action of acid gases.
_Burning_ is a common name for all cases where the leaves turn red or
red-brown in hot, dry weather, and many varieties are distinguished in
different countries and on different plants, because species react
dissimilarly. The primary cause is usually want of water--drought.
_Foxy leaves_ are a common sign of drought on hot soils, and the disease
may usually be recognised by the gradual extension of the drying and
fox-red colour proceeding from the older to the younger leaves, and from
base to apex--_e.g._ Hops.
_Coppery leaves._--The leaves of the Hop, etc., may show yellow spots
and gradually turn red-brown--copper-coloured--as they dry; the damage
is due to _Tetranychus_, the so-called Red Spider. These cases must of
course be carefully distinguished from the normal copper-brown of
certain varieties of Beech, Beet, _Coleus_, etc.
_Silver-leaf._--The leaves of Plum, Apple, and other fruit trees often
obtain a peculiar silvery appearance in hot summers, the cause of which
is unknown.
Discolorations in the form of confluent yellow and orange patches, etc.,
resembling variegations, are not infrequently due to the ravages of Red
Spider and mites--_e.g._ on Kidney Beans.
_Sun-spots._--Yellow spots, which may turn brown or black according to
the species of plant affected and the intensity of the action, are often
caused by the focussing of the solar rays by lens-like thickenings due
to inequalities in the glass of greenhouses, or by drops of water on
them or on other leaves, _e.g._ Palms, _Dracaena_, etc. The action is
that of a burning glass, and extends throughout the leaf-tissues. Young
grapes, etc., may also be injured in this way. Water-drops on the glass
can only act long enough to produce such injuries if the atmosphere is
saturated. The old idea that a drop on a leaf can thus focus the sun's
rays into the tissues beneath is not tenable.
Here again we see that the disease-agencies concerned in producing the
symptoms described in this chapter, agree for the most part in so far
that the principal effect is generally the disturbance of chlorophyll
action in the spots or flecks on the leaves, and the rendering useless
of these areas so far as providing further food-supplies is concerned.
The effects may be due merely to the shading action of a
parasite--_e.g._ epiphytic fungi--or to actual destruction of the
tissues invaded--_e.g._ by endophytic fungi--or the tissues may be
burnt, poisoned, etc. In so far the results are again quantitative and
cumulative, and the amount of damage depends on the number and size of
the spots or other areas affected, and the proportion of foliage
involved, as well as the length of time the injurious action is at work.
But, again, it must be remembered that several symptoms may co-exist,
and matters may be complicated by the spread of the destructive agent,
or its consequences, to other parts, and in some cases we are quite
uninformed as to the true nature of the disease.
NOTES TO CHAPTER XX.
Further information regarding these "leaf-diseases" will be
found in special works dealing with the fungi and insects
which cause them. In addition to works already quoted, the
reader may also be referred for Fungi to Massee, _A Textbook
of Plant-diseases caused by Cryptogamic Parasites_, London,
1899; or Prillieux, _Les Maladies des Plantes Agricoles_,
1895. See also Marshall Ward, Coffee-leaf Disease, _Sessional
Papers_, XVII., Ceylon, 1881, and _Journ. Linn. Soc._, Vol.
XIX., 1882, p. 299.
The question of "Sun-spots" has been dealt with by Joennson in
_Zeitschr. f. Pflanzenkrankh._, 1892, p. 358.
CHAPTER XXI.
ARTIFICIAL WOUNDS.
_The nature of wounds and of healing processes--Knife wounds--
Simple cuts--Stripping--Cuttings--Branch-stumps and pruning--
Stool-stumps--Ringing--Bruises._
_Wounds._--All the parts of plants are exposed to the danger of wounds,
from mechanical causes such as wind, falling stones or trees, hail,
etc., or from the bites of animals such as rabbits, worms, and insects,
and although such injuries are rarely in themselves dangerous, they open
the way to other agencies--water, fungi, etc., which may work great
havoc; or the loss of the destroyed or removed tissues is felt in
diminished nutrition, restriction of the assimilative area, or in some
other way.
We have seen that living cells die when cut, bruised, or torn; and that
the cells next below in a layer of active tissue are stimulated by the
exposure to increased growth and division, and at once produce a layer
of cork, the impervious walls of which again protect the living cells
beneath. This is found to occur in all cell-tissues provided the cells
are still living, and it matters not whether the wound occurs in the
mesophyll of a leaf, the storage parenchyma of a Potato-tuber, the
cortex of a root or stem, or in the fleshy parts of a young fruit, the
normal effect of the wound is in all cases to call forth an elongation
of the uninjured cells beneath, in a direction at right angles to the
plane of the injured surface, which cells then divide by successive
walls across their axis of growth: the layers of cells thus cut off are
then converted into cork, by the suberisation of their walls. Further
changes may then go on beneath the protective layer of wound-cork thus
produced, and these changes vary according to the nature of the cells
beneath: the cambium forms new wood, the medullary rays similar rays,
cortex new cortex, and so on.
_Knife-wounds._--Artificial cuts in stems are easily recognised and soon
heal up unless disturbed. Several cases, differing in complexity, are to
be distinguished. The simplest is that of a longitudinal, oblique, or
horizontal short cut in which the point of the knife severs all the
tissues of the stem down to the wood. The first effect usually observed
is that the wound gapes, especially if longitudinal, because the cortex,
tightly stretched on the wood cylinder, contracts elastically. This
exposes the living cortex, phloem and cambium to the air, and such
tissues at once behave as already described above: the cells actually
cut die, those next below grow out under the released pressure, and
these give rise to cells which become cork. As the growth and
cell-division continue in the cells below this thin elastic cork-layer,
they form a soft herbaceous cushion or _callus_ looking like a thickened
lip to each margin of the cut. Each lip soon meets its opposite
neighbour, and the wound is closed over, a slight projection with a
median axial depression alone appearing on the surface. The depression
contains the trapped-in callus-cork squeezed more and more in the plane
of the cut as the two lips of callus press one against the other, and
sections across the stem and perpendicular to the axis of the cut show
that this thin cork, like a bit of brown paper, alone intervenes between
the cambium, phloem and cortex respectively of each lip, as each layer
attempts to bridge over the interval. If the healing proceeds normally,
these layers, each pressing against the trapped cork-film, and growing
more and more in thickness, shear the cork-layer and tear its cells
asunder, and very soon we find odd cells of the cambium of one lip
meeting cambium cells of the other, phloem meeting phloem, and cortex
cortex, and the normal thickening of the now fused layers previously
separated by the knife goes on as if nothing had happened, the only
external sign of the wound being a slight ridge-like elevation, and,
internally, traces of the dead cells and cork trapped here and there
beneath the ridge. When the conjoined cambium resumes the development
of a continuous layer of xylem and phloem, no further trace of the
injury is observable, unless a speck of dead cells remains buried
beneath the new wood, and indicates the line where the knife point
killed the former cambium and scored the surface of the wood in making
the wound.
_Stripping._--Now suppose that, instead of a mere slit with the
knife-point, a strip of bark is removed down to the wood. Exactly the
same processes of corking and lip-like callus formation at the edges of
the wound occur, but of course the occlusion of the bared wood-surface
by the meeting of the lips occupies a longer time. Moreover, the living
cells of the medullary rays exposed by the wound on the wood-surface
also grow out under the released pressure, and form protruding callus
pads on their own account. In course of time the wood is again
completely covered by the coming together over its face of these various
strips of callus, but two important points of difference are found, as
contrasted with the simpler healing of the slit-wound. In the first
place the exposed wood dries and turns brown, or it may even begin to
decay if moisture and putrefactive organisms act on it while exposed to
the air; and, in the second place, the normal annual layer of wood--or
layers, as the case may be--formed by the cambium only extends over that
part of the stem where the cambium is still intact, and is entirely
wanting over the exposed area. Thus, if it takes two years for the
cambium to extend across the wound, a layer of wood will be formed all
round the intact part of the stem, from lip to lip of the cut tissues
during the first year; then a second annual layer outside this will be
formed during the second year, but extending further over the edges of
the wound, and nearly complete, because the cambium has now crept
further across the wounded surface to meet the opposite lip of cambium;
and during the third year, when the cambium has once more become
continuous over the face of the wound, the annual wood layer will be
complete. But, of course, this last layer covers in the edges of the two
previously developed incomplete wood-layers as well as the exposed and
brown, dry, or rotten dead face of the wood. It also covers up the
trapped-in brown cork and any debris that accumulated in the wound, and
this "blemish," though buried deeper and deeper in the wood during
succeeding annual deposits of wood-layers, always remains to remind us
of the existence of the wound, the date of which can be fixed at any
future time by counting the annual rings developed subsequently to its
formation. Obviously, also, the deficiency of wood at this place makes
itself visible on the outside by a depression.
_Cuttings._--When a cutting of _Pelargonium_, Willow, or other plant is
made, we have a typical knife-wound, the behaviour of which is very
instructive in illustration of plant-surgery, and may be most easily
seen by keeping it in damp air instead of plunging it into sand or
soil.
All the living cells actually cut or bruised turn brown and die as
before; those beneath--_e.g._ the living pith, medullary rays, cambium,
phloem, and cortex, grow out under the released pressure and form a
callus, the outermost layer of which becomes cork, while those below,
abundantly supplied with food-materials, proceed to spread, as if
flowing over the surface of the cut wood, and rapidly occlude the wound.
Meanwhile new roots are formed adventitiously from the cambium just
above the plane of section, and push out through the cortex into the
damp air, and if the cutting had been in soil it would now be capable of
independent existence. It is important to keep cuttings upright, as the
roots only spring from the lower end. Such cuttings can be obtained not
only from stems, but also from roots and even leaves.
Callus-formation is not confined to the basal end of a cutting; it has
nothing to do with position, but is a reaction to the wound stimuli,
independent of light, gravitation, etc. As time goes on, however, the
internal organisation of the erect cutting usually reacts on the callus
at either end, and roots only rise from the lower one, while shoot-buds
may form in the upper one, though it is possible to bring about the
formation of buds from the lower end also.