off short some distance--say a foot--from the base, where it springs
from the trunk. As before, the immediate effect of the section is the
formation of a callus from the cambium, phloem and cortex, which begins
to rise as a circular occluding rim round the wood. The transpiration
current in the trunk, however, is not deflected into the 12 inches or so
of amputated branch, because there are no leaves to draw the water up
it, and so the stump dries up and the cortex and cambium die back to the
base, leaving the dead wood covered with shrivelled cortical tissues
only. This dead stump gradually rots under the action of wet, fungi, and
bacteria, and since the pith and heart-wood afford a ready passage of
the rot-organisms and their products into the heart of the trunk, we
find in a few years a mere stump of touch-wood and decayed bark, which
falls out at the insertion like a decayed tooth, leaving a rotten hole
in the side of the trunk.
If, however, instead of allowing the basal part of the amputated branch
to protrude as a stump, we cut it off close to the stem, and shave the
section flush with the normal surface of the latter, the callus formed
by the cambium, etc., rapidly grows over the surface, and soon forms a
layer of cambium continuous with that of the rest of the stem. The wound
heals, in fact, much as if it were a strip-wound, and beyond a slight
prominence for a year or two no signs are visible from the outside after
the occlusion. Of course these matters depend on the relative thickness
of branch and stem, and if much wood is exposed the dangers of rot and a
resulting hollow in the stem are increased. It is interesting to note
how much thicker the callus lips are at the sides of the wound than
above and below, owing to differences in the distribution of the
nutrient materials.
_Stool-stumps._--When a tree is felled, the stump may, if the section is
close to the ground and kept moist, begin to form a thick rim-like
callus round the wood, in which adventitious buds soon make their
appearance, and grow out into so-called _Stool-shoots_. The products of
assimilation of these, and the stores accumulated in the stump, often
suffice to feed the callus sufficiently to enable it to grow over and
completely occlude the wound, if the wood surface is not too large, or
so long exposed that rotting processes have meanwhile set in.
_Ringing._--If the strip of cortical tissues and cambium is removed all
round the stem, exposing the wood in a form of a ring, complications may
ensue owing to the following circumstances. A well-marked callus appears
at the upper edge of the wound, because, the transpiration current up
the young wood not being stopped, plenty of water and salts from the
soil can reach the leaves; but the nutritive materials supplied by the
latter are accumulated at the upper lip of the wound owing to the
stoppage there of their descent in the phloem, cortex, etc. No such
callus-lip appears at the lower margin of the wound owing to want of
these supplies. Consequently the occlusion and healing of the ring-wound
only takes place from above downwards, and if the ring of cortical
tissues removed is a broad one, the healing may be a long process, or
may even be indefinitely delayed, a thicker and thicker callus
projecting over from above. For similar reasons no annual wood layers
are formed below, but only above the wound, and thus the branch or tree
may die. The latter contingency is the more likely the further up the
tree the ringing takes place, owing to the risk of drying up which
threatens the exposed wood, and to the consequent interruption of the
transpiration current, and the likelihood that lateral shoots below the
wound may divert the water to their own leaves. If the ringing occurs
low down on a stem, and the environment remains damp, the upper thick
callus may put out new roots; the part above the wound then behaves like
a cutting. If the ringing is done on a young and vigorous branch of an
old tree, the lower lip may receive supplies from the leaves of branches
below the wound, or from shoots which spring from adventitious buds
close to it, and the wound may heal over normally. Such healing may be
rendered more certain by keeping the wounded surface moist--_e.g._ by
means of damp moss, and so encouraging the formation of callus-bridges
from the medullary rays.
If on ringing a tree or a branch the young wood is removed as well as
the cambium and cortical layers, the death of the parts above the wound
is almost certain, owing to the stoppage of the transpiration current:
the exceptions to this rule depend simply on the existence of other
channels of communication, such as internal phloems, very thick
sap-wood, and so forth.
_Bruises._--If a branch or woody stem is struck sharply, with a hammer,
for instance, the bruised cortex, phloem and cambium are killed by the
blow, and the general effect is as if these tissues had been removed at
that spot by the knife, but with the following complications. The
bruised cortical tissues rapidly dry as they perish, and may adhere to
the wood below. Consequently the still sound parts bordering on the
wound are not released from pressure, but, on the contrary, have to
advance towards each other over the surface of the wood under still
greater pressures, in part due to the tightening of the whole cortex as
the dead parts dry and contract, and in part due to the above-mentioned
adherence of the latter to the wood. It results from this that such
wounds heal very slowly and badly, and when the killed patch at last
ruptures, wound-fungi, insects, and other injurious agencies may get in
and do irreparable damage, as has been found to occur in cases where
such wounds have been made in striking trees to shake down insects,
fruit, etc.
NOTES TO CHAPTER XXI.
The essential facts regarding wounds and healing by occlusion
are given in Marshall Ward, _Timber and some of its Diseases_,
1889, chapters viii. and ix., and in Laslett, _Timber and
Timber Trees_, 1894, chapters iv. and v. More detailed
treatment will be found in Frank, _Krankh. d. Pflanzen_, B. 1.
cap. 2, where the special literature is collected. The reader
may also consult Hartig, _Diseases of Trees_, Engl. ed. 1894,
pp. 225-269.
CHAPTER XXII.
NATURAL WOUNDS.
_Burrows and excavations. Bark-boring--Wood-boring--Wood
fungi--Leaf-miners--Pith flecks--Erosions. Skeleton leaves--
Irregular erosions--Shot holes. Frost cracks--Strangulations--
Spiral grooving._
Natural wounds are produced in a variety of ways during the life of the
plant, and, generally speaking, are easily healed over by the normal
process if the area destroyed is not too large, and the parts remaining
uninjured are sufficiently provided with foliage, or with supplies of
food-materials stored up in the roots, rhizomes, medullary rays, etc.,
to feed a vigorous callus.
The nature of such wounds and the mode of healing are explained by what
we know of artificial wounds, and it only remains to point out that the
principal danger of ordinary wounds is not so much the direct traumatic
action, because the simpler organisation of the plant does not involve
matters connected with shock, loss of blood, etc., as in animals; the
danger consists, rather, in their affording access to other injurious
agents, especially fungi, and the treatment of wounds frequently
resolves itself into cutting or pruning in order to get clean surfaces
which can heal readily.
Wounds on leaves imply loss of foliar surface--_i.e._ of chlorophyll
action--and the remarks on page 193 apply.
_Burrows_ may be taken as comprising all kinds of tunnel-like
excavations in the various organs of plants, including those cases where
insects burrow into hollow stems of grasses, etc., as indicated by the
perforations they make in the outer tissues.
_Bark-boring_ is done by many species of beetles, especially
_Scolytidae_, which excavate characteristically formed branching
passages tangentially in the inner bark of Conifers and other trees.
Some of them also bore down to the surface of the sap wood (_e.g._
_Tomicus bidentatus_) or even burrow right into the latter (_e.g._ _T.
lineatum_). It commonly happens that the external apertures show up
clearly, owing to the brown dust and excrement, sometimes accompanied by
turpentine, which exude from them. Many of these Bark beetles only
attack trees which are already injured by fire, lightning, etc.;
possibly they cannot bore through a cortex which swamps them with sap,
as a vigorous one might do.
_Wood-boring_ is also done by many of the bark-beetles as well as by
Longicorns, _e.g._ _Saperda_ in Poplars and Willows, the young shoots
of which often show characteristic swellings with lateral holes
indicating the points of exit. From the external apertures comminuted
wood, like saw-dust, is frequently ejected in quantity and betrays the
presence of the insects. Certain wood-wasps (_Sirex_) and the larvae of
moths (_Cossus_) also make large perforations in the wood of Willows and
other trees, often destroying it completely. In the case of these larger
borers, whose tunnels may be as broad as the little finger, the foul
smell as well as abundant "saw-dust" betray the evil.
Excavations in wood are by no means caused only by insects: several of
the larger Hymenomycetes--_Stereum_, _Thelephora_, _Polyporus_,
etc.--tunnel the timber in characteristic ways and often after a fashion
very suggestive of insects. They usually obtain access through
fractures.
_Tunnels_ in leaves are invariably due to the activity of miners
belonging to the smaller moths and beetles--_e.g._ _Tinea_, _Orchestes_,
etc.--the larvae of which eat out the mesophyll but leave the covering
epidermis or cuticle untouched, and since the insect bores forwards
only, in an irregular track, and leaves its excrement in the winding
passage, the effect is very characteristic.
Whitish leaf tunnels in Peas are excavated by _Phytomyza_.
Characteristic foxy-red tunnels are mined in the leaves of Apples by
_Lyonetia_, _Coleophora_, etc.
_Falling of fruit_, of Apples, Plums, Apricots, etc., before they are
ripe, is frequently due to insects, of which the various species of
_Grapholitha_ or _Carpocapsa_ are conspicuous: the fallen fruits show a
small hole leading by a labyrinth of passages to the "core" or "stone,"
and in which the grub and its excrement are visible. The cutting off of
the vascular bundles and disturbance of the water supply only partly
explain the premature fall.
_Pith-flecks_ are minute brown specks or patches found in the
wood-layers of many trees, and consist of dead parenchymatous
thick-walled cells, reminding one of the structure of pith. They are
explained as due to the borings of minute insects, _Diptera_ or Beetles,
the larvae of which pierce the cortex and phloem and bore their way into
the cambium. The latter then occludes the tunnels by filling them up
with cells, and continuing its wood-forming activity gradually buries
them deeper and deeper in the wood. Such pith-flecks are common in
Willow, Birch, Alder, _Sorbus_, etc. It is possible that they may be due
to other causes also in other trees.
_Erosions_ or _irregular wounds_ on leaves are caused by large numbers
of grubs and caterpillars and other insects, such as earwigs, as well as
slugs, snails, and other animals; but it must by no means be assumed
that all marginal leaf wounds, for instance, are caused by animals,
since many fungi which rot the tissues, as explained below (p. 208),
also cause such erosions, the putrescent parts falling out--_e.g._ the
Potato disease.
_Skeleton leaves_ frequently result from the ravages of caterpillars,
which leave the coarser ribs and veins untouched, but much finer
skeletons with the minute veins almost intact may be found on plants
infested with certain insects--_e.g._ _Selandria_ on Cherries.
Skeletonised patches on Cherry leaves, often pink or brown-pink, are
eaten out by this grub.
_Shot-holes_ are perforations in leaves presenting the appearance, from
their more or less rounded shape, of gunshot wounds. They may be due to
insects which bore through the young leaves while still folded in the
bud--_e.g._ Willow Beetle--or which gnaw out the tissue--_e.g._ the
Beech Miner. Similar but usually more torn and irregular holes are eaten
out by many caterpillars--_e.g._ the Cabbage Moth.
Shot-holes on Peas may be the work of Thrips.
Leaf perforations are commonly caused by severe hail-storms, the
hail-stones beating right through the thin mesophyll. Certain chemicals
used for spraying have also been known to cause shot-holes by killing
the tissue beneath the standing drops.
There is, however, a class of shot-holes in thin leaves which are due to
the action of minute fungi, the mycelium of which so rots the tissues in
a more or less circular area round the point of infection, that, in wet
weather, the decomposing mass falls out and leaves a round hole--_e.g._
certain Chytridiaceae, Peronosporeae, _Gloeosporium_, _Exoascus_, etc.
If dry weather supervenes these holes frequently dry at the edges, and
the leaves appear as if eaten out.
Shot-holes in Cherry, Walnut, Tobacco, and Plum leaves are due to
_Phyllosticta_, in Cherry leaves also to _Clasterosporium_, and in
Potato leaves to _Haltica_.
_Frost-cracks._--The trunks of trees exposed to the north-east, and
occasionally with other aspects, are apt to show longitudinal ridges
which realise on a larger scale the features of healed wounds scored
with a knife. These wounds are due to the outer layers of wood losing
water from their cell-walls as it congeals to ice in their lumina, more
rapidly than do the warmer internal parts of the trunk; as this drying
of the wood causes its shrinkage, especially in the tangential
direction, the effect of a sudden frost and north-east wind is to rend
the wood, which splits longitudinally with a loud report, as may often
be heard in severe winters. Since the cortex and bark are ruptured at
the same time the total effect resembles that of a deep knife-cut, and
the same healing processes result on a larger scale when the wood swells
and closes up the wound again in spring. But this recently-closed lesion
is evidently a plane of weakness, and if a similarly severe winter
follows the wound reopens and again heals, and so on, until after a
succession of years a prominent _Frost-ridge_ results, which may finally
heal completely if milder winters ensue or the tree be eventually
protected.
_Strangulations._--We are now in a position to understand the so-called
strangulations which result when woody climbers, telegraph wires, etc.,
kill or injure trees by tightly winding round them. If strong wire is
twisted horizontally round a stem, the growth in thickness of the latter
causes the trapping of the cortex and cambium, etc., between the wire
and the wood, and a ringing process is set up in consequence of the
death of the compressed tissues. A callus then forms above the wound, as
in the case of true ringing by means of a cut, and eventually bulges
over the upper side of the wire: in the course of years this overgrowth
may completely cover in the wire, and, pressing on to the lower lip of
the wound, may at length fuse with the cambium below. Hereafter the
thickening rings of wood are continuous over the buried wire. The
process is obstructed by all the impediments referred to in dealing with
ringing, and of course the stem thickens more above than below the wire.
If the sapwood is thin, and the bark is so thick as to put great
obstacles in the way of the junction of the upper and lower cambiums,
death may result--the tree is permanently ringed. (See p. 201.)
_Spiral grooves_ are frequently met with where Wood-bine or other woody
climbers have twined round a young stem or branch, the upper lip of the
groove always protruding more than the lower. If a kink or a crossing of
two plants or branches of the twiner results in a complete horizontal
ring, the results are as in the above cases of ringing and
strangulation. Naturally grooved walking sticks are often seen.
_Buried letters, etc._--These processes of healing by occlusion enable
us to understand how letters of the alphabet, cut into the wood of
trees, come to be buried deep in the timber as successive annual rings
cover them in more and more. Chains, nails, rope, etc., have frequently
been found thus buried in wood.
NOTES TO CHAPTER XXII.
In addition to the notes to the last chapter, the reader may
be referred to Fisher in Vol. IV. of Schlich's _Manual of
Forestry_, Chap. VI., for an account of Hess' excellent work
on Boring Beetles, etc.
The authority on Wood-fungi is Hartig, see especially his
_Zersetzungs-erscheinungen des Holzes_, the principal results
of which are condensed in his _Diseases of Trees_ already
referred to. As regards "Pith-flecks," the reader should
consult Frank, _Krankh. der Pflanzen_, B. I., p. 212: the
subject needs further investigation.
CHAPTER XXIII.
EXCRESCENCES.
_Herbaceous excrescences, or galls--Erineum--Intumescences--
Corky warts, etc.--Pustules--Frost-blisters--Galls and Cecidia
--Root nodules._
_Excrescences_, or out-growths of more or less abnormal character from
the general surface of diseased organs, are very common symptoms, and
widely recognised. They are due to hypertrophy of the tissues while the
cells are young and capable of growth, and may be induced by a variety
of causes, among which the stimulus of insect-punctures and of the
presence of insect eggs are best known; but that of fungi, though less
widely recognised, plays an equally important part, and, as we shall
see, galls and other excrescences may be due to widely different agents.
_Galls_ or _Cecidia_ are protuberances of the most varied shapes,
colours, and sizes found on herbaceous parts attacked by insects, fungi,
etc. In the simplest cases the insects only pierce and suck the young
cellular tissue--_e.g._ _Phytoptus_, Aphides, etc.--but in others the
stimulus to hypertrophy starts by the puncture of the embryonic tissue
of a leaf, root, etc., by the ovipositor of the female insect, which
then lays an egg--_e.g._ _Cynips_, _Cecidomyia_, etc.--the presence of
which appears to intensify the irritating action, or such only occurs
when the young larva escapes.
Our knowledge of the primary cause of gall-formation amounts to very
little. Generally speaking, only embryonic or very young cellular tissue
reacts, and galls on adult leaves and branches have usually been
initiated long before. The same gall-insect may induce totally different
galls on different plants, or even on different parts of the same plant,
and different insects call forth different galls on any one plant. These
facts point clearly to the co-operation of both plant and insect in the
gall-formation, and the best hypothesis yet to hand is to the effect
that a gall is a hypertrophy of cells, the normal nutrition, growth, and
division of which have been disturbed owing to the action of some poison
or other irritant derived from the insect, or fungus, or other organism.
Attempts have been made to reproduce galls by injecting the juices of
similar galls into the tissue, but as yet without success, and this may
point to the co-operation of mechanical irritation during the
hypertrophy in normal gall-formation.
Galls, in the broad sense, are not always preceded by a wound, however.
Insects on the outside of young tissues may cause such irritations that
the parts in contact with the animal are arrested in their growth, while
those further away grow more rapidly--_e.g._ where Mites, etc., cause
puckers and leaf-rolling. In true galls the hypertrophy may consist
merely in the enlargement of cells already present, and no new
cell-divisions and, still less, changes in the nature of the tissues
result--_e.g._ some pocket galls on _Viburnum_, _Pyrus_, etc., and the
hairy outgrowths of the epidermis known as _Erineum_. In other cases
there is not only hypertrophy of existing cells, but new cell-divisions
are instituted: these cell-divisions may be confined to the direction
perpendicular to the epidermis, and the tissues grow only in the
direction of the surface, producing puckerings--_e.g._ the Aphis galls
on _Ribes_, Phytoptus galls of _Salvia_, leaf galls on _Tilia_, _Acer_,
_Alnus_, etc., and the curious galls on Plums due to _Cecidomyia Pruni_,
and which must not be confounded with the "pocket plums" and similar
galls due to Exoasci.
In a third series of cases, cell-divisions occur parallel to the surface
of the leaf, and galls are formed which grow in thickness, and develop
the most extraordinary and complicated new tissues--proteid-cells
surrounding the egg or larva deposited inside, followed by a protective
layer of sclerenchyma encasing this food layer, and around this again
softer tissues which may assume the structures and functions of
respiratory tissues, water-storing tissues, starch reservoirs,
assimilatory, or protective tissues of various kinds, and over all may
be a well-marked epidermis, with stomata, or cork with lenticels.
The chief seat of these hypertrophies and--what is more
remarkable--development of new tissue elements not found elsewhere in
the leaves, or even in the species, is the mesophyll, and various
speculations and hypothesis have been founded on these curious
phenomena.
_Erineum._--The simplest excrescences on plants are certain hair-like
developments of epidermal cells due to the irritation of species of
_Phytoptus_, and similar insects which rise in clusters on the surfaces
of leaves and by their colours, consistence, arrangement in patches,
spots, etc., so simulate fungi that Persoon was deceived by them and
gave them the genus name _Erineum_. They occur on most of our trees,
_e.g._ Poplar, Lime, Oak, and are very common in the Tropics. Usually
pale or even white at first, they turn brown as the hair-like outgrowths
die and lose their sap, but since the latter may be bright
coloured--yellow, red, purple,--the patches are sometimes very
conspicuous objects on smooth leaves.
In many cases these hairs exactly resemble in shape and other characters
the abnormal root-hairs found on roots exposed to the effects of
poisonous reagents, or of unsuitable food-materials, or the rhizoids
developed from wounded Algae, etc.
_Intumescences_ are similar trichomatous outgrowths not associated with
insects or fungi, and due to some disturbance of the balance between
transpiratory and assimilatory functions of their leaves, as indicated
by the less localised occurrence and by their non-appearance when the
plant is under favourable cultural conditions. Structures not unlike
these have been artificially induced by exposure to particular lights,
and also by painting spots with dilute corrosive sublimate, indicating
that poisons may impel the epidermis cells to grow out abnormally.
_Corky warts._--Several forms of disease are known in which the
pathological condition is expressed by the formation of cork in unwonted
places and quantities. The _Scab_ or _Scurf_ of Potatoes is a case in
point. The tissue of the lenticels absorbs water and the outermost cells
are cut off by cork and die: the cells below them burst the dead
bark-like masses thus formed, and again cork is formed and cuts off the
outer masses, and the rough cork warts--_Scab_ or _Scurf_--are the
result.
The causes predisposing to scab have been variously assigned to
dampness, want of lime, action of bacteria and fungi--_e.g._
_Sorosporium_, _Oospora_, _Spongospora_,--the latter making their way
into the ruptured tissue of the lenticels and irritating the cells to
further growth.
It seems probable that several different kinds of scab exist in
Potatoes, as well as in roots--_e.g._ Beets, and the whole subject needs
further investigation. The scab-like rough scaly bark of Pear trees in
dry districts may also be mentioned here.
_Cork-wings_ are well known on the young branches of Elms, Maples, etc.,
some varieties of which have received specific names on this account.
_Corky excrescences_ on leaves occur occasionally in the Gooseberry,
Holly and other plants, for which no cause has been discovered.
Lenticels are also formed on some leaf-galls, and are remarkable as
being structures not normal on leaves.
_Pustules._--This term may be employed generally for all slight
upheavals of the surfaces of herbaceous organs, which subsequently burst
and give egress to the spores, etc., of the organism causing them, or
merely fray away at the top if no organism is discoverable. They are
often due to fungi--_e.g._ _Synchytrium_, _Protomyces_, _Cystopus_, and
Ustilagineae,--and we may extend the use of the general term also to
those cases where the _stroma_ of the fungus itself bursts through the
cortex of older parts and forms the principal part of the
pustule--_e.g._ _Monilia_, forming white or grey pustules on Apples,
_Roestelia_ and other AEcidia, forming yellow or orange pustules on
leaves, etc.; _Cucurbitaria_ and _Nectria_ (red) breaking through the
cortex of trees, and _Phoma_ and numerous other Ascomycetes which form
black cushions. _Pustules_ on the leaves of _Lysimachia_, _Ajuga_, etc.,
are due to the parasitic Alga _Phyllobium_.
Cylindrical stem swellings are caused by _Calyptospora_: they are due to
the hypertrophy of the cortex of Bilberry stems permeated by the
hyphae. _Epichloe_, which clothes the sheaths and halms of grasses with
its stroma, at first snowy white and later ochre-yellow as the
perithecia form, is another example.
The cylindrical layer of eggs of a moth such as _Bombyx_ on a twig must
not be confounded with these cases.
_Frost-blisters_ are pustule-like uprisings of the cortex, where the
living tissues below have formed a callus-like cushion into the cavity
beneath the dead outer parts of the cortex which were killed by the
frost; they occur on the stems of young Apples, Pears, etc.
_Galls_ in the narrower sense are tissue outgrowths usually involving
deeper cell-layers. They are so varied and numerous that classification
is difficult. For symptomatic purposes we may divide them as follows:
_Leaf-galls._--A well-marked type is that of the _pocket-galls_ or
_bladders_ in which the whole thickness of the leaf is as it were pushed
up like a glove-finger at one spot, so that if the upper surface of the
leaf forms the outside of the gall the lower surface is its lining. Such
galls are common on Limes (_Phytoptus_), _Glechoma_ (_Cecidomyia_), Elms
(_Tetraneura_), etc. Similar localised extension of the leaf surface,
compelling it to rise up like a pocket, are caused by fungi--_e.g._
_Taphrina_ on Poplars, _Exoascus_ on Birches, etc., _Exobasidium_ on
Bilberries, Rhododendrons, etc.
Another type is that of the _Gall-apple_, so well known on Oaks, where
the spherical swelling is solid--except for the inner cavity containing
the eggs--_Neurotus_, _Cynips_, _Hormomyia_, etc. These are comparable
in general characters to the nodules on roots.
Fungus galls with similar external features when young are found on
Maize (_Ustilago Maydis_), and betray their nature by the black powdery
spores as they mature.
Bud galls on Willows are due to _Cecidomyia_, which causes several
internodes to swell out into a greenish barrel-shaped mass, from which
leaves may spring.
Small irregular excrescences on Willow stems are referred to
_Phytoptus_, and another species of the same insect induces similar
swellings on Pines which are not surcharged with resin.
_American Blight_, or Woolly Aphis, on Apples especially, causes the
tumour-like swellings covered with sticky white fluff, which is a waxy
excretion of the insect. Galls on _Pilea_, in Java, are due to an
Alga--_Phytophysa_.
_Root-nodules_ or _nodosities_ are frequently caused by insects--_e.g._
_Centhorhynchus_, a beetle which attacks Crucifers, _Cynips_ and allied
"gallflies" of Oaks, and the notorious _Phylloxera_. But similar
root-galls are produced by Nematode worms, _Heterodora_, on Beets,
Tomatoes, Cucumbers and numerous other plants, and by the Slime fungus
_Plasmodiophora_, and it is not always easy to distinguish such cases
from the fungus-galls (_Mycocecidia_) on the roots of Alders, _Juncus_,
and Leguminoseae where the symbiosis of bacteria or fungi with the
roots are of benefit to the plant. _Urocystis Leimbachii_ forms similar
nodules at the collar of young plants of _Adonis_.
_Heterodora javanica_ passes into the cortex of sugar-cane roots through
fissures, and makes its way to the place where a young rootlet is about
to emerge; here it sticks its beak into the growing-point and remains
fixed.
Molliard has shown that in the roots of Melons, _Coleus_, etc.,
_Heterodora_ causes the cells in immediate contact with its head, and
which would normally become vessels of the xylem, to swell up into huge
giant-cells, with their walls curiously folded, and containing large
supplies of proteids and numerous nuclei, reminding us of the food-layer
of insect galls and of the tapetal layer of pollen-sacs. While the
stimulus exerted by the Nematode thus induces hypertrophy and storage
with food-substances of these cells, those of the next layers undergo
reticulate thickenings of their walls. Again instances of the evolution
of new tissue elements by the action of the foreign organism.
So far as galls on leaves are concerned the amount and kind of damage
done are in proportion to the area of chlorophyll action put out of play
for the benefit of the plant, and the remarks already made on p. 193
apply here also. Where buds are destroyed the effects may of course
extend further, but it rarely happens that leaf-galls are so abundant as
to maim a tree permanently. Nevertheless we must remember that cases
like _Phylloxera_ are notorious.
Far more dangerous, however, are the root-galls due to such insects,
because here the damage is not so local: the water-supplies are cut off,
and injurious consequences result from the absorption of the products of
decomposition in the soil.
NOTES TO CHAPTER XXIII.
In addition to the literature on galls quoted in the Notes to
Chapter XIV., the reader should consult Dale "On certain
Outgrowths (Intumescences) on the green parts of Hibiscus,"
_Proc. Cambr. Phil. Soc._, Vol. X., 1899, p. 192, and _Brit.
Ass. Rep._, Bradford, 1900.
The detailed study of the anatomy and histology of Galls has
been recently undertaken by Kuester, "_Beitrage zur Kenntniss
der Gallenanatomie_," Flora, B. 87, 1900, p. 117, where the
principal references will be found.
On the root-galls due to Nematodes see Atkinson in _Science
Contributions from the Agric. Expt. Station, Alabama_, Vol.
I., p. 1, 1889; Percival, "An Eel-worm disease of Hops" in
_Natural Science_, Vol. VI., 1895, p. 187; and Molliard in
_Revue generale de Botanique_, Apl., 1900, p. 157, where the
histology is dealt with.
The nodules of the roots of Leguminoseae are not part of the
subject of this work: the literature is collected in _Science
Progress_, 1895, Vol. III., p. 252, and Dawson, _Phil.
Trans._, 1900.
CHAPTER XXIV.
EXCRESCENCES (_continued_).
_Cankers--Burrs--Sphaeroblasts, and other excrescences of
woody tissues--Witches' Brooms._
_Cankers_ are irregular excrescences due to the perennial struggle
between tissues attempting to heal up a wound, and some organism or
other agent which keeps the lesion open. A canker always originates in a
wound affecting the cambium, and usually in a small wound such as an
insect puncture or frost nip; if undisturbed the dead parts would heal
over by cork and callus, but if recurring frost-cracks break open the
coverings, or if insects or fungi penetrate the callus and invade the
cambium, irregularities of growth due to the occluding tissue on the one
hand, and continued growth of the still unimpaired cambium on the
opposite side of the injured shoot on the other, result in the canker.
Frost cankers occur on fruit-trees, Vines, Beeches, etc.
Cankers due to insects are found on Apples, the cortex of which is
punctured by the woolly Aphis (_Schizoneura_) while the twigs are young,
and the wound is kept open by the insects nestling in crevices in the
occlusion tissues. Species of _Coccus_, _Lachnus_, and _Chermes_ also
produce cankers on forest trees.
Cankers due to fungi usually originate in a wound primarily due to an
insect puncture or bite, or to frost, the invading fungus hyphae making
their way into the wounded tissues and gradually extending more and more
into the cambium and the occluding callus. Among the best known of these
wound fungi which cause cankers are _Dasyscypha Willkommii_ the peziza
of Larch disease, _Nectria ditissima_ and _N. cucurbitula_ on Beech and
Conifers; less common are _Scleroderris_ on Willows, _Aglaospora_ on
Oaks and some others.
_Peridermium Pini_ and _Aecidium elatinum_ also cause cankers under
certain conditions, as also does _Gymnosporangium_, but in these cases
the fungi are more truly parasitic.
In some cases--_e.g._ Ash, Pine, Olives--bacteria are concerned as
associated organisms in the cankering of trees.
_Burrs_ or _Knauers_ are irregular excrescences, principally woody, with
gnarled and warted surfaces. They are frequently due to some previous
injury, such as the crushing or grazing of cortical tissues by
cart-wheels. The excitation of the tissues thus wounded results in the
development of shoots from adventitious or dormant buds at the base of
old tree trunks, or in the starting of the same process where a branch
has been broken off. The new bud begins to develop a shoot, but soon
dies at its tip owing to paucity of food-supplies to the weak shoot,
while new buds at its base repeat the process next year with the same
result, and each of these again in turn, and so on. The consequence is
an extremely complex nest of buds, all capable of growing in thickness
and putting on wood to some extent, but not of growing out in length. In
course of time this mass may attain dimensions measurable by feet,
forming huge rounded and extremely hard-knotted burrs, the cross-section
of which shows the vascular tissues running irregularly in all
directions, and, owing to the very slow growth, extremely dense and
hard. The dark spots in such sections--_e.g._ Bird's-eye Maple--are the
cut bud-axes all fused together, as it were. On old Elms such burrs are
common at heights on the stem which preclude the assumption of any
coarse mechanical injury, and similar structures occur on the boles of
other forest trees suddenly exposed to light by the felling of their
companions, which suggests that these epicormic shoots result from some
disturbance due to the action of light.
_Witches' Brooms_ are irregular tufts of twigs often found among the
branches of trees such as Birches, Hornbeam, etc., where they look like
crows' nests, and similar structures are to be found on Silver Firs and
other conifers. In the former case they are due to _Exoascus_, in the
latter to _Aecidium_, fungi which are perennially parasitic in the
shoots, and stimulate the twiggy development of a number of buds which
would normally have remained in abeyance, or not have been formed at
all, and only do so now in a fashion different from that of normal
branches.
Rosette-like formations, depending on similar disturbing causes on the
part of insects, occur in conifers--_e.g._ _Gastropacha Pini_.
Dense tufts of twiggy shoots may be developed on many trees by pruning
in such a way as to stimulate the shooting out of basal buds which would
otherwise remain dormant, _e.g._ Elm, Ash, and thus it occurs that
injuries such as frost, insect bites, etc., may induce the production of
such tufts in a tree crown. The dense nests of stool-shoots thrown up
from felled tree-stumps are of essentially the same nature--partly
adventitious and partly dormant buds being enabled to grow out because
they can now be supplied with materials previously carried beyond them
while the trunk was still there. Suckers, if repeatedly cut down, may
also behave similarly.
_Wood-nodules_ or _Sphaeroblasts_ are curious marble-like masses of wood
which protrude with a covering of bark from old trunks of Beeches, etc.,
and can be readily dug out with a knife. The nodule has arisen by the
slow growth of the cambium of a dormant bud, the base of which separated
at an early date from the wood beneath; the cambium then closed in over
the base and laid on thickening rings all round the axis of the bud
except at the extreme apex. When the separation occurred the cambium of
the wood beneath covered over the previous point of junction, and thus
the woody bud was pushed out with the bark, and now protrudes covered
with a thin layer of the latter. Similar nodules are occasionally found
on Apple trees.
NOTES TO CHAPTER XXIV.
For further information on Cankers the student should read
Marshall Ward, _Timber and some of its Diseases_, Chapter X.
Further, the discussion as to the causes of canker in Frank,
_Krankheiten der Pflanzen_, B. I., p. 207, and B. III., pp.
167 and 172, and various papers in _Zeitschrift fuer
Pflanzen-krankheiten_.
CHAPTER XXV.
EXUDATIONS AND ROTTING.
_Tumescence--Rankness--Bursting of fruits, etc.--Root rot--Rot
of fruits--Bulb diseases--Flux--Honey-dew--Slime flux--
Resinosis--Gummosis--Manna._
I put together in one artificial class a varied group of diseases, the
principal symptom of which is the escape of fluids from the tissues,
under circumstances which betray an abnormal state of affairs, often
obvious, but sometimes only to be inferred. In many of these cases
bacteria abound in the putrefying mass, and some evidence exists for
connecting these microbes causally with the disease in a few of the more
thoroughly investigated cases, but in no case has this been sufficiently
demonstrated; and considering the ease with which bacteria gain access
_via_ wounds caused by insects and fungi, as well as by other agents,
the necessity for rigid proof must be insisted upon before we can accept
such alleged examples of _Bacteriosis_.
_Tumescence._--It occasionally happens that herbaceous parts of plants
pass into a condition of over-turgescence from excess of water in the
tissues, an abnormal state which indicates pathological changes
resulting from various causes, often not evident and therefore regarded
as internal. Such disease was formerly termed _Oedema_ or _Dropsy_.
This disease is frequently due to the excessive watering of pot plants
with large root systems and deficient foliage, in hot-houses with a
saturated atmosphere: it is, therefore, primarily referable to
diminished transpiration. It can sometimes be brought about by covering
potato plants, for instance, with a bell-jar in moist, hot weather; and
this, and the prevalence of the disease in hot-houses as compared with
plants grown out of doors, point to the above explanation. Similar
phenomena do occasionally occur out of doors in hot, moist situations or
during wet seasons, however, and the watery shoots of rank vegetation
are merely particular cases of the same class. Moreover, the well-known
tendency to succulence of sea-side varieties of plants which have thin
herbaceous leaves when growing inland, points to the action of the
environment in these matters, excess of salts being no doubt one factor
in such cases.
_Rankness_ affords another example where superfluity of water is
concerned, though it does not involve simply this, because the plant may
also contain excessive quantities of nitrogenous and mineral matters
taken up by the roots.
Rankness is, in fact, in many respects analogous to etiolation in so far
as the tissues are soft and surcharged with water, but it differs
fundamentally in the deep green of the chlorophyll: this may lead to
abundant assimilation if free access of air and drier conditions can be
gradually brought about. Any sudden drying, however, may be fatal to the
tender tissues.
Rankness commonly depends on excess of food materials, especially
nitrogenous manures, as may be seen in meadows and cornfields where the
manure heaps have remained on the ground and saturated it to excess as
compared with the rest of the soil; this may often be observed with
weeds, etc., in the neighbourhood of farm-buildings. If the period of
rank growth is accompanied and followed by days of suitably bright
sunshine and dry air, the increase of vegetative structures usually
results in increased flowering, heavy crops, or strong wood; but if the
rankness continues too long, or is accompanied by wet and dull weather,
the watery tissues are peculiarly susceptible to attacks of fungi and
insects, and to damage by sudden frosts or chilly winds. Rankness
affords, in fact, a typical illustration of predisposition to disease.
_Damping off._--When seedlings are too closely crowded in beds kept too
damp, or in moist weather, they are very apt to rot away, with all the
symptoms--spreading from a centre, contagious infection, mycelia on and
in the tissues, etc.--of a fungus attack. The commonest agent concerned
is one of the species of _Pythium_, the propagation of which is favoured
by the rank, over-turgid, and etiolated conditions of the plants.
Species of _Mucor_, _Botrytis_, and other fungi, may also be met with.
_Bursting_ of fleshy fruits, such as Tomatoes, Grapes, etc., is due to
over-turgescence in rainy weather or excessively moist air. But the
phenomenon is by no means confined to such organs. Hot-house plants when
oedematous not infrequently put out watery blisters from the cortex or
leaves, which rupture; and the stems of fleshy fasciated (_e.g._
Asparagus) or blanched and forced plants (_e.g._ Celery, Rhubarb) are
particularly apt to crack here and there from the pressure of the
turgescent tissues on the strained epidermis. Beets, Turnips, and other
fleshy roots show the same phenomena in wet seasons. That these ruptures
and exposures of watery tissues afford dangerous points of entry for
parasites and moulds will be obvious--_e.g._ _Edelfaeule_, a rotten
condition of the grapes in the Moselle district.
_Root-rot_ is a common disease in damp, sour clay soils after a
continuance of wet weather--_e.g._ Wheat, especially if root-drawn and
exposed to thaw water.
In the disease known as Beet-rot, the roots turn black at the tip, where
the tissues shrivel and become grooved and wrinkled extensively. Inside
the flesh also blackens and finally rots. In earlier stages, only the
vascular bundles are brown and blocked with gum-like substances. In
advanced stages there is much gummy material in the lumina, and even
large cavities filled with this gum may be found.
The rot of Cherries, Pears, Apples, Plums, etc., in store may be due to
several fungi, of which _Botrytis_, _Monilia_, _Mucor_, _Penicillium_,
and _Aspergillus_ are the chief. The fruit may be attacked while still
on the tree, but very often fungi and bacteria gain access to the
tissues, through bruises, cracks, etc., formed in the fruit lying in the
storage baskets or on the shelves.
Rot in Onions, Hyacinth bulbs, etc., is frequently due to the access of
_Botrytis_ or _Sclerotinia_, followed by moulds, yeasts, and bacteria in
the stores.
_Sour-rot_ in Grapes, and other fleshy fruits which need much sun to
ripen them, is probably a usual result of continued cold, wet weather at
the cropping season, setting in when the fruits are beginning to swell.
_Flux._--It is a common event to see fluids of various kinds issuing
from wounds in trees, or congealing in more or less solid masses about
them; and owing to the prevailing tendency to compare plant diseases
with those of animals, we find such expressions as _Gangrene_, _Ulcer_,
and so forth, applied to these "open sores." In so far as such
outflowings frequently indicate diseased states of injured tissues which
are incapable of healing up, the analogy is perhaps a true one; but it
must be remembered that very different structures and processes in
detail are concerned. Moreover, liquid excretions more or less
indicative of diseased states are by no means confined to wounds or
definitely injured tissues, in which case such terms are wholly
misapplied.
_Honey-dew._--The leaves, or other organs, of many plants are sticky in
hot weather, owing to the excretion of a sweet liquid containing sugar,
the consistency and colour of which vary according to circumstances.
This honey-dew must not be confounded with the normal viscidity of
certain insectivorous plants--_e.g._ Sundew--or with the sticky
secretion on the internodes of species of _Lychnis_, etc., where it
plays the part of a protection against minute creeping things.
Honey-dew is often met with on Lime trees, Roses, Hops, etc. In many of
these cases the honey-dew is excreted by Aphides, which suck the juices
of the leaves and pour out the saccharine liquid from their bodies. The
sweet fluid is in its turn sought after by ants, and also serves as
nutritive material for various epiphytic fungi--_e.g._ sooty mould,
_Capnodium_, _Fumago_, and _Antennaria_--which give the leaves and
honey-dew a brown or black colour. Certain _Coccideae_ also excrete
honey-dew, especially in the tropics.
At least one case is known where honey-dew is formed as the result of
the parasitic action of a fungus, namely _Claviceps purpurea_ in its
conidial stage on the stigmas of cereals, and this may be compared with
the sweet odorous fluid excreted by the spermogonia of certain
_Aecidia_. In both cases the sweet fluid attracts insects which
disperse the spores.
Honey-dew may also be formed without the agency of fungi or insects,
when hot and dry days are followed by cool nights, with a saturated
atmosphere, _e.g._ _Caesalpinia_, _Calliandra_ and other trees in the
tropics, which are called rain trees owing to the numerous drops of
fluid which drip from the leaves under the abnormally turgescent
conditions referred to.
_Cuckoo-spit._--The leaves of Willows, Meadow grasses and herbs, etc.,
are often seen with froth on them, in which is a green insect,
_Aphrophora_, which sucks the juices from the tissues and excretes the
frothy watery cuckoo-spit from its body.
_Slime-flux._--The trunks of trees may sometimes be observed to pour out
a slimy fluid from cracks in the bark, or from old wounds, or branch
scars. In some cases, _e.g._ in Oaks, the slime has a beery odour and
white colour, and abounds in yeasts and other fungi to the fermentative
activity of which the odour and frothiness are due. In other cases the
slime is red _e.g._--Hornbeam; or brown--_e.g._ Apple and Elm; or
black--_e.g._ Beech, the colour in such cases being due to the mixture
of yeasts, bacteria, and fungi with which these slimes abound. The
phenomenon appears to be due to the exudation of large quantities of sap
under pressure--root pressure--and is primarily a normal phenomenon
comparable to the bleeding of cut trees in spring: the fungi, etc., are
doubtless saprophytes, but their activity is concerned with the
putrefactive processes going on in the diseased wood, and which may lead
to rotting of the timber.
The origin of the wounds in the bark and cortex, and which extend into
the wood and other tissues as the putrefactive and fermentative
processes increase, appears to be in some cases at least due to
lightning.
_Resin-flux_ or _Resinosis_.--The stems of Pines and other conifers are
apt to exude resin from any cut or wound made by insects, or by the
gnawing of other animals; but in many cases the flow is due to fungi,
_e.g._ _Peridermium_, the hyphae of which invade the medullary rays and
resin canals and thus open the way to an outflow through cracks in the
bark. _Agaricus melleus_ not only invades the resin passages, but
stimulates the tree to produce abnormal quantities of resin, which flows
down to the collar and roots, and exudes in great abundance at the
surface of the soil. Various other plants also exude resin from wounds,
and in some cases the flux seems to be increased by degeneration of the
tissues, _e.g._ _Copaifera_.
_Gummosis._--Cherries, Apricots, Acacias, and many other trees are apt
to produce abnormal quantities of gum, which flows from any wound or
exudes through cracks in the bark. Degeneration of the wood-cells, and
especially of the cell-walls of a soft wood formed by abnormal activity
of the cambium, points to its origin being due, in some cases at any
rate, to a conversion of the cellulose, and fungi are sometimes found in
the masses of gum; but beyond the fact that _gummosis_ is a pathological
phenomenon we know very little of the disease.
With regard to such gumming, it is significant how frequently pruned
trees--Cherries, Oranges, Lemons, Plums, etc.--suffer.
_Manna flux._--Certain trees, such as the Manna Ash, species of
Tamarisk, etc., yield manna from wounds, and in some cases the latter
are due to insects, _e.g._ _Cicada_.
The Potato-disease is best known by the pale whitish fringe, giving an
almost mealy appearance to the margins of the brown to black patches in
damp weather. In dry weather the brown patches shrivel and dry, and as
they are apt to be at the edges and tips of the leaflets, these curl up.
The young disease spots are yellowish, and the leaves of badly affected
plants are apt to be sickly yellow throughout.
This Potato-disease due to _Phytophthora_ must be distinguished from the
curling and puckering, with wilting and browning of the leaves and
yellow glassy look of the stems, due to the invasion of the vessels by a
fungus which lurks in the tubers, and gains access thence to the shoots.
In the disease traceable to _Phytophthora_ the stock remains green and
the leaves plump and plane, and only the brown patches slough out in wet
or shrivel in dry weather, and are bordered by the pale whitish zone of
conidiophores.
In the leaf-curl the yellow and flaccid appearance of all the leaves of
a stalk, or even of the plant, is the striking symptom, and the stem
soon droops and blackens just above the soil, a white mould appearing
also at the black spots. Subsequently black spots appear higher up, and
bacteria gain an entrance. The stolons rot, and eventually the roots and
the leaves wither. The tubers appear sound, but are small; they are apt
to rot in the store, the vascular zones turning brown.
This leaf-curl has been ascribed to _Pleospora_, _Polydesmus_,
_Verticillium_, and other parasites, as well as to excessive manuring
and other agencies, but it still needs explanation.
Rot of Potato tubers in the soil, or in store, may be brought about by
very different agents.
If _Phytophthora_ has obtained access, the fungus hyphae spread between
the cells, starting from the haulm, and cause the flesh to turn
yellowish and then brown in patches. On the exterior are discoloured
patches, depressed, with the flesh beneath brown and soft. The mycelium
spreads mostly in the outer layers, which though they turn deep brown
remain firm.
Wet rot of potatoes may be due to various fungi, and, in excess of
water, to putrefactive bacteria (_e.g._ _Clostridium_), which destroy
the cell-walls. The flesh becomes soft, then soup-like, and finally
putrefies to a liquid mass with a vile smell of butyric acid, etc., in
which the starch grains may be seen floating.
Tubers are often found with the cork burst and peeling in shreds, the
flesh more or less converted into a putrid and stinking pulp, with a
spotted brown boundary of partly destroyed but firmer tissue between the
dark utterly rotten and the white and still firm healthy flesh. The
principal agent in the destruction of the tissues is _Clostridium_, an
anaerobic bacillus which consumes the cell-walls but leaves the starch
intact. Hence a thoroughly decomposed tuber consists of a cork bag full
of starch and foetid liquid. In the dried condition the flesh shows a
brown marbling; this passes into a soft soupy starchy part, and here and
there may be violet grey cavities lined with _Spicaria_, _Hypomyces_,
etc., the white stromata of the latter often appearing externally. The
excavations are filled with loose starch grains, and bounded by cork and
cambium formed in the peripheral cells. The cell-walls eventually
undergo slimy decomposition.
_Spicaria_, _Fusisporium_, various moulds, and bacteria may all be
associated with wet-rot.
Dry-rot of Potatoes is also due to various fungi and bacteria, but the
destructive action goes on slowly, owing to there being no more moisture
than the tissues afford. The flesh becomes excavated here and there,
owing to the slow destruction of the cell-walls by _Clostridium_: the
destroyed tissues are brown, and the uninjured starch grains powder them
all over. Finally the whole shrunken mass has a crumbly consistency.
When the flesh remains white, but assumes a powdery consistency and
dry-rot, with the cork destroyed here and there, Frank refers the
damage to _Phellomyces_. Where the dry-rot is due to _Fusarium_ the
chalk-white stromata may often be detected breaking through the
periderm; but it must be remembered that the soil-contaminated, broken
skin of a potato-tuber is a favourable lurking spot for many fungi, and
_Periola_, _Acrostalagmus_, and others have been detected therein.
Brown spots, depressed into the flesh, sometimes result from the ravages
of _Tylenchus_, the minute worms being found in the diseased tissues.
In some cases the flesh turns watery and soft, grey, almost glass-like,
starting at the haulm end, and this may be owing to the invasion of
_Rhizoctonia_.
NOTES TO CHAPTER XXV.
The rotting of bulbs, roots, etc., has been much discussed
during the last few years in the pages of the _Gardeners'
Chronicle_, _Zeitschrift fuer Pflanzenkh._, and elsewhere. The
principal references to Bacteriosis--the rot in which bacteria
are stated to be the primary agent causing these and similar
diseases--may be found in Massee, _Diseases of Plants_, pp.
338-342, and more fully in Russell, _Bacteria in their
Relation to Vegetable Tissue_, Baltimore, 1892; and in Migula,
_Kritische Uebersicht derjenigen Pflanzen-krankheiten, welche
Angeblich durch Bakterien verursacht werden_, Semarang, 1892.
The most convincing accounts, however, are since that date;
see Smith, "Pseudomonas Campestris," _Cent. f. Bakt._, B.
III., 1897, p. 284, and Arthur and Bolley, _Bacteriosis of
Carnations_, Perdue University Agr. Expt. Station, 1896, Vol.
VII., p. 17. Woods has lately shown that this disease is due
to Aphides only, the bacteria having nothing to do with the
disease primarily, _Stigmonose_, _Bull. 19_, U.S. Dept. Agr.,
1900; but it is necessary to bear in mind that actual
penetration of the cell-walls from without must be proved, as
De Bary proved it for germ-tubes of fungi, before the evidence
that Bacteria are truly parasitic in living plants can be
called decisive. This is a difficult matter, but until it is
settled we do not know whether these organisms are really
parasitic in the sense that _Phytophthora_ is, or merely gain
access by other means--I have traced them through dead
fungus-hyphae--to the vessels, dead cell-walls, etc. The proof
of infection _via_ water pores and vessels is given for one
species by Harding, "Die Schwarze Faulnis der Kohls," etc.,
_Cent. f. Bakt._, Abh. II., B. VI., 1900, p. 305, with
literature.
Concerning the "Damping off" of seedlings, see Marshall Ward,
"Observations on the Genus Pythium," _Quart. Journ. Microsc.
Soc._, Vol. XXIII., 1883, p. 485, and Atkinson, _Bull. 94 of
Cornell University Agric. Expt. Station_, 1895, p. 233.
On Bacteriosis in Turnips, see Potter, _Proc. R. S._ 1901,
Vol. LXVII., p. 442.
CHAPTER XXVI.
NECROTIC DISEASES.
_Patches--Frost-patches--Bruising due to hail, shot, etc.--
Fire--Sun-burn or scorching--Sun-cracks. Dying-back--Frost--
Fungi--Wound fungi--Defoliation by insects--Defoliation by
hand--Staghead._
_Necrosis._--This is a general term for cases where the tissues
gradually turn brown or black in patches which die and dry up, the dead
area sometimes spreading slowly and invading the usually sharply
demarcated healthy tissues around. It is a common phenomenon on the more
slender stems or branches of trees, especially those with a thin cortex,
and the terms _Brand_ or _Scorching_ sometimes applied signify the
recognised resemblance between burnt patches and these dead areas of
necrotic tissue.
Necrosis is often due to frost, which kills the cortex of Pears, Beech,
etc., in patches of this kind. The dead cortex and cambium stick to the
wood beneath and contract as they dry. The living cambium and cortex
around them then begin to push in callus towards the centre of the
necrotic area; but since this callus is formed under the pressure of the
cortical tissues it does not form a thick lip or margin to the healing
wound, as it does in a Canker, but insinuates itself with thinned-off
edges between the wood and the dead tissue, or at most traps a little of
the latter in the final closing up of the wound. It is easy to see how
such an area of Necrosis may become a Canker if the dead tissues split
or slough off, and fungi or insects obtain access to the callus at the
margins of the area, setting up the disturbances described on p. 222. As
matter of fact many Cankers--_e.g._ those of the Larch disease, and
those due to _Nectria_, or Aphides, etc.--often begin as flattened or
depressed areas of Necrosis started by frost, and many small necrotic
patches would eventually become Cankers if not healed up by the callus.
Necrosis may also be due to the bruising of the tissues by large
hailstones, to gun-shot wounds, or to any form of contusion which kills
the living cells of cortex and cambium.
Necrosis is a natural and common result of fire, and it frequently
happens after forest-fires which have run rapidly through the dry
underwood, fanned by steady winds, that the lower parts of the boles are
scorched on one side only. The killed cambium and cortex then dry up in
black necrotic patches, which may eventually heal up by intrusion of
callus from the uninjured parts.
_Sun-burn_ or _Scorching_.--If thin-barked trees, such as Hornbeam,
Beech, Firs, etc., which have been growing in partial shade owing to
dense planting, are suddenly isolated by thinning, the impingement of
the sun's rays on the south-west side during the hottest part of summer
days may kill the cambium, and produce necrosis of the cortical tissues,
and such necrotic patches heal very slowly or not at all, because the
dead tissues have contracted so tightly on to the wood below that the
callus cannot readily creep between.
_Sun-cracks_ are due to intense insolation on the south side of trees in
clear weather in early spring, causing the drying and contraction of the
wood and its coverings down that side of the tree: the contracted
tissues consequently split, as in the case of frost-cracks, the healing
up of which is very similar.
_Dying-back._--All that is true of the necrosis of cortical tissues in
small patches also applies to cases where the whole of the outer tissues
of thin twigs and branches die of inanition owing to a premature fall of
leaves--_e.g._ after a severe attack of some insect or fungus pest. The
consequent arrest of the transpiration current and the proper supply of
nutriment to the cambium and cortex explain the phenomena. The younger
branches of Coffee trees suffering from severe attacks of leaf-disease
are often denuded of leaves and die back from the causes mentioned, the
whole of the outer tissues becoming necrotic, and drying up tight on to
the wood, because other branches with functionally active leaves on them
divert the transpiration current, and drought and inanition supervene.
Dying-back is frequently also a direct effect of early frosts, which
kill the thin twigs before the "wood is ripened," as gardeners say.
Dying-back is also a frequent result of direct frost action on thin
watery shoots or "unripe wood," and is apt to occur every year in
certain varieties of Roses, for instance, in particular situations, such
as "frost-beds," or aspects exposed to cutting winds, and so forth. The
necrosis which results may affect all the tissues, or only the cortex
and cambium, and the frequent accompaniment of all kinds of saprophytic
_Ascomycetes_ and moulds or other fungi is in no way causal to the
phenomenon.
Dying-back may also be caused by fungi, and not necessarily parasites,
for cases are often observed where saprophytes only are to be found in
the necrotic tissues of the cortex, having made their way in through
minute cracks, lenticels, etc.
A simple case is often seen in Chrysanthemums, Roses, etc., chilled and
wetted to danger point, but not frozen, during the nights of autumn. The
lowered resistance of the chilled tissues enables fungi like _Botrytis
cinerea_ to gain a hold, and the peduncles die-back with all the
symptoms of Necrosis, the fungus gaining power more and more as its
mycelium spreads in the dead tissues.
Many other cases are known where wound-fungi, such as _Nectria_,
_Cucurbitaria_, _Phoma_, etc., in themselves incapable of true
parasitism, gain a hold on the necrotic tissue of a wounded twig, and
having laboriously accumulated a vigorous mycelium saprophytically,
extend into other parts. In many of these cases the dying-back of the
twigs is expedited owing to the mycelium invading the medullary rays and
wood vessels, and so obstructing the transpiration current. The much
more rapid spread of the hyphae up into the parts thus killed
sufficiently indicates the fundamentally saprophytic character of such
fungi.
Dying-back in all its forms is a common result of defoliation by
insects, _e.g._ caterpillars, especially if it occurs when the wood is
depleted of reserve materials, and thus cannot supply the auxiliary buds
and enable the twigs to clothe themselves with a new flush of foliage, a
common danger in Conifers.
Any form of defoliation--_e.g._ excessive plucking of tea and mulberry
leaves, browsing of animals, etc.--exposes the twigs to the dangers of
dying-back, the accessory phenomena being similar to those already
described.
_Stag-head._--Old trees, though vigorous and in full foliage throughout
the crown generally, frequently lose the power of bearing leaves on
their topmost branches and twigs, which stand out bare and brown, and
fancifully resemble the antlers of a stag: hence the forester's name
"stag-head." This "top-dry" condition is frequently due to the removal
of litter, or to excessive draining, or to the roots having gradually
penetrated into unsuitable soil. The consequence is that some dry
summer the drought causes the breakage of the water columns above, and
the twigs die back.
Tropical trees may also become _stag-headed_ owing to the attacks of
_Loranthus_ and other parasites, the portions above the point of
attachment dying back from inanition.
Cases also occur in the tropics where the _stag-head_ condition is due
to the persistent roosting of frugiferous bats--"flying foxes"--which
tear the bark and foliage with their claws, and befoul the twigs
generally.
NOTES TO CHAPTER XXVI.
The principal literature as regards frost is given in the
works of Frank, Sorauer, and Hartig already referred to. An
excellent summary will be found in Hartig's _Diseases of
Trees_, p. 282, and in Fisher "Forest Protection," Vol. IV. or
Schlich's _Manual_, p. 423.
CHAPTER XXVII.
MONSTROSITIES AND MALFORMATIONS.
_Monstrosities--Teratology--Atrophy of organs--Shanking of
grapes--Barren fruit trees--Dwarfing--Distortions and
malformations--Fasciations--Flattened roots--Torsions--Curling
and puckering--Leaf rolling--So-called "spontaneous"
teratological changes._
_Monstrosities._--In a wide sense this term is applicable to many cases
here treated under other headings, and signifies any departure from the
normal standard of size, form, arrangement, or number of parts, and so
forth, due to arrest of growth, excessive growth of parts, or of the
whole organs, etc.
Such _teratological_ conditions are however by no means always
_pathological_: that is to say, they may be variations which do not
threaten the existence of the plant. In some cases they are clearly due
to exuberant nutrition, and although they may occasionally predispose to
disease, in others they show no evidence of doing so. The whole
practice of horticulture and agriculture abounds in examples of
teratological sports or varieties which are transmissible by seeds,
budding and grafting, and other means--_e.g._ double flowers,
hypertrophied floral organs (cauliflowers), seedless grapes and oranges,
crested ferns, etc.; and even when such varieties could not live as such
in a state of nature, there is evidence to show that many of them
readily revert to the original seed-bearing or single condition, and
adapt themselves to the altered environment.
Every part of the plant may exhibit teratological changes, and I shall
for the most part select cases in illustration which indicate approach
to pathological states, and group with them cases known to be
pathological in origin.
_Atrophy_ is a common phenomenon denoting dwindling or reductions in
size of organs due to insufficient nutrition, or arrest of growth from
various causes.
Atrophy of leaves is a common result of the attacks of parasitic fungi,
even when the latter induce local hypertrophy--_i.e._ excessive growth
of particular parts, _e.g._ _Synchytrium_ on Dandelions and Anemones.
_Puccinia suaveolens_ causes partial atrophy of the leaves of Thistles,
_Aecidium Euphorbiae_ of those of _Euphorbia_.
The carpels of Anemone are atrophied in plants attacked by _Aecidium_,
and the whole flower is suppressed in Cherries infested with _Exoascus
Cerasi_, while other fungi--_e.g._ _Cystopus_, _Exoasci_, etc.--cause
atrophy of the seeds, and numerous instances of atrophied grain occur in
plants infested with Ustilagineae.
Atrophy of the grains of cereals is sometimes due to the direct attack
of animals, _e.g._ eel-worms (_Tylenchus_) eat out the grains of Corn;
weevils and other beetles (_Curculio_, _Bruchus_, etc.) similarly devour
the contents of grain and nuts, the flowers of Peas and Apples, and so
forth, inducing atrophy of the parts left. Still more striking cases are
afforded by small insects which bore into the halms of cereals, and
cause atrophy of the whole ear--_e.g._ _Cephus_ in Wheat and Rye. Barley
occasionally withers after flowering, the grain atrophying from no known
cause, terms like _consumption_ given to the disease conveying no
information.
Atrophy of young fruits is commonly due to the flowers not
setting--_i.e._ some agent has interfered with the normal transference
of the pollen to the stigma. This may be due to excessive rain washing
out the pollen (_e.g._ Vine), to a lack of the necessary insects which
effect pollination, often seen in greenhouse plants; to the stamens
being barren--_e.g._ certain varieties of Vine--or to the premature
destruction of the stigmas by frost, as in Cherries, Pears, etc., or by
insects, as in Apples, or fungi, _e.g._ the infection of bilberries with
_Sclerotinia_; or even by poisonous gases, as is sometimes seen in
Wheat, etc., growing near alkali works. Drought is also a common cause
of atrophy of young Plums.
_Shanking of Grapes_ is a particular case of atrophy and drooping of the
immature fruits, due to the supplies being cut off by some agency. It
may arise from very various causes which bring about disease in the
leaves or roots, and should always be looked upon as a sign of weakness
in the Vine, the structure of which is affected, _e.g._ poor wood--or
the functions interfered with, _e.g._ water supplies deficient owing to
paucity of roots.
Barren Apple, Pear, Plum, and other flowers are often found to have been
bored through the petals while in bud, and the whole "heart" of the
flower eaten out by the grubs of _Anthonomus_, leaving the unopened buds
brown and dead, as if killed by frost or drought, and often erroneously
supposed to be so.
The wilting and shrivelling of Clover is sometimes due to _Sclerotinia_,
the mycelium of which pervades the roots and stock, on which the
sclerotia may be found. Lucerne is similarly killed in Europe by the
barren mycelium of _Leptosphaeria_, which may be found as a purple mat
on the roots.
_Dwarfing_ consists in partial atrophy of all the organs, and is a
common result of starvation in poor, dry, shallow soils, as may often be
seen in the case of weeds on walls or in stony places. Dwarfs which are
thus developed in consequence of perennial drought are not, however,
necessarily diseased, in the more specific sense of the word; their
organs are reduced in size proportionally throughout in adaptation to
the conditions, and simply carry out their functions on a smaller scale.
Dwarfing is frequently a consequence of the lack of food materials, or
of some particular ingredient in the soil, and in such cases is a
diseased condition of some danger; similar results may ensue in soils
containing the necessary chemical elements, but in unavailable forms.
Dwarfing may also be brought about by repeated maiming, nipping off the
buds, pruning, etc., as in the miniature trees of the Japanese; and the
case of trees continually browsed down by cattle, or of moor plants
perennially dwarfed by cutting winds, are further illustrations in the
same category, as are also those of certain alpine and moraine plants,
whose only chance of survival depends on their adapting themselves to
the repeated prunings suffered by every young shoot which rises into the
cutting winds, since there is no question of lack of food-materials in
these cases.
The practice of the Japanese is to pinch out the growing tips of the
shoots wherever they wish to prune back, and it is by the judicious use
of this heading in, and suitable pot-culture, that the dwarfs are made,
6-20 inches high at from 30-80 years old.
Dwarfing is often brought about by grafting on a slow-growing stock, and
this method is employed in practice, as are also heading in, pruning of
roots, and confinement in pots.
Dwarfing may also be due to poor or shrivelled--partially
atrophied--seeds or such as have had their endosperms or embryos injured
by insects or fungi, and although it is possible to nurse such dwarfs
into normal and vigorous plants with good culture, they do not usually
recover under natural conditions in competition with more vigorous
plants.
_Distortions_ or _Malformations_ may be defined as abnormalities in the
form of organs which concern all, or nearly all the parts, and do not
refer merely to swellings or excrescences on them or excavations, etc.,
in them.
_Fasciation._--Shoots of Asparagus, Pine, Ash, and many other plants are
occasionally expanded into broad ribbon-like structures often studded
with more than the normal number of buds or leaves, etc., such as would
be found on the usual cylindrical shoots. Such _fasciations_ are due to
several buds fusing laterally under compression when young and the whole
mass growing up in common, or, in a few cases, to the unilateral
overgrowth of one side of the terminal bud. Fasciations appear to depend
on excessive nutrition in rich soils. They may spread out above in a
fan-like manner, exaggerating the abnormality, or they may revert to the
original form. Some cases are more or less fixed by heredity--_e.g._
_Celosia_. Fasciated stems are frequently curved like a crozier, owing
to one edge growing more rapidly than the other.
Cauliflowers are really cultivated monstrosities. Fasciated Dandelions,
_Crepis_, monstrous Chrysanthemums, peloric _Linaria_, five-leaved
Clovers, spiral Teazels, etc., may all, if grown with care, be kept more
or less constant in the monstrous state. That is to say, the particular
kinds of variation here manifested can be maintained in proportion as
the external conditions controlling the variation are maintained. Such
conditions are chiefly rich supplies of food-stuffs, plenty of water and
air, suitable temperature and lighting, etc. Mutilations, favouring the
development of abnormal buds may also induce fasciations.
_Torsions_ or spiral twistings of stems also frequently arise among
plants grown in rich soils, and are often combined with
fasciations--_e.g._ Asparagus, _Dipsacus_; and De Vries has shown that
the peculiarity is not only transmissible by seed, but may be more or
less fixed by appropriate culture.
_Contortions_ of stems are often due to the unequal growth on different
sides of the stems owing to the presence of fungi--_e.g._ _Caeoma_ on
Pines, _Aecidium_ on Nettles, also _Puccinia_ on petioles of Mallow,
_Cystopus_ on inflorescences of _Capsella_, etc.
_Distortions_ of roots may be brought about in various ways by the
hindrances afforded by stones.
_Spiral roots_ occur occasionally in pot plants.
_Flattened roots_ usually result from compression between rocks, the
young root having penetrated into a crevice, and been compelled to adapt
itself later. The distortions of stems by constricting climbers, wire,
etc., have been described, and fruits--_e.g._ Gourds--are easily
distorted by means of string tied round them when young.
Distortions of leaves are very common, and are sometimes
teratological--_i.e._ due to no known cause--_e.g._ the pitcher-like or
hood-like _cucullate_ leaves of the Lime, Cabbage, _Pelargonium_, etc.,
and of fused pairs in _Crassula_. Also coherent, bifurcate, crested,
displaced and twisted leaves occasionally met with, and in some cases
fixed by cultivation, may be placed in this category.
_Puckers_ must be distinguished from pustules, since they consist in
local upraisings of the whole tissue, not swellings--_e.g._ the
yellowish green pockets on Walnut leaves, due to _Phyllereum_.
Puckered leaves in which the area of mesophyll between the venation is
increased by rising up in an arched or dome-like manner are sometimes
brought about by excessive moisture in a confined space.
_Leaf-curl_ is a similar deformation caused by fungi, such as _Exoascus_
on Peaches.
Wrinkling or puckering of leaves is also a common symptom of the work of
Aphides--_e.g._ Hops.
Characteristic curling and puckering, with yellow and orange tints, of
the terminal leaves of Apples, Pears, etc., are due to insects of the
genera _Aphis_, _Psylla_, etc.
Small red and yellow spots with puckerings and curlings of the young
leaves of Pears, the spots turning darker later on, are due to
_Phytoptus_.
_Leaf-rolling._--The leaves of Beeches, Poplars, Limes, and many other
plants, instead of opening out flat, are often rolled in from the
margins, or from the apex, by various species of _Phytoptus_,
_Cecidomyia_, or other insects, which puncture or irritate the
epidermis in the young stages and so arrest its expansion in proportion
to the other tissues. According as the lower or upper surface is
attacked the rolling is from the morphologically upper surface
downwards, or _vice versa_. Very often the mesophyll is somewhat
thickened where rolled and _Erineum_-like hairs may be developed--_e.g._
Lime. Many caterpillars also roll leaves, drawing the margins inward to
form shelters--_e.g._ _Tortrix viridana_, the Oak leaf-roller. Certain
beetles--_Rhynchitis_--also roll up several leaves to form a shelter in
which the eggs are laid.
Webs are formed among the mutilated leaves of Apples by the caterpillars
of _Hyponomeuta_.
It must be borne in mind that instances can be found of teratological
change of every organ in the plant--_e.g._ stamens transformed into
carpels or into petals; anthers partly polliniferous and partly
ovuliferous; ovules producing pollen in their interior, and so on, being
simply a few startling examples of what may happen. Such abnormalities
are frequently regarded as evidence of internal causes of disease, and
this may be true in given cases; in a number of cases investigated,
however, it has been shown that external agents of very definite nature
bring about just such deformations as those sometimes cited as examples
of teratology due to internal causes, and the question is at least an
open one whether many other cases will not also fall into this category.
The study of galls has shown that insects can induce the formation of
not only very extraordinary outgrowths of tissues and organs already in
existence, but even of new formations and of tissue elements not found
elsewhere in the plant or even in its allies; and Solms' investigations
on _Ustilago Treubii_ show that fungi can do the same, and even compel
new tissues, which the stimulating effects of the hyphae have driven the
plant to develop, to take part in raising and distributing the spores of
the fungus--_i.e._ to assume functions for the benefit of the parasite.
Molliard has given instances of mites whose irritating presence in
flowers causes them to undergo teratological deformations, and Peyritsch
has shown that the presence of mites in flowers induces transformations
of petals into sepals, stamens into petals. Similarly De Bary, Molliard,
Magnus, Mangin, and Giard have given numerous cases of the
transformation of floral organs one into another under the irritating
action of fungi, of which the transformation of normally unisexual
(female) flowers into hermaphrodite ones, by the production of stamens
not otherwise found there, are among the most remarkable.
These and similar examples suffice to awaken doubts as to whether any
teratological change really arises "spontaneously," especially when we
learn how slight a mechanical irritation of the growing point may induce
changes in the flower; _e.g._ Sachs showed that a sunflower head is
profoundly altered by pricking the centre of the torus, and Molliard got
double flowers by mechanical irritation.
NOTES TO CHAPTER XXVII.
For the details and classification of the multitude of facts,
the student is referred to Masters' _Vegetable Teratology_,
Ray Society, 1869, and the pages of the _Gardeners' Chronicle_
since that date.
Concerning torsions, etc., the student should read De Vries,
"On Biastrepsis in its Relation to Cultivation," _Ann. of
Bot._, Vol. XIII., 1899, p. 395, and "Hybridising of
Monstrosities," _Hybrid Conference Report_, _Roy. Hort. Soc._,
1900, Vol. XXIV., p. 69.
The reader will find an excellent account of the abnormalities
in flowers due to the action of parasitic insects and fungi in
Molliard, "Cecidies Florales," _Ann. des Sc. Nat._, Ser.
VIII., Bot., T. 1, 1895, p. 67.
CHAPTER XXVIII.
PROLIFERATIONS.
_Proliferations--Vivipary--Prolepsis--Lammas shoots--Dormant
buds--Epicormic shoots--Adventitious buds--Apospory and
apogamy._
_Proliferation_ consists in the unexpected and abnormal on-growing or
budding out of parts--stems, tubers, flowers, fruits, etc.--which in the
ordinary course of events would have ceased to grow further or to bear
buds or leaf-tufts directly. Thus we do not expect a Strawberry--the
swollen floral axis--to bear a tuft of leaves terminally above the
achenes, but it occasionally does so, and similarly Pears may be found
with a terminal tuft of leaves, Roses with the centre growing out as a
shoot, Plantains (_Plantago_) with panicles in place of simple spikes,
and so on.
We regard such cases as _teratological_, because they are exceptional
for the particular species, and as _pathological_ because they appear to
be connected with over-feeding in soils with excessive supplies of
available food-materials; but it should be noted that conditions quite
comparable to proliferation are normal in the inflorescences of
Pine-apples, some Myrtaceae, Conifers, etc., and that many instances of
proliferations come under the head of injurious actions of fungi,
insects, and other agents.
_Proliferation_ of tubers is sometimes seen in Potatoes still attached
to the parent plant in wet weather following a drought. The eyes grow
out into thin stolons, or forthwith into new tubers sessile on the old
tuber. Similarly in store we sometimes find the eyes transformed
directly into new tubers, and cases occur where the growth of the eye is
directed backwards into the softening tuber, and a small potato is
formed inside the parent one.
Threading is also occasionally met with in the "sets" when ripened too
rapidly in hot dry soils.
_Vivipary_ is a particular case of proliferation, in a certain sense,
where the seeds appear to germinate _in situ_, and we have small plants
springing from the flowers, reminding us of wheat which has sprouted in
the shocks in damp weather. In reality, however, the grains are here
replaced by bulbils which sprout before they separate from the
inflorescence. In varieties of _Poa_, _Polygonum_, _Allium_, _Gagea_,
etc., this phenomenon is constant in plants growing in damp situations.
_Prolepsis._--It frequently happens that branches or whole plants are
suddenly defoliated in summer,--_e.g._ by caterpillars or other
insects--at a time when considerable stores of reserves had already been
accumulated during the period of active assimilation. In such cases the
axillary buds, which would normally have passed into a dormant condition
over the winter had the leaves lived till the autumn-fall, suddenly
shoot out into _proleptic_ shoots (also termed Lammas shoots), and
reclothe the tree with foliage. The wood of the year in which this
occurs may exhibit a double annual ring, and the vigour of the tree is
likely to suffer in the following season and no fruit be matured.
Proleptic branches may also be due to the shooting out of accessory
buds--_i.e._ extra buds found in or near the leaf-axils of many plants,
such as Willow, Maples, _Cercis_, _Robinia_, _Syringa_, _Aristolochia_,
etc.--which do not normally come to anything, or do so only if a surplus
of food materials is provided.
_Dormant buds_, or _preventitious buds_, are such as receive no
sufficient supply of water and food materials to enable them to open
with the other buds in ordinary years, for in most trees only the upper
buds on the branches develop into new shoots. The lower buds do not die,
however, but merely keep pace with the growth in thickness of the parent
branch, and may be elongated sufficiently each year to raise the minute
tips level with the bark, their proper cambium only remaining alive but
not thickening the bud.
When, by the breaking of the branch above the insertion of the dormant
bud--or by pruning, defoliation by insects, etc.--the transpiration
current and supplies of food materials are in any way deflected to the
minute cambium and growing points of the dormant buds, they are
stimulated to normal growth, and may grow out as _epicormic shoots_ or
"shoots from the old wood." In many cases such epicormic shoots are
stimulated to grow out by suddenly exposing an old tree to more
favourable conditions of root-action and assimilatory activity, owing to
the felling of competing trees which previously hemmed it in from light
and air, and restricted the spread and action of its roots in the soil.
This is often seen in old Elms, Limes, etc.
It is by such means as the above that substitution branches are obtained
when a leader is broken or cut away.
_Adventitious buds_ are such as are newly formed from callus or other
tissues in places not normally provided with buds, as is often seen on
occluding wounds--_e.g._ stool shoots. They may also be developed on
roots, a fact utilised in propagating _Bouvardias_, Horse-radish, etc.,
by means of root-cuttings, and the _suckers_ of Plums and other fruit
trees are shoots springing from adventitious buds on roots.
Adventitious buds are also common on leaves (_e.g._ _Bryophyllum_,
Ferns, etc.), and are frequently induced on them by wounds--_e.g._
_Gesneria_, _Gloxinia_, etc. Even cut cotyledons may develop them, and
pieces of leafless inflorescence (Hyacinth), hypocotyl (_Anagallis_),
and in fact practically any wounded tissue with a store of reserve
materials may be made to develop them: thus they have been found arising
from the pith of Sea-kale, and are commonly developed from the cut bulb
scales of Hyacinths.
_Apospory_ and _Apogamy_ are particular cases of the production of
vegetative buds on the leaves in place of sporangia in Ferns (Apospory),
and on prothallia in place of Archegonia (Apogamy), in the latter case
induced by dry conditions and strong illumination.
NOTES TO CHAPTER XXVIII.
In addition to the literature quoted in the notes to Chapter
XXVII., the student should consult the works on Forest Botany
for the scattered information regarding adventitious buds. A
good account may be found in Buesgen, _Bau und Leben unserer
Waldbaeume_, Jena, 1897.
For Apospory and Apogamy, see Lang "On Apogamy and the
Development of Sporangia upon Fern Prothalli," _Phil. Trans._,
vol. 190, 1898, p. 187, where the literature is collected.
CHAPTER XXIX.
GRAFTS.
_Grafting--Comparison with cuttings--Effects of environment--
Relations between scion and stock--Variation in grafts--
Grafting and parasitism--Infection--Pollination--Grafts-hybrids
--Predisposition of Natural grafts--Root-fusions._
Grafting is a process which consists in bringing the cambium of a shoot
of one plant into direct union with that of another, and is practised in
various ways, the commonest of which is as follows:
One plant--the _stock_--rooted in the ground, is cut off a short
distance above the surface of the soil, and a shoot from the second
plant--the _scion_--cut off obliquely with a sharp knife, is inserted
into a cleft in the stock, so that the two cambiums (and sometimes the
cortex and pith of each as well) are in close contact: the scion is then
tied in position, the wounds covered with grafting wax, and the whole
left until union of the tissues is completed. This union depends on the
formation of _callus_ at the cut surfaces, and the intimate union of the
ingrowing cells from each callus.
The development of the callus follows the course described for wounds,
cuttings, etc., and the union is exactly comparable to the union of the
two lips of a healing callus over a wound (see p. 197).
Grafting was known and practised far back in the ages. Virgil was well
acquainted with the process, and Theophrastus compared it with
propagation by cuttings.
The scion differs from a cutting, however, in having no roots of its
own: it is parasitic upon, or rather is in symbiosis with the stock, the
root and tissues of which intervene between it and the soil.
Consequently the selective absorption, size and number of vessels, and
innumerable other physiological and anatomical peculiarities of the
stock determine what and how much shall go up into the scion, while the
latter supplies the former with organic materials and rules what and how
much food, enzymes, and other secretions, etc., it shall receive to
build up its substance. Surely, then, if such factors as the nature of
the soil, the water and mineral supplies, the illumination, and the
various climatic factors of altitude can cause variations on a plant
direct, these and other factors are still more likely to be effective on
stock and scion, and each must affect the other.
Nevertheless opinions have differed much as to whether any important
effect is to be seen, and on no point more than on whether the scion
can affect the stock, in spite of such examples as _Cytisus Adami_,
_Garreya_ on _Aucuba_, Sunflower on Jerusalem Artichoke, etc. Recent
results, especially of experiments with herbaceous plants, show that not
only can the stock affect the scion (and _vice versa_) directly, but the
effect of the changes may be invisible on the grafted plant and only
show itself in the progeny raised from the seed of the grafted plant. In
other words, variation occurs in grafts either _directly_, as the
results of the effects of the environment on the graft, or owing to the
interaction of scion and stock, showing as changes in general nutrition
in the tissues concerned, etc., owing to special reactions of the
protoplasm of the uniting cells one on the other, and of the results of
the further protoplasmic secretions, sortings, and so forth, on the
cells developed as descendants of these in the further growth of the
graft: or _indirectly_, in that some of these changes so alter the
nature of the special protoplasm put aside for reproductive purposes,
that the resulting embryo in the seed transmits the effects, and they
show as variations in the seedling. If these results are confirmed they
should meet all objections that have been urged against the transmission
of acquired characters.
In fact there are analogies between grafting and parasitism which cannot
be overlooked, and should not be underestimated, their commonest
expression appearing in the alterations in stature, habit, period of
ripening, and so forth. These analogies are easily apprehended when we
compare parasites like the Mistletoe, _Loranthus_, or even such
root-parasites as the Broom-rapes and the Rhinanthoideae with grafts;
but they also exist in the case of many fungus-parasites, and we might
almost as accurately speak of _grafting_ some fungi on their hosts as of
_infecting_ the latter with them, especially when it is borne in mind
that the effect of the scion on the stock is by no means always to the
benefit of the latter, and that there are reasons for regarding the
action of some such unions as that of a sort of slow poisoning of the
stock by the scion. Why do we not here say that the stock has been
_infected_ by the scion?
The resemblances between pollination and the infection by fungus hyphae
may also be insisted upon. If we take into account Darwin's remarkable
experiments showing that in "illegitimate unions" the pollen exerts a
sort of poisonous action on the stigmas or ovules, it is possible to
arrange a series of cases starting with perfectly legitimate
pollinations where the pollen tube feeds as it descends the style on
materials provided by the cells, and proceeding to cases where the
pollen is more and more merely just able to penetrate the ovary and
reach the ovules, to the extreme cases where no union at all is
possible.
Side by side with such series could be arranged analogous cases where
fungus spores can enter and infect the cells of the host, and live
symbiotically with or even in them, or can penetrate only with
difficulty, or with poisonous effects, and finally cannot infect the
plant at all.
Less obviously, but nevertheless existing, are gradations in grafting to
be observed, where one and the same stock may be successfully combined
with a scion which improves it--or which is improved by it--or the scion
may unite but acts injuriously on it, or, finally, cannot be induced to
unite.
But we may go further than this in these comparisons. Just as the
results of pollination frequently induce far-reaching effects on distant
tissues--_e.g._ the swelling of Orchid ovaries, and rapid fading of the
floral organs--so also the effects of hyphae in the tissues may induce
hypertrophies, deflection of nutrient materials, and the atrophy of
distant parts--_e.g._ the curious phenomena observed in _Euphorbia_
attacked by _Uromyces_--and some of the distant actions in grafts may be
compared similarly.
Going still further, we may compare the effects of cross-breeding or of
hybridisation, where the _progeny_ show that changes have resulted from
the mutual interactions and reactions of the commingled protoplasm, with
Daniel's results, in which he obtains proof of such interactions of the
commingled protoplasmic cell-contents of grafts in the seedling progeny;
although there is no probability--we may even say possibility--in this
latter case that the effects are due to nuclear fusions, but only that
the germ-plasm of the seed-bearing plant has been affected by the
changes in the cell-protoplasm which nourishes it when the reproductive
cells are forming.
In the case of graft-hybrids the matter appears to be somewhat
different, and we may well suppose, with Strasburger, that the
commingling of characters observed in flowers, fruits, foliage, etc., on
shoots borne after grafting are due to the occurrence of nuclear fusions
during the union of the grafted tissues; though it is by no means
impossible that what has really happened is profound alterations in the
nuclear substance (germ-plasm) owing to its being nourished by
cell-protoplasm (somato-plasm) which has been itself affected by the
interchanges of substance between scion and stock, and therefore itself
furnishes a different nutrient medium from the unaltered cytoplasm of
either.
But even here we can find parallels among the ordinary phenomena of
plant reproduction. Maize plants with white endosperm containing starch,
if crossed by pollen from other plants with purple endosperm containing
sugar, bear seeds with purple endosperm containing sugar, and such
_Xenia_ may be compared to graft-hybrids in many respects.
I know of no case among fungus infections which could be compared
directly with these examples, and it is not at all likely that we shall
meet with any instance of a fungus-hypha handing over nuclear substance
to an egg-cell, and so affecting the latter that an embryo results. But
the case is not hypothetically impossible, although the distant
relationships of the two groups of organisms render it extremely
improbable among the higher plants. It is by no means so improbable,
however, that further research may show cases where the egg-cell of a
lower cryptogam--_e.g._ another fungus--may be affected either directly,
or indirectly, by the protoplasm of a parasitic or symbiotic hypha, as
suggested by the extraordinary phenomena of symbiosis.
Some of the variations in grafted plants are found to predispose the
plant to disease, or the reverse, and cases may be cited where the
resulting shoots, foliage, or fruits, or seedlings more readily fall a
prey to, or resist, parasitic fungi and insects than the ungrafted
plants. Daniel gives instances of such--_e.g._ among other examples,
Peas grafted on Beans yield seeds which suffer more from Erysipheae than
the normal seedlings. But the best known cases are those of Vines in
their relations to _Phylloxera_, already referred to (p. 155).
Several instances are also known where grafted plants show more or less
resistance to such factors of the environment as low temperatures;
grafted or budded Roses often suffer much from Erysipheae, and so forth.
Much research is still needed to determine how far these matters depend
on real alterations in the nature of the graft, or _are only true for
the localities in which the experiments have been made_, a point which
has, I think, been overlooked by all observers.
Grafted plants are apparently very much exposed to injury by slugs,
insects, and the invasions of parasites during the healing of the callus
and the fusion process. Here again it must not be overlooked that the
callus is, so to speak, a tit-bit of luscious, thin-walled, succulent
tissue; and, like all wounds, the graft affords entrance to parasites
such as _Nectria_ and Ascomycetes of various kinds, under circumstances
very favourable to their invasion.
_Natural Grafts._--It is by no means an uncommon event to find the
branches of Beeches, Limes, and other trees which have been accidentally
brought into contact during growth, joined where they cross. As they
press one against the other, they become naturally grafted, by that form
of the process known as _inarching_: except that in artificial inarching
the operator cuts off the cortical tissues of the two branches and
brings their cambial surfaces together, whereas in nature the cambiums
only come into contact after the destruction by pressure, or slight
abrasion, of the entrapped intervening tissues. The fusion occurs, in
fact, exactly as in the burying-in of a nail or wire, referred to on p.
211.
Natural grafts are very common among the roots of trees, and possibly
explain some queer cases of the apparent revivification of stumps of
trees not usually given to forming abundant stool shoots. It is regarded
as probable in some old forests that the majority of the roots of trees
of the same species are linked up together by such natural grafts, a
probability not diminished by the fact that such roots cross at many
points, and are easily grafted.
NOTES TO CHAPTER XXIX.
The student should read Bailey, _The Nursery Book_, 1896, for
details regarding the practice of grafting, and facts in
abundance can be obtained from the pages of the _Gardeners'
Chronicle_.
Concerning graft-hybrids and the variations of grafted plants
see Jouin, _Can Hybrids be obtained by Grafting?_ and
especially Daniel, "La Variation dans la Greffe," in _Ann. des
Sc. Naturelles_, S. VIII., Vol. 8, 1898, p. 1, and the
literature there collected. The whole subject is largely
controversial, and much work remains to be done.
CHAPTER XXX.
LIFE AND DEATH.
_Protoplasm--Hypothesis as to its structure and behaviour--
Assimilation--Growth--Respiration--Metabolism--Action of the
environment--Nuclear protoplasm--Pollination--Grafting--
Parasitism--Graft-hybrids--Life--Death--Variation--Disease._
We have seen that all the essential phenomena of disease concern only
the living substance--the protoplasm--of the plant, and that however
complex the symptoms of disease may be, the occurrence of
discolorations, lesions, hypertrophies, and so forth are all secondary
matters subsidiary to the fundamental alterations of structure and
function constituting the disease. It remains to see if we can adopt any
hypothesis as to the nature of this physical basis of life--the
protoplasm--which shall help us to understand still more clearly in what
must reside those processes which, so long as they proceed harmoniously
and uninterruptedly, constitute life and health, and which when
interfered with result in disease and death. The protoplasm of the
living plant-cell looks like a slimy translucent mass which has been
superficially compared in appearance to well-boiled sago or clear gum.
Fifty years of observations and experiments with it have convinced
physiologists that it is not a mere solution or emulsion, however, or
even a chemical compound in the ordinary sense of the term, although
chemical analysis gets little out of it beyond water, proteids,
carbohydrates and fats, and traces of certain mineral salts; for living
protoplasm does not respond to the laws of physics and mechanics in
obeying them, simply as do ordinary solutions and liquids. On the other
hand, the most delicate chemical manipulation fails us, because when
killed it is no longer protoplasm. Nor does the microscope advance
matters far, beyond convincing us that this marvellous material must
have a structure far more intimate than anything visible to the highest
magnifying powers at our disposal.
Nevertheless, some information is forthcoming from the comparative
examination of the protoplasm of numerous different kinds of organisms,
for we have learnt that certain ingredients and no others are necessary
for its composition--namely, carbon, hydrogen, oxygen, nitrogen,
phosphorus, sulphur, calcium[Note: See note at end of chapter.],
magnesium, potassium--and it is as a rule of no use trying to foist on
to it any substitute for any one of these. Moreover, these chemical
elements must be given in certain definite proportions and forms: for
instance it is of no use to offer the carbon and sulphur in such a form
as carbon disulphide, or the nitrogen and hydrogen in that of
hydrocyanic acid, but the carbon must be given to the protoplasm in the
form of a carbohydrate or in some similar form, the nitrogen as an
ammonium salt, nitrate or proteid, the sulphur as a sulphate, and so
forth, and thus water, air, carbohydrates, and the nitrates, sulphates,
and phosphates of potassium, calcium, and magnesium become the chief
natural sources of the essential ingredients. Again, we have learnt that
while there are different forms of protoplasm in the cell, and that
these react on each other, and go through cycles of arrangement and
rearrangements, the intimate structure must be of that kind termed
molecular--beyond the region of vision, just as is the microscopic
structure of a crystal; but, while like the latter affording evidence of
order and sequence when properly examined, the structural arrangements
and changes must be infinitely more complex.
All these, and numerous other results of enquiry, have led to the
conclusions that we must regard living protoplasm as a complex made up
of very large molecular units, each containing atom-groupings of the
elements named; and, partly on account of the large number of atoms they
contain, and partly due to the vibrations of absorbed heat, these units
must be extremely labile. Moreover, they are linked up into an
invisible and intricate meshwork, bathed in a watery liquid held in the
interstices somewhat as water is held in a sponge. In this imbibed
liquid are dissolved the substances, consisting of the same elements,
which are to serve as food, and which are to be taken up into the
molecular framework and built up into the structure of new molecular
units--or, as they may be shortly termed, molecules of protoplasm: in
the bathing liquid are also dispersed the fragments--again containing
the elements named--which have resulted from the breaking asunder of
some of the complex protoplasm molecules, and which are partly destined
to be used up again, partly to be burnt off in respiration, and partly
to be put aside as metabolic products such as reserves, secretions,
permanent structure, etc. Among the elements carried into this liquid
and dissolved in it the free oxygen of the air also plays an important
part.
As new molecules are formed, by mutual combinations of the
food-materials selected by molecular attractions, they are taken up into
the protoplasmic framework, and built in between those already in
existence, thus distending the whole, and we say that the protoplasm
_Assimilates_ food-materials and _Grows_. When distended beyond a given
degree, or disturbed in various other ways, the molecular framework
breaks, and some of the molecules are shattered, and as they fall to
pieces certain of their constituent parts containing carbon and hydrogen
forcibly combine at the moment of liberation with the oxygen in the
fluid around and are burnt off in the form of carbon-dioxide and water,
heat being of course evolved. This is the fundamental process of
_Respiration_.
It is probably the alternation of these processes of _Assimilation_--the
building up into the protoplasmic structure of new complex labile
molecules--and _Destruction_--the shattering of such molecules
with redistribution, oxidation, etc., of their fragments--which
constitute the fundamental process of life. Different authorities
attempt to explain the details of these processes in various ways,
but there is practical agreement on the one point, that life
consists in the alternate building up of new protoplasm from the
food-materials--_Assimilation_--and the breaking down of the molecular
complexes to simpler ones--_Disintegration_, or _Dis-assimilation_, as
we may call it. During the periods when assimilation prevails, and the
protoplasm increases in mass, we recognise _Growth_, and since this is
usually associated with the vigorous imbibition of water, owing to the
powerful osmotic attractions for that liquid exhibited by some of the
products, and with consequent further stretching of the invisible
molecular plexus, the growth may be so evident in increased size, that
we are accustomed to look upon the visible increase in volume alone as
growth; but it is essential to understand that growth of the protoplasm
is always proceeding during life, even when as many older molecules are
being shattered and dispersed as new ones are being formed by
assimilation, and when, therefore, no visible permanent enlargement
occurs. Similarly, during periods when disintegration of the molecules
prevails, we must not assume that the assimilation of new molecules is
not occurring and that growth is not proceeding. The two processes are
always going on during the active life of the protoplasm: in fact life
consists in the play of these processes, as already said.
That numerous chemical rearrangements of the atom-complexes take place
outside the protoplasmic molecules--both of those left unemployed in
assimilation and of those rejected during the destructive
processes--will be readily understood: many of the bye-products found in
plants, such as vegetable acids, alkaloids, colouring matters,
crystalline bodies, etc., etc., are due to these, so to speak,
fortuitous combinations and re-combinations.
The part played by respiration has often been misunderstood. It consists
in the burning off of some of the carbon and hydrogen of the shattered
protoplasm molecules, by means of the oxygen of the air, which finds its
way into the fluids around the protoplasm, and when it is active every
act of combustion--which is here an explosion--leads to the shattering
of more protoplasm molecules, and consequently to more respiratory
combustion of the products. If the supply of oxygen is limited the
breaking down of the molecules of protoplasm does not cease, but the
carbon and hydrogen which would otherwise have been oxidised are now in
part left to form other compounds in the surrounding liquid, and thus
incompletely oxidised bodies, such as vegetable acids, alcohols, etc.,
accumulate. Even in the complete absence of atmospheric oxygen the
protoplasm may go on breaking down and accumulating various compounds
containing relatively much carbon and hydrogen--so-called intramolecular
respiration; but in ordinary plants this process soon comes to an end,
because the blocking up of the molecular plexus leads to obstruction and
interferes with the normal assimilation and dis-assimilation, and, if
prolonged, leads to pathological conditions, and eventually death.
Here, then, we meet with a cause of disease, or of predisposition to
disease. The deprivation of oxygen interferes with the normal processes
of building up and breaking down of the protoplasmic molecules, and
bodies we term poisonous accumulate and may lower the vitality or even
bring life to an end.
During normal life other products of the disruption of the protoplasm
molecules are nitrogenous bodies, such as proteids, and these we have
reason to believe are used up again, acting as the nuclei, so to speak,
of the new molecules, and so being built up again with fresh
food-materials into the plexus, to be again set free, and again used up,
and so on. Others are the carbohydrates, such as cellulose, which pass
out of the molecule into an insoluble form, and are accumulated outside
the protoplasm in the form of cellulose membranes, and so forth. It is
these formed products of metabolism (Metabolites), especially cellulose
and bodies which result from its subsequent transformation, which
constitute the main permanent mass of the ordinary plant.
We are now in a position to see how another fundamental cause of disease
or predisposition to disease exists in the deprivation of the protoplasm
of any of the elements needed to supply--in the food-materials--the
place of those which have been permanently put aside in the form of
cell-walls, or burnt off in respiration, passed out as excretions, or in
other ways lost.
It is clear that the indispensability of an element must mean that the
protoplasmic molecule cannot be completed without it: the same
conclusion is supported by the experimental proof that these elements
cannot be replaced by chemically similar elements.
It does not follow, however, that the protoplasm molecule must always
have the same number of atoms of these elements, and grouped always in
the same atom-complexes before being assimilated; nor that the
protoplasm molecule, when once built up, always breaks down in exactly
the same way. On the contrary, while the protoplasm of corresponding
parts of a daisy and of a rose must contain all the elements named, we
must believe that the atom groupings are different in the protoplasm
molecule in each case; and though the molecules of the cell-protoplasm,
of the nucleus, of the chlorophyll-corpuscles, etc., of one and the
same plant must have all these elements, the atom groupings and modes of
building up and breaking down may be very different in each case.
Again, the cell-protoplasm, bathed by the sap taken in by roots from the
soil or fed directly by that derived from the leaves, must be exposed to
very different stimuli and modes of nourishment, etc., from those
incurred by the protoplasm of the nucleus which it encloses: and similar
conclusions must apply in turn to the protoplasm of the root in the dark
moist soil and of the leaf in the light dry air, or to that of the
superficial epidermis cells as contrasted with that of the deeply
immersed pith, and so on.
It is no doubt in these directions that we must seek for the explanation
of many life-phenomena at present quite beyond explanation. Thus, it is
tolerably easy to modify the action of the cell-protoplasm of a plant,
by exposing it to differences of illumination, temperature, moisture,
and so forth, within certain limits; at least, since the changes in
stature, tissue differentiation, cell-secretions, flowering capacity,
etc., of plants affected by such factors of the environment--_e.g._
alpine plants brought into the plains--_must_ be due to changes in the
mode of activity of the protoplasm, we must assume that the above
factors affect the latter. But it is extremely difficult to reach the
nuclear-protoplasm directly by such stimuli, as proved by the experience
that even where we allow the factors to act for a long time, no
permanent change can be detected in the behaviour of the
nuclear-protoplasm--the essential material in the reproductive organs
and reproductive process. At least we must infer that no change has been
permanently stamped on this nucleo-plasm from such facts as the
characters of the seedlings of the progeny of the plain-raised plants:
if they are again sown in an alpine situation they forthwith behave
again as alpines.
Must we not conclude, then, that this difficulty of reaching the
nuclear-protoplasm is owing to the fact that it is nourished and
influenced directly only by the cell-protoplasm? That the
cell-protoplasm is its environment, and not so directly the outer world?
We may influence the cell-protoplasm--we may make it work harder or less
actively, respire vigorously or slowly, build up and break down in
various different ways, or at different rates, and so forth, _within
limits_; but it is nevertheless cell-protoplasm of its specific kind,
with its own range of molecular variations and activities within these
limits, and it supplies the nuclear-protoplasm with what it wants so
long as these limits are not exceeded. Consequently, while it is very
easy to make the cell-protoplasm vary within the limits of its range, it
is not easy to induce it to vary its effects on the nuclear-protoplasm
to such an extent or in such a way that the latter is permanently or
materially altered in constitution.
Nevertheless it would appear that cases do occur where the
nuclear-protoplasm _is_ reached and affected by external stimuli, as
evinced by some of the phenomena of hybridisation and of cross-and
self-fertilisation, because we find the results expressed in the
mingling of the characters of parents, in strengthened or enfeebled
progeny, and even in the appearance of unexpected properties, which,
from the facts of Reproduction, we know must have taken their origin in
some alteration of the nuclear substance of the embryo.
Here, however, we know in most cases that the principal agent
which has reached the nuclear-protoplasm, is another portion of
nuclear-protoplasm. In hybridisation, one which has been fed and
influenced by cell-protoplasm of a very different plant; in
cross-fertilisation, one fed and influenced by the cell-protoplasm of a
different plant of the same species, and in self-fertilisation, one fed
and influenced by the same cell-protoplasm.
That somewhere, and somehow, such nuclear-protoplasm as induces the
changes in the characters of hybrids, etc., has been influenced by its
immediate environment--the cell-protoplasm of the plant--appears to be a
conclusion from which there is no escape. We may obtain similar evidence
from the experience of grafting. It is relatively easy to influence the
cell-protoplasm of a scion by a suitable stock, obviously because the
latter, while handing on to the former all necessary materials from the
soil, presents the indispensable elements and compounds in somewhat
different proportions, dilutions, etc., from those which its own roots
would have done, and probably mingles with them a certain amount of its
own peculiar products, as well as affects the modes of working and
interaction of both by the molecular impetus impressed on them.
Consequently the cell-protoplasm of the scion, while obtaining from the
stock all it needs within the limits of its own variations of structure
and activity, nevertheless builds up and breaks down in ways or at rates
slightly different from those hitherto normal to it, and perceptible
variations result when the sequences and correlations of these material
and mechanical changes have affected a sufficiently large mass for the
accumulation of visible effects. The limits to grafting suggest not that
an inappropriate stock does not offer to the protoplasm of the scion the
right materials, but that it presents them in proportions and in forms
which are unsuitable for the assimilable powers of the latter, or,
possibly, mingled with substances poisonous in themselves or capable of
becoming so in conjunction with bodies in the scion.
What has been said of the action of stock on scion, will also be true,
_mutatis mutandis_, of the reciprocal action of scion on stock. Here
again we may have causes for disease, or predisposition to disease.
It occasionally happens, however, that the nuclear protoplasm
of the stock or scion _is_ affected in grafting, and we infer
from the difficulty of modifying it in any other way in ordinary
reproduction than by means of other nuclear protoplasm--_e.g._ in
hybridisation--that in such cases a fusion of the nuclei of stock and
scion has occurred during the grafting, and a graft-hybrid has
resulted--_e.g._ _Cytisus Adami_.
It is not impossible however that the nuclear protoplasm has in such
graft-hybrids been subsequently modified by the differences in nutrition
to which it has been subjected, in the modified cell-protoplasm affected
by the mingling of the juices, etc., of scion and stock; for it is quite
conceivable that such materials may affect the protoplasm far more
profoundly than anything derived directly from the environment.
If Daniel's researches are confirmed, however, it appears that in some
cases, at any rate, the nuclear-protoplasm is so altered by the grafting
that when the new embryo is developed, after fusion with nuclear
substance from another plant of the same species, the results are
apparent only in the progeny, and _the effects of alteration in the
cell-protoplasm have been transmitted to the nuclear protoplasm of the
germ-cells_--_i.e._ acquired characters have been transmitted and fixed
by heredity. Should this prove true the importance of the results can
hardly be over-estimated. The matter is too problematical for further
discussion here, but we see that any such action may profoundly affect
the "constitution" of the resulting plant.
Turning now to the case of fungi or other organisms which obtain access
to the cell-protoplasm. At the one extreme we have cases where the
protoplasm of the diseased plant is rapidly and directly poisoned and
destroyed, as in the killing off of seedlings in "Damping Off": near the
other extreme we have cases where the foreign protoplasm of the
parasite, although it gains complete access to that of the host, merely
stimulates the latter to greater activity and itself works for its own
ends in conjunction with it--_e.g._ _Plasmodiophora_. In such instances
we must figure to ourselves the cells of the root of the Crucifer
handing on food-materials to both masses of protoplasm--that of the
_Plasmodiophora_ and that of the cell into which it penetrates; and it
is immaterial whether both obtain the food-materials directly, or, what
seems more likely, the fungus only at second hand and by the medium of
the host's protoplasm. In any case, the latter is for a long time at
least not poisoned or maimed, or in any perceptible way injured by
excreta from the fungus-protoplasm, although it is evident that each
must excrete various metabolites which may soak into and be taken up by
the other: on the contrary the host-protoplasm grows larger, attracts
more food supplies, makes larger cells, and is evidently stimulated to
greater activity for the time being, its behaviour reminding us of the
stimulation of cells by means of slight doses of poison referred to
previously. We must therefore assume that the general course of building
up and breaking down of its protoplasm-molecules go on as usual--or
nearly so--in both the host cell and the invader; and that the
assimilatory, respiratory, excretory and other functions are carried on
in the former as in the normal cell, or are but slightly modified to an
extent which does no immediate injury to its life. But we must further
assume that the same is also true of the invading protoplasm, and that
the _Plasmodiophora_ is also supplied with suitable atom-complexes to
build up its protoplasm molecules, as fast as they are shattered and the
rejecta burnt off in respiration.
A step further, and we come to instances of _Symbiosis_, where the
commingled masses of protoplasm of host and invader continue this
harmonious action during life. Clearly there are resemblances between
these latter cases and successful grafts, and between both and
successful sexual unions where the resulting embryo-cell gives rises to
a vigorous and healthy plant; and the more these resemblances are
examined in the light of what we know of symbiosis the more they support
our contention.
Such considerations as the foregoing suggest, then, that life consists
in the regular and progressive building up and breaking down of the
complex protoplasm molecules, and is necessarily accompanied by the
influx of the indispensable food-elements in certain combinations and
atom-complexes for assimilation, and by the combustion of some of the
debris of the shattered molecules, which combine with the oxygen in
respiration and so afford explosions which raise the temperature and
enhance the lability of existing molecules, and act as stimuli to the
shattering of further molecules. The results of these rhythmical
buildings up (assimilation) and shatterings (dis-assimilation) of the
protoplasm molecules are the growth of the protoplasm, with further
intercalations of water and new food-supplies, etc., on the one hand,
and the formation of metabolic products (proteids, cellulose, sugars,
fats, etc.), some of which are again used up, others respired, others
deposited as stores, cell-walls, etc., on the other.
That the building-up process depends on the action of molecular forces
comparable to those by which a growing crystal goes on selecting
atom-complexes of its particular kind from the solution around seems
highly probable, and this being the case we can understand how under
certain circumstances _substitutive_ selections may occur. That is to
say, just as a crystal will sometimes build up into its structure
atom-complexes of a kind different from its normal molecules, so, given
the proper conditions, a protoplasmic molecular unit will build up into
its structure atom-complexes somewhat different from those it had
hitherto taken up--_i.e._ assimilated--with consequent modifications of
its behaviour. If this occurs, the modes of further building up and
breaking down will be affected by the subsequent action of these
slightly modified protoplasm units, _and it may well be that the whole
significance of variation turns on this_. Whether the resulting
variation makes for the welfare or otherwise of the organism will then
be decided by the struggle for existence, and the natural selection
which ensues. Such a view also implies that the energy concerned is
primarily what is usually termed chemical energy, and that every
compound entering into the protoplasm carries in a supply of this,
available in various ways.
_Death_, on the contrary, is the cessation of these rhythmical processes
of building up and breaking down of the protoplasm molecules. It does
not imply the cessation of chemical changes of other kinds, but that
these rhythmical constructions of the complex and labile protoplasm
molecules breaking down on stimulation to bodies partly re-assimilable,
partly combustible in respiration, and partly excretory, etc., have
ceased, and that further chemical changes in the material are
thenceforth simpler and different in kind and degree, eventually leading
to total disintegration so that no units are left capable of restoring
the rhythm.
If these ideas are correct, we may define _Disease_ as dangerous
disturbances in the regularity, or interference with the completeness or
range of the molecular activities constituting normal Life--_i.e._
Health--and it is evident that every degree of transition may be
realised between the two extremes. Now, if we further assume, as I think
we must do, that a considerable range or "play" must exist in the
molecular activities of the protoplasm constituting life, we obtain a
sort of expression of what we mean by limits of variation. The fact that
life can go on in a given plant at temperatures between from 1 deg.-5 deg. and
35 deg.-40 deg. C., or in lights of different intensity, or within considerable
ranges of water supply, concentration of salts, partial pressure of
oxygen, etc., implies that the molecular activities of the protoplasm
are of the normal _kind_ all the time, though they may differ in
rapidity, and even in _quantitative_ and _qualitative_ respects within
certain limits; and the meaning of the _optimum_ temperature,
illumination, oxygen pressure, etc., is, from this point of view, not
that the molecular activities differ in kind from those nearer the
minima and maxima, so much as that they are running at the best rates
for the welfare of the plant--_i.e._ for permanent health.
If we transcend the cardinal points limiting the range of this play,
however, and we get variations in the _kind_ as well as _rates_ of
molecular constructions and disruptions, then we pass by imperceptible
gradations into ill-health--_i.e._ _Disease_.
And similarly in relation to other protoplasm. That of the right kind of
pollen grain from another plant of its own species, stimulates the
contents of the ovule to produce a vigorous embryo and healthy seedling:
that of a similar pollen grain in its own flower either does no positive
harm, but has a feebler effect, or it may act like a poison. That of
another pollen grain again may refuse to unite at all; while that of a
fungus hypha--_e.g._ of _Sclerotinia_ on _Vaccinium_--may run down the
style as does the pollen tube and produce death and destruction
throughout the ovule.
Or again, in Clover, we may have the hypha of a _Botrytis_ with its
protoplasm unable to do more than penetrate into the cellulose walls
and diffuse a poison into the adjacent cells, being utterly incapable of
directly facing, or mingling with the living protoplasm of such cells,
whereas the protoplasm of another organism--_e.g._ _Rhizobium_--will
penetrate directly into the cells, live in them for weeks or months
without injury--nay even with advantage to their life. And hundreds of
similar cases can be selected.
We may, therefore, conclude that _Variation_ depends fundamentally on
alterations in the structure or mode of building up and disintegration
of the protoplasmic molecular unit, brought about either by direct
modifying action of the inorganic environment--nutrition, temperature,
oxygen supply, light, etc., etc.--or by the mingling with it of other
protoplasm, the molecules of which since they have already a slightly
different composition, configuration, mode of breaking down and building
up, etc., affect its molecules by supplying them with altered nutritive
atom-complexes, by competing with them for oxygen, etc., etc. Once these
molecules are affected, we must assume that long sequences of other
chemical and molecular changes will be also modified; and although we
have no conception of _how_ these changes bring about changes in form,
that they do so is only a conclusion of the same order as that which we
hold regarding the much simpler changes concerned in the formation of
crystals.
That such variations may be of every degree as regards profundity,
permanence, kind, etc., may well be imagined; and there is nothing
surprising in our being able to induce them more easily by the action of
external factors _in the readily accessible cell-protoplasm_ than in the
_less exposed nuclear-protoplasm_; because the latter is only accessible
through the former, or through the agency of _other nuclear protoplasm
already modified_. On these and similar phenomena depend the relative
permanency and transmissibility of the variations. Our measure of the
latter only begins when the effects referred to have become manifest in
large masses of cells, because only then do they become appreciable to
our senses.
Further, variations thus induced may be of advantage to the continued
life of the plant, or in all degrees disadvantageous or threatening to
its existence. These latter variations are _Disease_, and if their
interference with the normal rhythmical play of the building up and
breaking down of the protoplasm molecules proceeds beyond certain
limits, life ceases, and we have death supervening on disease.
NOTES TO CHAPTER XXX.
It appears probable that calcium is not always needed by
living cells, and may not enter into the composition of
protoplasm; on the other hand traces of iron are perhaps
necessary.
The criticisms and summary of facts on which the hypothesis
regarding protoplasm here adopted is based are developed at
length in Kassowitz, _Allgemeine Biologie_, Wien, 1899, B. I.
and II., where the collected literature may be found, and the
reader introduced to the huge mass of controversial writings
put forward since Darwin and associated with the names of
Weismann and others.
It will probably be noticed that I have employed the term
molecular unit of protoplasm, and have not discussed the
question of organised structure in the latter: this is because
it seems clear to me that living protoplasm as such does not
possess "organised structure" in the true sense of that
term--it is, rather, busy preparing and making "organised
structure," and a molecular constitution would have to be
ascribed to all "physiological units" of the nature of
micellae, pangens, ids, etc., as truly as to the structural
units of a starch-grain or cell-wall, or even of a crystal. In
this connection, the student will find the necessary points of
view put forward in Pfeffer, _Physiology_, pp. 37-83.
INDEX.
Absorption by roots, 49.
Absorption of energy, 23.
Absorption of light, 27.
Absorption of water, 50.
_Abutilon_, 183.
_Acarus_, 88.
Accessory buds, 259.
_Acer_, 214.
Acid gases, 181, 191.
Acids, 130, 136.
Acquired characters, 283.
_Acrostalagmus_, 238.
Action of the environment, 271.
Adaptation, 176.
Adapted races, 177.
_Adonis_, 220.
Adventitious buds, 224, 225, 257, 260.
_AEcidium_, 88, 114, 116, 187, 188, 189, 217, 223, 225, 232, 247, 252.
Aeration, 104.
Aerobic organisms, 57.
Aetiology, 89, 100.
_Agaricus melleus_, 115, 143, 145, 234.
Agents of disease, 113.
_Aglaospora_, 223.
Agriculture, 65.
Agricultural Chemistry, 2.
_Ajuga_, 217.
Albinism, 179, 182, 183, 186.
Alder, 207, 219.
Aleurone layer, 173.
Algae, 215.
_Allium_, 258.
Almond, 168.
_Alnus_, 214.
_Aloe_, 134, 161.
Alpine plants, 250, 279.
American blight, 164, 219.
American vines, 155, 169, 172.
Amides, 31.
Amoeba, 144.
Amount of energy stored, 25.
Amygdalin, 173.
_Anabaena_, 128.
Anaerobic bacteria, 58, 237.
_Anagallis_, 261.
Analyses, 65.
Analyses of waters, 58.
Anemone, 247.
Animals, 99, 108, 142, 207.
_Antennaria_, 232.
_Anthonomos_, 249.
Anthrax, 144.
Antiseptics, 162.
Ants, 232.
_Aphis_, 88, 109, 161, 165, 188, 213, 214, 232, 241, 253.
_Aphrophora_, 233.
Apogamy, 257, 261.
_Aporia Crataegi_, 187.
Apospory, 257, 261.