SOME INTERESTING BRITISH PLANT GROUPS
In the preceding chapters glimpses have been obtained of some of the
wider aspects of plant life, particularly as seen on the hills and
plains of our own country. The species composing our flora have been
seen mostly, not as individuals, but as portions of regiments and
armies, particular plants being mentioned but seldom, where required for
purposes of illustration. In the final chapter it will be well to
abandon this collective treatment, and glance at a few individual
species or genera or small natural groups which possess features of
interest of one sort or another. No systematic arrangement need be
attempted: it will be pleasanter to ramble on, allowing our points of
inquiry to turn up as they might on a country walk.
A consideration of abnormalities in the manner in which plants obtain
their food-supply--irregular nutrition, as it has been called--will
raise some interesting questions, and will bring us up against some of
the most remarkable species which are found in the British flora. The
outlines of the method by which plants manufacture their food are
familiar to all, and have been referred to already (pp. 75, 132). The
roots absorb from the soil water containing dissolved salts, which is
passed up by the stems into the leaves. The leaves extract from the air
carbon dioxide. The chlorophyll, or green colouring-matter of the
leaves, possesses the remarkable power in the presence of sunlight of
breaking up and recombining these substances into the compounds which go
to build up the plant-body. As has been pointed out, it is this power of
forming organic out of inorganic matter that especially distinguishes
plants from animals. But not all plants manufacture their food in this
way. A large number feed like animals, finding their sustenance
sometimes in living, more often in dead, organic material, either animal
or vegetable. The whole enormous group of the Fungi do not possess
chlorophyll, and in consequence are dependent on organic materials for
their food. Some of the most familiar of the lower Fungi live on cheese,
leather, bread, or any other damp animal or vegetable material. The
higher forms, which decorate our woods and pastures, find their
sustenance largely in leaf-mould. The groups of the Mosses, Hepatics,
and Ferns, which are more highly organized than the Fungi, possess
chlorophyll, and manufacture their own food; and it is with some little
surprise, therefore, that when we come to the Seed Plants, the highest
group of all, we find, though in relatively few cases, a reversion to
the animal trait of using organic food. Some of our woodland plants have
taken so entirely to a diet of leaf-mould that they have discarded the
apparatus which would enable them to manufacture their own food.
Chlorophyll, the magic wand by means of which the inorganic is
transformed into the organic, and also leaves, the mills wherein the
transformation takes place, are absent from these plants. For instance,
the Bird’s-nest Orchis (_Neottia Nidus-avis_),
[Illustration: FIG. 31.--BIRD’S-NEST ORCHIS (NEOTTIA NIDUS-AVIS). 1/2.]
sends up from a mass of fleshy roots a bare brown stem about a foot
high, bearing a spike of brown flowers, the whole being so much of the
same colour as the dead beech leaves among which the plant is usually
found that it may easily be passed over. It is quite incapable of
manufacturing its own food, but feeds on the decaying vegetable material
which was manufactured by the trees under whose shadow it grows.
It is but a step from _saprophytes_ such as this to _parasites_, which
feed, not on dead, but on living organic matter. In the case of the
higher plants, the hosts are always themselves plants, though, as
pointed out on p. 78, they are, in the case of the Fungi, sometimes
animals. One of the most interesting of these parasites is, like the
Bird’s-nest Orchis, found in woods--the Yellow Bird’s-nest (_Monotropa
Hypopitys_). This is, like the last, a leafless plant devoid of
chlorophyll, sending up from a tangled root-mass one or more pale yellow
stems, each bearing a drooping raceme of flowers of the same colour. The
flowers show affinities to the Heath family (_Ericaceæ_), but the plant
differs much from any other member of that Order. The Yellow Bird’s-nest
is always found associated with the _mycelium_, or cobwebby underground
portion, of a fungus, on which it appears to be parasitic. The fungus is
in turn a saprophyte, and the Seed Plant feeds at second hand, so to
speak, on decaying vegetable matter. This parasitism of a seed plant on
a fungus is a very exceptional case. A more frequent type is offered by
the Broomrapes (_Orobanche_), which we may find in meadows, etc.,
growing on Clover, Thyme, Ivy, and so on. These resemble the
Bird’s-nest Orchis in sending up a stout leafless stem crowned with a
spike of flowers. The different species display almost every colour
except green, being red or brown or purple or yellow, and one blue.
These plants live by attaching themselves to the roots of their host,
and drawing in the nourishment they need for their own growth--robbery
pure and simple. The seeds of the Broomrapes are very numerous and very
light, and of singularly primitive structure. When they develop, they
produce, not a young plant with root and stem, but a delicate spiral
filament which grows down into the ground. Should this meet with a root
of its host-plant, it adheres to it closely, and grows into a swollen
knob at the point of attachment, which when mature sends up the
flowering stem already described. Should a suitable root not be met
with, the filament withers away and dies as soon as it has exhausted the
small amount of reserve food stored in the seed. A parasite of a less
sedentary habit, to be found in spring in our copses and hedgerows, is
the Toothwort (_Lathræa Squamaria_). This curious plant has underground
creeping stems clothed with whitish, tooth-like, fleshy scales
(curiously modified leaves). In autumn and winter the stems lie dormant.
In spring they send out delicate roots which attach themselves to the
roots of trees of various kinds and suck nourishment from them, with the
aid of which the plant sends up into the air fleshy cream-coloured stems
bearing many drooping flowers of the same hue, the structure of which
shows that the plant is closely allied to the Broomrapes. The Toothwort
is a very harmless parasite, and the species of Broomrape also, though
sometimes abundant on Clover, etc., do not do much damage; but the same
cannot be said for the Dodders (_Cuscuta_), one of which is parasitic on
Flax, another on Clover, and so on. These are little annual plants whose
seeds lie dormant in the soil throughout the winter and well into the
spring. Then the young plant, which has remained coiled up inside this
seed like a spring, pushes forth in the form of a tiny thread. While one
extremity fastens itself to the soil, the other rises up into the air,
and its point slowly revolves. Should it come in contact with a living
stem of a suitable plant, it attaches itself to it by means of disc-like
suckers, penetrates the tissues of its victim, draws out nourishment,
and, growing rapidly, spreads from plant to plant, taking a couple of
close turns round each stem after the manner of a lasso, and then
sending in rootlets from the attaching disc, and sucking the life out of
each as it goes. It has no roots, no leaves, no chlorophyll, being of a
red or yellow tint, and is entirely dependent for its nourishment on the
plants which it attacks. In course of time--about August--an abundance
of pretty little waxy-white flowers are produced, which produce the next
year’s supply of seed. A few seedlings of Dodder, developing under
suitable conditions, will form a colony which is capable in its few
months of life of sweeping over a large area, wrecking the vegetation on
which it has battened.
A parasite of a quite different sort may be studied in the familiar
Mistletoe (_Viscum album_). It is the only parasitic native plant which
is shrubby, or which perches itself on trees (the seeds being spread by
birds, which devour the white berries). It is not, like some parasites,
particular as to the species upon which it grows, flourishing equally
upon a number of hosts, and even capable of living upon its own species.
It differs from those parasites which we have been considering in
possessing an abundance of green leaves, and being therefore capable of
manufacturing its own food. At the same time, it has no roots which can
penetrate the soil, and is incapable of an independent existence. It
seems probable that its relations with its host are to some extent
symbiotic--that is, each giving to the other--rather than purely
parasitic, where the benefit is entirely on one side. The Mistletoe,
retaining its leaves and manufacturing food throughout the year, is
clearly capable of aiding its host, which loses its leaves in autumn,
and cannot form fresh nourishment until spring is well advanced.
Before leaving this question of abnormal methods of procuring food as
found among the higher plants, we may return for a few moments to the
consideration of carnivorous plants, to which reference was made in
Chapter IV. Of these the Sundews (_Drosera_), Butterworts
(_Pinguicula_), and Bladderworts (_Utricularia_) supply very interesting
examples within our own flora, which anyone may study on a holiday spent
on the moors or mountains. The Sundews are familiar to all plant
lovers--little plants of the bogland, usually growing among Sphagnum,
and well distinguished by their leaves decked with spreading red hairs,
each of which is tipped with a little drop of sparkling sticky fluid. It
is these hairs or tentacles and their movements which place the Sundews
among the most interesting of all plants. It is important to note that
they are not hairs in the ordinary sense, which are organs of very
simple structure arising from the epidermis or skin of the leaf. The
tentacles of _Drosera_ have a complicated structure resembling that of
leaves, and the tip is occupied by a gland which produces the sticky
secretion already mentioned. These glands are exceedingly sensitive,
and, moreover, sensitive in a selective way. They are unaffected by the
drops of rain which frequently fall on them, but the touch of any solid
body, especially of organic material, immediately affects them; most of
all nitrogenous substances of any kind. Darwin found that a morsel of
human hair weighing only 1/78,740 of a grain was sufficient to set the
machinery of _Drosera_ in motion, and that immersion of a leaf in a
solution of phosphate of ammonium so weak that each tentacle could
absorb only 1/20,000,000 of a grain acted as a strong stimulus. In
nature the stimulus is usually given by some unwary insect--a midge or
other small flying creature--which, attracted by the bright colour or by
the odour of the leaf, ventures too close, and becomes entangled among
the sticky hairs. Then a most interesting series of events takes place.
Almost at once the tentacles--first the ones actually touched, and then
the adjoining ones--bend towards the point of disturbance, closing down
one by one on the unfortunate victim till the leaf resembles a closed
fist. At the same time the production of secretion increases, so as
further to entangle the victim. When it is firmly secured, the secretion
changes in character. Digestive ferments, closely resembling those by
which animals digest their food, are poured out. These dissolve the
animal’s body, all except the horny parts; the digested materials are
then absorbed into the plant, which, as experiments show, benefits
considerably by the addition to its diet of this animal food. When
digestion is completed, the tentacles open again and prepare for a fresh
victim. While the details of this remarkable process have been worked
out only by careful and minute research in the laboratory, the main
movements may be watched by anyone on any British moorland; or, bringing
home a few plants in the damp moss in which they grow, we may amuse
ourselves by experiments in feeding them.
In comparison with the Sundews, the other insectivorous plants which are
included in the British flora are of less interest. The Butterworts
(_Pinguicula_), of which four species are known in these islands, have a
rosette of smooth, broad, yellowish leaves covered with glands which
exercise the same functions as those of _Drosera_. To the touch of
raindrops, sand-grains, or other inorganic substances they are
indifferent; but a tiny insect alighting on the sticky leaf at once
provokes an outpouring of secretion, while the leaf rolls inward from
the edges till the victim is securely caught; it is then digested as in
the Sundew.
The Bladderworts (_Utricularia_), of which several species may be found
floating in boggy pools, are rootless, limp plants with finely divided
leaves, among which are numerous little bladders (in reality strangely
modified leaflets), and upright stems bearing pretty yellow
Snapdragon-like flowers. The bladders do not help the plant to float,
and appear to have for their sole function the securing of animal food.
In the Common Bladderwort (_U. vulgaris_) they are about 1/10 inch long.
At the upper end is a little hinged door, which is kept closed as by a
spring against a thickened rim or door-frame. Outside the door are a few
stiff hairs, a convenient perching-place for small aquatic creatures
such as the minute Crustaceans known as Water Fleas. Should one of these
try to explore the bladder, the door opens easily, but closes at once
behind the rash wanderer, imprisoning it. The Bladderworts do not
_digest_ the victims which they secure in this manner, but when the
bodies are decomposed by means of bacteria, the products of
decomposition are absorbed. How fatal this mousetrap arrangement is to
Water Fleas can be determined by dissecting the bladders of the plant.
Thus far, then, as regards some of those peculiar members of our flora
which make their living by the unusual method of stealing their
neighbour’s goods, or which eke out their existence by the capture of
animal food. Let us now take another line of exploration and consider
the conditions which prevail on the loftiest portions of our islands,
and how these affect the vegetation. Mountain-tops are always attractive
and interesting places--the keen rarefied air, the freedom and openness
of the summits, fill us with exhilaration. Our own mountains are not
lofty; nowhere in the British Islands is a height of a mile attained.
But we have only to ascend to a couple of thousand feet to note a great
change in the vegetation. The plants of the lower grounds to a great
extent die out (though some accompany us to our highest summit), and the
vegetation takes on a low compact form, which becomes more emphasized as
we ascend farther, till in sheltered nooks alone do we find any plants
more than a few inches in height. Furthermore, we notice an incoming of
new plants unknown at lower levels, which search will show us to be
confined to the mountains, each of them having a more or less definite
limit below which (also above which, though our mountains are not high
enough to render this point well marked) it is not found.
Among the plant formations and associations of the lower grounds which
we considered in Chapter II. it was noted that the controlling factors
were mainly connected with the nature of the soil and the amount of the
water-supply. Here on the mountains another factor, the climatic, comes
in emphatically, and takes charge. The temperature of the atmosphere
falls one degree centigrade for about every 200 feet of elevation, so
that a sharp frost on the lowlands may easily mean zero Fahrenheit on a
4,000-foot hill. The rarefaction of the atmosphere, too, tends to
produce a much greater range of temperature, both diurnal and seasonal.
Again, the velocity of the wind is much higher on the summits than on
the plains, where friction is greatly increased by trees and other
obstacles. These high winds have a very great cooling effect, as we may
notice on our own bodies even in summer. In fact, as regards climatic
change, an ascent of a thousand feet is comparable to a journey of
several hundred miles northward. Anyone who has, on a winter tramp, been
caught in a snowstorm on a 3,000-foot hill is forcibly reminded of what
he has read of winter conditions in the Arctic regions. In ascending Ben
Nevis we travel, in a sense, to the Arctic Circle. But the analogy is
false, for conditions, especially in summer, are very different in the
two places. The plants of our mountains have all the advantages of the
high summer elevation of the sun, very different from the weak, sloping
sunlight of the
[Illustration: FIG. 32.--ALPINE PLANT-BOSS (_SILENE ACAULIS_,
_HYMENOPHYLLUM UNILATERALE_, _MNIUM HORNUM_).
[_To face p. 191._]
Arctic. On our loftier hills, indeed, the heat is on occasions
oppressive.
Again, the mountain climate, with its heavy rainfall and long cold
period, tends to the formation of peat; and the acids thus engendered in
the soil, as well as the low temperature prevailing during most of the
year, render difficult the absorption of water by the roots of plants.
The conditions under which alpine plants, then, live may be summed up as
follows: a long cold winter, a short summer; great exposure; scarcity of
food-supply. The modifications which plants have undergone to meet these
conditions are very marked, and render alpine plants a source of
constant interest to the traveller and of delight to the gardener. The
effect of low temperature (also of peaty soil) in rendering difficult
the absorption of food materials, and causing extensive root production
and limited stem and leaf growth, is immediately observable. In Fig. 33
is seen an alpine Stonecrop (_Sedum primaloides_) as growing on the
Chinese Alps at some 12,000 feet. The root is out of all proportion to
the aerial parts. The same plant in the garden forms a little bush with
branching stems half a foot long, and flowers borne on leafy axillary
shoots a couple of inches long, while the roots are short and tufted.
The most characteristic form which alpine plants assume may be called
the cushion type. This is produced by excessive branching of the stems
of small-leaved plants, accompanied by but little longitudinal growth;
and it is excellently shown in many well-known plants such as the Mossy
Saxifrages, the Kabschia Saxifrages, the Cushion Pink (_Silene
acaulis_), and a number of others. The same type of
[Illustration: FIG. 33.--SEDUM PRIMULOIDES. 1/1.]
plant growth is characteristic of semi-desert regions, where the points
of similarity of environment to those of the mountain-tops are evident.
This cushion form has many advantages for the alpine plant. It keeps it
warm in winter and cool and damp in summer; it allows it to produce a
great amount of blossom without the necessity for extensive growth; it
resists the utmost efforts of furious gusts of wind almost as well as
would a half-buried stone; on the most storm-swept cliffs its fresh
green blobs “welcome every changing hour, and weather every sky.” Fig.
32 shows a boss of this kind, composed of the Cushion Pink (_Silene
acaulis_), with an admixture of Filmy Fern (_Hymenophyllum unilaterale_)
and a Moss (_Mnium hornum_). The shrubs of the alpine zone are mostly
small and creeping, weaving themselves among the vegetation, and with
low grasses and sedges forming a mat which is equally resistant to all
inimical conditions. Their leaves are small, to avoid damage by wind or
by excessive transpiration. In some genera--for instance,
_Veronica_--the diminution of leaf surface accompanying more elevated
habitat is very striking. In the New Zealand lowlands broad-leaved forms
(Fig. 34, _left_) are met with, which give way, as one ascends to 8,000
feet, to such forms as _V. Hectori_ (Fig. 34, _right_), in which the
leaves are reduced to mere scales, and the plant much resembles some of
the Cypresses or other Conifers with marked xerophile characters.
Other plants, again, escape climatic rigours by burrowing underground
and throwing up short aerial stems in summer; the spindly plants of the
lowland, with diffuse stems, and also the light-rooted annuals,
[Illustration: FIG. 34.--NEW ZEALAND SHRUBBY VERONICAS, SHOWING, FROM
LEFT TO RIGHT, REDUCTION OF LEAF WITH INCREASING ELEVATION OF HABITAT.
2/3.]
are conspicuous by their absence. The brief summer and long winter are
unsuitable to the economy of annual plants; and the alpine perennials
are so constructed that with the passing away of the cold, flowering and
fruiting may be accomplished quickly, before winter descends again. The
abundance and vividness of the flowers of alpines is almost proverbial.
Several explanations have been put forward to account for these
features, and probably there is some truth in each of them. It has been
held that the brilliancy of the sunlight is accountable; the shortness
of the period available for seed-production, and the consequent need of
prompt pollination by insects, have been suggested, as leading to
urgent advertisement by means of brilliant coloration; while the fact
that the pollinating insects are largely Butterflies, the most æsthetic
of flower visitors, has also been put forward as accounting for it. Be
that as it may, the glowing patches of colour produced by many quite
minute alpine plants are among the most delightful things in nature. Our
own flora contains but few of the more striking of these jewels; but
where will one find a more delightful sight than a well-flowered patch
of Spring Gentian (_G. verna_) or Mountain Avens (_Dryas octopetala_) or
Purple Saxifrage (_S. oppositifolia_)?
[Illustration: FIG. 35.--MOUNTAIN AVENS (DRYAS OCTOPETALA). 1/1.]
As we mount higher and higher on the hills, plants become fewer and more
stunted, but hardy forms persist even long after the level of perpetual
snow is reached. In the Alps, _Ranunculus glacialis_ occurs up to an
elevation of about 14,000 feet. In West Tibet, strange stunted species
of _Saussurea_, a genus of _Compositæ_ allied to the Thistles, exist at
elevations of 17,000 to 19,000 feet. Some of the Cryptogams go higher
still. Lichens grow on the summit of Kilimanjaro (over 19,600 feet); and
Schimper suggests[11] that this may by no means represent the absolute
limit of vegetation. The prevalence of snow and ice does not of itself
inhibit the lower forms of life. Since “red snow” was shown, nearly a
century ago, to be due to colonies of a minute Alga, many microscopic
organisms of like habitat have been discovered, and these algal
colonists of snow and ice are now known to extend far over the frozen
deserts of the highest hills, and to penetrate into the remotest regions
of the Arctic and Antarctic.
As we get up to the level of perpetual snow on the higher mountains, or
go northward within the Arctic Circle, the conditions under which plant
life exists become very severe. It has been pointed out that in spite of
a superficial similarity, wide disparity exists between the sets of
conditions prevailing in the two kinds of habitat just mentioned. In the
Arctic the winter is continuously dark and the summer continuously
light; and in summer the sun is never far above the horizon, so that the
temperature remains low, though it rises amply far enough above
freezing-point to allow of plant life. On high mountains, on the other
hand, there is the same succession of day and night which prevails on
the plains below, the height of the sun above the horizon being a
question of latitude. On mountain-ranges situated within the Temperate
Zone, such as the European Alps, and much more on those nearer the
Equator, the day temperature in summer is very high wherever the sun
strikes, and while plants may have to withstand at night a temperature
comparable to that borne by the Arctic flora, they must endure by day
the most intense insolation.
Neither in the Arctic nor on the high hills does plant life cease merely
on account of low temperature. Species belonging to many families
venture even beyond the limit of perpetual snow. The coldest known area
on the earth’s surface lies in Siberia, actually within the limits of
forest growth, and trees and herbs of many species survive winter
temperatures which may fall below -60° C. (76 degrees of frost
Fahrenheit). They freeze into solid lumps of ice without injury, and
indeed the thawing process in spring is more dangerous to them than
their congealment in autumn. Many of the high alpine plants are frozen
solid every night only to be roasted alive by day; it seems amazing that
any living organisms can endure under such circumstances. Yet it is not
only species confined to areas where such extremes exist, and specially
adapted thereto, which can resist them successfully. In Central Europe
the Common Chickweed and Common Daisy are often frozen solid, so that
leaves and stems snap between the fingers like sealing-wax, yet with a
rise of temperature they continue growth quite unperturbed, just as they
do in areas where frost is unknown. The main difficulty induced by cold
would appear to be the withdrawal of available water; if that goes on
for too long, life ceases. Of course the suspension of activities which
accompanies freezing cannot continue indefinitely, and in the cold
regions of the Earth plants are found only where for a sufficient
portion of the year the maximum temperature rises above freezing-point
enough to allow of ordinary vital functions being resumed. A curious
point in this power of resistance in plants to extremes of temperature
is that they display no obvious protective adaptations. “Our present
powers of investigation,” Schimper concludes,[12] “do not enable us to
recognize in plants any protective means against cold. The capacity of
withstanding intense cold is a specific property of the protoplasm of
certain plants, and is quite unassisted by protective means that are
external.”
It is a far cry from the high Alps to the seashore, but it will be of
interest to examine next the lower limit of the range of the Seed
Plants. While the upper limit varies much in different latitudes,
according to the distribution of temperature, the lower is controlled by
sea-level, which (for our purpose at least) is uniform over the whole
globe. The level of the fresh waters, whose margin marks the limit of
the bulk of the Seed Plants, is, on the other hand, various, lakes being
situated at different heights above (and occasionally below) sea-level,
while rivers slope across the lands down to the ocean. While the sea
margin forms a very real barrier to the spread of Seed Plants, the lakes
and rivers, on the other hand, yield many inhabitants, and we must
examine the relations existing between the aquatic and the terrestrial
species.
As has been stated on a former page, the evidence points to life having
originated in the water, at a period extremely remote. The most lowly as
well as the most minute of all organisms are the bacteria, some of which
are in size beyond the limit of the most powerful microscope to detect,
their presence being known only by their chemical actions. The most
primitive groups of bacteria, known as prototrophic, are able to live
without light, deriving their nourishment by the breaking up of
inorganic chemical compounds. It is difficult to conceive of any living
organism more primitive than these, and quite possibly they recall that
dim borderland where merely chemical structure and action mysteriously
advanced into the cell structure and purposive chemical changes which we
call life. From that lowly stage the evolution of plant life has been
marked especially by three great forward bounds, of inestimable
importance. The first of these was the “invention” of chlorophyll, which
allowed plants to use for their life-processes the vast supply of energy
furnished by the Sun. Sunlight then became essential to life, and the
Algæ, the probable ancestors of all the higher plants, were developed,
presumably through the peculiar _Cyanophyceæ_, or “Blue-green Algæ,” in
which the chlorophyll is in a somewhat undifferentiated condition. Much
later than this stage, yet far back in the history of evolution,
occurred the second of the great forward steps. This was the desertion
of the water for the land, which opened up for the plant world vast new
fields and a great variety of new conditions. The final stage was
reached by the abandonment of the aquatic mode of pollination by means
of swimming spermatozoids, as still found in the Maidenhair Tree
(_Ginkgo_), Cycads, Ferns, and groups lower in the scale, and the
adoption instead of pollination through the medium of the air, “which”
(to quote Mrs. Arber’s happy phrase) “has won for them the freedom of
the land.” The Seed Plants, then, achieved their wonderful abundance and
variety owing to the highly stimulating conditions offered by a
terrestrial existence; we must assign to all the existing types a long
terrestrial ancestry. How, then, about the water plants whose leaves and
flowers so decorate our lakes? There seems no doubt[13] that they are
species which have left the land to resume the aquatic habits of their
remote ancestors. With few exceptions they retain the aerial mode of
pollination which is the pride of the specialized land plants. The
pressure of competition has probably driven them into the water, where
they descend as far as the lessening light-supply will allow.
Some--presumably the earliest to take to an aquatic life--have all their
relations to keep them company, the remote ancestor which adopted an
aquatic habit being now represented by many species, or even by many
genera. In other cases a terrestrial genus or order has few or only a
single aquatic representative. It may be assumed that in such a case the
aquatic habit has been recently acquired. The great majority of water
plants send their flowers up above the surface to be pollinated by wind
or (more rarely) by insects. It may be noted that few of the more highly
evolved groups of Seed Plants are represented in the aquatic flora;
wind-pollinated flowers of a rather primitive type of structure are the
rule in our lakes and rivers; which points to an early assumption of the
aquatic habit, and suggests that the land is more favourable than the
water for the evolution of higher types.
While the fresh waters of the globe have thus acquired from the land an
abundant population of higher plants, the presence of salt, in water as
on land, has had a deterrent effect. The sea was at first fresh. The
primitive ocean derived by condensation from a cooling atmosphere in the
early days of the world’s history contained no excess of salts. Whether
life arose while this condition still persisted it is not possible to
say; but as the sea grew salter owing to the rivers bringing into it
incessantly salts derived from the land, the Seaweeds alone of the great
groups of plants adapted themselves to saline conditions, and the ocean
is now their unchallenged kingdom. The divisions which are represented
by the Mosses, Liverworts, Club-mosses, Horsetails, and Ferns, have not,
and so far as is known never had, a single representative in the sea.
Only one or two Fungi--often symbiotically combined with Algæ to form
Lichens--and a very few Flowering Plants, have attempted marine
colonization, after long ages spent on land; and they have met with
indifferent success. As we pass from fresh to brackish water, the
population decreases rapidly, till in the seas surrounding our islands
only one Seed Plant--the Grass-wrack, _Zostera marina_--has adopted a
habitat which is thoroughly marine, and very few are found in other
parts of the world. A study of the meeting-ground of the land and sea
plants, such as we may make on rambles along the coast, supplies us with
some interesting material. On sandy shores, the wave-trampled beach,
shifting under the influence of winds and currents, offers a stretch of
“no-man’s-land”--a desert strip untenanted alike by terrestrial or
marine plants. The former do not descend below spring-tide mark, if they
go so far; the latter cannot obtain foothold on the unstable substratum.
The peculiar characters of the terrestrial beach plants has been
referred to on a previous page (p. 36). On rocky shores the “desert”
strip is much narrowed, and a certain overlap may often be found, for
the Lichens--essentially a terrestrial group--descend from the
plant-covered slopes into the spray-swept zone below, and on to mix with
the Seaweeds which occupy the belt under high-water mark, some of them,
species of _Verrucaria_ and _Arthropyrenia_, continuing downward till
the low-water mark of spring tides is reached. On steep rocky shores the
dividing-line between the Flowering Plants and the Seaweeds is quite
narrow, and varies in elevation with the exposure. On cliffy coasts open
to the Atlantic waves the uppermost Seaweeds, such as _Pelvetia_, which
only asks to be wetted periodically by spray, occur far above high-water
mark, the lowest Seed Plants perching on the rocks much higher
still--sometimes not venturing to within 100 feet of the water-level.
Under such extreme conditions none of the higher land plants venture
down towards the unfriendly sea. To see the overlap of the terrestrial
and maritime vegetation well developed we seek conditions entirely
different, where amid shallow inlets and salt-marshes land and sea merge
imperceptibly. Here the absence of higher plants from the areas below
high water, as compared with their abundance above water-level, is a
conspicuous feature. This is a noteworthy point, because if we assume
that the presence of salt is the main factor which has prevented the
land plants from spreading downwards, we are faced with the fact that
the soil of the salt-marsh, where many such plants occur, may by
evaporation of water become much more highly charged with salt than the
sea itself. Yet the salt-marsh flora includes representatives of many
Natural Orders, including some of the most highly specialized
families--_Ranunculaceæ_ (_R. sceleratus_), _Cruciferæ_ (_Cochlearia_
spp.), _Caryophyllaceæ_ (_Alsine_), _Umbelliferæ_ (_Apium graveolens_,
_Œnanthe Lachenalii_), _Compositæ_ (_Aster Tripolium_, _Artemisia
maritima_), _Primulaceæ_ (_Glaux maritima_), _Plumbagineæ_ (_Statice_,
_Limonium_). It seems clear that it is the assumption of the marine
habit which is the stumbling-block, not the presence of salt. The
Grass-wrack or _Zostera_, our only marine Seed Plant, comes of one of
the oldest stocks of aquatic plants, and its nearest relatives have long
been toying with the idea of a maritime habitat. The Order to which it
belongs, the _Naiadaceæ_ or Pondweed family, from their worldwide range,
their number, their variety, and their uniformly aquatic habit, may be
set down as among the earliest Seed Plant colonists of lakes and rivers;
some of them favour brackish water, while others besides the Grass-wrack
have taken to marine life. Without going beyond the limits of our native
_Naiadaceæ_ we can study the various stages, and form a picture of how
the Grass-wrack migrated to the sea. First we have the numerous
Pondweeds which grow in our lakes and rivers--plants with leaves broad
and floating, or narrow and submerged, and inconspicuous flowers which
rise above the water and are pollinated by the wind. Next we find
several narrow-leaved Pondweeds which grow in brackish pools; and with
them are some allies, the Tassel Pondweed (_Ruppia_) and Horned Pondweed
(_Zannichellia_), with more reduced flowers and often a more nearly
marine habitat, as they sometimes mix with Seaweeds on the open shores
of estuaries; in these plants we find the stages of a most interesting
return to the archaic method of water-pollination, so long discarded by
the great mass of the Seed Plants. In the flower of _Ruppia_, which
consists merely of two stamens and four carpels without corolla or
calyx, the pollen is liberated under water, and, being light, rises to
the surface; older flowers have already, by growth of the flower-stalk,
reached the surface, and they become pollinated by the floating grains.
In _Zannichellia_ the process is in general similar, save that the
flowers are either male or female, the former consisting of nothing but
a single stamen. The Naiads (_Naias_) form an allied genus, and are
slender annual herbs, growing completely submerged in fresh or brackish
water. One of them (_N. flexilis_) occurs in lakes at rare intervals
along the western edge of the British Isles; and another, _N. marina_,
is found living in only one spot in Britain--Hickling Broad in Norfolk;
their fossil seeds embedded in old lake deposits show that in former
times both were more widely spread than now in Western Europe, and that
other species of the genus also occurred. In the Naiads complete
reversion to water-pollination is found. When the very simple male
flowers shed their pollen, the grains, which are heavy owing to the
presence of starch, fall through the water on to the female flowers
which are borne below them, or are carried by currents to other flowers.
Lastly we come to the Grass-wracks, a small group of submersed marine
plants. While some of them, like our little native _Z. nana_, haunt
muddy sands between tides, our more familiar species, the common _Z.
marina_, is thoroughly marine, growing tall and vigorous among the large
Seaweeds down to far below low-water mark (to over 30 feet in the
Baltic). The plant has, nevertheless, not yet developed submersed
pollination, the pollen-grains rising to the surface, where they are
caught by the stigmas of floating female flowers. It follows that the
individuals rooted in the deeper water, though growing vigorously, do
not mature seed, for the production of which the species has to rely on
plants which, at least at low water, are rooted sufficiently near the
surface to allow the flowers to rise above it. Could the species achieve
submersed pollination, it appears quite capable of colonizing throughout
the Laminarian zone, wherever there is a soft substratum for its
creeping stems.
The land plants of the salt-marsh, as well as the aquatic species,
furnish interesting examples of overlap with the sea flora, but a brief
reference must suffice. The Glasswort (_Salicornia_), for instance, has
furnished itself with a very complete equipment for the difficult
conditions of salt-marsh life (see pp. 17, 18), and grows far out on the
mud-flats in green colonies, often below the upper limit of the
Bladder-wrack or _Fucus_, the common brown Seaweed of our shores. The
Glasswort has discarded leaves, its stems have become thick and
succulent, and its flowers, reduced to the minutest and simplest
dimensions, are almost buried in the fleshy branches. Thus armed, it
braves the salt-desert of the mud-flats, and repeated submersion by the
tides leaves it uninjured. Under the peculiar conditions of its life, it
relies neither on insects nor wind nor water for pollination, the
flowers being self-pollinated. A more surprising commingling is that
which is illustrated by A. D. Cotton in his report on the Seaweeds of
the Clare Island district (_Proc. Royal Irish Academy_, vol. xxxi.,
1912), where, on peaty soil a little above mean high-tide level, the Sea
Pink is shown forming a sward with a peculiar dwarf form of _Fucus_ (_F.
vesiculosus_, var. _muscoides_) and a few other salt-marsh Seed Plants,
such as _Schlerochloa_ (_Glyceria_), _Glaux_, _Salicornia_. The Sea Pink
is highly evolved florally, and differs widely from the Saltwort in its
abundant production of leaves and showy flowers, the absence of any
conspicuous xerophile characters, and the fact that it is not confined
to the coasts, being often a member of the alpine flora of our higher
hills. In its association with _Fucus_ it may be claimed that it is the
latter which is “out of water,” as it never produces fruit, increasing
solely by means of vegetative growth. At the same time, so closely does
it press its partner in the struggle for room, that the Sea Pink fails
to form its usual robust clumps, its stem being mostly unbranched and
its stature dwarfed.
Viewing generally the migration of the Seed Plants from land to water,
we see that the fresh waters of the world, untenanted by other large
plants, have been fully colonized, generally a long time ago, and by
plants of rather early types. But as regards the sea, the luxuriant
Algal vegetation which is in possession of our shores has no reason to
tremble for its supremacy. Beautifully adapted for their life, whether
in sheltered bays or on stormy rocks, the Seaweeds show no sign of
relinquishing the domain that has been theirs since the earliest rocks
which still display traces of organic life were laid down in Cambrian
seas.
INDEX
Agriculture, 135
Alien plants, 143
Alpine plants, 190
Animal-eating plants, 77
Animals and plants, 75
and seed-dispersal, 69
dependence on plants, 155
Arctic deserts, 19
plants, 196
Bog plants, 41
British flora, 167
Isles, vegetation, 25, 30
Chlorophyll, 180
Cultivated plants, 145
Deserts, 16
Fertilization, 82
Flowers, 126
display, 85
structure, 81
Fruit, 131
Fruits, explosive, 55
Glacial Period, 165
Grassland, 25
Insectivorous plants, 78, 186
Insects and flowers, 81
Leaves, 119
Life, origin of, 15
Man and vegetation, 135
Marine plants, 201
Migration, 48
Mountain plants, 189
Mud-flats, 17
Mycetozoa, 156
Myxomycetes, 156
Ocean depths, 19, 76
Origin of life, 199
Parasites, 183
Peat flora, 41
Planets, question of life on, 11
Plant associations, 30
economy, 98, 141
formations, 32
migration, 48
Plants, cultivated, 145
earliest, 154
Pollination, 82
Roots, 105
Salt-marshes, 23, 40
Saprophytes, 181
Seed-dispersal, 49
Seeds, 50
Semi-deserts, 22
Shingle beaches, 39
Soil, 99
Stems, 109
Symbiosis, 79
Types of vegetation, 31
Vegetation, closed, 24
Vegetative reproduction, 53
Water, dispersal by, 61
flora, 43, 199
Wind, dispersal by, 62
Woodland, 25
Xerophytes, 36
PRINTED IN GREAT BRITAIN BY BILLING AND SONS, LTD., GUILDFORD AND ESHER
FOOTNOTES:
[1] SVANTE ARRHENIUS: “The Destinies of the Stars.” Translated
by J. E. Fries. Putnam, 1918.
[2] F. SODDY: “Matter and Energy,” 1912, p. 194.
[3] A. G. Tansley: “Types of British Vegetation,” 1911, p. 63.
[4] H. B. GUPPY: “Plants, Seeds, and Currents in the West
Indies and Azores,” 1917, p. 425.
[5] W. B. BARROWS: “Seed-planting by Birds.” Report of the
Secretary of Agriculture, U.S.A., 1890, p. 281.
[6] See A. H. CHURCH: “The Plankton-phase and the
Plankton-rate,” _Journal of Botany_, June, 1919, supplement.
[7] G. H. CARPENTER: “Insects: Their Structure and Life,” p.
300.
[8] W. B. BOTTOMLEY in “The Exploitation of Plants,” edited by
F. W. Oliver, 1917, p. 12.
[9] To be accurate, certain groups of Bacteria, the lowest forms of
organized life, must be excluded. They appear capable of building up
their bodies directly out of inorganic substances.
[10] F. J. HANBURY and E. S. MARSHALL: “Flora of
Kent,” 1899, p. xxxv.
[11] A. F. W. SCHIMPER: “Plant Geography” (English
translation, 1903), p. 719.
[12] A. F. W. SCHIMPER: “Plant Geography” (English
translation, 1903), p. 41.
[13] See AGNES ARBER: “Aquatic Angiosperms: the Significance
of their Systematic Distribution,” _Journal of Botany_, 1919, p. 83.