Popular Science Monthly/Volume 47/October 1895/The Life of Water Plants
|THE LIFE OF WATER PLANTS.|
WHEN we mentally survey the floral dress that variegates the solid crust of our globe, the world of plants appears to be divided into a few large groups. We think of the primitive forest with its mossy trees, the slender vines, and the multiform beauties of the orchids; the steppe comes to our minds with its hard, sharp-cutting grasses; and the moist carpeting of the Alpine flora, with gentian and fragrant herbs. The eye lingers longest on the native group; the colored population of our meadows rises before us, the forest with its berried undergrowth, and possibly the bushy river bank and the undulating insulated plant-covering of a pond. Each of the plant groups we have named bears a special expression distinguishing it from all the others. The members of each have certain common features the aggregation of which constitutes the characteristic of the group. In them are included plants which are not at all connected by natural relationship. Ivy is not related to wintergreen or the strawberry to the huckleberry, and all these, again, are far removed from ferns, mosses, and fungi; yet we are satisfied as to the connection of these plants, so that we regard them as members of a definite group, as is represented by our wood flora. So with the other forms we have named, those of the tropical forest, of the steppe, and of the Alpine fields; the plants constituting them are not grouped by blood relationship. Outer circumstances, the conditions of life, have impressed their special characters upon them. The shadow of the wood, the tropical rains, the short summer of the Alps, the aridity of the steppe—all these are factors which have produced in the plants exposed to them common properties more perceived than defined, because they have had effect upon their outer figure as well as upon their vital processes. Thus these groups of plants are developed by the community of their life conditions. They furnish illustrations of Goethe's saying that the manner of life works powerfully on all forms.
We shall study more closely in this paper one of these communities of life conditions—the plant world of the water—and inquire into the connection existing between its most marked peculiarities and the conditions of life afforded by the water. If we walk along the shore of a large pond sheltering a rich growth of plants, or of a bush-lined stream, the vegetable inhabitants will be divided, at first sight, into three groups—the shore plants on the banks; the floating leaves and flowers of the surface plants; and in the depths, hardly visible to the eye, the submerged flora, composed of a few curiously shaped flowering plants, and many of the lower orders belonging for the most part to the little microscopic world. The shore plants form a transition class between the vegetation of the land and that of the water. Taking root in damp soils, or perhaps under water, they lift the greater part of their stems with their leaves and flowers above the surface, joining the land flora in their methods of growing, respiring, and feeding. Among the shrubbery of the meadow, overtopped by single gray-stemmed alders, rise little forests of rustling reeds, both interspersed with variegated masses of various herbs, among which sharp-edged sedges and round-stalked rushes take the first places, by the side of the fragrant calamus, irises, and the umbel—flowers of the tall water violets. Farther ashore rise the beautiful white panicles of the swamp meadowsweet, with the grayish—green leaves and violet flowers of the bittersweet. We must not overlook the white stars of the willow-leaved aster, signs of the beginning of autumn, and the great bindweed, whose threadlike stems find welcomed support on the hard stalks of the reeds. Altogether a variegated picture, the characteristic points of which are hard to separate from the impression of the whole. This is easier to do with two other forms of shore flora which have been developed under peculiar conditions furnished by our waters—the flora of the sandy sea-beaches and that of the unfathomable, unstable morasses of the mouths of tropical rivers. In the former instance a striking appearance is given to the vegetation by the salt contained in the soil. Plants with usually inconspicuous flowers, and also a pretty blue aster, have adapted themselves to life by the salt water. They are sometimes distinguished by their fleshy leaves, the properties of which stand in so close relation to the presence of salt in their habitat that when one is far from the sea he can judge by their presence whether there is salt in the soil. Characteristic of the tropical morasses are the mangroves, a group of arborescent plants which stand as if on stilts on long, bracing roots sent out from all parts of their stems. The young shoots are hard, dagger-shaped bodies about a metre long, which finally drop down and bore perpendicularly into the slime so that they shall not be disturbed by the current, and may become fixed in the mud. In both of these shore regions the special forms appear to be developed in connection with the peculiar features of the locality.
These adaptations to special conditions thus easily recognized in the shore vegetation are greatly multiplied in the water plants proper. The better to understand them, we must, before going into particulars, devote a few words to the origin of the water flora. Among them are representatives of various orders and classes. They may be divided into plants that have strayed from the land into the water, and those whose original home is that element. The doctrine of the Greek philosophers, that everything is derived from water, is so far correct for the vegetable world that the first plants that appeared on the earth were water plants. There were probably little microscopic forms inhabiting the barely cooled waters of the primitive seas before there was any land to afford a suitable home for any living beings—formless albuminous masses, like "organisms without organs," which, like some of the bacteria, drew their food from the dead stone. Like their living kindred, the lower Algæ, they were of too tender nature to be preserved in the cavities of the sea slime. The first remains seemingly of vegetable character preserved in the oldest strata of the earth's crust are therefore of relatively large fucoids. Their existence justifies our supposing an already richly developed flora of Algæ such as is now found in the deepest parts of our lakes. Mosses, ferns, and flowering plants are absent. They appear later, and under conditions which prove that they were produced not in the sea but on swampy land. Geological evidence shows us that only the Algæ, and the fucoids originated in water and were water plants from the first. The other water plants, especially flowering plants growing in water, were driven into the water by increasing competition among the growing number and variety of the land plants, and assumed the properties that now distinguish them from land plants during their compulsory emigration, and in consequence of their water life. This process is now going on in our sight in a certain plant—the wandering knotgrass—a relative of the small-flowered, spreading swine grass and of the adderwort. This plant grows on the borders of ditches and ponds, often half on land and half in the water; and it can not escape the attentive observer that it presents a quite different appearance in the water from that upon land. Stiffly haired, and having short-stemmed leaves on land, it is in water bald and smooth, and develops very long leafstalks which terminate on the surface in flat, floating expansions. Here there is a plant which only occasionally, and usually only partly, makes its home in the water, and is in a position to suffer such remarkable changes that it is no longer a wonder that plants which have become entirely at home in the water are very little like those of their genus which remain land plants. Of many of them, in fact, it can no longer be determined from what family of land plants they are really derived. Even such well-defined plants as seaweed and duckweed would not at first sight suggest to any one relationship with the Arum family. So with the water crowfoot, which we shall take as our starting point in the discussion of the properties of water plants. Well known are its little white flowers, which adorn the ponds and even the swift streams in the summer time.
These little blossoms, thickly massed on the surface of the water, or sitting on long stems, spread out their five petals, which, with their numerous stamens and styles, mark them as relatives of the buttercups. In the other parts we look vainly for resemblances with the ranunculuses. If, for instance, we take one of them which we find floating in running water, out of its element, the whole plant falls together, and we hold in our hand nothing but a bunch of long threads, in which no difference can be perceived between stems and leaves. If we spread a part of the bunch upon a stone, we may discover branching shoots beset with leaf forms; but both organs are widely different from those of their nearest generic relatives. The stems of the ranunculuses of the fields are upright, stiff, skeletonlike, strong enough to defy wind and storm, and able to bear the weight of their leaves, flowers, and fruits. The stems of the water ranunculuses are slack and weak. They are swung around helplessly by the waves, winding hither and thither in the direction toward which the run of the stream carries them. They are stable only in the direction of their length, because in any other case the current would carry them away. In other respects the stem does need cohesive power. The whole plant is pierced with connected air passages, and all its parts are adapted to floating or swimming. The water here takes the burden upon itself which is imposed on the stems of land plants. Floating plants need no skeletons; and dissection and microscopic examination show that all those forms are wanting in their interiors which, like the bones of animals, give stability and tenacity to their structure.
Many water plants lack organs still more closely associated with the life processes. We can not conceive of a higher animal without veins and lymph-vessels. But in water plants we not seldom miss the long and broad ducts of which the vascular system of land plants is constituted. At all events the vessels do not perform so important a part in the vegetable kingdom as the circulation of the life juices in the animal kingdom. Their principal service is to carry water from the roots to the leaves. From this we can understand how organs essential to the life of land plants can be dispensed with in water plants. They do not need a special conducting of water, because they are surrounded by that element on every side. The most marked instance of the absence of internal organs is met in an alga which forms green fields in the deeper parts of the Mediterranean Sea. It has slender, branching, horizontally creeping stems which develop above in the water into leaves and below in the sand into fine thread roots. But the whole plant, often many feet in length, consists only of single gigantic cells. A tough skin incloses its juices, which flow in a continuous stream through the stem, leaves, and roots of the curious growth, here taking up through the skin and assimilating mineral substances, there producing and transforming organic matter, and at the same time advancing the growth and increase of the whole. Not less peculiar than the inner structure and appearance of the stem is the form of the leaves of water plants. Their service to the organism is the same with that of the leaves of other plants. They supply, with the help of the sunlight, matter which the plant needs for building up its body. The conditions under water are not very favorable for this work, for the rays of light suffer considerable loss of intensity in passing through even a thin sheet of water. In connection with this there are leaves growing under the water, as in the floating crowfoot, as a rule not flat or oval or cordate or round, like most other kinds of leaves, but divided into the thinnest threadlike strips, which, with the largest surface development, obstruct the least possible light from one another and easily yield to the current.
The leaves that are destined to live on top of the water are otherwise constructed. They will not overshadow one another, and they are exposed to the full light of the sun. They need only to receive it on as broad a surface as possible, and so to float that the weight of the food-stuff accumulated within them all the day long shall not cause them to be submerged. These leaves consequently do not present divisions or ramifications like the leaves of roses and acacias. They form reniform or oval disks, which lie flat upon the water. Every one will recollect this who has seen the yellow and the white pond lilies. The brownish spawn weed and the beautiful white flowing frog spittle likewise have swimming leaves; and there is a marsh crowfoot which has these and submerged threaded leaves all on the same stalk. In the duckweeds stem and leaf are not distinguished, and the plant is only a flat disk with a few insignificant rootlets on the under side; and in one species these are wanting. The plant is only a little floating leaf, with a pocket for the reception of the scantily endowed flowers.
The floating leaves of the Victoria Regia are beautifully developed. They have the form of flat plates with a narrow, upturned border—a form more favorable to their notation. The green leaves, a yard or more across, with the pink flowers resembling gigantic lilies scattered among them, present a remarkable spectacle.
One of the most remarkable peculiarities of the floating leaves of our water plants is that they never grow up above the surface of the water. The plant appears to know when that point is reached. As we shrink from sudden contact with cold water, these leafstalks suspend their growth on contact with the air. They grow just long enough for the leaf expansion to reach the surface of the water. There it unfolds itself in the full light, and finally lies flat on the water, protected against the wet by a fine, bright coating of wax. It is wonderful how large masses of organized material a water plant can lay up with the help of these floating leaves. The great Victoria Regia grows to its full size in a single year from a small seed. Nearly its whole mass is prepared during growth in the leaves, which can perform such a work only in the strong light and the warmth of the tropical zone.
Submerged plants, as we have already said, are less well provided as to the reception of light than those with floating leaves. Hence those are chiefly small forms which we find at the bottom of our waters. Of these the Algæ are the most numerous. There are, indeed, on the whole earth no wet, only moderately light places where Algæ have not established a home. Insensible in a high degree alike to heat and cold, they are capable of growing on the snow of the Alps and on the edges of hot springs. We find them on the stones of rushing mountain torrents, in the plunge of the steepest waterfalls, and in the surf of the seacoast, and again at the bottom of the nearly motionless waters of ditches and ponds. The diversity of their habitats corresponds with the immense multitude of their forms. In the form of microscopic dots they will gradually change all the water of a pond or lake into a disagreeable turbid, green, often rank-smelling fluid; sometimes floating on its surface as green or yellow wads dotted with air bubbles; sometimes they appear at the bottom of the water as thick, roundish bundles of green, tangled threads; sometimes as slippery brown coatings.
The Algæ, of the sea, or seaweeds, are strikingly rich in coloring. Besides green, there are in the sea black, brown, and red forms, the last, under favorable conditions of light, often attaining great size. They seem to be adapted by their peculiar coloring to the tempered blue light of the deeper strata of water. In the great deeps the plant life of the sea is extinguished for want of light. At a hundred metres beneath the surface only a few Algæ are found. These are plants of the shade, needing little light. Some of them continue to grow without interruption through the three months' polar night of Spitzbergen, and develop their invisible flowers and fruits at this season with the temperature of the water below the freezing point. The giants of the Algæ seek the enjoyment of uninterrupted sunlight. They grow in the deep waters near the shore, and send up slender stalks which pass at the surface of the water into long, shredded leaf forms. Algæ of this kind form, on the Chilian coast, for instance, forests in which specimens may be found more than two hundred metres long.
We return, after this excursion into the curious world of the Algæ, to the higher plants, and inquire whether there are not special adaptations to a life in the water in the conditions of their blossoming.
The flower of the phanerogam is adapted in general to life in the air and the light. We find, therefore, that the flowers of many water plants with floating leaves aim to reach the surface of the water. They require for the transfer of the pollen to the ovule the aid of the wind or of the insects which hover thickly over the water. Their emergence is effected with the help of floats of various kinds, of which it is sufficient to recall here a much-cited example. In the Vallisneria, the long, grasslike leaves of which form a kind of turf at the bottoms of some of the south European lakes, the inconspicuous male flowers rise in knobby bunches protected by a turgid envelope at the bottom of the water. The female flowers stand singly on very long, threadlike stems which rise to the top of the water. When the pollen has matured, the covering of the male flowers opens and the flowers escape in the form of little balls, which, being very light, rise at once to the surface of the water. Here are unfolded three white leaves, which, to use a figure of Kerner's, float around like pollen-laden canoes, and are so wafted by the wind as to convey their freight to the female flowers. While the fruit is forming, the stems of the female flowers roll up spirally and draw the seeds down into the protecting deeps, to remain there undisturbed till the time of germination. The flowers of many water plants, except for these processes, remain concealed in the deep throughout their lives. They do not there bring all their functions to fruiting, which can be accomplished only in the air. Without color or fragrance, they are inconspicuous; their structure is distinguishable only under close examination, and they are proved to be real flowers only by their pollen-shedding and their formation of seeds. Again arise in water plants special tasks. It is incumbent upon one to spread itself as widely as possible and establish its posterity in new places, where it may obtain room for free development and will not be dwarfed under the shadow of larger plants. The seeds, therefore, must not stay where they have fallen when ripened. They must be scattered, and by all means carried away from the immediate neighborhood of the mother plant. The fruits or seeds of water plants are therefore largely endowed with aids to swimming, by the help of which they can accomplish long distances. They share this provision with many shore plants, the whole existence of which is connected with the water in more than one respect. Among these is the cocoa palm, the gigantic fruits of which are comparatively very light. Filled within with cocoanut milk and cocoa butter, food for the young sprout, they possess a thick hull of loosely woven fibers, which forms a most excellent raft to carry the fruit far over the sea till it is stranded on some island to grow there into a new tree. Hence the cocoa palm is the first settler on all coral reefs that rise above the sea, and becomes the most characteristic element of tropical landscapes.
Other water plants employ animals as means of transport. They are swallowed by fishes, from the intestines of which they pass undigested.
The method of dispersion of many of the Algæ, is especially interesting. They produce wandering cells which, endowed like the infusoria with free motion, shoot around in the water till they find a point on which they can settle themselves and grow. The lens will show us on a stone in the brook or on a dry limb that has fallen into the water a small group of slender threads, perhaps about a third of an inch high, each of which consists of a row of cylindrical cells set one upon another. If we take the plants home, wrapped with their support in round paper or moss, and put them in a dish of fresh water, we shall often be able in a short time to observe the spectacle of the formation of wandering cells. With a microscope we can see the individual cells breaking up and their contents creeping out of the cleft in the form of oval bodies. At the forward end of these swarms we may observe a number of fine threads which swing rapidly back and forth till they acquire a rotary motion, by means of which they swim spirally forward. The Austrian botanist Unger, who was among the first to observe the formation of wandering cells, believed at the time that he had surprised the plants in the act of becoming animals. The vegetable wanderers are, in fact, wonderfully like minute water animals. Many of them have red spots in front, which some have not irrationally supposed were light-perceiving organs or eyes of the simplest sort.
The question whether plants have a consciousness meets us here more impressively than anywhere else in the vegetable kingdom. As we observe how wandering cells swim toward food-stuffs and avoid poisons, seek moderate light and retire from strong light, and distinguish their own likes from the wandering cells of other plants, we find also really no difference. We have to concede that the same feelings and expressions are apparent in both; and if we ascribe a kind of soul to the animals, we can not deny it to the plants. We can not expect to find thought and reason in these circles of simplest light. Those are the prerogatives of the highest inhabitants of earth. The whole existence of these lower beings consists in the unconscious reception of impressions and the unconscious movements occasioned by it. The vital career of the annual water plants closes with the formation of the seed. They die and fall into decay through the agency of the water bacteria, whose activity carries their substance on to renewed life in the circulation of matter. The vegetation of most of our northern perennial water plants is interrupted during the winter. Some of them, like the pond lily, have long rootstocks, in which superfluous food is deposited as in a storehouse in the course of the summer, to serve in the spring for the formation of the new leafy growth; others form special winter buds which, likewise filled with food, separate from the mother plant, sink to the ground, or are frozen in the ice, till the returning warmth of the sun revives them.
Having sought to uncover the mysteries of the household life of the water plants, we now turn to their relations to other families. No group of organisms has ever been able to develop itself independently of all other living beings. Individuals have to acquire the useful properties we admire in them in constant conflict—plants especially, in conflict with the animal world; and the vegetation of the water is as much subject to it as any other. Besides the fishes, there are the water snails and innumerable crustaceans, large and small, turning to water plants for food. Some plants are protected against these creatures by the presence of substances that give their leaves a bitter taste; some have many pointed prickle cells in the interior of their leafstalks which make it impossible for their enemies to bite through them. Most of the seaweeds are furnished with slimy cell walls, on which the water snails try their teeth in vain. The calcareous Algæ of the sea enjoy the best protection in the shape of a knotty or coralline form which has little resemblance to a plant, and through the deposition of carbonate of lime in their cell walls, almost turning them into real stone. Only a few marine animals know how to attack them. Among these is a snail that dissolves the lime by means of a secretion of sulphuric acid. With the same material, as Semon has shown, these snails also make sea urchins and starfish digestible, and are therefore brought in reach of an unusual variety of food, in which they are rivaled only by the lobster with his strong cutting jaws.
There are, besides, animal-catching plants among the water vegetation as the common bladderwort (Utricularia), a yellowflowering plant, with slender stem and finely dissected leaves, which is abundant in still waters in summer. Its leaves bear on and between their points round bladders, about as large as the head of a pin, which serve as animal traps. The most interesting part is an elastic lid, which opens only toward the interior. A wreath of glandular hairs surrounding the entrance of the bladder secretes a slimy material which entices the smaller water Crustacea to their destruction. They swim up greedily and collect at the mouth of the bladder to enjoy their feast. One of the guests ventures to get upon the lid. He remains there at first quietly held; but upon his making a more vigorous motion, the lid opens suddenly, swallows the little animal, and then closes again. The captive struggles awhile to escape from his prison, but gradually his movements become weaker, and he dies at last. Now the hairs clothing the interior walls of the bladder begin their work of imbibing food from the softer parts of the animal. When we reflect that a length of a finger and a half of a branch of the bladderwort can thus entrap two hundred of the little crustaceans, we can easily comprehend how the plant can do without roots, upon which it would otherwise depend for its supply of nitrogenous food.
In view of certain investigations which have been made of water flora and fauna for special purposes, I add a few words on the place of the water flora in the economy of Nature. The lower vegetation is of very great importance, especially the microscopic plants, innumerable plantations of which inhabit extensive tracts of all, and especially of northern, lakes. They move around in the water in masses or singly, changing the deep blue color of the spots destitute of organisms into green or dirty yellow. Most numerous among these minute plants wandering in lakes are the diatoms or siliceous Algæ, the richness of the forms of which surpasses all imagining. They consist of a nucleus of living substance inclosed as in a box between two siliceous shells. These shells bear markings so fine that they are used, in the same way as the dust of butterflies' wings, as tests for the delicacy of our best microscopes. The diatoms move through the water by the aid of a peculiar propulsory apparatus till they dying sink to the bottom and there go to help form the slime which is of so great importance in the evolutionary history of the crust of the earth. There are also diatoms in fresh water. They are the principal constituents of the brownish-yellow slippery coating of the stones in the beds of brooks.
The work of these and other similar living beings in the economy of Nature is not therefore lost, because they help with their dead remains to build up the dry land and prepare the ground for future generations. Their existence is thus of benefit to their fellow-creatures. As on the land, so there are plants in the sea which elaborate the unenlivened substances of the air and of the mineral kingdom and convert them into matter to become constituents of the bodies of living beings. Inasmuch as they serve as food, or as animals living upon them fall victims to the larger animals, they are the support of the whole animal life of the ocean. When we consider that twelve million individuals of one species of these sea plants hardly contain a half gramme of organic substance, the endless mass of life that perishes to form the material for a whale, for instance, becomes inconceivable.—Translated for The Popular Science Monthly from the Deutsche Rundschau.