Popular Science Monthly/Volume 63/July 1903/A Comparison of Land and Water Plants
|A COMPARISON OF LAND AND WATER PLANTS.|
By Professor GEORGE JAMES PEIRCE, Ph.D.,
LELAND STANFORD JR. UNIVERSITY.
THE aquatic origin of all living things is now a generally accepted conception. The arguments in its favor are: (1) morphological, based on comparative studies of the vegetative and reproductive parts; (2) biological, based on observation of the habits of plants and animals, especially at the breeding season; (3) paleontological, based on the now known fossil remains of formerly living organisms; (4) physiological, based on the absolute dependence of all living things on water. These last arguments appeal to me more strongly than any others. When we realize that all food, all the materials of which the body is constructed, and all the substances which its cells use, can enter the cells only in solution in water, we see at once how indispensable water is. When we realize besides that the form and size of the cells, and therefore of the body, depend upon the pressures within the cells which are due to the presence of aqueous solutions therein, we see how necessary water is in another way. Upon the tension of the cells depends the mechanical force which they, the tissues, and the organism, can exert. The absolute dependence of all living things upon water is one of the two most important characters which they possess. The amount of water which different cells, organs and organisms use varies greatly, but they all require some water. The ease with which different organisms, organs and cells obtain water also varies, though not necessarily in a degree corresponding with the amounts used. If we compare the conditions under which water and land plants live, we shall see some reasons for the differences in the structure and habits of these two classes.
The constantly submersed aquatic, whether fresh or salt water, is buoyed up with a very considerable force. A solid mass of plant tissue from which all air and water had been pressed would be buoyed up in water by a force from seven to eight hundred times as great as would be exerted if it were in the air. This is in accordance with Archimedes' well-known law in physics—a body in a fluid is buoyed up with a force equal to the weight of the volume of fluid which it displaces. Any part of a land plant, therefore, which rises into the air is supported with say only one seven hundredth of the force which supports the submersed aquatic. This difference is met by the land plant in two ways. It develops tissues which mechanically support it, which carry that part of its weight which the air can not carry; and it so constructs certain parts, for instance the leaves, of nearly all but the pines and their allies, that their form is best fitted for floating. The leaves are the organs in which most food is made. Their efficiency depends upon the amount of light which they can absorb, and they will evidently absorb most light if they are flat and placed at right angles to the rays as they come from the sun. This may be the main reason for the expanded form of the leaves, and it is the only reason which has been proved by experiment. But it is evident that a leaf is buoyed up more strongly and, therefore, requires less mechanical support if it is flat and more 01 less horizontal than if it were vertical or if it were cylindrical or cubical. Comparing weight for weight, we find more mechanical tissue in the pine-needle than in the flat leaf. And we find no such mechanical tissues even in the largest and longest submersed aquatics, some of which are as long as trees are tall.
The amount of mechanically strengthening tissue in a part or a plant has been proved by experiment to depend upon the amount of mechanical strain to which it is exposed. Garden plants which ordinarily carry the weight of their branches will be mechanically much weaker if supported on trellises. Conversely climbers and prostrate plants, if subjected to mechanical pull, will develop strengthening tissues which they ordinarily do not form. In these cases, the so-called inherited tendency to form or not to form mechanically strengthening tissues is so promptly overcome in the individuals experimented upon as to suggest some doubt whether there is such a tendency at all, whether the structure and behavior of living things is not more due to the influence of their surroundings than to inheritance.
We may conclude, then, that the presence in erect land plants of mechanically supporting tissues which are never found in submersed aquatics is not mere coincidence. The difference in the mechanical tissues of these plants is due, not to the differences in their places in any scheme of classification or to their degree of evolution, but to the differences in the buoyancy of air and water. Aquatic plants do develop mechanical tissues, but they resist the pullings, bendings and blunt blows which the waves give. These tissues can not support much weight.
The strength of the submersed aquatic will vary greatly according as it is a floating or an attached organism. All submersed aquatics which are unattached are mechanically weak and they are usually small, whereas those which are attached must develop a certain amount of mechanical strength to resist the tugging of the free parts against the holdfasts. Compare, for instance, Spirogyra and fresh-water Cladophora, plants of somewhat similar size, structure and situation. A Cladophora filament will break only under a much stronger pull than a Spirogyra filament of the same size and general structure growing in the same pool. Cladophora grows attached, Spirogyra is free. Compare Nereocystis and Macrocystis, the great kelps of the Pacific, with the Sargassum of the Atlantic. Sargassum begins life as an attached plant but is mechanically weak, is broken away and is for most of its life free. Our Pacific kelps are always attached and are tremendously tough. The comparison is not fair, however, for Sargassum is smaller than our giant kelps.
The attached plants between the tide-marks are among the most interesting as to mechanical strength. The rock weeds (Fucus), the Irideas, the Gigartinas, etc., of our Pacific shore withstand a tremendous amount of pulling and buffeting and are very hard to pull, though comparatively easy to tear, to pieces. These and other thinner and more delicate plants, e. g., the Ulvas, Porphyras, etc., escape destruction by their extreme pliancy rather than by toughness.
The most striking example of mechanical strength displayed by any plant living between the tide-marks is furnished by the sea palm (Postelsia), which is peculiar to the Pacific coast. This plant grows to a height of twelve to eighteen inches. The erect and smooth tapering trunk rises from the tangled mass of holdfasts attaching it to the flat or shelving ledge. The leaves, often over half as long as the trunk, narrow and corrugated, spring from its top. The trunk is like that of an erect land plant in being able to support a considerable weight applied vertically. The sea palm resembles in carrying power the land plant which gave it its name, but its remarkable strength is shown by its living where almost nothing else can, where the constantly beating surf is too much even for barnacles unless they take hold in some crevice. The spores must germinate very rapidly in the short times of comparative quiet, taking fast hold of the rock, for in most places where T have seen the sea palm growing, the waves were constantly in motion, and usually so violent, even at low water, that a man would be carried off his feet almost instantly. The sea palm bows before a breaker, bends away from it, resists its downward crushing force, holds on and holds together in spite of the shoreward thrust and seaward pull, thrives only where the sea is roughest, is the only plant where it grows every part of which has not fast hold of the rock.
Turning from the relative buoyancy of air and water and the effect of this difference in the supporting tissues of land and water plants, we may examine the relative ease with which land and water plants obtain their food-materials. The means by which any organism takes food or food materials into its living cells are simple though not generally enough understood. Only when the aqueous solution m the cell, permeating all its parts including the wall, is in contact with an aqueous solution outside the cell, can there be any absorption. The submersed aquatic has many or all of its cells in direct contact with the water. The land plant has only those cells which touch, or are in, the soil which are regularly in direct contact with water. Except those plants living in swamp or marsh, and except immediately after heavy rain, land plants are able to obtain only those thin films of water held on the surfaces of the soil particles. To reach these films, to bring the solution within the cells into contact with the water (also a solution) on the soil particles, land plants develop hairs—the rhizoids of the lower forms, the root-hairs of the higher. An aquatic composed of a chain or of a film of cells has all its cells directly in contact with the water, which holds in solution oxygen, carbon dioxide, and those mineral salts which constitute its food materials. An aquatic composed of a mass of cells, on the other hand, has only some cells which are able directly to absorb food materials from the water, those cells on the surface. The surface cells constitute the absorbing organ. Under these are other cells, containing chlorophyll, which manufacture the absorbed food materials into foods. If the plant is small, there may be besides only those cells which are used for storing the manufactured product and those concerned with reproduction. If the plant is larger, like the rock weeds and kelps, there must be in addition a system of cells for conducting the foods from the cells manufacturing them to others needing them. In all aquatics, even the largest, unless some are land plants retaining the structures characteristic of land plants even after becoming aquatic, there is only this one system of conducting tissues, the one which distributes food.
As we pass from the submersed aquatics to those only periodically submersed, from these to plants living prostrate on the ground, like most liverworts, and from these to erect plants, we see progressive changes in absorbing and conducting systems. The plants living between the tide-marks, for example the rock weeds and devil's apron (Laminaria), possess a conducting system similar to the submersed kelps, but the absorbing system is reduced in extent to prevent the plant from losing water by evaporation while exposed at low tide. Jn these plants there is need of two sets of qualities, those adapted to life under water, those fitted to life in the air—essentially, enough cells for absorbing water, and enough cells so placed and of such composition as to keep evaporation within safe limits.
The prostrate land plants, for example the liverworts, possess tissues similar to the small though massive algæ living between the tide-marks—an absorbing system and a protective system. But as, for most of the time, the prostrate land plant can absorb water only from the soil underneath it, and lose water by evaporation only from its upper surface, the absorbing and protective systems are separated, the food-manufacturing tissue lying between the other two. These prostrate i)hints are all so small that no conducting system is needed.
So soon as a plant turns up into the larger and unoccupied space above the soil, the part which grows up cuts itself off from a direct supply of water and mineral food materials and exposes itself to greater loss by evaporation. The absorbing system of the part still in contact with the soil must be extended, the part above must be covered with material less permeable to water, and a conducting system which will supply the part above with water, which can come only from below, must develop. This we find in the erect mosses, and also in these cells which mechanically support the parts the weight of which is not wholly or directly carried by air and soil. The larger mosses, Polytrichum for instance, show these different tissues.
When a plant assumes the erect posture, its structure must correspond with its changed habit. The anatomical changes in man's body, which supposedly took place when he assumed the erect posture, have been explained by zoologists. Similarly there are changes in the bodies of plants which take on the erect habit of growth. These changes enable them to conform to the new relations and degrees of mechanical strains, the different relations to absorption and loss of water, the different relations to light, etc. The simpler, larger, erect plants, for instance the grasses, have worked out the relations of absorbing, protecting, food manufacturing, conducting, and mechanically supporting systems in very definite fashion. In these plants, absorbing and food-manufacturing systems are remote from each other, connected, however, by conducting tissues which carry the mineral salts and water needed for food manufacture, plus the amount of water which must inevitably be lost by evaporation, an amount constantly varying everywhere, but differing greatly according to situation, climate, etc. In these plants there must be the other conducting system, the one for distributing the food made in the leaves to all the living cells in other parts. Here we encounter, as in the ferns and their allies, which might equally well have been selected as illustrating these points, the double conducting system. The food-distributing system is found in all larger plants in which there are other living cells than those engaged in food manufacture. This is the primitive conducting system, the one first needed, as our consideration •of the larger aquatics showed. Only when absorbing and food-manufacturing tissues are remote from each other is another conducting system needed and developed, and the dimensions of this correspond with the volume of water to be carried to supply food materials and to make good the loss by evaporation.
In the ferns and their allies, and in the grasses, the tissues mechanically supporting the parts above ground are combined into what may be called an external skeleton. This is distinct from the conducting tissues. It forms a cylinder close under the epidermis and enclosing the conducting and storing tissues. Each strand of conducting tissue may also be inclosed in a strengthening cylinder. This kind of skeleton is strong for the weight and amount of material in it, but it has the serious disadvantage of limiting the size of the organ or organism. The lobster and crab can continue to grow only by splitting the external skeleton. They shed this periodically, forming a new and larger one. Till this is formed they are weak and defenseless. If an erect plant were to split its external skeleton it would be too weak to stand. The limit which it sets to the size of the plant, rather than the difficulty of branching which is sometimes alleged as the disadvantage, is the serious defect in an external skeleton.
The grasses show an approach to an internal skeleton in that the greater part of the strength of the stem is due to the cylinders of supporting tissue in which the strands of conducting tissue are enclosed. But if the whole plant were to continue to grow, the cylinders in which the conducting tissues are enclosed would have to increase in diameter to allow an increase in the conducting tissues and this can not be done without splitting the strengthening cylinders and thereby greatly weakening the whole plant.
In the pines (using the word broadly) support and the conduction of liquids are accomplished by the same tissues, the same cells. These are the lowest plants in which an internal skeleton, if I may call it so, is found. Such a skeleton sets no limit to growth. It can be added to year by year as there is need of increased strength, and at the same time increased conducting tissue is formed. But conduction and mechanical support can not both be attained with the utmost efficiency and economy of material in cells which must serve both purposes. The diameter of the conducting elements must be limited lest they be weak, they must be comparatively short for the same reason, there can be no continuous tubes through which liquids can be rapidly transported. To ensure the requisite mechanical strength to the whole plant, the walls of the conducting cells must be thicker than would otherwise be necessary.
In the highest flowering plants, the dicotyledons, conducting and mechanically supporting tissues are combined in the same strands, but the same cells do not serve both purposes. In these plants, conducting and strengthening cells are side by side, they increase in number according to the needs of the plant, the conducting cells roost rapidly when most needed—as in the early spring—the strengthening cells later, when the increasing weight of the growing parts makes increased support necessary. In this combination of conducting and strengthening tissues, with the distribution of the two functions among different cells, the highest efficiency with the greatest economy of material is possible. There is no limit to which the plant can increase in size, provided only it preserve, from year to year, a layer of reproductive cells (the cambium) from which new cells developing into new conducting and strengthening elements may be formed.
In comparing the conditions under which water and land plants live this must be added. In the water, conditions change slowly and in regularly recurring periods. On land they change not only in regularly recurring periods but also frequently and suddenly. Submersed aquatics fall into a smaller number of species than do the plants living between the tide-marks. These again are numbered in fewer species than are land plants. The vertical distribution of aquatics is limited by the light to a few feet; the vertical distribution of land plants is limited by the temperature to a few thousand feet. Within this greater vertical space there is far greater diversity of conditions than in the shallow layer of water in which plants can live. This greater diversity of environment has been the cause of the greater diversity among land plants. But land and water plants, were they not sensitive to all the influences which combined make their environments, and had they not reacted to these influences, would never have attained the diversity which they now possess. The dependence of all living things upon water, and their power of reacting to all the influences of their environment to which they are sensitive, are the most striking phenomena displayed by animals and plants.