( Originally Published 1909 )
NUTRITION thus, as has been pointed out, makes it possible to classify most organisms as animals or plants. Yet there are many unicellular forms in which both kinds of nutrition go on at the same time ; that is, the forms may possess a mouth. for the ingestion of solid food and green coloring matter, chlorophyll, for the manufacture of starchy food from gaseous matter.
Many of the lowest forms of life have long been puzzles and the beginner in biological study is surprised to find them described in text-books of both botany and zoology. The fact is that they are on the border line, are neither plants nor animals but simply organisms. Since they cannot be classified, it is necessary that they be listed both under botany and zoology, in order to make sure that they will not be omitted entirely. Because of these uncertain forms of life, Haeckel proposed once to include all one-celled animals and plants in a third kingdom to be called Protista (meaning the first of all life).
Parker's definition of animals and plants, based on the foregoing considerations, is convenient for distinguishing between animals and plants in all cases except the doubtful unicellar forms. He says :
"Animals are organisms of fixed and definite form, in which the cell-body is not covered with a cellulose wall. They ingest solid proteinaceous food, their nutritive processes result in oxidation, they have a definite organ of excretion and are capable of automatic movement.
"Plants are organisms of constantly varying form in which the cell-body is surrounded by a cellulose wall; they cannot ingest solid food, but are nourished by a watery solution of nutrient materials. If chlorophyll is present, the carbon dioxide of the air serves as a source of carbon, nitrogen is obtained from simple salts and the nutritive processes result in deoxidation; if chlorophyll is absent, carbon is obtained from sugar or some similar compound, nitrogen either from simple salts or from proteids, and the process of nutrition is one of oxidation. There is no special excretory organ, and, except in the case of certain reproductive bodies, there is usually no locomotion."
The important point to recognise is that these boundaries are artificial and that there are no scientific frontiers in Nature. As in the liquefaction of gases, there is a 'critical point' at which the substance under experiment is neither gaseous nor liquid; as in a mountainous country, it is impossible to say where mountain ends and valley begins; as in the development of an animal, it is futile to argue about the exact period when, for instance, the egg becomes a tadpole or the tadpole a frog, so in the case under discussion. The distinction between the higher plants and animals is perfectly sharp and obvious, but when the two groups are traced downward they are found gradually to merge, as it were, Into an assemblage of organisms which partake of the characters of both kingdoms and cannot without a certain violence be either included in or excluded from either. When any given 'protist' has to be classified the case must be decided on its individual merits; the organism must be compared in detail with all those which resemble it closely in structure, physiology and life history, and then a balance must be struck and the doubtful form placed in the kingdom with which it has, on the whole, most points in common.
It will no doubt occur to the reader that, on the theory of evolution, the fact of the animal and vegetable kingdoms being related to one another like two trees united at the roots may be accounted for by the hypothesis that the earliest organisms were protists and that from them animals and plants were evolved along divergent lines of descent. And in this connection the fact that some bacteria the simplest organisms known and devoid of chlorophyll may flourish in solutions wholly devoid of organic matter is very significant.
The lower plants and animals referred to above are so far from everyday observation and hence so unfamiliar that to most people the comparison made will mean little in terms of ordinary green flowering plants and common vertebrate animals. In order to emphasize the fundamental similarity of organic function in higher and lower animals and plants, let us compare any higher plant —e.g., a bean plant with a higher animal, e.g., frog or even man. In each the life is the sum total of a series of definite processes nutrition or food supply, circulation, metabolism, excretion, oxygenation (part of respiration), movement, irritability (nervous activity) and reproduction. In turn these will be compared for the animal and the plant, following in part the comparisons of certain animals and plants by Sedgwick and Wilson and others. These comparisons will, however, he translated into terms applicable to any species of higher plants or animals.
In the nutrition of the animal the most essential and characteristic part of the food supply is derived from vegetable or animal matter in the form of various organic compounds, of which the most important are proteids (protoplasm, albumen, etc.), carbohydrates (starch, cellulose) and fats. These materials are used by the animal in the manufacture of new protoplasm to take the place of that which ha been used up. It is, however, impossible for the animal to build these materials directly into the substance of its own body. They must first undergo certain preparatory chemical changes known collectively as digestion, and only after the completion of this process can all the food be absorbed into the circulation.
For this purpose the food is taken not into the body proper. but into a kind of tubular chemical laboratory called the alimentary canal, through which it slowly passes, being subjected meanwhile to the action of certain chemical substances or reagents, known as digestive ferments. These substances, which are dissolved in a watery liquid to form the digestive fluid, are secreted by the walls of the alimentary tube. Through their action the solid portions are liquefied and the food is rendered capable of absorption into the body proper.
The food supply of the higher plant, like that of the animal, is the source of the required matter and energy, but unlike that of the animal, it is not chiefly an income of foods, but only of the raw materials of food. Matter enters the plant in the liquid or gaseous form by diffusion, both from the soil through the roots (liquids) and from the atmosphere through the leaves (gases). We have here the direct absorption into the body proper of food-stuffs precisely as the animal takes in water and oxygen. Energy enters the plant, to a small extent, as the potential energy of food-stuffs, but comes in principally as the kinetic energy of sunlight absorbed in the leaves.
Of the substances the solids (salts, etc.) must be dissolved in water before they can be taken in. Water and dissolved salts continually pass by diffusion from the soil into the roots, where together they constitute the sap. The sap travels throughout the whole plant, the main tho not the only cause of movement being. the constant transpiration (evaporation) of watery vapor from the leaves, especially hrough the stomata. The gaseous matters (carbon dioxide, oxygen, nitrogen) enter the plant mainly by diffusion from the atmosphere, are dissolved by the sap in the leaves and elsewhere and thus may pass to every portion of the plant.
The green plant owes its power of absorbing the energy of sunlight to the chlorophyll-bodies or chromatophores, for plants which, like fungi, etc., are devoid of chlorophyll, are unable thus to acquire energy. Entering the chlorophyll-bodies, the kinetic energy of sunlight is applied to the decomposition of carbon dioxide and water. After passing through manifold but imperfectly known processes, the elements of these substances finally reappear as starch, often in the form of granules embedded in the chlorophyll-bodies and free oxygen, most of which is returned to the atmosphere. Thus the leaf of a green plant in the light is continually absorbing carbon dioxide and giving forth free oxygen.
Carbon dioxide and water contain no potential energy, since the affinities of their constituent elements are completely satisfied. Starch, however, contains potential energy, since the molecule is relatively unstable i.e., capable of decomposition into simpler, stabler molecules in which stronger affinities are satisfied. And this is due to the fact that in the manufacture of starch in the chlorophyll-bodies the kinetic energy of sunlight was expended in lifting the atoms into position of vantage, thus endowing them with energy of position. In this way some of the radiant and kinetic energy of the sun comes to be stored up as potential energy in the starch. In short, the green plant is able by cooperation with sunlight to use simple raw materials (carbon dioxide, water, oxygen, etc.) poor in energy or devoid of it, and out of them to manufacture food i.e., complex compounds rich in available potential energy. This power is possessed by green plants alone; all other organisms being dependent for energy upon the potential energy of ready-made food. This must, in the first instance, be provided for them by green plants, and hence without chlorophyll-bearing plants, animals (and colorless plants as well) apparently could not long exist.
The plant absorbs also a small amount of kinetic energy, independently of the sunlight, in the form of heat. This, however, is probably not a source of vital energy, but only contributes to the maintenance of the body temperature.
Food (starch) thus produced in the green leaves of higher plants and the inorganic foods (water, nitrites or nitrates and various mineral substances in solution in water) furnish the materials and energy required for the life and growth of the plant.
The circulatory system distributes these foods. In animals foods prepared for absorption in the stomach and intestine (by digestion) are absorbed by the circulating liquids (blood and lymph) and transported to all cells of the animal body. In the plant the inorganic mat-ter in water from the soil are absorbed by the roots and carried up definite tubes in the woody part of the stem. The causes of this ascent are not clear, but root-pressure due to osmosis, capillary action and evaporation from the leaves are factors. Just as the solid food of animals must be digested in preparation for absorption, so starch manufactured in the leaves must be digested (dissolved) before it can be transported. This is done by diastase, an enzyme of plant cells. The change is from starch to a sugar capable of diffusion. Dissolved in water, the sugar is transported down delicate tubes, chiefly in the growing bark region of the stem. It is clear that there are upward and downward currents of water containing food (comparable to blood of an animal), but no system of complete circulation as in the blood vessels of a higher animal. However, the result in distributed food is the same in the plant and in the animal.
In the cells the foods undergo metabolic changes. In an animal the foods in the circulating liquids, blood and lymph, are selected and absorbed by the cells. Only proteid foods form new protoplasm and even of proteids only a limited amount, seventy-five to one hundred grams a day for a man, is built into new protoplasm. The excess undergoes oxidation and forms nitrogen excretions. The foods containing only the elements carbon, hydrogen and oxygen (fats and carbohydrates) are directly oxidized to excretions and, lacking nitrogen, cannot serve for making new animal protoplasm. Fat and carbohydrate foods, then, never become living matter. They may be stored, especially as fat, until needed for oxidation to supply energy. The building up of the protoplasm from proteids is anabolism, constructive metabolism. The destruction of protoplasm, excess proteids or the fat and carbohydrate foods is katabolism, destructive metabolism. Katabolism is probably due to enzyme action, but the final result is chiefly carbon dioxide and water, which could be derived by the ordinary chemical evolution of protoplasm, proteid, sugar, starch or fats.
In the plant, starch, as has been seen, is first formed in the chlorophyll-bodies. But the formation of starch, all-important as it is, is after all only the manufacture of food as a preliminary to the real processes of nutrition. These processes must take place everywhere in ordinary protoplasm, for it is here that oxidation occurs and the need for a renewal of matter and energy consequently arises. Sooner or later the starch grains are changed into a kind of sugar (glucose), which, unlike starch, dissolves in the sap and may thus be easily transported to all parts of the plant. Wherever there is need for new protoplasm, whether to repair previous waste or to supply materials for growth, after absorption into the cells the elements of the starch (or glucose) are, by the living protoplasm, in some unknown way combined with nitrogen and sulphur (probably also with salts, water, etc.) to form proteid matter. The particles of this newly formed compound are incorporated into the protoplasm.
If a larger quantity of starch is formed in the chlorophyll-bodies than is immediately needed by the protoplasm for purposes of repair or growth, it may be reconverted into starch after journeying as glucose through the plant and be laid down as "reserve starch" in the cells of root or stem or elsewhere. Apparently when this reserve sup-ply is finally needed at any point in the plant, it is again changed to glucose and transported thither. It is probable that new leaves and new tissues generally are always formed in part from this reserve starch.
In the plant as in the animal metabolism must consist of anabolic and katabolic processes. The construction in the cells of new proteid from the absorbed carbohydrate and the materials from the soil is true anabolism. It is also clear that katabolism or oxidation for the liberation of energy occurs as in animals, but this process is slower. Probably foods containing carbon, hydrogen and oxygen. are the sources of energy in the higher plants as in animals.
In both plants and animals simple waste substances result from the katabolic processes in the cells. In the animal carbon dioxide, water and nitrogen compounds are the chief excretions. They are absorbed by the circulating liquids and carried to the eliminating organs, lungs . and kidneys chiefly, for elimination. In the higher plants the excretions are carbon dioxide, which escapes through the epidermis of root, stem and leaf and through the stomata ; water which is lost by evaporation, especially from the leaf surface through the stomata; excretions which are lost by osmosis through the roots and the accumulated but use-less mineral substances which are eliminated by leaf fall.
In both animals and plants oxygen is essential to the katabolic part of metabolism. Hence oxygen must be supplied to the cells. Oxygenation is the term used to denote the oxygen-supplying part of respiration ; the other part of respiration, elimination of carbon dioxide, has been treated under excretions. In the animal oxygen is absorbed by the blood, in excess by the hemoglobin of the red cells of the blood and later is absorbed from the blood and lymph by all the living cells. In the plant also oxygen is absorbed through the epidermis and stomata from the air. This process is, however, obscured during the day because of the oxygen freed in the manufacture of starch which goes on at that time. Probably this freed oxygen is used for the purpose of oxygenation, but more is freed in the photosynthetic process than is needed for oxygenation and hence the excess oxygen is eliminated while starch manufacture is in process.
In comparing a higher animal and a green plant con-fusion must be avoided regarding the part played by oxygen and carbon dioxide in true respiration with the part played by the same substances in starch formation (photo-synthesis). In non-green plants like the Indian pipe and mushrooms the breathing of oxygen and the excretion of carbon dioxide are as in the animal. This is true also of green plants in darkness and even in the light of all parts of green plants except the chlorophyll-bodies. These constitute a sort of extra mechanism, enabling green plants to make their own carbohydrate food. Imagine a higher animal with an attachment for turning the carbon dioxide and water excreted back to starch usable as food and the comparison of the green plant and the animal would be complete.
The power of movement or locomotion is curiously thought of as peculiar to animals, but biologists know dozens of examples of movement in plants. Some of the lower plants possess the power of locomotion and even in plants as high as mosses and ferns there is a locomotion of the male germ-cells. Among the flowering plants there is no actual locomotion, but there are numerous forms of movement. Most striking perhaps are the so-called "sleep-movements" of plants by means of which the leaves of some plants e.g., the oxalis, bean, clover, locust and others can assume decidedly different positions at night from those which they occupy by day ; the movements of the sensitive plant (Mimosa) when "shocked" by touch or in other ways, and the movements of the leaves of the Venus flytrap (Dionea), which close with a quick jerk when the sensitive hairs with which they are bordered are stimulated.
It is in nervous functions that the most striking difference between the animal and the plant is to be found. Plants have no true nervous system, nevertheless irritability or response to stimulus is well marked in many plants. It would carry us far beyond the limits of this volume to trace satisfactorily the powers of plants to respond to stimuli of gravity, light, heat, water, electricity, chemicals, contact, injury and so on. Suffice it to say that the total effect of such responses is for the plant equal to that of the nervous system for the animal.
Reproduction is a most important function. The life of every organic species runs in regularly recurring cycles, for every individual life has its limit. In youth the constructive processes preponderate over the destructive and the organism grows. The normal adult attains a state of apparent physiological balance in which the processes of waste and repair are approximately equal. Sooner or later, however, this balance is disturbed. Even tho the organism escapes every injury or special disease, the constructive process falls behind the destructive, old age ensues and the individual dies from sheer inability to live. Why the vital machine should thus wear out is a mystery, but that it has a definite cause and meaning is indicated by the familiar fact that the span of natural life varies with the species ; man lives longer than the dog, the elephant longer than man.
It is a wonderful fact that living things have the power to detach from themselves portions or fragments of their own bodies endowed with fresh powers of growth and development and capable of running through the same cycle as the parent. There is therefore an unbroken material (protoplasmic) continuity from one generation to another that forms the physical basis of inheritance and upon which the integrity of the species depends. As far as known, living things never arise save through this process. In other words, every mass of existing protoplasm is the last link in an unbroken chain that extends backward in the past to the first origin of life.
The detached portions of the parent that are to give rise to offspring are sometimes masses of cells, as in the separation of branches or buds among plants, but more commonly they are single cells, known as germ-cells, like the eggs of animals and the spores of ferns and mosses. Only the germ-cells (which may conveniently be distinguished from those forming the rest of the body or the somatic cells) escape death, and that only under certain conditions.
All forms of reproduction fall under one or the other of two heads, viz., agamogenesis (asexual reproduction) or gamogenesis (sexual reproduction). In the former case the detached portion (which may be either a single cell or a group of cells) has the power to develop into a new individual without the influence of other living mat-ter. In the latter, the detached portion, in this case always a single cell (ovum, oösphere, etc.), is acted upon by a second portion of living matter, likewise a single cell, which in most cases has been detached from the body of another individual. The germ is called the female germ cell, the cell acting upon it the male germ cell, and in the sexual process the two fuse together (fertilization, impregnation) to form a single new cell endowed with the power of developing into a new individual. In some organisms (e.g., the yeast-plant and bacteria) only agamogenesis has been observed; in others (e.g., vertebrates) only gamogenesis; in others still both processes take place as in many higher plants.
The earthworm, for example, is not known to multiply by a natural process of agamogenesis. It possesses in a high degree, however, the closely related power of regeneration, for if a worm be cut transversely into two pieces the anterior piece will usually make good or regenerate the missing portion, while the posterior piece may regenerate the anterior region. Thus the worm can to a certain limited extent be artificially propagated, like a plant, by cuttings, a process closely related to true agamogenesis. Its usual and normal mode of reproduction is by gamogenesis; that is, by the formation of male germ cells (spermatozoa) and female germ cells (ova). In higher animals the two kinds of germ-cells are produced by different individuals of opposite sex. The earthworm, on the contrary, is hermaphrodite or bisexual ; every individual is both male and female, producing both eggs and spermatozoa.
As in the animal so in the plant, whether the individual dies or not, ample provision against the death of the race is made in the act of reproduction. Altho reproduction appears to be useless to the individual and even entails upon it serious annual losses of matter and energy, yet to this function every part of the plant directly or indirectly contributes. The reproductive germs are carefully prepared, are provided with a stock of food sufficient for the earliest stages of development, and are endowed with the peculiar powers and limitations of each species which influence their life history at every step and are by them transmitted in turn to their descendants. They are living portions of the parent detached for reproductive purposes and they contain a share of protoplasm directly descended from the original protoplasm from which the parent came. In short, reproduction is the supreme function of the plant.
As in the animal, reproduction in the higher plants is by the process of gamogenesis. Male germ cells and female germ-cells are formed in separate organs. In many of the higher plants these organs are both found in the same individual, but often they are produced by different individuals of opposite sex. Agamogenesis, or asexual re-production, also takes place in many higher plants by means of runners, buds, bulbs, tubers, etc., but in most of these cases provision is made for sexual reproduction also.
In relation to the environment animals and plants are masses of living matter occupying definite positions in space and time and existing amid certain definite and characteristic physical surroundings which constitute their "environment." As ordinarily understood the term environment applies only to the immediate surroundings of the animal and plant. Strictly speaking, however, the environment includes everything that may in any manner act upon the organisms that is, the whole universe outside them. For they are directly and profoundly affected by rays of light and heat that travel to them from the sun; they are extremely sensitive to the alternations of day and night and the seasons of the year; they are acted on by gravity; and to all these, as well as to more immediate influence, they make definite responses.
The body of the animal is a complicated piece of mechanism constructed to perform certain definite actions. But every one of these actions is in one way or another dependent upon the environment and directly or indirectly relates to it. At every moment of its existence the organism is acted on by its environment; at every moment it reacts upon the environment, maintaining with it a constantly shifting state of equilibrium which finally gives way only when the life of the animal draws to a close.
The action of the environment upon the animal has been sufficiently stated. It remains to point out the changes worked by the animal on the environment. These changes are of two kinds, mechanical (or physical) and chemical. The general effect of the metabolism of the animal is the destruction by oxidation of organic matter ; that is, matter originally taken from the environment in the form of complex proteids, fats, and carbohydrates is returned to it in the form of simpler and more highly oxidized substances, of which the most important are carbon dioxide and water (both inorganic substances). This action furthermore is accompanied by a dissipation of energy that is, a conversion of potential into kinetic energy.
On the whole, therefore, the action of the animal upon the environment is that of an oxidizing agent, a reducer of complex compounds to simpler ones, and a dissipator of energy.
The actions of the environment upon the plant have been sufficiently dwelt upon. It still remains, how-ever, to consider the actions of the plant upon the environment. These are partly physical, but mainly chemical. By pushing its stems and leaves into the air and slowly thrusting its roots through the soil, the atmosphere and the earth are alike displaced. But it is by its chemical activity that it most profoundly affects its environment. Absorbing from the latter water, salts, carbon dioxide, and other simple substances, as well as sunlight, it produces with them a remarkable metamorphosis. It manufactures from them as raw materials organic matter in the shape of starch, fats, and even proteids. These it gives back to the environment in some measure during life, and surrenders wholly after sudden death. But the most striking fact is that the plant is on the whole constructive and capable of producing and accumulating compounds rich in energy and in this respect it is unlike the animal.
Thus, while animals are destroyers of energized compounds, green plants are producers of them. Animals, therefore, in the long run, are absolutely dependent on plants ; and animals and colorless plants alike upon green plants. But it must never be forgotten that most plants are enabled to manufacture organic from inorganic matter by virtue of the chlorophyll which they contain. Without this they are powerless in this respect.
It is evident that to the superficial observer the plant and animal seem to have little or nothing in common, except that both are what we call alive. But whoever has studied the preceding pages must have perceived beneath manifold differences of detail a fundamental likeness between the plant and animal, not only in the substantial identity of the living matter in the two but also in the construction of their bodies and in the processes by which they come into existence. "Each arises from a single cell," to quote Sedgwick and Wilson, "which is the result of the union of two differently constituted cells, male and female. In both the primary cell multiplies and forms a mass of cells, at first nearly similar but afterward differentiated in various directions to enable them to perform different functions i.e., to effect a physiological division of labor. In both, the tissues thus provided are associated more or less closely into distinct organs and systems, among which the various operations of the body are distributed. And in both the ultimate goal of individual existence is the production of germ-cells which form the starting-point of new and similar cycles.
"This fundamental likeness extends also to most of the actions (physiology) of the two organisms. Both possess the power of adapting themselves to the environments in which they live. Both take in various forms of matter and energy from the environment, build them up into their own living substance, and finally break down this substance more or less completely into simpler compounds by processes of internal combustion, setting free by this action the energy which maintains their vital activity.
And, sooner or later, both give back to the environment the matter and energy which they have taken from it. In other words, both effect an exchange of matter and of energy with the environment."
Nevertheless the plant and the animal differ. They differ widely in form, and the plant is fixed and relatively rigid, while the animal is flexible and mobile. The body of the plant is relatively solid; that of the animal contains numerous cavities. The plant absorbs matter directly through the external surface; the animal partly through the external and partly through an internal (alimentary) surface. The plant is able to absorb simple chemical compounds from the air and earth, and kinetic energy from sunlight; the animal absorbs, for the most part, complex chemical compounds and makes no nutritive use of the sun's kinetic energy. By the aid of this energy the plant manufactures starch from simple compounds, carbon dioxide and water; the animal lacks this power. The plant can build up proteids from the nitrogenous and other compounds of its food. And by manufacturing proteids within its living substance, the plant is relieved of the necessity of carrying on a process of digestion in order to render them diffusible for entrance into the body.
Still, great as these differences appear to be at first sight, all of them, with a single exception, fade away upon closer examination. This exception is the power of making foods. Plants and animals differ in form because their mode of life differs ; but a wider study of biology reveals the existence of innumerable animals (corals, sponges, hydroids, etc.) which have a close superficial resemblance to plants, and of many plants which resemble animals, not only in form, but also in possessing the power of active locomotion. The stomach of the animal, as shown by its development, is really a part of the general outer surface which is folded into the body ; and the animal, like the plant, therefore, really absorbs its income over its whole surface oxygen through the general outer surface, other food matters through the infolded alimentary surface.
In like manner it is easy to show that not one of the differences between the plant and animal is fundamentally important save the power of making foods. The animal must have complex ready made food, including proteid matter. So must the plant; but the plant is able to manufacture this complex food out of very simple compounds. In terms of energy, the animal requires ready-made food rich in potential energy ; the plant, aided by the sun's energy, can manufacture food from matters devoid of energy. Hence it appears, broadly speaking, that the plant by the aid of solar energy is constructive, and stores up energy; the animal is destructive, and dissipates energy. And this difference becomes of immense importance in view of The fact that it is true in this respect of all green plants, as of all animals.
Even this difference, great as it is, is partly bridged over by colorless plants like yeast, molds, bacteria, etc., which have no chlorophyll, are therefore unable to use the energy of light, and hence must have energized food. But these organisms do not, like animals, require proteid food, being able to extract all needful energy from .the simpler fats, carbohydrates, and even from certain salts. When it is considered that the distinctive peculiarities of animals can thus be reduced to the sole characteristic of dependence on proteid food, it cannot be doubted that the difference between plants and animals is of immeasurably less importance than their fundamental likeness, the more so when it is kept in mind that each of the principles of organic function will be found to apply to all animals including man himself and to all plants, however complex they may be.