Atkins, W. R. G.—"Some Recent Researches in Plant Physiology," 328 pages, 28 figs., London, 1916.Czapek, F.—"Chemical Phenomena of Life," 152 pages, New York, 1911.Czapek, F.—"Ueber eine Methode zur direkten Bestimmung derOberflächenspannungder Plasmahaut von Pflanzen," 86 pages, 3 figs., Jena, 1912.Höber, M. R.—"Physikalische Chemie der Zelle und der Gewebe," 671 pages, 55 figs., Leipzig, 1911.Livingston, B. E.—"The Role of Diffusion and Osmotic Pressure in Plants," 149 pages, Chicago, 1903.McClendon, J. F.—"Physical Chemistry of Vital Phenomena," 248 pages, Princeton University Press, 1917.MacDougal, D. T.—"Hydration and Growth," Publication No. 297, Carnegie Institution of Washington, 176 pages, 52 figs., Washington, D. C., 1920.Speigel,L.,trans. byLuedeking, C.andBoylston, A. C.—"Chemical Constitution and Physiological Action," 155 pages, New York, 1915.Thompson, D. A. W.—"On Growth and Form," 793 pages, 408 figs., Cambridge, 1917.Willows, R. S.andHatschek, E.—"Surface Tension and Surface Energy and their Influence on Chemical Phenomena," 116 pages, 21 figs., New York, 1919, (2d ed.).
Atkins, W. R. G.—"Some Recent Researches in Plant Physiology," 328 pages, 28 figs., London, 1916.
Czapek, F.—"Chemical Phenomena of Life," 152 pages, New York, 1911.
Czapek, F.—"Ueber eine Methode zur direkten Bestimmung derOberflächenspannungder Plasmahaut von Pflanzen," 86 pages, 3 figs., Jena, 1912.
Höber, M. R.—"Physikalische Chemie der Zelle und der Gewebe," 671 pages, 55 figs., Leipzig, 1911.
Livingston, B. E.—"The Role of Diffusion and Osmotic Pressure in Plants," 149 pages, Chicago, 1903.
McClendon, J. F.—"Physical Chemistry of Vital Phenomena," 248 pages, Princeton University Press, 1917.
MacDougal, D. T.—"Hydration and Growth," Publication No. 297, Carnegie Institution of Washington, 176 pages, 52 figs., Washington, D. C., 1920.
Speigel,L.,trans. byLuedeking, C.andBoylston, A. C.—"Chemical Constitution and Physiological Action," 155 pages, New York, 1915.
Thompson, D. A. W.—"On Growth and Form," 793 pages, 408 figs., Cambridge, 1917.
Willows, R. S.andHatschek, E.—"Surface Tension and Surface Energy and their Influence on Chemical Phenomena," 116 pages, 21 figs., New York, 1919, (2d ed.).
Reference has frequently been made, in preceding chapters, to the effect of various stimulating or inhibiting agencies upon the physiological activities of plant protoplasm. In the main, these agencies areexternalto the plant and are either physical, such as changes of temperature, amount of light received, etc.; or chemical, such as variations in the salts received from the soil, or common anæsthetics applied to the plants by man. A plant grows normally under certain conditions to which it has become adjusted by hereditary acquirements. When these conditions are altered, the effect upon the functioning of the plant protoplasm may be either stimulating or depressing. Extreme changes in environmental conditions generally result in the death of the plant; but changes which do not result in the lethal condition affect the plant by either stimulating it to more rapid physiological activity or by depressing its normal growth or functions. As has been pointed out, the same external influence, either chemical or physical, which acts as a stimulant if it differs only slightly from normal conditions, may become depressing, or positively toxic, if present to a larger extent.
There is also the possibility of the elaboration by the plant itself ofinternalagents, or substances, which may have a definite stimulating or inhibitory effect upon its metabolism and growth. The study which has been given to these matters has practically all been carried on within very recent years and is still in progress. Most of it is still in the experimental stage, in which no definite conclusions are as yet possible. Hence, the most that can be done at present is to give a brief review of the suggestions which have been made thus far, as indicative of the uncertainty of our present knowledge of these matters and of the general trend of the investigations which are now in progress.
Substances which are elaborated by plants and which are supposed to have a definite stimulating or beneficial effect uponthe activities of the plant which produces them, or to influence the physiological activities of other plants with which these substances come in contact through either the parasitic or the symbiotic relation, have been variously discussed under the names "hormones," "auximones," and "vitamines"; while injurious substances are generally known as "toxins." Whether these different terms actually represent different definite types of substances, or whether there are actually different groups of stimulating or inhibitory agents produced in plants, is uncertain; but the following brief statements will serve to indicate the general nature of the suggestions which have been put forward and of the experimental work which is now in progress.
The term "hormone" was first used to designate certain stimulating substances which are supposed to exist in the intestinal tracts of animals and to cause the glands to elaborate and secrete their characteristic enzymes. The supposed "hormones" are not themselves active in performing the digestive functions of the glandular secretions, but are the exciting, or stimulating, agents which cause the glands to secrete their active enzymes.
The same term has been used, by certain plant physiologists, to designate any agency, either external or internal, which stimulates plant protoplasm to abnormal activity. It has been pointed out that there are a variety of substances, which are themselves chemically neutral, that are powerful stimulants of vital activity if used in only minute proportions, but are powerful poisons if present in larger amounts. Many of the alkaloids act in this way upon the animal organism; while chloroform, toluene, and even some of the more complex hydrocarbons, act similarly upon the tissues of plants, and ether vapor is known to be a powerful stimulant in accelerating the flowering of plants and the ripening of fruits. It has been shown that the vapors of all such substances readily penetrate the protoplasm of leaves, seeds, etc., even when the same parts are impermeable to most mineral salts, sugars, etc.; and that upon entrance to the protoplasm of a leaf, or a seed, they tremendously stimulate its metabolic activity.These hormones, as a class, are chemical substances which have very little attraction for, or power of combinationwithwater; and it has been suggested that the ease with which they penetrate the protoplasm is due to the fact that they are not held at the surface by combination with the active water molecules which are present in the surface layer.
The principal effect which is supposed to be produced by these "hormones" is the stimulation of the enzymic activity, particularly that of the degenerative processes which take place late in the plant's life, at the flowering or ripening periods. Many of the changes which take place normally at ripening time, such as the change in color from green to yellow or red and finally to brown or black, when the fruit or vegetable is fully ripe, can be greatly accelerated by treatment with these substances. Hormones are similar in type to the ethereal salts, or esters, which constitute the natural essential oils that develop in many plants at this stage of their growth. Hence, it seems probable that these changes in plants which are maturing naturally may be hastened by the hormone action of the esters and similar bodies which are developed in largest quantities at that stage. It has been pointed out that the characteristic group which is present in many natural glucosides is of the same general type as the "hormone" substances which are used in the artificial stimulation of the flowering or ripening changes. This fact, together with the possibility of the liberation of greater percentages of these aromatic compounds from their glucoside combinations at the later periods of plant growth, is assumed, by some plant physiologists, to account for the change from synthetic to degenerative processes at this stage of the plant's development.
Further, it has been suggested that the autumnal coloration of leaves, and their dropping from the stems of the plant, as well as the ripening of seeds, is probably determined by the liberation in the plant, at that stage of its growth, or as a result of changed climatic conditions at that particular season of the year, of the hormones which either initiate or hasten the special enzymic changes which distinguish the degenerative from the synthetic processes of the plant.
Similarly, it has been suggested that parasitic fungi are able to penetrate the host plant by first excreting "hormones" which bring about degenerative changes in the tissues of the host plantand so make it more easily penetrable by the hyphae of the parasite.
It will be seen that, in general, "hormones" are a type of substances (possibly often present in plants in the form of glucosides) which are supposed to stimulate the degenerative (or katabolic) vital processes in contrast to the synthetic (or anabolic) changes. It has been suggested that they do this in either one of two ways; namely, by favoring the introduction of water into the protoplasm and so diluting the cell contents, changing the osmotic pressure, etc.; or by bringing about a separation of the colloidal layers, or films, of the protoplasmic complex, producing a result similar to that produced by freezing the tissues. These ideas have been suggested by studies of the changes in the equilibrium of protoplasm when foreign substances are introduced into it. These studies have not as yet been brought to the stage of final conclusions, and the ideas presented must be considered as suggestive rather than as conclusive.
"Vitamines," as contrasted with "hormones," are supposed stimulants of synthetic metabolic processes, or accelerators of growth, rather than of degenerative processes.
The term "vitamine" was first used to designate the substance, or substances, which must be present in the diet of animals in order that the animal organism may grow. Absence of these substances from the food of the animal results in the stoppage of growth of young animals and in various so-called "deficiency diseases" (such as beri-beri, scurvy, polyneuritis, etc.) of adults. This means that the animal organism is altogether unable to elaborate its own vitamines, and extended investigations have indicated that the vitamines necessary for animal uses are wholly of plant origin. The name "vitamine" was first used because it was supposed that these substances are chemical compounds of the amine type and, since they are necessary to normal life processes of animals, the name "vitamine" seemed to represent both their chemical character and their functions. Later investigations have caused doubt as to the accuracy of the first belief as to their chemical nature, and various other names have been suggested for the general group of substances which have the observed beneficial effects; while such specific names as "fat-soluble A," "water-soluble B," etc., have been used to designate individual types of these accessory food substances. However, the termvitamineis such a convenient one and is so generally recognized and accepted that it will probably continue to be used, at least until some more definite knowledge of the nature and composition of these growth-promoting, disease-preventing, and reproduction-stimulating food constituents is obtained.
The following definition of the term "vitamines" gives a satisfactory conception of the nature and functions of these substances, so far as they are yet known. "Vitamines; constitute a class of substances the individuals of which are necessary to the normal metabolism of certain living organisms, but which do not contribute to the mineral, nitrogen, or energy factors of the nutrition of those organisms." As sub-groups of the vitamines, there have already been recognized the growth-promoting, fat-soluble A; the antineuritic B, and the antiscorbutic C.
Until very recently, the investigations of vitamines have dealt exclusively with their relation to human nutrition; although it has been generally believed that the vitamines themselves are elaborated only by plants. It was generally recognized, however, that those plants, or parts of plants, which are capable of very rapid growth or metabolic changes, such as germs, spores, leaves, etc., are generally the richest source for vitamines for animal needs. Hence, there seemed to be considerable basis for the assumption that the elaboration of these substances by plants is definitely connected with their own metabolic needs. Recently, investigations of the functions of vitamines in the growth of plants have been begun. These are still in progress, but the following conclusions seem to be justified at the present time: (a) Potato tubers appear to contain growth-promoting substances which are essential to the proper growth of the sprouts. Whether these are the same substances which are efficient in the prevention of scurvy in men has not yet been investigated. (b) Baker's yeast is probably dependent upon a supply of vitamines in the medium in which it is to grow. Yeast itself, after having grown in barley wort, is one of the most important sources of vitamines for animal uses or for purposes of investigations of vitamine activity. But it has been reported that a yeast cell will not grow in an artificial medium which contains all the essential nutrients for yeast buthas no vitamines of other plant origin in it. The addition of barley wort, containing the vitamines from barley germs, or any other similar supply of vitamines, induces rapid growth and the storage of vitamines in the growing yeast masses. (c) The growth of many bacteria is either wholly dependent upon or greatly stimulated by the presence of vitamine-like substances in the medium upon which themicroorganismsgrow. (d)Sclerotinia cinerea, the brown rot fungus of peaches and plums, will grow only in a medium which contains, in addition to the essential sugar, salts, and nitrogenous material, vitamines derived from either the natural host plant tissues or other plant sources. These may be of two types (namely, a vegetative factor and a reproductive factor) or two different manifestations of activity of the same vitamine substance. But both of these factors must be provided before the fungus can make its characteristic growth.
There is, as yet, no conclusive evidence on many of the matters concerning the relation of vitamines to plant growth. But it seems that these substances are of almost universal occurrence in the organic world; that they are not of the same general type as other substances which are essential to the nutrition of plants or animals, but have specific stimulating or regulating effects upon the physiological activities of the organism; that the vitamines which are essential to animal life are elaborated by plant tissues, but that in the case of the bacilli of certain human diseases there seems to be some indication that the affected tissues of the animal host produce vitamines which are essential, or favorable, to the growth of the parasitic organism. There seems, therefore, to be evidence of a mutual relation between plants and animals with respect to their nutritional needs for the so-called "vitamines." But the evidence concerning the function of these substances in the tissues of the organism which elaborates them is, as yet, inadequate to provide any clear conception of the reason for their development or of the mechanism by which they are elaborated. Neither is there, as yet, any conclusive evidence concerning the chemical nature of the substances themselves.
Certain investigations have indicated that bacteria, at least, develop exogenous vitamines which are beneficial to the growth ofother plants. These are the so-called "auximones." For example, bacterized peat seems to contain auximones which may be isolated from the peat and exert a beneficial effect upon the growth of various seed-plants, including common farm crops. Neither the original experimental data, nor the theories which have been advanced to account for the observed beneficial effects of the supposed "auximones" have, as yet, sufficient confirmatory evidence definitely to establish their soundness. But it seems that there is a probability that some plants, at least, do elaborate vitamines, or auximones, which are useful to other plants.
Toxins are substances which affect injuriously the normal activities of the organism. As has been pointed out, they may be the same substances which, in lesser concentrations, exert a stimulating effect upon the same organism. Hence, it is probably inaccurate to discuss the toxins as a distinct group of substances.
There are, however, a large number of water-soluble chemical substances which are injurious to all living protoplasm, even at concentrations considerably less than the point of osmotic equilibrium in the juices of the protoplasm. These substances may act either directly or indirectly upon the protoplasm, but at certain concentrations they always affect it injuriously. In the main, these toxins areexternalagents of other than plant origin; although chemical substances developed by one plant may be toxic to other plants, or even to other organs of the same plant than those in which they are elaborated.
Toxins may be eithergeneral(i.e., injurious to all types of plants), orspecific(i.e., injurious to only certain species) in their action. Examples of specific toxicity are of only minor importance in plant studies. They seem to be generally explainable on the basis of some unusual lack of resistance or failure of the susceptible plants to be able to exclude the entrance of these injurious substances into the protoplasm by "selective adsorption," or to convert the injurious substances into insoluble and non-injurious forms, as is done by other plants which are not susceptible to injury by these "specific" poisons. Hence, particular attention need not be given to this type of toxins.
Toxic substances may act injuriously upon plant tissues in a variety of ways. Many electrolytes, especially the salts of the heavy metals of high valency, coagulate protein material and the entrance of such substances into the protoplasm causes disturbances in the colloidal condition which cannot be otherwise than injurious to its normal activities. Similarly, formaldehyde and many other organic compounds may affect the colloidal properties of the protoplasmic gel in such a way as to injure the plant tissues.
The same substance is sometimes much more injurious to the tissues of one part of a plant than it is to those of another part of the same plant. Thus, the rootlets of a young growing plant are much more susceptible to injury by many mineral salts than are the vegetative parts of the same plants; while anæsthetics of various kinds generally exhibit their greatest injurious effects upon the leaves, or synthetizing cells. Again, the mycelia of fungi are much more easily killed by toxic agents used as fungicides than are the spores of the same fungi. Some of these observed differences in toxicity may be due to differences in the physiological effect of the substance upon the protoplasm of the tissues which it enters, and others may be due to differences in the resistance of the protoplasm, or of its protective coverings, to penetration by the toxic material. Indeed, the possibilities of different types of toxic action, and of resistance to it by individual plants and species, are so varied that it is not possible to divide toxic agents into specific groups according to the nature of their injurious action upon the plant cell. They are, therefore, more commonly grouped into classes according to their chemical nature and economic significance as fungicides, as follows: inorganic and organic acids; caustic alkalies; salts of the heavy metals;hydrocarbongases; formaldehyde; alcohols and anæsthetics; nitrogenous organic compounds; and miscellaneous decomposition productions of organic origin. The following brief review of some of the results of the experimental studies of the toxicity of different compounds belonging to these several groups will serve to indicate the general trend of the investigations of these matters which have thus far been made.
Acids.—The common inorganic acids (hydrochloric, nitric, and sulfuric) kill the rootlets of common farm crops when the latter are immersed for twenty to twenty-four hours in solutions of these acids containing from three to five parts per million of free acid.Acetic acid must be about five times as concentrated as this, and other organic acids may be much more concentrated still before they produce the same injurious effects. The toxic effect of all these acids is greatly reduced in soil cultures, or if particles of sand, graphite, clay, filter paper, etc., are suspended in the solutions containing the acids, the reduction in toxic effect being probably due to the adsorption of the acids upon the solid particles. Hence, the concentrations which limit the toxic effects of these acids in water solutions cannot be taken as representing the condition with which the same plant will have to contend when growing under normal cultural conditions.
Alkalies.—The caustic alkalies must usually be present in from five to ten times as great concentrations as those of the mineral acids, in order to produce the same injurious effects upon the rootlets of common plants. The so-called "alkali" of soils is not alkali at all, but is neutral soluble salts present in sufficient concentration to exert a toxic effect.
Salts of the heavy metalsare especially toxic to rootlets of plants. Salts of copper, mercury, and silver, have been found to kill the roots of seedlings immersed in them for twenty-four hours when present in proportions of less than three parts per ten million, while salts of many other heavy metals are toxic when present in concentrations of less than one part per million. The salts of the alkali metals are considerable less injurious than are those of the heavy metals, but even these exert their familiar injurious effect if present in concentrations which, measured by the ordinary standards, would still be regarded as very dilute solutions.
Illuminating gas, and similar hydrocarbon gases, kill plants when present in the atmosphere in as little as one part per million. Leaves, buds, and roots are all alike sensitive to this toxic effect, the nature of which is not yet understood.
Formalin, or formaldehyde, is a penetrating toxic agent for nearly all plant cells, and is commonly used as a fungicide for the destruction of parasitic fungi. It probably affects the colloidal condition in some way similar to its hardening effect upon gelatin, etc.
The toxic effect of many differentorganic compoundsis so varied in its nature and extent that it is impossible to give any satisfactory brief review of its manifestations. Recent investigations appear to indicate that organic products of decompositionof plant residues in the soil may exert powerfully toxic effects upon succeeding generations of the same, or of different, plants growing on the land. But the experimental data and conclusions concerning these matters are not yet accepted without question by all students of plant science or of the problems of the productivity of the soil. In fact, it is yet an open question whether toxic soil constituents are really an important factor in the so-called "unproductivity" of certain soils.
Alkaloids, and even the amino-acids which are produced in the tissues of some species of plants, while not toxic to the plants or organs which elaborate them, sometimes exhibit strikingly toxic action upon other plant organs with which they are brought into contact. There is, as yet, no satisfactory explanation of this difference in behavior between plant tissues toward various organic toxic substances.
In fact, the whole subject of the toxic action of various substances upon plants needs much more study before it is brought to the point where it will afford definite knowledge of either the physiological problems involved or of their practical applications in questions of soil productivity, etc.
Most of the discussions which have been presented in the preceding chapters have dealt with the types of compounds, the kinds of reactions, and the mechanism for the control of these, which are exhibited by plants under their normal conditions for development. The results of the evolutionary process have produced in the different species of plants certain fixed habits of growth and metabolism. So definitely fixed are these that in each particular species of plants each individual differs from other individuals, which are of the same age and have had the same nutritional advantages and environmental opportunities for growth, by scarcely perceptible variations, if at all. Indeed, this fixed habit of development makes possible the classification of plants into genera, species, etc. Whiledifferent speciesof plants, given the same conditions of nutrition and environment, produce organs of the widest conceivable variety in form, color, and function; within thesame species, the form and size of leaves, the position and branching of the stem, the color, size, and shape of the flower, the coloration and markings of the fruit, etc., are relatively constant and subject to only very slight modifications.
It is unnecessary to say that the mechanism, or the impulses, which govern the morphological characters of the tissues which any given species of plants will elaborate out of the crude food material which it receives from the soil and atmosphere, are wholly unknown to science. It is the commonly accepted assumption that the fixed habit of growth of the species is transmitted from generation to generation through the chromosomes of the germ cells. But the nature of the elements, or substances, which may be present in the chromosomes, which influence the character of the organs which will develop months later, after the plant which grows from the germ cell has gone through its various stages of vegetative growth, is still altogether unknown. There can be no question,however, that some influence produces a fixity of habit of growth and development which is almost inevitable in its operation.
But while this unvarying habit of growth is one of the fixed laws of plant life, there are occasional deviations from it. A plant which, under normal conditions of growth, develops in a certain fixed way, when exposed to unusual environmental conditions, may, and often does, alter its habit of growth in what may metaphorically be said to be an attempt to adjust itself to the new conditions. Numerous examples of this phenomenon might be cited. Certainalgæ, which grow normally in water at a temperature of 20° to 30° and which are killed if the temperature rises above 45°, have been grown for successive generations in water the temperature of which has been gradually raised, until they produce apparently normal growth in water the temperature of which is as high as 78°; also, certain types ofalgænormally grow in the water of hot springs at temperatures of 85° to 90°, and others in arctic sea-water the temperature of which sometimes falls to -1.8° and never rises above 0° C. This phenomenon of the adjustment of a species of plants to new conditions, which in the case of farm crops is sometimes called "acclimatization," is of common occurrence and is often utilized to economic advantage in the introduction of new strains of crops into new agricultural districts. Again, the normal development of plants may be altered as the result of injury or mutilation. Thus, if the ear is removed from the stalk of Indian corn, at any time after flowering, there always results an abnormal storage of sucrose in the stalk, instead of the normal storage of starch in the kernels. Similarly, midsummer pruning of fruit trees generally results in the production of abnormally large number of fruit buds on the remaining limbs. Many other familiar examples of alteration of normal development in response to, or as the result of, abnormal conditions of growth might be cited.
To designate these different alterations of normal growth, several different terms have been used. Among these, "adaptation," "accommodation," and "adjustment" have been commonly used by different biologists. Sometimes these are used interchangeably, and sometimes different terms are used to designate different types of response to altered conditions of growth. Inasmuch as there seems to be no generally accepted usage of these different terms, only one of them, namely, the word "adaptation" will be used here; and different manifestations of this phenomenon will be distinguished by using appropriate adjectives, as "physiological adaptations," "chromatic adaptations," "morphological adaptations," etc.
Two markedly different types of responses to altered conditions, or of adjustment to environment, may be recognized. In the first of these, for which we will use the term "physiological adaptation," the species of plant simply acquires the ability to exist and grow normally under conditions which formerly inhibited its growth. Thus, we may speak of the phenomena mentioned above as "acclimatization" as thephysiological adaptationof the crop to the new conditions of growth. In general, physiological adaptations include such variations in the characters or habits of growth of plants as results in differences in resistance to heat or to cold, relations to water, aggressiveness in competition with other plants, etc. In such cases, no modification of the morphological characters of the plant can be observed, the changes which take place in the structure of the plant (if, indeed, there be any such changes) must be only minor adjustments of the protoplasm to meet the new environmental needs.
In the second type of adaptations, for which we will use the term "morphological adaptations," the structure, or color, or some other morphological character of the plant is actually changed in some easily recognizable way, in order that the plant may be better adjusted to its environment. As examples ofmorphological adaptations, there may be cited the change in color of sea-weeds with increasing depth in the sea, and other examples of chromatic adaptation which are discussed below; the development of fewer, or a larger number, of buds on the above-ground stems of plants, in response to decreases, or increases, in the available supply of food; the alteration in the size and shape of the leaves of many plants when they are grown in shade; the dwarfing of plants at high altitudes, or under conditions of severe drought; the development of underground storage organs for certain species of shrubs and trees which grow in regions that are subject to periodical burning-over, in such a way as to destroy the above-ground storage stems, etc.
Hence, the two terms, as we will use them here, may be defined as follows:morphological adaptationis a change in the structural character of the species in order that it may be better fitted to meet the needs of the new conditions of growth; whilephysiological adaptationis an acquired power to survive and develop under abnormal conditions, which is not accompanied by any visible change in the characteristic structure of the species.
Both of these types of adjustment may be either hereditary (or evolutionary), or spontaneous in their origin and development. Changes which are evolutionary are fixed by heredity and become definite habits of growth in the species. Their origin may be explained in either one of two ways; namely, the so-called "increase by use," and "the survival of the fittest." The hypothesis of "increase by use," as an explanation of adaptations, is based upon the well-known observation that, in animals, muscles and other organs increase in volume as they are extensively used; and the assumption of the application of this principle to the phenomenon of adaptation supposes that the modification of any given structure or composition is the result of the hereditary accumulations of increased size resulting from use, or of atrophy from disuse. The "survival of the fittest" theory supposes that individuals of a species differ from each other by spontaneous variations, and that in the competitive struggle for existence those forms which are best adapted to the environmental conditions survive while the others perish. The contrast between these two views is that the first holds that adaptation proceeds by development, and the second that it proceeds by variation and elimination; the first presupposes the existence in the organism of a mechanism for response to changing conditions, and the second assumes that there are chance variations followed by the death through competition of the forms which are not able to meet the needs of the environment.
Confusion arises whenever an attempt is made to apply either of these theories to all kinds of adaptations. The idea of increase by use can be applied with some satisfaction to certain morphological adaptations in animal structure; and to such phenomena as the increase in strength of the branches of fruit trees, either with or without corresponding increase in size, as the load of fruit increases. But it certainly cannot apply to color change in surface pigmentation of either animals or plants, which is one of the most commonforms of adaptation. Furthermore, it is difficult to conceive the general application of this idea to alterations of habits of growth of plants, since a plant cannot have any such thing as a voluntary control over the amount of "use" which it makes of its different organs in response to changes of environment. The common form of statement that a plant develops an organ, or a process to meet a certain need, or modifies its habits of growth to meet a change of environment are, of course, purely metaphorical, and can only be taken to mean that such processes are mechanical responses to changes in external conditions.
The nature of the mechanism by which these responses are accomplished is, as yet, wholly unknown. There is accumulating a large mass of experimental evidence which goes to show that, while both temperature and light are very important factors in determining the type of changes which will take place in a living organism, the so-called "photochemical action of light" is by far the most potent of all the climatic factors which influence the course of development of a plant. But we have, as yet, no inkling of how the protoplasm of the plant adjusts or controls its responses to variations in any of these external factors.
With these general considerations in mind, we may now proceed to the consideration of certain particular types of adaptations.
Adaptations have been observed in both the energy-absorbing pigments of the general tissues and in the ornamental epidermis pigments of plants. The former are by far the most important from the physiological point of view; while the latter may have interesting biological significance.
Under nearly all conditions of growth of land plants, the supply of the chlorophylls and their associated pigments provides for the absorption of solar energy far in excess of the amount necessary for the photosynthetic assimilation of all the carbon dioxide which is available to the plant. It has been shown that an active green leaf, on an August day, can absorb eight times as much radiant energy as would be required to assimilate all the carbon dioxide present in the air over its surface. No land plant, under normal conditions, develops supplementary pigments inorder to utilize other than the parts of the spectrum which are absorbed by chlorophyll and its associated pigments.
But deep-sea plants show quite a different phenomenon of pigment development. Water is a blue liquid. At depths of 40 feet or more, the light which penetrates is devoid of red rays, feeble in yellow, and is characteristically green or blue in color. Now, the red rays of the spectrum are the ones which are most efficient for photosynthesis. Sea weeds which grow at these depths are brilliantly red in color, at intermediate depths they are brown, and at the surface they are green, in the same latitudes. While it is possible that the temperature of the water at these different depths may have something to do with the chemical synthesis of the pigments, it appears plain that this color change at increasing depths is a definite adaptation to provide for the absorption of the solar energy which is available at these depths. It has been shown that these pigments of deep-sea plants are additional to, and not substitutes for, the chlorophylls, etc. The latter pigments are present in normal amounts, but are supplemented by those which absorb the green and blue portion of the spectrum. Hence, this type of adaptation might be conceived to be a "survival of the fittest," resulting in the "natural selection" of individuals of the highest total pigmentation. But, on the other hand, there is experimental evidence to show that plants possess some means of varying their pigmentation in response to the character of the light which comes to them. For, it has been found that a complete change in color of certain highly colored plants can be produced in a single generation, by growing the plants in boxes or chambers whose walls are composed entirely of differently colored glass, so that the plants within receive light of only a particular part of the spectrum. In such cases, the plant, starting with an initial "natural" color, changes through a succession of colors until it finally reaches equilibrium at one which provides for the proper absorption of the right kind of light from the new supply which is available to it. Hence, it seems proper to conclude that chromatic adaptation is not a process of "natural selection," but a definite result of an actual mechanism for adaptation to changed environmental conditions of supply of radiant energy.
Changes in structure to meet special conditions of growth may be of several different types.
One of these, which is often cited as an example of adaptation (in this case, the term is used with a significance quite different than that in which it is being used here) is that of the development of unusual and often fantastic shapes of flowers, which are so related to the anatomy of certain species of insects that visit these flowers in search of nectar, that provision for the cross-fertilization of the plants is insured, in that the pollen from the anthers of one flower becomes lodged on the body of the insect as it is withdrawing from the flower in such a way that it comes in contact with the pistil of a second flower as the insect enters it. Such flowers often have such peculiar shapes and lengths of nectar tubes, etc., that only a single species of insect, whose anatomical shape is "adapted" to that particular blossom shape can enter the flower in its search for nectar. It is clear that this form of "morphological adaptation" is a highly specialized one, which can only be the result of a long process of evolutionary development. It is obvious that the plant cannot possibly possess a mechanism, or ability, to alter its flower form in order to make it conform to the shape and length of the proboscis, or other body parts, of a particular species of insect. Either the insect or the plant, or both, must go through a process of evolutionary development in order to arrive at this form of mutual "adaptation."
A form of true morphological adaptation (in the sense in which we have been using the term) is exhibited by many species of plants, which are provided with many more buds, or growing points, than ever actually begin to grow. For example, the single plumule which develops from a germinating wheat embryo has at its upper end a hundred or more tiny growing points. At the proper stage of its growth, several of these tiny buds begin to grow into individual separate stems, and the new wheat plant thus produces several stems from one seed and root system, a process known as the "stooling." The number of stems in a single "stool" depends upon the number of the potential growing points which are stimulated into growth. It varies from only two or three up to as many as thirty or forty, and is apparently controlled by the favorable or unfavorable conditions of climate ornutrition at the time when the "stooling" takes place. The plant is thus provided with a mechanism for adapting its possibilities of growth to the supply of growth-promoting material which is available to it.
Many other plants produce far more buds than ever develop into growing tissues, and buds which, under normal conditions, remain dormant, under altered conditions start into growth and so provide for an "adaptation" of the total mass of the growing plant to correspond with the altered conditions of growth. The actual means by which certain buds are stimulated into growth while others remain dormant, or are inhibited from growing, are as yet unknown. Two theories have been advanced. One is that the growing buds absorb all available nutrition and the others remain dormant by reason of lack of growth-promoting material. The other is that the vegetating (growing) tissue elaborates and sends to other parts of the organism one or more substances, which actually inhibit growth of the other parts, as dormant buds, etc. The experimental evidence which has been presented thus far is inconclusive, but seems to favor the distribution of nutritional material as the governing factor, although there is some evidence which seems to indicate that a supposed growth-inhibiting substance is actually translocated from rapidly-vegetating tissues to other parts of the plant. There is, however, no explanation of how the buds, or other tissues, which do grow get their initial stimulus, while the dormant buds do not. After growth has once started, the changes in osmotic pressure due to the accumulation and translocation of synthetized materials can account for the movement of new nutritional material for the synthetic processes into the growing organ; but this would not account for the selective stimulation of only a part of the buds, or possible growing points, of a plant, or for an adaptational development of others under altered conditions of growth.
The form of morphological adaptation which has been discovered in the course of the study of the native vegetation of the campos of Brazil (which have a very dry season and have been regularly burned over by the natives for many generations) in which the papilionaceous shrubs have developed underground trunks, or stems, and seem actually to profit in luxuriance of growth when the rainy season comes on by reason of this morphological adaptation to the unusual environmental conditions,is wholly inexplicable by any present knowledge of the science of plant growth.
The type of adjustment to environmental conditions which does not result in any recognizable alteration in the structure of the plant, but simply permits it to grow under new conditions, manifests itself in many ways. These adjustments are usually associated with differences in temperature during the growing season, and for this reason, most such examples of adaptation have been studied in connection with possible temperature reactions upon the growing organism.
However, recent investigations seem to point strongly to the conclusion that the amount oflightrather than thetemperatureof the new surroundings is the most important influence in determining the physiological processes known as the "acclimatization" of plants. For example, a very elaborate series of investigations has shown that the flowering stage in the development of plants is determined by the length of the daylight period per day, irrespective of the actual amount of vegetative growth which the plant has made. Thus, tobacco plants, which during a period of long days grow to the height of 8 or 10 feet before blossoming, if grown at the same temperature in periods of short days (or if kept in the dark during a portion of the longer days) will blossom when less than 3 feet in height and when the total mass of vegetative material which has been produced is less than one-third of that of the "gigantic" plants of the same variety grown with longer periods of illumination per day. This same principle has been found to hold good for many widely different types of plants. In some species, however, flowering is favored by long days, and vegetative growth by short daylight illumination. But in all species which have been studied, there seems to be a direct relation between the length of day, or the total illumination per day, and the normal or abnormal functioning of the plant. It is apparent that at least the physiological function of sexual reproduction (flowering and seed-production) is determined by the length of daylight illumination. The duration of daylight per day which is necessary to induce the blossoming of the plants varies for different species, but it is constant for individuals of the samespecies. This adaptation of stage of growth to duration of daily illumination must, therefore, be an evolutionary character of the species.
Hence, it appears that in many cases physiological adaptation may be a direct response of the life-processes of the plant to the daily length of photochemical stimulation which it receives from solar light. But there is, as yet, no explanation of how this (or any other) influence actually changes the vital processes of the plant protoplasm so as to bring about either a morphological adaptation of structure or a physiological adaptation of functions to altered conditions of growth.
Enough has been said to show how very inconclusive and unsatisfactory is our knowledge of the phenomena known as "adaptation." Even the nomenclature used by different scientists to describe its various manifestations is confused and misleading. For example, certain crops are said to be "adapted" (i.e., suited) to certain types of soils, andvice versa; crops are said to be "adapted" to given agricultural districts, etc.
In this chapter, an attempt has been made to arrange in some semblance of order some of the known manifestations of alteration of fixed habits of growth of plants in response to changes of environment, and to point out some of the suggestions of possible explanations of these phenomena which have been presented by different investigators.
This presentation cannot be considered as anything other than an introduction to a field of study which is as yet almost entirely unexplored, and, like all other unexplored territory, is full of mysteries. If the study of this chapter serves to stimulate interest in these mysteries and wonders of plant life, its purpose will have been accomplished.