The Chemistry of Plant Life

by Roscoe Wilfred Thatcher

CHAPTER XVIII

ADAPTATIONS

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. While different species of plants, given the same conditions of nutrition and environment, produce organs of the widest conceivable variety in form, color, and function; within the same 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. Certain algæ, 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 of algæ normally 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.

TYPES OF ADAPTATIONS

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 the physiological adaptation of 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 of morphological 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 adaptation is 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; while physiological adaptation is 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.