Speigel, L., trans. by Luedeking, C. and Boylston, 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. and Hatschek, E.—"Surface Tension and Surface Energy and their Influence on Chemical Phenomena," 116 pages, 21 figs., New York, 1919, (2d ed.).
CHAPTER XVII
HORMONES, AUXIMONES, VITAMINES, AND TOXINS
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 are external to 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 of internal agents, 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 upon the 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.