CZAPEK, F.--"Chemical Phenomena of Life," 152 pages, New York, 1911.
CZAPEK, F.--"Ueber eine Methode zur direkten Bestimmung der Oberflächenspannung der 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 Rôle 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. 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.).
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.