processes. Its ionization effects are discernible in respiration in all of its separate stages. None of the measurements of the action of this important environic component have proven more interesting than those which have been carried out by Professor H. M. Richards and Dr. Spoehr upon the reduction of the acids in plants. Although formed and present in minute quantities in all plants, yet they accumulate and are present in such quantities in the succulent cacti that facile conditions for experimentations are found. The accumulation goes on during darkness so that at daybreak these plants may contain as much as ten times as much acid as at sunset, the diminution during the day being due to the action of light, the disintegration of the acid resulting in formaldehyde and carbon dioxide. Now growth has long been held to be directly retarded by light, it being supposed that the blue-violet rays especially exerted a fixing or destructive action on living matter which prevented growth. That such action did not take place was established by my own experiments on etiolation previous to 1903. The fact remains, however, that growth-expansions and elongations generally go on more rapidly at night than in daytime, and in the determination of the daily fluctuation of acidity we believe to have hit upon the cause of the difference in the rate of growth by day and by night.
Growth is correlated with hydratation, or increase of the water absorbing capacity and consequent swelling of living matter and cell walls in which osmotic pressure must also play a part. Acids may cause such swelling and increase, and this effect would accumulate with the increasing acidity through the night until daybreak, when light begins to break up the acids, and growth-extension would slacken. Light does, therefore, in finality, retard growth not by its action on the components of living matter as formerly supposed, but by breaking up the compounds which increase the water-absorbing power of protoplasm. The controlling environmental features in the growth and development of vegetation are water-absorption or hydratation and temperature.
Some isolated processes of plants, the course of which runs for a short time, such as the action of enzymes upon the starch, which may be accumulated in a tuber or a seed, the germination of seeds or the development of buds, which depends directly upon the hydrolysis of such food material, are found to conform fairly well with van't Hoff's rule by which the rate of activity is about doubled for every rise of 18° F. above the minimum temperature at which it begins. If the entire development of the plant could be interpreted in the same manner the task of estimating the effect of the temperature factor in environment would be a simple one. This is far from the case, however, as any change in temperature may disturb chemical equilibrium in a dozen ways.
The director began a study of this subject in 1900, and first formulated a method by which the total heat-exposure of a locality in which a plant was growing was calculated in hour-degrees, simply as the
