ENY1R0NIC FACTORS 427 



processes. Its ionization effects are discernible in respiration in all of 

 its separate stages. None of the measurements of the action of this im- 

 portant 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 pres- 

 ent in minute quantities in all plants, yet they accumulate and are pres- 

 ent in such quantities in the succulent cacti that facile conditions for 

 experimentations are found. The accumulation goes on during dark- 

 ness 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 formalde- 

 hyde 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 fluctua- 

 tion 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 ab- 

 sorbing 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 in- 

 creasing 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 com- 

 pounds 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 devel- 

 opment of buds, which depends directly upon the hydrolysis of such 

 food material, are found to conform fairly well with van't Hoffs 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 de- 

 velopment 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 formu- 

 lated 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 



