RATK UNDER CONSTANT CONDITIONS 873 



one — light intensity — affects this process only partly by direct action, and 

 partly through plasmatic stimulation. 



In the U.S.S.R., there is a tendency now to consider this point of view as the only 

 one in accord with dialectic materialism, and attempts to isolate photosynthesis from 

 other functions of the Uving organism and study it as an independent photochemical 

 reaction are criticized as "mechanistic."* Such dogmatic assertions, practically 

 banished from physics and chemistry, but still recurring in biological sciences, particu- 

 larly in the U.8.S.R., are strangely beside the point. A reaction in a living organism 

 is distinguished from that in a test tube by the complexity of the system in which it 

 takes place. This complexity is due to three causes: the impossibility of separating the 

 reacting system from other components of the organism; its inhomogeneous structure; 

 and its complex and largely unknown chemical composition. Unprejudiced experiments 

 alone can prove whether, despite these handicaps, direct relationships can be established 

 between external kinetic variables and the rate of the specific process under investigation. 

 If this proves possible, a promising approach to the understanding of the process is 

 opened, and it would be foolish to refuse to use it because of dogmatic objections. 



5. Rate of Photosynthesis under Constant Conditions. 

 Midday Depression and Adaptation Phenomena 



Obsei-vations that lend support to the "physiological" concept of photo- 

 synthesis include, among others: the difference in photosynthetic activity, 

 under identical external conditions, of plants grown in various habitats; 

 the adaptation of photosynthetic activity to changed conditions (stronger 

 or weaker light, higher or lower temperature — cf. for example, Harder 1933 

 and Brilliant 1940); the effects of aging; and the changes of photosyn- 

 thetic activity under constant external conditions (fatigue, midday depres- 

 sion etc.) . Not only do plants of different species behave differently under 

 identical external conditions, but variations are found also between "sun 

 plants" and "shade plants" of the same species, "sun leaves" and "shade 

 leaves" on the same branch and even between different parts of the same 

 leaf (c/. Drautz 1935). The photosynthetic activity of a plant often 

 changes strongly in the course of a smgle day, not to speak of a whole season. 

 Obsei-vations of diurnal changes have played an especially important role 

 in the development of Kostychev's "new concept" of photosynthesis. 



Offhand, one would expect photosynthesis to increase steadily after 

 sunrise until the light intensity has reached the saturating value, and then 

 remain more or less constant, unless cloudiness decreases the illumination 

 below the saturating intensity, until the evenmg decline sets in. The actual 

 behavior of the plants often follows, however, a much more complicated 



* An interesting monograph, Photosynthesis as Life Process of the Plant, by Miss 

 Brilliant (1947) seems to be the only comprehensive review of photosynthesis published 

 in Russian in recent years. It contains a survey of about 200 Russian and 350 other 

 papers, many of them not utilized in the present book. 



