210 PLANT PHYSIOLOGY 



The interaction of the separate factors influencing photo- 

 synthesis directly, as well as the stomatal movements regulating 

 the penetration of carbon dioxide into the leaf, lead to the result 

 that under natural conditions, even in the course of a steady, 

 clear day, the march of photosynthesis is of an irregular character 

 with alternate periods of abrupt increases and decreases of the 

 process. 



This course becomes still more irregular on days with changing 

 weather, when each passing cloud alters the condition both of 

 illumination and of temperature and thus reacts upon photo- 

 synthesis. At the same time, it must not be forgotten that the 

 action of the chloroplasts depends likewise on the condition of 

 the protoplasm in which they are immersed and a differentiated 

 part of which forms the stroma. Abrupt changes in temperature 

 and illumination may provoke a certain state of excitation of the 

 protoplasm, which naturally may affect the action of the chloro- 

 plasts. According to Harder's observations, a rapid change 

 from darkness to light may lead to a transitory increase of the 

 intensity of photosynthesis as a consequence of excitation. 

 According to the indications of Kostytchev and Lubimenko, it 

 is essential to consider the possibility of the influence of such 

 excitations in studying under laboratory conditions the effect of 

 different factors upon photosynthesis. It must be kept in mind 

 that photosynthesis is not simply a chemical reaction, of decom- 

 position of carbon dioxide, but a complicated chain of processes 

 taking place in the film of chlorophyll on the surface of the 

 chloroplasts, in the stroma, and in the surrounding protoplasm. 



48. Intensity of Photosynthesis and Yield of Dry Matter. 

 Significance of the Relation between Photosynthesis and 

 Respiration. — As the process of photosynthesis represents the 

 source of accumulation of organic substance in the plant, and 

 this accumulation determines the ultimate yield of this plant, 

 these two concepts are confused. It is very often assumed that 

 the size of the yield is wholly determined by the intensity of the 

 photosynthetic process. Actually, the interrelations between 

 photosynthesis and yield are considerably more complicated. 



First of all, it must be emphasized that the accumulation of 

 organic substance represents the result not only of photosynthesis 

 but also of respiration, proceeding uninterruptedly in all the 

 living cells of the plant. It has already been seen that the rela- 



