208 



PLANT PHYSIOLOGY 



leaf and, more generally, the conditions of the water supply of 

 the plant exert a very great and complicated influence upon 

 photosynthesis. This influence will be discussed farther on in 

 the chapter devoted to the water relations of plants. 



The complicated influence of each of the external factors upon 

 photosynthesis, as well as the complexity of the interrelations 

 between the separate factors, determines the extreme complexity 

 of the diurnal march of photosynthesis. This complexity is 



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Light Irrf-ensit^ in Mc. 



Fig. 64. — Interrelation of the intensity of light and the concentration of carbon 

 dioxide in their influence on assimilation in the aquatic moss, Fontinalis {after 

 Harder, from Lundegardh) . 



augmented by the continuous changes in the intensity of the 

 factors, more or less regular on clear days but most erratic on 

 days with variable weather. That is why the diurnal course of 

 photosynthesis is highly unstable on different days even for one 

 and the same plant and may sharply differ on the same day for 

 various plants. 



In a moderate climate with a sufficient water supply, the 

 general course of photosynthesis accords with the course of the 

 sun's radiation. Beginning in the morning with the rising of 

 the sun, photosynthesis reaches its maximum in the noonday 

 hours and then gradually decreases toward the evening, ceasing 



