ASSIMILATION OF CARBON BY PLANTS 209 



with sunset. But with considerable intensification of sunlight, as 

 on hot summer days even in moderate latitudes, a shift of the 

 maximum rate of photosynthesis to the morning hours may be 

 observed, while in the noon hours there is a depression of photo- 

 synthesis under the influence of loss of water. The stronger 

 the sun's heat, the more pronounced this depression; and in a 

 hot cUmate or on extremely hot days in the noon hours, a very 

 sharp decrease of photosynthesis may be observed, with a rise 

 toward the evening when the heat decreases. The curve of the 

 diurnal march of photosynthesis-'achieves in this case two sum- 

 mits, with a sharp maximum in the morning and a less pro- 

 nounced one in the evening. 



There is another factor that contributes to a decrease of photo- 

 synthesis after the morning maximum, viz., the inhibiting 

 influence of the accumulation in the leaf of assimilation products. 

 Like other chemical reactions, photosynthesis requires for its 

 successful continuation an uninterrupted removal of the products 

 of the reaction, and it has already been seen that the structure 

 of the leaf clearly reflects this necessity for the removal of organic 

 substances. But in periods of intensive assimilation, the produc- 

 tion of carbohydrates exceeds their translocation from the leaves, 

 and the cells of the assimilation tissue become overfilled with 

 them. With the insufficient rate of translocation, a part of the 

 assimilates is transformed into starch, which is insoluble in water 

 and is thus removed from the sphere of reaction. However, this 

 is not always sufficient entirely to preclude accumulation; and 

 in consequence, after energetic assimilation in the morning 

 hours, photosynthesis is inhibited by the excess of products by 

 their mass action. 



In still hotter regions, e.g., in the deserts of California, Arizona, 

 Africa, and Central Asia, another factor causes the decrease of 

 photosynthesis, viz., the overheating of the leaves by the sun. 

 On very hot days such an overheating, causing simultaneously 

 a decrease in photosynthesis and an increase in respiration, 

 affects the functioning of the leaf to such an extent that instead 

 of absorbing, it emits carbon dioxide. In this case, the harmful 

 effect of excess of insolation is observed, and under such climatic 

 conditions the total production of the leaves, contrary to that 

 in a moderate climate, proves to be higher on cloudy than on 

 clear days. 



