204 LIGHT, VEGETATION AND CHLOROPHYLL 



temperature of orange leaves can fall 18° C. in four minutes 

 and, when the sun reappears, rise 15° C. in two minutes. These 

 variations in the temperature of the leaf are the only variations 

 of temperature which need be taken into account. 



Other Influences 



Some investigators have held the opinion that water has an 

 influence on photosynthesis. Undoubtedly it is indispensable, 

 but it is indispensable to so many other processes in plant 

 Ufe that its exact influence on the rate of assimilation is difficult 

 to discern. Some lower plants succeed in living, although with 

 reduced vitality, after they have lost most of their water. If 

 the water content of their tissues increases, assimilation 

 increases up to a certain optimum point, too much water 

 being injurious to the plant and to assimilation. Such a curve 

 is easy to trace, but how can the optimum for healthy growth 

 be distinguished from the optimum for assimilation? 



Normally, however, a deficiency of water checks assimi- 

 lation. If the tissues lack their usual turgescence, the stomata 

 close and the penetration of carbon dioxide is reduced; in 

 addition, the glucosides accumulate as there is not sufficient 

 water to carry them into the tissues which would use 



them. 



Glucosides are the products of photosynthesis and it is a 

 general law that a reaction is accomphshed more slowly as 

 its products accumulate. This eff'ect can easily be demonstrated 

 by making an incision to prevent the leaf from exporting the 

 synthesized glucosides; the rate of photosynthesis, while 

 continuing to vary normally, is maintained weU below that 

 of the ordinary leaf. Conversely, photosynthesis is more 

 rapid after a period of darkness which has given the leaves 

 tune to rid themselves of glucosides, or after the application 

 of nitrates which favour their utilization, or in fruit trees 

 whose leaves increase in dry weight in the course of the day 

 more than the leaves of trees which have no outlet for their 

 glucosides. 



Another product of photosynthesis is oxygen, but it has 



