WATER SUPPLY: TEMPERATURE 141 



present in it. Now, as the water in the cell sap decreases — 

 through a failure of supply completely to cover evaporation 

 — it becomes a more concentrated solution, and will tend to 

 remove water from the plastids, perhaps to an appreciable 

 extent ; we do not know. Further, during assimilation, the 

 concentration of sugars in the plastid may change, and this 

 must mean a normally recurring alteration of the water 

 relations. The exact effect on the rate of assimilation is 

 obscure ; but it is known that increase in the products 

 of assimilation tends to inhibit further assimilation, and it 

 is possible that the water balance of the plastid is affected. 

 Thoday (191 o) has shown that as the sunflower leaf 

 loses turgor the rate of assimilation falls, till it reaches 

 zero in the wilted leaf. Referring to work of previous 

 investigators, he concludes that this may be a frequent 

 occurrence in hot weather with some plants, e.g. the beet. 

 The effect may be due to stomatal closure, but it may be 

 related to diminution of water content in the assimilating 

 cells. 



Temperature. — The other four factors we may call " exter- 

 nal." We may first take the relation to temperature. The 

 temperature of the chloroplast must be taken as the tem- 

 perature of the leaf ; this may be accurately determined by 

 thermo-electric methods. In dull weather it is practically 

 the air temperature ; the amount of heat liberated in respira- 

 tion is small and cannot appreciably raise the temperature 

 of a-thin freely radiating and transpiring organ like the leaf ; 

 the heat lost in evaporation must be quickly made good 

 by absorption from the air. In sunlight, on the other hand, 

 the leaf temperature may be considerably higher than that 

 of the air. 



It is a fact of common knowledge that, Vvithin a cer- 

 tain range, metabolic processes are more active at higher 

 than at lower temperatures. The favourable range of tem- 

 perature was looked upon as specific for given organisms 

 and for given functions, such as assimilation, although 

 liable to modification ; a certain point in it was called the 

 optimum temperature, it being supposed that there the 



