204 



PLANT PHYSIOLOGY 



exerts a profound influence on all vital processes. Like chemical 

 reactions, these processes follow van't Hoff's rule, according to 

 which the rate of chemical reactions approximately doubles 

 with every increase of 10°C. in temperature. One of the most 

 characteristic features of vital processes is that due to the 

 extreme complexity and instability of the protoplasm where these 

 processes take place, they are subject to the rule of van't Hoff 

 only within comparatively narrow limits of temperature, gen- 

 erally between 6 and 30 to 35°C. With further increase of tem- 

 perature, the acceleration of the process is markedly decreased, 



Fig. 62.- 



10 15 



40 45 50 55 



20 25 30 35 

 TemperCT+ure 



-Dependence of assimilation on temperature, in the leaves of potato, 

 tomato, and cucumber {after Lundegardh) . 



after which a rapid falling off takes place, the bend in the coeffi- 

 cient curve often showing an acute angle (Fig. 62). At 40 to 

 50°C., the process completely ceases. Therefore, on the tem- 

 perature coefficient curve of assimilation and other vital proc- 

 esses, three principal or, as Sachs called them, cardinal points 

 are found; the minimum, at which the process just begins; the 

 optimum, at which it goes on at its highest rate; and the maxi- 

 mum; after which the process stops again. 



According to the ingenious explanation by Blackman, the 

 sharp bend in the temperature curve shows that there are here 

 not one but at least two processes to deal with. Both of them 



