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TEXT BOOK OF PLANT PHYSIOLOGY 



them, cardinal points are found — the minimum, at which the proc- 

 ess just begins; the optimum, at which it goes on at its highest 

 rate; and the maximum, after which the process stops again. 



According to the ingenious explanation by Blackman, the 

 sharp bend in the temperature curve shows that we have to deal 

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

 are accelerated with the increase of temperature, more or less in 

 accordance with the rule of Van't Hoff , but inducing opposite effects 

 in the leaf. If one of them be the process of decomposition of 



10 15 20 25 Z>0 35 40 45 

 Temperature 



50 55 



Fig. 19. — Dependence of assimilation on temperature, in the leaves of potato, 

 tomato, and cucumber (after Lundegardh) . 



carbon dioxide and the other the process of the "inactivation" or 

 loss of the faculty of action of the chloroplasts, then with com- 

 paratively low temperatures the process of inactivation is almost 

 imperceptible and assimilation is subject to Van't Hoff's law. 

 But with a temperature of 20 to 25° the inactivation begins to tell, 

 it proceeds at a rapidly increasing rate, and at 30 to 35° it has over- 

 come already the process of assimilation and soon stops it. 

 These relationships are shown diagrammatically in Fig. 20, where 

 the curve A B shows the theoretical progress of the assimilation 

 process as controlled by temperature, the curve CD shows the 

 increase of the inactivation process, while the curve AE indicates 



