454 



EFFECT OF HEAT 



[Cn. XVIII 



Fig. 129, shows, more graphically than the tables from which 

 they are drawn, that, as the temperature rises, growth increases 

 up to a certain point, and then diminishes again. The falling 

 off is more rapid than the increase a condition which we 

 found also in the curves (Fig. 68 ; p. 226) giving the rate of 

 movement of protoplasm at different temperatures. We have 

 thus three critical points to distinguish : the minimum, or 

 lowest temperature at which growth occurs ; the optimum, or 

 the temperature of greatest growth ; and the maximum, or the 

 highest temperature at which growth can take place. These 

 critical points, then, are to be considered comparatively ; and 

 as an introduction to this consideration I present, in a table, 

 the points as they have been determined for various species : 



TABLE L 



SHOWING CRITICAL POINTS FOR VARIOUS PLANT ORGANISMS, ARRANGED 

 ACCORDING TO THE OPTIMUM 



