PLANT PHTSIOLOOT. 



91 



160. Heat. — For every cell there is a certain range of 

 temperature in which it is active, culminating in an opti- 

 mum temperature ; above this its activity decreases rapidly 

 to its maximum temperature, where all activity ceases. In 

 like manner below the optimum temperature activity de- 

 creases (not so rapidly, however) until the minimum is 

 reached, where activity ceases again. This range of activ- 

 ity is not the same for all plants, and in many-celled plants 

 it often differs considerably for different parts of the plant- 

 body. Sachs determined this range for the germination of 

 the following seeds : 



Minimum. 



Optimum. 



Maximum. 



Indian Corn 

 Scarlet Bean 

 Pumpkin , 

 Wheat.... 

 Barley. . . 



9° C. (= 48° F.) 



9°C. ( = 

 14° C. (= 57° 

 5°C. (=41° 



P.) 

 P.) 

 P. 



5' C. (= 41° P.) 



34° C. (= 93° P.) 

 34° C. (=98° P.) 

 34° C. (= 93° P.) 

 29° C. (= 84° P.) 

 29° C. (=84° P.) 



46° C. (= 115° F.) 

 46° 0. (= 115° P.) 

 46° C. (= 115° P.) 

 42° C. (= 108° P.) 

 37° C. (= 99° P.) 



161. Common observation shows that plants differ much 

 as to the degree of heat necessary for germination, as well 

 as for other activities; but we have little in the way of care- 

 ful measurements upon anything more than the germination 

 of seeds. Certain experiments appear to indicate that the 

 range in green parts of plants is much greater than has 

 usually been supposed, in some cases approaching 0° C. and 

 in others reaching 50° to 55° C. (132° to 131° P.), or even 

 more. On the other hand, it is certain that other parts of 

 plants will not endure such temperatures; e.g., roots and 

 underground stems. 



For our ordinary terrestrial flowering plants the mini- 

 mum temperature ranges from near 0° to about 10° C. (33° 

 to 50° Fahr.), the maximum from about 35° to 50° C. (95° 



