122 



BOTANY OF THE LIVING PLANT 



It is a common fact of experience that a rise of temperature from the 

 normal accelerates growth. Its influence in germination has already 

 been considered (p. 8). In order to force plants on, the gardener 

 cultivates them in hot-houses, or in frames slightly warmed by fer- 

 mentative changes in rotting manure upon which the frames are 

 placed. The rise of temperature in spring and early summer is a 

 leading factor in the stimulation of growth at that season, while 

 conversely the fall of temperature in autumn leads to a period of 



FIG. 85. 



The plant to the left side is grown under normal conditions of lighting : that 

 to the right is " etiolated." (After Errera.) 



dormancy in vegetation at large. But there are limits of temperature 

 outside of which growth will not take place. For most plants the 

 minimum is about o C., and the maximum lies between 40 C. and 

 50 C. Between the minimum and the maximum, but as a rule nearer 

 to the latter, lies an optimum temperature at which growth is most rapid. 

 This optimum varies for different plants, but it is usually between 

 22 C. and 37 C., being lower for arctic and temperate plants than for 

 those of the tropics. (Fig. 84 bis.) It may be noted for comparison 

 that 37 C. is the normal temperature of the body in man. 



Light has an opposite effect, acting as a retarding influence. If 

 similar seedlings be grown at the same temperature, the one in light, 



