122 



BOTANY OF THE LIVING PLANT 



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

 7iormal 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 

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



that 



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

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

 7ninimum is about 0° 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 1, 

 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 lor 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, 



