30 THE STORY OF PLANT LIFE 



is less in spring, autumn and winter than in summer, 

 and the duration of light is connected with the 

 seasons. 



In relation to light plants exhibit certain sleep 

 movements. The leaves close up at night and 

 assume a vertical position. This prevents loss of 

 water by transpiration, since under a cooler atmo- 

 sphere absorption is slower. Flowers also close up 

 at night and in wet weather. 



Light affects growth also. Plants grown in the 

 dark have long internodes. Too intense light has 

 the opposite effect, causing dwarfing as in arctic 

 plants. The relative amount of light causes plants 

 to exhibit different characteristics, and some plants 

 are called sun-plants when the leaves are small 

 and thick with chloroplasts in several layers of 

 tissue, but not in the epidermal cells. In the shade 

 plants the leaves are thin and large, and the chloro- 

 plasts are in the upper layers of cells. 



Heat, in the same way as light, is one of the most 

 important factors of plant life. The vital functions 

 of nutrition and growth, absorption, transpiration, 

 respiration, and assimilation depend on the mainten- 

 ance of a sufficient degree of heat. A rise in tempera- 

 ture advances, a fall retards, the activities of the 

 plant, and its growth as a result. 



It is the solar heat that is meant here. This varies 

 according to latitude. The tropics are the warmest 

 zones, the Polar regions the coldest, the temperate 

 regions intermediate in this respect. From this 



