136 PLANT LIFE 



The behaviour of plants at the different 

 seasons of the year is instructive from this 

 point of view. 



The habit of shedding the leaves on the 

 approach of winter which is so characteristic 

 of the majority of our trees and shrubs is 

 often regarded as an adaptation to physiologi- 

 cal drought rather than as directly due to 

 the action of the lowering of temperature on 

 leaves. Although there is plenty of water 

 in the soil in winter, the temperature of the 

 ground is too low to enable the trees to absorb 

 it freely enough. It is true there are evergreen 

 trees which do not throw off their leaves in 

 autumn, but they generally exhibit definite 

 structural features indicative of a normally 

 slow rate of transpiration, i. e. of water lost 

 as vapour through the stomata. The leaves 

 are leathery or small, the stomata are compara- 

 tively few, whilst various other features point 

 to an economy in the matter of water expendi- 

 ture. The deciduous trees and shrubs, which 

 shed their leaves in winter, are relatively more 

 prodigal of water during the warmer season, 

 thus compensating for the alternate periods 

 of inactivity. 



Without doubt there is much to be urged in 

 favour of the deciduous habit being regarded 

 primarily as an adaptation to a reduction of 

 the water supply. This argument is strength- 

 ened by a consideration of plants which 

 quite definitely respond to periodic drought, 

 physical or physiological, by casting off their 



