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, 7. 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 
