PLANTS AND WATER 137 



leaves. Thus, in the tropics the dry season 

 is marked by the leafless character of many 

 trees which renew their foliage as soon as the 

 rainy season sets in. Again, many evergreens, 

 when transplanted, frequently throw off their 

 leaves. This is the result of injury to, and 

 disturbance of, the root system, whereby 

 absorption is suddenly checked. Hollies and 

 laurels often display this reaction, and indeed 

 it is generally to be regarded as a favourable 

 sign for the future of the plant; individuals 

 that shed their leaves promptly always suffer 

 less than those which retain their foliage in 

 a flaccid or withered condition on the branches. 

 But if we look a little further into this 

 question of leaf fall, it turns out to be not so 

 simple as it appears at first sight. It must be 

 premised that the fall of the leaf is not a 

 matter of mere detachment, but it ensues in 

 consequence of definite changes which have 

 caused a layer of tissue to become differen- 

 tiated across the base (usually) of the leaf. 

 Thus, even before the detachment of the leaf, 

 the wound is practically healed in advance. 

 Although various functions connected with 

 nutrition are concerned in bringing about the 

 formation of this " separation layer," the most 

 powerful stimulus is unquestionably that of 

 physiological water starvation, whether this 

 starvation results from physical shortage or 

 from a physiological inability to absorb. The 

 intermittent periods of drought in summer 

 are often followed by early leaf fall on the 



