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 
