PLANTS AND WATER 143 
doubtless they further serve to protect the 
delicate leaves within from a variety of other 
injurious influences. Nevertheless, in spite 
of their wonderfully perfect adaptation to 
these functions, they may be shown to owe 
their existence in their present form to certain 
conditions which affect nutrition during a 
far earlier period of active vegetation, for 
they are simply modified leaves or parts of 
leaves. In a sense they may be said to have 
been starved and arrested, or rather dis- 
torted, in their development. This can easily 
be proved in such a plant as a young ash or a 
plum, by pulling off the young and active 
outer foliage leaves. The operation must be 
performed quite early in the summer while 
the bud scales are young, and before they are 
fully formed. The result of the experiment 
is to cause the young leaves that otherwise 
would have been destined to form the bud 
scales, to grow out, and provide a further crop 
of foliage leaves. The removal of the active 
leaves has diverted nutrition into the young 
bud scales that were to be, and has caused 
them to assume the form and character of 
foliage leaves; and this happens for precisely 
the same material reasons that the foliage 
leaves themselves are forced to assume their 
own proper form. 
In many parts of the world the climate is 
sharply marked into a wet and a dry season. 
During the wet period, vegetation of a meso- 
phytic type can exist; but unless it has some 
