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 



