CH. XIV] ADAPTATIONS 139 



grasped the main features of the botany of the higher 

 plants. For the leaf is the great plastic organ of the 

 plant, and responds more readily than any other organ 

 to the vicissitudes of the environment, the result being 

 alterations in its size, texture, form and functions, so 

 profound in many cases that its typical leaf-nature is 

 completely obscured. 



The fundamental principle to be grasped here is that 

 in the adaptation of plants to their environment, ad- 

 vantages are often gained by one and the same organ 

 undertaking several duties. We have already seen how 

 foliage-leaves vary on the plant in cases of heterophylly 

 (e.g. Ranunculus aquatilis)', and the differences between 

 the cotyledons and the leaves which follow (e.g. Horn- 

 beam), and those between the lower leaves of a plant and 

 the stem leaves higher up (e.g. Brassica) are all examples 

 of the same category. So also are the following. In many 

 cases the last leaves of the current year's twig of a tree 

 become bud-scales (e.g. Horse-chestnut) and as such are 

 reduced in size, altered in form and texture, and otherwise 

 so changed from the type that everyone agrees to give 

 them another name bud-scales. The scales of winter- 

 buds, however, are oftener stipules than leaves, though the 

 difference does not much concern the principle. Bulb- 

 scales, again, are merely altered leaves, and other examples 

 can be given. 



The student will find it useful to begin asking the 

 meaning of these variations in the structures and functions 

 of leaves, even at this stage, since no part of the plant 

 probably no organ in any organism affords better 

 material for facts whence deductions may be made in the 

 discussion as to the origin of specific forms. Leaves vary 

 both in direct response to external conditions e.g. light, 

 drought, temperature and in response to internal stimuli 



