CHAPTER VI 

 LEAVES 



54. Introductory. — Leaves are usually the most ex- 

 panded parts of plants. They are the principal organs 

 wherewith the plant estabhshes those relations to hght and 

 to air which are essential to its life. They are the prin- 

 cipal organs of photosynthesis. They have been called 

 the light-related organs of the plant, for, although their 

 relation to air is equally important, Hght appears to be 

 that external factor which chiefly affects their behavior. 



It is difficult to construct a perfect definition of leaf, 

 for it is a word which came into general use long before 

 any effort was made to give it an exact meaning. On 

 the basis of origin leaves may be defined as organs which 

 arise at nodes and have buds in their axils. According to this 

 definition, a leaf is not a leaf by virtue of the work it does, 

 but by virtue of its position. For example, the parts of 

 flowers and many thorns and tendrils have the origin just 

 described, and your attention has been called already to 

 the fact that these parts are often leaves as to origin. So 

 it appears that a definition of an organ based on its func- 

 tion may be far from agreeing with a definition of the 

 same organ based on its origin. It appears that we can 

 understand leaves best, and perhaps be nearer the truth, 

 by not trying to crowd our idea of them into one precise 

 group of words. Concerning leaves, as concerning many 

 other things, we can form a right idea even though we 



