CHAPTER III. 



FOLIAGE LEAVES: FUNCTION, STRUCTURE, AND PROTEC- 

 TION. 



A. Functions of foliage leaves. 



24. Functions in general. \Ve have observed that foliage 

 leaves are light-related organs, and that this relation is an 

 important one is evident from the various kinds of adjust- 

 ment used to secure it. We infer, therefore, that for some 

 important function of these leaves light is necessary. It 

 would be hasty to suppose that light is necessary for every 

 kind of work done by a foliage leaf, for some forms of work 

 might be carried on by the leaf that light neither helps nor 

 hinders. Foliage leaves are not confined to one function, 

 but are concerned in a variety of processes, all of which 

 have to do with the great work of nutrition. Among the 

 variety of functions which belong to foliage leaves some of 

 the most important may be selected for mention. It will 

 be possible to do little more than indicate these functions 

 until the plant with all its organs is considered, but some 

 evidence can be obtained that various processes are taking 

 place in the foliage leaf. 



25. Photosynthesis. The most important function of the 

 foliage leaf may be detected by a simple experiment. If 

 an active leaf or a water plant be submerged in water in a 

 glass vessel, and exposed to the light, bubbles may be seen 

 coming from the leaf surface and rising through the water 

 (see Fig. 23). The water is merely a device by which the 

 bubbles of gas may be seen. If the leaf is very active the 



