CHAPTEK III 



FOLIAGE LEAVES: FUNCTION, STRUCTURE, AND PROTEC- 

 TION 



A. Functions of foliage leaves 



24. Functions in general. — AVe have observed that fuliage 

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

 important one is evident from tlio various kinds of adjust- 

 ment used to secure it. AVe infer, tiierefore, tliat for some 

 important function of these leaves liglit 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 

 miglit lie carried on by the leaf that light neither hcljis nor 

 hinders. Foliage leaves are not confined to oiu' 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 

 bo possible to do little more than indicate these functions 

 until the ]ilant with all its orgajis is considered, but some 

 evidence can l)e obtained that various processes are taking 

 jilace in the foliage leaf. 



2.">. Photosynthesis. — The most important function of the 

 foliage leaf may 1ie detected by a simjjle cx]ierinu'nt. If 

 an active leal' or a water ]ilant be subnu-rged in watm- in a 

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

 coming fi'om the leaf surface and rising through the water 

 (see Fig. 23). The water is nun-ely a device by Avhich the 

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

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