PART I 'J 



CHAPTER III 

 THE MORPHOLOGY AND PHYSIOLOGY OP LEAVES 



1. The Distinctive Charactehistics of Leaves 



Leaves are the most abundant and conspicuous of plant 

 parts, collectively constituting foliage, the most distinctive 

 part of vegetation. Their essential features consist in their 

 green color, flat form, and growth towards light. Their 

 prominence is explained by their function, which consists 

 in the exposure of green tissue to light, under action of which 

 the plant forms its food out of water and mineral matters 

 drawn from the soil, and a gas received from the air. This 

 function is all the more important because the food thus 

 formed serves not only for plants, but ultimately for all 

 animals as well. 



Although uniform in their primary function, foliage 

 leaves show much diversity in various features. In size, 

 some are almost microscopic, most are a few square inches 

 in area, and a few are measured in feet. In shape, some are 

 nearly circular, others almost needle-form, and others of 

 diverse intermediate gradations. In color, while typically 

 green, some are gray, white, yellow, or red ; and in autumn 

 they often display a brilliant succession of colors. In tex- 

 ture, some are flaccid, as in water plants, others almost 

 leathery, as in evergreen trees, while most are intermediate, 

 with a flexible-elastic consistency. In duration of life, they 

 are typically temporary, lasting but one season, and even in 

 evergreens for only a few years ; but cases occur in which the 

 leaves persist as long as the long-lived stem. In only one 



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