CHAPTER III. 



STRUCTURE AND FUNCTIONS OF THE LSAF. 



THE leaf is an extension of the two outer layers of the bark 

 expanded into a broad thin net work: leaves constitute the 

 verdure of nearly all plants; their color is almost universally 

 green, which color they derive from a substance called chloro- 

 phylle, deposited just beneath the cuticle. Towards the end 

 of autumn, after the verdure of plants has matured, their color 

 is changed to various hues, as yellow, orange, red, <fec., by the 

 action of oxygen on their elements. 



Deciduous leaves are those which fade and fall off at the end 

 of autumn, annually. 



Evergreens are those which remain green throughout the 

 year. . % 



Leaves are arranged in various ways upon the stem and 

 branches of plants : in some, as in the potato, they are scattered 

 along the stem irregularly: in others, as the pea, they aie 

 alternate, or one above another on opposite sides of the stem : 

 when two are against each other at the same joint or node, 

 they are called opposite: when more than two are arranged 

 in a circle at the same node, as in the meadow lily, they are 

 verticillate : in the pepper and some others, they are arranged 

 spirally around the stem. 



The prolongation of the leaf-stalk, through the middle of 

 the leaf, is called the midrib; the smaller divisions, or ribs, 

 which radiate or go off from this, are called nerves. 



