CHAPTER XII 



THE LEAF AND ITS FUNCTIONS 



leaf 



dveln 



The leaf is the food-manufacturing organ of the plant. Typically, it is a 

 flattened structure, attached to a node of a stem, supported in such a 

 way as to receive an optimum amount of sunlight, and green in color 

 because of the presence of great numbers of chlorophyll-containing cells. 1 



Leaves are extremely varied in form 

 and size, and their differences are 

 among the conspicuous features 

 that aid in recognizing the various 

 kinds of plants. Nevertheless, most 

 leaves are built on a common plan, 

 which includes the following parts. 

 There is a broad, flat, expanded 

 portion, the blade, supported by a 

 stalk, the petiole. Where the petiole 

 is attached to the node, its base 

 may be flanked by a pair of small 

 leaflike appendages, the stipules. On 

 both surfaces of the blade there 

 may be seen numerous veins, which 

 are vascular bundles formed by 

 repeated division of the large 

 bundles that enter the blade from 

 the petiole. The venation, or ar- 

 rangement of the veins, forms a 

 branching network in most dicotyle- 

 dons, whereas in most monocotyle- 

 dons the veins run parallel, or 

 approximately so. 

 The structure of the leaf. In cross section the blade of a leaf is seen 

 to consist of several layers. The upper and under surfaces are covered 



1 All functional leaves contain chlorophyll. If they do not appear green to the 

 eye, it is either because of the presence of additional pigments that mask the green 

 of the chlorophyll or because the surface is covered with light-reflecting hairs or is 

 otherwise modified. 



166 



petiole 



temode 



ternod* 



lipulsl 



Fig. 12.1. Stem and leaves of the climbing 

 fig, a dicot. 



