STEMS, ROOTS, AND LEAVES 



113 



the phenomena of osmosis by living cells. It seems wiser, there- 

 fore, to consider absorption by roots later, in connection with 

 the absorption and movement of water in plants. 



Midveini 



Slamata, 



cells 



LEAVES 



Structure. Leaves are flat expansions of the plant body, 

 designed to present a large surface of green leaf tissue to air 

 and sunlight for the purpose of food-making. The leafstalk, or 

 petiole, connects the leaf 

 blade, or expanded green 

 portion of the leaf, with 

 the conducting and storage 

 portions of the main stem. 

 It also serves to display 

 the leaf effectively to sun- 

 light and air. The lamina, 

 or blade, of the leaf (Fig. 

 61, a) is composed largely 

 of green tissue cells called 

 mesophyll, supported and 

 subdivided into small areas 

 by the intricately branched 

 veins. The relation of 

 veins and mesophyll is 

 best appreciated by hold- 

 ing the leaf of an ordi- 

 nary plant toward the 

 light. The veins not only 

 form a supporting framework for the leaf but also serve to 

 irrigate the mesophyll cells with water and to remove sugar 

 and other foods which must be transported back into the stem 

 from the manufacturing mesophyll cells. The leaf, like other 

 organs of the plant, is therefore composed of layers and groups of 

 cells called tissues, each cell layer, or group, having a special func- 

 tion to perform for the leaf and for the entire body. The princi- 

 pal leaf tissues are the epidermis, the mesophyll, and the veins. 



FIG. 61. Gross structure of a leaf of the 

 milkweed (Asclepias) 



a, surface and sectional views of a milkweed 

 leaf; 6, microscopic structure of the epidermis 



