i68 THE STRUCTURE OF ECONOMIC PLANTS 



stomata lie parallel to, and on either side of, the bundles. The 

 parenchymatous cells of the mesophyll do not contain chloroplasts 

 except in the apical region and adjacent to the bundles. Two 

 large bundles are located in the angles of the prophyll where the 

 convex and flat sides meet, and the convex portion is traversed by 

 several smaller ones. The margins of the leaf are strengthened by 

 strands of mechanical tissue, and there may be a few supporting 

 cells abutting the smaller vascular bundles. 



The Foliage Leaves. — The structure of the foliage leaf is 

 more complex than that of the coleoptile and prophylls. The 

 epidermal cells are arranged in rows which run parallel to the 

 long axis of the blade; and, as in the stem, some of these are 

 elongated while others are approximately square in surface view. 

 The elongated cells may form continuous rows, or they may be 

 separated at intervals by the short cells; and they also occur 

 between the longitudinal rows of stomata. Epidermal hairs, when 

 present, are of varying lengths and arise from the surface in a 

 more or less regular arrangement. 



The adaxial surface is somewhat ridged, while the abaxial 

 one is nearly flat; and in the furrows of the former are single 

 or double lines of stomata with bands of motor or buUiform cells 

 between them. (Fig. 76.) Each band is three to seven cells in 

 width and the cells comprising it are thinner walled than the 

 adjacent epidermal cells. The motor cells are shorter than the 

 long cells of the epidermis; but, as seen in transection, are thicker 

 than any of the other epidermal cells. Since their outer surfaces 

 are not cutinized, they lose water under conditions which cause 

 high transpiration, and the decreased turgidity results in an incurv- 

 ing of the adaxial surface of the blade. The number of stomata is 

 greater on this surface than the abaxial one and the curling tends 

 to decrease the amount of water loss. The stomatal apparatus 

 consists of two narrow guard cells which surround the stoma, and 

 two accessory cells. (Fig. 5.) The development of the stoma is 

 similar to that described for Zea. 



The abaxial epidermis is like the adaxial with respect to the 

 arrangement of the cells, but differs from it in the absence of ridges 

 and motor cells. In addition to this, the cell walls may be some- 

 what thicker, the number of hairs fewer, and the stomata more 

 commonly occur in single rows. The abaxial epidermis of the 

 sheath resembles that of the blade, with thick sinuous walls, 



