336 



THE STRUCTURE OF ECONOMIC PLANTS 



tip. Wilson (35) observed beads of water accumulating on the 

 teeth under some conditions, and has suggested that these termina- 

 tions might be hydathodes serving as drip tips. 



The midvein of the leaflet forms a prominent ridge on its abaxial 

 surface. The branches from it lie much closer to the lower epi- 

 dermis than the upper, and, in some instances, the mechanical 

 tissue surrounding the secondary veins is in contact with the lower 

 epidermis. The midvein is almost completely surrounded by a 

 bundle sheath of thick-walled mechanical cells which become 



pal ■; 



Fig. 170. A, habit of foliage leaf showing stipules and character of venation; B, tran- 

 section of portion of leaflet cut through midvein showing distribution of chloroplasts and 

 intercellular spaces: hr, hair; mech, mechanical tissue; pa/, palisade region; ph, phloem; 

 spo, spongy tissue; stp, stipule; xy, xylem. 



sclerotic in the abaxial sector, and crystals of calcium oxalate are 

 numerous in the cells of this region. (Fig. 170, B.) Toward the 

 distal portion of the midvein and in the lateral veins, the degree 

 of lignification of the bundle sheaths is less pronounced, and the 

 smaller veins are surrounded by parenchymatous cells. The 

 vascular tissues of the midvein are similar to those of the stem, 

 and some secondary thickening occurs in the larger veins. 



The mesophyll usually consists of a palisade zone of two rows 

 of cells approximately twice as high as broad which together 

 constitute about half of the thickness of the leaf. The spongy 

 parenchyma is loosely arranged and air spaces communicate freely 



