36o THE STRUCTURE OF ECONOMIC PLANTS 



smaller ones consist of a few primary xylem and phloem elements, 

 and the ultimate veinlets end blindly in the mesophyll. (Fig. 

 i8i, B.) 



The Petiole. — The petiole is terete to subtriangular and con- 

 tains three main bundles. The largest one is located on the abaxial 

 side of the petiole, the other two lie toward the adaxial surface, 

 occupying points about equidistant from the abaxial bundle; and, 

 between the main bundles, two or more smaller vascular strands 

 may occur. (Fig. i8x. A.') The epidermis resembles that of the 



Fig. 182.. A, diagrammatic transection of petiole showing arrangement of bundles. Cross- 

 hatched portions indicate fibers; stippled portions, phloem; lined regions, xylem. B, 

 transection of leaf tendril showing arrangement of bundles ; C, detail of a sector of petiole 

 showing bundle : «^, epidermis ; /^.fibers; /»A, phloem; atj, xylem. 



blade with a thin cuticle and stomata similar to those described 

 for that organ. The subepidermal region consists of several layers 

 of chlorenchyma, and the central portion is made up of large thin- 

 walled parenchymatous cells which ultimately disintegrate so that 

 thp mature petiole is usually hollow. The main bundles resemble 

 those of the stem. (Fig. i8i, C.) 



The Tendril. — The leaf tendril is subtriangular in transection, 

 its adaxial and lateral faces being somewhat grooved. Like the 

 petiole, it has three main bundles, the abaxial one which frequently 

 has a subtending cap of mechanical tissue being larger than the two 

 adaxial bundles. (Fig. iSi, B.) The remainder of the tissue is 

 chlorenchymatous. 



