i66 THE STRUCTURE OF ECONOMIC PLANTS 



in the course of the bundles which, together with the entrance of 

 additional bundles from the leaf, result in the formation of the 

 nodal plate. Percival (lo) has observed that, in most cases, 

 the hypodermal bundles extend through a single internode, anasto- 

 mosing with other bundles at the first node below. The number 

 of bundles in the inner ring is reduced by anastomoses to about 

 one-half their original number as they pass through the second 



a b e d e f g h i 



Fig. 74. A, longisection of a portion of the stem from epidermis to lacuna. The plane of 

 the section with respect to the vascular bundle is indicated by the dotted line i-i in B. B, 

 transection of a mature stem bundle : a, the epidermis showing two types of epidermal cells ; 

 h, chlorenchyma ; c, mechanical tissue centrad to the chlorenchyma; d, parenchyma; e 

 and /', mechanical tissue of the bundle sheath ; /, phloem ; g, primary xylem elements consist- 

 ing of a pitted tracheid, spiral vessel, and an annular or annular-spiral vessel ; g', metaxylem 

 vessel, not shown in ^; h, xylem parenchyma surrounding protoxylem elements; ;, paren- 

 chyma; k, margin of the central lacuna. 



internode. Thus, in any given internode, one-half of the large 

 bundles can be traced to the leaf that is diverged at the node 

 immediately above; and the remainder to the leaf diverged from 

 the second node. 



In the nodal region, the bundles are surrounded by a sheath 

 of chlorenchyma, and differ from the internodal bundles in the 

 number and character of the xylem elements. In general, the 

 xylem elements are short, pitted or reticulate tracheids; and the 

 bundles may be amphi vasal rather than collateral. 



