138 



TEXTBOOK OF BOTANY 



FIG. 8 1. A small part of a cross sec- 

 tion through a young portion of the stem 

 of a bean ; a, epidermis ; b, cortex ; Stem. For 

 c, bast ; d, cambium ; e, wood ; /, pith. may use a 



end to swing about in a 

 way that will be ex- 

 plained more fully in 

 Chapter XV. In all va- 

 rieties the stem branches 

 freely, each branch begin- 

 ning as a bud in the axil 

 of a leaf, and practically 

 all that will be said about 

 the stem applies also to 

 the branches. 



162. Inner Structure of the 

 this study we 

 cross section 

 through the younger part either 



of the main stem or of a good-sized branch (Fig. 81) ; the section 



should be cut between the 



points of attachment of two 



leaves. In such a section we 



shall observe that the vascular 



bundles are arranged, like the 



bundles of the pine, in a hol- 

 low cylinder, which in the 



cross section looks like a ring. 



Between the wood and the 



bast of each bundle are one or 



two layers of cambium cells. 



In a section through an older 



part of the stem (Fig. 82), we 



find that the cambium makes 



a complete ring, and that its 



cells have begun to divide so 



as to form more wood on the 



inside and more bast on the 



outside of the ring. The bean FIG '82. -Part of a cross section 

 through an older portion of the bean 



stem therefore grows in thick- stem> in which the division of the cam . 



ness just as the pine stem bium cells has beguri and new bast and 



does ; but since the bean plant woo d are being formed ; a, epidermis ; 



lives only one season, its stem b, cortex; c, bast; d, cambium; e, wood; 



never grows to be very thick, /, pith. 



