13^ 



TEXTBOOK OF BOTANY 



Fig. 8i. — A small part of a cross sec- 

 tion through a young portion of the stem 

 of a bean ; a, epidermis ; b, cortex ; 

 c, bast; d, cambium; e, wood; /, pith. 



end to swing about in a 

 way that will be ex- 

 plained more ftally 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 Structxu'e of the 

 Stem. — For this study we 

 may use a cross section 

 through the youngerpart either 



of the main stem or of a good-sized branch (Fig. 8i) ; 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 



stem therefore grows in thick- 

 ness just as the pine stem 



does ; but since the bean plant 



lives only one season, its stem 



never grows to be very thick. 



Fig. 82. — Part of across section 

 through an older portion of the bean 

 stem, in which the division of the cam- 

 bium cells has begun and new bast and 

 wood are being formed; a, epidermis; 

 b, cortex ; c, bast ; d, cambium ; e, wood ; 

 /, pith. 



