Ch. IV, 2] 



STRUCTURE OF STEMS 



119 



into the buds. Just below the leaves some of the bundles 

 fork, and each sends one branch, called a leaf trace, into 

 a leaf, and the second up the stem, as indicated in the 

 typical example here pictured (Fig. 73), and as can be 

 seen directly in a translucent stem like that of the Balsam. 

 This branching and rejoining of the bundles produces the 

 node, which is thus explained, while thereby the bundles 

 are united into one great 

 cylindrical network or 

 system. In this cylinder 

 the turning of bundles out 

 into the leaves results in 

 gaps just above them; and 

 around these gaps the new 

 developing fibro-vascular 

 cylinders of the axillary 

 buds establish their connec- 

 tion with the main cylinder 

 (Fig. 71). 



While in most herbaceous 

 stems the bundles are so 

 arranged as to form a ring 

 when seen in cross section, 

 in others they are scattered irregularly, as illustrated here- 

 with (Fig. 74). In such cases the bundles anastomose in the 

 stems and extend out into the leaves in a manner differing in 

 details, but not in principle, from the methods just described 

 (Fig. 72). Thus the bundles collectively constitute a con- 

 tinuous conducting system for water and food throughout 

 the plant. 



The tissues above considered are all formed in the buds, 

 and belong to the primary growth of the plant. Later the 

 cambium, and other growth layers, add new tissues, which 

 thus belong to the secondary growth. 



Woody stems develop from an herbaceous condition, 

 through stages easily observed in the twigs of our common 



Fig. 74. — Stem of Corn, in cross 

 section; X 5. (Drawn from a photo- 

 micrograph by Stevens.) 



