THE STEUCTITRE OF THE STEM 



365 



A little further from the apex the differentiation of the pro- 

 cambium bundles in the stele can be seen. By the time this is 

 evident the leaves have elongated to a greater extent, and the 

 differentiation of their internal tissue is progressing. Each pro- 

 cambium bundle of the stele is found to be continuous with one 

 of those of the leaf, so that the bundle appears to originate in the 

 leaf and to pass dov^nwards into the stem. It is said on this 

 account to be a common bundle, that is, common to leaf and stem. 

 The first appearance indeed of protoxylem is usually seen near the 



Fig. 747. 



Fig. 748. 



Fig. 747. Phaseolus multiflorus. Longitudinal section 

 througli the apical region at the stem of an embryo, 

 s s. Apex, pi, pi. Parts of the two first leaves, k, I: 



Commencement ol their axillary buds. After Sachs, 



Fig. 748. Diagram of course of bundles in stem of 

 Dicotyledon. After Naegeli. 



point of junction of the two members. These primary common 

 bundles are known as leaf -trace bundles on account of their 

 origin as described. 



Tracing these leaf-trace bundles down the stem they are 

 found to pass independently down the internodes, joining at the 

 nodes other bundles entering there from the leaf vertically below 

 them, or bifurcating and joining those entering the stem from 

 the orthostichies next in order. If a leaf sends many bundles 

 into the stem, their courses become compUcated; they some- 

 times join the other primary brmdles of the stelar ring, or less 



