PRACTICAL BOTANY 



elements essentially similar to those described above in the 

 older stem ; but the bundles are here much smaller, they con- 

 sist of fewer elements, and the tissues are immature : thus the 

 sclerenchyma which lies directly within the bundle-sheath has 

 its walls as yet thin ; it may already be distinguished from the 

 soft bast by the more uniform character of its elements : the 

 cambium may be distinguished by the regularity of arrange- 

 ment of its cells in radial rows. Of the wood only the vessels 

 nearest to the pith will be matured, and between them and the 

 cambium various young vessels may be seen, having large 

 cavity, but the walls not yet thickened or lignified. 



Note that, if the stem be young enough, the bundles are 



FIG. 2. A, B. Diagrams illustrating the formation of interfascicular cambium. 

 A shows the primary vascular bundles isolated, and embedded in quiescent 

 ground tissue, before the interfascicular cambium begins to be formed ; B shows 

 the interfascicular cambium () forming with the fascicular cambium (ff) a con- 

 tinuous ring : p = phloem ; ,v= xylem ; b, 6, groups of bast-fibres at the periphery 

 of the phloem. (After Sachs.) 



not joined laterally as in the older stem, but are separated from 

 one another by broad bands of ground tissues. In slightly 

 older stems the cells of this tissue may be found actively dividing, 

 by tangential and occasionally by radial walls. An interfascicular 

 cambium is thus formed, and by the tissues derived from it the 

 vascular ring, as seen in the older stem, is completed. Centrally 

 lies- 



4. The pith, consisting of thin-walled cells, with sparing 

 cell-contents : thus these cells have not yet lost their activity j 



