GROWTH OF VASCULAR BUNDLES 



53 



the tropics on the approach of winter, and when these plants 



again clothe themselves with leaves a -ring of growth is formed 



as already described. Such facts as 



these strengthen the conception that 



the formation of the ring of growth is 



at first stimulated by the demand for 



water on the part of the leaves. 



Secondary increase in thickness in 

 roots does not differ essentially from 

 that of stems, and the slight differ- 

 ence that occurs is due to the pecu- 

 liar arrangement of the phloem and 

 xylem in the root bundle. It will be 

 remembered that the phloem and 

 xylem strands in roots stand side by 

 side and not in radial line as in 

 stems. (Compare diagrams in Fig. 

 21.) When secondary increase in 

 thickness begins, the cambium flank- 

 ing the phloem on the inside or 

 toward the center lays down xylem 

 elements, so that, with the already 

 existing phloem, a collateral bundle, 

 such as is typical in stems, is pro- 

 duced. At the same time the cam- 

 bium in front of the original or 

 primary xylem commonly forms a 

 medullary ray (Fig. 26), but it some- 

 times makes phloem elements and 

 thus completes a collateral bundle 

 here also. The cambium then con- 

 tinues to add new phloem and new 

 xylem in both cases, and secondary 

 medullary rays as the dimensions 



of the xylem and phloem wedges increase, and the root soon 

 comes to look quite like a stem, the discernible difference being 



FIG. 26. Cross section of a 

 young root of Phasetlus multi- 

 florus. A, pr, cortex; m, pith; x, 

 stele (all tissues within the endo- 

 dermis collectively) ; g, g, g, g, 

 primary xylem bundle; b, b, b, b, 

 primary phloem bundle; the cam- 

 bium, not indicated here, has the 

 same location as indicated in C 

 and D, Fig. 21. B, cross section 

 through older portion of root of 

 the same plant, b', b', secondary 

 bast; k, k, periderm. The remain- 

 ing letters stand for the same 

 tissues as in .4. Notice that the 

 cambium has laid down medullary 

 rays in front of the primary xylem, 

 but has made secondary xylem 

 behind the primary phloem. 

 (After Vines.) 



