GROWTH OF VASCULAR BUNDLES 



53 



the tropics on the approach of winter, and when these p'ants 



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 p,^. .6.-cross section of a 

 elements, so that, with the already young root of Phaseoius muiti- 



florus. ^4, pr, cortex; m, pith; x, 



existmg phloem, a collateral bundle, stele (aii tissues within the endo- 

 such as is typical in stems, is pro- ''''™' ''°"r''T'''^;/'/; ^; f' 



■^ ^ ' r primary xylem bundle; b, b, b, b, 



duced. At the same time the cam- primary pWoem bundle; the cam- 



, . . ;. ^ c ,1 • • 1 bium, not indicated here, has the 



blUm m front of the original or ^^^ne location as indicated in c 



primary xylem commonly forms a and d, Fig. 21. b, cross section 



. _ _ through older portion of root of 



medullary ray (Fig. 26), but it some- the same plant, y, 6'. secondary 



times makes phloem elements and f^ i«.t'ttr J'TJ'":. 



thus completes a collateral bundle tissues as in a. Notice that the 



. cambium has laid down medullary 



here also. 1 he cambium then con- rays in front of the primary xylem. 



tinues to add new phloem and new !^"'. '\^' T'^^ secondary xyiem 



'■ behind the primary phloem. 



xylem in both cases, and secondary (After vines.) 



medullary rays as the dimensions 



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



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



