THE WALLFLOWEJi l 



xylem directed straight outwards. From this point 

 onwards, the hypocotyl contains two groups of xylem 

 only, and these groups now converge, and ultimately 

 unite to form the diarch xylem-plate of the root itself. 

 We see, then, that this xylem-plate forms the downward 

 prolongation of the two pairs of xylem strands from 

 the cotyledons. Consequently the xylem-plate lies in 

 the plane which passes through the insertion of the 

 two cotyledons. 



In the meantime, we find a corresponding change 

 in the position of the phloem strands, but in the 

 reverse direction. The two phloem strands of each 

 pair of vascular bundles diverge more and more from 

 each other, until the right-hand group of one pair 

 meets the left-hand group of the opposite pair, and 

 vice Tersd (see Fig. 31,_pA). Tracing them still farther 

 down, we find that these two groups become one, so 

 that at last there are only two groups of phloem alto- 

 gether, one on each side of the xylem-plate. Each 

 phloem strand of the root, then, is continuous with two 

 phloem strands of the hypocotyl, one belonging to each 

 cotyledon, and thus we see that in spite of the great 

 difference in the arrangement of the xylem and phloem 

 strands, the vascular system of the main root is the 

 direct downward continuation of that of the stern ; the 

 parenchyma of the central cylinder undergoes a great 

 reduction in the transitional region, the pith gradually 

 disappearing as the bundles converge. The pericycle 

 forms the external layer of the cylinder throughout, 

 whether in stem or root. The continuity of the 

 endodermis and other cortical tissues between stem 



6 



