4i6 THE STRUCTURE OF ECONOMIC PLANTS 



remains parenchymatous, thick-walled pericyclic cells abut the 

 endodermis, which becomes discontinuous, retaining its identity 

 only over the transition bundles. (Figs, xiy B; 119, F.) Second- 

 ary xylem and phloem are formed except outside the protoxylem 

 points where parenchymatous rays occur. The protoxylem points 

 of the divided transition bundles occupy a position more in line 

 with the metaxylem cells so that the primary xylem arms are tan- 

 gentially oriented. 



Somewhat higher in the hypocotyledonary axis, there is a 

 definite decrease in cambial activity, and the phloem is arranged 

 in smaller groups separated from one another by parenchymatous 

 cells. (Fig. XI 8, A.^ At successively higher levels, the progres- 

 sive differentiation of the protoxylem in a more centripetal posi- 

 tion, and of the metaxylem in a centrifugal one, results in an 

 endarch orientation of the primary xylem tissues. (Fig. ii8, B.) 

 This reorientation is gradual, but eventually the endarch arrange- 

 ment is reached in all bundles. The parenchymatous rays, separat- 

 ing the halves of the original transition bundles, become progres- 

 sively wider; and there is evidence of the meristematic character 

 of the hypocotyl at this level in the relatively small number of 

 mature metaxylem elements. The sieve tubes and companion 

 cells lie in a collateral position external to the four pairs of transi- 

 tion bundles, but there are no thick-walled cells in the phloem. 



About 8 cm. above the soil surface, the rays that bisect the 

 transition bundles lying in the intercotyledonary plane widen to 

 such an extent that one-half of each bundle lies on either side 

 of the hypocotyledonary axis. (Fig. 119, G.) Thus, four units of 

 the original transition bundles are located on each side of the 

 axis. Each group consists of one original transition bundle in 

 which the two units are separated by a narrow ray, and one unit 

 from each of the transition bundles of the intercotyledonary plane, 

 the bundles ad'dc' and a'bb'c constituting the left and right 

 cotyledonary traces respectively. (Fig. 119, H, 7.) In the inter- 

 cotyledonary plane, there are from one to three endarch bundles 

 which in this stage of development consist of a few primary xylem 

 and phloem elements. (Fig. ±1^, /.) 



At the cotyledonary plate, the four bundles of each trace diverge 

 into their respective cotyledons. The endarch bundles are the 

 downwardly diverging strands of the traces of the foliage leaves 

 above. Ultimately they become continuous with the cotyle- 



