248 



MORPHOLOGY 



is of the most primitive type, being solid (with xylem at the center) and 

 exarch. However, it is not concentric, the xylem developing towards the 

 center from two or more protoxylem points near the periphery of the 

 stele, and between these radiating strands of xylem separate phloem 

 strands occur (fig. 555). This arrangement of phloem and xylem, in 

 which they occur on alternating radii, is called the radial arrangement. 

 In the secondary thickening of roots (figs. 556, 557), a cambium is 

 developed, which forms secondary xylem inside the phloem; the two, 



Figs. 553, 554. — Root haiis: 553, of corn, showing 

 relation to root tip; 554, of wheat, showing relation to an 

 epidermal cell and the close contact with soil particles. 



Fig. 5 5 8. — Endog- 

 enous origin of root 

 branches ; longitudi- 

 nal section of root of 

 arrow leaf, showing 

 the branches starting 

 from the vascular 

 c}'linder and pene- 

 trating the cortex. — 

 After Coulter. 



therefore, hold the same relation to one another as do the xylem and 

 phloem of a collateral bundle. Continued activity of this cambium 

 results in a cylinder of collateral bundles, made up of phloem and 

 secondary xylem; and the radiating arms of the primary xylem are at 

 the bottom of the primary pith rays. Of course, the cambium also 

 forms secondary phloem within the older phloem. By this secondary 

 growth the vascular cylinder of a root may soon lose any appearance 

 of its primitive radiate structure, and assume the appearance of a dicoty- 

 ledonous stem, with collateral bundles. 



