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 hairs: 553,0! corn, showing 
relation to root tip; 554, of wheat, showing relation to an 
epidermal cell and the close contact with soil particles. 
FIG. 558. Endog- 
enous origin of root 
branches : longitudi- 
nal section of root of 
arrow leaf, showing 
the branches starting 
from the vascular 
cylinder 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. 
