ROOTS 



257 



central axis like a spongy bark. If the section has passed 

 through the zone of root-hairs, they can be seen coming 

 from the epidermal cells. A longitudinal section of a root- 

 tip, in which these regions are very young, is shown in 

 Fig. 221. 



The xylem and phloem of the vascular cylinder of the root 

 do not hold the same relation to each other as in the stem 

 ( 133, p. 237). The vascular cylinder, instead of being 

 made up of vascular bundles with 

 xylem toward the centre and 

 phloem toward the outside, as 

 in the stems of Seed-plants, is 

 made up of xylem and phloem 

 strands alternating with each 

 other around the centre (Fig. 

 223). The xylem strands radiate 

 from the centre like the spokes 

 of a wheel, and the phloem strands 

 are between these spokes near 

 their outer ends. This arrange- 

 ment of xylem and phloem is 

 peculiar to roots. 



When roots increase in diam- 

 eter, a cambium soon begins to form new xylem and 

 phloem, as in the stems that increase in diameter ( 133, 

 p. 239). The new xylem, however, is not formed in con- 

 nection with the old wood, but just within the phloem, 

 that is, farther in between the " spokes " of old wood, 

 resulting in bundles like those of the stem (Fig. 224). In 

 this way a thickened vascular cylinder is formed, like that 

 of stems that increase in diameter ; and presently the cross- 

 section of the root resembles that of the stem. It is evident 

 (Fig. 224) that the principal pith rays that traverse the wood 

 zone formed by the cambium (secondary wood) extend in- 

 ward to the original radiating strands of wood (primary 



FIG. 223. Cross-section of a young 

 root, showing the solid vascular 

 cylinder, the extensive cortex, 

 and the epidermis ; observe that 

 the xylem extends from the cen- 

 tre in four strands, between 

 which the phloem strands are 

 seen. 



