ROOTS 



75 



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

 the stem ( 24). The vascular cylinder, instead of being 

 made up of vascular bundles 

 with wood toward the center 

 and bast toward the outside, 

 as in stems, is made up of 

 wood and bast strands alter- 

 nating with each other around 

 the center (Fig. 72). The 

 wood strands radiate from 

 the center like the spokes of 

 a wheel, and the bast strands 

 are between these spokes near 

 their outer ends. This ar- 

 rangement of wood and bast 

 is peculiar to roots. 



When roots increase in diameter, a cambium soon begins 

 to form new wood and bast, as in the stems that increase 



FIG. 72. Diagrammatic cross-section of 

 a young root, showing the innermost 

 layer of the cortex (c) and the vascu- 

 lar cylinder (t>) containing alternating 

 regions of xylem (z~) and phloem (p). 



B A 



FIG. 73. Diagram showing the method of thickening the vascular cylinder of a root : 

 A represents the cross-section of a young root in which four phloem strands (p) 

 alternate with four xylem strands (x), the whole bundle region being enveloped 

 by the thick cortex ; B represents an older root in which there is a continuous zone 

 of cambium (c), which is forming on the outside new phloem (np) in contact 

 with the old Op), and on the inside new xylem (nz) alternating with the old (x) 



