ROOTS 95 
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 Fic. 72.—Diagrammatic cross-section of 
their outer ends. This ar- a young root, showing the innermost 
layer of the cortex (c) and the vascu- 
rangement of wood and bast lar cylinder (v) containing alternating 
ig peculiar to roots. regions of xylem (z) and phloem (p). 
When roots increase in diameter, a cambium soon begins 
to form new wood and bast, as in the stems that increase 
A 
Fig. 73.—Diagram showing the method of thickening the vascular cylinder of aroot: 
A represents the cross-section of a young root in which four phloem strands (p) 
alternate with four xylem strands (zx), 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 (p), and on the inside new xylem (nz) alternating with the old (z). 
