148 ROOTS 
must follow. (Fig. 131.) The dead epidermis and cortex form 
the outer portion of the bark, which thickens as cork is added by 
the cork cambium, and in roots living a number of years, like 
those of shrubs and trees, may become quite thick, and broken 
and furrowed, as in the large roots of trees. As a provision for 
strength, fiber-like strands of strengthening tissue are commonly 
formed in the secondary cortex. 
In order that the vascular cylinder may have adequate con- 
ductive capacity in the older portions of the root, it, too, must 
enlarge, for as the absorptive surface of the root increases ahead 
by the multiplication of branches, not only is there an increase 
in the amount of absorbed substances which the xylem must 
carry to the shoot, but also an increase in the amount of food 
which the phloem must carry to feed the greater number of 
branches of the root. In the formation of xylem in roots, the 
portions first formed are the radiating strands or spokes which 
enlarge by developing toward the center where they usually come 
together, thus forming the solid central core of xylem as shown 
in Figure 132. However, in some plants, as in Corn and many 
other Monocotyledons, the xylem strands never come together, 
and consequently a central pith is left, around which the strands 
of xylem are arranged. In all roots the xylem strands are at first 
enlarged by this centripetal development. In some short-lived 
roots of Dicotyledons and in the roots of most Monocotyledons, 
the enlargement of the xylem is due to this centripetal develop- 
ment and the development of new vascular bundles between the 
old ones as the root becomes older. In Dicotyledons and also 
in the Gymnosperms, the group to which Pines, Firs, Spruces, etc., 
belong, the vascular cylinders of most roots are increased also 
as shown in Figure 182. It is seen in A of Figure 132 that the 
phloem and xylem are not in contact. Lying between them 
are cells which have not been modified into definite tissues. 
Some of these cells become meristematic and so arranged as to 
form a continuous band of cambium, known as the cambium 
ring, which by curving outward passes on the outside of the 
xylem, and by curving inward passes on the inside of the phloem, 
thus separating the xylem and phloem regions of the vascular 
cylinder as shown at B. The cambium cells, in the main, divide 
parallel to the surface of the root, and divide in such a way that 
the layers of new cells on the inside of the cambium are about 
