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 hving 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-Uke strands of strengthening tissue are commonly 

 formed in the secondary cortex. 



In order that the vascular cyUnder 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 multipUcation of branches, not only is there an increase 

 in the amomit 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 sohd 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 132. It is seen in A of Figure 132 that the 

 phloem and xylem are not in contact. Lying between them 

 are ceDs 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 



