272 ABSORPTION OF WATER 



most stems. The cambium layer is initiated in young roots in such a way 

 that it lies inside of the strands of phloem tissues, and outside of the xylem 

 strands. In cross section the original cambium layer appears as a wavy band, 

 passing inside of each phloem strand, and outside of each xylem strand. 

 Once differentiated this cambium layer produces secondary xylem on its inside 

 face, and secondary phloem on its outside, just as the cambium of stems does. 

 The initial formation of secondary tissues by a root cambium is usually more 

 rapid in the segments of the cambium internal to the primary phloem strands. 

 Due to this differential growth rate the cambium of a root rapidly attains a 

 circular aspect in cross section. The further differentiation of secondary 

 phloem and secondary xylem proceeds just as it does in stems. 



Most perennial roots sooner or later become encased in layers of cork 

 cells. The initial cork cambium often originates in the pericycle. As layers 

 of cork cells are produced by the cork cambium the cortex of the root, in- 

 cluding the endodermis, is ruptured and the cells of these tissues die and 

 decay away. Older roots therefore have a characteristic smooth, brownish, 

 corky covering which is pierced only by lenticels. With increasing age, sec- 

 ondary cork cambiums may arise progressively more and more deeply in the 

 phloem tissues. This results in the gradual loss of the pericycle and older 

 phloem tissues. The bark of older roots, therefore, is essentially similar to 

 that of the trunks or larger branches of trees (Chap. XXVIII). Thick 

 layers of bark do not as a rule accumulate on roots as they do on the trunks 

 of some species of trees because of the rapid decay of all dead underground 

 tissues. 



In the roots of species in which no secondary thickening occurs, as in many 

 monocots, the epidermal layer of cells may persist intact, usually becoming 

 suberized. In other such species the epidermis may die and decay, but when 

 this occurs an underlying layer of the cortex cells in turn becomes suberized. 



Lateral root branches of the primary root system originate in the peri- 

 cycle, most of them being formed in the region just above the root hair zone. 

 Usually the point of origin of a lateral root is opposite one of the primary 

 xylem strands. The first step in the formation of a lateral root in most 

 species is the development of a group of meristematic cells by the division of 

 several adjacent pericycle cells in the layer just inside of the endodermis 

 (Fig. 71). By successive divisions these cells rapidly form a growing point 

 with its characteristic root cap, region of cell division, etc. As this develops 

 the endodermis and tissues exterior to it are first stretched and later ruptured. 

 The elongating lateral root penetrates through the tissues external to it, partly 

 by mechanical pressure, and perhaps partly by digesting the tissues through 



