THE ANATOMY OF THE ROOT 



49 



points, and the continued differentiation and maturation of second- 

 ary xylem ultimately smooths out the curve. In some cases, not 

 all of the fundamental parenchyma lying between the primary 

 xylem and phloem is involved in this cambial activity and there is 

 a narrow zone of this tissue remaining between the primary and 

 secondary xylem. 



The elements which constitute the secondary tissues of the stele 

 are derived from the cambium in such a way that at first they tend 

 to lie in regular radial rows; wx^ 

 but these may become some- 

 what irregular as growth and 

 maturation proceed. This 

 occurs in part because of the 

 unequal growth rate of the 

 elements, some of which 

 attain larger size than others. 

 In addition, the subsequent 

 divisions and growth of some 

 of the parenchymatous ele- 

 ments derived from the 

 cambium may result in dis- 

 placement and reorientation 

 of the secondary tissues. 

 (Fig. 19.) The secondary 

 xylem consists of tracheids, 

 vessels (tracheae), paren- 

 chyma, and fibers. The 

 secondary phloem commonly 

 includes sieve tubes, compan- 

 ion cells, parenchyma, and 

 occasionally fibers and secre- 

 tory tissue. The relative proportions of the constituent elements 

 in the xylem and phloem, especially with reference to the paren- 

 chymatous cells, varies strikingly with the species of plant; and, 

 where there is considerable secondary thickening, the amount of 

 storage parenchyma may be large. 



The continued enlargement of the stele results in definite changes 

 in the outer portion of the axis. The primary phloem groups are 

 forced outward, and this tissue is usually crushed or digested. The 

 pericycle remains active and by radial divisions of its cells increases 



Fig. 18. Transection of a portion of the root of 

 Brassica showing the diarch primary xylem 

 strand, the broad pericyclic ray, and the secondary 

 xylem. 



