IPOMOEA BATATUS 



493 



The cortex is derived from the periblem, which consists of two 

 layers of cells overlying the plerome. The outer layer of this 

 histogen divides only anticlinally and forms the peripheral layer 

 of the cortex, while the inner layer divides in all planes to form 

 the remainder of this region, which is six to nine cells in thickness 

 at maturity. The stele is differentiated from a group of plerome 

 cells lying above the periblem. The lateral members of this group 

 produce the pericycle, which can be distinguished from the other 

 regions of the stele early in ontogeny since the cells composing it 

 are radially rather than vertically elongated. (Fig. Z53.) In the 

 development of the vascular portion of the stele, the primary 



pri Ix ves 



pri Ix ves 

 -lat rt 



Fig. 154. A, transection of young primary root in region of differentiation; B, primary 

 root sliowing origin of lateral root: lat rt, lateral root; pel, pericycle; pri Ix ves, primary 

 latex vessel; pri ph, primary phloem. 



phloem is first differentiated and four groups of elongated phloem 

 elements are formed by longitudinal divisions of plerome cells 

 abutting the pericycle. At the same time, a primary latex vessel 

 is differentiated adjacent to the pericycle by the enlargement of a 

 single vertical row of cells in each phloem group. (Fig. 154, A.') 

 The cells of these vessels retain their end walls and protoplasts. 

 The remainder of the primary phloem consists of long, very slender 

 sieve tubes with transverse or occasionally oblique sieve plates 

 and companion cells. 



The primary xylem differentiates centripetally and the first 

 protoxylem cells to mature are thin-walled with annular or annular- 

 spiral thickenings. While the protoxylem elements are develop- 

 ing, a vertical row of parenchymatous cells at the center of the 



