SPERMATOPHYTA 



369 



in diameter a cork cambium or phellogen arises beneath the epidermis, 

 forming cork tissue. This finally replaces the epidermis as a protective 

 covering. Communication between the atmosphere and the living tissues 

 beneath the cork is maintained through lenticels. 



The stele is bounded externally by the pericycle, which usually consists 

 of several layers of cells. Some of these may remain parenchymatous, 







Fig. 314. Conducting tissues, X200. Phloem elements: a sieve tube and a row of com- 

 panion cells from a squash stem, as seen in longitudinal {A) and transverse (5) sections. 

 Xylem elements: spiral (C), annular (D), and pitted vessels {E), and a wood fiber (F) from 

 a stem of castor bean. 



while others become sclerenchymatous. The development of the primary 

 xylem is endarch. Among gymnosperms, with the exception of the 

 Gnetales, tracheids are the only conducting elements present in the xylem, 

 but in angiosperms vessels are the chief elements. Tracheids are derived 

 from single cells, vessels from a row of cells whose end walls break down. 

 Both are lignified, the lignin being localized to form spirals, rings, an 

 irregular network, or it may be so abundant that the walls are pitted 

 (Fig. 314C-E). Spiral and annular elements are characteristic of proto- 

 xylem, reticulate and pitted elements of metaxylem and secondary xylem. 

 In addition to vessels, the secondary xylem of angiosperms may consist, 

 largely or in part, of tracheids, wood fibers, and wood parenchyma (Fig. 

 , 314F). The phloem is made up of sieve tubes, companion cells, and often 



