STEMS 



705 



mechanical elements (largely bast fibers). 

 The development of the cork cylinder 

 usually occasions the death of all cells 

 external to it, since it checks the move- 

 ment of material from within. 



Cork. Structural features. The 

 most important protective tissue of the 

 bark is the cork, which is developed 

 from a meristematic layer known as 

 the phellogen or cork cambium. Occa- 

 sionally this layer arises from the epi- 

 dermis, as in some Rosaceae and in 

 many herbs (fig. 1031), but much more 

 commonly the phellogen layer arises in 

 the primary cortex (figs. 1032, 1033), as 

 in most woody stems and in various 

 underground stems (e.g. potato tubers). The region usually involved 

 is the outermost cortical layer, the hypodermis, but phellogen may 

 develop in any of the deeper layers, net excluding the endodermis; 

 even the pericycle sometimes gives rise to cork. Cork is developed 

 outward from the phellogen layer, which toward the inside may give 

 rise to phelloderm or cork cortex; the phellogen, cork, and phello- 



FIG. 1031. A cross section o\ 

 the outer part of a stem of the bone- 

 set (Eupatorium per/olialum), show- 

 ing the development of epidermal 

 cork (c); a t the original epidermal 

 walls; b, later cross walls, whose ap- 

 pearance indicates the inception of 

 cork formation; note the thick- 

 walled hypodermis (h) which forms 

 a mechanical cylinder around the 

 cortical parenchyma; highly mag- 

 nified. 



1032 1033 



FIGS. 1032, 1033 1032, a partial cross section of a stem of Jussiaea peruviana from 

 a dry habitat, showing the development of cork tissue (c) underneath a stereome bundle 

 of thick -walled cells (s) ; from SCHENCK; 1033, a cross section of the outer part of a 

 bur oak twig (Quercus macrocar pa) , showing the layers of the periderm; p, the phello- 

 gen, from which cork (c) develops externally and phelloderm (d) internally; note that 

 the phelloderm contains chloroplasts, that the cork layer is without air spaces, and that 

 the tissues external to the cork are rupturing; both figures highly magnified. 



