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. 103 1), but much more 

 commonly the phellogen layer arises in 



the primary cortex (figs. 1032, 1033), as cortical parenchyma; highly mag- 

 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 oi 

 the outer part of a stem of the bone- 

 set {Eupatorium perfohatum), show- 

 ing the development of epidermal 

 cork (c); a, the original epidermal 

 walls; b, later cross walls, whose ap- 

 pearance indicates the inception of 

 cork formation; note the thick- 

 walled hypodermis Qi) which forms 

 a mechanical cylinder around the 



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 macrocarpa), 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. 



