STEMS 



70S 



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 



FIG. 1031. A cross section o\ 



bark js the cork, which is developed the outer part of a stem of the bone- 

 from a meristematic layer known as set (Eupatorium perfoliatum), show- 



the phdlogen or Cork cambium. Occa- in \ th ,\ development of epidermal 



cork (c); a, the original epidermal 



sionally this layer arises from the epi- walls; 6, later cross walls, whose ap- 

 dermis, as in some Rosaceae and in pearance indicates the inception of 



many herbs (fig. I0 3 l), but much more rk formation ; note the thick- 



walled hypodermis (h) which forms 



commonly the phellogen layer arises in a mec hanical cylinder around the 

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

 in most woody stems and in various D1 e ' 



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- 



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 (5); from SCHENTCK; 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. 



