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 



r ' FIG. 1031. A cross section o\ 



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



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 



set (Eupatorium perfoliatum), 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 (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 ' 



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 (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 externa) to the cork are rupturing^ both figures highly magnified. 



