OUTER CELLS AND TISSUES. 39 



under the heading of meristem, to be considered later. The 

 cork-cambium produces on its outer aspect fresh cork-cells, and 

 on its inner aspect, at times, a layer of cells possessing chloro- 

 plasts, known as the phelloderm (see supra). In some plants — 

 e.g., Qmrcus sessiliflom — several separate zones of cork may be 

 found at different depths in the cortex. Cork forms, at times, 

 a layer of considerable thickness, which affords no mean protec- 

 tion to the cortical tissues of the stem ; the walls of the freshly- 

 formed cork-cells are composed of pure cellulose and pectose, but 

 they soon become toughened and rendered more elastic by the 

 deposit in them of a substance known as suberin. Older cells of 



^^s 



Fig. 24. — Portion of a Teansvekse Section or this Young Stem or 

 Pinua TO show the fokmation of Cork. — e, Epidecmis ; a, cuticle ; 

 hy, hypodermis ; sc, cork-cambium ; c, cortical cells. 



the cork contain air only, so that cork-tissue is of low specific 

 gravity and very elastic in nature. In some stems openings are 

 formed in the " bark," through which the more superficial of the 

 cork-cells are continually shed; these apertures are known as 

 lenticels, and are caused by the thinning of the epidermis at 

 certain points and subsequent rupture, leading to an aperture 

 which is of value in admitting air into the intercellular spaces of 

 the cortex. 



