CORK 137 



walls. Suberised membranes, moreover, are highly resistant, being 

 insoluble both in cuprammonia and concentrated sulphuric acid. 

 Suberisation of the walls ensues soon after the cork-cells are cut 

 off from the cambium, and it is almost unnecessary to add that, 

 as a result, the cells die, their contents ultimately consisting of 

 air or more rarely of pigmented bodies (often tannins and their 

 derivatives). The characteristic white appearance of the surface 

 of the Birch (Betula) is due to the presence in the cells of the 

 cork of solid granules of a substance known as betulin. 



The cells of the cork vary considerably in shape, although 

 very commonly flattened. The walls are often relatively thin. 

 The radial walls are frequently thrown into folds, whilst the 

 tangential ones are often almost straight (cf. Fig. 65), facts that 

 can be related to the tangential tension and radial compression 

 set up by the increasing girth of the enclosed axis. 



In a few cases (e.g. Currant, Fig. 66, Laburnum, etc.) cells 

 are likewise cut off by the phellogen towards the inside, so that 

 a more or less extensive phelloderm (Ph.) is formed. The cells 

 of this tissue also are arranged in radial files, but the walls remain 

 unsuberised. The phelloderm consists of living cells, and thus 

 merely serves to augment the primary cortex, although where 

 the walls are thickened it has an additional mechanical value. 



It has already been mentioned above that the cork-cambium 

 most commonly develops in the subepidermal layer of cells, but 

 not infrequently it arises in deeper-seated cortical layers (Fig. 66) 

 or even in the peri cycle. A good instance is furnished by the 

 Barberry (Fig. 67), where the phellogen (C.C.) develops just inside 

 e ring of mechanical tissue (Sc.) occupying the inner part 

 of the cortex. In roots, too, it almost invariably arises in 

 cortical cells in the immediate neighbourhood of the pericycle 

 (Fig. 63, C, Co.). The origin of the cork-cambium from the 

 epidermis is seen in Willows, as well as in the Rose, Apple, Haw- 

 thorn, and other members of the Rosaceae. 



The cork not only prevents the excessive transpiration which 

 would ensue from rupture of the epidermis consequent upon 

 secondary thickening, but also takes over other protective func- 

 tions of that tissue. Suberised walls possess considerable strength, 

 though their elasticity is slight, and the cork consequently forms 

 a mechanical envelope whose efficiency is heightened by the 



