134 DERMAL SYSTEM 



and altogether more resistant, besides being capable of continued self- 

 regeneration. The epidermis fulfils these two conditions to a limited 

 extent, inasmuch as its cells on the one hand possess thickened 

 and cutinised outer walls, and on the other contain living protoplasts : 

 but the physiological requirements referred to have become so much 

 greater, when the more highly-developed type of dermal system is 

 brought into action, that a single kind of tissue is no longer able to 

 satisfy them all. Hence the principle of division of labour once more 

 comes into play. The periderm, as the secondary dermal tissue is 

 termed, accordingly comprises two different tissues. One of these, the 

 cork, is a permanent tissue which is concerned with the protection of 

 the underlying layers against excessive evaporation, mechanical injury 

 and deleterious influences in general ; the other, the phellogen, is a 

 meristematic layer, responsible for the regeneration of the dead cork, 

 which is constantly undergoing disruption owing to the growth in 

 thickness of the organ that it envelops. 



A. CORK." 



The cells of the cork are usually prismatic or tabular in shape, with 

 four-to six-sided end-walls. Their walls may be thin, or more or less 

 thickened, but in either case are usually of uniform thickness all over. 

 In a few instances the outer (Salix, Xantlwxylon fraxincum, Cytisus 

 Laburnum) or the inner (Mespilus germanica, Viburnum Opulus) walls 

 are thicker than the rest. According to De Bary, whose observations 

 have in many respects been extended by VonHohnel, the wall separating 

 two cork-cells generally comprises five distinct layers. The inner 

 surface on either side consists of a cellulose layer, which occasionally 

 undergoes lignification. Next follows, also on either side, a suberised 

 layer, the so-called suberin-lamella, to which the physiologically 

 important characteristics of the entire wall are due. The central 

 portion of the whole system is formed by the limiting membrane or 

 middle lamella, which is either lignified or else appears to consist of 

 unaltered cellulose. Where the wall is thin, the innermost (or 

 cellulose) layers may be absent, so that the whole membrane is 

 suberised, except for the middle lamella. 



It was at one time generally assumed that a corky wall represents 

 an ordinary cellulose membrane, the secondary thickening layers of 

 which are impregnated with a fatty substance termed suberin (hence 

 the term suberin-lamella). This view has been strongly opposed by 

 Gilson and by Van Wisselingh, who both maintain that suberised 

 layers contain no true cellulose at all. With regard to the chemical 

 nature of suberin, it may be remarked that Kugler, Gilson and Van 

 Wisselingh have all isolated phellonic acid from the cork of Qucrcus 



