CORK 



135 



Suber ; according to Gilson this substance is accompanied by two 

 other acids, namely suberic acid and pldoionic acid. It is still uncer- 

 tain whether these acids occur as glycerine-esters in suberised walls, 

 as Kiigler and Van Wisselingh assume, or whether, as Gilson asserts, 

 sidjerin is made up of compound esters or of other products of the 

 condensation or polymerisation of 

 the three acids. The suberin- 

 lamella may in addition be silicified, 

 as Von Hohnel first demonstrated ; 

 it is noteworthy that silicified cork 

 as a rule only occurs in plants 

 which also contain a lare amount 

 of silica in their epidermis. 



Among other physiologically 

 interesting features of corky walls 

 which are deserving of special 

 mention, is the circumstance that 

 pits are generally absent. Where 

 they do occur, they are, according 

 to Von Hohnel, confined to the 

 inner cellulose layer, and never 

 penetrate into the suberin-lamella. 

 These pits are most conspicuous 

 where the cellulose layer is 

 secondarily thickened, and are 

 evidently functional only so long 

 as the cork cells are alive and 

 undergoing differentiation ; they serve to facilitate the access of plastic 

 materials to the developing suberin-lamella. The fact that the suberin- 

 lamella of the outer wall of a cork-cell is sometimes thicker than the 

 corresponding layer of the inner wall is evidently connected with the 

 functions of the fully-developed corky tissue ; where this condition is 

 found, the inner wall usually has a thicker cellulose layer than the 

 outer wall. 



The cells comprising corky tissues are always dead, and hence as a 

 rule contain little besides air ; this statement applies more especially 

 to thin-walled cork. Whether remnants of the former protoplasts 

 persist in the form of thin films on the inner surface of the cell-wall 

 or not, is of little moment. Greater interest attaches to the fact that 

 the cavities of thick-walled cork-cells are often filled witli a yellow or 

 reddish-brown substance, which consists in part at any rate of tannins 

 and their decomposition-products (phlobaphenes). In special cases 

 other compounds may be present, such as betuline in the cork of the 



Fig. 40. 



T.S. through the periderm of a branch of Cytisus 

 Laburnum (in winter), e, dead epidermal cells con- 

 taining- Fungus-spores ; k, layers of cork ; pit, phel- 

 logen ; phd, phelloderm. 



