THE CORK MEMBRANE. 187 



Bottle cork (of Quercus suber, the cork-oak,) consists of 

 almost cubical, thin-walled, comparatively large cells ; which 

 gradually pass over into somewhat more strongly thickened, 

 flatter cells, marking the limits of the year's production ; to which 

 the cubical cells again succeed. Addition of potash solution 

 colours the section yellow, above all the somewhat thicker-walled 

 cells marking the year's limits. Upon these it can now be deter- 

 mined that here also each double wall consists of five layers, 

 just as we found it in Gytisus. The reactions for suberin occur 

 here even more beautifully than in Cytisus, especially the eerie 

 acid reaction. 



Periderm of Ribes. Often from the phellogen are formed not 

 only centrifugal cork cells, but also centripetal cortical cells, the 

 so-called Phelloderm ; but usually in much smaller quantity. 

 Rarely does this phelloderm attain such a decided thickness as in 

 the species of Ribes. If we prepare cross-sections through older 

 twigs of the Bed Currant (Ribes' rubrum), we find under the 

 thin-walled brown cork layer, first the phellogen, then a thick 

 layer of flat chlorophyll-containing, cortical cells. These latter 

 also are arranged in radial rows, which coincide with those of the 

 adjoining cork. In the inner part of the phelloderm the radial 

 arrangement is lost, as a result of tensions acting subsequent to 

 its formation. The innermost phelloderm cells adjoin the collen- 

 chyma of the primary cortex. All the structures proceeding from 

 the phellogen are collected under the term periderm ; in Ribes, 

 therefore, the periderm consists of cork (phellem) and cork- 

 cortex (phelloderm). Young twigs show that in Ribes the origin 

 of the phellogen is somewhat deep in the primary cortex. 



Origin and Function of Cork. While in most cases the cork 

 has origin in the cells immediately hypodermal, in not a few 

 it originates in the epidermis itself, as, e.g., the Willows (Salix), 

 and all species of the Pomeae, a section of Bosaceae ; or it may 

 even have a pericyclic origin, as on thickening roots. Successive 

 phellogenic layers may be produced, in the stems of dicotyledo- 

 nous trees, in the cortex, and at various depths in the bast : and 

 the dead tissues external to the innermost layer of cork form the 

 Bark, 



Cork is also the healing tissue of plants, in that by its means 

 wound surfaces are closed. A phellogen arises in the living cell- 

 tissue under the wound, by which the injured part is soon cut off 

 with cork. The process can be readily followed, experimentally, 



