522 PHYSIOLOGY. 



of cork from the outer layers of cortical parenchyma. The surface 

 of the corky layer is usually rough and irregular, and it peels off 

 in laminae periodically in certain plants, being renewed by deve- 

 lopment from the green cellular layer which it covers. 



In some plants the corky layer is little developed, in others very much, 

 as in the Cork-Oak. In the Vine and Clematis the corky layer is scarcely 

 distinguishable after the first year's growth, as the bark breaks away, down 

 to the liber, in stringy shreds. In Viscum no cork occurs ; even in shoots 

 eight or nine years old the epidermis remains, but completely consolidated 

 by secondary deposits, as noticed above. 



Cork is composed of tabular thin-walled cells, containing only air, 

 closely arranged in rows at right angles to the surface. The surface of 

 wounds in soft-growing tissue is usually covered with a layer or layers of 

 cork-cells, which form a sort of defence to the wounded tissues. In 

 chemical and physical properties, cork closely resembles the cuticular 

 substances just mentioned. Cork-cells are formed from a special set of 

 cells constituting the cork-cambium or phellogen. The cells divide hori- 

 zontally or parallel to the surface, but always in such a manner, that of 

 two newly-formed cells one remains full of protoplasm, with chlorophyll 

 contents &c., while the other is transformed into a permanent cork-cell. 

 The formation of cork, however, varies in different cases, and is sometimes 

 of a very complex character. According to Sanio, Rauwenhoff, and Vesque, 

 the growing cork-cells grow on the outer or on the inner side of the 

 phellogen or cork-cambium, the formation being centrifugal in the former 

 instance, centripetal in the latter. In other cases the growing cork-cells 

 are placed on both sides of the phellogen. But in this latter case it is 

 only the outermost cells of each layer of the phellogen which become 

 truly corky ; the inner cells in both cases retain their cellulose charac- 

 teristics, and become filled with chlorophyll, forming an herbaceous 

 envelope. Vesque has proposed that the term periderm be applied to 

 the whole of the cork-cells produced by the phellogen ; some of these 

 cells, as above described, become suberified, or converted into passive 

 cork-cells, while others retain their active character and constitute the 

 herbaceous envelope. The periderm layers occur, not only singly, but in 

 separate groups at different depths in the bark, causing the exfoliation of 

 plates or rings of bark, to which the.name rhytidome has been given by 

 Von Mohl and Hanstein. 



Lenticels are small local formations of cork-cells occurring on young 

 shoots in the form of little warts. The cork-cells are formed beneath 

 portions of the tissues which are decaying or dead, and which surround 

 the cavities beneath the stomata. Similar formations are consequent upon 

 the formation of cracks in the epiderm, the object being, in both cases, 

 to afford protection to the denuded tissues (Trecul). 



Liber. Every fully developed fibro-vascular bundle consists, as above 

 said, of liber, cambium or generating tissue, and wood encircled by cellular 

 tissue. The liber (phloem) part of the bundle is distinguished from the woody 

 (xylem) part of the bundle by its position outside the cambium, by the 

 larger size of its woody fibres, their different chemical properties, and 

 especially by the presence of latticed or sieve cells, &c. Ihe unlignified 

 liber-cells (called also soft bast cells) contain albuminoid materials, proto- 



