CORK. 



75 



formed by the division of the mother-cell into two tabular cells, by 



a partition parallel to the surface of the organ. In most cases 



the outer cell becomes 



cork, while the inner re- 



tains its power of division 



and in turn produces new 



cells. But with the first 



appearance of the cork- 



layer a change takes place 



in all layers Ivinff to the 



/ *> 



outside of it : they are cut 

 off from nutritive supplies 

 and soon die. The con- 

 tinuous layers of cork are 

 called, collectively, Peri- 

 derm, a name restricted 

 by Mohl to tough cork in 

 distinction from soft cork, 

 but now employed with a 

 wider sinification. 



246. Cork meristem 

 gives rise to successive 

 layers of cork-cells : if the 

 new layers differ much 

 from the preceding in the 



shape and size of their cells, an appearance of stratification 

 naturally results. Cork meristem may, in exceptional instances, 

 produce on its inner side permanent parenclryma, the cells of 

 which contain chlorophyll ; these green lavers are called Phel- 



j. ^j \j 



loderm, and are observed well in the beech, willow, etc. (see 

 Chapter III.). 



247. Cork-cells are tabular, or sometimes cubical, and with 

 few exceptions have no intercellular spaces. In the case of very 

 flat cells which cohere more firmly laterally than in the line of 

 the radius, the cork-tissue may be readily separated in films or 

 sheets. 



248. The walls of older cork-cells are cutinized or subenzed 

 throughout. The demonstration of cellulose in cork-cells is not 

 possible unless the cells have been first acted on by solvents, 





FIG. 56. Formation of cork in a branch of Ribes nigrum, one year old; part of trans- 

 verse section: h, liair; e, epidermis; pr, cortical parenchyma, somewhat distorted; 

 K, the total product of the phellogen c ; k, cork-cells ; pel, phelloderm ; 6, bast-cells. 

 I Sachs.) 



