CORK. 



75 



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

 ii jjartitioii parallel to the surface of the organ, in most eases 

 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 lying to the 

 outside of it : they are cut 

 off' from nutritive supplies 

 and soon die. The con- 

 tinuous layers of cork are 

 called, collectivel}', Peri- 

 derm, a name restricted 

 by Mohl to tough cork in 

 distinction from soft cork, 

 but now employed with a 

 wider signification. 



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 ma}', in exceptional instances, 

 produce on its inner side permanent parencli^-ma, the cells of 

 which contain chlorophyll ; these green layers are called Phel- 

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

 Chapter 111.). 



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

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

 flat cells which cohere more firml}' lateiall}' 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 suberized 

 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 brancli of liibes nigrum, one year old ; part of trans- 

 verse section; A, hair; e, epiilerniis; pry cortical [larencliynia, somewhat distorted; 

 K, tlie total product of the phellogen c; k, cork-cells; pd, phelloderm j b, bast-cells. 

 ISachs.) 



