THE CELL-MEMBRANE. 

 T. 



THE membrane by which the cells of vegetable tissues 

 are bounded has generally been described by most 

 authors as originating in a fairly uniform condition, and 

 consisting at first at any rate of an almost or quite homoge- 

 neous sheet of cellulose. After its first formation it increases 

 in surface and later in thickness, its composition remaining 

 unchanged. Finally, in most cases it undergoes chemical 

 transformations in its substance, becoming suberised, cutin- 

 ised, lignified or mucilaginous. As generally described, 

 therefore, cellulose alone must be regarded as the basis of 

 the cell membrane and the modifications met with are due 

 to changes in the cellulose, leading to the formation of other 

 bodies, which are then found mixed with, and largely re- 

 placing, the latter. 



The study of the cell- wall may thus be carried on under 

 the two heads of the primitiv^e membrane and such modifi- 

 cations of it as do not present the latter changes, and the 

 modified membranes which are characteristic of woody, 

 corky and mucilaofinous cells. 



In dealing with the former of these, what indeed may 

 for purposes of discrimination be called " unchanged" cell- 

 wall, it is well to recall the theories of its composition which 

 have been advanced in recent years. Without entering 

 into much detail we may mention the two hypotheses of 

 Naegeli and Strasburger. According to the former the 

 molecules of cellulose are aggregated together into quasi- 

 crystalline groups, the micellae, which are separated from 

 each other by delicate films of water. According to Stras- 

 burger, the theory of the micellar aggregations is unneces- 

 sary and the structure is regarded as having a network 

 arrangement, the meshes of the network containing water. 



A later theory, advanced by Wiesner ( i ), has however 

 much to recommend it, especially in the light of recent re- 

 searches made upon the structure and chemical composition 



