324 Mr W. Gardiner, On the constitution of the walls of [Nov, 9, 



vegetable tissue. Even from the very first the wall may be for the 

 most part mucilaginous, or may consist of some form of hydrated 

 cellulose ; but perhaps the more normal phenomenon that occurs 

 is, that in a thickening cell-wall the innermost layers consist of 

 cellulose, while the outer portion tends to degenerate into mucilage. 



The structure known as the middle-lamella is composed, in the 

 pure cellulose cell-wall, simply of cellulose layers, which have 

 become peculiarly dense and resistant owing to pressure and 

 tension, but more generally these layers undergo a hydrated or 

 mucilaginous change, and even incipient lignification and cuticu- 

 larisation. 



The occurrence of mucilaginous degeneration is one of the 

 deepest physiological interest, since by this means the separation 

 of cells in the formation of stomata, intercellular spaces, and the 

 like is made possible. During the separation from one another of 

 contiguous cells the external mucilaginous portions of the wall 

 frequently present a very rugged outline, and the two walls may 

 even be connected by strands of mucilage which traverse the inter- 

 cellular space. 



The external portion of the walls lining intercellular spaces 

 often degenerates into mucilage over their whole surface, but in 

 many cases this degeneration is confined to certain definite areas, 

 leading to the formation of mucilage rods and drops. 



The process of partial or entire mucilaginous degeneration 

 appears to be one of hydration, since (among other things) it can 

 be shewn that by employing suitable hydrating reagents, the area 

 of mucilaginous change can be increased at will. The author 

 believes that the phenomenon of swelling as exhibited by cell-walls 

 is made possible by, and is in the main dependent on, the forma- 

 tion of definite hydrates of cellulose, such hydrated forms being 

 very unstable and probably consisting of a large number of mole- 

 cules. 



In addition to the mucilaginous degeneration occurring in the 

 walls of normal cells, mucilage may be derived directly from the 

 protoplasm or from walls which are undergoing a pathological 

 change. 



Cuticularisation occurs in the outermost layers of the walls of 

 living cells, when the walls in question are freely exposed to the 

 external medium, but in the opinion of the author, lignification and 

 suberisation is in some way dependent on changes taking place in 

 consequence of the slow and gradual death of the cell, which must 

 however be situated in the vicinity of living tissue. In certain in- 

 stances a suberous degeneration may occur in the most external 

 layers of certain cellulose walls, in which normally a mucilaginous 

 change would have supervened, but this is apparently dependent 



