Mr. G. II. K. Thwaitcs on the Cell-Membrane of Plants. 21 



geneous gelatinous structure called the intercellular substance. 

 This substance is no doubt analogous to the mucus of the Pal- 

 mellece, and of similar character to it. That this is the fact may 

 be well-seen in attending to the mode of development of the 

 frond of Tetraspora, in which a quaternary division of the cells 

 takes place, as in some species of Pahnellea : around each of the 

 new cells, though principally on one side of them, is developed 

 a definite amount of gelatine ; and in this way the size of the 

 frond is increased. This genus, Tetraspora, forms a beautiful 

 connecting link between the Palmellece and the laminose Algae ; 

 for although the mass of cells is developed in the form of a frond, 

 still these cells have their individual development but slightly 

 modified by forming a part of an entire structure. In some Ulvce 

 the character of a whole compound structure is more manifest, 

 and the individual cell-life begins to appear secondary ; and as 

 we advance higher in the scale of vegetation, the latter ceases to 

 speak plainly to our senses. 



If what I have said respecting the intercellular structure is 

 true of the Algse, the same explanation would apply to that struc- 

 ture in the higher plants, where it is often very conspicuous ; and 

 it appears to me not improbable, that the deposits of sclerogen 

 as well as the firm portion of the spiral fibre may be considered 

 as structures of a similar character. The pellicle which covers 

 the cuticle is doubtless so. 



Now what is the character of the mucus which we have seen 

 to be developed in definite quantity outside and around the cell- 

 wall ? That it is not a mere chemical solution of starch would 

 appear evident from its persistence when mounted for the micro- 

 scope in water and other fluids. Its toughness and elasticity, 

 the readiness with which it allows water to permeate it, and its 

 recovering its original form and consistence upon being moistened 

 after desiccation, seem to warrant the belief that it possesses an 

 organized form of the same mechanical properties as sponge ; and 

 if we could resolve it under the microscope into its ultimate 

 structure, we should probably find that its texture would be best 

 expressed by the term spongy ; though I would not wish it to be 

 supposed I believe it to have the complicated structure of real 

 sponge, but to consist rather of a mat of delicate fibres. 



And as, in viewing a series of Algse, a transition may be ob- 

 served from a mucous structure to one possessing the external 

 characters if not the functions of cell-membrane ; it may be fairly 

 inferred that cell-membrane is of a very similar mechanical struc- 

 ture, and we should perhaps not be far from a right definition in 

 applying to it the term felt, as indicating its real characters. 



After duly weighing the foregoing phenomena and others of 

 a similar character, I have arrived at the conviction that the cell- 



