DIVISION OF THE VEGETABLE CELL. 23 



molecules, and therefore cannot possibly estimate how many layers 

 would be required to make up the thickness of even the thinnest 

 cell-wall. Still this is by no means necessary. In order there- 

 fore, to render the idea as simple as possible, let us imagine a wall 

 composed of say three layers, and the molecules, if you will, as 

 large as peas. Now imagine that these peas are floating in a 

 viscid fluid medium somewhat denser than themselves, but each 

 pea kept apart from, and in its relative position with regard to its 

 neighbours, by reason of an attractive force which is inherent in 

 itself ; so that when uninfluenced by external conditions, this 

 structure and all its individual parts are relatively in a state of 

 stable equilibrium. Now in imagination reduce the size of the 

 molecules as much as you like, but, preserving the general idea of 

 this extremely rough illustration, you have a conception of the 

 present view as to the structure of the cell-wall. That such a 

 theory represents the facts of the case is highly probable, seeing 

 that it, far better than any other which has yet been suggested, 

 enables us to account for the increase of the area of the cell-wall 

 during the process of growth. It should be always borne in 

 mind that nothing like life can be ascribed to the cell- wall, that 

 the substance of which it is composed has no power of increasing 

 its own bulk, and that any extension which takes place in it is due 

 solely to the protoplasm which it encloses. Growth of the cell- 

 wall takes place by intussusception, i.e., the intercalation or inser- 

 tion of new molecules between those already existing. Remem- 

 bering that between the existing molecules we have a layer of 

 water which envelops them on every side, it will be seen that if 

 by any means, we can obtain a force or pressure internally which 

 shall, so to speak, stretch the existing cell-wall, and therefore 

 separate the component molecules farther from each other, there is 

 nothing to hinder the insertion of new molecules amongst them. 

 And such a force exists, and results from the eagerness with 

 which young and growing cells imbibe water, thereby producing a 

 condition of great turgidity, and keeping the thin cell-wall in a 

 state of tension. 



The question naturally arises as to how these new molecules 

 which are required to build up the cell-wall are produced. Are 

 the elements of the cellulose held in solution in the water of 

 organization, as it is called, which occupies the interspaces between 

 the existing molecules, waiting, as it were, for room to combine ? 



