34 



the other hand, they approach one another in proportion as 

 water is withdrawn from between them. But it may be asked, 

 How can such a structure as the cell- wall hold together? 

 The forces by which these micellae, with their surrounding 

 watery areas, are held together, are : 



ist. The attraction existing between the separate particles of 

 each micella holding them together and rendering 

 them impermeable to water ; — since we saw that each 

 micella was itself probably an aggregate of chemical 

 molecules. 



2nd. The attraction of the micellae for one another, in conse- 

 quence of which they tend mutually to approach. 



3rd. The attraction which exists between each micella and 

 the water which surrounds it. This acts, of course, as 

 a counteracting influence to the second. 



It is obvious that the attraction existing between the micellae 

 and their surrounding watery areas must necessarily be greater 

 than that of the micellae for each other, otherwise the micellae 

 would attract one another and exclude the water ; but whereas 

 the latter attractive force varies inversely as the square of the 

 distance, the former must vary inversely as some higher power. 

 Thus, if A represents the attraction between two micellae, 

 B represents the attraction between a micella and the water, 

 while D is the distance between two micellae, the limit of 



B A 



swelling up or imbibition will be reached when twjl.^ 'jj^ 



between any two neighbouring particles. B, however, will 

 diminish more rapidly as the distance increases. 



This theory of Nageli affords a satisfactory explanation of 

 the mode of growth of a cell-wall. It is easy to understand 

 that when the limit of extensibility is nearly reached — that is, 

 when the micellae of the membrane are separated as far as it is 

 possible for them to be — new micellae can be deposited in the 

 interstices, and the extended condition can be thus rendered per- 

 manent, and made a basis for further extension. This mode of 

 growth is commonly known as growth by intussusception^ or by 



