THE VITAL PROPERTIES OF THE CELL 169 



means of the protoplasm, and, like the lime and silica particles, 

 are deposited as an insoluble modification in the cellulose mem- 

 brane, or both substances originate on the spot, through a chemical 

 transformation of the cellulose. This is another problem which 

 must be decided by means of physiological chemistry rather than 

 through morphological investigations (vide p. 153). 



The question as to how the cell membrane grows is a very im- 

 portant problem, and has led to much discussion ; it is very diffi- 

 cult to come to any decision on the subject. Two methods of 

 growth may be distinguished, a superficial and an interstitial 

 method. The delicate cellulose coating, which at first is scarcely 

 measureable, may by degrees attain a very considerable thickness, 

 growing by the addition of numerous laminae, the number of which 

 varies with the thickness. It is most probable that layer after 

 layer is deposited by the protoplasm of the outer layer which was 

 at first differentiated off. This method of growth is termed 

 " growth by apposition," in contradistinction to " growth by in- 

 tussusception," which, according to Nageli, is the way in which 

 the cell-wall grows, that is to say, by deposition of particles in 

 the interstices between the particles already present. 



The apposition theory is supported by the following three ob- 

 servations : (1) Before the ridge-like thickenings are formed upon 

 the inner surface of a cell-wall, the protoplasm is seen to collect 

 together at those places, where thickening of the wall is about to 

 occur, in masses, which exhibit active streaming movements. (2) 

 When, in consequence of plasmolysis, the protoplasmic body has 

 receded from the cell- wall, a new cellulose membrane is seen to 

 appear on its naked surface (Klebs IV. 14). If the plasmolysing 

 agent be removed, and the cell-body be made to increase in size by 

 the absorption of water, so that its new cellulose membrane comes 

 into close contact with the original cell-wall, they unite with one 

 another. (3) When a plant cell divides, it may often be plainly 

 seen that each daughter cell surrounds itself with a new wall of 

 its own, so that the two newly-formed walls of the daughter-cells 

 are enclosed by the old wall of the mother-cell. 



It is more difficult to explain the growth in superficial area of 

 the cell-wall. This may be effected by two different processes, 

 working either singly or in unison. The membrane may become 

 stretched, like an elastic ball which is inflated with air ; or it 

 may grow by intussusception, that is to say, by the deposition of 

 new cellulose particles between the old ones. 



