170 THE CELL 



That such a stretching of the cellulose membrane does actually 

 occur is proved by several phenomena. The turgescence already 

 mentioned causes distension. When a cell is plasmolysed it at 

 first contracts somewhat as a whole, in consequence of the loss of 

 water, before the outer layer of the protoplasm becomes separated 

 from the cell-wall. This indicates that it was subjected to in- 

 ternal pressure. It may be observed in many Algce, that the cellu- 

 lose lamellae, which are first formed, are eventually ruptured by 

 the stretching, and discarded (Rivularia, Glceocapsa, Schizochlamys 

 gelatinosa, etc). Each distension and contraction must be con- 

 nected with a change of position of the most minute particles, 

 which become located either on the surface or in the deeper layers. 



Thus the way in which a membrane increases in size when 

 stretched offers many points of resemblance to growth by intus- 

 susception. The difference consists in this, that in the first case 

 particles of cellulose already present are deposited in the surface, 

 whilst in the second case particles in process of formation are so 

 deposited. 



However, I do not wish to totally disregard growth through 

 intussusception, as Strasburger formerly did (V. 31). On the 

 contrary, I consider it to form, in addition to apposition, a second 

 important factor in the formation of the cell-wall, although it is 

 certainly not the only factor, as is dogmatically stated in Nageli's 

 theory. 



Many phenomena in cell-growth may be most easily explained 

 by means of intussusception, as has been done by Nageli, whilst 

 the apposition theory presents numerous difficulties. 



It does not often occur that the cell-wall becomes ruptured by 

 stretching, and yet the increase in size which occurs in nearly all 

 cells from their initial formation until their full growth, is quite 

 out of proportion to the elasticity of the cell- wall, which, as it is 

 composed of cellulose, cannot be assumed to be very great. Many 

 plant cells grow until they are a hundred or even two hundred 

 times as long as they were originally (Ghara). 



The fact that many cells are xery irregular in form would be 

 very difficult to explain if the cell membrane were considered to 

 increase superficially solely by stretching, like an indiarubber 

 bladder. For example, Caulerpa, Acetabularia, etc., are apparently 

 differentiated, like multicellular plants, into root-like, stem-like, 

 and leaf-like structures, although each plant consists of only a 

 single cell-cavity. The growth of each of these parts proceeds 



