GROWTH. 337 



as well as the growth in thickness, of cell-walls to apposition. 

 The distension of a cell-wall due to turgidity is rendered 

 permanent, not by the intercalation of fresh particles of cellu- 

 lose, but by the deposition of a layer of cellulose upon the 

 internal surface of the one already existing. So long as the 

 cell is turgid, this deposition of fresh layers of cellulose does 

 not produce any considerable thickening of the cell-wall 

 because, in consequence of the tension to which they are 

 subjected, the layers are very thin ; it is only when the cell 

 has ceased to be turgid and has ceased to grow that the 

 thickness of the cell-wall begins to increase materially 

 (p. 291). 



We will content ourselves with this brief statement of the 

 two most important views as to the growth of cell-walls, for it 

 would lead us too far were we to attempt to discuss the vast 

 mass of facts and of observations upon which they are based. 

 It is also impossible at this stage of the development of the 

 subject to pronounce definitely for the one or for the other. 

 Still, inasmuch as it has now been conclusively proved that 

 the growth of starch-grains (p. 181) and the increase in thick- 

 ness of cell-walls takes place by apposition, it is not improbable 

 that the growth in area of cell-walls is effected in the same way. 



In some cases the growth in area of the cell- wall is certainly not 

 effected by intussusception, but by a modified form of apposition ; for 

 instance, the intercalary growth of the cell-walls in CEdogonium. 



It has been already pointed out that the growth of the 

 protoplasm in a cell is commonly small as compared with the 

 growth of the cell-wall. We know so little about the struc- 

 ture of protoplasm that it is impossible at present to give any 

 account of its mode of growth. It appears, however, that its 

 growth is interstitial, that is, that growth goes on throughout 

 its substance. 



V. 22 



