23] CHAPTER I. THE CELL. 73 



mineral constituents. Its growth, as well as its first formation, is 

 the result of the vital activity of the protoplasm ; it is, in fact, 

 formed from and by the protoplasm. 



1. 'The Growth of the Cell- Wall. The cell-wall grows in surface 

 and in thickness. 



a. The growth in surface of the cell-wall may take place in 

 either of two ways, both of which are, however, dependent upon 

 pressure exerted from within upon the wall. In the one case the 

 stretched wall grows continuously by means of material supplied 

 to it by the cytoplasm, the wall remaining unbroken. In the 

 other, the stretched wall is ruptured at certain parts, new portions 

 of cell-wall being at once intercalcated to close the gap. The 

 former is of more common occurrence : the latter has been observed 

 in some Algae, for instance in the growth of the cells of (Edo- 

 gonium, and in connexion with the apical growth and with the 

 development of lateral members in Caulerpa, Cladophora, and 

 Polysiphonia. 



Growth in surface takes place to such an extent that the volume 

 of the cell not infrequently becomes a hundred-fold greater than it 

 was originally. Thus, for instance, in a leaf still enclosed in a 

 leaf-bud, the cells of which it will consist when fully developed 

 are all actually present, and it is simply by their increase in 

 volume that the leaf attains its full size. 



In the comparatively rare cases in which the superficial growth 

 of the cell-wall is equal at all points, the cell preserves its original 

 form : but more commonly the cell-wall grows more vigorously 

 at certain points than at others ; thus, 

 for instance, a primarily spheroidal or 

 cuboidal cell may become tubular, cylin- 

 drical, fusiform, stellate, etc. 



b. The growth in thickness of the cell- 

 wall is effected by the deposition of sue- 

 cessive layers on the internal surface of 

 the first-formed layer. The cell-wall does 



FIG. 43. Ripe pollen-gram 



not usually begin to thicken until after of cichorium, intybus ; the ai- 

 Its growth in surface has ceased, the cell m 8t 8 P herica ; 8urf ^ e of th 



, ? cell-wall is furnished with 



having then attained its definite size ; but ridge-like projections pro- 

 cases of simultaneous growth in surface lon g ed int spines, and form- 



j . , i i ing a network. (After Sachs.) 



and iu thickness have been observed. 



The growth in thickness of the cell-wall is also rarely uniform ; 

 the cell-wall commonly becomes more thickened at some points 



