24.] CHAPTER I. THE CELL. 103 



are to be found generally in the zoospores, zoogonidia, and planogametes of the 

 Alga?, as also in those of the oogamous Phycomycetes and of the Myxomycetes 

 among Fungi. 



These peculiar structures, the hyaline area, the cilia, and the contractile 

 vacuole, are not confined to isolated free-swimming cells, but are also character- 

 istic of cells forming part of a multicellular motile body (e.g. Volvocaceoc). 



24. The Cell-Wall is a non-protoplasmic membrane con- 

 sisting, at least at its first formation, of an organic substance 

 termed cellulose, of water, and of a small proportion of inorganic 

 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 intercalated to close the gap. The 

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

 in some Alga?, for instance, in the growth of the cells of QEdo- 

 gonium, and in connexion with the apical growth and with 

 the development of lateral members in Caulerpa, Cladophora, 

 Bryopsis, Derbesia, 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, cylindrical, fusi- 

 form, stellate, etc. 



6. The growth in thickness of the cell- wall is effected by the 

 deposition of successive layers on the internal surface of the first- 

 formed layer. The cell- wall does not usually begin to thicken 



