THE ALG^E 



103 



the top of the cell, a thick cellulose ring which, is attached to the 

 inner surface of the cell-ineinbraue. The division of the nucleus 

 follows, and between these new nuclei the division-wall arises. The 

 latter is not attached to the outer membrane of the cell, but is quite 

 free at the margin, and can therefore shift its position. The cellu- 

 lose ring at the top of the cell splits circularly, and is rapidly 

 drawn out into a cylinder, thus causing a sudden lengthening of 

 the cell. The division-wall is pushed up until it occupies the centre 

 of the divided cell, and then grows to the outer wall, completing the 

 division into the two cells. Of these cells, the lower has its lateral 

 walls composed of the original cell-wall, while the upper cell has its 

 much thinner wall derived mainly from the cylinder formed from 

 the cellulose ring. The small piece of the old wall above the ring is 

 evident as a little cap surrounding the upper end of the cell. This 

 process may be repeated several times, and the older cells of CEdogo- 

 nium show a series of these little caps, indicating the number of 

 times the cell has divided. 



Reproduction. Most of the filamentous Confervoideae may form 

 new individuals by the separation of portions of the plant-body, or 

 even by the separation of the individual cells in some cases. Most 

 commonly, however, 



A 



special reproductive 

 cells are produced. 



Zoospores. The 

 commonest forms of 

 non-sexual cells are 

 zoospores, or swarm- 

 spores, which arise 

 either singly, by the 

 escape of the whole 

 protoplast, as in (Edo- 

 gonium (Fig. 73), or 

 after a preliminary 

 division of the pro- 

 toplast into two or 

 more parts. The 

 swarm-spores may 

 escape through a 

 pore in the wall of the 



B 



FIG. 73. A, cell of (Edogonium sp., with a zoospore 

 about ready to escape. B, zoospore escaping from 

 the cell. C, free zoospore (X 500). D, formation of 

 zoospores in the germinating resting-spore of Bulbo- 

 ch&te intermedia (X 250) . (D, after PRINGSHEIM.) 



mother-cell, or the filament may break, so as to open the end of 

 the cell (Fig. 73). The escape of the swarm-spore from the mother- 

 cell is probably aided by the swelling of mucilage developed within 

 the mother-cell. 



The free swarm-spore has usually an oval form, with two or four 

 cilia (Fig. 71, E). Less frequently there is but a single cilium (Con- 



