THE ALGAE 



75 



and it is only when the cell is in a suitable position that the lobing of the 

 chloroplast can be seen. There is no pyrenoid present. 



Reproduction 



Reproduction is effected entirely by vegetative means ; by simple division 

 followed by the separation of the cells. There is no evidence that this species 



Fig. 57. — Pleurococcus tiaegelii. Various cell groupings 

 showing the tendency to filament formation. (After 

 FritscJi.) 



possesses any special resting stage, for the ordinary cells are able to withstand 

 long periods of desiccation without injury. The Alga can obtain what water 

 is required directly from atmospheric moisture. 



Accounts have not infrequently been given of a motile stage closely 

 resembling Chlamydomonas in appearance, with two apical flagella. Recent 

 work indicates that such organisms are not part of the life-history of Pleuro- 

 coccus but belong to other genera. 



Coleochaete pulvinata 



This Alga occurs not uncommonly in this country attached to the sub- 

 merged leaves of water plants such as Sparganium and Callitriche, or to the 

 fronds of Lemna. The thallus (Fig. 58) is green and heterotrichous, con- 

 sisting of a flat prostrate part from which vertically growing filaments arise 

 forming a cushion. Growth of the vertical filaments is apical, and of the 

 prostrate part marginal. 



The cells are all uninucleate and possess a single, large parietal chloroplast 

 of irregular shape, with one or two prominent pyrenoids. Usually every cell 

 bears a characteristically sheathed bristle which begins with the development 

 of a pore in the outer cell wall, which is followed by the secretion of a 

 membrane forming a tubular sheath extended outwards through the pore 



