190 Chlorophycece 



The two component parts of the cell-wall are sometimes evident 

 in the ordinary vegetative condition. The reproduction is similar 

 to that of the Chlamydomonadese. 



Genus Phacotus Perty, 1852. The cell-body is ovoid in form 

 and considerably flattened, so that when viewed from the side it is 

 relatively narrow and biconvex. The cell-wall is thick, dark in 

 colour, with a rough or rugulose exterior, and consists of two 

 valve-like pieces. The chloroplast is large and parietal, with one 

 or many pyrenoids. There is a clear space of some size at the 

 anterior end of the cell, and two long cilia are attached imme- 

 diately opposite. The reproduction takes place by the formation 

 of 2 8 zoogonidia in a vesicle which bursts apart the two halves 

 of the mother-cell-wall by a longitudinal split. 



P. lenticularis (Ehrenb.) Stein is not uncommon in stagnant water. 



Another genus of this sub-family is Pteromona* Seligo (1886) of which 

 there are seven or eight species known from continental Europe. No doubt 

 some of these occur in the lakes of the British Islands, but as yet there are 

 no records of them. 



Sub-family III. VOLVOCE.E. 



The plants of this sub-family are composed of a motile cosno- 

 bium of Chlamydomonadine cells, generally embedded in a 

 considerable mucous envelope, and sometimes connected by proto- 

 plasmic processes. Attached to each cell are two cilia which 

 project through the mucous coat and give the coenobium a rotatory 

 motion by their combined movements. The number of cells 

 present in a single cosnobium varies from four in one species of 

 Gonium to 22,000 in some forms of Volvox. In the lower forms 

 all the cells are equal and capable of reproducing the plant, but 

 in the higher forms there is a differentiation amongst the cells of 

 the coenobium, some of which are purely vegetative, whereas others 

 are solely reproductive cells. 



Reproduction in the lower forms takes place by the formation 

 of a daughter-coenobium from every cell of the mother-coenobium. 

 The daughter-cosnobium is formed within the wall of the mother- 

 cell, which swells up and ultimately sets the young colony free. 

 In the higher forms only certain of the reproductive cells, often 

 termed parthenogonidia, give rise to daughter-cosnobia. 



Sexual reproduction occurs in the lower forms by the union 

 of isogamous or heterogamous planogametes which arise by the 



