Protococcacece 



225 



diameter, and are generally attached by their apices to form 

 radiating colonies of small size. Each cell possesses a parietal 

 chloroplast furnished with a pyrenoid. Multiplication takes place 

 by the longitudinal division of the cell-contents, with the occasional 

 formation of another transverse wall. The products of division 

 diverge outwards, but remain attached by their proximal ends, 

 and the colonies sometimes reach a relatively large size owing to 

 the repeated new divisions of the contents of the radiating cells. 

 As a general rule four young cells (autospores) are produced in 

 each mother-cell. 



A. Hantzschii Lagerh. is a very rare Alga in Britain and is confined 

 principally to the plankton. Length of cells 10 24 p, ; breadth 3 6 p.. 



Genus Selenastrum Reinsch, 1867. In this genus the cells 

 are arcuate or lunate, attenuated to 

 fine points, and arranged back to back 

 to form a 4- or 8-celled colony of small 

 size. The cell-wall is thin and firm, 

 and the chloroplast is without a pyre- 

 noid. The multiplication is by auto- 

 spores which arise as in Ankistrodesmus 

 and Kirchneriella. 



The species of this genus are very un- 

 common, usually occurring amongst other 

 water-plants at the margins of ponds and 

 lakes. S. Bibraianum Reinsch, S. gracile 

 Reinsch (fig. 95 A D) and S. acuminatum 

 Lagerh. (fig. 95 E G) all occur in Britain. 

 Chodat considers the latter species to be a 

 Scenedesmus, but that I cannot agree with. 



Fig. 95. A D, Selenastrum 

 gracile Keinsch; A C, from 

 near Settle, W. Yorks.; D, 

 from Puttenham Common, 

 Surrey. E G, S. acumina- 

 tum Lagerh., from Bowness, 

 Westmoreland. (All x520.) 



Genus Kirchneriella Schmidle, 

 1893. [Indus. Selenoderma Bohlin, 1897.] The cells are arcuate 

 or crescent-shaped, attenuated or subcylindrical, often bent until 

 the apices almost touch each other ; they are loosely aggregated, 

 without any definite disposition, within a large enveloping mass of 

 jelly. The cell- wall is very thin and the chloroplast is parietal, 

 being situated on the convex wall of the cell. There is one 

 pyrenoid, but it is frequently absent. The multiplication is by 

 autospores, four or eight of which are produced in a mother-cell 

 by oblique or more or less transverse divisions. The genus differs 

 from Selenastrum in the loosely aggregated colonies of cells, which 



w. A. 15 



