202 



Tetrasporinese 



oblique divisions have very much the appearance of causing a fragmentation 

 of the cell (fig. 129 G). Sometimes the division-plane in Ankistrodesmus 

 is almost transverse (fig. 129 B c), and in Elakatothrias it is generally quite 

 transverse 1 . In Actinastrum the autospores diverge outwards but remain 

 attached by their proximal ends, and as a result of the formation of suc- 

 cessive generations of autospores the colonies may reach a comparatively 

 large size. In Selenastrum acuminatum, a species connecting the two genera 

 Selenastrum and Ankistrodesmus, and referred by Chodat to Scenedesmus, 

 the young cells attained maturity in from 15 to 20 days, at the end of 

 which period many of them again produced a new generation of auto- 

 spores (G. S. W., '12). 



Fig. 129. A, Ankistrodesmus falcatus (Corda) Ralfs. B and C, A. falcatus var. acicularis 

 (A. Br.) G. S. West; C shows division by oblique planes into four daughter-cells. D, 

 A. falcatus var. tumidus G. S. West. E, A. falcatus var. mirabilis G. S. West. F, A. 

 setigerus (Schrod.) G. S. West. G and H, A. Pfitzcri (Schrod.) G. S. West, the enveloping 

 mucus not shown. All x 520. auts, autospores ; p, pyrenoid. 



The coenobium of Scenedesmus is somewhat specialized, so that of all the 

 genera of the group Scenedesmus is the least representative. Scenedesmus 

 quadricauda possesses strong bristles attached to the end-cells of the coeno- 

 bium, and sometimes to the other cells also ; in S. denticulatus the poles of 

 all the cells are furnished with minute teeth ; in S. granulatus the walls are 

 granulated ; and in S. costatus and S. acutiformis the cells are longitudinally 

 ridged. It has been shown by Petersen ('11) that the cells of some species 

 of this genus are furnished with very delicate bristles (fig. 130 F). Chodat 



* 



1 Wille associates this Alga with Coccomyxa, but there is little doubt that it is most closely 

 allied to Ankistrodesmus, especially to those forms which are normally enveloped in mucus ; and 

 although the actual division-plane of the cell is transverse, the daughter-cells as often as not 

 grow by sliding obliquely past each other. Elakatotlirix gelatinosa occurs in the lakes of Norway 

 and the English Lake District (Wastwater). 



