Selenastrese 



201 



Sub-family SELENASTRESE. The Alga? of this group are characterized 

 by the elongation of the cells, which are often very narrow with the ex- 

 tremities attenuated to fine points. They are frequently lunate or arcuate, 

 and may be solitary or associated to form colonies often of a more or less 

 fragile character (Actinastrum, fig. 130 .A and B\ Selenastrum, fig. 131), the 

 cells in some instances being held in position only by an envelope of mucus 

 (Ankistrodesmus Pfitzeri, Kirclineriella, Elakatothrix). In many species of 

 Ankistrodesmus, in Dactylococcus, Closteriopsis, and most forms of Scenedesmus 

 there is no enveloping mucus. The cell-wall is firm but delicate. Each cell 

 contains a large elongated chloroplast which often fills almost the entire cell. 

 In some forms pyrenoids do not occur, but in others one or more pyrenoids 



aut. 



Fig. 128. A, Scenedesmus oblh/uus (Turp.) Kiitz. B, the culture-state of S. obliquus formerly 

 known as Dactylococcus infusionum Nag. C, S. bijugatus (Turp.) Kiitz. D F, S. quadri- 

 cauda (Turp.) Breb. G, S. quadricauda var. horridus Eirchn. H, S. quadricauda var. 

 maximm W. & G. S. West. I K, S. denticulatus Lagerh. var. line arts Hansg. L, S. spicatus 

 W. & G. S. West. All x 520. aut, autocolonies. 



are invariably present. In the ubiquitous species Ankistrodesmus falcatus, 

 and especially in its var. acicularis, pyrenoids may be present in some indi- 

 viduals but not in others (fig. 129 B). Scenedesmus and Tetradesmus are 

 the only genera with a definite coenobium ; in the former the cells are 

 arranged side by side (with their long axes parallel) in one plane, and in 

 the latter side by side in two planes (Smith, '13). 



% Reproduction takes place by the formation of autospores or, in Scenedesmus 

 and Tetradesmus, of autocolonies (fig. 128 C and K). The division of the 

 parent-protoplast is in one or more oblique planes, often nearly longi- 

 tudinal, and results in the formation of 2, 4 or 8 daughter-cells. In those 

 cases where the wall of the mother-cell does not become mucilaginous the 



