Selenastreae 



203 



& Malinesco ('93) and also Grintzesco ('02) have done considerable experi- 

 mental work on Scenedesmus obliquus (= S. acutus), and have confirmed the 

 observations of others that this Alga has a Dactylococcus-sfate (fig. 128 B) 



Fig. 130. A and B, Actinastrum Hantzschii Lagerh. x 400 (after Lagerheim) ; B shows the 

 division of the mother-cells. C E, Desmatractum plicatttm W. & G. S. West, x 520. 

 F, Scenedesmus obliqwis (Turp.) Kiitz. var. ditnorphus (Turp.) llabenh., showing the bristles 

 at the extremities of the cells, x 730 (after Petersen). 



which is identical with Dactylococcus infusionum Nag. Grintzesco also 

 showed that remarkable malformations are produced by the culture of 

 Scenedesmus obliquus on a nutritive medium of agar and glucose, and that 

 this species can liquefy gelatin. He attributes the extensive geographical 

 distribution of this plant to the ease with which it adapts itself to different 

 media and different temperatures. 



The members of the Selenastrese usually occur in the smaller ponds and pools, where 

 they are often found in great quantity ; some of them are 

 also constituents of the freshwater plankton, especially species 

 of Kirchneriella, Closteriopsis, and Ankistrodesmus. Actinas- 

 trum (fig. 130 A and B} and Lauterborniella are found in the 

 helioplankton of pools, canals, etc. ; and Desmatractum (fig. 

 130 C E) occurs in the paddyfields of Ceylon. 



One species of Ankistrodesmus (A. nivalis) is a consti- 

 tuent of the red snow flora, and other forms occur in the 

 yellow snow. Kirchneriella is a genus which, although 

 occurring in small pools and ponds, is a characteristic con- 

 stituent of the summer plankton of certain lakes, its much- 

 curved cells enveloped in wide mucous investments being 

 very distinctive. Closteriopsis longissima is by far the most 

 elongated member of the Selenastrefe, attaining a length of 

 530 p. ; but there is some possibility that this Alga is a 

 degenerate form of the Desmid Closterium aciculare var. 

 subpronum (vide W. & G. S. W., '06), in which case its 

 inclusion in the Protococcales would clearly be incorrect. 



A number of imperfectly described species of Reinschiella are without doubt the 

 resting-cysts of certain of the freshwater Peridiniese. 



Fig. 131. A D, Selenasti-um 

 gracile Eeinsch. E G, S. 

 acuminatum Lagerh. All 

 x520. 



