THE FRESH-WATER ALGAE 



141 



53 (52) Colony a compact cushion; 



stalks encrusted with lime. 



Oocardium Nageli. 



Cells broad, middle constriction slight, chromatophores two, 

 pyrenoid in each. Stalks closely placed so that the enveloping 

 cylindrical lime sheaths make a honeycomb-like structure. 



They are sometimes branched and imbedded in the free end 

 of each is a single cell, placed transversely. It occurs where 

 water trickles over limestone rocks, and is also reported as 

 being found in mountain streams. 



Fig. 



133. Oocardium stratum Nageli. X 485. Portion of figure. 

 (After Senn.) 



54 (49) Cells with spines 55 



55 (56) Two or four spines on each cell-half . . . Arthrodesmus Ehrenberg. 



General characteristics as in Cosmarium, except that each cell-half is fur- 

 nished with two or four long spines, and the end view shows no lateral 

 rounded prominences. 



The spines in Arthrodesmus are all arranged in one plane, while in Xanlhid- 

 iutn they may be arranged in two planes. 



Fig. 134. Arthrodesmus convergens Ehrenberg. X about 250. (Original.) 



56 (55) Two rows of strong spines on each cell-half 57 



57 (58) Spines simple Xanthidium Ehrenberg. 



Cells oval or nearly round, with deep, narrow, central constriction; end view 

 elliptical, often with protruding sides; membrane with two rows of strong, 

 horn-like spines; chromatophore parietal, more or less divided, with several 

 pyrenoids. 



As in Arthrodesmus the presence and the nature of the spines distin- 

 guish the genus from certain species of Cosmarium. 



Fig. 135. Xanthidium fasciculaium Ehrenberg. X about 300. (Original.) 



58 (57) Spines branched Schizocanthum Lundell. 



Characteristics similar to those of Xanthidium, except that the 

 spines are thick, short, and branched at the ends. 



West believes that Schizocanthum should be included under 

 Xanthidium as the only difference is in the spines, and there is too 

 much variation in these, he thinks, to make separate genera. 



Fig. 136. Schizocanthum armatum Lundell. X 106. (After Wood.) 



59 (3) Plant filamentous, cylindrical, only one individual originating from 

 a germinating zygospore . . Family Zygnemaceae . . 60 



Cells cylindrical, united into filaments, usually found near the surface of the water. Chro- 

 matophores different in different genera, but all with several pyrenoids. Reproduction .sexual, 

 occurring by the conjugation of cells in two parallel filaments, ladder-like, or lateral, between two 

 neighboring cells of the same filament. Parthenogenesis may occur. 



