HOW TO KNOW THE FRESH-WATER ALGAE 



319a Cells spherical. Fig. 224 BOTRYDIOPS1S 



Fig. 224. Botrydiopsis arhiza Borzi. 



In the same habitats where Ophiocytium 

 (Fig. 219) occurs one may usually find this 

 species; a solitary, free-floating cell which 

 is either spherical or spheroidal. Small cells, 

 when young, may contain but a single chro- 

 matophore, but in age develop many parietal, 

 yellowish-green bodies. Another species, B. 

 eriensis Snow, is larger and less commonly 

 seen. Apparently it is a truly planktonic 

 species whereas B. arhiza occurs in shallow 

 water (tychoplankton). 



Figure 224 



319b Cells broadly ovoid or pear-shaped. Fig. 225 LEUVENIA 



Fig. 225. Leuvenia natans Gardner. (Redrawn 

 from Smith). 



Although essentially unicellular, this spe- 

 cies occurs in a dense film at the surface of 

 a quiet pond. Young cells are spherical and 

 have but 1 or 2 chromatophores, whereas 

 older cells become pear-shaped or ovate, 

 and have numerous yellow-green chromato- 

 phores. There is but 1 species in the genus 

 and it seems to be rare, having been reported 

 only from California in this country. 



Figure 225 



320a (316) Cells rectangular, with a spine at each corner. Fig. 226. 

 PSEUDOTETRAEDRON 



Fig. 226. Pseudotetraedron neglectum Pascher. 



This rectangular cell with a spine at each cor- 

 ner clearly shows the chrysophycean character in 

 its 2-parted wall, the sections overlapping in the 

 midregion of the cell. This can be seen only when 

 the cells are turned on their 'side' of course. In 

 end view the cells are narowly elliptic. There are 

 several yellow-green chromatophores and oil bodies 

 as food reserve. The only species has been re- 

 ported but rarely from the United States. 



Figure 226 



137 



