Figure 1 1 1 



HOW TO KNOW THE FRESH-WATER ALGAE 



148a Cells with a shallow and broad, or a deep and narrow notch in 

 the apex of the semicell. Fig. Ill EUASTRUM 



Fig. 111. a, Euastrum pinnotum Ralfs; b, 

 E. pectinatum var. inevolutum West & 

 West. 



There are numerous species in this 

 ydesmid genus, varying greatly in size 

 and shape of cell. Most of them, how- 

 ever, have a polar notch in the semi- 

 cells and, characteristic of the genus, 

 have more or less prominent protru- 

 sions and swellings on the face of the 

 semicell. In filled cells the latter are 

 difficult of determination, especially in the smaller species. The speci- 

 mens need to be rolled so that they can be seen from the side or top 

 when making microscopic examinations. Nearly all of the species of 

 Euastrum are limited to an acid habitat. 



148b Cells without a notch in the apex of the semicell 149 



149a Apex of the cell extended into 2 or more arms or lobes, the arms 

 usually extending radiately so that the cells are star-shaped or 



triangular when seen from the top (end view). Fig. 112 



STAURASTRUM 



Fig. 112. a, Staurastrum rotula 

 Nordst., 'front' or side view; b, 

 end view; c, St. cornutum Arch.; 

 d, end view. 



This is a large desmid genus 

 with several hundred species 

 which vary in shape of semicell 

 and type of decoration on the 

 wall. The chief distinguishing 

 characteristic is the extension of 

 lobes or arms in at least 3 planes 

 so that the cell appears radiate 

 when seen from the top. Many species appear like Cosmarium (Fig. 

 113) when seen in front view and one needs to change the plane of 

 focus in order to see the semicell extending up toward the observer, 

 or down as the case may be. A few species have arms in 1 plane 

 only and these are retained in the genus by virtue of the fact that the 

 shape and decorations of the arms (spines, verrucae, etc.) are those of 

 Staurastrum. Some species are definitely euplanktonic and have long 

 arms which give them buoyancy, whereas others are tychoplanktonic 

 and are intermingled with other desmids in acid swamps. 

 149b Apex of semicell not extended into arms, or if with arms these 

 not radiating in 3 planes 150 



Figure 1 1 2 



75 



