HOW TO KNOW THE FRESH-WATER ALGAE 



Figure 136 



196a Cells barrel-shaped, with a slight notch-like median incision in 

 the broadest part of the cell where it is somewhat bulged. Fig. 

 136 GYMNOZYGA 



Fig. 136. Gymnozyga moniliformis Ehr. 

 There are 3 species of this genus 

 found in the United States, differenti- 

 ated mostly on the size and propor- 

 tions of the cell, but none are as com- 

 mon as the one illustrated. This species 

 occurs sometimes almost pure in pools within Sphagnum bogs, and is 

 a common component of the desmid flora of almost any soft or acid 

 water habitat. G. moniliformis is characterized by having barrel-shaped 

 cells that have longitudinal striations in the apical portion of the 

 semicells. These are sometimes faintly seen, especially in living cells 

 when the chloroplast obscures them. 



196b Cells cylindrical or somewhat rectangular, with a broad and 

 shallow emargination rather than an incision in the midregion. 



Fig. 137 HYALOTHECA 



(See also Desmidium Bailey i. Fig. 138b.) 

 Fig. 137. Hyalotheca dissiliens (Smith) 

 Breb. 



There are 3 or 4 common species of this 

 genus, differentiated by cell shape and 

 Figure 137 proportion, some being short and nearly 



quadrate whereas others are cylindrical. 

 In some individuals the constriction of the cell occurs only as a shallow 

 invagination in the median part. In Sphagnum bogs small pockets of 

 water or pools in the mat may be densely green with a pure growth 

 of H. mucosa (Dill.) Ehr., a species which has a conspicuous gelatin- 

 ous sheath. 



197a Cells wider than long or as wide as long, without a median in- 

 cision or with but a slight median notch; walls at the poles of 



young semicells infolded or replicate . Fig. 138 DESMIDIUM 



Fig. 138. a, Desmidium Gievillii 

 (Kuetz.) De Bary; b, D. Baileyi 

 (Ralfs) Nordst. 



Cells of this filamentous desmid 

 genus vary much in shape. Some 

 are oval and moniliform when 

 seen in end view, some are tri- 

 angular, and some are quadran- 

 gular. The shape of the cell may 

 be determined by careful focusing 

 up and down through the depth of a specimen. A characteristic habit 

 of some species is to show a spiral twisting of the cell arrangement 

 so that in any one view they do not have their processes in the same 

 plane throughout the length of the filament. Desmidium usually oc- 

 curs in the same habitats with Hyalotheca (Fig. 137). 



90 



Figure 138 



