HOW TO KNOW THE FRESH-WATER ALGAE 



436a. Cells arranged in rectilinear series. See Fig. 313 



MERISMOPED1A 



436b Cells irregularly arranged. Fig. 317 HOLOPEDIUM 





(0 o 



■;QO Q O0'OO«O«o o °o 



ooo 



o o 



°o* 



OOO On 8 W 



o°°; 



Fig. 317. Holopedium iriegulare Lag. 



This genus differs from Merismopedia by- 

 having the cells irregularly arranged with- 

 in a gelatinous plate. Only this species 

 has been reported for the genus from the 

 United States. 



Figure 31 7 



437a (435) Colony globular, rather definite in shape 438 



437b Colony irregular in outline 440 



438a Cells very numerous and crowded within the colonial mucilage 

 (usually showing false vacuoles which refract the light so that the 



cells appear brownish, black or purplish). Fig. 318 



MICROCYSTIS 



Fig. 318. a, Microcystis Hos-aquae 

 (Wittr.) Kirch.; b, M. aeruginosa 

 Kuetz. emend Elenkin. 



&g|s|||^ 4%. «S^ iBk The marble-like cells of this 



genus are closely compacted and 

 '*VJV irregularly arranged in definite- 



ly shaped but irregular colonies 

 W% inclosed in mucilage. M. Hos- 



aquae has more nearly globular 

 and symmetrically shaped colo- 

 b nies than M. aeruginosa. The cells 



Figure 318 often contain pseudovacuoles (gas 



pockets) and float high in the 

 water. Hence they produce surface scums and like Aphanizomenon 

 (Fig. 298) cause a great deal of disturbance in lakes and reservoirs. 

 Dense growths may lead indirectly to the death of fish through suf- 

 focation. It is rather curious that where these species occur (especial- 

 ly M. aeruginosa) the water is completely dominated by the plant to 

 the exclusion of almost all other forms. It has often been noted that a 

 lake may be densely overgrown with either Microcystis ox with Aphani- 

 zomenon, but not the two together. There are several species of the 

 genus differentiated by size and by details of the sheath structure, and 

 by form of colony. 



181 



