gPZLI 1 ucr 



dcz i i i i i i 1 1 i i i nanxi 



Figure 298 



HOW TO KNOW THE FRESH-WATER ALGAE 



412a Trichomes parallel, forming a free-floating flake or bundle, con- 

 taining a single heterocyst and a spore near the middle of each 

 trichome. Fig. 298 APHAN1ZOMENON 



Fig. 298. Aphanizomenon 

 fios-aquae (L.) Ralfs. a, 

 diagram of colony; b, 

 a few trichomes from 

 the colony; c-d, tri- 

 chomes in detail show- 

 ing medially located 

 cylindrical akinete. 



This is the only com- 

 mon species reported 

 from this country and is 

 very widely distributed in lakes which are rich in nitrogen and phos- 

 phorus. The trichomes He in parallel bundles and form flakes of 

 macroscopic size. Because of the gas vacuoles (pseudovacuoles) the 

 plants float high in the water and form surface scums and mats. 

 Hence they are able to cause serious trouble in lakes and reservoirs 

 used for water supplies and pleasure resorts. During summer months 

 the species may develop a "bloom" condition and be so abundant as 

 to give the water the appearance of "pea soup." Considerable eco- 

 nomic loss is suffered as a result of the disturbance caused by Aphani- 

 zomenon when it leads to the death of fish. It is a plant that usually 

 accompanies human settlement about lakes and rarely is the plant 

 found in abundance in lakes remote from habitation. 



412b Trichomes not parallel, or if so, forming indefinitely shaped flakes. 

 See Fig. 271 (in part) ANABAENA 



413a (410) Trichomes planktonic, solitary 414 



413b Trichomes colonial, in a gelatinous mass 415 



414a Vegetative cells and heterocysts disc-shaped, wider than long. 

 Fig. 299 NODULARIA 



IBHI 



c^^ SrooQJMxPGairo ^c^; 



Fig. 299. 



Figure 299 



Nodularia spumigena Mert. 



Filaments of this species are at once distinguishable by the very 

 short, compressed vegetative cells and heterocysts. The sheath is 

 rather thin and mucilaginous and sometimes is not immediately ap- 

 parent. The species illustrated is the most common of the 4 that are 

 reported from the United States. Plants of this genus are usually 

 found intermingled with miscellaneous algae from the tychoplankton. 



172 



