rectangular, constricted at the cross walls. Heterocysts cylindric, 

 usually but one (rarely 2) in each trichome. Gonidia cyhndrical, 

 truncate at the apices; only one in each trichome; located in the 

 median region but not adjacent to the heterocyst. 



Aphanizomenon flos-aquae ( L. ) Ralfs 1850, p. 340 

 PL 122, Figs. 6-8 



Trichomes parallel, tapering at both ends; united in bundles or 

 flakes to form macroscopic colonies of few or hundreds of plants. 

 Cells 5-6/x in diameter, 8-12/x long. Heterocysts oblong or cylindrical; 

 scattered in the midregion of the trichome; l/x in diameter, 12-20/i, 

 long. Gonidia cylindrical; formed near the middle of the trichome 

 but not adjacent to the heterocyst; 8//, in diameter, 60-75)u, long. 



This plant is a frequent component of water blooms and in favor- 

 able habitats may become super-abundant. Hard water lakes in 

 which there is a high nitrogen content and an adequate supply of 

 carbon dioxide, either free or available in half-bound carbonates, 

 may become biologically unbalanced by excessive growth of this 

 plant. The cells have pseudovacuoles which permit the trichomes 

 to float high in the water, where they form sticky masses, that are 

 sometimes many square feet in extent. Either alone or in accompani- 

 ment with Microcystis, aeruginosa and Anahaena spiroides, this 

 plant is not infrequently responsible for oxygen depletion in small 

 lakes and bays, resulting in great loss of fish. The occurrence of this 

 species is so consistently related to hard water lakes that it may be 

 used as an index organism for high pH, and usually a high nitrogen 

 and carbonate content (especially when the plant appears as a 

 water bloom). Aphanizomenon fios-aquae is rarely found except in 

 eutrophic lakes or in polluted, hard water, slow-flowing streams. An 

 exception to this was found in Rahr Lake, Vilas County, Wisconsin, 

 where there was a visible, although not abundant, bloom in August, 

 Rahr Lake is a semi-hard water body with an acid marginal mat. 

 This species may remain alive all winter in the vegetative state, 

 sometimes thriving under ice; as with Oscillatoria rubescens, there 

 is some evidence that such growths bring about depletion of oxygen 

 in shallow lakes, poorly illuminated because of coverage by ice and 

 snow. The blue-green algae are poor oxygenators in any case. Also 

 the gonidia may carry the plant over a period of unfavorable en- 

 vironmental conditions. Their germination and the relation of the 

 gonidium to bundle-formation have been carefully studied under 

 laboratory conditions by Rose (1934). 

 Mich., Wis. 



[528] 



