326 



Myxophycece 



ft Trichomes short, aggregated in parallel 

 bundles to form thin, feathery, plate- 

 like masses Aphanizomenon. 



itt Trichomes free; spores seriate, remote 

 from the heterocysts ; cells shorter 



than their diameter Nodularin. 



** Heterocysts terminal and the spores always 



contiguous to them Cylindrospermmn. 



Genus Nostoc Vaucher, 1803. The plants of this genus are 

 found as gelatinous masses of a rounded or expanded character, 



sometimes attached to a 

 substratum, at other times 

 floating freely in the water. 

 When young the plants 

 are almost always globose 

 or ellipsoid, but with ad- 

 vancing age many species 

 become exfoliated to form 

 flattened expansions, often 

 with lacerated margins. 

 The trichomes are much 

 contorted, forming an in- 

 tricate mass within the 

 gelatinous envelope, and 

 they are generally much 

 denser towards the ex- 

 terior. The sheaths are 

 never very distinct, and 

 often they have 



Fig. 141). A and B, Nostoc Liiickia Bornet, 

 from Ben Lawers, Perthshire; A, nat. size; 

 B, small portion of thallus, x 340. C, N. 

 cieruleinn Lyngbye, from B. Wharf e at Ilkley, 

 W. Yorks., nat. size. 



more 



become completely merged 



with the enveloping jelly. The cells may be spherical, barrel- 

 shaped, or cylindrical, and the heterocysts are either intercalary, 

 or in the young forms, terminal. The spores are spherical or 

 oblong, and are developed centrifugally in series between the 

 heterocysts 1 . 



Young Nostocs of different species greatly resemble each other. 

 They consist of a variously contorted trichome within a relatively 

 small, elongated, mucous investment, terminated at each end by a 

 heterocyst. 



1 Borzi in Gioruale Botan. Ital. 1878, x, p. 241. 



