NostocacecB 325 



heterocysts may be terminal or intercalary. The sheaths are very 

 delicate and invariably gelatinous in character. In some they are 

 more or less distinct, of variable diameter and closely adhering to 

 the trichomes; but in others they become confluent, forming a 

 watery mass of jelly which encloses a large number of trichomes. 

 It is in the genus Nostoc that this gelatinous mass becomes most 

 conspicuous. Whenever there is a fracture of a filament the 

 trichome and the sheath both break at the same point, and empty 

 sheaths, such as are commonly found in the Stigonemacea? and 

 Scytonemacese, are never met with. The trichomes are sometimes 

 straight and rigid, but they may be contorted to form densely 

 interwoven masses. 



Reproduction occurs by the formation of hormogones and 

 spores. The former are produced by the fragmentation of fully 

 grown trichomes, and in some genera they are set free by the 

 dissolution of the enveloping mucilage. Spores of a spherical, 

 ellipsoidal, or cylindrical form are developed from the vegetative 

 cells, and invariably arise in relation to the position of the hetero- 

 cysts. They may be developed singly or in series, and by reason 

 of their definite form and position they furnish a character of 

 primary importance in the discrimination of genera and species. 



About half the species of Nostoc and Cylindrospermum are 

 terrestrial in habit, but all the other members of the family are 

 aquatic. Some species of Nostoc, and also of Andbcena, are 

 endophytic in various parts of such plants as Azolla, Cycas, Lemna, 

 Sphagnum, Anthoceros, etc., and other species of Nostoc are con- 

 stituents of the thallus of many Lichens, especially those of the 

 CollemaceaB. Many species of Anabcena are a marked feature of 

 the freshwater plankton, and certain species of Nostoc are used as 

 food by the inhabitants of divers parts of the world. 



A. Trichomes flexuose and contorted within a defi- 



nite gelatinous investment Nostoc. 



B. Trichomes more or less straight, free, or forming 



a thin mucous stratum. 

 * Heterocysts and spores intercalary. 



t Trichomes nude, or with a thin mucous 

 sheath, free, or aggregated without 

 order to form a flocculent mass ; cells 

 equal or longer than their diameter; 

 spores solitary, geminate, or in short 

 series -1 ndbaena. 



