CYANOPHYCEJE 433 



float freely on the surface of the water, solitary or in interwoven masses, 

 forming but little mucilage, and destitute of heterocysts. 



Hansgirg (Bot. Centralbl., xxii. and xxiii., 1885) regards the genera 

 Nostoc, Anabaena, Cylindrospermum, and Sphoerozyga as stages of 

 development — analogous to certain zoogloea-conditions of the Schizo- 

 mycetes — of various species belonging to the Oscillariaceae, Rivulariaceas, 

 and Scytonemaceae. The relationship of Nostoc with Drilosiphon will 

 be alluded to under this last family (see p. 440). The Nostocaceae appear 

 also to approach the Schizomycetes through Leuconostoc (Van Tiegh.). 



Among the Nostocaceae are included, in addition to the genera 

 already mentioned, Nodularia (Mert)., Cylindrospermum (Ktz.), Spha^- 

 rozyga (Ag.), and Aulosira (Kirch.). 



Literature. 



Meneghini - Monogr. Nostoc. Ital., 1843. 



Thuret— Mem. Soc. Sc. Nat. Cherbourg, 1857, p. 23; Ann. Sc. Nat., 1875, p. 1)T^-. 



Janczewski — Ann. Sc. Nat., 1874, p. 123. 



Bornet and Thuret— Notes Algol., fasc. i. , pp. 2-3 ; fasc. ii., pp. 78- 1 32. 



Borzl— Flora, 1878, p. 465; and Nuov. Giorn. Bot. Ital., 1S78, p. 236. 



Fischer — Beitr. zur Kenntniss der Nostocaceen, 1853. 



Order 2. — Rivulariace^. 



In the Rivulariacete — including also the Calotrichaceae — the cells 

 or pseudocysts of which each filament is composed are not connected 

 together in a necklace-like form, but constitute a continuous thread 

 divided by transverse septa which are exceedingly thin and often 

 scarcely perceptible. Many of the species are extremely minute, the 

 individual filaments being quite microscopic, and grow attached in tufts 

 to a solid substratum, some water-plant or floating body, from which they 

 radiate in a star-shaped manner, forming small green discs or cushions, 

 often imbedded in copious mucilage. Each filament displays a distinct 

 differentiation of the two apices, the distal extremity being elongated and 

 attenuated into a hyaline hair, while at the base a portion is marked off 

 into a more or less globular colourless basilar cell, the rudimentary cell- 

 division being exhibited by the portion intermediate between the basilar 

 cell and the terminal hvaline hair. The terminal hair is, accordins: to 

 Gomont, in perfect continuity with the rest of the true membrane of the 

 filament, and is distinguished only by having fewer transverse septa, and 

 by the entire absence of granular protoplasm. The outer layers of the 

 walls of the filaments have a very strong tendency to become transformed 

 into mucilage, from which is formed not only the copious jelly in which 



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