THALLOPHYTA. 621 



Alliance II. — Cyanophyceae, the Blue-green Algae. 

 Families: Ghroococcacece, Nostocacece. 



Includes pigmented forms in which in addition to chlorophyll phycocyanin is 

 present, giving the cells a bluish, violet, or reddish tint. They occur in water or 

 in moist places, and their cells may be united together into aggregates of various 

 kinds. The cell- walls are usually mucilaginous, so that the cells or filaments cling 

 together in colonies, or they are inclosed in special sheaths. The simpler forms 

 included under the Chroococcacese are unicellular; the products of their division 

 may either remain united into colonies or become quite free from one another. 

 The rest are filamentous, and are included under the Nostocacese, whose filaments 

 can become segmented into small portions which move away by a peculiar motion 

 not yet fully understood (c/. vol. i. p. 40). At times also certain cells become 

 resting-spores and can endure climatic vicissitudes. They are widely dispersed 

 over the globe, and are met with in cold glacier-streams and have been found 

 living in hot springs at a temperature of even 85° 0. Some 800 living species are 

 distinguished. 



Ghroococcacece. — Includes the unicellular forms. Gloeocapsa (vol. i., fig. 25a, n, o) 

 forms little mucilaginous colonies, often found on the moist window-panes of hot- 

 houses. MerisTnopedia forms films on stagnant water, and Clathrocystis like 

 certain of the Nostocacese (alluded to below) arises in quantity in water. A 

 form probably referable to this group {Derm.oglcea LiTni) developed in 1874 in 

 such quantities off the Adriatic coasts as to seriously interfere with the fishing 

 industry. A commission was appointed to investigate the matter, but in six weeks 

 the Dermoglcea vanished as suddenly as it had appeared. 



NostocacecB are, for the most part, filamentous, though in some forms the cells 



may become isolated. Nostoc itself is common, and takes the form of irregular 



gelatinous colonies, which contain numerous interwoven necklace-like filaments. 



In some districts, owing to its sudden appearance after rain, it has received the 



name of " Falling Stars". This explains the allusion in the following lines from 



Dryden's CEdipus: — 



" The tapers of the gods, 

 The sun and moon, run down like waxen globes; 

 The shooting stars end all in purple Jellies, 

 And chaos is at hand ". 



A species common in China, iV. edule, is used as a thickening for soup, and an 

 allied form, Hormosiphon arcticus, abounds in the Arctic regions upon floating ice. 

 Anabcena Flos-aquce, Aphanizomenon Flos-aquce, &c., appear in fresh and brackish 

 water — sometimes in enormous quantities, and to considerable depths. The Tricho- 

 desmium Erythrceum — another of these " flowers of the sea " — referred to at vol. i. 

 p. 889, belongs also to this group. Very little is really known about the life- 

 histories of these interesting plants, which so frequently appear in great quantities 

 at or near the surface of the water and then as mysteriously disappear. But now 



