CYANOPHYCEAE 427 



light, and is known as phycocyanin. This colour is, in a few cases, re- 

 placed by a red, and, in some Scytonemaceae, by a brown endochrome, 

 the scytonemin of Nageli. 



Although the Cyanophyceae are described as unicellular, this term 

 must be used with some limitation in the sense employed with respect 

 to the Protococcoideae, in comparison with which some of the blue- 

 green algae display greater differentiation in their vegetative structure. 

 In several families, notably the Oscillariaceae and Rivulariaceae, the 

 protoplasm of the very long filament is broken up transversely into a 

 large number of imperfect cells known as pseudocysts, which are, how- 

 ever, never invested with a true cellulose cell-wall. In the Nostocaceae, 

 the breaking up of the filament into cells is much more complete ; but 

 even here it is doubtful whether the very thin membrane in which they 

 are invested is composed of true cellulose. It displays the strongest 

 tendency to deliquesce into a hyaline jelly, and, not unfrequently, as in 

 the Scytonemaceae and some Nostocaceae, this gelatinous envelope 

 becomes strongly coloured. Chromatophores with enclosed pyrenoids 

 and nuclei are stated to have been detected in some species ; while 

 other authorities assert that neither starch nor a true nucleus is found 

 in the Cyanophyceae. 



Multiplication by ciliated zoospores is, with a few doubtful excep- 

 tions, unknown in the Cyanophyceae. The ordinary mode of propaga- 

 tion in the lower families is by simple cell-division. Quiescent resting- 

 spores or cysts, with thicker cell- walls, are also produced in several 

 families. In the filiform orders, portions of the filament known as 

 hormogones, consisting of a varying number of pseudocysts, become 

 detached ; these hormogones are endowed for a time with a power of 

 spontaneous motion, after which they come to rest, and develop into 

 new filaments. In the Oscillariaceae this power of motion extends to 

 the entire individual. In the Rivulariaceae, Scytonemaceae, and Nosto- 

 caceae, a further differentiation is exhibited, of special cells within the 

 filament known as heterocysts ; but their function is unknown. 



The life-history of many of the Cyanophyceae is still but imperfectly 

 known, and their classification into families is very obscure. Some of 

 those who have most closely investigated their structure and develop- 

 ment are of opinion that a large number of the apparently distinct forms 

 are but stages in the development of the same pleomorphic organism, 

 often really belonging to a higher type. This will be further discussed 

 under the separate families. They may be conveniently divided into 

 two groups : the Chroococcacece, which are strictly unicellular, and the 

 Nostochinea, embracing those families in which the aggregation of cells 

 or pseudocysts assumes more or less of a filiform character. 



