CLASSIFICATION 



81 



residue, after the phycocyanin is extracted, yields a green solution 

 if treated with alcohol, but this solution is usually tinged with 

 brown on account of the mixture with phycoxanthin. The phyco- 

 cyanin may be precipitated in the form of blue crystals by the 

 action of ammonium-sulphate. 



Vacuoles. Vacuoles are usually absent from the vegetative cells, 

 but occur in the heterocysts. Vacuoles filled with gas, which ren- 

 ders the cells buoyant, are found in those species which float at the 

 surface of the water. 



Differentiation of the Plant-body 



The lowest of the Schizophycese, the Chroococcaceae, are strictly 

 unicellular forms, which live either isolated or in colonies. The 

 cells are globular, and separate 

 on division, although secondary 

 divisions may follow before the 

 cells have become completely 

 rounded off. The cell-wall is 

 usually gelatinous, and often 

 striated. 



The other forms are mostly 

 filamentous. In the Oscillatori- 

 acese, the lowest of these, the 

 cells are short-cylindrical, but 

 all alike, except that the tip of 

 the filament is often somewhat 

 attenuated. In the Nostocaceee, 

 the cells are rounded, and in 

 addition to the ordinary cells, 

 so-called " Heterocysts " are 

 formed, sometimes at irregular **<* 65. -4, four cells ^of Chroscoeeut 

 . turgiuus. surrounded by the gelatinous 



intervals, as in Nostoc and 



envelope ( X 500) . B, colony of Dermo- 

 ca'.-pa prasina, upou Gigartina spinosa 

 (X 500). C, a single cell containing 

 spores. 



Anabeena; sometimes at a defi- 

 nite place, as in Cylindrosper- 

 mum (Fig. 57). The heterocysts 

 are derived from the ordinary cells by their losing most of their pro- 

 toplasmic contents, and becoming enlarged, with thickened, usually 

 yellowish, cell-wall. In young heterocysts the chromatophore may 

 sometimes be detected, b\it it finally disappears. When complete, 

 the heterocyst is shut off from the adjacent cell by a sort of plug, 

 which projects into its cavity. The heterocysts serve to separate the 

 "Hormogonia" or segments into which the filaments finally separate. 

 The Nostocaceee may also develop thick-walled resting-spores 

 (Arthrospores). 



