CYANOPHYCE^E 251 



like a true nucleus, and it has not been observed 

 to display karyokinesis. Vacuoles do not appear 

 ordinarily in the young cells, but with age, obscura- 

 tion of light, or other unfavourable conditions, they 

 arise and occupy a considerable part of the cell. It 

 is not known, however, whether these vacuoles con- 

 tain a cell-sap like those of other plants. Glycogen 

 has been determined as present in the cells, but not 

 starch. 



If we consider the close relationship of the 

 Cyanophycecc to the Bacteria, it is not strange that 

 theories of their polymorphism have arisen. It has 

 been supposed for example that forms like those of 

 Chroococcacccc are often stages in the development of 

 the higher Cyanophyccce, and there are sufficient re- 

 semblances to give colour to such a view. But there 

 is no more proof of it than this slight ground of 

 speculation, except the equally slender support de- 

 rived from the fact that the forms frequently grow in 

 the same places. It requires actual observation of 

 development to establish such a matter. 



NOSTOCACE.E. 



This order is distinguished from the Chroococcacecc 

 by its multicellular thallus, and by the production of 

 hormogonia, formed by the fragmentation of the 

 filaments into mobile segments. The whole of a cell 

 row is called a trichome, and the trichome with its 

 envelope, which may be gelatinous, or even almost 

 cartilaginous in consistence, is called the filament. 



