214 STRUCTURAL BOTANY 



species gas-vacuoles are present in the cells, that is to 

 say, little cavities in the protoplasm containing a gas, 

 the nature of which has not been determined. These 

 gas-vacuoles appear to be important, as they make the 

 plant lighter and enable it to float. 



The apparent simplicity of the histological structure, 

 due to the want of well-defined nuclei and plastids, is 

 the chief reason why the Cyanophycese are often 

 separated from the Algae. Further observations may, 

 however, at any time abolish these distinctions. The 

 colouring matter appears to be a compound substance 

 consisting of blue-green, yellow, and pure green con- 

 stituents. The tint varies greatly in different forms, 

 but we never find the pure green of chlorophyll. In 

 Nostoc the filament is interrupted at intervals by larger 

 cells with thicker walls, apparently destitute of pro- 

 toplasm. These cells are called the heterocysts (Fig. 

 92, A, h), and are characteristic of Nostoc and its nearer 

 relations. Sometimes the filaments break across at the 

 heterocysts, and the short rows of living cells between 

 them become isolated. These detached filaments (called 

 the hormogonia) are capable of creeping movements; 

 though how they move is quite unknown. They escape 

 from the gelatinous mass, and start new colonies for 

 themselves. 



This is one mode of propagation. Another is by 

 means of resting-spores, formed directly from some of 

 the vegetative cells, which grow larger than the rest, 

 accumulate more abundant protoplasm, and surround 

 themselves with a thick cell- wall (Fig. 92, A, sp). The 

 spores (Fig. 92, B) can pass through a resting-stage, 

 and endure drought ; when water is supplied they 

 germinate, forming new filaments (Fig. 92, C). 



