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 Algse. 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 ; they may serve for the storage 

 of food-material, becoming emptied as the filament grows. 

 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 hormo- 

 gonia) 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). 



