THE 



213 



are open to doubt. Of the Cyanophyceae some are 

 terrestrial, some aquatic, occurring both in fresh water 

 and in the sea. The representative of the group which 

 we have chosen a species of Nostoc (see Fig. 92) is 

 filamentous. The threads are associated in colonies held 

 together by the soft gelatinous outer walls of the cells. 

 Such colonies often form conspicuous masses of bluish- 

 green jelly on the damp ground, 

 especially in wet weather. Within 

 the mass the filaments wind about 

 in every direction. The cells of 

 which they are made up are 

 rounded, so as to give the whole 

 thread a beaded appearance. 



The ordinary vegetative cell has 

 a thin inner cell-wall, which is alone 

 visible in Fig. 92, A, the confluent 

 gelatinous layers scarcely showing 

 under the microscope owing to their 

 transparency. The interior of each 

 cell is full of protoplasm, which 

 appears to be coloured throughout 

 its whole mass. No definite plastids 

 have been found to exist in Cyano- 

 phycese, nor has the presence of 

 a nucleus been finally determined, 

 though in certain cases a colourless central body certainly 

 exists, which resembles a nucleus in some of its characters 

 and reactions. In the protoplasm are numerous granules. 



Many of these plants float on the surface of water, 

 where they sometimes appear rather suddenly in vast 

 quantities in ponds and lakes, covering many acres with 

 a bluish-green scum. It is said that in 



A 



FIG. 92. Nostoc Linckii. 

 A, part of a filament ; 

 h, h, heterocysts ; sp, 

 sp, spores. B, isolated 

 spore beginning to 

 germinate. C, young 

 filament formed from a 

 spore, the burst cell- 

 vail of which is shown 

 at the ends. Magnified 

 470. (After Born et.) 



these floating 



