26 PLANT-LIFE 



The genus embraces a number of species, some of which 

 are found in water. The masses are quite evident to 

 the unaided eye. On examining them microscopically 

 we see that the transparent jelly protects a colony of 

 filaments which wind about in all directions, each 

 filament being composed of tiny round cells arranged 

 like beads on a necklace. The jelly in which the fila- 

 ments occur is, of course, secreted by the cells. The 

 cells are filled with protoplasm, and no defined chloro- 

 plasts exist; the pigment is diffused throughout the 

 protoplasm. As the diagram shows, somewhat larger 

 cells appear at intervals in the strings of cells ; these are 

 called " heterocysts." They are emptied during the 

 growth of the filaments, and it would seem that they 

 act as food stores, which are drawn upon when required. 

 Nostoc is propagated either by the breaking of the fila- 

 ments into sections, each section worming its way out of 

 the mass — a movement which has not been explained — 

 and giving rise to a new colony, or by the formation of 

 spores. These spores are developed from ordinary cells 

 which grow larger than their fellows and secrete a stout 

 cell- wall. They break away from the filaments in which 

 they grow, and, under suitable conditions of moisture, 

 germinate and give rise to new filaments. The spores 

 are not killed by drought ; they are evidently a provision 

 for tiding over adverse conditions. 



It is questionable whether the Cyanophyceae should 

 be classed with the Algae, and it is only from " use-and- 

 want " that we have referred to them under that name. 

 As already indicated, they are true Schizophytes, or 

 Fission-Plants. Like the Bacteria, they multiply by 

 fission and show no trace of sex, but they are dissimilar 



