NOSTOC. 23 



cell, in Nostoc many cells are held together in a row 

 partly by the unbroken portions of their cell-walls and 

 partly by the gelatinous material that envelops them. 

 The jelly is formed by a chemical alteration of the outer 

 parts of the cell-walls. They thus constitute a colony. 

 In Pleurococcus, colonies were developed when a number 

 of plants had formed from the division of one individual 

 and the resulting groups had not been disturbed; but 

 in Nostoc division occurs in parallel planes only, the 

 cells remain long united, and the jelly serves to hold 

 and protect the entire colony. 



In reproduction Nostoc differs considerably from the 

 unicellular Algae. The division of any of the cells in 

 the chain results in the growth of the filament, and not 

 directly in the production of a new chain. Finally, 

 when this chain has become large, heterocysts are formed 

 from certain cells, and through their agency the filament 

 is broken into two or more parts, each of which moves 

 independently and may even creep out of the jelly and 

 start a new colony. Even should one cell be cut off 

 from all others, in favorable conditions it might pro- 

 duce a new filament by successive divisions and may 

 reproduce itself in the normal way. 



This plant belongs to a group of Algae known as 

 Cyanophyceae. The group is characterized by the 

 presence of phycocyanin, a blue coloring-matter, which 

 together with chlorophyll gives the bluish-green color. 

 The plants in the Cyanophyceae are all quite simple, 

 few being more complex than Nostoc, and some even 

 simpler. 



