100 A TEXT-BOOK OF BOTANY 



cells like those of Glceocapsa, but they are strung together 

 so as to form chains of varying lengths (Fig. 93, A). The 

 jelly in which these chains are embedded is formed from 

 the cell walls, as in Glceocapsa, but it is much more abun- 

 dant. One notable fact in Nostoc is that the cells of a chain 

 are not all alike, for at irregular intervals there occur larger 

 colorless cells, called heterocysts (Fig. 93, A, a), a name 

 which means simply "other cells." It is observed that 

 when the chain breaks up into fragments, each fragment 

 is composed of the cells between two heterocysts. The 

 fragments wriggle out of the jelly matrix and start new 

 colonies or chains, each cell dividing to increase the length 

 of the chain. A common plant related to Nostoc shows 

 still more differentiation in the cells of the filament, the 

 heterocyst being at the base, and the end cells forming a 

 tapering and sometimes whip-like termination (Fig. 93, B). 



That each cell of Nostoc is an individual is evident from 

 the fact that a single cell separated from the chain continues 

 to live and divides; and therefore the chain is a colony of 

 individuals, each one reproducing by cell-division. 



60. Oscillatoria. These plants are found as bluish- 

 green slippery masses on wet rocks, or on damp soil, or freely 

 floating. They are simple filaments composed of very 

 short flattened cells (Fig. 94), and the name refers to the 

 fact that the filaments exhibit a peculiar oscillating move- 

 ment. A filament is really a row of independent cells 

 packed in a mucilaginous sheath, like coins in a coin-case. 

 The cells are evidently flattened by mutual pressure, for 

 the free face of the terminal cell is rounded (Fig. 94, B); 

 and if a filament is broken, and a new cell surface exposed, 

 it at once bulges out. If a single cell of the filament is 

 free from all the rest, both flattened faces become rounded, 

 and the cell becomes spherical. It is evident that pressure 

 within the cell distends the elastic wall whenever it is free. 

 Each cell is able to divide, forming new cells and thus 



