100 A TEXT-BOOK OF BOTANY 
cells like those of Gleocapsa, 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, asin Gleocapsa, but it is much more abun- 
dant. Onc 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 hy 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 
