BLUE-GREEN ALGAE 299 
a tiny worm in its creeping and bending to one side and then the 
other. This movement indicates that the cells of the colony of 
Oscillatoria work together as a unit and thus the. many-celled 
colony takes on the character of a many-celled plant where the 
cells are closely associated in the activities of the plant. 
Another filamentous form (Fig. 257) is Nostoc, which is common 
in fresh water and on moist soil. In this plant the cells are 
rounded and the filament re- 
sembles a chain of beads. Nostoc 
secretes an extraordinary amount 
of gelatinous substance and forms 
jelly-like lumps in which a large 
number of the plants are held. 
These jelly-like masses are often 
more or less rounded, and are of 
various sizes up to that of a 
marble or even larger. When 
growing on soil, they often swell 
up and glisten after a rain, on 
which account they have been 
called “fallen stars.” 
In Nostoc there is some differ- 
entiation of cells. At intervals — Fis. 257.— Nostoc. At the 
in the filament ordinary working fet ete jetty uke lamps. of Neston 
: consisting of numerous colonies. 
cells enlarge, lose their contents, about natural size. At the right 
and thicken their walls. Being is a single colony, showing the 
larger in size and almost colorless, gelatinous sheath and the hetero- 
they are quite distinct from the °YS*s the large cells shown empty, 
other cells of the filament, and a segment. the: Blament ibe 
pee 7 armogonia. XX 540. 
thus divide the filament into sec- 
tions called harmogonia. These special cells, called heterocysts, 
seem to be concerned with the multiplication of filaments, for 
it has been observed that the harmogonia break loose at the 
heterocysts, wriggle out through the jelly-like matrix, and de- 
velop new filaments. 
Another special kind of cell formed in Nostoc is the resting cell, 
which is formed when periods unfavorable for the growth of the 
plant appear. In this case certain cells of the filament enlarge, 
accumulate food, and thicken their walls. These cells are able 
to endure cold, drought, and other conditions which are destruc- 
