THE GREAT GROUPS OF ALG.E 
233 
together imbedded in the jelly-like matrix formed by the 
wall material (Fig. 201). These imbedded groups of indi- 
viduals are spoken of as colonies, and 
as colonies become large they break 
up into new colonies, the individual 
cells composing them continuing to 
divide and form new individuals. 
This represents a very simple life his- 
tory, in 'fact a simpler one could hard- 
ly be imagined. 
160. Nostoc. These forms occur in 
jelly-like masses in damp places. If 
the jelly be examined it will be found 
to contain imbedded in it numerous 
cells like those of Glmocapsa, but they 
are strung together to form chains of 
varying lengths (Fig. 202). The jelly in 
which these chains are imbedded is the 
same as that found in Glwocapsa, being 
formed by the cell walls becoming mucilaginous and swollen. 
One notable fact is that all the cells in the chain are not 
alike, for at irregu- 
lar intervals there oc- 
cur larger colorless 
cells, an illustration 
of the differentiation 
of cells. These larger 
cells are known as het- 
erocysts (Fig. 202, A), 
which simply means 
"other cells." It is 
observed that when 
the chain breaks up 
into fragments each 
fragment isCOmpOSed 
of the cells between 
FIG. 201. Glceocapsa, a 
blue-green alga, show- 
ing single cells, and 
small groups which have 
been formed by division 
and are held together by 
the enveloping muci- 
lage. CALDWELL. 
FIG. 202. Nostoc, a blue-green alga, showing the 
chain-like filaments, and the heterocysts (A) 
which determine the breaking up of the chain. 
