218 ESSENTIALS OF BOTANY 
the usual one in higher plants. In the latter every cell 
commonly possesses a distinct nucleus, and this generally 
goes through complicated changes of form during the pro- 
<p wee 
“LI c 
cbs 
realy? 
a 
ein 
ED 
a . 
Fic. 152. Oscillatoria. 
A few cells are shown 
treated with reagents 
to show the protoplas- 
mic cell-contents more 
clearly. At b the cell- 
wall is sending a par- 
tition inward, and the 
protoplasmic contents 
beginning to divide. 
At c the division (jfis- 
sion) has proceeded 
still further. (Much 
magnified.) 
cess of nuclear division (see Bergen and 
Davis’ Principles of Botany, Sect. 199). 
277. Summary of the Blue-Green 
Alge.— Many species of the blue-green 
algee occur in fresh water, especially in 
warm waters which are impure from 
decaying organic matter, and in hot 
springs, some are found in sea water, a 
good many flourish on damp earth, and 
a few inhabit cavities in other plants. 
The blue-green alge are plants of 
very low organization. Some of them 
consist of single cells, living in a soli- 
tary way, others have the individual 
cells held together in masses by a 
gelatinous substance, and still others, 
like Oscillatoria, consist of thread-like 
rows of cells. 
Blue-green alge differ from other alge in the nature of 
their coloring matter,! in the absence of well-defined nuclei 
in the cells, and in the simple mode of reproduction, by 
cell-division. 
From the readiness with which their cells 
split into new ones they are often called fission alge. 
1 Their color is never exactly that of chlorophyll, but ranges from some- 
thing not much unlike chlorophyll to blue-green, orange-yellow, brown, red, 
or violet. 
