234 
PLANT STUDIES 
two heterocysts. The fragments wriggle out of the jelly 
matrix and start new colonies of chains, each cell dividing 
to increase the length of the chain. This cell division, 
to form new cells, is the characteristic method of repro- 
duction. 
At the approach of unfavorable conditions certain cells 
of the chain become thick-walled and well-protected. These 
cells which endure the cold or other hardships, and upon 
the return of favorable conditions produce new chains of 
cells, are often called spores, but they are better called 
" resting cells." 
161. Oscillaria. These forms 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. 203), and the name 
Oscillaria refers to the fact that they 
exhibit a peculiar oscillating move- 
ment. These motile filaments are is- 
olated, not being held together in a 
jelly-like matrix as are the chains of 
Nostoc, but the wall develops a cer- 
tain amount of mucilage, which gives 
the slippery feeling and sometimes 
forms a thin mucilaginous sheath 
about the row of cells. 
The cells of a filament are all alike, 
except that the terminal cell has its FIG. 203. OscWaria, a uue- 
. - , , T . _. green alga, showing a 
free surface rounded. If a filament group O f filaments u>, 
breaks, and a new cell surface ex- and a sin g le filament 
-i ! i n i more enlarged (B). 
posed, it at once becomes rounded. CALDWEM.. 
If a single cell of the filament is 
freed from all the rest, both flattened ends become rounded, 
and the cell becomes spherical or nearly so. These facts 
indicate at least two important things : (1) that the cell 
wall is elastic, so that it can be made to change its form, 
and (2) that it is pressed upon from within, so that if free 
fT B 
