224 INTRODUCTION TO BOTANY 
When Spirogyra cells divide, the division wall is at right 
angles to the length of the plant. This results in an increase 
in the number of cells and usually in the growth in length 
Fie. 175. Spirogyra 
A, a vegetative cell, showing the form of the cell 
and of the spiral chloroplast (ci), also the nucleus 
(n) and cytoplasm (cy); B, beginning of conjuga- 
tion (a and b), and tubes which failed to conjugate 
(cand d); C, completed zygospores (z) 
of the whole plant. 
Growth occurs so rap- 
idly that, in a few 
days after the plants 
are first seen in the 
spring, they become 
so abundant that they 
pollute the water in 
which they grow, and 
it is often necessary 
to remove these and 
other algze, as is seen 
later (sect. 216). 
210. The reproduc- 
tion of Spirogyra. It 
is possible for a sin- 
gle plant to become 
broken into two or 
more pieces, when 
each one may grow 
into anew plant; this 
is vegetative repro- 
duction and resem- 
bles the vegetative 
reproduction that was 
seen in Pleurococcus. 
But this is not the 
usual method of repro- 
duction in Spirogyra. 
The cells of two plants that lie near one another may unite 
in pairs by means of tubes growing out from the walls of both 
of the uniting cells (fig. 175). These tubes meet and their 
end walls are absorbed, so that there is a continuous tube 
