306 
THE SIMPLEST GREEN PLANTS 
fine threads called filaments which float in the water in 
large masses. Spirogyra is only one of many kinds of fila¬ 
mentous algae. 
The individual cells of spirogyra are provided with one 
or more narrow green bands arranged spirally within the 
protoplasm. These spiral bands of chlorophyll are the 
special structures which manufacture food (Figure 284). 
The cells of the filament increase rapidly in size and divide, 
and thus the filaments 
increase in length. As 
each cell divides, the cell 
wall grows in at right 
angles to the length of 
the plank Spirogyra 
grows very rapidly in 
the spring. The bub¬ 
bles found among a mass 
of spirogyra contain the 
oxygen which the cells 
give off during photo¬ 
synthesis. 
During the summer there are times when spirogyra re¬ 
produces in another manner (Figure 285). Two cells of 
adjacent plants join by putting forth tubes which fuse on 
meeting. The contents of one cell pass through the tube, 
and flow into and unite with the contents of the other 
Figure 285. — Spirogyra Conjugating. 
A, zygospore; B, empty cell; C, canal 
formed by tubes ; D, tubes. 
