ZYGOPHYTA. 123 
simpler plants is called the asexual mode of reproduction. 
In the plants under consideration there is barely enough 
coherence of the cells to enable them to constitute a plant- 
body, and one can readily see that 
the same fission of the cells which 
here increases the size of the plant 
would, if the cells cohered less, sim- 
ply increase the number of individ- 
uals, 
260. As might be expected, the 
filaments occasionally separate 
Fia. 56.—A, beginning of the sexual reproduction of a Pond Scum (Spirogyra 
longata): a, beginning of the formation of lateral tubes; b, c, the tubes in con- 
tact B, the protoplasm passing from one cell to the other at a; b, the mass of 
protoplasm formed by the union of the protoplasmic contents of the two cells. 
Cc. two young resting spores (c), each with a cell-wall. They contain numerous 
oil-drops, and are still enclosed by the walls of the parent-cell. Magnified 550 
times. 
spontaneously into several parts of a considerable length, 
and the parts floating away give rise to new filaments. 
The separation takes place” by the cells first rounding off 
slightly at the ends, so that their union is weakened at 
