200 FIRST FORMS OF VEGETATION. 
new plants, and extend themselves with amazing 
rapidity, in a week or two producing thousands 
and tens of thousands of individuals. The lives 
of the fresh-water alge rarely exceed a year in 
duration, many of them dying in the course of a 
few months or weeks. ‘They complete the process 
of reproduction early in spring, and last during 
the summer, perishing in the autumn, and disap- 
pearing altogether in winter. No sooner does the 
ice, which had bound up the streamlet in its silent 
fetters, melt under the warm rays of the sun, al- 
lowing its water to flow merrily on, and flash and 
sparkle in the sunbeams, than every stone in its 
bed, though brown and naked before, is suddenly, 
as if by magic, invested with a green velvet coat- 
ing, whose long graceful filaments float freely 
with the water. Every ditch and marsh, every 
rivulet, every hoof-mark and rut on the road 
where water has accumulated, is filled with green 
clouds of these mysterious plants. The purposes 
which they serve in these situations are sufficiently 
obvious. Though associated in our minds with 
stagnation, putrefaction, and malaria, they are the 
scavengers, the water-filters of nature. Like the 
flowers and the trees, that on dry land remove the 
impurities with which the animal world is con- 
tinually tainting the atmosphere, they purify the 
waters, by assimilating the decaying matter which 
