certain
seasons of the year imparting a taste and odor to the water that
cannot be tolerated.

Algae--what are they? They are aquatic plants. Algae are not to be
confounded with the water vegetation common to the eye and passing by
the term weeds. Such plants include eelgrass, pickerel weed, water
plantain, and "duckmeat"--all of which have roots and produce flowers.
This vegetation does not lend a bad odor or taste to the water. In
itself it is harmless, although it sometimes affords a refuge for
organisms of a virulent type.

But when the aquatic vegetation of the flowering variety is eliminated
from consideration, there still remains a group of water plants called
algae. They comprise one-fifth of the known flowerless plants. They are
the ancestors of the entire vegetable kingdom. Those whose habitat is
the sea number the largest plants known in nature. Certain forms found
in the Pacific are supposed to be 800 feet in length; others are
reported to be 1,500 feet long. The marine variety are familiar as the
brown kelps and the wracks, which are very common along our Northern
coast.

_Plants Which Pollute Drinking Water_

The fresh-water algae are usually grass green in color. This green
variety is often seen as a spongy coating to the surface of stagnant
pools, which goes by the name of "frog spawn" or "pond scum." One of
this description, _Spirogyra_, has done thousands of dollars' worth of
damage by smothering the life out of young water-cress plants in
artificial beds constructed for winter propagation. When the cress is
cut the plants are necessarily left in a weakened condition, and the
algae form a thick mat over the surface of the water, thus preventing
the growth of the cress plants and oftentimes killing them. The
absolute necessity of exterminating these algae led to the perfection
of the copper-purification process.

It is, however, a variety of algae not easily detected that
contaminates the water. So long as they are in a live, healthy
condition they benefit drinking water by purifying it. Indeed, some
scientists have attributed the so-called self-purification of a stream
entirely to the activities of these plants. Of such, one form,
_Chlamydomonas_, is bright grass green in appearance. But the largest
group--the plants which have the worst reputation as polluters of
drinking water--are popularly known as the "blue-green algae"
(_Schizophyceae_). The common name tells the color of these plants,