CHAPTER XV 
THE ALGE 
203. Introductory. Since they usually grow in water, the 
_ algee are sometimes spoken of as pond scums, water mosses, 
sea mosses, and seaweeds. Both fresh and salt waters serve 
as growing places for the algee, and they often appear in such 
abundance that great mats of plants are formed upon or below 
the surface of the water. There are many different kinds of 
algee, and they are divided into four groups, the chief distin- 
guishing character being their color. The most common alge 
have a distinctly green color and are known as the green alge. 
Others, also quite common, have their chlorophyll mixed with 
a blue coloring matter, so that they are bluish green, and these 
are known as the blue-green alge. These two groups of alge 
are found chiefly in fresh waters. There are two other groups 
found usually in salt waters. In one of these, the brown alge, 
there is a brown coloring matter mixed with the green, and 
in the other, the red alge, the green is obscured by red, which 
often becomes most striking in its hues. 
204. The blue-green alge.! Masses of these blue-green 
plants may often be seen in pools of stagnant water or upon 
wet soil. Sometimes they appear as slimy céatings upon 
sticks, stones, and poorly cleaned watering troughs for farm 
animals, and they may also appear as free-floating, dirty mats 
or balls. The blue-green alge are very widely distributed, 
and there are many kinds, of which Nostoe and Oseillatoria 
are quite common and may serve to illustrate the group. 
1 In structural details blue-green alge are probably more closely related 
to bacteria than to other alge, but because of their general appearance and 
habits of living they are mentioned briefly with the alge. 
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