CHAPTER XV 

 THE ALGJB 



203. Introductory. Since they usually grow in water, the 

 algae are sometimes spoken of as pond scums, water mosses, 

 sea mosses, and seaweeds. Both fresh and salt waters serve 

 as growing places for the alga?, 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 

 algae, and they are divided into four groups, the chief distin- 

 guishing character being their color. The most common algre 

 have a distinctly green color and are known as the green algce. 

 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 algce. These two groups of algte 

 are found chiefly in fresh waters. There are two other groups 

 found 'usually in salt waters. In one of these, the broum algce, 

 there is a brown coloring matter mixed with the green, and 

 in the other, the red algce, the green is obscured by red, which 

 often becomes most striking in its hues. 



204. The blue-green algae. 1 Masses of these blue-green 

 plants may often be seen in pools of stagnant water or upon 

 wet soil. Sometimes they appear as slimy coatings 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 algse are very widely distributed, 

 and there are many kinds, of which Nostoc and Oscillatoria 

 are quite common and may serve to illustrate the group. 



1 In structural details blue-green algse are probably more closely related 

 to bacteria than to other algae, but because of their general appearance and 

 habits of living they are mentioned briefly with the algae. 

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