RELATIONS OF PLANTS TO EACH OTHEU 209 



Note the species floating on the surface. Among 

 these may be found the minute duckweeds, which 

 look like small leaves, and are of such size that 

 several of them may be laid side by side on the 

 thumb nail. The leaflike body represents both 

 leaves and stems, while but one slender root is 

 present that trails down in the water and serves as 

 a keel to prevent the upsetting of the leaf (§31). A 

 vivid coloring may be seen on the lower side of these 

 and other floating aquatic organs. In places the sur- 

 face may be taken up by numerous specimens of 

 the bladderwort with their curiously pouched leaves 

 full of minute entrapped animals. In quiet pools, 

 free from the action of currents, the surface will be 

 covered by heavy masses of a greenish scum, which 

 has a greasy feel when held between the fingers. 

 This is a very widely distributed alga, Spirogyra, 

 and the masses are composed of long threads made 

 up of cylindrical cells placed end to end. What 

 becomes of all these floating members of the com- 

 munity in the winter time ? How do they reappear 

 in the spring? 



247. Locations occupied by members of different 

 kinds of communities. — Low-lying areas may be 



