RELATIONS OF PLANTS TO EACH OTHER 207 



will be present; but the plant most at home in 

 such places is a flat-crusted one, of various shades 

 of greenish-brown, which lies flat on the surface, 

 growing vigorously in the wet seasons, and shrink- 

 ing to a hard brittle mass in dry weather. These 

 are the lichens, and other members of this group 

 sheathe the trunks of trees in the forest. Com- 

 pare the species found on different slopes of the 

 rock. 



246. Pond societies. Pond societies live in al- 

 most all waters except those of lakes and streams 

 polluted by the sewage of cities. The constitution 

 of such societies may be best studied in a pond, lake, 

 or sluggish stream, and a boat will be a useful 

 adjunct for the exploration of depths which cannot 

 be reached by wading. The soil above high water 

 will be very moist, offering suitable conditions for 

 a belt of cat-tails, then one of flowering plants, wil- 

 lows, and small shrubs, and back of these the trees 

 of a forest. 



On the first visit to the place chosen for study, 

 note the extreme height to which the water rises, 

 and examine first the plants growing between the 

 high-water mark and the edge of the water. Identify 



