206 THE NATURE AND WORK OF PLANTS 



by the open spaces in woodlands. Observe a spot 

 of this character, and note whether the young trees 

 from the forest are taking possession or not. 



245. Hock societies. — Find a set of cliffs or 

 exposed ridge of rocks, and note the character of 

 the vegetation that finds a foothold there. It will 

 be seen that the distribution of the rock plants is 

 limited quite exactly by the extent of the stone on 

 which they grow. This is perhaps the most strik- 

 ing feature about these communities. Forests may 

 cover many square miles in extent, but rock socie- 

 ties usually extend over a few square yards only. 

 Still another feature will be brought out in a com- 

 parison of the rock society and the forest ; the thou- 

 sands of individuals in the forest crowd each other 

 fiercely for space in which to live, while the mem- 

 bers of the rock societies are scattered about over the 

 hard surface, finding a foothold where they may in 

 any convenient crevice or roughened area. Observe 

 a rock society throughout a portion of the season, 

 and make a census of its members. The number of 

 seed plants will be small. Ferns will probably be 

 represented, the mosses will form clumps or coat- 

 ings on moist portions of the rock, some liverworts 



