BELATIONS OF PLANTS TO EACH OTHER 193 



are, — the loater supjily, temperature of the soil and air, 

 physical and chemical properties of the soil, winds, 

 and light. Tlie length of time during which these 

 conditions have continued unchanged in any given 

 piece of land or country is also a very important 

 factor. 



233. Communities change. — Communities are al- 

 ways in a state of being changed, and they may 

 disappear from any place and others may replace 

 them. A good example of this will be seen if a 

 marsh or pond is drained. During the existence of 

 the pond, water societies only could live there. The 

 mud left by the pond will allow societies of lower 

 plants to form a green coating over its surface. 

 After a time liverworts, mosses, and ferns may find 

 suitable conditions here. Later, when the mud has 

 dried and the soil becomes loose and loamy, flowering 

 plants will find a foothold, and if imdisturbed, young 

 trees may grow up, and thus the place once occupied 

 by a pond society will be the residence of a forest. 

 But even then the changes are not all past. The 

 action of the first kind of trees on the soil may fit 

 it for the growth of other trees, and the first forest 

 may be replaced by a different one. Similar move- 



