THE PLANT FORMATION 



217 



into the succeeding one, and during the process the vegetation 

 is really a mixture of the two. The same formation often appears 

 on several new or denuded soil areas. In some of these it is com- 

 posed of entirely different species, apparently indicating a number 

 of distinct formations. If a critical comparison is made of all 

 such areas, or their further development traced, it can be determined 

 whether they are parts of one formation. 



One part of a formation may lag behind the others in develop- 

 ment, or a physical change or difference covering a small area 



Fig. 80. a cliff formation of saxifrage herbs and 

 bushes {Heuchera and Edwinia). 



produces an alien group in a formation otherwise uniform. The 

 lichen and moss groups that are found on rocks in forest and grass- 

 land furnish a good illustration of this. The lichens which grow 

 on the forest floor and those which are found on the trees clearly 

 belong to the forest formation. They are present because of the 

 shade and moisture furnished by the trees. The same is true of 

 those found among the grasses and sedges of meadows. The 

 physical conditions are those of the formation, and the lichens 

 are as much at home as the other plants. In the case of rocks 



