THE PLANT FORMATION 



235 



altitude, as follows: (1) lowland formations, (2) upland forma- 

 tions, (3) foothill formations, (4) sub alpine formations, (5) alpine 

 formations, (6) niveal formations. 



249. Open and closed formations. These terms refer to the 

 completeness with which the ground is occupied by plants, and 

 indicate the relative ease with which newcomers may invade it. 

 In open formations the habitat is slightly or partiall}^ occupied, 

 and new plants enter readily without displacing those already 

 present. The species of closed formations occupy the ground com- 



FiG. 94. A closed spruce formation. 



],letely. The competition is intense and new plants can enter 

 only by displacing some of the original ones. Ojien formations 

 form the earlier stages in the development of a particular area 

 of vegetation. They are typical of new or denuded soils, such as 

 blowouts, dunes, flooded areas, gravel slides, burned places, etc. 

 Closed formations constitute the later stages of a .succession, and 

 especially the final or stable condition. Forest, thicket, meadow, 

 and prairie are good examples. 



250. Mixed formations. These are produced by the inter- 

 mingling of the species of two or more adjacent formations, or of 



