5l6 RELATION TO ENVIRONMENT. 



regions. They do not govern special types of plant communities. 

 Climate controls the general type of vegetation of a region. In 

 the sense of control there are two climatic factors, temperature 

 and moisture, especially soil moisture. Temperature exerts a 

 controlling influence only over the general vegetation type where 

 the total heat during the period of growth and reproduction 

 is very low. This occurs in polar lands, so that the arctic zone 

 (including alpine areas on high mountains) has a distinct gen- 

 eral vegetation type which is controlled by temperature. This 

 is the arctic vegetation of the Cold Wastes, or Tundra, the land 

 where the ground in many places is perpetually frozen and 

 only thaws out on the surface during the short summer period. 

 In the temperate and tropical regions of the globe, moisture, 

 not heat, is the controlling factor in determining the general 

 vegetation types. These are in general coincident with rain- 

 fall distribution, and there are here, according to Schimper, three 

 great vegetation types controlled by climate: the woodland, 

 the grassland, and the desert. There are, then, according to 

 Schimper, four climatic formations, as follows: 



986. (i.) The Arctic- Alpine Formation. The determining 

 climatic cause is temperature, as indicated above. 



987. (2.) The Woodland Formation. This is characterized 

 by woody plants, whether trees or shrubs. If a close formation 

 of trees, it is a jorest; if clumps of shrubs separate the trees so 

 the crowns do not meet, it is a "bush"; or if the shrubs dominate, 

 it is a thicket. The woodland formation is a close formation, 

 i.e., the climatic conditions are such that freedom of growth is 

 permitted over the entire area. The plants are not struggling 

 against climate, but compete with each other, and the entire 

 ground being occupied the formation is close. If there are 

 bare places here and there, or places occupied by herbage, they 

 are mere accidents, due to factors acting temporarily, or to 

 differences of soil (i.e., ground). There also may be spots 

 which are xerophytic or spots which are hydrophytic, but these 

 are due to soil, not climatic factors. The unfavorable condi- 

 tion of the soil here overcomes for the time the climatic influence. 



