DISTRIBUTION AND ADJUSTMENT 



97 



mining plant distribution on a large scale as well as locally. 

 This is apparent from a comparison of the accompanying maps, 

 one (Fig. 57) showing the ratio of rainfall to evaporation in the 

 various parts of the United States, the other (Fig. 58) the forest, 

 prairie, and plains areas. The general coincidence of forest 

 distribution and of the areas occupied by prairies and plains 

 with the areas of a decreasing rainfall is very striking. 



That physiographic features must affect the moisture content 

 of the soil is evident on reflection. A poorly drained area 



Intensity of evaporation 



Standard, open garden, Normal School 

 Sta. Ill, b. Mixed prairie and young forest 

 Sta. II, a. Grassy area, Panicum 

 Sta. II, a. Grassy area, Euphorbia 

 Sta. TV, a. Upland, open woods 

 Sta. HI, a. Silphium on black soil 

 Sta. II, a. Colony of 5. laciniatum 

 Sta. IV, b. Ravine slope, open woods 

 Sta. IV, c. Dense climax forest cover 



FIG. 59. Diagram of the relative evaporation in different prairie and forest 

 habitats, showing the great reduction in evaporation with the development of a 

 closed forest canopy of a climax forest; Charleston, Illinois. After Adams. 



develops swamps, bogs, and wet prairie. A rock surface is prone 

 to be xerophytic; so too will be the steep side of a clay bluff. 

 On the other hand, the side of a rock ravine may furnish hydro- 

 phytic conditions, for the sunlight penetrates so little that the 

 temperature is low, the evaporation is slight, and even though 

 water be not abundant, it may be so well conserved as to be 

 adequate to moisture-loving plants. The accompanying dia- 

 gram (Fig. 59), modified from Adams, will show how great differ- 

 ences there are in rate of evaporation in contiguous regions, and 

 also that temperature differences, while not great, may still be 

 sufficient to influence animal and plant distribution. 



