164 



TERRESTRIAL CONDITIONS 



eluding that the evaporating power of the air is probably the best index 

 of environmental conditions of land animals. 



b) Evaporation in different habitats. The evaporating power of the 

 air varies in different situations (Fig. 115). There are great differences 

 between open prairies and closed forests. Shimek (135) found that the 

 evaporation in the undisturbed groves in Eastern Iowa during July and 

 August was very much less than that in the prairies adjoining. From 

 the free surfaces of pans set in the ground so that the water which 

 they contained was level with the surface of the soil, the evaporation 

 of the groves was about 27 per cent of that of the prairie; with 

 cup evaporimeters about 37 per cent, and with Piche evaporimeters 



FIG. 115. Showing the comparative evaporation rates in the ground stratum of 

 several animal habitats on Long Island during July and August (after Transeau, 

 courtesy of the Botanical Gazette). 



about 47 per cent. This is about the same as the difference on Long 

 Island between the inner side of Transeau 's salt marsh dominated 

 by grasslike plants and his mesophytic forest. Sherff (135) found the 

 evaporation in a marsh forest to be a little less than that in the beech- 

 maple and from 1.8 to 2.6 times as great as in the lowest stratum of 

 marsh. 



c} Vertical differences in evaporating power and other conditions. The 

 evaporating power of the air is usually greater at the higher levels of a 

 habitat. 



