164 TERRESTRIAL CONDITIONS 
cluding 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 
Per cent. of standard 0 20 40 60 80 100 120 
1. Salt marsh, outer margin... 
to 
. Open gravel slide........... 
3. Carnegie garden, standard... 
ASU PPeEMDCACH tersielsinisisisiatelsteiote 
5. Salt marsh, inner margin.... 
6. Garden, high level .......... ee OS 
7. Gravel slide, partly invaded . | 
8. (Open'forests. .cs<ueec ese Se 
9. Fresh-water marsh.......... 
10. Typical mesophytic forest... 
EE Ravine forests..c-'- sisi noe 
D2 WAM COLeSbe teers eletemistore ss 
Fic. 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. 
