ECOLOGICAL SUCCESSION. 79 



so that it gives off a bad odor. Special investigation would be 

 necessary to determine the cause of this difference in the death 

 rate, yet difference in the rate of evaporation from the animals' 

 bodies is probably an important factor. After long and careful 

 experimental studies dating far back into the history of plant 

 physiology, plant ecologists have come to the conclusion, that the 

 evaporating power of the air is the most satisfactory index of 

 plant environments. 



(d) Evaporation in Forest Animal Habitats. Fortunately this 

 has been investigated (Fuller, 'n) in the five types of stations, 

 viz., cottonwood, pine, black oak, oak-hickory, and beech. 

 Fuller's first three stations were a little more mesophytic than 

 ours. The data were obtained by using a porous cup atmome- 

 ter. Evaporation from the atmometer is more nearly like that 

 from an organism than is evaporation from any other device; 

 it was devised by Livingston ('06, '08, '10, '10). It consists 

 of a hollow r cup of porous clay 12.5 cm. high, with an internal 

 diameter of 2.5 cm. and a thickness of wall of about 3 mm. It is 

 filled with pure water and connected by means of glass tubing to 

 a reservoir usually consisting of a wide-mouthed glass bottle of 

 one half liter capacity. The water, passing through the porous 

 walls, evaporates from the surface, the loss being constantly 

 replaced from the supply within the reservoir. Readings are 

 made by refilling the reservoir from a graduated burette to a 

 certain mark scratched upon its neck. For convenience in 

 handling a portion of the base of the cup is coated with some 

 impervious substance and before being used in the field, the 

 instrument is standardized by comparing its loss of water with 

 that from a free water surface of 45 sq. cm. exposed under uni- 

 form conditions. As a further check against error this standardi- 

 zation is repeated at intervals of six to eight weeks throughout 

 the season (Fuller, 'n). In Fuller's work, the bottles were 

 sunk so that the evaporating surface of the instrument was 

 20-25 cm. above the surface of the soil. 



Figure 2 shows the results of a season's study by Fuller. 

 "The graph for the pine dunes is decidedly lower and more 

 regular in its contour than that of the association which it 

 succeeds. Its four nearly equal maxima would indicate that 



