ECOLOGICAL PAPERS. 



125 



the critical portion of the growing season with reference to its 

 water supplies, a comparison like the preceding might be made 

 for those months only, when it would be found that the compara- 

 tive evaporation in the oak dune would be i/j per cent, in the 

 pine dune 146 per cent, and in the cottonwood dune 2^0 per cent. 



Figure 4. Diagram showing the comparative evaporation rates in 

 different plant associations on the basis of the maximum average amount 

 per day for any week between May 6 and October 31, 1910. 



SUMMARY. 



1. These data represent the evaporation rates in the lower but 

 critical stratum of the plant associations. 



2. Evaporation at different stations in the same plant associa- 

 tion exhibits variations similar in character and degree. 



3. The rate of evaporation in the cottonwood dune association 

 both by its great amount and by its excessive variation seems a 

 quite sufificient cause for the xerophytic character of the vegeta- 

 tion and for the absence of undergrowth. 



4. Evaporation in the pine dune association exceeds that in the 

 oak and beech associations except when the latter are devoid of 

 foliage. 



5. The vernal vegetation of the pine dune is quite as meso- 

 phytic as that of the succeeding association, thus agreeing with 

 its lower evaporation rate during that portion of the year. 



6. Evaporation in the various association varies directly with 

 the order of their occurrence in the succession. 



7. The differences in the rate of evaporation in the various 

 plant associations studied are sufficient to indicate that the atrruos- 

 pheric conditions are most efficient factors in causing succession. 



