138 ILLINOIS ACADEMY OF SCIENCE. 



association has been so well made by Cowles^ that little could be 

 added to his account. To summarize : it is characterized by the 

 one tree species only and extends from immediately within the 

 fore-dune to the region where the dunes become established, com- 

 prising the greater part of the moving dune-complex and forming 

 a zone from 100 to 300 meters wide. In the dune region com- 

 paratively few cottonwoods are found beyond these limits, min- 

 gling with the members of other associations. 



It is the purpose of this paper to discuss briefly two of the 

 physical ecological factors of the association which have been 

 quantitatively studied, viz. : the evaporating power of the air,2 and 

 the range of soil moisture during the growing season,- and to 

 attempt to relate the germination and growth habits of the prin- 

 cipal tree member of the association to these factors. 



EVAPORATING POWER OF THE AIR. 



This factor has been measured by means of the Livingston 

 atmometer during the past two growing seasons, and some of the 

 results for the summer of 1910 have been reported to this Acad- 

 emy.- The readings of the atmometers were taken weekly and 

 corrected by the application of the coefficients necessary to express 

 the results in term of loss from the standard instrument adopted 

 by Livingston. When these results are plotted as graphs having 

 the weekly intervals as abscissae and the loss per day in cubic 

 centimeters as ordinates (Fig. i), they show that the evaporating 

 power of the air in the lower stratum of the association is excess- 

 ive and subject to extreme variations, indicating that the demands 

 for water made upon the aerial parts of the vegetation are very 

 great. The average rate for the season of 1910 was 21. i cc. per 

 day and that for 191 1 amounted to 24.6 cc. per day. A further 

 comparison of the conditions existing during these two years 

 may be obtained by a study of these two graphs, representing as 

 they do the mean of the observations taken at three stations from 

 May I to October 31 of these years. The graphs will be seen to 

 be similar in character, the differences being traceable to differ- 

 ences in atmospheric conditions peculiar to each particular year, 

 and they demonstrate that the evaporation conditions are rigorous 

 and indicate a xerophytic response. The amount of xerophytism 

 may be indicated by a comparison with the evaporation occurring 



^ Cowles, H. C. The ecological relations of the vegetation of the sand dunes of 

 Lake Michigan. Bot. Gaz., 27: 95-117, 1899. 



2 Fuller, G. D. Evaporation and plant succession. Trans. Jll. Acad. Sci. 4: 119-1J5, 

 191 1. 



