SEVENTH ANNUAL MEETING 73 



SUMMARY 



1. These data represent the range of soil moisture in the 

 upper subterranean strata of the vegetation of the various as- 

 sociations. 



2. The rate of evaporation in the cottonwood dune asso- 

 ciation both by its great amount and by its excessive variation, 

 seems a quite sufficient cause for the xerophytic character of 

 the vegetation and for the absence of undergrowth, in spite of 

 the constant presence of growth-water. 



3. The pine and oak dune association resemble one another 

 closely in their supply of growth-water. The former is slightly 

 more xerophytic during the midsummer weeks. 



4. The amount of growth-water in the various associations 

 varies directly with the order of their occurrence in the suc- 

 cession, the pioneer being the most xerophytic. 



5. The ratios between the evaporation and growth-water 

 in the beech-maple forest, oak-hickory forest, oak dune, pine 

 dune, and cottonwood dune associations have been shown 

 to have comparative values of 100, 65, 20, 17, and 15 respec- 

 tively, and the differences thus indicated are sufficient to be 

 efficient factors in causing succession. 



LITERATURE CITED 



1. Clements, F. EL, Research methods of ecology, pp. 334. Lincoln, Neb., 

 1905. 



2. Livingston, B. E., The Relation of desert plants to soil moisture and 

 evaporation. Carnegie Institution of Washington. Publication No. 50. pp. 78. 

 1906. 



3. Briggs, L. J., and McLane, J. W., The moisture equivalents of soils. U. S. 

 Dept. of Agr., Bur. of Soils, Bull. 45; 1907. 



4. Briggs, L. J., and Shantz, H. L., The wilting coefficient for different plants 

 and its indirect determination. U. S. Dept. of Agr., Bur. Plant Ind., Bull. 230; 

 1912. Bot. Gaz. 51; 210-219. 1911. 



5. Fuller, G. D., Germination and growth of the cottonwood upon the sand 

 dunes of Lake Michigan, near Chicago. Trans. III., Acad. Sci. 5; 137-143. 1912. 



6. McNutt, W., and Fuller, G. D., The range of evaporation and soil moisture 

 in the oak-hickory forest association of Illinois. Trans. 111. Acad. Sci. 5; 127-137. 

 1912. 



7. Cowles, H. C., The physiolgraphic ecology of Chicago and vicinity. Bot. 

 Gaz. 31; 73-108, 145-182. 1901. 



8. Fuller, G. D., Evaporation and soil moisture in relation to the succession of 

 plant associations. Bot. Gaz. 58: 193-234. 1914. 



