BIOLOGICAL PAPERS 13/ 



there is. instead of a surplus, a deficiency in the water supply 

 amounting to an average of 3 per cent at the 7.5 cm. and 0.5 per 

 cent at 25 cm. 



The writers gratefully acknowledge their indebtedness to ^liss 

 Laura Gano for making the atmometer readings and soil mois- 

 ture determinations during July and August. 



SUMMARY. 



The evaporating power of the air in the lowest stratum of the 

 oak-hickory forest, as determined for the 'growing season of 

 1911, places the association midway between the black oak dune 

 forest association and the beech-maple forest association, a posi- 

 tion which exactly corresponds with its place in formerly observed 

 succession. 



The data on soil moisture show that the water-retaining powers 

 and wilting coefficients of the soils studied varied considerably 

 and appeared largely to be determined by the amount of humus 

 present. 



The soil of the depression showed the most favorable moisture 

 conditions throughout the season. 



The soil moisture determinations afford too meager data to 

 permit any but the most tentative conclusions, but it is believed 

 that they will tend to confirm the position assigned to the oak- 

 hickory forest association upon the basis of its evaporation rate. 



The Unircrsity of Chicago. 



GERMIXATIOX AND GROWTH OF THE COTTON- 

 WOOD UPON THE SAXD DUXES OF LAKE 

 MICHIGAX XEAR CHICAGO. 



George D. Fuller. 



extent of the associatiox. 



The first tree association to become established upon the sand 

 dunes of Lake ^lichigan is one composed of the cottonwood 

 Populus deltoides and a few shrubs, among which species of 

 Sali.v and Corniis are conspicuous. Occasionally Populus del- 

 toides is replaced by P. halsamifera; its ecological equivalent. 

 The detailed examination of the composition and extent of this 



