SEVENTH ANNUAL MEETING 69 



from soil whose moisture content is below the wilting coeffi- 

 cient is used for the growth of the organism, has used growth 

 water for the soil moisture in excess of the wilting coeffi- 

 cient. 



METHODS 



The soil moisture determinations here reported were made 

 in the plant associations described in this paper during th. 

 years 1911-12, from the first of May to the end of October. 

 In making the determinations, weekly samples, each consisting 

 of 200 to 250 grams of soil, Were taken in each of the asso- 

 ciations at depths of 7.5 cm. and 25 cm. In order to provide 

 against unnecessary error, each sample consisted of two por- 

 tions of some 100 grams each, taken from spots several meters 

 apart, care also being taken that no soil was taken nearer than 

 a meter to holes where previous samples had been dug. The soil 

 was placed in wide mouthed jars, tightly sealed, brought to the 

 laboratory, weighed and dried at a temperature of 100° to 

 104° C. until it reached a constant weight (about 5 days). The 

 percentage of water to the dry weight of the soil was then cal- 

 culated. 



The wilting coefficients of the same soils were determined 

 by both the direct and indirect methods of Briggs and 

 S chants*. 



THE ASSOCIATIONS STUDIED 



The series of associations studied consists of a succession 

 beginning with a pioneer association dominated by the cotton- 

 wood, Populus deltoides, developed upon the sand dunes at 

 Millers, Ind., and extending through succeeding associations, at 

 the same place, dominated by Pinus Banksiana and Quercus 

 velutina, known respectively as the pine and oak dunes. In 

 the usual order of succession in the Chicago area the black oak 

 forest is succeeded by one composed of the more mesophytic 

 white oak, Q. alba; the red oak, Q. rubra, and the hickory, 

 Carya ovata, and usually designated the oak-hickory forest. 

 The example of this association studied is situated at Palos 

 Park, 111., and the results from this station have already been 

 reported*. The climax of the succession is found in the meso- 

 phytic beech-maple forest which is found well developed at 

 Otis, Ind. A description of the vegetation of these associations 

 has been given by Cozvles and also by the writer in his more 

 detailed report of these studies. 8 



An examination of the accompanying table (Table I) will 

 show the growth-water for the ten weeks of midsummer for 



