70 ILLINOIS ACADEMY OF SCIENCE 



1911-1912. The results for the Cottonwood dune as already 

 published for 1911 s show two rather surprising- things, namely, 

 a remarkably small wilting coefficient, only 0.8 per cent at 

 both depths, and a surprisingly constant supply of growth- 

 water. Considered in relation to the wilting coefficient, the 

 soil moisture actually present in the sand is continuously at 

 least twice the amount of water necessary for the growth of 

 such a plant as wheat. This is in striking contrast with the 

 desert-like aspect of the association due to the almost complete 

 absence of herbaceous undergrowth. Doubtless this con- 

 stancy is largely due to the dry mulch maintained by the 

 action of the wind and to the small quantities of water with- 

 drawn by the sparse vegetation. The record for 1912 differs 

 in no important particular from that of the previous year. 



In the next association, the pine dune, the accumulation of 

 humus increases the wilting coefficient to 1.1 per cent at 7.5 

 cm. and 1.0 per cent at 25 cm. The amount of growth-water 

 is less than in the previous association and at three separate 

 weeks during both of the years it quite disappears. Reasons 

 for this failure are to be found in the comparative absence of 

 the conserving dust mulch and in the much larger demands 

 made by the denser stand of vegetation. 



In the oak dune a slight decrease of humus seems to be indi- 

 cated by the slightly lower wilting coefficient, but on the whole 

 there is a somewhat greater supply of growth-water. 



The water supply for the oak-hickory forest has already 

 been reported in detail 8 and it is only necessary here to point 

 out that the growth-water more than doubles that in any pre- 

 vious association. 



The climax beech-maple forest shows the accumulation of 

 humus in its wilting coefficient of 9.5 per cent at 25 cm., being 

 increased to 13.5 per cent at 7.5 cm. The large and unfailing 

 supply of growth-water is indicative of truly mesophytic con- 

 ditions. As in the other associations, the upper stratum 

 shows the larger amount of variation. Comparing the two 

 seasons, it will be seen that while the averages are about the 

 same, the year 1912 shows a much greater uniformity in the 

 range of moisture supply. 



GENERAL CONCLUSIONS 



An examination of Table I as well as a general knowledge 

 of conditions of growth in these associations indicates that 

 only during midsummer is there any scarcity in the supply of 



