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ILLINOIS ACADEMY OF SCIENCE. 



seems to coincide with miaxima in the other associations. On the 

 whole, it is more moderate during the summer months than that 

 of the pine dune, but the difference is not so great as to make 

 it surprising that its undergrowth differs but Httle from that 

 found in the pine dune association. 



The graph from the beech-maple forest stations is one of mod- 

 erate height and great regularity. At no point does it reach to 

 half the height of that from the cottonwood dune but surpasses 

 that of the pine dune in October. 



The data of these observations relate only to the stratum of 

 vegetation immediately above the surface of the soil and would 

 be quite different at a height of one or two meters. This lower 

 stratum is, however, the critical one for a forest association for 

 the development of tree seedlings occurs within its limits and it 



Figure 3. Diagram showing the comparative evaporation rates in 

 diflferent associations on the basis of the average daily amount from 

 May 6 to October 31, 1910. 



is therefore the portion of the habitat which determines the forest 

 succession and hence the most important ecologically. 



The rates of evaporation in the different plant associations may 

 be compared in other ways. If the average amount of water 

 lost by the standard atmometer daily throughout the season be 

 taken as a basis represented in a diagram giving the loss in cubic 

 centimeters (Fig. 3), a graphic representation results which, how- 

 ever, tells little more than what has been shown differently in 

 the graphs. Likewise, the maximum daily rates for the week 

 of greatest evaporation during the season gives a similar repre- 

 sentation of the conditions in the several plant associations 

 (Fig. 4). Upon a percentage basis, with the average rate per day 

 throughout the season in the beech-maple forest as a unit, the 

 comparative evaporation rate in the oak dune is 127 per cent; 

 in the pine dune, 140 per cent, and in the cottonwood dune, 260 

 per cent. As the months of July and August probably represent 



