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ILLINOIS ACADEMY OF SCIENCE. 



instructive. The pine dune association with a maximum of 17.5 

 cc. per day and a daily average of 11.3 cc. for the season com- 

 pares closely with the record of the grazed oak-hickory forest, 

 with an average of 12.74 cc. daily for the season. The oak dune 

 forest he studied, which has been regarded as less mesophytic 

 than the one at Palos Park, gave similar evaporation rates with 

 midsummer maximum of 16 cc. per day and an average for the 

 entire season of 10.3 cc. daily. The beech-maple forest, the most 

 niesophytic of our deciduous forests, gave a maximum of 12.0 cc. 

 daily, with an average of 8.1 cc. daily for the season. 



The three stations at Palos Park located where the under- 

 growth was undisturbed gave a seasonal average of 9.35 cc. 

 daily. This figure is very significant, as it places the oak-hickory 

 forest association midway between the black oak dune associa- 

 tion and the climax beech-maple forest (Fig. 3), the position 

 already assigned to it by Cowles and others in the forest sue- 



Figure 3. Diagram representing the average evaporating power of 

 the air in various associations. 



cession of Indiana and Illinois. That the atmometer measure- 

 ments, expressing as they do an integration of the atmospheric 

 factors determining plant growth, assign to the oak-hickory for- 

 est a position immediately below and but slightly inferior to that 

 occupied by the climax beech-maple forest, in absolute agreement 

 with the conclusion already reached by a consideration of other 

 data, must be regarded as of the greatest importance. 



Expressing the same result upon a percentage basis, with the 

 average rate throughout the season of 1910 in the beech-maple 

 forest as a unit,^ the comparative evaporating power of the air 

 in the oak-hickory forest is 115 per cent; in the oak dune asso- 

 ciation, 127 per cent; in the pine dune association, 140 per cent; 

 and in the cottonwood dune association 260 per cent. 



