194 BOTANICAL GAZETTE [September 



words, the evaporating power of the air, and the amount of mois- 

 ture in the soil available for the use of plants. Such measurements 

 have been carried on in certain parts in the Chicago region in an 

 effort to establish a relation between the moisture conditions and 

 the succession of plant associations. 



The researches of Livingston (i) and others have shown that 

 the evaporating power of the air is a rather satisfactory summation 

 of all the atmospheric factors that determine and limit the growth 

 of plants, since it indicates the power of the atmosphere to extract 

 water from their aerial parts, and has been shown in general to 

 vary almost directly with their rate of transpiration. He has also 

 devised the porous cup atmometer and shown that it is able to 

 measure this power with a very considerable degree of accuracy 

 during the growing season, or rather during that portion of the 

 year free from frost, which in the Chicago region is practically the 

 same as the growing season. Hence, during the three seasons of 

 19 10-19 1 2 stations with these atmometers were established and 

 maintained in six distinct plant associations in the Chicago region. 

 Since nowhere in this region is a succession of associations more 

 clearly marked or more easily determined than upon the sand 

 dunes immediately south of Lake Michigan, it seemed that a large 

 number of the determinations should be made within this area. 

 This vegetation has been described by Cowles (2), who has shown 

 that the forest succession consists principally of associations domi- 

 nated by cottonwood, pine, black oak, white and red oak, and beech 

 and maple, in the order named. These represent a continuous 

 series extending from pioneer trees to the mesophytic climax forest 

 of the region. The physiographic factors, the detailed composition 

 of the associations, the variety of the transitional forms, and the 

 frequency of retrogression have been so well explained by Cowles 

 that further detailed discussion at present seems unnecessary. 

 While all the associations mentioned occur upon the sand dunes in 

 the order indicated, it was found to be impossible to obtain easy 

 access to the two final stages of the succession upon sand, and 

 hence the oak-hickory and beech-maple forests were upon clay soil. 

 Objections may be urged against such a selection, and many are 

 recognized by the writer, but it is maintained that the comparisons 



