120 ILLINOIS ACADEMY OF SCIENCE. 



During the spring and summer of 1910, an attempt was made 

 to obtain such a quantitative determination of the atmospheric 

 conditions within the vegetation upon the sand dunes of Lake 

 Michigan, in order to discover any existing causal connection 

 between such conditions and the plainly marked succession of 

 plant associations within these areas. The region selected for 

 study was about twenty miles south and east of Chicago, near the 

 little village of Millers, Ind. Here typical localities in each of 

 the several plant associations were carefuly chosen for the evap- 

 oration stations which were maintained from May 6 to October 31, 

 readings being made weekly. 



On the moving dunes the pioneer tree association is one of the 

 Cottonwood, Popnlus deltoidcs, with a scanty undergrowth of two 

 species of willow, the sand cherry and various xerophytic grasses, 

 Ln this association three stations were established about 100 meters 

 apart, nearly 200 meters south of Lake Michigan and 12 meters 

 above the level of its waters. At each the instruments were some- 

 what shaded during a few hours of the day ; one possessing some 

 shelter from the northwest wind and another from the southwest. 

 The mean of the standardized readings were plotted with the daily 

 average evaporation in cubic centimeters as ordinates and the 

 intervals between the weekly readings as abscissae. The graphs 

 for the Cottonwood stations were found to agree in their general 

 direction and in the time of their maxima and minima, the minor 

 differences being probably due to the differences in the direction 

 of the winds to which the stations were unequally exposed. The 

 mean of the readings of these stations is used in comparing the 

 Cottonwood dune with the other plant associations. (Fig. 2.) 



The maximum average evaporation for any week is just above 

 35 cc. per day and the minimum less than 10 cc, while the average 

 for the 178 days is 21.1 cc. per day. 



As the dunes become fixed, a pine association succeeds the 

 Cottonwood. Here it is composed principally of Finns Banksiana, 

 Junipenis virginiana, and /. communis with an undergrowth of 

 Arctestaphylos Uva-ursi, Rhus canadensis, seedlings of black oak, 

 and various other shrubs and xerophytic herbs. Within it sta- 

 tion No. 4 was located about 50 meters south of stations Nos. 2 

 and 3. The instruments were shaded for about two-thirds of 

 the day. The resulting graph is much lower than that of the 

 Cottonwood dune, the maxima are smaller but occur at the same 

 time. The minima are also synchronos but smaller, especially 



