426 . BOTANICAL GAZETTE [November 



previous paper.* The observations were made and the results have been 

 plotted graphically for three different strata, with the intervals between 

 the weekly readings as abscissae and the daily evaporation from the 

 standard Livingston atmometer as ordinates (fig. i). The interme- 

 diate graph (fig. ib) represents the mean of three stations situated upon 

 the forest floor, with the atmometers 25 cm. above the surface of the 

 soil, in conditions of average vegetation. Here the average rate for the 

 season was 7.4 cc. per day. The highest rate (fig. id) is that given by 

 an instrument elevated 2 m. above the forest floor and shows an average 

 of 13.5 cc. daily, or very nearly twice the amount of the stations imme- 

 diately above the soil surface. The third record (fig. id) is for a station 

 situated upon the slope of a ravine 10 m. deep, cut in the clay soil by a 

 wet weather stream, and having a V-shaped outline in cross-section. 

 The atmometer was placed 4 m. below the edge of the ravine and gave 

 an average for the season of 5 . 9 cc. daily. 



If the average rate of evaporation at the stations upon the forest 

 floor be taken as unity, the proportional evaporating power of the air in 

 the three strata will be found to be very nearly 1 . 84 : 1 . 00 : o . 80 f or the 

 season, and these figures may be taken to represent, more exactly than 

 any previously available data, the measure of the mesophytism of these 

 three several regions. It may also be noted that the elevated station 

 has a much higher rate proportionally during the first half of the season. 



The object of this paper being to indicate the amount of difference 

 existing in the atmospheric conditions of some of the different strata of 

 the same association, and to show how these differences may vary 

 throughout the growing season, no attempt will be made to relate the 

 vegetation to the different rates of evaporation. More extensive records 

 must be obtained and an intensive study of the composition of the 

 association undertaken before any satisfactory conclusions can be 

 reached. It is interesting and important, however, to note the different 

 atmospheric conditions to be encountered by forest tree seedlings during 

 the first two years of their existence and at a later period when they 

 reach the height of a meter or more. This may indicate one of the most 

 important reasons why so many of the beech seedlings die before attain- 

 ing the height of two meters. It may also be remarked that the lower 

 evaporation in the ravine may be a sufficient explanation for the presence 

 upon its slopes of a much greater abundance of such delicate forms as 

 Dicentra canadensis, D. Cuadlaria, Impatiens biflora, and Asplenium 

 angustij olium — George D. Fuller, University of Chicago. 



< Fuller, G. D., Evaporation and plant succession. Bot. Gaz. 52: 193-208. I9»« 



