200 BOTANICAL GAZETTE [SEPTEMBER 
The comparative 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 and represented in a diagram giving 
the loss in cubic centimeters (fig. 5), a graphic representation 
results which, however, tells little more than what has been shown 
differently in fig. 4. Likewise, the maximum daily rates for the 
week of greatest evaporation during the season gives a similar 
representation of the conditions in the several plant associations 
(fig. 6). Upon a percentage basis, with the average rate per day 
throughout the season in the beech-maple forest taken 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 the 
critical portion of the growing season with reference to its water 
supplies, a comparison like the preceding might be made for those 
months only, when it would be found that the comparative evapo- 
ration in the oak dune would be 113 per cent, in the pine dune 146 
per cent, and in the cottonwood dune 230 per cent. 
Summary 
1. These data represent the evaporation rates in the lower but 
critical stratum of the plant associations. 
2. Evaporation at different stations in the same plant associa- 
tion exhibits variations similar in character and degree. 
3. The rate of evaporation in the cottonwood dune association, 
both by its great amount and by its excessive variations, seems a 
sufficient cause for the xerophytic character of the vegetation and 
for the absence of undergrowth. 
4. Evaporation in the pine dune association exceeds that in the 
oak and beech associations except when the latter are devoid of 
foliage. 
5. The vernal vegetation of the pine dune is quite as mesophytic 
as that of the succeeding association, thus agreeing with its lower 
evaporation rate during that portion of the year. 
6. Evaporation in the various associations varies directly with 
the order of their occurrence in the succession. 
