208 BOTANICAL GAZETTE [SEPTEMBER 
both, so no unit seems so well suited for this purpose as the beech- 
maple forest association or its ecological equivalent. Thus it may 
be said that the atmospheric conditions in the lower stratum of 
the cottonwood dune association during the growing season are 
260 per cent as severe for plant life as those in the same stratum 
of the standard association (the beech-maple forest) during the 
same period. 
The writer hopes to continue and extend these investigations 
during the coming seasons. 
Grateful acknowledgment is made of the helpful advice and 
suggestions of Dr. Henry C. Cow Les, under whose direction this 
investigation has been conducted. 
THE UNIVERSITY OF CHICAGO 
LITERATURE CITED 
1. Cow gs, H. C., The ecological relations of the vegetation of the sand dunes 
of Lake Michigan. Bor. Gaz. 2'7:95-117, 169-202, 281-308, 361-391. 1890. 
2. Livincston, B. E., Evaporation and plant habitats. Plant World 11:1-10. 
1908. 
———, The relation of desert plants to soil-moisture and to evaporation. 
Parceple Institution of Washington, Publication no. 50. 1906. 
4. TRANSEAU, E. N., A simple vaporimeter. Bort. Gaz. 49:459-460. 1910. 
5. Lrvineston, B. E., Operation of the porous-cup atmometer. Plant World 
6. Brown, Wm. H., Evaporation and plant habitats in Jamaica. Plant 
World 13:268-272. 1910 
. Lrvincston, B. E., A aie cospeecting atmometer for ecological instrumenta- 
tion. Plant World 13:79-82. 1910. 
8. TrANSEAU, E. N., The relation of plant societies to evaporation. Bor. 
AZ. 452217-231. I 
