1gto]} DACHNOWSKI—ARID HABITATS 339 
ing the character of it. It is on the University campus that the 
greatest rate of evaporation was constantly recorded. The differences 
in the rate of evaporation between the campus and the central station 
on the bog island varied from 16.6 per cent to 50.4 per cent—a 
relative rate of evaporation at times more than twice the rate observed 
in the bog habitat. This relation seemed wholly unexpected and 
very surprising at first, for the campus area on the basis of its vegeta- 
tion cannot be considered xerophytic. The general indications, 
therefore, point to the conclusion that at Buckeye Lake the evapora- 
tion from the water surface and from the vegetation produces a vapor 
blanket. The action of this invisible vapor blanket influences to a 
great extent the rate of transpiration of bog plants, for it conserves 
the moisture that would otherwise be lost by evaporation. The 
character of the flora and the structural conditions of bog plants, 
therefore, cannot be ascribed to a greater evaporating power of the 
air, nor to alterations in temperature conditions as has been stated 
so frequently. The weight of evidence from this and other investi- 
gations is again decidedly in favor of the conclusion that the real 
determining factor in the bog habitat is the ratio of the possible rate 
of water absorption to the rate of transpiration, and that the toxicity 
of the bog habitat has a primary r6le in bringing about bog conditions. 
The extent of incontrovertible facts is still scanty. Only of late 
has the question of the physiological aridity of a bog habitat been 
removed from opinion, scientific and unscientific, to the field of 
€xperiment; and further experiments will be necessary to set clear the 
Many complicated questions still connected with this inquiry. 
Botanica LABORATORY 
Ogio State UNtIversity 
