328 BOTANICAL GAZETTE [APRIL 
the condition of incipient drying. Judging from the rates of tran- 
spiration shown by the tables of this contribution (no attempt is 
made to define the aerial conditions), it appears that the evaporating 
_ power of the air during the experiments there described was rela- 
tively low. This fact rendered the process of incipient drying, 
which leads eventually to actual wilting, very much prolonged, and 
it is perhaps not surprising that these authors failed to detect 
incipient drying in the plants which actually wilted, making the 
mistake of supposing that incipient drying in the non-wilting forms 
(as cactus, lemon, etc.) is to be considered as identical with actual 
wilting (which is relatively a much later occurrence) of ordinary 
thin-leaved plants. It is greatly to be regretted that such an 
expensive and elaborate series of determinations as Briccs and 
SHANTz have made should have been carried out without records of 
the intensity and duration aspects of the evaporating power of the 
air under which the experiments were performed. Such records, 
which are perhaps the most readily obtained of all the climatic 
records which are as yet available for the study of plant processes, 
would have made possible the duplication of the aerial conditions 
of these experiments and would have aided greatly in the further 
analysis of some of the important findings of these authors. 
Conclusions 
We conclude from our measurements and comparisons that 
there can remain little question that green plants when subjected 
to relatively great diurnal evaporation intensity, at least frequently 
exhibit a marked fall in foliar moisture content by day and a 
corresponding rise by night. The daily march of evaporation 
remains still to be studied in other climates than that of summer 
in southern Arizona, so that we are unable to compare our condi- 
tions with those of more humid or cooler regions. From our 
experience with cloudy weather, we are inclined to the prediction 
that the diurnal decrease in leaf moisture here established for high 
evaporation rates may fail to occur in regions of low evaporation 
when accompanied by relatively high rates of soil moisture supply. 
Our studies also indicate that some non-succulent, small-leaved 
xerophytes (such as Covéllea and Prosopis) fail more or less com- 
