I40 BOTANICAL GAZETTE [AUGUST 
THE ROLE OF THE EVAPORATING POWER OF AIR 
As a preliminary study to the transpiration value of bog plants 
and to the question also whether the xerophytism and the stunted 
growth so manifest on maples, poison sumach, and various other 
plants in the central zone is brought about by an excessive evapo- 
rating power of the air, quantitative measurements have been made 
by the volumetric method to determine the saturation deficiency 
of the air in three representative stations. 
is one of the most important factors of the meteoro- 
logical ‘efcle of a locality. To the student of agriculture and of 
plant physiology it is a problem the study of which aids in supply- 
ing much of the desired information on the growth of plants in 
irrigated and uncultivated fields. Hann (16), who studied 
evaporation chiefly from the point of view of the meteorologist, 
has pointed out that the amount of water which the atmosphere 
is capable of taking up to become saturated is one of the indices 
of the influence of climate. The highly important observations 
made by LIvINGsTON (21, 23) op ome the fact that the effect 
of an atmosphere of great wer y influences 
the geographical distribution of atastl, and through its local 
variations exerts an equally determining effect as a physiological 
and an ecological factor. The problem of evaporation has been 
but imperfectly appreciated, and though the bibliography of 
evaporation is extensiye (24), the correlation between the evapo- 
ration under different conditions has not been satisfactorily 
formulated. The evaporating power of the air is generally under- 
stood to comprise a resultant of temperature, humidity, and wind. 
But evaporation is very sensitive to soil as well as to air relations, 
and since a multitude of local factors may influence either of the 
two conditions, the amount of evaporation integrates the effect 
of numerous variables. Evaporation is a rather complex resultant, 
therefore, and in preparing for an investigation which has in view 
the measurement of the amount of evaporation in plant societies, 
it is important to keep in mind the several conditions entering 
into the problem. It is necessary to recognize that the essential 
details of the phenomenon of evaporation are different in the great 
variety of conditions, and require separate and special study 
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