Jan. 3,1916 
Hourly Transpiration Rate on Clear Days 
641 
piration rate. In arriving at this conclusion it is again necessary to 
consider the correlation not only between wind and transpiration, but 
also between wind and radiation. If the wind influences transpiration 
independently of its association with radiation, the wind velocity must 
show a higher correlation with transpiration than with radiation. This 
occurs only during the long alfalfa period, in which there appears to 
be a slight effect due to wind. In all other cases the wind correlation 
with transpiration differs from the wind correlation with radiation by 
an amount not greater than the probable error of the difference. Here, 
again, we are making the specific assumption that the radiation is the 
primary causative factor, so that if wind is associated with transpiration 
to an extent no greater than with radiation its effect on transpiration 
is slight. This assumption is here again supported by the fact that the 
transpiration is extremely low during the night hours, although the wind 
is blowing. 
If transpiration and evaporation are largely determined by the same 
factors or, in other words, if transpiration is essentially a physical process, 
then a high correlation between transpiration and evaporation is to be 
expected. Reference to Table XXXVII will show that the correlation 
of evaporation with transpiration ranges from 0.84 to 0.95. The latter 
value is slightly higher than the maximum correlation (0.89) of radia¬ 
tion with transpiration and shows that 0.9 of the transpiration was in 
this instance determined by the same factors which determined the 
transpiration. 
The relation of evaporation to transpiration is to be considered as 
associative rather than causative, both responding to the same environ¬ 
mental factors, but not necessarily in precisely the same way or to the 
same degree. The extent of this association furthermore depends upon 
the manner in which evaporation is measured. For example, the evapora¬ 
tion rate from a free-water surface in a very shallow tank conforms much 
more closely to the transpiration rate than when a deep tank is used, 
since the latter, on account of its large heat capacity, stores up a large 
amount of energy which is dissipated through evaporation during the 
night. It is evident that the evaporimeter must simulate the plant 
system as nearly as possible in absorption and heat capacity if a high 
degree of correlation between the two is to be attained. 
LEAST-SQUARE RELATIONSHIPS BETWEEN TRANSPIRATION (OR EVAP¬ 
ORATION) AND ENVIRONMENTAL FACTORS 
The method of least squares affords a means of determining the rela¬ 
tive influence of the various environmental factors upon the transpiration. 
In these least-square reductions (Merriman, 1893, and Bartlett, 1915) 
the mean hourly values have been used, and it has been assumed that 
the relationship is linear in character—i. e., that the transpiration varies 
directly in proportion to the intensity of the environmental factors. 
