Jan. 3, 1916 
Hourly Transpiration Rate on Clear Days 
645 
similar to those of the former series. The ratio of the radiation to the 
saturation-deficit coefficient for the different plants is as follows: Rye, 
0.73; alfalfa, 0.97; amaranthus, 1.12. The equation for rye again shows 
the radiation to have the lesser influence of the two factors considered, 
while in the case of the other two plants, the radiation has an equal or 
greater influence. The equations for the latter plants are in fair agree¬ 
ment, but in all cases discrepancies occur between the observed and com¬ 
puted curves, particularly during the early afternoon and early evening 
hours. 
EVAPORATION AS DETERMINED BY RADIATION AND SATURATION DEFICIT 
The evaporation rate from the shallow, blackened tank for the three 
transpiration periods just considered has also been computed, assuming 
the vertical radiation and the saturation deficit to be the controlling envi¬ 
ronmental factors. The observed and computed evaporation graphs are 
given in figure 22. The agreement during the rye and alfalfa periods is 
very satisfactory, but during the amaranthus period the departures are 
greater. The evaporation equations for the several periods are as 
follows: 
Rye period.0.787 R v +0.292 D=E; 
Alfalfa period.0.680 i^+0.360 D = E; 
Amaranthus period.0*563 R v -f 0.411 D=E; 
in which E represents the evaporation expressed as a percentage of the 
maximum. 
It will be observed that the radiation has a preponderating influence 
in each instance. 
DISCUSSION OF BEAST-SQUARE REDUCTIONS 
The least-square reductions again emphasize the fact that the trans¬ 
piration response to changing environmental conditions is not the same 
for different plants. In other words, the distribution of the transpira¬ 
tion loss through the day varies with different plants. Furthermore, the 
distribution of the transpiration loss differs from the distribution of the 
evaporation loss from a shallow tank. As a whole, the agreement 
between observed and computed evaporation is much closer than between 
observed and computed transpiration. Either some factor operative in 
transpiration yet remains to be accounted for or the transpiration system 
changes its coefficient during the day. The latter condition may result 
from stomatal control or through the inability of the plant to secure 
sufficient water to maintain complete turgidity during the day. The fact 
that the evaporation on clear days can be satisfactorily accounted for 
from a consideration of radiation and saturation deficit indicates that 
the essential environmental factors have been considered and suggests 
that the outstanding differences between observed and computed trans- 
