Oct 23 ,1916 Daily Transpiration during Normal Growth Period 187 
The shadow areas of the other crops were computed from the pro¬ 
jection of the geometrical figures which they most closely approximated, 
such as cylinders, spheres, or inverted truncated cones. 
While measurements of this kind are at best approximations, the 
results given in Table XVII are consistent in showing that the energy 
received directly from the sun is insufficient to account for the energy 
dissipated by the plants through transpiration during the midday 
hours. Only in the case of alfalfa in the open is the direct solar radia¬ 
tion intercepted by the plant sufficient to account for the observed 
transpiration. Even on bright days, therefore, other sources of energy, 
such as the indirect radiation from the sky and from surrounding objects 
and the heat energy received directly from the air, contribute mate¬ 
rially to the energy dissipated through transpiration. Comparative 
transpiration measurements of shaded and unshaded plants also show 
that the energy consumed in transpiration is only partially attributable 
to direct radiation. 
RELATION OF TRANSPIRATION TO THE WEATHER 
If transpiration is determined absolutely by the intensity of any one 
of the weather factors, the ratio of the daily transpiration to the daily 
intensity of this factor should give a regular graph when plotted. This 
graph, if the correlation were perfect, would be an expression of the 
relative transpiration coefficient of the plant considered. 
In figure 5 are plotted the ratios of the daily transpiration of Kubanka 
wheat to the daily evaporation from the shallow tank. This graph 
shows a gradual increase in the transpiration coefficient to a maximum 
on July 13, followed by a somewhat more rapid decrease to harvest. 
While the graph shows many irregularities a similar response in trans¬ 
piration and evaporation to changes in weather is indicated. 
The second graph shows the ratio of transpiration to wet-bulb de¬ 
pression. Outstanding points in this graph indicate, of course, that on 
certain days the transpiration values were influenced by some factor 
other than the dryness of the air. Reference to the radiation graph 
will show that on such day's the transpiration-radiation ratio is normal 
or shows a departure in the opposite sense to the ratio of transpiration 
to wet-bulb depression. In other words, both factors enter into the 
determination of the transpiration. 
Similar departures are in evidence in the transpiration-temperature 
graph, and it will be noted that such departures are usually in an oppo¬ 
site sense to one or the other of the graphs just discussed. Tempera¬ 
ture, therefore, also enters into the determination of the transpiration. 
The ratio of transpiration to wind velocity is plotted at the bottom of 
figure 5. The change in the transpiration coefficient is evident in this 
graph, but no marked correlation is indicated. 
