Jan. 3,1916 
Hourly Transpiration Rate on Clear Days 
623 
The transpiration measurements (see Table XXIX) include six day 
records on three successive days in July. The corresponding physical 
measurements are given in Tables XXX to XXXIV, inclusive, and the 
hourly means are plotted in figure 8. 
While these measurements were made during what we have termed 
“clear days,” the sky was not wholly free from cumulus cloud during the 
period, and this is reflected in the radiation curve, which does not quite 
reach its normal value during the late morning hours. 
Comparison with the conditions prevailing during the rye transpira¬ 
tion period, which extended over the two preceding weeks, will show that 
the evaporation was distinctly higher during the amaranthus period. 
The temperature during the latter period was slightly lower, but the 
saturation deficit was greater. Yet the transpiration graph of Amaran¬ 
thus retrofiexus gives no indication of the flattening which is so marked 
in the transpiration graph of rye. There appears then to be a marked 
difference in this respect in the response of the two plants to the march 
of radiation and other cyclic factors. 
GENERAL DISCUSSION 
It seems desirable at this point to summarize briefly the prevailing 
climatic conditions at Akron during the growth period of plants and 
more particularly during the transpiration periods included in the above 
determinations (Table XXXV). Akron is located in the rolling short- 
grass plains of northeastern Colorado. Absolutely clear days seldom 
occur, but often there are days with only a few light cumulus clouds in 
the sky, and during such days the plants are rarely shaded from the 
direct rays of the sun. Such brief interruptions in the direct radiation 
appear to have little influence on the hourly transpiration rate. On the 
other hand, there are many days during which cloudiness develops, espe¬ 
cially in the afternoon, not infrequently accompanied by light rain and 
high wind. The number of days which may be classified as clear in the 
above-defined sense forms consequently a relatively small part of the 
growth period of the plants. The measurements presented in this paper 
have been made on practically cloudless days. The radiation intensity 
at midday on clear days in midsummer is normally about 1.4 calories 
per square centimeter per minute on a surface normal to the sun's rays. 
In the 1912 experiments the hazy condition of the atmosphere, together 
with the shading effect of the hail screen, combined to reduce the maxi¬ 
mum radiation to 0.8 calorie during the wheat transpiration period, 1.02 
calories during the oat transpiration period, and 1.05 calories during the 
sorghum measurements. The plants during the 1912 measurements 
were consequently obliged to dissipate only from 60 to 75 per cent as 
much solar energy as in the 1914 experiments. 
