Transpiration as a Factor in Crop Production. 13 
A comparison of plants grown in the potometers and in the 
field under normal conditions shows that the yield in the potom- 
eters was entirely normal. 
4. The limitation of the amount of soil thru the size of the 
potometer may be a great source of error in pot experiments. 
It seriously affects not only the transpiration relationships but the 
entire development of the plant. 
5. The soil in pots fully exposed to the sun acquired a some- 
what higher temperature than normal field soil, but the water 
requirement per pound of dry matter was affected only slightly. 
It appears that comparable data may be secured from exposed 
pots. 
6. Much variation exists in the results obtained from dif- 
ferent pots receiving similar treatment. Error due to such 
individuality is greatly reduced by repetition. 
7. The "probable error" has not been calculated for the 
results from these experiments, because it is evident in actual 
practice that knowing the "probable error" for such a small 
number of cases has little value. 
8. Transpiration is essentially evaporation. The rate of 
water loss from a corn plant is affected in a rather similar manner 
by changing climatic conditions as it is from a physical free water 
surface. 
9. The amount of water transpired from a given leaf-area of 
corn (based on expanse of leaf rather than both surfaces) is 
approximately one-third as great as the evaporation from a 
free water surface of the same area. 
10. The maximum transpiration as well as the maximum 
evaporation rates occur between 1 and 3 p. m. Prior to this time 
the increase is gradual from early morning, and following it the 
rate gradually recedes until late in the evening. The rate of 
water loss follows the same general sequence as the diurnal 
climatic factors. 
11. Transpiration is reduced relatively more during the night 
than is evaporation. This may be explained, perhaps, by the 
fact that the leaf stomata actually close almost completely at 
night, and as a result the opportunity for diffusion is reduced. 
12. Variation in the water requirement from day to day is 
very marked. Occasionally this daily variation amounts to 300 
or 400 per cent in successive days. The maximum variation ob- 
served in two successive days has been 600 per cent. 
On days of extreme temperature in very dry years there may 
be an atmospheric demand of 10 pounds of water from a single 
average corn plant during 24 hours. The greater part of this 
exists for a period of about seven hours in the driest, hottest 
