Transpiration as a Factor in Crop Production. 119 
field itself. The extensive data concerning the relative evapora- 
tion and transpiration rates reported in pages 87 to 116 have 
shown that these two phenomena (which are really only one 
phenomenon acting on two somewhat different media) are 
strikingly parallel, and respond in a rather similar degree to similar 
influences. It is entirely probable that, soil conditions being 
equal, the relative evaporation rate in two conditions, localities, 
or regions is a very good index as to the relative water require- 
ments of similar crops grown under those conditions. 
Two of the chief objects sought in the tests which follow were: 
(1) In what degree is the vegetation in a field a protection against 
excessive transpiration as measured in terms of evaporation from 
a free water surface at different elevations within the growing 
crop; and (2) to what degree is such a crop likely to serve as a 
protection against moisture evaporation from the soil. 
The method used was to place a number of evaporation jars 
of 36 square inch surface area and 1 gallon capacity at different 
elevations in corn and small-grain fields. The lowest jar was 
placed with its surface level with the ground (unless otherwise 
stated) and the highest jar stood above the crop even after it 
had attained its maximum growth. Studies were made in the 
cornfield in 1912, 1913, and 1914, in spring wheat in 1912, and in 
oats in 1913. 
In the cornfield, the evaporation jars which regularly stood at 
different elevations at the north end of one of the potometer pits 
were not used for these determinations. But a separate series 
of jars was placed out in the cornfield under strictly normal 
field conditions where Hogue's Yellow Dent corn was grown 
three plants in a hill, with hills 3 feet 6 inches apart. Moving 
leaves and birds were prevented from disturbing the water by 
arching woven chicken wire over the jar. Normally, the water 
was \ inch below the surface of the jars, which were weighed in 
grams each evening and the evaporated water replaced to a 
standard weight. 
The results are contained in Tables 44 to 50. It must be 
borne in mind that crops do not make exactly the same growth 
each year and are lodged more some years by wind than others. 
Such influences disturb somewhat the normal relationships from 
year to year. 
The evaporation rate within the crop increases with an 
increase in elevation. The crop doubtless causes a higher atmos- 
pheric humidity and lower wind velocity at the bottom of the 
vegetation, which change with the elevation. It certainly would 
appear that a plant afforded the protection of fellow plants 
under field conditions, against the water-dissipating influences 
