Transpiration as a Factor in Crop Production. 69 
protected tubes with watering jars which stood in the pit. Four 
others were surrounded by a 4-inch water jacket, which was kept 
filled with water. The water jackets as used were not exactly 
what water-jacket advocates recommend in that they were not 
sunk in the ground, but rather stood on top of the floor of the 
pit. A buried water jacket would have kept the potometer 
temperature more nearly the same as the field soil, and in future 
tests of the method this will be provided for. The advantage of 
surrounding the potometer with a water jacket rather than 
filling soil around it is that it may facilitate occasional weighing. 
Table 10.— Summary of effect of degree of exposure of potometer 
upon growth and water requirements of corn. 191^. 
Exposure of potometers 
Dry matter 
Total 
water 
tran- 
spired 
Transpiration per 
Ear 
Total 
gram dry 
wt. of ear 
gram total 
dry matter 
Grams 
Grams 
Kilos. 
Grams 
Grams 
(1) 
(2) 
(3) 
(4) 
(5) 
(6) 
Potometers fully expos- 
ed in pit to atmos- 
phere 
287.5 
557.6 
151.7 
527.8 
272.1 
Potometers buried in 
soil 
257.7 
545.3 
152.7 
592.6 
280.0 
Potometers in pit sur- 
rounded by 4-inch 
water jacket 
293.0 
602.5 
162.6 
555.0 
270.0 
The exposed, buried, and water-jacket potometers in the corn- 
field respectively produced an average yield of 557.6, 545.30, and 
602.5 grams of dry matter; transpired 151.7, 152.7, and 162.6 
kilograms of water; and used 272.1, 280.0, and 270.0 grams of 
water per gram of dry matter. The differences obtained from 
exposed and buried potometers are not large, and it would appear 
that reliable comparative data may be secured from large exposed 
potometers. 
The potometer soil temperatures were taken 15 inches below 
the surface both in the center and at a distance of 1 inch from the 
edge. The results are recorded in Table 11. The daily average 
soil temperatures of the exposed potometers was 4.1° F. higher 
than of those buried in the soil, which in turn were 2.4° F. warmer 
than the field soil at a depth of 1 foot. The water jacket raised 
the average temperature 0.5° F. The soil did not get quite as 
warm in the daytime but cooled off less at night. With 4.1 
