EFFECT OF SOIL TEMPERATURE ON BEAN PLANTS 515 
small, to retard the flow of water into the cups. This was accom- 
plished by introducing mercury columns of equal heights into all the 
supply tubes. Later the mercury was not needed and was removed. 
The water baths employed were three in number, each 60 cm. in 
diameter and 25 cm. deep (ordinary galvanized iron laundry tubs) 
thus giving space for seven culture vessels each. A wood grating at 
the bottom supported the culture vessels and allowed theni to be sub- 
merged to within one centimeter of che top. 
Three temperatures, 34°, 22°, and 15° C, were arbitrarily decided 
upon, but this choice was governed somewhat by the facilities avail- 
able. The highest temperature was obtained by means of an electric 
heater under thermostatic control, and was maintained uniformly 
throughout the course of the experiment. 
The medium temperature followed that of the culture room; there 
was no special control in this bath. Because of the great bulk of 
water, the fluctuation in temperature was not very great. The range 
was from 20° to 23° C. (usually 21° to 22°) whereas the diurnal varia- 
tion in the temperature of the greenhouse room was large, 12° to 28° C. 
In our present state of knowledge of the influence of soil temperature 
on host or parasite this fluctuation is to be regarded as of little con- 
sequence but obviously some constant temperature might have been 
maintained with very little difficulty, by employing such an outfit as 
was used for the highest temperature. 
A constant water level was maintained in each of the two warmer 
baths by means of a Mariotte flask. 
The lowest temperature was obtained by passing a continuous 
stream of tap, water through the bath. When the water flowed at the 
rate of 1,500 cc. per minute a temperature of 15° was maintained, 
during the winter months. On \evy warm days a rise of two or three 
degrees sometimes occurred. The total rang« was from 14° to 18°. 
The surface of the water in the baths was covered with a thick 
paraflin oil to reduce loss of heat by evaporation and to eliminate the 
vapor blanket Aat would otherwise have been present over such an 
exposed body of water. Later it was found that a covering of ordi- 
nary parafifin (melting point about 50°) was very much better for the 
purpose. This was melted and poured on the water, where it was 
allowed to spread and harden. 
The soil in four of the culture vessels of each series was contami- 
nated by sprinkling in it, when nearly full, some soil heavily laden with 
