462 
Journal of Agricultural Research 
Vol. XXI, No. 7 
state existing between C. vagum and its numerous hosts is not known. In 
the present soil temperature studies the use of the same soil and a single 
strain of host plant made the problem of controlling the various influ¬ 
encing factors relatively simple. The methods employed are discussed 
below. In general, an attempt has been made to depart as little as pos¬ 
sible from the natural conditions of potato culture. 
GENERAL METHODS AND MATERIAL 
APPARATUS 
The experiments were conducted in especially constructed water jackets 
now in use for soil temperature studies in the greenhouses of the Depart¬ 
ment of Plant Pathology of the University of Wisconsin. The essential 
features of these were described and illustrated by Jones ( 6 ) in 1917* 
Johnson and Hartman (5) in 1918 gave further details as to their structure 
and convenience for temperature control. It should be mentioned here 
that each unit or tank as used in the experiments consists of an insulated 
water chamber 3X feet long, 2 feet wide, and 1 }4 feet deep. Eight cans 
7 inches in diameter and 12 inches deep were so arranged in each water 
jacket as to form two parallel rows of four cans each. The cans were sup¬ 
ported in the water to a depth which just permitted of safety from over¬ 
flow. The y 2 inch to 1 inch of the cans thus remaining above the water 
projected into the hole in the cover in such a way as to provide for ample 
lateral support. Cold water and steam pipes with outlets were arranged 
in connection with the tanks so as to permit of the convenient use of 
either steam or water. Electric heating units were also placed at the 
bottom of the water jackets that were kept at the higher temperatures. 
Each water jacket thus equipped became an independent unit, with a pos¬ 
sible eight pots of soil all held at the same temperature for the growth of 
plants. The range of temperatures within the temperature limits of 
plant growth, therefore, was limited only by the number of units em- 
employed. 
TEMPERATURE CONTROL 
The large number of tanks rendered the use of automatic temperature 
regulation impracticable. It was found, however, that by personal 
adjustments three times during the 24 hours at intervals of approximately 
8 hours each, the temperature could be controlled within a range of 
variation sufficiently narrow to be adequate for obtaining the required 
data. At each visit the various water temperatures were recorded and 
proper adjustment was made. At those temperatures which approached 
tlie temperature of the greenhouse, the water was adjusted to the exact 
degree desired for the soil. At the lower and higher extremes, however, 
it was found necessary to raise or lower the temperature of the water 
