442 Wisconsin Academy of Sciences , Arts, and Letters. 
surface of the soil about the plants in order to maintain the same 
temperature in the surface layer as in the deeper portions. Ex¬ 
perience has shown that no single method is best for all purposes, 
the details being varied to meet particular needs. The main feat¬ 
ures are illustrated in text figures 2 and 3, and the essential details 
are given in the descriptions of these figures. By these methods 
the various soil temperatures desired have been maintained with a 
sufficient degree of constancy (in general within l°-2°) to meet 
our experimental needs while giving the plants otherwise favorable 
environmental conditions. In most of the series run the tempera¬ 
ture range has been as follows, including at the extremes the cold¬ 
est and the warmest soil temperatures at which the various experi¬ 
mental host plants will grow satisfactorily: 12°, 15°, 18°, 21°, 24°, 
27°, 30°, 33° C. (approximately equivalent to 53°, 58°, 64°, 70°, 
75°, 80°, 86°, 91° F.) 
Of course, different hosts respond differently as also do the vari¬ 
ous parasites. Hence temperatures both higher and lower than 
any in the series above given have been used in certain cases. It 
is to be noted that in all the trials, at least in the initial experi¬ 
ments, the aerial parts of the plants in each series have developed 
at the same air temperatures. The aim has been so to control this 
as to secure the best normal host development. This means, for 
example, that the experimental tomato and tobacco plants have been 
carried in a relatively warm greenhouse with air temperature dur¬ 
ing the day approximating 27°-30° C. (80°-85° F.) and falling 
several degrees lower at night. Potatoes, peas, etc., have been 
grown in a relatively cool house where the air temperature has 
ranged some 10°-20° F. below that of the warmer house. It is 
clearly recognized, of course, that some correlation between soil 
and air temperature is essential for the normal plant functions, and 
likewise that the degree of soil moisture may have a relation to the 
influence of temperature. Attempts have been made in connection 
with the work with certain diseases to determine in some degree 
these interrelations. While it does not seem wise to introduce the 
details in the present paper, it should be noted that the results are 
such as to restrain one from too hasty and broad generalizations 
concerning the influence of soil temperature as an isolated factor. 
As a matter of fact, in nature no one factor varies alone. The 
totality of evidence, however, leaves us confident that for certain 
types of soil parasitism, and within the usual limits of environ- 
