442 
Journal of Agricultural Research 
Vol. XXIV. No. 6 
data, considerable overlapping of the temperature range of one group 
onto that of another. If spores of all the species were present on the 
sweet potatoes, one might expect the species found within one of these 
groups to predominate to a greater or less degree in the amount of infec¬ 
tion they provided, within the temperature ranges of the particular 
group, unless the number of spores of some of the species present that 
belonged to the other groups greatly exceeded that of the particular 
group under consideration. Within the limits of their temperature 
ranges one would expect to find some infection by all the species present. 
The results of the investigation in this field will be discussed later in detail. 
APPARATUS 
Much of the investigational work was carried out in a series of insulated 
infection chambers, the construction and dimensions of which are given 
in figure i. Heat is supplied by means of a heating coil composed of 
Fig. I. —^Infection chamber. A, front view; B, horizontal cross section. 
No. 27 nichrome wire wound around ^-inch transite board 2 inches wide 
and 14 inches long. The floor and end wall immediately adjacent to the 
heating coils are insulated with transite board. Evaporating pans for 
the control of the humidity are located one-half inch above the heating 
coils. Each chamber is provided with standardized dry and wet bulb 
thermometers, the latter being covered with thin muslin kept wet by a 
capillary stream of distilled water, the excess of which is drained off by 
