78 
AMERICAN JOURNAL OF BOTANY 
[Vol. io, 
With this disease, since the fungus is growing in the xylem vessels, toxic 
products could be promptly carried to the leaves. 
To consider the manner in which the toxin affects the host, as a starting- 
point there is the fact that a wilt is produced. Caldwell (2), discussing 
wilting, says: 
Permanent wilting may result from the operation of either one or both of two factors: 
(a) decrease in the soil moisture content, ( b ) loss of water from the plant by transpiration. 
Attainment of the permanently wilted condition indicates a certain degree of reduction in 
the amount of water contained within the organism. 
The primary effect of a toxic substance such as has been supposed to 
exist here would probably be upon the permeability of cell membranes. 
The permeability of the membranes might be increased, in this case facili¬ 
tating loss of water from the cells, and, as a result, there might be a rapid 
increase in the rate of transpiration from the leaf as a whole. On the other 
hand, it is quite possible that permeability is decreased and that there is a 
slowing up of the movement of water from cell to cell. As a result of this, 
those cells farthest away from the veins might dry up completely while cells 
adjacent to the veins remained turgid. 
Livingston (8) has shown that permanent wilting may occur with the 
stem and roots functioning normally, when the rate of transpiration from 
the leaves is greatly increased. 
Methods of Experimentation 
The experiments here described were all conducted in the greenhouses 
at the University of Wisconsin, and the Wisconsin temperature tanks were 
used in all cases in which soil temperature was controlled. In these tanks 
cylindrical cans of galvanized iron are immersed in water, the temperature 
of which is controlled by an electrical heating device. The most uniform 
infection occurred when a good loam soil was sterilized by steaming for 
three hours at six pounds’ pressure and subsequently inoculated with 
Fusarium lycopersici. Neither the same loam not sterilized but artificially 
inoculated, nor imported naturally infected soils gave such consistent infec¬ 
tion as was secured when soils were first sterilized and then inoculated. 
Unless otherwise indicated, freshly sterilized soil was inoculated by mixing 
with it a quantity of the mycelium of the fungus. For this purpose the 
fungus was so grown on rice in 500-cc. Erlenmeyer flasks as to obtain an 
abundant mycelial growth. The inoculated soil was then incubated at 
about 25 0 C. for a week or ten days in order to give the fungus time to 
permeate thoroughly and uniformly the soil mass. The galvanized-iron 
containers used in the Wisconsin temperature tanks were partially filled 
with sterilized soil, the upper four inches were filled with the inoculated soil 
prepared as described above, and the whole was thoroughly mixed. These 
containers were then incubated for several days at the temperatures to be 
maintained throughout the experiment. Thrifty tomato plants (4-5 inches 
