142 
AMERICAN JOURNAL OF BOTANY 
[Vol. 10, 
plants were found to be much decayed, only a few roots thrown out at the 
very surface of the soil being white and clean in appearance. Portions of 
partly decayed roots were plated out, and cultures of saprophytic fungi 
and bacteria were secured. In one instance, however, a root was plated 
out which gave pure cultures of F. lycopersici , the fungus apparently growing 
out from the decayed root cortex only. These cultures were preserved and 
proved through inoculation experiments to be virulently pathogenic. 
These and other considerations have led to the view that the immunity 
induced by soil saturation is probably a host-plant factor. Thus the fungus 
grows very well in liquid culture media, and it may be present in the par¬ 
tially rotted roots. Furthermore, these plants throw out many new roots 
at the surface of the soil; through these roots infection could readily take 
place if infection were possible. 
From microchemical analysis it was found that the saturation plants 
were markedly different from normal plants in their nitrogen relations. 
Resistance in these plants seemed to be correlated with the absence of 
nitrate nitrogen. This relationship was tested by growing plants in 
sand culture and adding nutrient solutions. A complete nutrient solution 
was supplied to part of the plants, and a solution lacking nitrate to the 
remainder. The temperature conditions in the greenhouse where this work 
was conducted did not permit a virulent development of the disease, but it 
was conclusively shown that plants grown without nitrate, the tissues of 
which plants give no nitrate test, are not infected by the fungus. Plants 
grown with a complete nutrient solution were readily infected. 
Experiments Dealing with Resistance Induced by Low 
Soil Moisture 
It has been shown by carefully controlled experiments that succulent, 
rapidly growing plants produced under optimum moisture conditions (23-33 
percent) are subject to the disease, while the woody, slowly growing plants 
which result from low moisture (13-20 percent) are not readily attacked. 
Rapidly growing, susceptible plants were made resistant by allowing 
the soil to dry, the growth being checked in this way. Slowly growing, 
resistant plants were made susceptible by making the soil moist, and thus 
inducing a rapid, succulent growth of the host. Thus, at the conclusion of 
experiment V there were set aside eleven plants which had been growing 
with the soil very dry and at a temperature of about 27 0 C. for five weeks 
without showing the wilt. The soil of four had been made moist; the re¬ 
mainder were allowed to continue dry. In a week the four plants in the 
moist soil had changed over into a succulent, rapidly growing condition, 
and in three days more all were showing the initial symptoms of wilt. The 
disease progressed rapidly, and soon the plants were completely wilted. 
The seven plants which had been allowed to remain dry during this time 
did not develop any disease and were in the same condition at the end of the 
experiment that they had been in at the beginning. 
