RELATION OF SOME RUSTS TO THEIR HOSTS 
201 
at 110° C. for 30 minutes. It was found that there was much less 
contamination of cultures if the flasks were autoclaved just before 
using and allowed to cool in the autoclave. If this was not possible, 
the flasks were usually wiped off with a corrosive sublimate solution 
before using, since the roots of the corn plants often touched the out- 
side of the flasks while they were being placed in the solution due to 
the small neck of the flask. The cotton plugs used to stopper the 
flasks were replaced after flaming and as they were made rather loose 
and somewhat larger than the necks of the flasks, they fitted rather 
closely around the corn stems (Plate IV, figure 2). 
The flasks with plants thus prepared were placed in a moist dark 
chamber. This chamber was prepared by covering a galvanized tank 
(i^fl X I X 3 ft.) with a cover made of heavy black paper. The 
cover was a little larger than the tank and reached to the bottom on 
the sides, so that light was excluded and ventilation permitted. A 
layer of water was kept in the bottom of the tank while the work was 
being carried on in the greenhouse; this was enough to maintain a 
saturated atmosphere in the chamber. Later when the experiments 
were conducted in the drier air of the laboratory, coarse woven cloth 
which was kept wet was spread over the tank under the black paper 
lid in order to maintain the saturated atmosphere. 
After the plants had been in this dark chamber for several days, 
they were inoculated by spraying with a spore suspension and in 
addition a small quantity of spores was placed on certain leaves. 
They were kept at 20° C. for 24 hours, at which temperature the uredo- 
spores of Puccinia Sorghi germinate vigorously. 
The results of Experiments 24 to 31 are given in the following table. 
In all the cases where infection took place upon plants in Knop's 
nutrient solution or distilled water, the pustules were poorly developed 
and few in number. In the three experiments where such infection 
occurred, the infected portion of the leaves were cut off and the plants 
were reinoculated. No infection took place the second time on such 
plants in distilled water or Knop's nutrient. Plants in sugar solution, 
however, were infected, although to a somewhat less extent than in 
the previous experiments. The reinfected plants were left until all 
of them died. It was found that the plants which were infected in 
Knop's and distilled water in the original experiments lived as a rule 
longer than the others. This would indicate that infection in the 
original experiments upon these plants was due to a supply of food 
