RELATION OF SOME RUSTS TO THEIR HOSTS 203 
"steeping in various antiseptics, or by heating to 60-70° C." The 
sterile seeds were placed in sterile drying towers, supplied with Knop's 
mineral solution and aerated with a continuous current of air or were 
placed in large sterile test tubes which contained the solution. When 
the first leaf was well developed it was inoculated with uredospores of 
Puccinia dispersa. Good infection was obtained in the inoculated 
area. 
Ward's object in developing this method was to be sure he was 
working with only one race of Puccinia dispersa and not so much to 
free the rust from saprophytic fungus spores. He does not say that 
the spores with which he inoculated his sterile plants were free from 
other fungus spores yet he assumes that he had a pure culture as far 
as fungi were concerned. He says (p. 459), however, that the method 
does not exclude harmless bacteria. In order to be sure that spores 
of saprophytic fungi were not present in the sowing of the rust spores, 
it would have been necessary to sow the spores of the resulting rust 
upon nutrient media. This is necessary, since many saprophytic 
fungus spores do not germinate except in the presence of such nutrient 
media and so would remain dormant upon the surface of the plant in 
the infected area and be removed with the spores of the resulting rust. 
Two methods have been developed for obtaining pure cultures of 
the rust. The first method is a modification of Ward's. Large test 
tubes (30 X 5 cm.) were prepared by filling the lower end with absor- 
bent paper and adding Knop's mineral solution. These were then 
stoppered with cotton plugs and autoclaved at 10 pounds pressure for 
30 minutes. Two or three seeds sterilized in .5 percent HgCl2 solution 
for 30 minutes were placed in each test tube and the tubes placed in a 
well lighted window. After the seedlings had developed one or two 
leaves, they were inoculated with uredospores of Puccinia Sorghi 
(Plate V, figure i). 
The uredospores were obtained from well-developed pustules of 
Puccinia Sorghi and were placed in a capsule of sterile distilled water 
and thoroughly mixed up. The uredospores of Puccinia Sorghi, as 
well as most other rusts, are much lighter than water and float on the 
surface, from which they can be removed with a looped platinum wire 
and placed upon the leaves of the corn. The first trials to obtain 
infection in this way were failures because of the lack of adhesion of 
the water drops to the waxy surface of the corn leaves. In ordinary 
infection work, the leaves are gently drawn between the fingers before 
