May, 1922] 
RAINP:S — VEGETATIVE VIGOR OF THE HOST 
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advantage of such a method of maintaining stock cultures of cereal rusts 
is that it eliminates the most technical operation, that of sowing or applying 
the fungous spores to the new host, and reduces the problem of maintaining 
cereal rusts in culture in the greenhouse to a non-technical routine such as 
can be entrusted to the average gardener or greenhouse man. 
In growing the rust under aseptic conditions on seedlings in test tubes, 
the method developed was to treat the seed with chlorine water {cf. Wilson, 
1915), put the seed to germinate on filter paper in Petri dishes, and transfer 
the germinated seed to a test tube plugged with cotton. Half an inch of 
sterile water was put into the test tube with the plant. The reserve food 
materials of the endosperm are capable of bringing the seedling to the third 
leaf stage, which is sufficient to raise a generation of the rust on it. P. 
coronifera was cultivated for 10 generations in this manner, transfers being 
made once a month ; P. Sorghi for 8 generations; P. triticina for 8 generations; 
and P. secalina for 6 generations. A small platinum spatula was employed 
for making transfers; spores were applied to the upper surface of the first 
leaf in each case, and material for inoculum was taken from the under 
surface. That a cereal rust can thus be grown under conditions free from 
accidental contamination was indicated by the total absence of organic 
growth, bacterial or fungous, when a rust-infected seedling was deposited 
on sterile beef-peptone agar. 
Studies on the Incidence of Infection by Measured Doses of Uredospores 
of Puccinia Sorghi on Zea Mays 
An effort was made to determine the minimal dose of uredospores of 
P. Sorghi that (i) can possibly, and (2) will certainly produce infection in 
Zea Mays. 191 tests were made on corn seedlings growing under aseptic 
conditions in twelve-inch test tubes. 
The method employed to determine the dose and to inoculate was as 
follows: A dilute suspension of uredospores was made in a vial of sterile 
water. A small drop from this suspension was put on a piece of sterile 
cover slip, and the number of uredospores in the drop was counted under 
the microscope. The piece of cover glass was then inverted and deposited 
on the upper surface of the first leaf of the young corn seedling in the test 
tube, about one half inch below the tip, bringing the drop of water con- 
taining the known number of uredospores in contact with the host tissue. 
The work was done in the winter in the laboratory, with no rust growing 
free anywhere in the building, so that the danger of accidental contamina- 
tion was negligible. No infection ever developed on the seedlings except 
on the spot where the plant had been inoculated. 
The inoculated seedlings were kept under observation for 21 days. 
If the inoculated leaf yellowed or withered before 15 days, the plant 
was discarded. The data on 191 tests are shown in table 4. 
