May, 1922] 
RAINES 
VEGETATIVE VIGOR OF THE HOST 
225 
Soil Cultures 
From the studies of Sheldon, Ward, and Stakman, as also from the 
experiments described above, it may be considered as established that, 
within the range of forms worked with, conditions unfavorable to the 
growth of the host cause an increase in the incubation period of the rust and 
depress the luxuriance of growth of the fungous mycelium as indicated by 
the size of the pustules and the number and size of the spores produced 
in them. 
Concerning the effect of conditions unfavorable to the growth of the 
host on the incidence of rust infection — the number of successful infections 
produced on a unit area of host tissue by a given dose of inoculum — our 
knowledge must be regarded as not so definite. The data extant are sub- 
ject to criticism because of the relatively small number of variables studied 
and because of the irregularity of dosage inherent in the method of inocula- 
tion employed. Ward (1902Z?) applied spores to the leaf by means of a 
swab of cotton, and Stakman (1914, p. 11) employed a fiat inoculating 
needle for this purpose. 
Studies on the relation between host vigor and incidence of infection, 
to be of critical value, must be made with numbers of variables sufficient 
to preclude undue distortion of the results by fluctuations in condition of 
host and fungus, and by errors in the taking and studying of data; the 
method of inoculation employed must stand criticism as to the uniformity 
of dosage for the variables compared; and, if any but the grossest relations 
between the variables studied are to be made apparent, a more exact basis 
than visual observation and judgment must be employed for determining 
vigor of growth of host plant and degree of rust infection on it. 
In the experiments described below on the relation between host vigor 
in the oat plant and its susceptibility to crown rust, data were obtained on 
1450 individual plants receiving different nutritive treatment and exhibiting 
wide variation in vigor of growth. The plants were grown in pots in 
the greenhouse. Inoculation was effected under natural field conditions 
by placing the pots containing the experimental plants out of doors near a 
stand of oats heavily infected with crown rust. Analysis of the data indi- 
cates that the dosage for the variables compared was uniform. The experi- 
ments were concluded and the readings taken before the rust on any of the 
plants approached the maximum that the leaf tissue could support, so 
that the infection present at the time may be considered an index of the 
response of the host tissue to the conditions of inoculation to which it was 
subjected, and variation in this response between host tissues receiving 
similar doses of inoculum was presumably due to differences in the condi- 
tion of the tissues compared. 
Values for the vigor of the host plant and for the amount of rust in- 
fection present on it were obtained as follows: At the conclusion of the 
experiment the plant was cut off at the base and observations were taken 
