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 (1902) applied spores to the leaf by means of a 

 swab of cotton, and Stakman (1914, p. n) employed a flat 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 



