540 
Journal of Agricultural Research 
Vol. XXIV. No. 7 
EXPERIMENTAL METHODS 
The aecial material used for making spore measurements was obtained 
from several species of barberry (Berberis spp.) artificially inoculated 
with teliospores obtained from wheat, quack grass, oats, and redtop, 
respectively. The infected plants were collected in the spring of the year 
at University Farm, St. Paul, Minn. Young, tender leaves of separate 
barberry plants were inoculated with the teliospores from each of the 
hosts. Sets of seedlings of wheat, rye, oats, and redtop were inoculated 
with the mature aeciospores. The infection results thus obtained 
verified the identity and purity of each of the biologic forms. The 
aeciospores used for the determination of the spore size of each biologic 
form were grown under identically the same cultural conditions and on 
plants of the same barberry, species, namely, Berberis vulgaris L. In the 
study of the effect of the host on the dimensions of aeciospores, the cul¬ 
tural conditions were exactly alike, but the barberry species differed in 
each case. The aeciospores obtained in this manner were first put away 
as herbarium material and were not measured until several months after 
maturity. Fifty aeciospores picked out at random from a considerably 
larger population were measured for both length and width. 
Pure strains of urediniospores of each of the biologic forms of P. 
graminis were obtained from specimens of viable uredinial material on 
grains and grasses, collected at University Farm, St. Paul, Minn., and 
cultured continuously in the greenhouse on seedlings of different host 
plants. The measurements in each case were made of healthy uredinio¬ 
spores from the superficial layer of mature uredinia, care being taken to 
measure spores from uredinia in the same stage of development. In 
determining the urediniospore dimensions of the individual biologic 
forms, 200 spores in each population were measured for length and width, 
respectively, whereas in the study of environmental and host effects loo 
spores were measured in every case. For the latter experiment, however, 
200 measurements were made of the controls. In the first instance, that 
is, in the study of the comparative morphology of the urediniospores, the 
biologic forms concerned were grown on congenial host plants and under 
uniform and favorable conditions. In the study of relationship of host 
to urediniospore dimensions, both susceptible and resistant varieties 
were used, but the environment was maintained uniform and favorable. 
In the study of the effect of cultural conditions on the size of uredinio¬ 
spores, only one biologic form, P. graminis tritici, andone variety of wheat, 
Haynes Bluestem, were used, the environmental conditions, such as 
temperature, light, and soil moisture, being interchangeably altered. 
Control series were conducted simultaneously with the experimental 
series. 
The telial material used in this study was collected at St. Paul, Minn., 
and kept in the herbarium for nearly a year before the measurements were 
made. The rust on the common wheat (Triticum vulgare Vill.) was 
P. graminis tritici; on the quack grass {Agropyron repens (L.) Beauv.) 
it was P. graminis secalis; on the oat {Avena saliva L.) it was P. graminis 
avenae; on the timothy {Phleum pratense L.) it was P. graminis phleipra- 
tensis; and on the redtop {Agrostis alba L.) it was P. graminis agrostis. 
One hundred teliospores were measured in each test and the statistical 
results computed from these. 
All of the spore measurements were made with the same microscope, 
carefully calibrated and standardized. The Zeiss screw micrometer was 
