5^6 
Journal of Agricultural Research 
Vol. XXIV, No. 7 
SUMMARY 
(1) In studying the nature and behavior of biologic forms careful 
uniform technic and pure rust strains have been used in order to 
reduce to a minimum the pgssibility of experimental error. 
(2) Spore measurements can be employed as an additional aid in 
identifying the biologic forms of P. graminis, provided a sufficiently 
large number of spores are measured for both length and width, and the 
spores studied are developed on congenial hosts and under favorable 
conditions. The different measurements should be grouped in classes 
for length and width, respectively. 
(3) On account of the variability in size of the different kinds of spores 
of each biologic form, a more ready comparison can be made by the use 
of biometric constants than by a mere mass of individual measurements! 
By the use of these constants the identity and relationship as well as the 
nature and behavior of a given biologic form may be further confirmed. 
Such biometric constants have been calculated and are incorporated in 
the respective tables. 
(4) Numerical differences in spore dimensions may or may not be 
significant in themselves. The important factor in determining the value 
of numerical differences consists in the probable error of the differences 
and in the relation of these differences to their probable errors. 
(5) The biologic forms of stem rust differ markedly and significantly 
in their various kinds of spores when produced under uniform conditions. 
An interesting parallelism is found to exist between the means of the 
spore dimensions of the different biologic forms. 
(6) In general, the wheat rust, P. graminis triticiy has larger spores of 
each type than any other biologic form. The oat rust, P. graminis 
avenaCy occupies the second place; the rye rust, P. graminis secalisy the 
third; and the timothy rust, P. graminis phleipratensisy the fourth. The 
redtop rust, P. graminis agrostisy has the smallest spores of all. 
(7) The differences in spore sizes do not occur in any consistent direc¬ 
tion, nor do they follow a logical sequence. In some cases the spores of 
two biologic forms may vary in both length and width, in other cases 
the differences may be in one dimension only. 
(8) Congenial hosts do not in any way change the morphology of stem- 
rust spores. A single host plant, common to several biologic forms, 
lacks the ability to unify them either in size or shape. Nor can several 
host plants, which are equally susceptible to a single biologic form, exert 
any influence on the spore morphology of this form. 
(9) Resistant host plants and adverse environmental conditions inva¬ 
riably tend to decrease the size of any type of spores of any biologic form. 
The reduction in size occurs most frequently in spore length only, but 
sometimes both length and width are affected. As soon as the unfavor¬ 
able conditions of host and environment are removed, the spores in ques¬ 
tion will attain their normal dimensions in a single generation. 
(10) It is of interest as well as of importance to establish the quan¬ 
titative measure of the spore shape of the different biologic forms. For 
this purpose the ratio of length to width should be determined and grouped 
into classes, the constants calculated and the significance of the difer- 
ences ascertained. 
