554 “ 
Journal of Agricultural Research 
Vol. XXIV, No. 7 
P. GRAMINIS AVENAK 
The means for this form were determined as46.i5 ±0.43 X 15.84 ±0.12/1. 
The difference in means between this biologic form and P. graminis iriiici 
is obviously quite significant; not so in the case of the difference in the 
means of the avenae and the secalis forms, however. Here the difference 
in the means of the length of the biologic forms is only 1.94 times greater 
than its probable error and therefore insignificant, because such a differ¬ 
ence is likely to occur naturally about once in every five random samples. 
There is, however, a significant difference in the means of the width of 
these two forms, it being 6.30 times greater than its probable error. 
P. GRAMINIS PHI.E)IPRATENSIS 
A reverse condition prevails in the timothy rust, as seen by examining 
its constants and differences in means. The means of this biologic form 
fall at 41.30 ±0.32 X 15.63 ±0.10 iu. In other words, the teliospores of this 
form are considerably shorter than those of the preceding three forms, 
but of the same or of nearly the same width as the teliospores of the avenae 
form. Table VIII shows that the difference in the means of the width of 
P. graminis phleipratensis and P. graminis avenae divided by its probable 
error is only 1.31, a very insignificant difference. The chances for an 
occurrence of such a difference are about 38 out of 100. 
Tabee VIII .—Summary of differences in the means of the sizes of teliospores of biologic 
forms of P. graminis 
Biologic forms. 
Difference in means (in 
microns). 
Difference in 
means divided 
by probable error 
of die difference. 
Length. 
Width. 
Length. 
Width. 
P. graminis tritici and P. graminis secalis. 
4 - 45 67 
I. 90±o. 17 
6. 64 
II. 23 
P. graminis tritici and P. graminis avenae. . 
P. graminis tritici and P. graminis phlei¬ 
S-6S± .65 
.83± .17 
8. 69 
4. 88 
pratensis. 
10. 5o± .59 
I. 04 ± . 16 
17. 80 
6. 50 
P. graminis tritici and P. graminis agrostis. 
II. 5 o± .63 
2. 03± . 17 
18.25 
II- 93 
P. graminis secalis and P. graminis avenae. 
P. graminis secalis and P. graminis phlei¬ 
I. 2 odb . 62 
I. o7db . 17 
I. 94 
6. 30 
pratensis . 
6.os± .5S 
. 86± . 16 
II. 00 
5-37 
P. graminis secalis and P. graminis agrostis. 
P. graminis avenae and P. graminis phlei¬ 
7. o5± .60 
.i 3 ± -17 
II. 74 
•77 
pratensis . 
4. 85± .34 
. 2I± . 16 
8. 98 
I-31 
P. graminis avenae and P. graminis agrostis .. 
P. graminis phleipratensis and P. gra¬ 
S-8S± -59 
I. 20 ± . 17 
9. 92 
7.06 
minis agrostis . 
I. oodb . 51 
. 99 ± . 16 
I. 96 
6.18 
P. GRAMINIS AGROSTIS 
The redtop rust has the smallest teliospores of any of the biologic 
forms, although the difference is not direct or absolute. These spores 
are much shorter than those of either the wheat, rye, or oat rust, but only 
a little shorter than those of the timothy rust. On the other hand, they 
are appreciably narrower than those of the wheat, oat, and timothy 
forms, but much the same in width as the teliospores of the rye rust. 
