May 19,1923 
Statistical Stvdy of,Biologic Forms 
563 
Table XII .—Summary of differences in the means of urediniospores of P. graminis tritici 
subjected to various environmental conditions 
Environmental conditions favorable except for— 
Difference in means (in 
microns). 
Difference in 
means divided by 
probable error of 
the difference. 
Excessive heat (average 81.8° F.). 
Length. 
3. I5±0.28 
3. 48± .28 
I. 89± .30 
Width. 
0. 08 io. 12 
. lOib . IO 
. 48 ± .09 
Length. 
11. 2 $ 
12. 42 
6. 30 
Width. 
0. 67 
I. 00 
5 - 34 
Deficient light (average 3.1 per cent).... 
Drought (water content 5.38 per cent).... 
general discussion 
No definite statistical information concerning the dimensions of the 
different types of spores of the biologic forms of stem rust existed 
prior to this study. 
Measurements of aecio- 
spores, urediniospores, 
and teliospores, devel¬ 
oped under different 
sets of conditions and 
on different host plants, 
were made with the aim 
of supplying such infor¬ 
mation. Biometric 
methods were employed 
with a view of ascer¬ 
taining the significance 
of the variations dis¬ 
covered. These meth¬ 
ods were used merely as 
a means to ascertain 
more about the nature 
and behavior and the 
identity of the different 
biologic forms. That 
results thus obtained a^e more reliable than a mere mass of individual 
measurements is now generally conceded. 
The statistical data presented in this paper, in addition to corrobgrating 
the empirical results obtained by previous workers concerning the 
differences in the size of urediniospores, also indicate very strongly the 
existence of similar differences in the aeciospores and teliospores of the 
biologic forms of Puccinia graminis. The differences in magnitude of 
the various types of spores seem to be quite marked, although they do 
not occur in any definite order or logical sequence. Sometimes a signifi¬ 
cant difference appears in both length and width of spores; on other 
occasions the .difference can be detected in the spore length only, the 
width being almost identically the same, and vice versa. 
A numerical difference alone may or may not be significant in itself. 
A variation of i micron in the means of two measurements, for instance, 
may in one case be ascribed to experimental error and on another occasion 
constitute a fundamental difference. The difference in the means of 
teliospores of P. graminis phleipratensis and P. graminis agrostis is a 
Fig. 12.—^Differences in widths of urediniospores of Puccinia graminis 
tritici cultured under favorable and adverse environmental con¬ 
ditions. 
