Feb. 1, 1925 
Urediniospores of Cronartium Rilicola 
289 
Table V.— Summary of measurement data; urediniospores of Cronartium ribicola 
and Cronartium occidentale 
Length 
Width 
Mean b 
wall 
thick¬ 
ness 
Ratio 
mean length 
mean width 
Species 
Mean 
length 
Stand¬ 
ard 
devia¬ 
tion 
Coeffi¬ 
cient of 
vari¬ 
ability 
Mean 
width 
Stand¬ 
ard 
devia¬ 
tion 
Coeffi¬ 
cient of 
vari¬ 
ability 
Field series: 
Cronartium ribicola_ 
23.5 
2.6 
11.1 
15.0 
1.8 
12.1 
1. 70 
1.57 
Cronartium occidentale_ 
25.2 
3.5 
13.7 
15.3 
1.8 
11.4 
2.05 
1.64 
Block Island series: 
Cronartium ribicola.... 
23.1 
2.6 
11.3 
15.1 
1.6 
10.8 
1.73 
1.53 
Cronartium occidentale_ 
24.9 
2.9 
11.8 
14.9 
1.7 
11.5 
2.18 
1.67 
Washington, D. C., greenhouse 
series: 
Cronartium ribicola. ... 
24.1 
3.4 
13.9 
15.4 
2.0 
12.9 
2.02 
1.57 
Cronartium occidentale_ 
25.0 
3.1 
12.3 
15.7 
1.9 
12.0 
2.16 
1.60 
° The length and width means are based on 1,000 measurements for each species in each series. 
b The mean wall thickness is based on 500 measurements for each species in each series. 
DISCUSSION 
Each of the three series of measure¬ 
ments serves a different purpose; for 
the mean measurements of the sets 
in the Field series evidently are better 
species indices than those in the two 
other sets, being based on a broader 
sampling system; and the means in the 
Block Island series show the difference 
in size of the urediniospores of the two 
species on a few Ribes hosts growing in 
the same locality, as well as the practi¬ 
cal utility of 100 spore means for 
diagnostic purposes; whereas the means 
in the Greenhouse series represent the 
variation in the urediniospores of the 
two species on the same host ( Ribes 
aureum) grown under experimental 
conditions in the greenhouse. 
It is evident from the graphs of the 
length means and wall thicknesses that 
the two species are distinct, as far as 
these two factors are concerned, in the 
Field series and Block Island series, 
but that they are not separable on the 
same bases in the Greenhouse series . 
The respective value of the measure¬ 
ments for diagnostic purposes appears 
to be in the order of length mean, 
mean wall thickness, and ratio of mean 
length divided by mean width. The 
width mean has no diagnostic value. 
The results indicate that measure¬ 
ments of the urediniospores from hosts 
growing in the greenhouse should be 
used with extreme caution, if at all, 
in any attempt to diagnose field col¬ 
lections of either species. The data 
obtained in the Field series and the 
Block Island series , however, appear to 
warrant the conclusion that field col¬ 
lections can be separated in most cases 
on the basis of spore size, spore shape, 
and wall thickness. The following 
discussion of diagnostic division points 
is accordingly confined to the data 
from sets 1-20 inclusive. 
The horizontal line drawn at 24.1 //, 
in Figure 1, separates 37 out of the 40 
length means correctly, all 20 means for 
C. ribicola being below the line and 17 
means for C. occidentale being above the 
line. One mean for C. occidentale falls 
on the line. 
The horizontal line drawn at 1.88//, 
in Figure 2, separates all 40 of the mean 
wall thicknesses correctly, the 20 for 
C. ribicola being below the line and the 
20 for C. occidentale being above the 
line. 
The horizontal line drawn at 1.59, 
in Figure 3, separates 29 out of 40 of 
the ratios of mean length divided by 
mean width correctly, 14 out of 20 for 
C. ribicola being below the line and 15 
out of 20 for C. occidentale being above 
the line. 
In other words the diagnostic division 
points, 
24.1 for the length mean, 
1.88 for the mean wall thickness, 
and 
1.59 for the ratio of mean length 
divided by mean width, 
would have correctly identified 92.5 
per cent, 100 per cent, and 72.5 per 
cent, respectively, of the sets measured. 
Examination of the graphs shows that 
the measurements for C. occidentale in 
set 5 are abnormal, that is, on the 
wrong side of the diagnostic division 
points for both length mean and ratio; 
that the figures are low for the length 
mean of the same species in set 6, low 
for the wall mean in sets 5 and 6, and 
below the line for the ratio in set 6. 
However, there is not a single case 
where the figures for any one set for 
either species fall on the wrong side 
of the line for all three criteria, and 
only one case, the one in set 5 men- 
