Feb 1,1925 
Urediniospores of Cronartium Bibicola 
291 
and the means for length and width 
illustrated in Figure 1 were based on 
the resultant 100’s. This procedure 
undoubtedly resulted in a set of means 
showing less variation than would be 
expected if the means had been based 
on the measurements of 100 spores 
from single specimens, in which case 
there would have been more danger of 
crossing the dead line 24.1. The 
points on the graph for the length 
means of sets 1-10 in the Field series are, 
therefore, not exactly comparable with 
the points for sets 11-20 in the Block 
Island series. As a matter of fact, the 
Block Island series is the only one of 
the three which illustrates the way in 
which biometric diagnosis methods 
based on measurements of 100 spores 
might work out when applied to un¬ 
knowns. 
The data indicate that 200 measure¬ 
ments would have been better than 100 
as a basis for determining the means; 
and, in cases where the species are close 
together as these rusts are, it would 
seem wiser to use the higher number. 
Means based on 200 measurements 
would probably have shown less tend¬ 
ency to cross the diagnostic division 
point than the means based on 100 
measurements. 
It would be preferable also in 
diagnosing unknown specimens to take 
spores from as many different leaves or 
sori as possible for each specimen 
studied; in other words, to sample the 
specimen with the aim of getting the 
best possible representative lot of spores 
for the mount. Unfortunately it was 
not practicable to follow this course 
in all the cases reported in this paper. 
The following examples indicate the 
futility of attempts to use measure¬ 
ments of fresh spores as size standards, 
particularly in the case of C. ribicola , 
unless, of course, they are to be com¬ 
pared with other measurements made 
on fresh spores. The mean measure¬ 
ments for 137 fresh urediniospores of C. 
ribicola from Ribes gracillimum, mounted 
in water, were found to be 28.7 by 
18.4, with a standard range of ap¬ 
proximately 25 to 33 by 16 to 23m- 
The walls of this set were not measured. 
The means of 50 fresh urediniospores, 
mounted in water, from a specimen 
collected on Ribes aureum growing in 
the greenhouse were 33.2 by 21.5m, 
with a standard range of approximately 
30 to 37 by 20 to 24/x. The mean 
wall thickness was 0.91m- These means 
and standard ranges are far out of 
line with the figures given in the 
tables for C. ribicola. On the other 
hand the means for 330 urediniospores 
from herbarium specimens of C. ribicola 
collected in the field on various hosts 
were 22.3 by 15.4m, with a standard 
range of 18 to 27 by 13 to 18m. The 
mean wall thickness was approximately 
2.00m- These figures agree fairly well 
with those for sets 1, 2, and 3 of C. 
ribicola in Table I, except for the wall 
mean; yet the results are not really 
comparable, except in a very general 
way, because the measurements were 
made some years ago by methods 
which were not comparable with 
those used by the writer. 
On the basis of the data presented, it 
would be going too far to expect bio¬ 
metric diagnosis methods to yield 100 
per cent correct results; but they are 
the only methods applicable in cases 
where one is dealing with herbarium 
material, and where inoculation ex¬ 
periments are impossible. The diag¬ 
nostic division points given in this 
paper can not be expected to hold good 
unless the mounting methods are 
rigidly followed. The measuring must 
be done with great care—as all real 
measuring should be done—and the 
measurements should be analyzed by 
statistical methods. Under such con¬ 
ditions the investigator can separate 
the uredinial stages of C. ribicola and C. 
occidentale in most cases with com¬ 
parative ease. 
In a later paper a biometric com¬ 
parison of the aeciospores of the two 
species will be presented. 
SUMMARY 
The above analysis of measurements 
on 3,000 urediniospores of Cronartium 
ribicola and Cronartium occidentale 
indicates that the two species may be 
separated in the uredinial stage with 
practical certainty on the basis of 
spore size, shape, and wall thickness. 
The most important criteria for 
biometric diagnosis are the length 
mean, the mean wall thickness, and 
the ratio of mean length divided by 
mean width. 
The diagnostic division point for the 
length mean is 24.1m, for the mean 
wall thickness 1.88m, and for the ratio 
of mean length to mean width 1.59. 
In the cases of collections from the 
field and from experimental plots these 
three diagnostic division points proved 
good in 92.5 per cent, 100 per cent, and 
72.5 per cent, respectively, of the trials. 
The dimensions of urediniospores 
produced under greenhouse experi¬ 
mental conditions do not appear useful 
for distinguishing these two species. 
The data presented in the paper are 
applicable only when the conditions of 
mounting, measuring, and analysis are 
strictly comparable. 
O 
