220 
AMERICAN JOURNAL OF BOTANY 
[Vol. 9 
Table 4 
No. of 
No. of 
No. of 
No. of 
No. of 
No. of 
Spores 
Inoculations 
Infections 
Spores 
Inoculations 
Infections 
I 
5« 
2 
52 
0 
2 
T ft 
0 
53 
0 
3 
D 
I 
55 
0 
4 
4 
0 
50 
0 
5 
1 0 
^ 
0/ 
J 
0 
4 
0 
50 
0 
/ 
-7 
0 
0 
oV 
J 
0 
0 
5 
^ 
0 i 
0 
9 
3 
I 
D2 
0 
1 0 
0 
I 
03 
0 
1 1 
2 
0 
04 
0 
1 2 
2 
0 
^0 
0 
13 
I 
07 
0 
T A 
2 
68 
J 
16 
2 
J 
6q 
0 
T 0 
Q 
72 
J 
22 
Q 
7/1 
0 
23 
I 
77 
2 
'J ? 
-0 
2 
2 
70 
Q 
27 
I 
0 
86 
I 
0 
25 
3 
I 
98 
0 
31 
I 
0 
100 
32 
I 
0 
103 
35 
I 
0 
104 
36 
2 
119 
40 
3 
0 
142 
42 
3 
0 
135 
44 
2 
0 
425 
47 
I 
0 
700 
48 
3 
0 
800 
50 
I 
0 
1000 
The results cannot be convsidered as entirely convincing in view of the 
many factors involved in a successful inoculation. The evidence indicates, 
however, that, as to the minimal number of uredospores which can possibly 
produce infection, it is possible for a single uredospore to produce infection. 
In each of the two cases in which infection was produced after inoculation 
with a single uredospore, the rust appeared after the usual incubation period 
as a very small pustule at the point of inoculation. In one case the in- 
fected leaf withered shortly after the appearance of the pustule; in the other 
instance the mycelium showed normal growth, and about a week later 
seven new uredosori were produced in a ring around the first pustule. 
As regards the second question, the minimal dose of uredospores which 
will certainly produce infection, the data indicate this to be, for the con- 
ditions under which the work was done, between 100 and 125. This is 
high. In view of the varying viability of spores taken at one time from the 
same pustule, and of their further variation with the age of the pustule, 
it was not considered possible actually to test the germination for each 
sample used. However, the spores were always taken from the surface of 
a large and pulverulent sorus, and, considering that the index of germination 
