230 
AMERICAN JOURNAL OF BOTANY 
[Vol. 9 
the dosage for the variables compared in any experiment was uniform and 
that the variation in the amount of rust observed on the different groups 
of plants in the experiment is due to differences in the reactions of the plant 
tissue to the infection to which they were subject. 
The explanation of the apparent reversal of the result in soil-culture 
experiment V as compared with the others is probably to be found in the 
age of the plants and in the length of time they were exposed to infection. 
The experiments are compared in table 12. 
Table 12 
Experiment 
I 
II 
III 
IV 
V 
Age of plants at conclusion of experiment (days) 
49 
49 
49 
47 
45 
Age of plants when set out of doors to he in- 
oculated 
35 
35 
35 
31 
24 
Number of days out of doors and exposed to 
infection 
15 
15 
15 
16 
21 
66 
70 
120 
600 
600 
Average dry weight of top of plants (mg.) 
127 
278 
538 
264 
123 
Average infection (total leaf surface) 
1.8 
1.8 
2.3 
•9 
4.6 
Average infection (most severely infected leaf) . . 
5-4 
47 
57 
3-0 
10.4 
Experiment V differs from the other four experiments in that (i) when 
set out of doors to be inoculated the plants were from 7 to 1 1 days younger. 
Even at the conclusion of the experiment these plants had only half the 
dry weight of the plants of experiment IV and were evidently much less 
mature. (2) When the experiment was concluded the plants had been 
out of doors and subject to infection 6 days longer. If we allow an incuba- 
tion period of 10 days for the rust, then the rust present on the plants of 
experiment V at the conclusion of the experiment represents inoculation 
through a period of time twice as long as in the case of the other experiments. 
(3) The amount of rust on the plants at the conclusion of the experiment was 
several times greater in experiment V than in any of the other experiments. 
The last-mentioned fact immediately brings into view an aspect of the 
method of experimentation used tending to limit the value of the pustule 
count as a criterion of the frequency of penetration and infection by the 
uredospore germ tube. It is probable that only in cases of very sparse 
infection is there a pustule for every focus of infection, and that only in 
cases of very sparse infection is the number of pustules counted an accurate 
index of the number of infections which have taken place. With abundance 
of infection there appears a tendency for the coalescence of foci of infection, 
for two or more mycelia the result of contiguous infections to coalesce and 
produce only one pustule; and this tendency would be highly accentuated 
on the more vigorously growing host plants where the parasite finds a 
favorable nidus and develops more luxuriantly. In experiment V the 
error introduced by the coalescence of mycelia may well have masked a 
