224 
AMERICAN JOURNAL OF BOTANY 
[Vol. 9 
Exp. 4. Effect of Varying Concentration of Nutrient Solution. Plants Inoculated April 2, 
1919 
Concen- Dry Weight 
No. of tration of j of Top 
Plant Nutrient ! of Plant 
Solution i in Mg. 
Observations 
April 8 
April 9 
April 14 
2. 
3- 
4- 
5- 
6. 
0.012 
0.009 
0.006 
0.003 
0.0015 
Distilled 
water 
140 
no 
no 
! No infection 
Rust showing 
on second leaf 
No infection 
Rust showing 
on third leaf 
100 j No infection 
(destroyed) 
No infection 
12 pustules on 
second leaf 
3 pustules on 
second leaf 
19 pustules on 
second leaf; 
10 pustules 
on third leaf 
No infection 
No infection; plant dying 
Large number of pustules 
on first, second, and 
third leaves 
5 pustules on second leaf; 
plant dying 
5 pustules on first leaf; 20 
pustules on second leaf; 
10 pustules on third leaf 
17 pustules on first leaf; 
8 pustules on second leaf 
In all the water-culture experiments (table 5) an increase in the incuba- 
tion period of the rust with depression in the vigor and rate of growth of the 
host plant was apparent. Coincident with the increased incubation period 
of the rust on host plants of poor growth and little vigor went always a 
marked depression in the luxuriance of the fungus. The pustules were 
appreciably smaller, and produced decidedly fewer spores. 
Comparing the incubation period of the rust on the leaves of the same 
plant, it is found to be shorter on the younger leaves. Comparing the 
first and second leaves, we find: 
Infection noted on the first leaf before it appeared on second leaf. . . . o 
Infection noted simultaneously on first and second leaves 5 
Infection noted on second leaf before first 7 
Infection on first leaf ; none on second 2 
Infection noted on second leaf; none on first 7 
The older host tissue, it would seem, provided a less congenial environ- 
ment for the development of the rust. 
Incidental to the above-described water-culture experiments was the 
demonstration of the ability of the rust to develop on chlorotic tissue. 
Some corn seedlings were grown in iron-free nutrient solution, and the 
fourth and fifth leaves produced by the plants were completely blanched. 
The plants were sprayed with a spore suspension to test the susceptibility 
of these leaves to the rust. Nine days after inoculation the chlorotic leaves 
showed abundant rust infection. Giddings (personal communication) 
has obtained infection with Gymnosporangium juniperi-virginianae on apple 
leaves blanched by being kept in the dark room while unfolding from the 
bud. It may be concluded that the presence of chlorophyll is not a neces- 
sary condition for rust development. 
