22 4 



AMERICAN JOURNAL OF BOTANY 



[Vol. 9 



Exp. 4. Effect of Varying Concentration of Nutrient Solution. 



1919 



Plants Inoculated April 2, 



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 Gymno sporangium 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. 



