18 
W. L. Balls . 
INFECTION OF PLANTS BY RUST-FUNGI. 
URING some work on the Uredineae it was noticed that the 
germ-tubes produced by spores on the surface of infected 
leaves would radiate from it in a velvety pile, if the surrounding 
atmosphere was kept saturated with water vapour. This suggested 
that water vapour might be the body in search of which the fungus 
entered the stoma. For many years it had been known that germ- 
tubes will enter the stomata of leaves which they are quite 
incapable of infecting. The earliest reference to this fact which I 
have been able to find is in 1853. 
While the experiments described below were being tried, a 
paper by Miss Gibson 1 appeared in the New Phytologist 
recording a detailed investigation of these “ futile infections,” 
and concluding that the first entrance of the germ-tube was due to 
some body, to which the hyphae were positively chemotropic, 
common to all plants and possibly gaseous. 
The identity of this common substance was already indicated 
in my own experiments by the behaviour of the germ-tubes in 
saturated air, and was tested directly by using a membrane of thin 
indiarubber, perforated with holes comparable in size with stomata. 
This membrane was arranged with one side exposed to air saturated 
(at 23°C) with water vapour, the other to the air of the laboratory. 
On the latter side were sown spores of Puccinia glumarum, var. 
hordei Eriks. 
After two days the membranes were fixed, and microscopically 
examined, when germ-tubes were found entering the majority of 
these artificial stomata. That this entrance was not due to chance 
was shown by the many tubes which, when once inside, passed 
straight over other holes, in no case manifesting any tendency to 
return to the dry outer air. 
The only apparent difference between these and natural 
infections was the absence of any marked vesicle at the end, which 
in nature is probably caused by chemical stimuli from the mesophyll 
cells. 
Other substances may possibly share in attracting the germ- 
tube to the interior of the plant, but this conception of the germ- 
tube as growing from an area having a low partial pressure of 
water vapour to one with a higher pressure, accounts for all the 
facts known about the first entrance of the germ-tube. 
1 C. M. Gibson. Notes on Infection-Experiments with various 
Uredineie, New Phytologist, 3 , p. 184, Oct. 1904. 
