NEGATIVE HELIOTROPISM OF UREDINIOSPORE GERM-TUBES 85 
nature of the stimulus, or combination of stimuli, that is responsible 
for the stoma tal entrance of germ-tubes of rust spores and of other 
parasitic fungi has not been definitely ascertained. The more general 
belief is that the host exerts a positive chemotropic influence on the 
germ-tube, but the failure of attempts to prove that chemotropism 
plays an important part in this process has been practically universal. 
If a chemotropic attraction by the host is to.be assumed in the rusts 
it must be considered a very general property of plants and not a 
specific property of the hosts alone, since urediniospore germ-tubes 
of a number of rusts have been shown by Gibson^ to enter the stoma ta 
of non-hosts as well as those of their hosts. That positive hydro- 
tropism may partially explain stomatal entrance is suggested by the 
work of Balls^ and of Fulton*^ but it is doubtful that this can be a factor 
of primary importance. What part the action of light may play in 
bringing about the stomatal entrance of the urediniospore germ-tube 
is as yet a matter of conjecture. It seems quite probable, however, 
that a continued turning away from light may serve to bring the 
germ-tube into close contact with the surface of the host and be chiefly 
responsible for its entrance into the stomatal opening. This is, of 
course, only a preliminary stage of infection. The success or failure 
of the attack on the host tissue from the substomatal chamber cannot 
be directly influenced by light. 
Purdue University, 
Lafayette, Indiana 
4 Gibson, C. M. New Phytol. 3: 1 84-191. 1904. 
5 Balls, W. L. New Phytol. 4: 18-19. I905- 
5 Fulton, H.R. Bot. Gaz. 41: 81-108. 1906. 
