340 Robinson . — Experiments on Pnccinia malvacearum . 
5. In the case of certain leaves {Pelargonium , Eucalyptus , Primula) 
with glandular secretions, a definite toxic effect was evidently exerted on 
sporidia and germ tubes in the immediate neighbourhood of the fragment of 
the leaf. This is a special phenomenon presumably related to the presence 
of glandular secretions and cannot be extended generally to leaves that are 
not susceptible to infection. 
So far as the results bear on the question of why the normal host 
should be infected whilst other plants are not, they simply confirm this fact 
without explaining it. They do not support any of the explanations so far 
offered of this natural immunity or susceptibility, nor do they indicate the 
direction in which an explanation is to be sought. 
The experiments, however, result in indicating that the germ tube is 
irritable to light, to a certain degree of moisture, and to contact. These 
irritabilities are possibly advantageous in bringing the tip of the germ tube 
of a sporidium into the most favourable position for infection, but the 
problem as to what permits or determines actual infection still remains. 
In conclusion, I wish to acknowledge my indebtedness to Professor W. H. 
Lang for suggesting this investigation and for his continual advice and 
assistance in the course of the work. I am also grateful to Mr. D. Thoday 
for his helpful interest in the experiments. 
Cryptogamic Research Laboratory, 
University of Manchester. 
