317 • 
“ Predisposition ” and “ Immunity ” in Plants. 
case (out of 13 tried) where i?. madritensis ( Stenobromus ) showed 
pustules. Moreover, there was only one exception, viz. if. commu- 
tatns [9] to the generalisation that the spores from B. secalinus 
infect every species tried, nine in all, of its own group — Serra- 
falcus. However, in view of the fewer series of infections (see 
Table III.) with these spores I refrain from speculating on these 
results. 
In the note quoted, I also referred to successful pure-cultures 
of this Uredo in glass tubes, aerated or not, and using sterilised 
“ seeds ” of the Bromes employed ; to certain points not hitherto 
known, apparently, regarding the effects of temperature; to the 
fact that no explanation of the differences in infection could be 
derived from my examinations of the anatomy of the plant ; and 
to the fact that the uredospores will germinate in extracts of 
leaves which they are unable to attack successfully in the living 
state. 
The conclusions arrived at were, substantially, that the mutual 
relations between host and parasite depend not only on the 
influence — nutritive and otherwise — of the previous host on the 
spores themselves, making the latter “virulent” or “weak” towards 
any prospective host, but also are to be referred to positive re- 
actions on the part of this prospective host, so that the latter is 
“ predisposed ” or “ immune ” to the attacks of the spores ; and 
some remarks were added on the nature of the evidence supporting 
such conclusions. 
The more immediate object of the present paper is to give some 
account of an attempt to bring this question of predisposition 
and immunity to a test of another kind. 
It has been asserted and denied that certain structural pecu- 
liarities in the host, such as the amount of wax on the leaves, the 
number of stomata, the thickness of the cell-walls, and so forth 
are the determining factors as to whether infection can or cannot 
take place in such cases as we are considering here ; but nothing 
has been done, so far as I can discover, in the way of a thorough 
comparative examination of these points in this connection. 
The plan decided upon was as follows. The leaves — first green 
leaf of the seedling — of two series of pot-plants employed for 
comparative infection experiments, were cut off and preserved at 
the end of the experiments, and subjected to rigorous examination 
during the autumn and winter to determine the following points. 
The numbers and sizes of the stomata and' the hairs per square 
millimetre of surface, both above and below; the breadth and 
thickness of the leaf, and the proportions of vascular tissue 
to chlorophyll-tissue ; the character of the motor cells, and so 
forth. 
Care was taken to employ the same part of the same organ — 
