"Predisposition" and ''Immunity" in Plants. 317 



case (out of 13 tried) where B. madritensis (Stenobromus) showed 

 pustules. Moreover, there was only one exception, viz. B. commu- 

 tatits [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 — 



