“ Predisposition ” and “ Immunity ” in Plants. 323 
general average trend of corresponding parts of these curves, and 
so test the question, Does the predisposition or immunity of a 
given species bear any relation to the size of the stomata ? And, 
similarly, to the numbers of stomata, the sizes or numbers of hairs, 
the chlorophyll area, and so forth ? 
So far as the work has gone I find such glaring and irrecon- 
cilable dissimilarities between the curve of infection and any of 
the curves- of measurements, together with such obvious general 
agreements in the latter curves among themselves, that it is 
impossible to regard the predisposition or immunity of any par- 
ticular species as a function of the sizes or numbers of stomata or 
hairs, or, at any rate at present, of the area of the leaf and volume 
of chlorophyll-tissue exposed to the attacks of the fungus, and 
unless further research shows some much greater variations in the 
factors mentioned than have appeared up to the present, must 
conclude that we have here a method which enables us to say at 
once that the infecting tube of the fungus does not enter the leaf 
of, e.g., B. mollis sixty times out of 85 (70 - 6°/ o ) cases tried, and 
the leaf of B. maximus only once out of 74 (1*3 °/ 0 ) times, simply 
because the stomata of the former are more numerous than those 
of the latter — which is the case — because if that were so it seems 
impossible to understand why B. sterilis with more stomata than 
either is not also more susceptible. Nor can the predisposition 
of B. mollis be due to the fact that its stomata are larger than 
other species : those of B. maximus and B. racemosus are larger 
still. 
Appended are diagrams of the curves for stomata (Tables YI. 
and VII.), which illustrate the principles involved. 
It seems clear from the foregoing that we are face to face with 
one issue only, viz. that infection does not depend on the ease of 
access afforded to the germ-tubes by the number and sizes of the 
stomata, and since similar negative results have been met with 
in trying the curves of other structural features, the temptation 
to the following generalisation is irresistible : — 
The capacity for infection, or for resistance to infection, is in- 
dependent of the anatomical structure of the leaf, and must depend 
on some other internal factor or factors in the plant 
If this is accepted, however, we are driven back on to those 
mysterious factors, the properties of the cell or the constitution 
of the plant, for an explanation of the relative immunity from or 
predisposition to the disease. The failure to find any structural 
or mechanical explanation of the phenomenon, in the sense here 
implied, does not necessarily involve the assumption that there 
is no mechanism in the living plant which is answerable for the 
obstruction, or aid, to infection exhibited by the species. It 
only points to the conclusion that the mechanism is of that more 
VOL. XI. PT. V. 24 
