" 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 %) cases tried, and 

 the leaf of B. maximus only once out of 74 (1*3 °/ ) 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 VI. 

 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 inter nal 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 



