S. p. WiLTSHIRK 349 



of germination were gradually improved until in media of which the 

 ratio of sap to water was 1 : 10 no ditt'erence from the water controls 

 could be observed. A few further experiments were made with Newton 

 Wonder (resistant) and Cap of Liberty (susceptible) which as far as 

 they went coincided with the results given above. In these sets of 

 preparations the conidia appeared to grow better in the sap of the 

 susceptible varieties than in that of the resistant kinds : but the 

 approach of winter rendered it impossible to carry this work any 

 further for the time being. The little which has already been done 

 seems to support the view obtained from the study of microscopical 

 preparations that the host is antagonistic to rather than attractive 

 towards the parasite. 



The facts which are recorded in this paper are therefore considered 

 to suggest that the appressorium, being formed as the result of mechanical 

 stimulus as shown by Aderhold, penetrates into the cuticle, feeding on 

 it as it goes and j&nally reaching its normal habitat between the cuticle 

 and epidermis where it flourishes, if the attacked variety is susceptible. 



Immunity is shown not to depend on the protection of the cuticle 

 and indications are recorded which suggest the probability of the cell 

 sap of the host being in all cases antagonistic to the fungus. 



In conclusion I should like to express my deepest thanks to Professor 

 B. T. P. Barker, M.A., at whose suggestion the work was begun, and 

 who throughout has rendered invaluable aid by helpful criticism and 

 advice. Lam also indebted to him for facilities for working at the 

 Agricultural and Horticultural Research Station of the University of 

 Bristol. 



LIST OF FIGURES. 



Fig. 1. Spore of Venturia inaeqnnlii' germinating on the fruit of apple Stirling Castle. 



X 1600. 

 Fig. 2. Sijore of Venturia pirina germinating on leaf of pear Louise Bonne de Jersey. 



X 1600. 

 Fig. 3. Same showing later stage. x 1600. 

 Fig. 4. Mycelium of Venturia inaequalis in leaf of apple Cap of Liberty, surface view 



showing delimitation of mycelium juncture of the epidermal walls. x KiOO. 

 Fig. 5. Appressorium of Venturia inaequalis on the leaf. x 1600. 

 Fig. 6. Entrance of infection hypha into the cuticle on leaf of apple Cap of Libeity. 



X 1600. 

 Fig. 7. Same. x 1600. 

 Fig. 8. Same, later stage. x 1600. 

 Fig. 0. Same. x 1600. 

 Fig. 10. Penetration of mycelium of Venturia lyirina from cuticular to subcuticular 



position in young leaf of pear William's Bon Chretien. x 1600. 



