348 Apple ami Pear Scab Fungi 



subcuticular position. But although the cuticle of organs susceptible 

 to attack is very often so thin as to prevent the horizontal growth of 

 the infection hyphae in it, yet occasionally one finds stages as represented 

 in the Figs. 6 and 7. These figures suggest that there is no guiding 

 influence towards the epidermis, but that the fungus simply eats its 

 way slowly through the cuticle until it arrives between the cuticle and 

 epidermal cell- wall. With mature organs of both susceptible and 

 resistant varieties where the cuticle is thicker, we have still more 

 interesting cases. How can the long horizontal growth of the infection 

 hyphae in the cuticle (a very frequent occurrence) be explained? Does 

 it not rather suggest in place of an attracting substance the presence 

 of something which is repulsive to the invading hypha? The physical 

 layering of the cuticle may have something to do with the direction of 

 growth, but this horizontal habit is not confined to fruits but is also 

 exhibited in mature leaves, where the layering of the cuticle is not nearly 

 so evident on staining. Of course, the tendency is most clearly 

 developed in resistant organs because the young susceptible parts of 

 plants have very thin cuticles, but that it should occur at all strongly 

 negatives the view of the germ tube being attracted by the cell sap of 

 the host. The conclusion seems unavoidable that the fungus infects 

 in spite of the host, and that chemotropism if it acts at all must play 

 a very insignificant part in the process. 



It has been shown that immunity does not depend on freedom 

 from the initial attack, any more than in the case of the rusts ; immunity 

 there depends upon the non-entry of the infection tube into the host. 

 What the Hmiting factor in immunity is, we can only speculate — it 

 may be simple starvation of the fungus, as suggested by the gradual 

 reduction of growth, or it may possibly be the formation of some toxic 

 substance. With such possibilities existing it might be well to mention 

 here some experiments which were made on the growth of the conidia 

 of V. inaequalis in the expressed sap of the young leaves of certain 

 varieties of apples. Young leaves of King of the Pippins (susceptible) 

 and Bramley's Seedling (resistant) were pounded in a mortar and the 

 sap then squeezed out of them. Conidia were then sown in the 

 sap and hanging drop preparations made which, although by no means 

 free from bacteria and yeasts, remained sufficiently pure for the one 

 or two days necessary for the experiment. Control drops were set up 

 with water instead of the sap medium. In all cases the growth of the 

 conidia in the sap was very much slower than in water and a much 

 larger percentage did not germinate. On diluting the sap, the conditions 



