342 Apple and Pear Scab Fungi 



characteristic behaviour of the disease may be due to variations in the 

 cHmatic conditions at the time when the hosts are in most susceptible 

 condition, is not a sufficient explanation, and Wallace thinks that the 

 fungus itself is adapting itself to life on the "resistant" hosts. Again, 

 too, there is great variation in the parts of the host attacked. Some- 

 times it is especially the stem, sometimes the leaves, sometimes the 

 fruit. Again, the part of the host especially attacked, whether leaf, 

 stem, or fruit, varies considerably under conditions of which we know 

 little. The absence of adequately definite facts makes it at present 

 unprofitable to formulate hypotheses to account for these phenomena, 

 but on fuller knowledge it seems probable that much of the accessory 

 factors connected with the immunity of these fungi might be explained. 

 In the following instance some such explanation is attempted. It is 

 well known that the pear fungus appears on the fruit first and on the 

 foliage afterwards, whilst in the case of the apple the conditions are 

 reversed. Now it seems likely that the very thick woolly coating of 

 hairs on the young fruit of the apple very efficiently protect it from 

 becoming wetted and consequently infected. Later when this covering 

 disappears infection very readily takes place. There are many fungi 

 in common with Venturia which attack their hosts in a more or less 

 localized portion, and it is usually the young organs which are most 

 susceptible. There seems to be a definite correlation between the age 

 of the leaf and its powers of resistance. We might point to a parallel 

 amongst animals in the everyday fact that the young are more sus- 

 ceptible to certain diseases than adults. In the case of plants it is not 

 known whether this increased resistance accompanying age is due to 

 the better protection of the host against attacks of fungi, such as might 

 be afforded by the thickening of the cuticle, or whether the limiting 

 factor is intimately associated with the Hfe of the living cell, and the 

 fungus is killed after penetrating the cuticle. Definite experiments 

 were therefore made to find out what happens when spores attack the 

 mature leaves of various varieties of some of which the young organs 

 are susceptible. For this purpose the mycelium from a pure culture 

 was used for inoculations, in order that the fungus might have a larger 

 reserve of food material to sustain itself in the attack than that 

 contained in the spore. In the case of the pear WilHam's Bon Chretien, 

 a susceptible variety, the fungus developed normally up to the stage 

 of the formation of the appressorium and the issuing of the hypha 

 from its underside into the cuticle. Instead, however, of the infection 

 hypha penetrating completely through the cuticle, and developing 



