XL] LEAVES, AND LEAF-DISEASES 253 



two chief causes (and many minor ones) are co- 

 operating to favour the fungus in the struggle for 

 existence : in the first place, a continuously wet 

 summer means loss of sunlight and diminished trans- 

 piration, &c., to the leaves, and so they form smaller 

 quantities of food materials ; and secondly, the damp 

 in the atmosphere and leaves favours the fungi pro- 

 portionally more than the leaves, and so they destroy 

 and occupy larger areas of leaf surface. 



It should be mentioned here, by the way, that all 

 leaves of all trees are apt to have fungi on them in a 

 wet summer, but many of these are only spreading 

 their mycelia in all directions over the epidermis, in 

 preparation, as it were, for the fall of the leaf: they 

 are saprophytes which feed on the dead fallen leaves, 

 but cannot enter into them while they are yet alive. 

 In some cases, however, this preparation for the fall is 

 strikingly suggestive of adaptation towards becoming 

 parasites. Lwill quote one instance only in illustration 

 of this. On the leaves of certain trees in Ceylon, 

 there was always to be found in the rainy season the 

 much-branched mycelium of a minute Sphmvia : this 

 formed enormous numbers of branches, which, on the 

 older leaves, were found to stop short over the stomata, 

 and to form eventually a four-celled spore-like body 

 just blocking up each stoma on which it rested. So 



