252 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



attempts to deal with these " leaf diseases " is at least 

 twofold ; for not only are the leaves so numerous and 

 so out of reach that he can scarcely entertain the idea 

 of doing anything directly to them, but (and this is 

 by no means so clearly apprehended as it should be) 

 they stay on the tree but a short time as a rule, and 

 when they fall are a continual source of re-infection, 

 because the spores of the fungi are developed on 

 them. It is a curious fact that those fungi which are 

 known to affect the leaves of forest-trees nearly all 

 belong to two highly-developed groups the Uredineae 

 and the Ascomycetes and the remarkable biological 

 adaptations which these parasites exhibit for attack- 

 ing or entering the leaves, passing through periods 

 of danger, and so on, are nearly as various (and one 

 might almost say ingenious) as they are numerous. 

 Some of them, such as the Erysiphece or mildews on 

 beeches, oaks, birches, ashes, &c., only form small ex- 

 ternal patches on the leaves, and do little if any harm 

 where the leaf-crown is large and active ; others, such 

 as many of the very numerous Sph&riacea and their 

 allies, which form small dark-coloured flecks and spots 

 on leaves, may also be looked upon as taking only a 

 slight tax from the leaves. Even in these cases, how- 

 ever, when the diseases become epidemic in certain 

 wet seasons, considerable damage may accrue, because 



