254 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



long as the leaf remained living on the tree, nothing 

 further occurred ; but wherever a part of the leaf died, 

 or when the leaf fell moribund on the ground, th< 

 spore-like bodies at once began to send hyphae inl 

 the dying tissue, and thus obtained an early place ii 

 the struggle for existence among the saprophyl 

 which finished the destruction of the cells and tissue 

 of the leaf. 



There is another group of fungi, the Capnodiecs^ 

 which form sooty black patches on the leaves, and 

 which are very apt to increase to a dangerous extent 

 on leaves in damp shady situations ; these have no 

 connection with the well-known black patches of 

 Rhytisma from which the leaves of our maples are 

 rarely free. This last fungus is a true parasite, its 

 mycelium penetrates into the leaf tissues, and forms 

 large black patches, in and near which the cells of the 

 leaf either live for the benefit of the fungus alone, 

 or entirely succumb to its ravages : after the leaf has 

 fallen, the fungus forms its spores. Nevertheless, 

 although we have gone a step further in destructive- 

 ness, foresters deny that much harm is done to the 

 trees no doubt because the foliage of the maples is 

 so very abundant. Willows, pines, and firs suffer from 

 allied forms of fungi. 



But it is among the group of the Uredinece, or rusts, 



