THE LIVING ENVIRONMENT. 115 



to the rapid spread of Phytophtlwra in the leaves 

 and throughout the plant, which it blackens and 

 rots in a few days. 



Many fungi not in themselves very virulent 

 or aggressive do enormous harm owing to the 

 secondary effects they induce. Some of the 

 tree -killing hymenomycetes, such as Agaricus 

 melleus, for instance, penetrate the wood of a 

 pine at the collar, and the result of the large 

 flow of resin which results is to so block up 

 the water passages that the tree dies off above 

 with all the symptoms of drought. Similarly, the 

 Peziza causing the larch disease, having obtained 

 .access to the stem about a foot or so above the 

 ground, will gradually kill the cambium further and 

 further round the stem, and so girdle the tree as 

 effectually as if we had cut out the new wood all 

 round. In all such cases and the same applies 

 to the leaf-diseases referred to above the fungus 

 may be compared to an army which is not strong 

 enough to invade the whole territory, but which, 

 by striking at the lines of communication, cuts off 

 the supplies of water, food, etc., and so brings the 

 struggle to an end. Indeed we might compare 

 the cases of fungi which attack the root and 

 collar, and so strike at and cut off the water 

 supply, to a compact army which at once cuts off 

 the enemy from his narrow base ; whereas the 

 innumerable units which bring about an epidemic 

 attack on the leaves, and so surround the enemy 

 and cut off his food supplies all round, is rather 

 like a much larger army which cannot get in 



