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 beyond the natural barriers of the tissues, and so puts a _cordon_ all round the territory and seizes the multitudes of food-stuffs at the frontiers. The end result is similar in both cases, but the methods of warfare differ. Many fungi, however, though they make their presence noticeable by conspicuous signs, cannot be said to do much damage to the individual plant attacked. The extraordinary malformations induced by parasites like _Exoascus_, which live in the ends of twigs of trees and stimulate the buds to put out dense tufts of shoots, again densely branched--Witches' brooms--are a case in point. Also the curious distortions of nettle stems swollen and curved by _Æcidium_, of maize stems and leaves attacked by _Ustilago_, and of the inflorescences of _Capsella_ by _Cystopus_, etc., are not individually very destructive; it is the cumulative effects of numerous attacks, or of large epidemics, which tell in the end. Some very curious effects are due to fungi such as _Æcidium elatinum_, which, living in the cortex of firs, stimulate buds to put out shoots with erect habit, and with leaves which are radially disposed, annually cast, and differently shaped from the normal--characters quite foreign to the species of fir in its natural condition.