INTRODUCTION TO CRYPTOGAMIC BOTANY. 239 



no other habitats, though analogous species exist in less abnor- 

 mal localities. 



229. Having disposed of these exceptional cases, there can 

 be no question that the great mass of Fungi is really either 

 hysterophytal or epiphytal, restricting the latter term to those 

 species which affect living vegetables. Fungi are indeed one 

 of the great instruments which keep up the balance between 

 animal and vegetable life. No sooner does death take posses- 

 sion in any vegetable, than a host of Fungi of various kinds, 

 are ready to work its decomposition. This is at once evident 

 in all softer structures, which are soon reduced to humus by 

 the combined action of putrescence and Fungi ; the one, in 

 fact, being frequently the handmaid of the other. The hardest 

 wood, however, yields, though more slowly, to the same agent, 

 and, indeed, far more rapidly than it would do under the 

 action of mere climatic conditions. A stump of one of our 

 largest trees, if once attacked by Fungi, will, in a short time, 

 present a mere mass of touchwood, which is nothing more than 

 woody tissue traversed and disorganized by mycelium. The 

 same stump, if simply left to the action of the weather, might 

 be half a century before it was fairly decayed. The appear- 

 ance of such a Fungus as Polyporus squamosus is the sure 

 harbinger of speedy decay. Nor is the case much mended, 

 supposing vegetation still to exist in the stump ; for though 

 the mycelium cannot prey on cells full of vital energy, life is 

 so depressed by the presence and contact of tissues already 

 diseased, that the healthiest soon fall a prey to the spreading 

 mycelium. There are, indeed, hundreds of Fungi of the most 

 varying size, form, and ajDpearance, Avhich more or less speedily 

 accomplish the same end, and there is sometimes a host 

 equally fatal to some individual species. 



230. But not only are there numerous species which grow 

 upon vegetable matters already decomposed, or which, by their 

 presence, promote the decomposition of materials already 

 divested of Hfe, but there are very many also which are 

 the express enemies of living tissues ; species of which it can- 

 not be said that they are at first propagated on plants, or in 

 tissues already in a low state of vitality, but which induce such 



