The fleshy fungi, _Agaricini_ and _Boleti_ especially, are largely dependent upon the character of woods and forests. When the undergrowth of a wood is cleared away, as it often is every few years, it is easy to observe a considerable difference in the fungi. Species seem to change places, common ones amongst a dense undergrowth are rare or disappear with the copsewood, and others not observed before take their place. Some species, too, are peculiar to certain woods, such as beech woods and fir woods, and their distribution will consequently depend very much on the presence or absence of such woods. Epiphytal species, such as _Agaricus ulmarius_, _Agaricus mucidus_, and a host of others, depend on circumstances which do not influence the distribution of flowering plants. It may be assumed that such species as flourish in pastures and open places are subject to fewer adverse conditions than those which affect woods and forests.
Any one who has observed any locality with reference to its Mycologic Flora over a period of years will have been struck with the difference in number and variety caused by what may be termed a "favourable season," that is, plenty of moisture in August with warm weather afterwards. Although we know but little of the conditions of germination in Agarics, it is but reasonable to suppose that a succession of dry seasons will considerably influence the flora of any locality. Heat and humidity, therefore, are intimately concerned in the mycologic vegetation of a country. Fries has noted in his essay the features to which we have alluded. "The fact," he says, "must not be lost sight of that some species of fungi which have formerly been common in certain localities may become, within our lifetime, more and more scarce, and even altogether cease to grow there. The cause of this, doubtless, is the occurrence of some change in the physical constitution of a locality, such as that resulting from the destruction of a forest, or from the drainage, by ditches and cuttings, of more or less extensive swamps, or from the cultivation of the soil--all of them circumstances which cause the destruction of the primitive fungaceous vegetation and the production of a new one. If we compare the fungal flora of America with that of European countries, we observe that the former equals, in its richness and the variety of its forms, that of the phanerogamous flora; it is probable, however, that, in the lapse of more or fewer years, this richness will decrease, in consequence of the extension of cultivation--as is illustrated, indeed, in what has already taken place in the more thickly peopled districts, as, for example, in the vicinity of New York."