One of the most unaccountable circumstances in the history of the lower fungi is their sudden appearance in immense numbers, and the rapid extension of disastrous and destructive epidemics caused by them among plants and animals, as the potato murrain and the vine disease, which, though widely spread through Europe and Madeira, leaves the North American vines unscathed, whether grown at home or abroad. The black mildews at one time raged so much in the Azores and Ceylon as to threaten the complete annihilation of the orange and coffee plantations. Their ravages have been scarcely less among the olive trees in some parts of Europe; and the Lanosa nivalis, which grows in the melting snow in spring, is supposed to be the cause in many instances of the death of the germs of the sprouting rye. The destructive course of most of these has abated, but the silkworm disease still continues. The fungi require warmth and a moderate degree of moisture for their development, but the unwonted multitudes in which the parasites occasionally appear, possibly indicate some meteoric influences of which we are ignorant. SECTION IV. LICHENS. LICHENS are essentially air plants, being nourished, like the Algæ, by the medium in which they grow. They vary from a pulverulent or dry papillose crust, to a leathery or horny expansion, and even acquire an erect stem. They are independent of the matrix to which they are attached. Hence they spread their coloured frond, or thallus, in circular or indefinite patches on old walls, the tiles of houses, stones, and rocks. They appear in large expansions of red, golden yellow, grey or white, on barren heaths, under plantations, and on the stems of aged trees; while others of them often hang from the branches like long shaggy grey hair, and many form forests of miniature bushes on the northern plains. The lichen is the last trace of vegetation on the tops of the mountains, and on the arctic deserts. Some lichens are patient of severe cold, yet in general they prefer heat and moderate warmth, and they love bright light so much, that they are usually barren, or else yield little fruit under shade. Though differing greatly from fungi in slowness of growth, length of life, and the power of forming chlorophyll, they resemble them in having a mycelium in their youth, and in their ascigerous fructification. A perfect lichen without an ascus would be an anomaly, for the asci contain the true fruit, associated with vertical threads or elongated cells called paraphyses, which sometimes bear secondary spores on their summit. The asci with their paraphyses inclosed in vessels of various shapes, called perithecia, are aggregated in discs or shields, which form projections on the surface of the plant. Some of these discs are closed, and give egress to the spores through a fracture or pore on their surface; others