may be simply wrinkled; sometimes it is warty or prickly, and in Fig. 5 it is represented as covered with spines. Again, in some species of fungi, the hymenium, instead of being situated externally, is inclosed in a membrane which bursts when its spores are ripe, and scatters them like a cloud of smoke to the winds. Of such is the puff-ball. Fig. 6, with which everybody is familiar.
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| Fig. 5.—Hydnum Repandum. | Fig. 6.—Scleroderma Vulgare. |
These fleshy forms, however, although very numerous, constitute but a small part of this immense group. But most of the species included in it are either quite invisible, or else the parts which characterize them as fungi are so small as to be indistinguishable. The feature by which a fungus may always be known is the mycelium. Every plant of which this structure forms a part, spreading its web through-out the substance on, or in, which it grows, belongs among fungi. They differ among themselves in such comparatively unimportant respects as the mode of growth of the hymenium, or the degree of complexity of the reproductive system, but mycelia and spore production are their essential characters. In these diminutive organisms, the delicate mycelium is so minute as to traverse living plants and the pores of solid wood. The potato-rot is such a fungus—a sort of mould—the mycelium of which grows rapidly, penetrating the leaves, stem, and tubers, and causing quick decay. Dry-rot in timber is occasioned by the penetrating mycelium of fungi. The yeast and vinegar plants are submerged mycelia. The mildews, rusts, and smuts of grain—those scourges of the farmer—are all fungi. Their minute mycelium penetrates and destroys the tissues of plants, and, bursting through the cuticle, covers them with myriads of their orange, brown, and black spore. All those black, pustular growths seen on dead wood, bark, twigs, and leaves, and the whole tribe of moulds that cover every substance exposed to dampness, are fungi. Not only do these fungi ravage the living and the dead, but they fill the air with the countless myriads of their spores. These subtile particles, "invisible to the naked eye, and light almost as vapor, are continually floating in the air we breathe, or swimming in the water we drink, or lying amid the impalpable dust and sand of the soil, waiting the presence of warmth
