Polyporus
(from the Greek polyporus, having many outlets; in allusion to the many openings or pores on the lower surface of the pileus). A genus of Fungi, belonging to the Hymenomycetes, or those in which the spores are formed on the tips of small projections from larger cells (of which each gives off four spores) on the surface (hymenium) of a definite part of the Fungus. See Mushrooms. The species of Polyporus differ from the true Mushrooms in that, while the latter bear the hymenium on the gills, the former bear it in a number of small tubes packed together to form a layer of peculiar aspect and texture on the lower surface of the cap, or pileus. They are usually dry and hard in texture, after a short time, and are rather long lived. They, at first, emit an acid smell, but afterwards are nearly without smell. They vary in form, and grow indefinitely. Those parasitic on trees usually have the pileus sessile, and fixed by one side to the trunk of the tree. They often continue to grow slowly for many years, and reach a size of from 1in. or 2in. to 3ft. across, by several inches in thickness in the middle. From their dry texture, it is easy to preserve them as herbarium specimens; but insects are very apt to eat and destroy them when dried. Old trees of various kinds very frequently have Fungi of this genus growing on their stems, the mycelium penetrating and drawing nourishment from the wood, and the pileus often remaining for many years on the stem, very often near its base. It seems probable that the species of Polyporus do not live on quite healthy trees, but on those already weakened by some other cause. Our knowledge of their importance as parasites is due largely to R. Hartig, the well-known authority on the diseases of forest-trees. He has traced and described the effects produced by P. annosus, Fr. (under the name of Trametes radiciperda), on various trees, by P. fulvus on the Silver Fir, by P. borealis on the Spruce, by P. vaporarius on Spruce and Firs, by P. mollis on Firs, by P. igniarius on numerous forest-trees (Dicotyledons) and fruit-trees, by P. dryadeus on Oaks, and by P. sulphureus on many forest-trees (Dicotyledons) and on Pear-trees. Numerous other instances of parasitism could be added. Further details need not here be entered into, it being sufficient to say that the species of Polyporus are not of frequent occurrence in gardens and pleasure-grounds. The wood diseased by the presence of the Fungus becomes soft and rotten, and a tree infested with Polyporus may be regarded as doomed, sooner or later, to perish from the action of the Fungus on the wood. It is well, if the tree can be at once removed, to cut it down and have it used as firewood, rather than to allow the Fungus to distribute its myriads of spores to injure other trees.