OAK FUNGI. The species of Fungi that live upon the common Oak are numerous; but by far the larger proportion either do comparatively little injury, or make their appearance only on dead parts of the trees; e.g., on bark or wood of dead branches, or on leaves or acorns. Many of the species are very minute, but only a few of the more injurious will be mentioned here. Those of chief importance belong to the Hymenomycetes (see Mushrooms), and, for the most part, to the genus Polyporus, which has pores instead of gills on the lower surface of the cap, or spore-producing structure. The following are among the more noteworthy species, inasmuch as their mycelium traverses the living wood of the Oak, and withdraws the nourishment from the cells of the wood, which, in consequence, becomes decayed and brittle. The cap, in most of the species, grows to a breadth of several inches. Of Polyporus, several species attack living wood of Oak, e.g., P. dryadeus, P. sulphureus, P. igniarius, and P. intybaceus; so also does the nearly-allied Fistulina hepatica, or Beefsteak Fungus. In each case, the mycelium of the Fungus usually gains entrance by the uncovered end of a branch, or a wound, and spreads through the wood cells. Careful pruning, and protection of wounds by tar, or some other dressing, are of very great service in preventing the entrance of mycelium, and the consequent development of the Fungus, with its disastrous results to the trees. The methods of pruning, and of protecting wounds, cannot be treated of under this head, but will be found fully described in this work under the appropriate headings. Trees suffer much injury from Fungi in their roots, which become attacked by the mycelium spreading in the soil from adjoining roots of dead trees; hence, care should be taken to remove, as far as possible, all decaying wood from soil in the neighbourhood of healthy trees, lest the Fungi should extend to the roots of the latter, and, after a time, cause their destruction. Trees, when badly attacked by these Fungi, should be cut down and destroyed, to prevent the spread of the Fungi to healthy trees. See Polyporus. A soot-like crust very frequently covers the leaves of Oaks, as well as of many other trees and shrubs. When examined microscopically, it is seen to be formed of cells and threads of a Fungus, belonging to a group included under the name Fumago, called also, when fully developed, Capnodium. In Capnodium, the spores for reproduction are produced in the interior of larger cells (asci), many of which are inclosed in a globular or flask-shaped vessel (perithecium), of microscopic size; but this stage of development is not often met with. The Fumago stage is much the more common. In this, the reproductive cells are separated from the tips of threads on the exposed surface, or are produced in minute flask-like perithecia, but not in asci. The crusts are much like soot in general appearance, like it, resting only on the upper surface of the leaves. They have frequently been attributed to injury from smoke, leading more than once to lawsuits for compensation for such alleged injury. The microscope shows the cell walls to be dark brown. The Fungus is readily removable from the leaves by rubbing the surface. It does not seem to derive nourishment from them--at least, to any marked extent--but feeds in large degree, probably, on the honeydew excreted abundantly by various kinds of Aphides, or Green Flies. As these insects live on the lower surface of the leaves, their excretions fall on the upper surface of leaves below them, which explains the presence of the Fungus on that surface. If present only in small amount, the crust does not seem to do much injury to the trees; but when thick, it partially chokes the leaves, and cuts off the light from them, so that they become sickly, and fail to nourish the trees. This seldom occurs before autumn, which is fortunate, as there is hardly any practicable method of removing Fungi from trees such as Oaks. The species found on Oak has received various names, that generally adopted being Capnodium quercinum.