One of the commonest of these is _Polyporus sulphureus_, which does great injury to all kinds of standing timber, especially the oak, poplar, willow, hazel, pear, larch, and others. It is probably well known to all foresters, as its fructification projects horizontally from the diseased trunks as tiers of bracket-shaped bodies of a cheese-like consistency; bright yellow below, where the numerous minute pores are, and orange or somewhat vermilion above, giving the substance a coral-like appearance. I have often seen it in the neighborhood of Englefield Green and Windsor, and it is very common in England generally. If the spore of this _Polyporus_ lodges on a wound which exposes the cambium and young wood, the filaments grow into the medullary rays and the vessels and soon spread in all directions in the timber, especially longitudinally, causing the latter to assume a warm brown color and to undergo decay. In the infested timber are to observed radial and other crevices filled with the dense felt-like mycelium formed by the common growth of the innumerable branched filaments. In bad cases it is possible to strip sheets of this yellowish white felt work out of the cracks, and on looking at the timber more closely (of the oak, for instance), the vessels are found to be filled with the fungus filaments, and look like long white streaks in longitudinal sections of the wood--showing as white dots in transverse sections. It is not necessary to dwell on the details of the histology of the diseased timber; the ultimate filaments of the fungus penetrate the walls of all the cells and vessels, dissolve and destroy the starch in the medullary rays, and convert the lignified walls of the wood elements back again into cellulose. This evidently occurs by some solvent action, and is due to a ferment excreted from the fungus filaments, and the destroyed timber becomes reduced to a brown mass of powder.