I cannot leave this subject without referring to a remarkably interesting museum specimen which Prof. Hartig showed and explained to me last summer. This is a block of wood containing an enormous irregularly spheroidal mass of the white felted mycelium of this fungus, _Polyporus sulphureus_. The mass had been cut clean across, and the section exposed a number of thin brown ovoid bodies embedded in the closely woven felt; these bodies were of the size and shape of acorns, but were simply hollow shells filled with the same felt-like mycelium as that in which they were embedded. They were cut in all directions, and so appeared as circles in some cases. These bodies are, in fact, the outer shells of so many acorns, embedded in and hollowed out by the mycelium of _Polyporus sulphureus_. Hartig's ingenious explanation of their presence speaks for itself. A squirrel had stored up the acorns in a hollow in the timber, and had not returned to them--what tragedy intervenes must be left to the imagination. The _Polyporus_ had then invaded the hollow, and the acorns, and had dissolved and destroyed the cellular and starchy contents of the latter, leaving only the cuticularized and corky shells, looking exactly like fossil eggs in the matrix. I hardly think geology can beat this for a true story. The three diseases so far described serve very well as types of a number of others known to be due to the invasion of timber and the dissolution of the walls of its cells, fibers, and vessels by hymenomycetous fungi, i.e., by fungi allied to the toadstools and polypores. They all "rot" the timber by destroying its structure and substance, starting from the cambium and medullary rays. To mention one or two additional forms, _Trametes Pini_ is common on pines, but, unlike its truly parasitic ally, _Tr. radiciperda_, which attacks sound roots, it is a wound parasite, and seems able to gain access to the timber only if the spores germinate on exposed surfaces. The disease it produces is very like that caused by its ally; probably none but an expert could distinguish between them, though the differences are clear when the histology is understood. _Polyporus fulvus_ is remarkable because its hyphae destroy the middle lamella, and thus isolate the tracheides in the timber of firs; _Polyporus borealis_ also produces disease in the timber of standing conifers; _Polyporus igniarius_ is one of the commonest parasites on trees such as the oak, etc., and produces in them a disease not unlike that due to the last form mentioned; _Polyporus dryadeus_ also destroys oaks, and is again remarkable because its hyphae destroy the middle lamella. With reference to the two fungi last mentioned I cannot avoid describing a specimen in the Museum of Forest Botany in Munich, since it seems to have a possible bearing on a very important question of biology, viz., the action of soluble ferments.