The family Hymenomycetes is divided into the following six orders: Agaricini, Polyporei, Hydnei, Thelephorei, Clavarei, Tremellini.
In the order Agaricini the hymenium is found on the under surface of the mushroom cap, covering pleats or gills, technically called lamellæ. These gills vary in character in the different genera, being "persistent in such as the Agaricus, Russula, and Lentinus, deliquescent (melting) in Coprinus, Bolbitius, etc. The edge of the gills is acute in Agaricus, Marasmius, etc., but obtuse and vein-like in Cantharellus, longitudinally channelled in Trogia, and splitting in Schyzophyllum."
In the Polyporei, pore-bearing mushrooms, the gills are replaced by tubes or pores. The tubes are little cylinders, long or short, pressed one against another, forming by their union a layer on the under surface of the cap, and the sporiferous membrane or hymenium lines their inner walls. Their upper end is always closed, while the lower extremity is open to permit the outward passage of the spores. The tubes are generally joined together and are not easily disunited. They are free, i. e., separable, in the sole genus Fistulina. As regards their attachment to the cap, the tubes may be firmly adherent as in the genus Polyporus or easily detached in a single mass as in Boletus, the fleshy form of the order Polyporei. They frequently leave a circular space of greater or less dimensions around the stem, or they adhere to or are prolonged upon it in such a manner that the orifices rise in tiers one above another. The color of the tubes, although not offering as characteristic varieties as that of the gills, changes nevertheless according to species and according to the age of the plant. The tubes may sometimes be of a different color from their orifices, as in Boletus luridus. In some of the Boleti the color of the flesh is changed on exposure to the air and the tubes often assume the same tints. The tubes, generally called pores, are sometimes closely adherent to the substance of the cap, which is often hard, corky, or coriaceous, as seen in most of the Polyporei.
In the Hydnei, spine-bearing mushrooms, the hymenium is seen covering the spines or needle-like processes which take the place of gills in this order, and which project from the under surface of the cap. These spines may be divided or entire, simple or ramified, and are formed of the substance of the cap. In the early stages of development they appear like small projecting points or papillæ, those on the margin of the cap and at the apex of the stem being always less developed, frequently remaining in this rudimentary state. They are rounded in the species Hydnum imbricatum, sometimes compressed in Hydnum repandum, sometimes terminating in hairs or filaments, as in Hydnum barba Jovis, or very much divided, as in Hydnum fimbriatum.
In the Clavarei, the whole plant consists of solid fleshy masses without any stem of a distinct substance, sometimes club-shaped, sometimes branched with the hymenium smoothly covering the entire surface, never incrusting or coriaceous.
In the Thelephorei, the lower surface of the cap presents neither gills, pores, nor spines, but instead the hymenium covers an uneven or slightly wrinkled surface, partially striate, sometimes obscurely papillose. The plants of this order assume a great variety of shape, from that of a perfect cup with a central stem to an irregularly and much branched frond. They are generally dry and tough. Very few are recommended as edible. Prof. Peck says of this order that probably no edible species will be found in any of its genera outside of the genus Craterellus.
In the order Tremellini we have a great departure from the character of the substance, external appearance, and internal structure of the other orders of the Hymenomycetes. The substance is gelatinous; the form is lobed, folded, or convolute, often resembling the brain of some animal. It is uniformly composed throughout of a colorless mucilage, with no appreciable texture, in which are distributed very fine, diversely branched, and anastomosing filaments. Towards the surface the ultimate branches of this filamentous network give birth to globular cells, both at their summits and laterally, which attain a comparatively large size. These cells are filled with a protoplasm, to which the plant owes its color. The fertile threads are not compacted into a true hymenium.
Representative types of the above-described orders of the Hymenomycetes are shown in [Plate B]. The various genera, and species of these orders, will be described more in detail in connection with the species illustrated.