According to their form, lichens are sometimes divided into the bushy (fruticose), leafy (frondose), incrusting (crustaceous), and gelatinous. Of the first, the long gray _Usnea_ (Fig. 45, _A_, _B_), which drapes the branches of trees in swamps, is a familiar example; of the second, _Parmelia_, _Sticta_ (Fig. 45, _C_) and _Peltigera_ (_D_) are types; of the third, _Graphis_ (_G_), common on the trunks of beech-trees, to which it closely adheres; and of the last, _Collema_ (Fig. 44, _C_, _D_, _E_), a dark greenish, gelatinous form, growing on mossy tree trunks, and looking like a colony of _Nostoc_, which indeed it is, but differing from an ordinary colony in being penetrated everywhere by the filaments of the fungus growing upon it. Not infrequently in this form, as well as in other lichens, special cavities, known as spermogonia (Fig. 44, _D_), are found, in which excessively small spores are produced, which have been claimed to be male reproductive cells, but the latest investigations do not support this theory. [Illustration: FIG. 46.--Branch of a plum-tree attacked by black knot. Natural size.] The last group of the _Ascomycetes_ are the "black fungi," _Pyrenomycetes_, represented by the black knot of cherry and plum trees, shown in Figure 46. They are mainly distinguished from the cup fungi by producing their spore sacs in closed cavities. Some are parasites; others live on dead wood, leaves, etc., forming very hard masses, generally black in color, giving them their common name. Owing to the hardness of the masses, they are very difficult to manipulate; and, as the structure is not essentially different from that of the _Discomycetes_, the details will not be entered into here. Of the parasitic forms, one of the best known is the "ergot" of rye, more or less used in medicine. Other forms are known that attack insects, particularly caterpillars, which are killed by their attacks. CHAPTER X. FUNGI--_Continued_. CLASS _Basidiomycetes_. The _Basidiomycetes_ include the largest and most highly developed of the fungi, among which are many familiar forms, such as the mushrooms, toadstools, puff-balls, etc. Besides these large and familiar forms, there are other simpler and smaller ones that, according to the latest investigations, are probably related to them, though formerly regarded as constituting a distinct group. The most generally known of these lower _Basidiomycetes_ are the so-called rusts. The larger _Basidiomycetes_ are for the most part saprophytes, living in decaying vegetable matter, but a few are true parasites upon trees and others of the flowering plants. All of the group are characterized by the production of spores at the top of special cells known as basidia,[8] the number produced upon a single basidium varying from a single one to several. [8] Sing. _basidium_.