=472. Order Plectobasidiales (Plectobasidiineæ).=—The basidia do not form a definite hymenium, but are interwoven with the threads inside, or are collected into knot-like groups. (Examples: Calostoma, Tulostoma, Astræus, Sphærobolus, etc.) =472a. Lichens.=—The plant body of the lichens (see paragraphs 200, 201) consists of two component parts, the one a fungus, the other an alga. The fructification is that of the fungus. The fruit body shows the lichens to be related some to the Ascomycetes, others to the Hymenomycetes, and Gasteromycetes. They are usually classified as a distinct class or order from the fungi, but a natural arrangement would distribute them in several of the orders above. Their special relationship with these orders has not been satisfactorily worked out. For the present they are arranged as follows: =Ascolichenes.= _Pyrenocarpous lichens_ (those with a fruit body like the Pyrenomycetes). _Gymnocarpous lichens_ (those with a fruit body like the Discomycetes). =Hymenolichenes= (those with a fruit body like the Hymenomycetes). =Gasterolichenes= (those with a fruit body like the Gasteromycetes). From a vegetative standpoint there are two types according to the distribution of the elements. 1st. Where the fungal and algal elements are evenly distributed in the plant body the lichen is said to be _homoiomerous_. There are two types of these: _a. Filamentous lichens_, example, Ephebe pubescens. _b. Gelatinous lichens_, example, Collema (with the alga nostoc), Physma (with the Chroococcaceæ). 2d. Where the elements are stratified, as in Parmelia, etc., the lichen is said to be _heteromerous_. In these there are three types: _a. Crustaceous lichens_, the plant body is in the form of a thin incrustation on rocks, etc. _b. Foliaceous lichens_, the plant body is leaf-like and lobed and more or less loosely attached by rhizoids: Parmelia, Peltigera, etc. _c. Fruticose lichens_, the plant body is filamentous or band-like and branched, as in Usnea, Cladonia, etc. [Illustration: Fig. 251_a_. Rock lichen (Parmelia contigua).] FOOTNOTES: [20] As suborder in Engler and Prantl. [21] As suborder in Engler and Prantl. [22] As suborder in Engler and Prantl. [23] The Uredinales and Auriculariales in Engler and Prantl are placed in order, Auriculariineæ. [24] The Uredinales and Auriculariales in Engler and Prantl are placed in order, Auriculariineæ. CHAPTER XXIII. LIVERWORTS (HEPATICÆ). =473.= We come now to the study of representatives of another group of plants, a few of which we examined in studying the organs of assimilation and nutrition. I refer to what are called the liverworts. Two of these liverworts belonging to the genus riccia are illustrated in figs. 30, 252. Riccia.