According to the nature of the thallus, these Lichens may be divided into:—
a. Thallus homoiomerous, but not gelatinous, branching according to the mode of growth of the Algæ: Cœnogonium.
b. Thallus homoiomerous, gelatinous: Collema (Fig. [132]), with Algæ of the genus Nostoc; Leptogium.
c. Thallus heteromerous, crustaceous: Pertusaria (Fig. [140]), Lecidea, with apothecia open from the beginning; Lecanora, with apothecia, which in the beginning are closed, later on open, but with a rim formed by the thallus (Fig. [138]); Bæomyces, whose apothecia are borne on a stem formed by the thallus.
d. Thallus heteromerous, foliaceous: Parmelia (P. saxatilis; P. parietina, Wall-Lichen, Fig. [141], is yellow, very frequent on tree-stems, stone-walls, tiles); Physcia (P. ciliaris, frequent on tree-stems); Sticta (S. pulmonacea, Lung-Lichen, on tree-stems); Peltigera, especially on the Moss among trees; Umbilicaria, on rocks.
e. Thallus heteromerous, fruticose: Cetraria (C. islandica), “Iceland Moss,” with an olive-brown, flat, furrowed, fringed thallus, on heaths; C. nivalis, white, in the Polar regions; Evernia, Ramalina, Usnea (U. barbata, Beard-Lichen, Fig. [143]); Roccella, Stereocaulon, Cladonia, of which the genus C. rangiferina, Reindeer-Moss (Fig. [142]) is important; Cladonia has two kinds of thallus, one scaly and leaf-like, the other erect, which bears the apothecia and may be fruticose (Fig. [142]), or cupular (Fig. [144]); they grow in soil in forests and on heaths.
Fig. 142.—Cladonia rangiferina: s ascocarp.
Fig. 143.—Usnea barbata: s ascocarp. (Slightly magnified.)