[Illustration: FIG. 141.--_A_ A portion of the thallus of _Parmelia parietina_ with ascocarps (_a_) and pycnidia (_b_). _B_ A portion of the thallus of _Cetraria islandica_ with pycnidia at the end of small lobes. _C_ A lobe with pycnidia and ejected microconidia. (Magnified).] GEOGRAPHICAL DISTRIBUTION. The Lichens are the most hardy plants, and are the first to appear on hitherto bare rocks which they gradually disintegrate, and hence prepare the way for the growth of other plants. They are to be found from the Polar regions to the Equator; from the highest snow-free mountain-peaks down to the level of the sea; on the stems of trees; on rocks, soil, some even on inundated places; on stones in woodland streams, and on beaches; but they are never found upon rotten organic remains. Some grow gregariously in enormous masses, and form wide-stretching carpets, _e.g._ Reindeer Moss (_Cladonia rangiferina_), species of _Cetraria_ and other fruticose Lichens. USES. On account of the cell-wall being composed of Lichenstarch (Lichenin), the Iceland-Lichen and Manna-Lichen (_Lecanora esculenta_) are used as food; the latter grows on stones, in the deserts of Asia and North Africa, and is often torn loose in large masses and carried away by the wind. The Reindeer-Lichen is not only the principal food of the reindeer, but it is also used in the manufacture of Danish brandy. _Cetraria islandica_ (Lichen islandicus) is OFFICINAL. Colouring materials (lacmus, orseille, persio) are made from several species, especially from _Roccella tinctoria_ (from the rocky coasts of the Mediterranean). _Parmelia saxatilis_ and particularly _Lecanora tartarea_ are used for colouring purposes in the Northern countries. About 2,000 species of Lichens have been described. If we disregard the Basidiolichenes, which will be considered on page 176, the remaining Lichens (Ascolichenes) may be divided into the two following orders according to the structure of the fruit-bodies:-- Order 1. =Pyrenolichenes.= The ascocarps (apothecia) are spherical or flask-shaped, as in the Pyrenomycetes, more rarely linear (_Graphis_). 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: _Ephebe_ (Fig. 133), with Algæ of the genus _Stigonema_. _b._ Thallus homoiomerous, gelatinous: _Lichina_. _c._ Thallus heteromerous, crustaceous: _Verrucaria_, _Pyrenula_; _Graphis_ (Fig. 139), which may be considered as Hysteriaceæ with gonidia; several species of _Graphis_ are common on bark. _d._ Thallus heteromerous, foliaceous: _Endocarpon_. _e._ Thallus heteromerous, fruticose: _Sphærophorus_. Order 2. =Discolichenes.= These, as in the Discomycetes, have open apothecia, which, as a rule, are cupular, more rarely hemispherical (_Cladonia_). According to the nature of the thallus, these Lichens may be divided into:--