_b._ EXOGENOUS THALLUS. In this group, composed almost exclusively of heteromerous lichens, Zukal includes all those in which the fungus takes the lead in thalline development. He counts as such _Leptogium_, a genus closely allied to _Collema_ but with more membranous lobes, in which the short terminal cells of the hyphae have united to form a continuous cortex. A higher development, therefore, becomes at once apparent, though in some genera, as in _Coenogonium_, the alga still predominates, while the simplest forms may be merely a scanty weft of filaments associated with groups of algal cells. Such a thallus is characteristic of the Ectolechiaceae, and some Gyalectaceae, etc., which have, indeed, been described by Zahlbruckner[314] as homoiomerous though their gonidia belong to the non-gelatinous Chlorophyceae. Heteromerous lichens have been arranged by Hue[315] according to their general structure in three great series: 1. _Stratosae._ Crustaceous, squamulose and foliose lichens with a dorsiventral thallus. 2. _Radiatae._ Fruticose, shrubby or filamentous lichens with a strap-shaped or cylindrical thallus of radiate structure. 3. _Stratosae-Radiatae._ Primary dorsiventral thallus, either crustaceous or squamulose, with a secondary upright thallus of radiate structure called the podetium (Cladoniaceae). II. STRATOSE THALLUS 1. CRUSTACEOUS LICHENS A. GENERAL STRUCTURE In the series “Stratosae,” the plant is dorsiventral, the tissues forming the thallus being arranged more or less regularly in strata one above the other (Fig. 37). On the upper surface there is a hyphal layer constituting a cortex, either rudimentary or highly elaborated; beneath the cortex is situated the gonidial zone composed of algae and hyphae in close association; and deeper down the medulla, generally a loose tissue of branching hyphae. The lower cortex which abuts on the medulla may be as fully developed as the upper or it may be absent. [Illustration: Fig. 37. Vertical section of crustaceous lichen (_Lecanora subfusca_ var. _chlarona_ Hue) on bark. _a_, lichen cortex; _b_, gonidia; _c_, cells of the periderm. × 100.] The growing tissue is chiefly marginal; the hyphae on the outer edge remain “meristematic”[316] and provide for horizontal as well as vertical extension; and there is also continual increase of the algal cells. There is in addition a certain amount of intercalary growth due to the activity of the gonidial tissue, both algal and fungal, providing for the renewal of the cortex, and even interposing new tissue. B. SAXICOLOUS LICHENS _a._ EPILITHIC LICHENS. The crustaceous lichens forming this group spread over the rock surfaces. The support must be stable to allow the necessary time for the slowly developing organism, and therefore rocks that are friable or subject to continual weathering are bare of lichens.