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.
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.
aa. Hypothallus or Prothallus. The first stage of growth in the lichen thallus can be most easily traced in epilithic crustaceous species, especially in those that inhabit a smooth rock surface. The spore, on germination, produces a delicate branching septate mycelium which radiates on all sides, as was so well observed and recorded by Tulasne[317] in Verrucaria muralis ([Fig. 14]). Zukal[318] has called this first beginning the prothallus. In time the cell-walls of the filaments become much thicker and though, in some species, they remain colourless, in others they become dark-coloured, all except the extreme tips, owing to the presence of lichen pigments—a provision, Zukal[319] considers, to protect them against the ravages of insects, etc. The prothallic filaments adhere closely to the substratum and the branching becomes gradually more dendroid in form, though sometimes hyphae are united into strands, or even form a kind of plectenchymatous tissue. This purely hyphal stage may persist for long periods without much change. In time there may be a fortuitous encounter with the algae ([Fig. 38 A]) which become the gonidia of the plant. Either these have been already established on the substratum as free-growing organisms, or, as accidentally conveyed, they alight on the prothallus. The contact between alga and hypha excites both to active growth and to cell-division; and the rapidly multiplying gonidia are as speedily surrounded by the vigorously growing hyphal filaments.
Fig. 38 A. Hypothallus of Rhizocarpon confervoides DC., from the extreme edge, with loose gonidia × 600.
