Fig. 58. Usnea barbata Web. (S. H., Photo.).
The radiate type of thallus occurs in most of the lichen groups but most frequently in the Gymnocarpeae. In gelatinous Discolichens it is represented in the Lichinaceae. It is rare among Pyrenocarpeae: there is one very minute British lichen in that series, Pyrenidium actinellum, and one from N. America, Pyrenothamnia, that are of fruticose habit.
2. INTERMEDIATE TYPES OF THALLUS
Between the foliose and the fruticose types, there are intermediate forms that might be, and often are, classified now in one group and now in the other. These are chiefly: Physcia (Anaptychia) ciliaris, Ph. leucomelas and the species of Evernia.
In the two former the habit is more or less fruticose as the plants are affixed to the substratum at a basal point, but the fronds are decumbent and the internal structure is of the dorsiventral type: there is an upper “fibrous” cortex of closely compacted parallel hyphae, a gonidial zone—the gonidia lying partly in the cortex and partly among the loose hyphae of the medulla—and a lower cortex formed of a weft of hyphae which also run somewhat parallel to the surface. Both species are distinguished by the numerous marginal cilia, either pale or dark in colour. These two lichens are greyish-coloured on the upper surface and greyish or whitish below.
Evernia furfuracea with a basal attachment[374], and with a partly horizontal and partly upright growth, has a dorsiventral thallus, dark greyish-green above and black beneath, with occasional rhizinae towards the base. The cortex of both surfaces belongs to the “decomposed” type; the gonidial zone lies below the upper surface, and the medullary tissue is of loose hyphae. In certain forms of the species isidia are abundant on the upper surface, a character of foliose rather than of fruticose lichens. E. furfuracea grows on trees and very frequently on palings.
Fig. 59. Evernia prunastri Ach. (M. P., Photo.).