1. CHARACTERS OF RADIATE THALLUS In the stratose dorsiventral thallus, there is a widely extended growing area situated round the free margins of the thallus. In the radiate thallus of the fruticose or filamentous lichens, growth is confined to an apical region. Attachment to the substratum is at one point only—the base of the plant—thus securing the exposure of all sides equally to light. The cortex surrounds the fronds, and the gonidia (mostly Protococcaceae) lie in a zone or in groups between the cortex and the medulla. It is the highest type of vegetative development in the lichen kingdom, since it secures the widest room for the gonidial layer, and the largest opportunity for photosynthesis. [Illustration: Fig. 57. _Roccella fuciformis_ DC.] Shrubby upright lichens consist mostly of strap-shaped fronds, either simple or branched, which may be broadened to thin bands (Fig. 57) or may be narrowed and thickened till they are almost cylindrical. The fronds vary in length according to the species from a few millimetres upwards: those of _Roccella_ have been found measuring 30 cm. in length; those of _Ramalina reticulata_, the largest of all the American lichens, extend to considerably more. Lichens of filamentous growth are more or less cylindrical (Fig. 58). They are in some species upright and of moderate length, but in a few pendulous forms they grow to a great length: specimens of _Usnea longissima_ have been recorded that measured 6 to 8 metres from base to tip. [Illustration: 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.