1. Intricate: an intricate dense layer of gelatinous-walled hyphae, branching in all directions, but not coalescent ([Fig. 44]). This rather unusual type of cortex occurs in Sphaerophorus and Stereocaulon, both of which have an upright rigid thallus (fruticose).
Fig. 44. Sphaerophorus coralloides Pers. Transverse section of cortex and gonidial layer near the growing point of a frond × 600.
Fig. 45. Roccella fuciformis DC. Transverse section of cortex near the growing point of a frond × 600.
2. Fastigiate: the hyphae bend outwards or upwards to form the cortex. A primary filament can be distinguished with abundant branches, all tending in the same direction; anastomosis may take place between the hyphae. The end branches are densely packed, though there are occasional interstices ([Fig. 45]). Such a cortex occurs in Thamnolia; in several genera of Roccellaceae—Roccellographa, Roccellina, Reinkella, Pentagenella, Combea, Schizopelte and Roccella—and also in the crustaceous genus Dirina. The fastigiate cortex corresponds with Zukal’s palisade cells.
3. Decomposed: in this, the most frequent type of cortex, the hyphae that travel up from the gonidial layer become irregularly branched and frequently septate. The cell-walls of the terminal branches become swollen into a gelatinous mass, the transformation being brought about by a change in the molecular constituents of the cell-walls which permits the imbibition and storage of water. The tissue, owing to the enormous increase of the wall, is so closely pressed together that the individual hyphae become indistinct; the cell-lumen finally disappears altogether, or, at most, is only to be detected in section as a narrow disconnected dark streak. The decomposed cortex is characteristic of many lichens, crustaceous ([Fig. 46]) and squamulose, as well as of such highly developed genera as Usnea, Letharia, Ramalina, Cetraria, Evernia and certain Parmeliae.
Fig. 46. Lecanora glaucoma var. corrugata Nyl. Vertical section of cortex × 500 (after Hue).
Zukal took no note of the decomposed cortex but the omission is intentional and is due to his regarding the structure of the youngest stages of the thallus near the growing point as the most typical and as giving the best indication as to the true arrangement of hyphae in the cortex. He thus describes palisade tissue as the characteristic cortex of Evernia, since the formation near the growing point of the fronds is somewhat palisade-like and he finds fibrous cortex at the tips of Usnea filaments. In both these instances Hue has described the cortex as decomposed because he takes account only of the fully formed thallus in which the tissues have reached a permanent condition.