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Lichens, Genera and Species

The Genus CETRARIA, (Ach.) Fries

The Spanish-shield Lichens.--The members of this genus are generally found on trees, although sometimes they are found on rocks or even earth. The colour on the upper surface is bright yellow, greenish-yellow, straw, olive, or brown.

The thallus is expanded and leaf-like or shrubby (fruticulose or fruticose) with branches compressed or channelled; the lobes are flattened and broad; root-like growths on the under surface (rhizoids) are few or wanting.

Fine hairs (cilia) are frequently present and sometimes are numerous.

Soredia are rare.

The fruiting disks (apothecia) are medium in size or large; they are attached on or near the margin of the lobes; the disk itself is shield-shaped, light chestnut to dark reddish-brown. The shape of the disk suggested the name Cetraria, from cetra, a Spanish shield.

The Pitted Cetraria, Cetraria lacunosa, Ach.--See Plate II.

Habitat.--On trees and old fence-rails, easily detached.

Vegetative organ (thallus).--Leaf-like (foliaceous) the lobes rounded, the surface pitted (lacunose); pearl to slate colour above, white to darker below, margins scalloped.

Fruiting disks (apothecia).--Abundant on the lobes, some-what elevated, the disk light chestnut, growing darker, the margin entire.

Name.--The specific name lacunosa is from the Latin lacus, a saucer, referring to the pitted surface of the thallus.

Iceland Moss, Cetraria Islandica, (L.) Ach.--See Colour Plate VII.

Habit and habitat.--On earth.

Vegetative organ (thallus).--Cartilaginous and shrubby (fruticulose); usually brown above, lighter toward the base where there is sometimes a red stain.

Fruiting disks (apothecia).--Not numerous, shield-like and dark chestnut.

Branches.--Flattened, not hollow, rolled in on the margin which is beset with a fringe of dark-coloured hairs 0.3 to 0.5 mm. long.

Name.--The specific name (Islandica) refers to the fact that this lichen is very abundant in Iceland.

Genus USNEA (Dill.) Ach.

Thallus shrub-like (fruticulose) or pendulous, in cross-section round or angular, grayish-green, to greenish-white, sometimes straw-coloured; the interior consisting of a central tough cord surrounded by a cottony layer.

The fruits (apothecia) shield-shaped (scutellae-form), pale, with a fibrillous margin, spores more or less ellipsoid, undivided, colourless.

The name is derived from achneh, the Arabic for lichen.

The Bearded Lichen, Usnea barbata, (L.) Fr.--See Colour Plate I, frontispiece, also Plate III.

Habitat.--On trees, both living, and dead.

Vegetative organs (thallus).--Shrubby (fruticulose) often pendulous, greenish, covered with numerous radiating fibrils of the same colour; the interior tissue (medulla) consisting of a central cord surrounded by a cottony layer. The branches are circular (terete) in cross-section and are rough with papillae.

Fruiting organ (apothecia)--Shield-like (scutellaeform) with a pale disk.

Name.--The specific name barbata is the Latin for "bearded."

The Hair-like Usnea, Usnea trichodea, Ach.

Habit and habitat.--On trees in long waving tufts.

Vegetative organ (thallus).--Pendulous, greatly elongated. The few secondary branches are smooth, bearing numerous lax fibrils of variable length, cross-section circular (terete).

Fruiting organ (apothecia).--Small, disk pale flesh-colour with margin bearing very few fibrils.

Name.--The specific name trichodea is derived from the Greek, resembling hair, and refers to the thallus.

Genus THELOCHISTES, Norm., emend.

The thallus is leaf-like (foliaceous) or scale-like; usually yellow, appressed or sometimes ascending and scrub-like; the fruits (apothecia) are yellow and shield-like (scutellaeform). The spores are colourless, ellipsoid, simple, or with the ends divided off by partitions (polar-bilocular) the end spaces sometimes united by a tube running through the middle space.

The Yellow Wall-lichen, Thelochistes parietinus (L.) Norm. (Xanthoria parietina)--See Colour Plate II.

Habit and habitat.--On trees and rocks usually near bodies of water.

Vegetative organs (thallus).--Leaf-like, pale yellow to orange above, white below; loosely appressed to the surface on which it grows, the margins sometimes ascendant, not gelatinous when moist.

Fruiting organs (apothecia).--The disk orange, the margin (thalline exciple) entire.

Spores.--Colourless, ellipsoid, polar-bilocular.

Name.--The specific name parietina, is from the Latin parie (t-) s, a wall, referring to its habit of growing on stone walls.

Genus PARMELIA, (Ach.) De. Not.

Parmelias usually grow as horizontal mats, gray, blue-green, dark brown, or brown tinged with green. They are closely attached by black rhizoids to rocks and trees are distinctly branched and lobed. If the thallus is torn across and viewed with a hand lens, the torn edge shows that the thallus is composed of a layer of long, loose, white fibres (the medullary layer) between thin but more substantial surface tissues (cortical layers). Over the surface of the mat are scattered flat or saucer-shaped disks (apothecia) generally brown with a thin margin. These are numerous toward the centre. The apothecia have suggested the generic name Parmelia from parma, a small round shield. Many species have also a powdery appearance due to the bursting of the surface to emit little bodies (soredia) which may grow into lichens.

Parmelia conspersa, (Ehrh.) Ach.--See Colour Plate V.

Habit and habitat.--On rocks and stones; degenerate on dead wood.