THE Lichens are found growing upon the bark of trees, old palings, &c. Those most easily seen with the naked eye form grey or coloured dryish patches or pendulous tufts; while the smaller ones are singly easily overlooked, from their minute size and close adhesion to the mÁtrix or body upon which they grow, forming, by their aggregation, the grey or otherwise-coloured dry and brittle coatings of almost every tree or decaying branch.

The Lichens derive their nourishment from the air, and not from the matrix—in this respect differing from the Fungi, with some of which, as we shall presently see, they agree in the structure of the fruit.

The structure of the Lichens is simple, no distinction of root, stem, and leaves existing in them, although certain dry root-like fibres exist in many of them, by which the plant is fixed to the matrix. The whole consists mainly of a frond or thal´lus (?a????, a leaf). This is either raised above the surface of the matrix in a shrubby form, or spread upon the surface as a flexible lobed layer (Pl. II. fig. 2), or it is dry and brittle (crustaceous) and closely adherent (Pl. II. fig. 26).

The fructification consists of little saucers, disks, or streak-like furrows, often of a different colour from the thallus, the structure of which will be best illustrated by reference to a few common species.

Parmelia´ceÆ.—ParmÉlia parietÍna (Pl. II. fig. 2) is a very common Lichen, found on the bark of trees, on old palings, &c. It is of an orange-yellow colour, the thallus being flat, lobed, and scalloped (crenate) at the margins. The structure of the thallus may serve to represent that of most of the Lichens. It consists of three layers,—an upper cortical or epidermic layer, which is continued over the margins to the under surface of the thallus, to form the under layer; and between these is the middle or medullary layer. The medullary layer consists of interwoven fibres, which are more closely packed towards the upper and under surfaces, so as to give them a cellular appearance. Near the upper part of the medullary layer, a number of minute rounded green cells exist, lying loosely in its meshes (Pl. II. fig. 4 a). These green cells (gonidia) appear to correspond to the buds of the higher plants, and, when detached from the plants, they are capable of growing into new individuals.

On the upper surface of the thallus of Parmelia (fig. 2) the fructification may be observed. This consists of saucers or shields (apothÉcia, ?p?????, a repository), formed of a raised and expanded portion of the thallus (fig. 3), and containing the spores. The spores are enclosed in closely set upright cells, or spore-sacs (figs. 4 b and 5 b), called as´ci (?s???, a bottle) or thecÆ; and intermingled with the asci are filaments, enlarged and coloured at the ends (paraphyses), which are probably abortive asci.

In systematic works upon the Lichens, the saucers and their contents are included in the term apothecia, the saucer alone being called the excip´ulum (excipulum, a receiver); the mass of asci and paraphyses forming the nucleus or thalÁmium (???a??, a bed).

The yellow spores are very minute, each ascus containing eight of them, and they are divided by a transverse partition or septum.

Near the margins of the lobes of the thallus are small dark points. These are the pouting mouths of little capsules (spermogonia) sunk in the substance of the thallus, and containing numerous filaments, terminated by very minute stick-shaped bodies (spermatia), which break off and escape through the orifices of the capsules. These are probably the representatives of the anthers of flowering plants and of the antheridia of the ferns. They are, however, so difficult to find and examine, that I must refer to the Dictionary for a further description and figures of them.

Lecidin´eÆ.—This family contains the genus CladÓnia, three or four species of which are common on boggy heaths, banks, &c., viz. Cladonia coccif´era, the Scarlet Cup-moss (Pl. II. fig. 21); C. pyxidÁta, the Common Cup-moss (Pl. II. fig. 23); C. vermiculÁris (Pl. II. fig. 22); and C. rangiferÍna, the Reindeer Moss (Pl. II. fig. 24). The thallus of these Lichens forms little rounded irregularly overlapping scales, with scalloped edges, overgrowing the surface upon which the Lichens are found. The fruit-stalks, or podÉtia (p???, a foot), are hollow (fistulose), and either simple and dilated into cups (Pl. II. fig. 23), or branched with the corners or angles between the adjacent branches perforated. The apothecia in the young state resemble those of Parmelia on a small scale; but as they approach maturity, the centre becomes pushed up, so that the spore-layer is extended over the ends of the stalks. In C. coccifera and pyxidata the cups are proliferous at the margin; i. e., branches upon which the apothecia are placed arise from it. The asci and paraphyses are very minute, but do not differ essentially in structure from those of Parmelia. In C. vermicularis the podetia are pointed and more solid than in the other species, the apothecia forming very minute spots at their apices.

Graphid´eÆ.—To this family belong GrÁphis scripta (Pl. II. fig. 26) and Opeg´rapha betulÍna (Pl. II. fig. 30). These little Lichens are easily overlooked, from the thin and but slightly raised thallus being only visible to the naked eye as a discoloration of the bark of the trees upon which they grow; while the fructification is very minute, forming little black streaks or lirel´lÆ (lira, a furrow), irregularly arranged, and resembling somewhat the letters of some of the Oriental alphabets.

In Graphis scripta (fig. 26) the thallus is thin, somewhat membranous, smoothish, shining, greyish white, and faintly bordered with black. The lirellÆ (fig. 27) are partly sunk in the bark, winding and narrow, some being simple, others branched; and they are surrounded by a raised border, formed by the thallus. The lirellÆ are lined at the sides with a black (carbonaceous) layer or excip´ulum, within which are situated the asci and paraphyses. The spores (Pl. II. fig. 29) are 8-cleft, the segments being again divided longitudinally into little spores or sporidia.

In Opegrapha betulina (Pl. II. fig. 30), which is found on the bark of the birch-tree, the thallus is thin, dirty yellowish white, bordered with black. The lirellÆ (figs. 31, 32) are mostly simple, without a raised border of the thallus, and the excipulum forms a complete lining to them. The spores (fig. 33 a) are 3-cleft, and taper at the ends.

Calici´eÆ.—Calic´ium clavel´lum (Pl. II. fig. 6) is a pretty little Lichen, growing upon old boards and farm-buildings. The thallus is granular and greyish white. The apothecia (fig. 7) are stalked and black, but of a lighter colour than the mass of spores forming the nucleus. The spores are very minute, black, oblong, and divided by a transverse septum.

The Lichens are divided into two Orders, according to whether the apothecia are open before the spores are ripe, as in the species noticed above, or whether the apothecia only open to discharge the ripe spores. The first Order forms the Gymnocar´pi (?????, naked, ?a?p??, fruit); the second forms the Angiocar´pi (???e???, vessel, capsule).

Preservation.—The Lichens are readily preserved, on account of their dry nature; they need simply be kept in a dry place, and glued to pieces of card. If room is an object, they may be dried under pressure, as in the case of the flowering plants. When re-moistened, the minute structures may be easily made out by sections. The smaller ones may be mounted dry, in cells made of the wax cement (p. 16). The minute structures keep well in chloride of calcium or glycerine.