The thallus or frond of many of these lichens is irregularly covered with thick convex scales, in each of which are concealed white or grey bodies, which become flask-shaped, with pores in their extremities. When mature, they exactly resemble the bottle-shaped perithecia in the genus Sphæria among fungi. They are lined with branching filaments or other supports, ending in minute ovoid particles, which escape in myriads from the flask-shaped vessels long before the spores appear. They are motile, though not provided with cilia, and are supposed to be fertilizing particles like pollen, since they do not germinate as spores do. In this vast family of open-shielded lichens, which comprises every form and habit of the plant, the preceding type, which is the highest, undergoes many modifications; but the essential character remains the same, and the group is so natural, that the species run into one another so as to leave no very striking distinction.
Commencing with the Gymnocarpei, the Parmeliacei first claim notice as containing the highest types which lichens are capable of assuming, and as abounding in species. The disc, which is the hymenium, is orbicular or kidney-shaped, and surrounded by the frond, without any proper excipulum or cortical covering. A large portion of our most widely diffused lichens, whether growing on the ground or attached to rocks and trees, belong to this tribe; they form patches two or three feet in diameter, which are often of marvellous antiquity, and they grow so slowly that even small patches are of great age. This tribe consists of three distinct groups: in two of these the plants are horizontal and sometimes foliaceous; in the third they are vertical, often branched, and occasionally pendulous.
The latter group of this large division of lichens comprises the genera allied to Usnea, which are erect, centripetal lichens, that is to say, their body or thallus is an upright and generally cylindrical stem, in the centre of which the layer of marrow, the origin of fructification, is condensed, and the open discs, or hymenia, are in connection with it, whether they be situated at the ends of the branches, or on the surface of the upright stems. The Usneæ have three forms, the pendulous, the inflated and erect, and the branched or shrubby. The Evernia jubata is an example of the first, and remarkable for its long, pendulous, cylindrical branches hanging down like bunches of bluish grey hair. It is often attached to the branches of aged larch trees, and is exceedingly picturesque. With the exception of Evernia vulpina and flavicans, which are brightly coloured, all the rest of the genus have dull tints. The genus Ramalina has the dull tints of the Evernia, and some species approach very nearly in form to that genus. The Ramalina polymorphum and R. scopulorum abound in dyes, while the R. calicaris is rich in gluten.
Lichens have lost much of their importance since the discovery of the coal tar colours; nevertheless they afford dyes still in use. In many species the dying principle is colourless like aniline; but it yields the most brilliant colours by means of alkalies. The Parmelia parietina, common on walls and the roofs of houses, gives the chrysophonic acid, a colourless liquid which becomes bright yellow when treated with an alkali; and in like manner the vulpinic acid, given by Evernia vulpina, gives a brown dye. Notwithstanding the quiet greyish green tints of the Rocella fuciformis and R. tinctoria, no lichens are richer in the purple substance known as orchil, from which, by means of soda or potash, the valuable blue substance litmus is manufactured, so important as a test for acidity. Many of the small moss-like lichens yield orchil, but none in such abundance as the R. fuciformis, which grows in Madeira, Angola, Madagascar, and South America. Oxalic and usnic acids are produced by lichens; indeed the usnic acid combined with green and yellow resins seems to be more or less a constituent of various lichens. It is evident that the colour of the dyes is altogether independent of the colour of the lichen from whence they are obtained.
These aërial plants have a marvellous power of decomposing the light, and adorning themselves in brilliant hues. In our own country, where the colouring is often cold from the excess of verdure, the lichen affords a happy relief by giving a little warmth to the landscape. Even at Rome, where nature is so gorgeously coloured, the ruins owe much of their picturesque beauty to the red, white, and golden lichens with which they are clothed.
The Usneæ are perhaps the most beautiful of the lichens, the colours being sometimes brilliant, the forms elegant, and when the broad discs are amply ciliated, the appearance is very striking. The same species are widely diffused, but the colours are brighter in exotic specimens. Usnea melaxantha and Usnea Taylori are splendid productions.[[63]] The Usneæ are cosmopolitan in genera and species. The genus Cetraria, of which the Iceland moss is a well known species, forms a connection between the vertical and horizontal lichens; its thallus is neither cylindrical nor quite erect, though it becomes more so towards maturity. The Cetraria tristis has only that degree of inclination which arises from its crowded mode of growth, and springs like a sea-weed from a little peltate disc.
The most typical species of lichens occur in the second group of Euparmeliaceæ, or Parmeliacei proper, in which the disc is at first closed, and surrounded after expansion by a border arising from the thallus or frond. The thallus is always horizontal, and expands from the centre towards the circumference. The genus Sticta, belonging to this group, is often highly foliaceous, and is not excelled by any horizontal lichen in brightness of colouring or elegance of form. Even in our own country the Sticta pulmonacea spreads over a wide area, and is remarkable for its pitted frond. In this genus the under-side of the plant is covered with a delicate velvety down, amidst which are scattered round white spots, which penetrate to the medullary strata.
The erratic lichens are among the peculiarities of the genus Parmelia. The Parmelia saxatilis, common on stones and boulders of the primary and metamorphic formation, curls up into a ball, only fixed to its matrix by a slender thread, which soon gives way, and the ball being dry and as light as a balloon, is driven bounding by the winds, over the sheep walks and downs of England. The globular Lecanora esculenta sometimes suddenly covers large tracts in Armenia, Persia, and Tartary, where the plants are eaten by the cattle and by the nomade tribes. This species, with Lecanora affinis, is largely used by the inhabitants of the countries east of the Levant, and in some parts of Africa is mixed with wheat in grinding. These species are found, scattered over the ground without any attachment, in the form of rugged truffle-like objects of the size of walnuts. Sometimes they are piled together in strata a few inches thick, by the whirlwinds, after traversing the air for many miles, which gives rise to the histories of the miraculous descent of food. During a scarcity, a shower of these lichens fell at Erzeroum, and there are other well-authenticated instances recorded.[[64]]
The Peltigeri, or the third group of Parmeliacei, are so named from the target-like discs on their surface, covered by a veil, which afterwards disappears. The species frequently spread their beautifully foliaceous fronds upon the ground, and as the fruit is marginal, it gives the thallus a digitate appearance. They are often spotted by a little red fungus. The genus Solorina has the fruit scattered over the frond; in the Solorina saccata it is at first superficial, but a number of fibres are formed on the under-side of the discs, which penetrate the soil, and draw them down below the general surface—a very peculiar structure, in strong contrast with that of the Solorina crocea, which has veins on the under-surface but no fibres, and as no rootlets are sent out from the base of the disc, it remains superficial. M. Tulasne discovered fertilizing particles in several species of this group.
The order of the Lecidinei contains numerous species of the most varied habits. There is a distinct and regular series upwards in the genus Lecidea, the species of which are always crustaceous, and often form merely a thin, close, adherent fibrous stratum on the hardest flint or quartz. Some of them disintegrate the hard rocks on which they spread, possibly by the action of some acid which they contain. From this low type, the genus rises to the exalted forms with erect branching stems clothed with foliaceous scales, and brightly coloured. The order is distinguished by an orbicular disc, contained in a distinct excipulum or cortical envelope, which is open from the earliest age; but it is frequently obliterated afterwards by the development of that part of the medullary stratum where the fruit is formed, the disc in consequence becoming convex and capitate. This transformation takes place in the highest forms of the genus Lecidea. The genus Bæomyces has rose or chestnut coloured convex discs, supported on a stem called a podetium. The genus Cladonia has a foliaceous thallus producing free scale-like fronds, from the midst of which spring cylindrical or cup-shaped podetia, which are sprinkled with leaves. The margins of the cups or tips of the branches bear an abundant crop of convex, irregular brown, or deep red discs, often as brightly coloured as sealing wax.