Lichens are essentially air plants, being nourished, like the AlgÆ, by the medium in which they grow. They vary from a pulverulent or dry papillose crust, to a leathery or horny expansion, and even acquire an erect stem. They are independent of the matrix to which they are attached. Hence they spread their coloured frond, or thallus, in circular or indefinite patches on old walls, the tiles of houses, stones, and rocks. They appear in large expansions of red, golden yellow, grey or white, on barren heaths, under plantations, and on the stems of aged trees; while others of them often hang from the branches like long shaggy grey hair, and many form forests of miniature bushes on the northern plains. The lichen is the last trace of vegetation on the tops of the mountains, and on the arctic deserts. Some lichens are patient of severe cold, yet in general they prefer heat and moderate warmth, and they love bright light so much, that they are usually barren, or else yield little fruit under shade. Though differing greatly from fungi in slowness of growth, length of life, and the power of forming chlorophyll, they resemble them in having a mycelium in their youth, and in their ascigerous fructification. A perfect lichen without an ascus would be an anomaly, for the asci contain the true fruit, associated with vertical threads or elongated cells called paraphyses, which sometimes bear secondary spores on their summit. The asci with their paraphyses inclosed in vessels of various shapes, called perithecia, are aggregated in discs or shields, which form projections on the surface of the plant. Some of these discs are closed, and give egress to the spores through a fracture or pore on their surface; others are open cups of various forms, either with stalks or sessile on the frond, and through these the spores have egress. Hence the whole order of lichens is naturally divided into two groups, according as their discs are open (Gymnocarpei), or closed (Angiocarpei), the first being incomparably the most important. Fig. 37 shows open, and sections of closed cups, or perithecia.

The highest type of horizontal lichens, of which fig. 37 b is a perpendicular section, has a firm, spreading, superficial crust or surface, formed of oblong coloured cells, or of coloured filaments closely aggregated, and which covers two distinct layers of cellular tissue. In the layer immediately below the surface, the cells are globular and of a paler colour; the second layer, or marrow, which is the origin of fructification, consists of lax, detached, branching rows of elongated cells and gelatinous matter. These branches of cells spring up from a fourth layer, which is the base of the plant. It is of a strong, firm, and tough nature, composed of interlaced filaments and is often ribbed on the under-side. White fibres fix the base of the plant to the surface on which it is spread; they are the remains of a mycelium or matted mass of fibres, from whence lichens spring, which vanishes when the plant is full grown.

Globular bodies of vegetable green called gonidia, like those at the base of fig. 39 a, are arranged in regular parallel rows, and placed between the surface and the base of the lichen, as in fig. 38 b and c; they are said to proceed from the medullary layer, though Mr. Berkeley has seen them springing from the threads of the mycelium of Parmelia parietina. The gonidia find their way to the air through rents in the surface of the plant, and are washed off by the rains, after losing a little of their green colour. When they germinate they only produce a facsimile of the mother plant, as buds do in the highest classes. But according to the microscopic observations of M. Tulasne, the true fruit differs little if at all from the asci-bearing fungi. Through the open discs of the higher lichens, sporidia are discharged from perpendicular septate asci which, with their paraphyses, are imbedded in the substance below. The asci are formed by the elongation of some of the cells of that layer into cylindrical septate vessels, generally containing from four to eight sporidia, the ultimate result of as many free cells. In whatever part of a lichen the perithecia may be placed, the asci and paraphyses invariably originate in the medullary layer.

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.

The species of Lecidea bear severe cold, and are wonderfully long lived. Mr. Berkeley mentions that there are patches of the Lecidea geographica which probably date from almost fabulous periods.

The fronds of the order Collemacei are gelatinous; the medullary stratum seems to occupy the whole, though in two species of the genus there is a distinct epidermic cellular coat. There are moniliform strings of minute gonidia in the gelatinous matter, and others that are single, arising from the division of each gonidium and its supporting thread into two: both kinds differ from the gonidia in other lichens. The fruit-bearing discs on the surface are open. Members of this order occur in Europe, Africa, and Australia.

The order Coccocarpei is mainly distinguished by having orbicular discs entirely deprived of the cortical envelope called an excipulum, or, if it does exist, it is confounded with the thin membranaceous thallus. The discs spring at once from the medullary stratum, and contain asci and sporidia, similar to those of minute fungi (SphÆriÆ). Some species of the genus Coccocarpia only differ from Lecidea in the total absence of an excidium. The order is chiefly parasitic, and in some cases the whole plant is little more than a mass of fructification, parasitic upon and continuous with the substance of other lichens, at whose expense they live, thus forming an exception to the general habit of lichens, which are fed by the atmosphere alone. M. Tulasne has discovered that the genera Abrothallus and Scutula, though consisting almost entirely of fruit, produce secondary spores—they are the only lichens in which they occur; while Phacopsis and Celidium bear spermatogonia, which, analogous to antheridia, contain minute fertilizing particles. The parasitic genera occur in most parts of Europe and North America.

The Pyxinei are horizontal foliaceous lichens, for the most part fixed by the centre. They have orbicular discs, and form one of the most singular groups, both with regard to the superficial fruit, and the curious convolutions of the perithecia. The fruit-cup, or excipulum, is at first closed, and in the genus Gyrophora the disc produces a number of partial fructiferous discs on the original fruit-bearing surface. As usual the cells from whence the asci spring belong to the medullary stratum; the border on the contrary is cortical. The thallus is always foliaceous, and more or less peltate, but it becomes dry and brittle when exposed to drought. Several of the species have tufts of strong rootlets by which the frond is fixed to rocks, and some have deep pits, with corresponding swellings on the upper-surface, from whence they have their popular name of Tripe de Roche. These lichens, together with some species of Gyrophora, afforded a miserable sustenance to Sir George Back and his companions during their journey along the frozen regions of Arctic America in quest of the north-west passage. It is evident that plants which derive their sustenance solely from the medium in which they live, whether air or water, can never become a permanent and wholesome food for man, though erratic lichens and laver may be eaten from necessity, certainly not from choice. In the Gyrophora the perithecia are convolute, in the Umbilicaria they are not; but both of these genera, which have their seat in Arctic Europe and America, possess fertilizing particles like most of their class.

In the order Graphidei the disc is linear, simple, or branched, with or without an excipulum, which is carbonaceous. Many of these lichens are crustaceous, and in most of the genera the perithecia are much elongated, pointed at both ends. When they are parallel, or placed at different angles to each other, they form groups like Japanese or Chinese characters, whence their name. The genera are determined by the position of these perithecia, which are much varied. M. Tulasne has found linear male particles immersed in the crust of some of these lichens. They have their principal seat in tropical America, though a few species occur in the frigid and temperate zones of both hemispheres.

The order Glyphidei has no true excipulum; the coloured discs are at first immersed in the medullary stratum of a crustaceous thallus, the crust then rises into distinct expansions, in the centre of which the coloured discs are set like gems in a mosaic. ‘There is in fact no true border to the disc, the perithecium being reduced to a thick conical base, from which proceed immediately the asci and paraphyses; each individual hymenium being surrounded by the intervening medullary matter injected, as it were, into the interstices.’ The whole surface of Chiodecton monostichum is productive, and in that genus M. Tulasne found vessels in the form of little scattered perithecia, containing filiform curved fertilizing particles. The species of this order are almost wholly tropical, though the Chiodecton myrticola has been found in Ireland.

The order Caliciei consists of horizontal lichens, with generally an ill-developed crust; the discs, which are at first covered by a veil, are contained in a stalked, or more rarely sessile, excipulum, looking like little flat-headed pins stuck into the crust; the veil at length vanishes, and exposes a pulverulent mass of spores, which adhere so loosely in the Calicium inquinans, that they soil the finger if touched; in other cases they come out of their ascus like little necklaces. The species of these lichens are almost entirely confined to Europe and North America.

In the second division of LichenaceÆ, the Angiocarpei, the discs are enclosed in an excipulum, which projects from the surface of the plant, and ultimately discharges the spores from a rupture or pore in its surface. Besides the fructification consisting of perithecia containing paraphyses and asci with their enclosed sporidia, there is a nucleus in each disc, either of firm or deliquescent matter.

Some of the lichens of this group are parasitic, others are aquatic. The order Limboriei, like parasitic fungi, begin their existence under the thick skin of the leaves of tropical plants, spread their crustaceous thallus over their surface, and destroy their beauty, by stopping up their pores, and preventing the admission of light to their tissues. The excipulum and perithecia are black, and the latter burst in an irregular fissure, and are in most cases covered by a beautifully sculptured crust.

The crustaceous fronds of the Verrucariei are often so thin as to be inseparable from the substance over which they spread. The excipula are closed, the walls of the perithecia are often black, and in some species more or less crowded round a columella. In the Verrucaria muralis fertilizing particles have been discovered. The plants are widely distributed, and at least one species spreads its crust over the smooth stones in running streams.

In the group Endocarpei, the perithecia are immersed in the substance of the plant, which has, for the most part, a foliaceous horizontal crust, and a gelatinous nucleus. Some species grow on stones, perpetually or periodically submerged, or, if not under water, continually wet with its spray. The Lichina, a genus of the group Lichinei, lives on marine rocks, and is often dripping with salt-water, and often suddenly dried up.

The SphÆrophorei, or sphere-bearing lichens, have upright stems bearing globular fruit at the extremity of their numerous branches. At first, the fruit is only indicated by a swelling, but in time the outer bark bursts, and exposes the contents of the perithecium, which consist of asci and paraphyses seated on a central columella. The sporidia are beautiful objects under the microscope on account of their spherical form, which is rare among lichens, and their more or less deep blue tint—a colour by no means peculiar to the sporidia of this order of plants, for in other orders they are bright scarlet, olive, golden yellow, or brown.