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.