In the classification of plants we have the two great sub-kingdoms of flowering and flowerless plants; the flowerless or cryp-togamous plants are subdivided into acrogens, which are mostly herbaceous, with a distinct axis of growth, having foliaceous appendages and growing from the apex, and thallogens, which are seldom herbaceous or with foliaceous appendages, the growth taking place periphically or horizontally. To the thallogens belong the algae, the fungi, and the lichens. These orders are usually readily distinguished. But there are some lichens which approach alga?, and others so near fungi as to make their classification difficult without careful study. The vegetative portion of a lichen is the thallus, which may be regarded as the plant proper, as it performs all the functions of root, stem, and leaves; it is exceedingly variable in form, texture, and color. When the thallus forms a flat expansion it is called foliaceous, as in sticta (fig. 1); if erect and cylindrical, as in cla-donia (fig. 2), it is fruticulose; in some it forms a mere crust on the soil or other surface, when it is called crusta-ceous; and when concealed beneath the fibres of the bark of trees, it is hypophleous.

Whatever the form of the thallus, it consists wholly of cellular tissue, and its surface is destitute of stomata. (See Leaf.) The structure of the thallus is not homogeneous, but the microscope shows several distinct layers. A magnified cross section, as given in fig. 3, presents first a layer of cells of colorless cellular tissue, the cortical layer (c I). Beneath this is the gonidial layer, made up of opaque cells, not altogether continuous, called gonidia (g); these are usually bright green or olive-green, and their presence is characteristic of lichens, serving to distinguish them from fungi. Next below the gonidia is the medullary layer (m l), consisting of elongated cells, which are either (1) felted or interlaced to form a loose web, (2) crustaceous, when the filaments are fewer, accompanied by white granules and crystals of oxalate of lime, or (3) cellular, when rounded or angular utricles are associated with the filaments. The lower layer of the thallus, called the hypothallus (h), is of cells or filaments, and is usually darker than the upper surface; it sometimes gives off rootlike hairs (rhizines) which serve to attach the lichen to its matrix.

Various modifications of this structure are met with; in some the cortical and in others the lower layer is wanting; in the erect lichens the medullary layer serves as an axis around which the gonidia are arranged, and in some of the crustaceous lichens it is difficult to distinguish any other elements than the gonidia. - The organs of fructification, called apothecia, are sometimes concealed within the tissues of the thallus, but are commonly upon its surface or margin, where they appear as variously shaped disks, to the different forms of which the descriptive names of peltate, scutel-late, etc, are given; sometimes, as in what are called the " written " lichens, grapliis (fig. 4), for example, the apothecia are elongated or branching irregular spots, which have been compared to Japanese letters; similar lichens of related genera are quite common on the bark of oak and other forest trees. Apothecia are rarely of the same color as the thallus, and are black, brown, yellow, or red of various shades. A general idea of the structure of an apothecium may be had from fig. 5, showing a magnified section.

The microscope shows a number of oblong or club-shaped bodies, the spore cases (s c) or asci, to which all other parts of the apothecium are subordinate; these spore cases (also called thecal and sporangia) are surrounded by numerous filaments, the paraphyses (par); the exterior of the apothecium has a cortical layer (c l), below which are gonidia (g), as in the thallus. Other apothecia vary in form and details from the one figured, but their office is the same, the protection and development of the spore cases, which contain the spores. Though lichens multiply by other methods, that by spores is regarded as the normal one, corresponding to the reproduction of flowering plants by seeds, while the other ways in which they are multiplied are equivalent to propagating by means of cuttings, etc. The common number of spores in each spore case is eight, but there are in some species six, four, or two, and even a solitary spore; on the other hand, they are (rarely) more numerous, up to 100 or more. The spores vary greatly in size, being from 27/1000 to 157/1000 of an inch in length; in form they vary from globose to needle-shaped, and in color from brownish yellow to nearly black.

Some spore cases with their paraphyses are shown in fig. 6. One or more subdivisions are to be seen in the spores themselves, characters that are made use of in describing these plants; the color of the spores is very constant, and furnishes good distinguishing marks for genera and species. When the spores are perfected they are thrown out from the apothecium, either with or without the enclosing spore cases. - Many lichens bear upon the thallus small black dots, either upon the margin only or scattered over its surface, called spermo-gonia. A magnified cross section (fig. 7) shows a great number of needle-shaped, extremely minute bodies, the spermatid, which are borne by branched or simple filaments called sterig-mata. The spermatia are present or absent in different species, and vary considerably in form, which is constant for each species in which they occur. Reasoning from analogy, the spermatia have been supposed to be equivalent to the antheridia in ferns and mosses, and to play the part of the male organ in fertilizing the spores; but as their action has not been observed, and they have not the locomotive power of the antheridia, their office is inferred rather than proven.

Pycnidia are still other protuberances upon the surface of the thallus of some lichens; they are not of common occurrence, and their office is not well understood; as they contain spore-like bodies, they are regarded by Tulasne as supplementary means of propagation. - The close relationship of lichens with algae and fungi has already been alluded to; indeed, Schwendener goes so far as to deny to lichens the rank of a distinct order, but regards them as compound organisms consisting of algae held in captivity by the meshes of a fungus; a view which meets with but few adherents. - The essential elements of terrestrial vegetation are to be found in these plants, which hold such a subordinate rank in the scale of creation, being in fact rootless and cellular, subsisting upon the air, but furnished with stems, branches, and parts which correspond to fruits and seeds; their position in the vegetable kingdom being intermediate between the floating tribes of the algae and the fugacious forms of the fungi, or, as Fries expresses it, "having the vegetation of the algals and the fructification of the fungals." Thus ingeniously contrived and admirably fitted for an especial office, we should expect to find them in situations suited to no other vegetation.

Lichens play an important part in the economy of nature, and it is probable that they were the first forms of vegetation upon the dry rocks; and that by their decay and accompanying disintegration of the rocks they began the accumulation of soil. It is now well known that the pulverulent lichens are the first plants that clothe the bare rocks of newly formed islands in the midst of the ocean; the foliaceous lichens follow these, then mosses and liverworts, which by their decay prepare a soil for the growth of plants of the higher orders. The crustaceous lichens affect the very summits of mountains, growing near the limits of perpetual snow, and are seen very far north, so seemingly rudimentary as to appear like colored spots of the solid rocks. They are not, however, exclusively confined to such regions, being common in some instances on the margin of the sea in countries where granitic strata especially are to be found. The sides of buildings and the surfaces of sandstone rocks are favorite situations for many kinds. The larger and more conspicuous are found in temperate and moist climates, choosing in the northern hemisphere northern and western exposures; and even at the equator there are species rich and gorgeous in colors.

The prevailing tints in lichen are gray, white, black, dark brown, rich green, pale yellow, and orange red. From mere specks or patches of hard, seemingly inanimate matter, the lichens assume sizes of considerable magnitude. The change produced by moisture in the same plant is very striking: dull and inconspicuous in dry weather, it assumes bright colors in a prolonged season of dampness, and appears endowed with life. Lichens grow upon almost every substance where alternate dryness and moisture can be found, a very few only passing much of their existence in a submerged state. Destitute of roots and dependent upon the atmosphere for their nutrition, it seems to matter little with them upon what matrix they fix. Some have even been found attached to the glass in the windows of old and deserted buildings. In so wide a geographical range as that over which they are spread, the same identical species must be found occurring upon very distinct kinds of trees and soils, yet maintaining their specific characters. Thus there are some species which are most commonly to be expected upon rocks, yet which frequently grow upon the bark of trees.

Many species are excessively polymorphous, and present themselves under so many varieties as to render it difficult to reduce them to an original type, the color of the thallus being often affected by the chemical composition of the rock on which they grow, or the color of the disk of the apothecia remarkably diverse. Several species are parasitical upon others, occurring upon their thalli in the reduced forms of mere fruits or of spermagonia; their own vegetative functions being supplied by the subject to which they have attached themselves. - The value of the lichens to man may be estimated from their uses as articles of food and of medicine, and from their employment in the arts. According to Linnaeus, in the arctic regions of Lapland the reindeer lichen (cladonia rangi-ferina) grows in the utmost profusion, and overspreads plains hundreds of miles in extent. These are the fertile fields of the Laplanders, so that the possessor of such a barren tract thus covered with lichens considers himself fortunate; for when the cold of winter has withered up every sort of herbage, this lichen becomes the principal aliment of the herds in which consists his wealth, and on which depends the very existence of the natives of that country.

The reindeer lichen was at one time by edict of Gustavus III. of Sweden used in the manufacture of flour, when grain was scarce. It also grows in this country as far south as Pennsylvania, and is especially abundant northward to Canada and arctic America. The Iceland moss (cetraria Islandica) fattens cattle, sheep, deer, and swine; and out of this and of the G. nivalis the Icelanders make soup and even bread. According to Olafsen, one ton of Iceland moss is equal to half a ton of meal. (See Iceland Moss.) Lecanora esculenta, of the steppes of Asia, is eaten by the nomadic tribes of those regions. This occurs in masses about the size of a filbert, and so like the stones in appearance that it needs a practised eye to detect it; as it has never been found attached to any object, it is regarded as having fallen from heaven, like the manna of the Israelites. The tripe de roche (umbilicaria Muhlenbergii), mixed with the roe of fishes, assists in making nutritious food for the North American Indians. Sir John Franklin was indebted to this lichen for subsistence after a four days' abstinence when on his journey to the shores of the polar sea. Lichens afford valuable materials for dyeing, of which the parelle (lecanora parella) and cudbear (L. tartarea) may be cited as familiar instances.

To these may be added urceolaria scruposa and cinerea, with parmelia saxatilis, omphalodes, conspersa, etc. Rocella tinctoria, fuciformis, intricata, etc, inhabitants of the shores of the Mediterranean or of the coast of Africa, Chili, etc, yield archil. Even the common yellow wall lichen (parmelia parietina), so abundant near our sea-coasts, possesses a peculiar principle called pa-rietine (Thompson), which forms a bright yellow coloring matter; this is heightened by nitric, sulphuric, or muriatic acid, and alkalies change it to a rich purplish red. - The chemical constituents of lichens are phosphate of lime, salt, manganese, iron; several principles, as picrolichine, variolarine, orceine, cetrarine, inu-line, erythrine, rocelline, picroerythrine; several acids, as parellic, usnic, orceic, and ery-thrynic acid, and others; uncrystallizable sugar, oil, waxy matter, resinous matter; crystals, and oxalate of lime in the tissues especially of lecanora tartarea. - The name lichen was originally given by the ancient naturalists to certain species, because of a fancied resemblance to the cutaneous disease so called, whence they were supposed to be specifics for it. - The works of Acharius, though published early in the present century, are still employed by students in lichenology; his Lichenographia Universalis (Gottingen, 1810) and Synopsis Methodica Lichenum (Lund, 1814) are useful works of reference.

Among other important European works upon the subject are Fries's Lichenographia Europropcea reformata (Lund, 1831); Nylander's Synopsis Methodica Lichenum (Paris, 1858); and Tulasne's Memoire sur les lichens (Paris, 1852). Berkeley's "Introduction to Cryptogamic Botany ' (London, 1857) treats of the structure in this and the related families. The first enumeration of American lichens is to be found in Gronovius's Flora Virginica (1761); Muhlenberg's Cata-logus Plantarum (Lancaster, Pa., 1813) enumerates 184 species; and several other works record American lichens. Halsey's "Synoptical View of the Lichens of New York" appeared in the "Annals of the New York Lyceum of Natural History," 1823. Various papers upon lichens by Profs. Edward Tucker-man and J. Lewis Russell are to be found in the " Boston Journal of Natural History " (1838 et seq.). Prof. Tuckerman, now of Amherst college, who has devoted himself more thoroughly than any other American botanist to these obscure plants, besides the memoirs above referred to and others in Silliman's "Journal of Science and Arts" (1858-'9), has published an " Enumeration of North American Lichens" (Cambridge, 1845), and a "Synopsis of the Lichens of New England and of the Northern States and British America " (Cambridge, 1848). The same author's Lichenes America Septentrionalis Exsiccati (1848-'51) consists of six fascicles of specimens, and is an important contribution to American lichenology.

In an enumeration of this author's labors we should not omit a memoir in Agassiz's "Lake Superior" (Boston, 1850), and one in the "Botany of Wilkes's Exploring Expedition," recently published. Since the publication of Prof. Tuck-erman's earlier works much progress has been made in the study, and his promised work on the "Genera of North American Lichens" will no doubt embody whatever is now known of these obscure forms of vegetable life.

Fig. 1. - Tree Lungwort (Sticta pulmonacea).

Fig. 2. - Cladonia coccinia.

Fig. 3. - Microscopic View of Transverse Section of the Thallus of a Lichen, c l, the cortical layer; g. gonidia; ml. medullary layer; h, the lower layer or hypothallus.

Fig. 4. - Graphis elegans.

Fig. 5. - Microscopic View of the Cross Section of an Apothecium. s c, spore cases; par, the filaments or paraphyses accompanying them; c I, cortical layer; g, gonidia.

Fig. 6. - Spore Cases with paraphyses, highly magnified.

Fig. 7. - Microscopic View of the Cross Section of a Pycnidium.