Lichens

LICHENS(Lichenes) may briefly be defined as cellular perennial plants, furnished with a vegetative system containing gonidia, and with a reproductive system consisting of female thecasporous fruits and male spermo-gonous organs. They constitute a distinct class of cellular cryptogams, intermediate between algae and fungi, to both of which in some respects they present certain affinities. By the earlier authors they were regarded as being Aerophycx or terrestrial algae, while of recent years they have been viewed by some writers as being Ascomycetous fungi. From both of these, however, they are sufficiently distinguished and separated by the special structure of their thallus, by the presence of certain immediate principles proper to their tissues, and by their mode of life and nutrition. Their relations to these neighbouring classes, and their true systematic place, will be best elucidated on considering their structure and its bearings upon some recent speculations.

Structure of Lichens.

A complete lichen consists of a nutritive and vegetative system termed the thallus, and of reproductive bodies borne upon it in the form of apothecia and spermogones. Occasionally, however, there is no thallus present (e.g., Sphinctrina, various Lecidex, Endococcus), in which case the fructification is parasitical on the thalli of other lichens.

I. Vegetative System.— The thallus is very variable in external form and colour, as also in internal structure.

1. In external form it presents the following modifications. (a) The foliaceous thallus, which may be either peltate, i.e., rounded and entire, as in Umbilicaria, &c., or variously lobed and laciniated, as in Sticta, Parmelia, &c. This is the highest type of its development, and is sometimes very considerably expanded, (b) The fruticulose thallus, which sometimes is filamentose, as in Ephebe, and may be either erect, becoming pendulous, as in Usnea, Pamalina, &c., or prostrate, as in Alectoria jubata, var. chalybeiformis. It is usually divided into branches and branchlets, bearing some resemblance to a miniature shrub amongst the Phanerogamia. An erect cylindrical thallus terminated by the fruit is termed a ρodetium, as in Cladoηia. (c) The crustaceous thallus, which is the most common of all, forms a mere crust on the substratum, varying in thickness, and may be squamose (in Squamaria), radiate (in Placodium), areolate, granulose, or pulverulent (in various Lecanorx and Lecidex). In its pulverulent state it is either the rudimentary or the abortive condition of many species, (d) The hypophloeodal thallus is often concealed beneath the bark of trees (as in some Verrucarix and Arthonix), or enters into the fibres of wood (as in Xylographa and Agyrium), being indicated externally only by a very thin film or macula. To this may also be referred the evanescent thallus which is denoted solely by gonidia sparingly scattered on the stone or wood (as in some Caliciei and Lecidex:). This is the simplest form under which lichenose vegetation occurs. These two latter forms of thalli may be either determinate, i.e., of a definite shape with a distinct margin or boundary, or effuse, i,e., spreading extensively over the substratum with no visible limits. The differences in these forms are no doubt connected with differences in the chemical composition of the thallus. In colour also the thallus externally is very variable. In the dry and more typical state it is most frequently white or whitish, and almost as often greyish or greyish glaucous. Less commonly it is of different shades of brown, red, yellow, and black. These various colours do not originate from any colouring matters contained in the cells themselves, but, according to investigations made by Nylander (see Flora, 1879, p. 558) they depend upon such as are deposited in the granulations and cellular walls, whence they appear on the epithallus. In the moist state of the thallus these colours are much less apparent, as the textures then become more or less translucent, and the epithallus usually presents the greenish colour of the gonidia (e.g., Parmelia Borreri, Peltidea aphthosa, Umbilicaria pustulata, and pulverulent Lecidex).

2. The internal structure of the thallus presents two principal modifications, viz., the stratified thallus, having its different elements (hyphae and gonidia) arranged in layers, and the unstratified thallus, in which these different elements are confused in a homogeneous tissue.

A. The stratified thallus. —On making a vertical section this is seen in a foliaceous lichen to consist of three layers constituting a cortical, a gonidial, and a medullary system, to which in the case of many cnrsta-ceous lichens is to be added a fourth, viz., a hypothalline stratum.

(a) The cortical stratum occupies the whole of the external surface of filamentose and fruticulose lichens, both the upper· and under surfaces of some foliaceous and squamulose species, while it is found only on the upper surface of crustaceous lichens. It usually consists of a colourless cellular tissue, in which the cellules are closely compacted and form a pseudo-parenchyma. Its most superficial portion, termed by Nylander the epithallus, a sort of cuticle, is amorphous, often more indurated and coloured. In some lichens (e.g., Collenia) it is the only portion of the cortex present, while in pulverulent crustaceous thalli it is entirely wanting, (ό) The gonidial stratum is situated immediately beneath the cortical stratum, and consists usually of greenish spherical cellules, or of granules destitute of a cellular membrane. It is not always continuous, but is often interrupted,'the gonidia occurring in dissociated masses. Sometimes it is situated on the upper part of the medullary stratum, in which case the gonidia are arranged either between or amongst its exterior elements. In general its limits may readily be distinguished from the others by its peculiar colour. Various important matters relating to the gonidia will more appropriately be afterwards discussed at length, (c) The medullary stratum is more variable in its constituent elements, but, being always colourless, is easily recognized. It presents the three following principal modifications, (a) The woolly medulla consists of simple or branched filaments, which in foliaceous species are loosely intersected and entangled, and in fruticulose species are more or less congluti-nated, assuming a longitudinal direction, and constituting, as in Usnca, a kind of solid axis for the support of the thallus, (β) The cretaceous medulla occurs only in crustaceous lichens, and is generally characterized by its tortareous appearance. It is more compact than the preceding, and consists for the most part of molecular granulations often intermixed with octahedral crystals of lime, and presenting but few traces of filamentose elements, (γ) The cellulose medulla consists of a tissue of angular, rounded, or oblong cellules containing gonidia in their interior or in their interstices (e.g., Pannaria, Endocarpon). In some species (e.g., Verrucaria fuseula) the cellules have a tendency to reunite into filaments and then to separate again into rows of cellules, (d) The hypothalline stratum is the inferior one of the thallus and that upon which the other strata are developed, though it is not always visible, and is sometimes entirely wanting. - It usually presents itself under a twofold aspect, viz., the hypothallus and rhizinae. ( α ) The hypothallus proper, which is immediately developed upon the prothallus (Le., the filaments of the germinating spore), is a horizontal stratum consisting of interlacing filaments or of elongated, short, or rounded cellules, and is sometimes of a white or whitish colour, but usually dark or blackish. In many crustaceous lichens it is represented only by a black or dark-coloured border limiting the thallus (c.g. Lecidca geographica, &c.). (β) The rhizinae consist of vertical rhizoid fibrillae, usually branching and tufted at their extremities, blackish or greyish in colour, rarely white, which occur on the lower surface of foliaceous lichens. They consist of several filamentose elements which are most frequently articulated and agglutinated (e.g., Parrnclia, Physcia), or sometimes simple and then always articulated (e.g., Sticta). It is to be observed that the hypothallus and the rhizinae serve merely as bases of attachment for the lichens to the substratum, and do not in any way aid in its nutrition.

B. The unstratified tluillus.— This occurs amongst the Lichenacci (which, however, are most frequently stratified as above), and in various species belonging to the inferior genera, which have a pulverulent or hypophloeodal thallus. In these the constituent elements are more or less mixed together, though the gonidial stratum generally remains distinct, and is often visible when the others are absent. It is, however, the families of the Byssacei and Collemacei that are more especially characterized by an unstratified thallus. Here the cortical stratum is chiefly represented by a greenish (vn. Collema), rarely brown (in Synalissa, &c.). non-cellular epithallus, or in others (Leptogium) by a thin stratum of angulose cellules distinct from the other elements of the thallus. The gonidial granules are also disposed in a different manner to those of the Lichenacei. In the majority of the Collemacei they are strung together moniliformly, and distributed without order in a gelatinous pellucid substance; while sometimes they are agglomerated into small groups, and situated for the most part next to the epithallus. In Ephebacei they are not moniliformly arranged, but are tunicated or involved in a gelatinous cellulose stratum. The rest of the thallus consists of the medullary system (except in Ephebacei, in which there is no medulla), and is composed of tubular or hollow filaments, with roundish cavities containing the gonidial granules, and imbedded in the gelatinous substance, which very readily imbibes water. There are a few lichens in which there is no trace whatever of stratification, as the genus Canogonium, in which the entire thallus is composed of filamentose membranous elements, and the peculiar family of the Myriangiacei (doubtfully, however, referable to lichens), in which it is equally cellulose throughout.

In addition to the hyphal and gonidial anatomical elements which thus enter into the structure of the thallus, there is another to be 'noticed, which, however, is to be regarded rather as an immediate principle. This is the molecular granulations, which are extremely small and (in form) irregular corpuscles, OO01-0 ’ 002 millim. in diameter, and visible only when very highly magnified (300-400 diameters). They occur in all parts of the thallus, especially in the younger cellules, from the epithallus to the hypothallus, being especially abundant in the medulla of crustaceous species. In the epithallus they are variously coloured according to the colours which it presents, but in all other parts they are colourless. They occur also in the apotheca, in the epithecium, the thecae, and the spores, and constitute the famous “microgonidia” of Dr Minks. By the application of sulphuric acid many of them are transformed into small acicular crystals, and in the spores they are frequently agitated by a Brownian movement.

We may here also, in connexion with the vegetative system of lichens, refer to certain peculiar excrescences which are sometimes presented by the upper or under surface of the thallus. Of these the principal are the following. (1) Sorcdia are pulverulent eruptions on the cortical stratum, varying in form, being rounded or diffuse, and either are scattered upon the upper surface of the thallus or border its margins. They are of a lighter colour than the thallus, and consist of a mass of gonidia and of molecular granulations intermingled with filamentose elements. They occur in many frιιticulose, foliaceous, and crustaceous lichens, and their protrusion through the cortical stratum is owing most probably to an excessive development of the gonidial element. Occasionally also they appear on the disk of apothecia (in Pcrtusaria), which they render abortive, and in this case constitute the pseudo-genus Variolaria of older authors. When detached from the thallus they are capable under certain favourable circumstances of giving rise to new plants, and thus act the part of bulbils in the Phancrogamia. It is no doubt by their means that many species which are never-found in a fertile state (e.g., Thamnolia vermicularis) are propagated. (2) Cyphcllae are small, urceolate, pale excavations which occur abundantly on the under surface of many species of Sticici. They are generally naked, but are often also pul vendent or sorediiferous, in which latter case they are called pseudo-cyphellae. Their physiological function is not definitely known, but they are most likely connected with the nutrition of the plants. (3) Isidia constitute an exuberant condition of some foliaceous and crustaceous thalli. They consist of elevated, coralloid, stipitate excrescences, which are sometimes branched, and which are always of the same colour and texture as the thallus itself. This isidioid condition in crustaceous thalli is the basis of the old pseudo-genus Isidium, which is merely an anomalous state of some species of Pertusaria. Isidia have the same functions as the soredia, and serve as pro-pagula. (4) Cephalodia are orbicular and flattened or globular tubercles of a peculiar texture which occur on the thallus of many species belonging to different genera. They are usually epigenous, as in Stereocaulon, Usnea, Lecanora gelida, Leddea panaeola, &c. In a few species, however, they are hypogenous, as in Peltidea venosa and Psoroma euphyllum, while in various Stictei, Nephroma expallidum, &c., they are endogenous, forming pyrenoid protuberances on the lower surface. Recently Nylander has detected both epigenous and hypogenous cephalodia on Psoroma arenariu/m and Lecanora allorhiza. They are of a paler colour than the rest of the thallus, from which they differ also in structure, being confusedly cellulose, and containing gonidial granules. According to Th. Μ. Fries (in Flora, 1866, p. 19) they are only morbid excrescences caused by algals intruding themselves under the cortex; but this is at once refuted by the fact of their forming constant characters of so many different species occurring in various situations. In these they evidently constitute normal organs, the use of which, however, is unknown.

II. Reproductive System.— Thia consists of apotlceda or the female organs, of sρermogones or the presumed male organs, and probably also of ρycnides or a secondary kind of fructification.

1. The apothecia, like the thallus, are very variable in external form and colour, as also in their internal structure. In external form they present three principal modifications, viz., (1) disciform (or gymnocarpous), in which the shape is that of a disc (as in all the higher genera); (2) nucleiform (or angiocarpous), in which the shape is that of a rounded tubercle with an apical ostiole (as in Endocarpon, Verruearia) ; and (3) ρeridiiform, similar in shape to the preceding, but closed, with no ostiole (as in Thelocarρon, Endococcus). The last two are but little variable in figure, and consequently do not in this respect admit of different designations. The disciform apothecia, however, present various shapes, of which the following are the principal :—(a) peltate, which are large, rounded, without any distinct thalline margin (e√7∙, Usnea, Peltigera) · (b) lecaηoriηe, or scutelliform, which are orbicular and surrounded by a distinct, more or less prominent thalline margin (e.g., Parmelia, Lecanora), having sometimes also in addition a proper one {e.g., Thelotrema, Urceolaria) ; (c) lecideine, or patelliform, which are typically orbicular, with only a proper margin (e.g., Lecidea), sometimes obsolete, and which are occasionally irregular in shape, angular or flexuose {e.g., Lecidea jurana, L. myrmeciηa), or complicated and gyrose (e.y., Gyrophora) i and even stipitate (e.g., Baeomyces) ; (d) lirelliform, which are of very irregular figure, elongated, branched or flexuose, with only a proper margin (e.g., Xylographa, Graphis, &c.) or none {e.g., some Arthoniae), and are often very variable even in the same species. It may be here observed that young disciform apothecia are more or less nucleiform. In colour the apothecia are extremely variable, and it is but rarely that they are concolorous or subconcolorous with the thallus (p.g., Usnea, Ramalina). Usually they are discolorous, and may be black, brown, yellowish, or also less frequently rose-coloured, rusty-red, orange-reddish, saffron, or of various intermediate shades. Occasionally in the same species their colour is very variable (e.g., Lecanora metaboloides, Lecidea decolorans), while sometimes they are white or glaucous, rarely greenish, pruinose. Lecideine apothecia, which are not black, but otherwise variously coloured, are termed biatorine.

The two principal parts of which an apothecium consists are the hypothecium and the thecium.

(1) The hypothecium, which corresponds to the hypothallus, is the conceptacle of the^apothecia. It is composed of cellular tissue, generally very dense, *and often presenting an indistinct stratification. This tissue may in general be distinguished from that of the neighbouring parts of the thallus by its cellules being smaller, more compact, and differently coloured, though in some instances (as in certain Pannariae) the limits are not determinable. In the apothecia of such genera as Calidum, Baeomyces, &c., the hypothecium te composed of hollow cemented filaments arranged longitudinally and constricted into a stipe for the support of the fruit. The hypothecium in disciform apothecia is usually termed the excipulum proper, while in nucleiform apothecia it is termed the pyrenium, and in peridiiform apothecia the peridium. When the pyrenium quite covers the nucleus it is said to be entire, dimidiate when it covers only the upper portion. The hypothecium is either colourless or dark, or reddish or yellowish, according as its cellules are tinged.

(2) The thecium, or as it is more frequently termed the hymenium, is that part of the apothecium which contains the organs of the fruit, viz., the thalamium and the thecae, which are placed perpendicularly to the hypothecium. It is penetrated by an amyloid substance, colourless and very greedy of water, termed the hyrnenial gelatine, formed of the lichenine, which becomes bluish or wine-reddish when tinged with iodine. The thecium itself corresponds to the gonidial-medullary stratum, while its superficial portion, termed the epithe∙ cium, corresponds to the epithallus, (a) The thalamium generally consists of paraphyses which are erect colourless filaments arising from the hypothecium, and whose function is to aid in the expulsion of the spores by the pressure which they exercise upon the thecae. They are of nearly equal height, closely placed together, usually very slender, though slightly variable in thickness, frequently articulated, and rarely branched or anastomosing. Internally they are hollow and filled with protoplasm, which sometimes is separated into little globules. Their apices are generally coloured, in most instances dilated, sometimes clavate, and are cemented together by gelatin. They are frequently confused together; occasionally they are but little evolute; while in many of the Pyrenocarpei they are entirely wanting, though in these the ostiolar filaments of the hypothecium have sometimes been mistaken for them, (b) The thecae are large, oblong, cylindrical or ovoid cellules or vesicles containing the spores, and are usually more or less attenuated towards the base. In size and shape they vary considerably in the different genera and species according to the size, number, form, and arrangement of their spores. They differ also in the same species, within certain limits, according to age, the young theca being more slender than those which are older. In some genera which have very large spores (e.g., Variedlaria, Pertusaria) the thecae are distended in proportion, and generally present a saccate or oblongo-ventricose form. The theca itself is a thin membranous cellule, the walls of which are at first of an equal thickness throughout, but in process of development they become gradually thinner, except at the summit, where they retain their original thickness. In some species the wall is remarkably thick at the apex {e. g., Arthonia), and in others it is invested throughout with a kind of external cuticle {e.g., Pertusaria). The thecae are resorbed after the expulsion of the spores; though where their walls are extremely thin (as in Calidum) they are ruptured and disappear at a very early stage. The spores are the special reproductive organs of lichens, and are produced in the thecae by free cell-formation, i.e., by the separation and subsequent condensation of the protoplasm of the fully developed theca around certain points in its interior, corresponding in number to that of the spores to be formed. After the formation of the external spore-wall has taken place, the spores have a definite outline, and may consist of only one cellule or loculus {simple spores), or may be divided by one or more transverse partitional membranes {septate spores). Sometimes the loculi are restricted to the two ends or poles of the spore (one in each) and are said to be polari-bilocular, the two loculi being occasionally united by a longitudinal tube. At other times the transverse partitions are further divided by several longitudinal partitions, in which case the spore is said to be murali-diwted, from the resemblance it then bears to the stones in a wall. The contents of the spores are a homogeneous protoplasm, molecular granulations (often abundant), and a pale-yellow oily substance, which in the fully developed spore often becomes condensated into one or more globules, visible generally near its ends or in the middle. The number of the spores in each theca varies very much in different species, being in most lichens 8, occasionally 6, 4 or 2, or rarely only 1. In some species, however, they are 20-100, when the thecae are said to be polysporcd. In form they are also very variable, the principal shapes assumed by them in order of frequency being ellipsoid, fusiform, oblongo-cylindrical, sphaeroid, and acicular, with several intermediate forms. They are still more variable as to size, being smallest in polyspored (e.g., Lecanora fuscata, &c. ), and largest in monospored species (e.g., Pertusaria communis, &e.). The largest spores of all are found in Varicellaria amicrosticta, where they are 0‘225-0 , 030 millimetre long, and 0 - 095-0∙115 millim. thick. As to colour, they are either colourless (white, nearly hyaline or pale yellow under the microscope), or coloured (brownish, or brown, or blackish), while in Sphoerophoron there is also a bluish-black superficial pigment. The spore-wall varies in thickness, and is typically composed of two strata, the outer of which is termed the episρore and is coloured, while the inner is termed the endospore and is colourless, gelatinous, and generally less distinct. Notwithstanding that in these respects the spores are so variable in different species, they are when mature remarkably constant in the same species, so that their characters afford valuable specific diagnoses. Apparently it is only those lichens which germinate from spores that occur in a fertile condition.

2. The spermogones, which are the presumed male organs of reproduction, at once differ in appearance from the apothecia in being very minute corpuscles. In many cases their outline is invisible to the naked eye, unless the thallus has been previously moistened, when they appear as minute points or papillae. When magnified they externally bear a resemblance to the apothecia of the Pyrenocarpei, but internally, on microscopical examination, they are seen to differ essentially from these. In form they are nucleiform, round, or oblong, and are either sessile on the surface of the thallus, or more or less immersed in its substance, or sometimes enclosed in prominent thalline verrucae. Usually they are simple, though occasionally two or several become confluent or aggregated into little groups. They almost always occur on the same thallus as the apothecia, or rarely on different thalli (e.g., Pρhebe pubescens), so that lichens are consequently monoecious and dioecious. In colour the spermogones are black or brown, or concolorous with the thallus itself. They are composed of two parts, viz., a shell or conceptacle and a nucleus.

(1) The conceptacle, which is analogous to the hypothecium of the apothecium, is composed of a tissue formed of very small cellules, which are cemented together and have thick walls. The ostiole at its summit is generally similar to that of nucleiform apothecia, and in the case of entirely immersed spermogones is the only portion visible.

(2) The nucleus consists of the sterigmata and spermatia, and of a mucilaginous substance (the spermatic gelatin) in the cavity between them, which very greedily imbibes water and aids in the expulsion of the spermatia. Sometimes also there are present in the nucleus a few elongated, articulated, and occasionally branching filaments intermixed with the sterigmata which are comparable with the paraphyses of the apothecia {e.g., Ramalina). (a) The sterigmata are elongated cellules filled with a colourless fluid, and are attached to the interior face of the conceptacle growing from it con-vergently towards the centre, and often nearly filling its entire cavity. They present two important modifications, viz., simple sterigmata and articulated sterigmata or arthrosterigmata, both of which may become somewhat branched. When arrived at maturity they possess the faculty of producing from their apices or also from the articuli the spermatia, one at a time, though many in succession, as may be inferred from the immense number of them lying in the cavity of the nucleus. (5) The spermatia are very small, slender, colourless corpuscles, which at first appear as minute protrusions on the apices of the sterigmatic cells. They subsequently become gradually elongated, and, on reaching maturity, detach themselves from the sterigmata and lie free in the cavity of the nucleus, till in wet weather they are expelled through the ostiole of the spermogones. In form they present two principal modifications, viz., acicular and ellipsoid, of which the former, the more frequent, is rather variable. They may be slightly fusiformi-incrassate at one apex <,e.g., Usnea), or at both apices {e.g., Parmelia), or they may be cylindrical and straight, the most common shape, or cylindrical and arcuate {e.g., Roccella). The spermatia of arthrosterigmata are cylindrical and straight, but slightly thickened and obtuse at the apices. In size they also vary, though more in length than in thickness, the arcuate spermatia being sometimes very Tong (0Ό40 millimetre). These differences in form and size are often very useful in the discrimination of species, just as the two types of the sterigmata are sometimes of great service in the distinction of genera. The spermatia frequently exhibit a Brownian movement, but they possess no faculty of germination, their functions, as generally acknowledged, being the fertilization or fecundation of the spores. That the spermogones are rightly presumed to be the male organs of reproduction may legitimately be inferred alike from their relation to and their antagonism with the apothecia. In the former respect their position on the thallus relatively to the apothecia is similar to that of the sexual organs in other classes of plants; while in the latter respect spermogones are plentiful on thalli destitute of apothecia, and on those with apothecia are much more sparingly present. The manner, however, in which the spermatia impregnate the spores has not yet been definitely ascertained. Most probably, as suggested by Nylander (in Syn,, p. 40, note 1), the spermatia do not exercise any direct influence on the female organ in the hymenium, but their fecundating influence is rather exercised on the prothalline elements of the growing thallus. Μ. Stahl indeed has recorded and illustrated {Beitr. zur Entwick. der Flechten, 1877) the result of certain researches on Collema microphyllum, in which he supposed that he had detected sexual organs in the shape of an ascogonium and a trichogyne, regarded by him as a kind of carpo · gonvum. These observations, however, have not been confirmed by subsequent researches on the same or any other species of Collema, while his attributing to the hyphae a faculty of “contortion” or spirally coiling themselves, which from their nature they do not and cannot possess, is calculated to invalidate all that he otherwise observed and depicted.

3. The pycnides are minute, dark-coloured pyrenodean conceptacles which occur on the thalli of various lichens, especially such as are crustaceous. In external appearance they resemble to some extent the spermogones, from which, however, they at once differ in their internal organization. They consist of simple filaments composed of narrow (often short) cellules, termed basidia, bearing on their apices bodies called stylospores, which are colourless, usually oblong, but variable in form and size, and filled with organic matter (in part at least oily) similar to the spores. Each basidium produces only a single stylospore, which, unlike the spermatia, has a germinative property. Their occurrence in lichens was first pointed out by Tulasne, who showed their affinity to certain analogous fruits {Diplodia, PJιoma t Septοria i &c.) in various thecaspored fungi, and regarded them as supplementary or secondary sporiferous reproductive organs. Considering the number of parasitic fιmgilli which frequently occur on lichens, it might be supposed that the pycnides in reality belonged to the same category. From their constant occurrence, however, on the same species, and the evident correlation between them and the accompanying fructifications, as also from the resemblance of their stylospores to the spores of the apothecia, there are good grounds for adopting the conclusion come to by Tulasne.

They are very common on the margin of the thallus of isidiiferous states of Peltigera canina and Ρ. rufescens, where they have often been mistaken for spermogones, which in this genus have not yet been detected. Pycnides occur also in Lecidea rermifera, and abundantly in L. tantilla, in Habrothallus, in several species of Stmgula, in Spilonema revertens, and will probably be yet observed in other lichens.

The Gonidia of Lichens.

In view of the important place occupied by the gonidia in the structure of lichens, and of the discussions that have recently taken place concerning them, they require to be considered somewhat in detail in order that their real nature and relation to the hyphae, or the thalline filaments, may become apparent. The gonidia are spherical, ellipsoid, or variously rounded cellules, with thin, colourless walls composed of cellulose, containing chlorophyll (or a subsimilar colouring matter), homogeneous or granulose, with generally a solid nucleus in the centre. As to the origin of the chlorophyll, it may be observed in passing that this is the same in lichens as in other cryptogamie plants, e.g., mosses and Hepaticae, in which it occurs, the only visible difference being that gonidia often occur as discrete cellules. The gonidia increase by binary (very rarely by ternary or quarternary) division, the nucleus also dividing into two portions, each of which forms the centre of a secondary gonidial cellule. In the gonidial stratum, where they are arranged between the radicles of the hyphae, their division necessarily proceeds only slowly, but in ecorticated thalli, leprose and others, in which they are free, they are readily multiplied by repeated division. In gonidia isolated from the thallus of some species belonging to Cladonia, Evernia, and Physcia, zoospores have been detected by M. Famintzin and M. Boranetzky (see Ann. Sc. Nat., 1868, p. 137), and, although Nylander failed to perceive such in subsequent experiments (Flora, 1877, No. 23), he adds that it is possible they may be generated in free gonidia (i.e., in unstratified thalli), which could not be the case in gonidia closely surrounded by thalline filaments. The subject will well repay further investigation. Other matters relating to the character and relations of the gonidia will be best elucidated by considering the forms which they present, and their origin in the thallus.

1∙ The Forms of the Gonidia.— These have been fully treated by Nylander in the Flora, loc. dl,, where also the first scientific exposition and classification of them have been given. According to the views there propounded, gonidia in their wider acceptation include three very distinct types :—(1) Eugonidia (or gonidia proper), which are involved in a distinct cellular membrane, and are usually bright green; (2) Gonimia (or the gonidial granules already mentioned), which are naked, pale greenish, glaucous greenish or bluish; and (3) Gorndιmιa.(or Leptogonidia'), which are intermediate between the two preceding, smaller, and of an oblong form. Of these the distinction between eugonidia andgonimia is fundamental, and of “so great weight that lichens seem to present a twofold parallel series” • according to the presence of the one or the other in their texture These two different anatomical elements, as observed by Nylander, have a certain biological analogy with the blood globules in animals, •and similarly afford absolute characters. The principal forms presented by these three kinds of gonidia are the following. 1. Eugonidia consist of—(a) Ηaplogonidia, the most frequent, simple, of a protococcoid form, or sometimes glomerulose (as in granuloso-leprose thalli); (δ) Platygonidia, being depressed and variously membranosely connated gonidia {Syngonidia}, as in some foliicolous species {e.g., Platygramma phyllosema) ; (e) Chroolepogonidia (or Ohrysogonidia∖ containing chlorophyll and orange grains (endo-chrorne) in the same cellule, more or less similar to Chroolepa (as in Gyaleeta chlorobaea, Arthonia pruiosa, Platygraρha periclea, Verrucaria insiliens, and the genus Thelopsis) ; (d) Confervogonidia, somewhat resembling Confervae, and forming the chief element of the thallus of Coenogonium. 2. Gonidimia are smaller than gonidia proper, with the wall of the cellules less distinct. They occur in Peltidea, Solorina, and Nephroma expallidum. To these belong also hymenial gonidia, which are often very minute, and are present in the thalamium (destitute of paraphyses) of various Pyrenocaτpei {e.g., Verrucaria pallida, V. umbrina, and V. hymenogonia), τa,τe}γ of Arthoniae as in A. chroolepida. 3. Gonimia (including the gonimia of Cephalodia) consist of—(a) Haplogonimia, which are somewhat large (very large in Phylliseuni), and either simple, or two or several aggregated; (δ) Sirogonimia, which are scytonenιoid or sirosiphoid gonimia, distinguished by the gonimia being tunicated, and are characteristic of the family Ephebacei ; (c) Hormogonimia, the most common form, smaller, moniliformly arranged, and contained in syngonimia, especially characteristic of the family Collemacei, whence Collenia (or Nostoc) itself, according to Nylander, is to be considered but as a single syngonimium; (d) Speirogonimia, which are similar to the preceding, but are not moniliform, with the syngonimia sιιbglobose, smaller and more scattered, as in Omρha∙ laria and Synalissa. It will be perceived from the above that many of these forms are more or less similar to “gonidioid” algae, though, as we shall presently see, they are not identical with these.

II. The Origin of the Gonidia.— By pre-microscopic authors this was a subject necessarily ignored, and indeed it is only within the last thirty years that it has been investigated by lichenists. The earliest theory as to their origin was that propounded by Bayrhoffer {Einiges ub. d. Lichenen und deren Befruchtung, 1851), confirmed by observations of Speerschneider {Bot. Zeit., 1853, &c.), and supported by Schwendener {Untersuch, ub. d. Flechtenthallus, 1868). This was to the effect that the gonidia derived their origin from the hyphae {i.e., the thalline filaments), in the way succinctly detailed by Μ. Flies (in Scand., 1871, p. 7, where it is fully endorsed). “The hyphae,” he says, “are not only elongated into filaments, but also put forth short branchlets, the terminal cell of which is gradually dilated, becomes subglobose, and is at length filled with chlorophyll (or a subsimilar matter); in a few that (terminal cell) is changed into a gonidium, and then by varied division germinates other gonidia.” For several years this theory was accepted at second-hand by most authors who referred to the subject, though a different origin of the gonidia, presently to be noticed, was indicated by the celebrated Tulasne so early as 1852, in his “Mémoire sur les lichens” {Ann. Sc. Nat. ). The erroneous nature of this theory was well pointed out by Schwendener, who {Die Algentypen d. Flechtengonidien, 1869) very correctly affirmed that the actual development of a gonidium from the terminal cell of a hypha had not been observed, though, strange to say, he had previously himself observed this phenomenon. Not being able otherwise to account for the origin of the gonidia, and following up one of two alternatives put forward by De Bary {Morpholog. und Physiolog. der Pilze, Flechten, kc., 1865, p. 291), he promulgated the hypothesis now familiarly known as Schwendenerism. The conclusion to which De Bary came on noticing the resemblance between the gonidia of Collemaccae and certain algae was as follows :—“Either the lichens in question,” he says, “are the perfectly developed states of plants, whose imperfect forms have hitherto stood amongst the algae, as Nostocaceae and Chroococcaceae, or these latter are typical algae which assume the forms of Collema, Ephebe, &c., through certain parasitic Ascomycètes penetrating into them, spreading their mycelium into the continuously growing thallus, and frequently attached to their phycochrome-bearing cells.” Taking this latter suggestion as his starting point, and assuming the identity of certain algal types with the gonidia of lichens, and the identity of the mycelium of fungi with their hyphae, Schwendener extends the said alternative to various other groups of lichens than the Collemaccae, and comes to the conclusion that a lichen is composed of a parasitic fungus (the hyphae) and a number of low algae {Chlorophyllaccae and Phycochromaccae), the former of which produces the reproductive bodies and is nourished by the latter. This theory was subsequently expanded and illustrated at length by Bornet {Recherches sur les Gonidies des Lichens, 1873), who affirms, as the result of numerous investigations, “that the connexion of the hyphae with the gonidia is of such a nature as to exclude all possibility of the one organ being produced by the other,” and that the theory of parasitism can alone explain it satisfactorily. To give any detailed account of the investigations of these authors, and the arguments by which they endeavour to support the hypothesis, would exceed the limits of the present article, even were a∏ other matters relating to lichens to be excluded. Suffice it to state briefly that, according to Schwendenerisrn, a lichen is not an individual plant, but rather a community made up of two different kinds of individuals belonging to two distinct classes of cryptogams, viz., a master-fungus and colonies of algal slaves, which it has sought out, caught hold of, and retains in perpetual captivity in order to provide it with nourishment. To such a singular theory, which from its plausibility has met with considerable support in certain quarters, various a priori objections of great validity may be taken. Amongst others which have been adduced two may especially be noticed, having reference to the nature of this alleged parasitism, and the situations in which lichens are found. (1) The parasitism described is of a kind unknown in the vegetable kingdom, inasmuch as the host (the Algae), instead of suffering any injury, only flourishes the more vigorously. Moreover, the algal slaves being entirely enclosed in the masterfungus, can evidently supply no nourishment to it whatever, while direct observation shows that it is through the surface of the thallus of the lichen that nourishment is conveyed to the gonidial stratum, where the active life chiefly has its seat. (2) As is well known, lichens shun such habitats as are most frequented by algae and fungi, and occur in situations where neither of these are seen. Where then are the algal colonies which, according to the hypothesis, the fungus goes forth in quest of, and “presses into its service”? Either of these arguments is sufficient to throw more than doubt upon Schwendenerisrn. At the same time, as will be perceived, the origin of the gonidia in the thallus remains to be accounted for, in order that the hypothesis may be utterly subverted. It might at first sight be supposed that this was a matter of no very great difficulty, since by cultivating lichens from the spores it would readily become apparent. All attempts, however, in this direction (and many such have recently been made) have, owing to the peculiar character and conditions of lichen-growth, confessedly proved futile, and the experiments instituted have been productive of no definite results. This is not altogether to be regretted, since in nature itself, when lichens occur on certain substrata, we have in various instances the whole process of the evolution of their vegetative system placed before our eyes, from the first germinations of the spores to the formation of the perfect thallus. It is to N ylander, whose services here as elsewhere in lichenological science— structural, physiological, and systematic—are so valuable, that we owe the first clear enunciation as to the origin of the gonidia in the lichen - thallus.

This, though previously indicated by him, as we shall presently see, was at greater length and still more definitely stated in several important papers in the Flora, viz., “De gonidiis et eorum formis animadversiones” (1877, No. 23), “Circa lichenes vitricolas notula” (1879, No. 19), and “De hypothallo notula” (1879, No. 36). In these he incontrovertibly demonstrates that the gonidia originate, not from the filaments themselves, but in the cellules of the first cortical glomerules which are produced upon the young hypothallus. This may very easily be studied in the earlier stages of development of crustaceous lichens growing on pure quartz rocks (e.g., Lecidea geographica), on the smooth bark of young trees (e.g., Lecanora subfusca), and more especially on the surface of old glass (e.g., Lecanora galactina and Lecidea alboatra). In the case of the young thalli of Lecanora cinerea this was sufficiently well delineated by the celebrated Tulasne in 1852, the year subsequent to the promulgation of the Bayrhofferian theory, in his elaborate “Mémoire sur les Lichens,” pl. iii. f. 3. Afterwards, as the evolution of the cortical stratum advances, its lower portion is resorbed, and the gonidia there become free, giving rise to the gonidial stratum, to the hyphae in which they are not adnate, as has been represented, but only adherent by means of the gelatin which penetrates all the elements of the lichen. Often also growing gonidia, young and adult, may be observed in the pseudo-parenchymatous cortical cellules in lichens which in this respect are best adapted for examination, such as Umbilicaria (vide Nylander in Flora, 1875, p. 303), Physcia pulverulenta, Psoroma hypnomιm, &c. Similarly, with respect to the origin of thegonimia, Nylander observes (Flora, 1868, p. 353) that the isidia in the Collemacei (more especially in Collema) “show very clearly under the microscope the entire history of the evolution of the thallus from its first origin from a cellule containing a single gonimium to a minute true nostoc, and ultimately to the perfect texture of a Collema.” With these facts before us, to which various others might be added, such as the entire absence of any algals on pure substrata, where lichen growth presents itself in all its stages, we are compelled to come to the conclusion that the gonidia constitute a true organic system in lichens, and in nature are nowhere seen outside the thallus. Consequently those free algal forms, such as Protococcus, &c., regarded as the free gonidia of lichens, are true algae, there being indeed a parallelism between them but no identity. On the other hand, as to the alleged identity of the liehen-hypha with a fungus-mycelium, it is to be observed that the two are totally different in their nature. The hyphae of lichens (myelohyphae of Nylander) are rigid, elastic, containing lichenine, not becoming putrid by maceration, with no faculty of penetrating or involving, while the hyphae of fungi are caducous, soft, flexile, with thin walls, &c. Hence, as there is no algal in the lichen, so neither is there any fungus, though there is a parallelism between the fructification of lichens and the ascifer∙ ous section of fungi (Crombie in Pop. Sc. Rev., 1874).

Nutrition and Life of Lichens.

As already intimated, lichens derive their nourishment directly from the atmosphere, in the shape of rain (or dew) with the materials contained in it. Here, as elsewhere, water is the condition of life, and through its medium is conveyed to them the nutrient substances requisite for their existence and growth, from the clouds, from rivers, and, in the case of maritime species, even from the sea. Where, however, the atmosphere is impregnated with smoke, soot, or other deleterious ingredients, lichens will not grow nor flourish. Hence in our larger cities, or even in smaller manufacturing towns, scarcely any lichen vegetation, or none whatever, is seen. Even in their more immediate suburban districts they occur only in a gonidial or rudimentary state, constituting the pseudo-genus Lepraria of the older botanists, and increasing through long periods by bisection, but never developing into perfect plants. Indeed it is now a well-known fact that their fully developed condition is a sure indication of the purity of the air and the salubrity of the districts in which they occur. It has sometimes been stated that they draw some portion at least of their nutriment from the substratum to which they are affixed. For this, however, their structure is by no means well adapted, and such inorganic substances as iron and lime, which enter into their composition, are only as if mechanically derived in solution from the substratum. This in very many instances, e.g., bare quartzose rocks, dead sapless wood, and pure naked glass, can evidently supply no nutriment whatever. Moreover, in the case of crustaceous species, such as Lecanora tαrtarea, <fcc., and also of terricole fruticulose species, such as Cetraria islaηdica, &c., the portion of the thallus next to the substratum is dead, so that no nutrient substances can be conveyed through it to the upper layers of the thallus. A very simple, but at the same time convincing, illustration of this is adduced by Nylander. “By immersing,” he says, “any fruticulose thallus, such as Usnea, by the base in water, it remains entirely dry (with the exception of the part submersed), but if water be poured over the other portions, it quickly absorbs it, softens, and revives.” The same thing may be seen in nature itself, in the case of such species as Cladina rangiferina, Alectoria ochroleuca, Platysma nivale, <fcc., growing on temporarily wet substrata, when the base of their thalli is soft and moist, and all the rest dry and rigid. It cannot therefore be doubted that the nutritive elements contained in the rain or other water are conveyed to the lichen through the surface of the thallus. It is in the superficial parts also, as Nylander has well observed (in Plora, 1874, No. 4), that “the active life has its seat, chiefly around the gonidia, manifesting itself in the putting forth of young parts (lobes, laciniae, branches, isidia), and in the functions of the apothecia and the spermogones, so that the nourishing humours necessary for all the actions of life are especially and directly poured upon these.” The vital activities, however, in lichens thus nourished are, as might be expected from the nature of the source whence their nutriment is derived, very intermittent, and in dry weather cease to operate, and become entirely dormant. Hence their life, unlike that of all other plants, is twofold, viz., one active, in which when moistened all the vegetative and reproductive functions are at work, and the other passive, in which when dry these functions are completely in abeyance. For such a peculiar duplex existence, at one time vegetating, at another lethargic, their organization in all its parts, gonidial and otherwise, is admirably adapted. More especially is this the case with respect to the lichenine found in their textures, which, being readily dried and as readily moistened, enables them to resist with impunity the greatest extremes of temperature, alternate periods of drought and wet, the scorching heat of the sun, the vehemence of stormy winds, and the nipping frosts of winter.

In this fitful and abnormal life of lichens we have the explanation in a great measure of their almost indefinite duration of existence. It is well known that they are perennial plants in the widest sense of the term, and that, though in the earlier stages of their existence their growth is comparatively rapid, yet this becomes extremely slow when they arrive at a certain age. The time required for the development of even the most rapidly growing species may be calculated by the appearance of such of these as are met with on gravestones, mortar of houses, stone walls, wooden palings, and such like, the date of whose erection is known. Amongst other instances which have come under the present writer’s own observation may be adduced the case of Physcia parietina, growing in fair quantity on the stones of a granite wall built in 1836 in a maritime district where the plant is extremely abundant, and where atmospherical and other conditions are well suited for its growth. In a recent visit to the spot it was found that, although the thallus is now well developed, no fructification whatever is visible, though traces of spermogones are beginning to appear, so that in the space of forty-five years this plant has not yet attained full maturity. But slow as is the growth of lichens after a certain stage of their development, their tenacity of life is very remarkable, as might a priori be inferred from their capacity of enduring without injury the greatest extremes of temperature and of hygrometrical conditions. It is on record that, after the lapse of nearly half a century, the same specimen on the same spot of the same tree has been observed without any change in its condition. On this point also E. Fries (in L. E., p. xlv.) notices that certain species such as Physcia ciliaris, kept in houses for upwards of a year, revive when again exposed to the influences of the atmosphere,—an observation which in the case of Cladina rangiferina similarly kept for a still longer period the present writer can fully corroborate. Endowed then with this singular intermittent vitality, we can easily understand how many individuals which occur on hard mountainous rocks or on the trunks of aged trees in ancient forests are in all probability many hundreds of years old. Nor does age seem in any way to weaken their fecundity, even when the thallus has apparently ceased to grow. This, as observed by Nylander (in Syn., p. 5), is shown from the circumstance that were it otherwise “the already old fruits would be destitute of spores, which is never the case,” unless in plants of some lower tribes, e.g., Graρhidei and Verrucaiiae, in which the thallus is but sparingly gonidiose, and the life consequently is shorter. In other instances the central portion of the thallus sometimes normally perishes in old plants, as in Pterygium centrifugum, Collema melaenum, Parmelia centrifuga, and P. saxatilis, leaving only peripherical circles, in which, however, the life of the individual still continues for ages. In fact, “the life of lichens bears in itself no cause of death, and is only to be ended by external injuries ”(E. Fries, L. E., loc. citf), or by the alteration of climatic and atmospherical conditions. Hence the assumption is not unwarrantable that individuals of such confessedly long-lived species as Lecidea geographica, growing on rocks upon the summits of lofty mountains, date from more than “fabulous epochs,” and probably outrival in longevity the ages assigned to the oldest trees on the surface of the globe.

Chemistry of Lichens, and Chemical Reactions. Chemistry of Lichens.— This is still but little understood, notwithstanding that the subject has been more or less investigated by authors. Their examinations, however, have been too limited and desultory to enable us to give any detailed account of the different principles which enter into the composition of the lichen tissues. Moreover, with respect to those species which have been more particularly analysed, they have sometimes employed not only the same terms in different senses, but also different terms to denote the same substance. There can, however, be no doubt that the chemical composition of lichens not only produces great modifications in their form, but also considerable diversities in their properties.

The principal substance which occurs in lichens, especially in such as are foliaceous and fruticulose, is lichenine— a special kind of gelatin peculiar to them. It is intermediate in character between dextrin and starch, and very eagerly imbibes water, though if boiled in water it is dissolved and lost. Starch also very rarely occurs in large lenticular grains scattered in the tissues. In crustaceous lichens oxalate of lime predominates, and forms a large proportion of the thallus, e.g., 65 per cent, in Lecanora esculenta according to Goebel. Chlorophyll and its modification phycochrome are found in lichens only in relatively small quantities, as is also the case with most of the other principles they present. These are enumerated by Nylander (in Syn., p. 51) according to the affinities they bear, as follows:—(1) phosphate of lime, sea-salt, manganese, iron; (2) picrolichenine, variolarine, orceine, cetrarine, inuline, erythrine, roccelline, picroerythrine; (3) gyτophoric acid, parellic acid, usneic acid, orceic acid, erythrinic acid; (4) sugar not crystallizable, oil, waxy matter, resinous matter. Several others, especially phosphates and acids, have been enumerated by authors, which in some instances have not been corroborated, and in others are doubtfully distinct. In illustration of the manner in which several of these principles enter into the composition of lichens, we may adduce the analyses given in one or two species which have been more fully subjected to experiment. In Cetraria islandica there have been detected starch, including lichenine and inuline, to the extent of 80 per cent.; gummy and waxy matters; a bitter principle, cetraric acid ; a fatty principle, lichestearic acid; fumaric acid, which exists also in Fumaria officinalis; gallic acid, the astringent principle of galls; uncrystallizable sugar ; various salts, such as bitartrate of potash, and tartrate and phosphate of lime. Again, in Physcia parietina there have been found several alimentary principles, as gliadine, starch, sugar, gum ; several medicinal substances, as resin, bitter matter, gallic acid, and a peculiar ethereal oil ; a yellow colouring matter called chrysophanic acid ; also wax, stearine, and some salts, as carbonate of lime. The ash of lichens is said to constitute from 8 to 10 per cent, of their bulk, and consists of various earthy bases, such as potash, soda, lime, magnesia, alumina, silica, manganese, and peroxide of iron, in combination with various acids, such as carbonic, phosphoric, sulphuric, and hydrochloric. The whole subject, however, of the chemistry of lichens requires to be investigated in a more extensive and methodical manner than has hitherto been done either by chemists or lichenists.

Chemical Reactions in Lichens.— These have reference to the thallus and the apothecia, and in both respects afford valuable assistance in the systematic study of lichens, (a) Thalline reactions depend upon the presence in the thallus of certain colourable materials in the form of acids, and are manifested on the application of hydrate of potash and the hypochlorite of lime either on the tortical stratum or the medulla. Of these reagents, hydrate of potash (K), composed of equal weights of caustic potash and water, and hypochlorite of lime (CaCl), composed of chloride of lime and water of any strength, may give certain reactions or none, according to the presence or absence of particular acids in the thallus. If no reaction takes place, this is denoted by K -, CaCl -, and similarly if there is reaction by K +, CaCl +. A very convenient mode of symbolizing the positive and negative reactions of the cortical layer and the medulla of the same species is to place the reaction of the former above the latter, e.g., K±, CaClψ, denoting that with K the cortical stratum shows reaction and the medulla none, while with CaCl the cortical stratum gives none, but the medulla a distinct reaction. Again, there is often no reaction produced by K alone, but if CaCl be added to it while still moist, a decided reaction appears, for which K(CaCl) + is the symbol; or, on the other hand, the reaction given by K may be neutralized by the immediate application of CaCl, in which case it is expressed by K + (CaCl) -. The positive reactions are due to the presence of particular acids in the thallus, such as erythrinic acid giving a crimson reaction with CaCl, chrysophanic acid giving a purple reaction with K, glaucinic acid giving a yellow reaction with CaCl, and lecaιwric acid giving a citrine reaction with K. In most cases where the natural colour of the thallus (often also of the medulla) is yellow or orange, chrysophanic acid is present, and with K gives a purple reaction ∖e.g., Physcia lychnea), but in others externally similar in colour and general appearance, where only lecithophanic acid is present (p.g., Lecanora laciniosa), K gives no reaction whatever. Sometimes K produces at first a yellow colour which immediately changes into a red or purple, as in Lecanora cinerea, which at once enables us to distinguish it from the closely allied species L. gibbosa and L. calcarea. A solution of iodine (I) is also in certain cases useful as a test on the medulla, and in the discrimination of some species of Collenia. The value of these chemical tests in lichens was first pointed out by Nylander in Flora, 1866, pp. 223, 224, was subsequently illustrated by him in Flora, 1869, passim, in the case of species belonging to various genera, and has since been generally acknowledged by lichenists and employed by them in all specific diagnoses. They are unquestionably most important, not only in the discrimination of many difficult and closely allied species, but also in enabling us to refer with certainty varieties to the species to which they properly belong. Nay, even a small fragment of the thallus, whether sterile or fertile, may by their aid be readily determined, while otherwise it would be either quite indeterminable or doubtful. Generally they may be obtained on any portion of the thallus, but they are frequently more vivid, as might be expected, in the growing or circumferential portions. It is, however, only immediate reactions which depend on the colouring matter contained in the cortical stratum and the medulla that are to be attended to, and not secondary or tardy reactions which may otherwise originιte, e.g., from the dissolution of the chlorophyll of the gonidia. At the same time it is to be observed, in order to prevent a misconception which has occasionally been entertained, that they do not per se constitute a special specific character, but only an additional and confirmatory specific character. As such their value is clearly apparent in cases where the external characters are similar or approximate, and doubt necessarily exists as to their specific value, (δ) Apothecial reactions for the most part take place either externally on the epithecium or internally on the hymenial gelatin. The reactions of the epithecium are generally produced by K on species which have yellow or orange apothecia (e.g., Lecanora auranliaca, Leeidea leucoxantha), while others with concolorous apothecia (e.g., Lecanora epanora, Lecidea lucida) give no reaction whatever, owing to the presence or absence of chrysophanic acid. In certain cases also the reaction with CaCl is very useful in enabling us to separate closely allied species with similar apothecia, e.g., Lecanora subcarnea (epithecium CaCl-) from L. glaucoma (epithecium CaCl + yellow). Again, with respect to the reaction of the hymenial gelatin, this, as already observed, depends on the application of a solution of iodine. The formula by which the solution is to be prepared is—iodine, gr. j.; iodide of potash, gr. iij; distilled water, ⅜ ounce. In most lichens, where the spores are mature, this solution will tinge the hymenial gelatin in some cases only blue, in others at first blue and then wine-red, and in others wine-red or tawny wine-coloured only, without any preceding blue tinge. In such instances the reaction obtained is often very useful as a confirmatory specific character. Sometimes, however, the blue reaction takes place only on the thecae {e.g., Pertusaria, Cladonia, &c.), and at other times only on the epispore (e.g., Graphis, Thelotrema, kc. ). In the case of some species belonging to the inferior genera the reaction with I is especially valuable for the assistance it lends in distinguishing them from certain pyrenomycetous fungi, to which otherwise they might be supposed to belong. True, in some fungi (c.g., Peziza) we obtain a reaction with I, and in some lichens we have no reaction visible; but otherwise in such exceptional instances their respective anatomical characters readily show to which class they belong.

Economic Uses of Lichens.

These are intimately connected with their chemical constituents, and are in some respects very important. In the arts, as food and as medicine, many of them have been highly esteemed, though others are not now employed for the same purposes as formerly.

1. Lichens Used in the Arts.— Of these the most important are such as yield, by maceration in ammonia, the valuable dyes known iu commerce as archil, cudbear, and litmus. These, however, may with propriety be regarded as but different names for the same pigmentary substance, the variations in the character of which are attributable to the different modes in which the pigments are manufactured. Archil proper is derived from several species of Roccella (e.g., R. Moηtaguei, R. tiηctοriα), which yield a rich purple dye and fetch a high price in the market. Of considerable value is the “perelle” prepared from Lecanora parella, and much used in the preparation of a red or crimson dye. Inferior to this is “cudbear,” derived from Lecanora tartarea, which was formerly very extensively employed by the peasantry’of north Europe for giving a scarlet or purple colour to woollen cloths. By adding certain alkalies to the other ingredients used in the preparation of these pigments, the colour becomes indigoblue, in which case it is the litmus of the Dutch manufacturers. Amongst other lichens affording red, purple, or brown dyes may be mentioned Ramalina scopulorum, Parmelia saxatilis and P. omj)halodes, Umbilicaria pustulata and several species of Gyrophorα, Urceolaria scruposa, all of which are more or less employed as domestic dyes. Yellow dyes, again, are derived from Chlorea tηιlρina, Platysma jτιηiperiηum, Parmelia caperata and P. conspersa, Physcia flavicαns, Ph. parietina, and Ph. lychnea, though like the preceding they do not form articles of commerce, being merely used locally by the natives of the regions in which they occur most plentifully. In addition to these, many exotic lichens, belonging especially to Parmelia and Sticta (e.g., Parmelia tinctorum, Sticta αrgyracea), are rich in colorific matter, and, if obtained in sufficient quantity, would yield a dye in every way equal to archil. These pigments primarily depend upon special acids contained in the thalli of lichens, and their presence may readily be detected by means of the reagents already noticed. In the process of manufacture, however, they undergo various changes, of which the chemistry is still but little understood. At one time also some species were used in the arts for supplying a gum as a substitute for gum-arabic. These were chiefly Ramalina fraxinea, Evernia pi'unastri, and Parmelia physodes, all of which contain a considerable proportion of gummy matter (of a much inferior quality, however, to gum-arabic), and were employed in the process of calico-printing and in the making of parchment and cardboard. In the 17th century some filamentose and fruticulose lichens, viz., species of Usnea and Ramalina, also Everniafurfuracea and Cladina rangiferina, were used in the art of perfumery. From their supposed aptitude to imbibe and retain odours, their powder was the basis of various perfumes, such as the celebrated “Poudre de Cypre” of the hairdressers, but their employment in this respect has long since been abandoned.

2. Nutritive Lichens.— Of still greater importance is the capacity of many species for supplying food for man and beast. This results from their containing amylaceous substances, and in some cases a small quantity of saccharine matter of the nature of mannite. One of the most useful nutritious species is Cetraria islandica, “Iceland moss,” which, after being deprived of its bitterness by boiling in water, is reduced to a powder and made into cakes, or is boiled and eaten with milk by the poor Icelander, whose sole food it often constitutes. Similarly Cladina rangi-feriηa and Cl. sylvatica, the familiar “reindeer moss,” are frequently eaten by man in times of scarcity, after being powdered and mixed with flour. Their chief importance, however, is that in Lapland and other northern countries they supply the winter food of the reindeer and other animals, who scrape away the snow and eagerly feed upon them. Another nutritious lichen is the “Tripe de Roche” of the Arctic regions, consisting of several species of the Gyrophorei, which when boiled is often eaten by the Canadian hunters and Red Indians when pressed by hunger. But the most singular esculent lichen of all is the “manna lichen,” which in times of drought and famine has served as food for large numbers of men and cattle in the arid steppes of various countries stretching from Algiers to Tartary. This is derived chiefly from Lecanora esculenta, which grows unattached on the ground in layers from 3 to 6 inches thick over large tracts of country in the form of small irregular lumps of a greyish or white colour. Speaking of the distribution of these nutritive lichens, whose qualities depend on the presence of amylaceous matter, Dr Lindsay (in Pop. Hist. Brit. Lich., p. 82) very appropriately remarks that, “by a beautiful provision of nature, they occur precisely under the circumstances where they are most wanted—in northern or arctic countries, or. on arid steppes, where grain stuffs are unknown, and food of a better kind is often scarce or deficient.” In connexion with their use as food we may observe that of recent years in Scandinavia and Russia an alcoholic spirit has been distilled from Cladina raηgiferiηa and extensively consumed, especially in seasons when potatoes were scarce and dear. Formerly also Sticta pulmonaria was much employed in brewing instead of hops, and it is said that a Siberian monastery was much celebrated for its beer which was flavoured with the bitter principle of this species.

3. Medicinal Lichens.— During the Middle Ages, and even in some quarters to a much later period, lichens were extensively used in medicine in various European countries. Many species had a great repute as demulcents, febrifuges, astringents, tonics, purgatives, and anthelmintics. The chief of those employed for one or other, and in some cases for several, of these purposes were Cladonia pyxidata r Usnea barbata, Ramalina farinacea, Evernia prunastri, Cetraria islandica, Sticta pulmonaria, Parmelia saxatilis, Physcia parietina, and Pertusaria amara. Others again were believed to be endowed with specific virtues, e.g., Peltigera canina, which formed the basis of the celebrated “pulvis antilyssus” of Dr Mead, long regarded as a sovereign cure for hydrophobia ; Platysma juniperinum, lauded as a specific in jaundice, no doubt on the similia similibus principle from a resemblance between its yellow colour and that of the jaundiced skin; Peltidea aphthosa, which on the same principle was regarded by the Swedes, when boiled in milk, as an effectual remedy for the aphthae or rash on their children. Almost all of these virtues, general or specific, were imaginary; and at the present day, except perhaps in some remoter districts of northern Europe, only one of them is employed as a remedial agent. This is the “Iceland moss” of the druggists’ shops, which is undoubtedly an excellent demulcent in various dyspeptic and chest complaints. Probably also Pertusaria amara, from the intensely bitter principle which it contains, might still with propriety be employed as a febrifuge. No lichen is known to be possessed of any poisonous properties, although Chlorea νulpina is believed by the Swedes to be destructive to wolves when powdered and “mixed with pounded glass.” Nor are lichens, as has sometimes been alleged, injurious to the trees upon which they grow, except to a very limited extent. Not being parasites properly so called, the only injury they can inflict upon them is by slightly interfering with the functions of respiration, or, when growing very crowdedly upon the branches of orchard trees, by checking the development of buds.

Classification of Lichens.

From the time of Acharius, the father of lichenological science, different authors have proposed different classifications of lichens, according to the degree of importance attached by them to one or other of their vegetative and reproductive organs. Most of these classifications, however, whether proposed by microscopical or pre-microscopical lichenists, have been too artificial and arbitrary, and indeed less natural in various ways than that originally propounded by Acharius. Of recent years they have been entirely superseded by other two systems, viz., that of the Massalongo-Koerberian and that of the Nylanderian school. With respect to the former of these, its characteristic feature is the prominence which it assigns to the form and structure of the spores not only in the differentiation of species but also in the foundation of genera. Though it has been adopted, with various modifications, by many Continental lichenists, yet essentially it also proceeds on an artificial principle, and necessitates the adoption of far too many genera, distinguished from each other merely by slight differences in the spores. The other system—that of Nylander, which was first proposed by him in his Essai d ’ une Nouvelle Classification des Lichens (1854-55),—has since then commended itself more and more to the acceptation of lichenists, so that even the disciples of the opposite school (the sporologists) have in many respects gradually approximated towards it in their most recent writings. Not only is it the only complete system of classification yet wrought out; it is also the most natural and philosophical of any hitherto propounded. In its main outline it proceeds upon the principle of showing the near relation of certain lichens to some genera of algae on the one hand, and of certain other lichens to some genera of fungi on the other hand, and connects these three great classes of cryptogams together by a sort of twofold chain, commencing with those genera of lichens nearest allied to the algae, working up to those genera best developed (Stictei), and thence retrograding and terminating with those nearest allied to the fungi. His genera also are principally founded, not upon a single special character, but upon the combined anatomical characters presented by the thallus, the apothecia, and the spermogones. It may here be further observed that we are indebted to the same accomplished lichenist for the succinct but comprehensive diagnoses, generic and specific, of the different parts of a lichen, which have tended so much to facilitate their systematic study. The following is a conspectus of the Nylanderian classification of lichens, with the leading characters of the different families and tribes, and an enumeration of all the principal genera of which these are composed.

Family I.— Eρhebαcci, Nÿl.

Thallus but little turgid when moist, gonidial stratum consisting of gonimia which are tunicated; medullary filaments none.

Tribe 1. Sirοsiphei, Nyl.—Thallus filamentoso-fruticulose, gonimia variously connate. Apothecia biatorine or lecideine. Spermogones with sterigmata or arthrosterigmata.

Genera: Sirosiphon, Ktz.; Gonionema, Nyl.; Spilonema, Born.

Tribe 2. Pyrenoρsei, Nyl.—Thallus thinly granulose, rubricose within, gonimia simple or connate. Apothecia lecanorine or pyrenocarpous. Spermogones with simplish sterigmata.

Genera: Buopsis, Nyl.; Pyrenopsis, Nyl.

Tribe 3. Homopsidd, Nyl.—Thallus either fruticolosö with the gonimia seriated, or squamuliform or granulose with the gonimia subsolitary. Apothecia pyrenocarpous with or without paraphyses. Spermogones with simple sterigmata.

Genera: Ephebe, Fr.; Eρhebeia, Nyl.; Phylliscum, Nyl.; Phylliscodium, Nyl.; Collemopsidium, Nyl.

Tribe 4. Magmopsei, Nyl.—Thallus pyrenopsidian, containing syngonimia, the gonimia arranged without order. Apothecia ρeridieine.

Genus: Magmopsis, Nyl.

Family II.— Collemacei, Nyl.

Thallus turgid when moist, gonidial stratum consisting of gonimia moniliformly arranged; medulla not discrete.

Tribe 1. Lichind, Nyl.—Thallus fruticulose or radiately laciniated, gonimia elongato-seriately moniliform, subconnate. Apothecia lecanorine or lecideine. Spermogones with sterigmata or arthrosterigmata.

Genera: Lichina, Ag.; Pterygium, Nyl.; Leptogidium, Nyl.; Lichinodiurn, Nyl.

Tribe 2. Gollemei, Nyl.—Thallus membranaceous, lobate, rarely fruticulose, granulose or subsquamulose, gonimia moniliformly arranged, cortical stratum none or distinct. Apothecia lecanorine or rarely pyrenocaq>ous. Spermogones with simplish sterigmata or arthrosterigmata.

Genera: Leciophysma, Fr. fil.; Synalissa, DR.; Ornphalaria, DR.; Ancma, Nyl.; Paulia, Fée; Schizorna, Nyl.; Collema, Ach.; Leptogium, Ach.; Ramalodium, Nyl.; Lichinella, Nyl.; Am· phidium, Nyl.; Collemopsis, Nyl.; Hydrothyria, Russ.

Tribe 3. Pyrenidiei, Nyl.—Thallus fibrillose, gonimia moniliformly coherent, cortical stratum distinct. Apothecia pyrenoid.

Genus: Pyrenidium, Nyl.

Family III.— Lichenacei, Nyl.

Thallus not gelatinous, with a gonidial, rarely gonimic stratum; medullary stratum more or less distinct.

Series I.— Epiconiodd, Nyl.

Apothecia with the spores usually naked and pulverulent on the surface of the fructification.

Tribe 1. Calidd, Nyl.—Thallus horizontally expanded, sometimes none. Apothecia stiρitate, capitulifoιm or sessile. Spermogones with simple sterigmata.

Genera: Sphinctrina, Fr.; Calidu,m, Pers; Stenocybc, Nyl.; Coniocybe, Ach.; Pyrgidium, Nyl; Trachylia, Fr.; Pyrgillus, Nyl.

Tribe 2. Tylophorci, Nyl.—Thallus thinly crustaceous. Apothecia at first pyrenodean and then lecanorine with sporal mass. Spermogones with somewhat branched sterigmata.

Genus: Tylophoron, Nyl.

Tribe 3. Sfhaerophorei, Nyl.—Thallus fruticulose, branched. Apothecia at first nucleiform, becoming variously dehiscent, with sporal mass. Spermogones with simplish sterigmata or arthrosterigmata.

Genera: Sphaerophoron, Pers.; Acroscyphus, Lev.; Tholwrna, Norm.

Series II.— Cladodd, Nyl.

Apothecia terminal on podetia, rarely sessile, biatorine or rarely lecanorine.

Tribe 4. Baernnycetei, NyL—Thallus horizontally expanded. Apothecia substipitate. Spermogones with sterigmata or arthrosterigmata.

Genera: Gomphillus, Nyl.; Baeomyces, Pers.; Glossodium, Nyl. j Thysanotheciτιm, Berk.; Stercocauliscum, Nyl.

Tribe 5. Piloplwrei, Nyl.—Thallus granulose, cephalodiiferous, with rigid podetia. Apothecia cenhalodine, on the podetia, with the paraphyses prolongated into the hypothecium. Spermogones with simplish sterigmata.

Genus: Pilophoron, Tuck.

Tribe 6. Stereocauld, Nyl.—Thallus caespitose, podetiiform, solid. Apothecia terminal or lateral, lecideine or rarely lecanorine. Spermogones with simple sterigmata.

Genera: Stereocaulon, Schreb.; Stercocladium, Nyl.; Argopsis, Fr. fil.; Oxocladium, Mnt.

Tribe 7. Cladonid, Nyl.—Thallus foliaceous or fruticulose, with fistulose podetia. Apothecia biatorine on the podetia, rarely sessile on the leaflets. Spermogones with simplish sterigmata.

Genera: Meter odea, Nyl.; Pycnothelia, Ach.; Cladonia, Hffm.; Cladina, Nyl.; Cladia, Nyl.; Ramalea, Nyl.

Series III.— Ramalodci, Nyl.

Thallus efoliolose, fruticulose, or filamentose. Apothecia generally lecanorine.

Tribe 8. Roccellei, Nyl.—Thallus simplish or branched, internally with filamentose medulla. Apothecia irregular (normally lecanorine), adnate, terminal, or lateral. Spermogones with simplish sterigmata.

, Genera: Combca, DN.; Roccella, DC.

Tribe 9. Siphulei, Nyl.—Thallus podetiiform, simple or fruticulose, internally with filamentose or fistulose medulla. Apothecia unknown. Spermogones (where seen) with arthrosterigmata.

Genera: Siphula, Fr.; Endocena, Cromb.; Thamnolia, Ach.

Tribe 10. Ramalinei, Nyl.—Thallus fraticuloso-foliaceous, rounded or compressed, with woolly medulla. Apothecia lecanorine, scutellate. Spermogones with arthrosterigmata.

Genus: Ramalina, Ach.

Tribe 11. Usned, Nyl.—Thallus much branched, rounded, or compressed, with firm medullary axis. Apothecia parmelioid, peltate. Spermogones with simplish sterigmata.

Genera: Usnea, Hffm.; Neuropogon, N. and Fl.; Chlorea, Nyl.

Tribe 12. Alectoriei, Nyl.—Thallus branched, rounded, or compressed, with woolly medulla. Apothecia parmelioid, scutelliform. Spermogones with simplish sterigmata or arthrosterigmata.

Genera: Alectoria, Ach.; Bactylina, Nyl.; Buf aurea, Ach.

Tribe 13. Cetrariei, Nyl.—Thallus fruticulose or foliaceous, with woolly medulla. Apothecia parmelioid, marginal, obliquely affixed. Spermogones with simple sterigmata or arthrosterigmata.

Genera: Cetraria, Ach.; Platysma, Hffm.

Series IV.— Phyllodd, Nyl.

Thallus foliaceous, usually depressed, lobate. Apothecia generally peltiform or lecanorine.

Tribe 14. Parmelid, Nyl.—Thallus frondosely dilated, or lobate, or laciniated, with woolly, rarely solid, medulla. Apothecia parmelioid, scutelliform. Spermogones with simple sterigmata or arthrosterigmata.

Genera: Evcrnia, Ach.; Everniopsis, Nyl.; Parmclia, Ach..; Parmeliopsis, Nyl.

Tribe 15. Stictci, Nyl.—Thallus large, lobate, cyρhellate, or ecyphellate beneath; gonidial stratum composed either of nodulose gonimia or of true gonidia. Apothecia lecanorine, rarely parmeleine. Spermogones with arthrosterigmata.

Genera: Stictina, Nyl.; Lobarina, Nyl.; Sticta, Ach.; Lobaria, Nyl.; Ricasolia, DN.

Tribe 16. Peltigerci, Nyl —Thallus frondosely dilated, the cortical stratum often wanting beneath; gonidial stratum consisting either of gonidia or (usually) of gonimia, rarely of gonidimia. Apothecia peltiform, adnate, or innate. Spermogones (where seen) with arthrosterigmata.

Genera: Nephroma, Ach.; Nephromium, Nyl.; Peltidea, Ach.; Pcltigera, Hffm.; Solorina, Ach.

Tribe 17. Physciei, Nyl. Thallus stellato-orbicular, rarely fruticulose, internally with woolly medulla; gonidial stratum consisting of true gonidia. Apothecia lecanorine. Spermogones with arthrosterigmata.

Genus: Physcia, Nyl.

Tribe 18. Pyxind, Nyl.—Thallus stellato-laciniated, with woolly medulla and true gonidial stratum. Apothecia lecideine. Spermogones with arthrosterigmata.

Genus: Pyxine, Fr.

Tribe 19. Gyrophorei, Nyl.—Thallus umbilicately affixed, with woolly medulla and true gonidial stratum. Apothecia lecanoroid, or lecideine and gyrose. Spermogones with arthrosterigmata.

Genera: Umbilicaria, Hflm.; Gyrophora, Ach.

Series V.— Placodd, Nyl.

Thallus variously cnιstaceous, sometimes evanescent, rarely hypophloeodal, without any filamentose medullary stratum. Apothecia lecanorine, or lecideine, or lirellaeform.

Tribe 20. Lccanorei, Nyl.—Thallus crustaceous, rarely evanescent or obsolete; gonidial stratum consisting of gonidia, rarely of gonimia. Apothecia lecanorine, rarely subbiatorine or typically biatorine. Spermogones with simple sterigmata or arthrosterigmata.

Genera: Pannaria, Del.; Gymnoderma, Nyl.; Erioderma, Fée4 Hcppia, Naeg.; Amphilama, Fr.; Psoroma, Fr.; Lccanora, Ach.; Glypholecia, Nyl.; Peltula, Nyl.; Dermatiscum, Nyl.; Birina, Fr.

Tribe 21. Pertusariei, Nyl.—Thallus crιιstaceous, continuous; gonidial system consisting of true gonidia. Apothecia endocarpoid or lecanoroid. Spermogones with simple sterigmata.

Genera: Pertusaria, DC.; Varicellaria, Nyl.

Tribe 22. Thelotrcmei, Nyl.—Thallus crustaceous, or pulverulent, or areolate, with true gonidial stratum. Apothecia urceolato-impressed with double margin. Spermogones with simple or somewhat branched sterigmata.

Genera: Phlyctis, Wallr.; Tremotyliurn, Nyl.; Thelotrema, Ach.; Urceolaria, Ach.; Bclonia, Krb.; Gyrostomum, Nyl.·; Ascidium, Fée; Gymnotrcma, Nyl.

Tribe 23. Leddeci, Nyl.—Thallus variously crustaceous, pulverulent, evanescent or none proper, with the gonidial stratum consisting of gonidia (rarely chrysogonidia), rarely gonimia. Apothecia lecideine (or biatorine). Spermogones with simple or simplish sterigmata.

Genera: Coenogonüim, Ehrh.; Byssocaulon, Mnt.; Pannularia, Nyl.; Coccocarpi, Pers.; Lecidca, Ach.; Gyrothedum, Nyl.; Epiphora, Nyl.

Tribe 24. Graphiclei, Nyl.—Thallus thinly crustaceous, or hypophloeodal, or rarely none proper; gonidial stratum consisting of gonidia (rarely chrysogonidia). Apothecia lirelline or rotundate. Spermogones with simple sterigmata.

Genera: Xylographa, Fr.; Agyrium, Fr.; Inthographa, Nyl.; Graphis, Ach.; Thelographis, Nyl.; Helminthocarρon, Fée; Leuco-grapha, Nyl.; Opegrapha, Ach.; Platygrapha, Nyh; Stigmati-dium, Mey.; Arthonia, Ach.; Melaspilea, Nyl.; Lecanactis, Eschw.; Schizographa, Nyl.; Glyphis, Ach.; Chiodedon, Ach.

Series VI.— Pyrenodci, Nyl.

Thallus peltate, or crustaceous, or hypophloeodal, or evanescent. Apothecia nucleiform, with an apical ostiole.

Tribe 25. Pyrenocarpei, Nyl.—Thallus various, often macular or obsolete; gonidial system consistingof gonidia(rarely chrysogonidia), sometimes of gonimia. Apothecia pyrenodeine, often without paraphyses. Spermogones with simple sterigmata or arthrosterigmata.

Genera: Cora, Fr.; Dichonema, N. ab Es.; Normandina, Nyl.; Endocarpon, Hedw.; Verrucarina, Nyh; Verrucaria, Pers.; Thelenella, Nyl.; Thelopsis, Nyl.; Obryzum, Wallr.; Strigula, Fr.; Parathelium, Nyl.; Melanothcca, Fée; Trype∙ thelium, Ach.; Astrothelium, Eschw.

Series VII. — Peridiodei, Nyl.

Thallus thin, often wanting. Apothecia peridieine, without any ostiole.

Tribe 26. Peridei, Nyl.—Thallus thin, macular, or none proper. Apothecia consisting of a peridium. Spermogones (where seen) with simple sterigmata.

Genera: Thelocarpon, Nyl.; Thclococcus, Nyl.; Endococcus, Nyl.; Mycoporum, Flot.

Family IV.— Myriangiacei, NyL

Thallus unstratified, entirely and equally cellulose. Fructification not discrete.

Tribe 1. Myriangiei, Nyl.—Thallus noduloso-pulvinate. Apothecia sublecanorine. Spermogones unknown.

Genus: Myriangium, Mnt. and Berk.

In the Nylanderian arrangement, most of these tribes and genera are again divided into subtribes and subgenera, the latter being further subdivided into sections according to the affinities of the different species.

Habitats and Distribution of Lichens.

These two subjects are intimately related and present many interesting features which here we can only very generally notice without entering into details.

1. Habitats of Lichens.— These are extremely varied, and comprehend a great number of very different substrata. Chiefly, however, they are the bark of trees, rocks, the ground, mosses, and, rarely, perennial leaves. (a) With respect to corticole lichens, some prefer the rugged bark of old trees {e.g., Ramalina, Parmelia, Stictei) and others the smooth bark of young trees and shrubs (e.g., Graphidei and some Lecideae). Many are found principally in large forests (e.y., Esnea, Λlectoria jubata) ; while a few occur more especially on trees by roadsides {e.g., Physcia parietina and Ph. pulverulenta). In connexion with corticole lichens may be mentioned those lignicole species which grow on decayed or decaying wood of trees and on old pales {e.g., Caliciei, various Lecideae, Xylographa). (b) As to saxicole lichens, which occur on rocks and stones, they may be divided into two sections, viz., calcicole and calci-fugous. To the former belong such as are found on calcareous and cretaceous rocks, and the mortar of walls (e.y., Lecanora calcarea, Lecidea calcivora, and several Verrucariae), while all other saxicole lichens may be regarded as belonging to the latter, whatever may be the mineralogical character of the substratum. It is here worthy of notice that the apothecia of several calcicole lichens {e.g., Lecanora Prevostii i Lecidea calcivora) have the power (through the carbonic acid received from the atmosphere) of forming minute faveoli in the rock, in which they are partially buried, (c) With respect to tcrricole species, some prefer peaty soil {e.g., Cladonia, Lecidea decolorans), others calcareous soil (e.g., Lecanora crassa, Lecidea decipiens), others argillaceous soil or hardened mud {e.g., Collema limosum, Peltidea venosa) ; while many may be found growing on all kinds of soil, from the sands of the sea-shore to the granitic detritus of lofty mountains, with the exception of course of cultivated ground, there being no agrarian lichens, {d) Muscicole lichens again are such as are most frequently met with on decayed mosses and jungermannias, whether on the ground, trees, or rocks {e-g., Leptogium muscicola, Gomphillus calicioides). {e) The epiphyllous species are very peculiar as occurring upon perennial leaves of certain trees and shrubs, whose vitality is not at all affected by their presence as it is by that of fungi. In so far, however, as is known, they are very limited in number (e.y., Lecidea Douteillei, Strigula). With the exceptions of these last, it is to be observed that all the rest may, under different conditions of locality and climate, be found growing for the most part indiscriminately on the substrata mentioned, a normally saxicole species becoming corticole, a terricole one becoming muscicole, and vice versa. Amongst other instances of this that might be adduced, the case of Lecidea geographica, a peculiarly saxicole species, growing on the stems of Rhododendron in the Tyrolese Alps, and that of Lecidea mvulosa, a like peculiarly saxicole species, growing on the bark of trees in Germany, are especially striking. Sometimes also various lichens occur abnormally in such unexpected habitats as dried dung of sheep, bleached bones of reindeer and whales, old leather, iron, and glass, in districts where the species are abundant. Consequently it is apparent that in many cases lichens are quite indifferent to the substrata on which they occur, whence we infer that the preference of several for certain substrata depends upon the temperature of the locality or that of the special habitat. Thus in the case of saxicole lichens the mineralogical character of the rock has of itself little or no influence upon lichen growth, which is influenced more especially and directly by their physical properties, such as their aptitude for imbibing and retaining heat and moisture. As a rule lichens have a propensity for open exposed habitats, though some are found only or chiefly in shady situations; while, as already observed, scarcely any occur where the atmosphere is impregnated with smoke. Many species also prefer growing in moist places by streams, lakes, and the sea, though very few are normally, and probably none entirely, aquatic, being always at certain seasons exposed for a longer or shorter period to the atmosphere {e.g., Lichina, Leptogium rivulare, Endocarpon fluviatile, Verrucama maura). Some species are entirely parasitical on other lichens (e.g., various Lecideae, and Pyrenocarpei), and may be peculiar to one {e.g., Lecidea vitellinaria) or common to several species (e.y., Habrothallus ρarmeliarum). A few, generally known as erratic species, have been met with growing unattached to any substratum {e.g., Parmelia revoluta, var. concentrica, Lecanora esculenta) ; but it seems somewhat doubtful if these are really free ab initio {vide Crombie in Jourη. Bot., 1872, p. 306). It is to the different characters of the stations they occupy with respect to exposure, moisture, &c., that the variability observed in many types of lichens is to be attributed. The fact also that in numerous instances they are so indifferent to the nature of their habitats clearly shows that they do not at all depend upon the substratum for their nourishment.

2. Distribution of Lichens.— From what has now been said it will readily be inferred that the distribution of lichens over the surface of the globe is regulated, not only by the presence of suitable substrata, but also and more especially by atmospherical and climatal conditions. At the same time it may safely be affirmed that their geographical range is more extended than that of any other class of plants, occurring as they do in the coldest and warmest regions—on the dreary shores of the palaeocrystic sea and in the torrid valleys of tropical climes, as well as on the greatest mountain elevations yet attained by man, on projecting rocks even far above the snow-line {e.g., Lecidea geographica), where, as in many other situations, fungusmycelium and gonidioid algal are unknown. Our knowledge of the distribution of lichens in various countries, like that of other plants, is derived from general and local floras, from special works on lichenology, from the observations of scientific travellers and the collections made by them in distant parts of the world. Most of these, however, of an earlier date, are to be used with great caution, as the species recorded were, in the absence of microscopical examination, not sufficiently discriminated, and in many cases are prima facie entirely erroneous. Amongst botanical travellers whose contributions are most trustworthy may be mentioned pre-eminently Sir Joseph D. Hooker, whose remarks on this subject in his Flora Antarctica and in various papers in Trans. Linn. Sοc., voL xiv., <fcc., are most valuable and suggestive. Again, in purely lichenological works, the general principles regulating lichen distribution (with statistical tables) are admirably expounded by Nylander in the introduction to his Synopsis Methodica Lichenum, while the only complete record of the distribution of species is that given by the same author in his Enumeration generale des Lichens. Since the date of this latter work (1858) our knowledge of the range of lichens, both European and exotic, has been greatly extended, apart from the discovery of numerous species previously unknown. No special treatise, however, on the subject has been published, though there is now ample material, very much scattered no doubt, available for the purpose. It may, however, be observed that of the four thousand species (exclusive of numerous varieties and forms) which have been described, the geographical distribution is known almost as well as that of phanerogams and filices, as well, if not more so than that of mosses and hepatics, and far better than that of algae and fungi. In arctic regions lichens form by far the largest portion of the vegetation, occurring everywhere on the ground and on rocks, and fruiting freely; while terricole species of Cladοnia and Stereocaulon are seen in the greatest luxuriance and abundance spreading over extensive tracts almost to the entire 4 exclusion of other vegetation. The lichen flora of temperate regions again is essentially distinguished from the preceding by the frequency of corticole species belonging to Lecanora, Lecidea, and Graphidei. In intertropical regions lichens attain their maximum development (and beauty) in the foliaceous Stictei and Parmeliei, while they are especially characterized by epiphyllous species, as Strigula, and by many peculiar corticole Thelotremei, Graphidei, and Pyreηocarpei. Some lichens, especially saxicole ones, seem to be cosmopolitan (e.g., Lecanora subfusca, Cladοnia pyxidata) ; and others, not strictly cosmopolitan, have been observed in regions widely apart. A considerable number of species, European and exotic, seem to be endemic, but further research will no doubt show that most of them occur in other climatal regions similar to those in which they have hitherto alone been detected. To give any detailed account, however, of the distribution of the different genera (not to speak of that of individual species) of lichens would necessarily far exceed the limits at our disposal. Suffice it to say that both in horizontal and vertical range, they sufficiently correspond with the distribution of phanerogams in the several regions of vegetation into which the surface of the globe has been divided. The proportion of lichens to phanerogams in different regions increases in a regular ratio from the equator to the poles, and from the base to the summit of lofty mountains, till at length in more arctic and alt-alpine tracts lichens constitute almost and sometimes entirely the sole vegetation. (j. μ. c.)