Lichens

Lichens, in botany, compound or dual organisms each consisting of an association of a higher fungus, with a usually unicellular, sometimes filamentous, alga. The fungal part of the organism nearly always consists of a number of the Discomycetes or Pyrenomycetes, while the algal portion is a member of the Schizophyceae (Cyanophyceae or Blue-green Algae) or of the Green Algae; only in a very few cases is the fungus a member of the Basidiomycetes. The special fungi which take part in the association are, with rare exceptions, not found growing separately, while the algal forms are constantly found free. The reproductive organs of the lichen are of a typically fungal character, i.e. are apothecia or perithecia (see Fungi) and spermogonia. The algal cells are never known to form spores while part of the lichen-thallus, but they may do so when separated from it and growing free. The fungus thus clearly takes the upper hand in the association.

Owing to their peculiar dual nature, lichens are able to live in situations where neither the alga nor fungus could exist alone. The enclosed alga is protected by the threads (hyphae) of the fungus, and supplied with water and salts and, possibly, organic nitrogenous substances; in its turn the alga by means of its green or blue-green colouring matter and the sun’s energy manufactures carbohydrates which are used in part by the fungus. An association of two organisms to their mutual advantage is known as symbiosis, and the lichen in botanical language is described as a symbiotic union of an alga and a fungus. This form of relationship is now known in other groups of plants (see Bacteriology and Fungi), but it was first discovered in the lichens. The lichens are characterized by their excessively slow growth and their great length of life.

Until comparatively recent times the lichens were considered as a group of simple organisms on a level with algae and fungi. The green (or blue-green) cells were termed gonidia by Wallroth, who looked upon them as asexual reproductive cells, but when it was later realized that they were not reproductive elements they were considered as mere outgrowths of the hyphae of the thallus which had developed chlorophyll. In 1865 De Bary suggested the possibility that such lichens as Collema, Ephebe, &c., arose as a result of the attack of parasitic Ascomycetes upon the algae, Nostoc, Chroococcus, &c. In 1867 the observations of Famintzin and Baranetzky showed that the gonidia, in certain cases, were able to live outside the lichen-thallus, and in the case of Physcia, Evernia and Cladonia were able to form zoospores. Baranetzky therefore concluded that a certain number, if not all of the so-called algae were nothing more than free living lichen-gonidia. In 1869 Schwendener put forward the really illuminating view—exactly opposite to that of Baranetzky—that the gonidia in all cases were algae which had been attacked by parasitic fungi. Although Schwendener supported this view of the “dual” nature of lichens by very strong evidence and identified the more common lichen-gonidia with known free-living algae, yet the theory was received with a storm of opposition by nearly all lichenologists. These workers were unable to consider with equanimity the loss of the autonomy of their group and its reduction to the level of a special division of the fungi. The observations of Schwendener, however, received ample support from Bornet’s (1873) examination of 60 genera. He investigated the exact relation of fungus and alga and showed that the same alga is able to combine with a number of different fungi to form lichens; thus Chroolepus umbrinus is found as the gonidia of 13 different lichen genera.

The view of the dual nature of lichens had hitherto been based on analysis; the final proof of this view was now supplied by the actual synthesis of a lichen from fungal and algal constituents. Rees in 1871 produced the sterile thallus of a Collema from its constituents; later Stahl did the same for three species. Later Bonnier (1886) succeeded in producing fertile thalli by sowing lichen spores and the appropriate algae upon sterile glass plates or portions of bark, and growing them in sterilized air (fig. 1). Möller also in 1887 succeeded in growing small lichen-thalli without their algal constituent (gonidia) on nutritive solutions; in the case of Calicium pycnidia were actually produced under these conditions.

The thallus or body of the lichen is of very different form in different genera. In the simplest filamentous lichens (e.g. Ephebe pubescens) the form of thallus is the form of the filamentous alga which is merely surrounded by the fungal hyphae (fig. 2). The next simplest forms are gelatinous lichens (e.g. Collemaceae); in these the algae are Chroococcaceae and Nostocaceae, and the fungus makes its way into the gelatinous membranes of the algal cells and ramifies there (fig. 3). We can distinguish this class of forms as lichens with a homoiomerous thallus, i.e. one in which the alga and fungus are equally distributed. The majority of the lichens, however, possess a stratified thallus in which the gonidia are found as a definite layer or layers embedded in a pseudo-parenchymatous mass of fungal hyphae, i.e. they are heteromerous (figs. 8 and 9). Obviously these two conditions may merge into one another, and the distinction is not of classificatory value.  
  
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After Bonnier, from v. Tavel.  From Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer.  
Fig. 1.—Xanthoria parietina. By the fusion of the hyphae in the middle of the mycelium a pseudo-parenchymatous cortical layer has begun to form.  
  
1, Germinating ascospore (sp) with branching germ-tube applied to the Cystococcus cells (a).

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2, Thallus in process of formation.

sp, Two ascospores.

p, Cystococcus cells.  
  
In external form the heteromerous thallus presents the following modifications. (a) The foliaceous (leaf-like) thallus, which may be either peltate, i.e. rounded and entire, as in Umbilicaria, &c., or variously lobed and laciniated, as in Sticta, Parmelia, Cetraria (fig. 4), &c. This is the highest type of its development, and is sometimes very considerably expanded. (b) The fruticose thallus may be either erect, becoming pendulous, as in Usnea (fig. 5), Ramalina, &c., or prostrate, as in Alectoria jubata, var. chalybeiformis. It is usually divided into branches and branchlets, bearing some resemblance to a miniature shrub. An erect cylindrical thallus terminated by the fruit is termed a podetium, as in Cladonia (fig. 7). (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 Lecanorae and Lecideae). (d) The hypophloeodal thallus is often concealed beneath the bark of trees (as in some Verrucariae and Arthoniae), or enters into the fibres of wood (as in Xylographa and Agyrium), being indicated externally only by a very thin film (figs. 3, 4, 5, 6, 7 and 8). 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. In the moist state of the thallus these colours are much less apparent, as the textures then become more or less translucent, and the thallus usually prevents the greenish colour of the gonidia (e.g. Parmelia Borreri, Peltidea aphthosa, Umbilicaria pustulata and pulverulent Lecideae).

The thallus may be free upon the surface of the substratum (e.g. Collema) or may be fixed more or less closely to it by special hyphae or rhizoids. These may penetrate but slightly into the substratum, but the connexion established may be so close that it is impossible to remove the thallus from the substratum without injury (e.g. Physcia, Placodium). In some cases the rhizoids are united together into larger strands, the rhizines.

The typical heteromerous thallus shows on section a peripheral, thin and therefore transparent, layer, the cortical layer, and centrally a mass of denser tissue the so-called medullary layer, between these two layers is the algal zone or gonidial layer (figs. 8 and 9).

The term epithallus is sometimes applied to the superficial dense portion of the cortical layer and the term hypothallus to the layer, when specially modified, in immediate contact with the substratum; the hypothallus is usually dark or blackish. The cylindrical branches of the fruticose forms are usually radially symmetrical, but the flattened branches of these forms and also the thalli of the foliaceous form show a difference in the cortex of the upper and lower side. The cortical layer is usually more developed on the side towards the light, while in many lichens this is the only side provided with a cortical layer. The podetia of some species of Cladonia possess no cortical layer at all. The surface of the thallus often exhibits outgrowths in the form of warts, hairs, &c. The medullary layer, which usually forms the main part of the thallus, is distinguished from the cortical layer by its looser consistence and the presence in it of numerous, large, air-containing spaces.

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After Sachs, from De Bary’s Vergleichende Morphologie und Biologie der Pilze, Mycetozoen und Bacterien, by permission of Wilhelm Engelmann. |  Fig. 3.—Section of Homoiomerous Thallus of Collema conglomeratum, with Nostoc threads scattered among the hyphae.  
Fig. 2.—Ephebe pubescens, Fr. A branched filiform thallus of Stigonema with the hyphae of the fungus growing through its gelatinous membranes. Extremity of a branch of the thallus with a young lateral branch a; h, hyphae; g, cells of the alga; gs, the apex of the thallus. |    
  
Gonidia.—It has been made clear above that the gonidia are nothing more than algal cells, which have been ensnared by fungal hyphae and made to develop in captivity (fig. 1). Funfstuck gives ten free living algae which have been identified as the gonidia of lichens. Pleurococcus (Cystococcus) humicola in the majority of lichens, e.g. Usnea, Cladonia, Physcia, Parmelia, Calicium, many species of Lecidea, &c., Trentepohlia (Chroolepus) umbrina in many species of Verrucaria, Graphidieae and Lecidea; Palmella botryoides in Epigloea; Pleurococcus vulgaris in Acarospora, Dermatocarpon, Catillaria; Dactylococcus infusionum in Solorina, Nephromia; Nostoc lichenoides in most of the Collemaceae; Rivularia rutida in Omphalaria; Lichina, &c., Polycoccus punctiformis in Peltigera, Pannaria and Stictina; Gloeocapsa polydermatica in Baeomyces and Omphalaria; Sirosiphon pulvinatus in Ephebe pubescens. The majority of lichens are confined to one particular kind of gonidium (i.e. species of alga) but a few forms are known (Lecanora granatina, Solorina crocea) which make use of more than one kind in their development. In the case of Solorina, for example, the principal alga is a green alga, one of the Palmellaceae, but Nostoc (a blue-green alga) is also found playing a subsidiary part as gonidia. In L. granatina the primary alga is Pleurococcus, the secondary, Gleococapsa.

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From Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer. | From Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer.  
Fig. 4.—Cetraria islandica. (Nat. size.) ap, Apothecium. | Fig. 5.—Usnea barbata. (Nat. size.) ap, Apothecium.  
  
Cephalodia.—In about 100 species of lichens peculiar growths are developed in the interior of the thallus which cause a slight projection of the upper or lower surface. These structures are known as cephalodia and they usually occupy a definite position in the thallus. They are distinguished by possessing as gonidia algae foreign to the ordinary part of the thallus. The foreign algae are always members of the Cyanophyceae and on the same individual and even in the same cephalodium more than one type of gonidium may be found. The function of these peculiar structures is unknown. Zukal has suggested that they may play the part of water-absorbing organs.

The exact relation of gonidia and hyphae has been investigated especially by Bornet and also by Hedlund, and very considerable differences have been shown to exist in different genera. In Physma, Arnoldia, Phylliscum and other genera the gonidia are killed sooner or later by special hyphal branches, haustoria, which pierce the membrane of the algal cell, penetrate the protoplasm and absorb the contents (fig. 11, C). In other cases, e.g. Synalissa, Micarea, the haustoria pierce the membrane, but do not penetrate the protoplasm (fig. 11, D). In many other cases, especially those algae possessing Pleurococcus as their gonidia, there are no penetrating hyphae, but merely special short hyphal branches which are in close contact with the membrane of the algal cell (fig. 3).

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From Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer. | From Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer.  
Fig. 6.—Cladonia rangiferina. (Nat. size.) | Fig. 7.—Cladonia coccifera. Podetia bearing apothecia. (Nat. size.)  
  
A, Sterile.

B, With ascus-fruit at the ends of the branches.

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t, Scales of primary thallus.  
  
Reproduction.

There are three methods of reproduction of the lichen: by fragmentation, by soredia, by the formation of fungal spores. In the first process, portions of thallus containing gonidia may be accidentally separated and so may start new plants. The second method is only a special process of fragmentation. The soredia are found in a large number of lichens, and consist of a single gonidium or groups of gonidia, surrounded by a sheath and hyphae. They arise usually in the gonidial layer of the thallus by division of the gonidia and the development around them of the hyphal investment; their increase in number leads to the rupture of the enclosing cortical layer and the soredia escape from the thallus as a powdery mass (fig. 12). Since they are provided with both fungal and algal elements, they are able to develop directly, under suitable conditions, into a new thallus. The soredia are the most successful method of reproduction in lichens, for not only are some forms nearly always without spore-formation and in others the spores largely abortive, but in all cases the spore represents only the fungal component of the thallus, and its success in the development of a new lichen-thallus depends on the chance meeting, at the time of germination, with the appropriate algal component.

Conidia.—Contrary to the behaviour of the non-lichen forming Ascomycetes the lichen-fungi show very few cases of ordinary conidial formation. Bornet describes free conidia in Arnoldia minitula, and Placodium decipiens and Conidia-formation has been described by Neubner in the Caliciae.

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After Sachs, from De Bary’s Vergleichende Morphologie und Biologie der Pilze, Mycetozoen und Bacterien, by permission of Wilhelm Engelmann. | From Beiträge zur Wissenschaftlichen Botanik.  
Fig. 8.—Usnea barbata. (Mag. nearly 100 times.) | Fig. 9.—Section of Heteromerous Lichen Thallus.  
  
A, Optical longitudinal section of the extremity of a thin branch of the thallus which has become transparent in solution of potash.

B, Transverse section through a stronger branch with the point of origin of an adventitious branch (sa).

r, Cortical layer.

m, Medullary layer.

x, Stout axile strand.

g, The algal zone (Cystococcus).

s, Apex of the branch.

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a, Upper cortical layer.

d, Lower cortical layer.

c, Medullary layer.

b, Gonidial layer.  
  
Spermatia.—In the majority of genera of lichens small flask-shaped structures are found embedded in the thallus (fig. 13). These were investigated by Tulasne in 1853, who gave them the name spermogonia The lower, ventral portion of the spermogonium is lined by delicate hyphae, the sterigmata, which give origin to minute colourless cells, the spermatia. The sterigmata are either simple (fig. 13, C) or septate—the so-called arthrosterigmata (fig. 13, B). The spermogonia open by a small pore at the apex, towards which the sterigmata converge and through which the spermatia escape (fig. 13). There are two views as to the nature of the spermatia. In one view they are mere asexual conidia, and the term pycnoconidia is accordingly applied since they are borne in structures like the non-sexual pycnidia of other fungi. In the other view the spermatia are the male sexual cells and thus are rightly named; it should, however, be pointed out that this was not the view of Tulasne, though we owe to him the designation which carries with it the sexual significance. The question is one very difficult to settle owing to the fact that the majority of spermatia appear to be functionless. In favour of the conidial view is the fact that in the case of Collema and a few other forms the spermatia have been made to germinate in artificial cultures, and in the case of Calicium parietinum Möfler succeeded in producing a spermogonia bearing thallus from a spermatium. For the germination of the spermatia in nature there is only the observation of Hedlund, that in Catillaria denigrata and C. prasena a thallus may be derived from the spermatia under natural conditions. In relation to the view that the spermatia are sexual cells, or at least were primitively so, it must be pointed out that although the actual fusion of the spermatial nucleus with a female nucleus has not been observed, yet in a few cases the spermatia have been seen to fuse with a projecting portion (trichogyne) of the ascogonium, as in Collema and Physcia, and there is very strong circumstantial evidence that fertilization takes place (see later in section on development of ascocarp). The resemblance of the spermatia and spermogonia to those of Uredineae should be pointed out, where also there is considerable evidence for their original sexual nature, though they appear in that group to be functionless in all cases. The observations of Möller, &c., on the germination cannot be assumed to negative the sexual hypothesis for the sexual cells of Ulothrix and Ectocarpus, for example are able to develop with or without fusion. The most satisfactory view in the present state of our knowledge seems to be that the spermatia are male cells which, while retaining their fertilizing action in a few cases are now mainly functionless. The female sexual organs, the ascogonia, would thus in the majority of cases develop by the aid of some reduced sexual process or the ascocarps be developed without relation to sexual organs. A further argument in support of this view is that it is in complete agreement with what we know of the sexuality of the ordinary, free-living ascomycetes, where we find both normal and reduced forms (see Fungi).  
  
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After Bornet, from De Bary’s Vergleichende Morphologie und Biologie der Pilze, Myceiozoen und Bacterien, by permission of Wilhelm Engelmann.  
Fig. 11.—Lichen-forming Algae. (A, C, D, E mag. 950, B 650 times.) The alga is in all cases indicated by the letter g, the assailing hyphae by h.  
  
A, Pleurococcus, Ag. (Cystococcus, Näg.) attacked by the germ-tube from a spore of Physica parietina.

B, Scytonema from the thallus of Stereocaulon famulosum.

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C, Nostoc from the thallus of Physma chalazanum.

D, Gloeocapsa from the thallus of Synalissa Symphorea.

E, Pleurococcus Sp. (Cystococcus) from the thallus of Cladonia furcata.  
  
Fruit Bodies.—We find two chief types of fruit bodies in the lichens, the perithecium and apothecium; the first when the fungal element is a member of the Pyrenomycetes division of the Ascomycetes, the second when the fungus belongs to the Discomycetes division. In the two genera of lichens—the Basidiolichens—in which the fungus is a member of the Basidiomycetes, we have the fructification characteristic of that class of fungi: these are dealt with separately. The perithecium is very constant in form and since the gonidia take no part in the formation of this organ or that of the apothecium it has the general structure characteristic of that division of fungi. The apothecia, though of the normal fungal type and usually disk-shaped, are somewhat more variable, and since the variations are of value in classification some more details may be added.  
  
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After Schwendener, from De Bary’s Vergleichende Morphologie und Biologie der Pilze Mycetozoen und Bacterien, by permission of Wilhelm Engelmann.  
Fig. 12.—Usnea barbata. (Mag. more than 500 times.)  
  
c, An isolated mature soredium, with an algal cell (Pleurococcus) in the envelope or hyphae.

d, Another with several algal cells in optical longitudinal section.

e, f, Two soredia in the act of germinating; the hyphal envelope has grown out below into rhizoid branches, and above shows already the structure of the apex of the thallus (see fig 9).  
  
They present various shapes, of which the following are the principal: (a) peltate, which are large, rounded, without any distinct thalline margin1 (e.g. Usnea, Peltigera); (b) lecanorine, 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 one¹ (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. myrmecina), or complicated and gyrose (e.g. Gyrophora), 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 often very variable even in the same species. In colour the apothecia are extremely variable, and it is but rarely that they are the same colour as the thallus (e.g. Usnea, Ramalina). Usually they are of a different colour, 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.  
  
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After Tulasne, from De Bary’s Vergleichende Morphologie und Biologie der Pilze, Mycetozoen und Bacterien, by permission of Wilhelm Engelmann.  
Fig. 13.—A, B, Gyrophora cylindrica. (A mag. 90, B 390 times, C highly magnified.)  
  
A, A vertical median section through a spermogonium imbedded in the thallus.

o, Upper rind.

u, Under rind.

m, Medullary layer of the thallus.

B, Portion of a very thin section from the base of the spermogonium.

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w, Its wall from which proceed sterigmata with rod-like spermatia (s).

m, Medullary hyphae of the thallus.

C, Cladonia novae Angliae, Delise; sterigmata with spermatia from the spermogonium.  
  
The two principal parts of which an apothecium consists are the hypothecium and the hymenium, or thecium. The hypothecium is the basal part of the apothecium on which the hymenium is borne; the latter consists of asci (thecae) with ascospores, and paraphyses. The paraphyses (which may be absent entirely in the Pyrenolichens) are erect, colourless filaments which are usually dilated and coloured at the apex; the apices are usually cemented together into a definite layer, the epithecium (fig. 14). The spores themselves may be unicellular without a septum or multicellular with one or more septa. Sometimes the two cavities are restricted to the two ends of the spore, the polari-bilocular type and the two loculi may be united by a narrow channel (fig. 15). At other times the spores are divided by both transverse and longitudinal septa producing the muriform (murali-divided) spore so called from the resemblance of the individual chambers to the stones in a wall. The very large single spores of Pertusaria have been shown to contain numerous nuclei and when they germinate develop a large number of germ tubes.  
  
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After Darbishire, from Berichte der deutschen botanischen Gesellschaft, by permission of Borntraeger & Co.  
Fig. 14.—Diagram showing Apothecium in Section and surrounding Portion of Thallus, and special terms used to designate these parts.  
  
Development of the Ascocarps.—As the remarks on the nature of the spermatia show, the question of the sexuality of the lichens has been hotly disputed in common with that of the rest of the Ascomycetes. As indicated above, the weight of evidence seems to favour what has been put forward in the case of the non-lichen-forming fungi (see Fungi), that in some cases the ascogonia develop as a result of a previous fertilization by spermatia, in other cases the ascogonia develop without such a union, while in still other cases the reduction goes still farther and the ascogenous hyphae instead of developing from the ascogonia are derived directly from the vegetative hyphae.  
  
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Fig. 15.—Vertical Section of Apothecium of Xanthoria parietina.  
  
a, Paraphyses.

b, Asci (thecae) with bilocular spores.

c, Hypothecium.  
  
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After E. Baur, from Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer.  
Fig. 16.—Collema crispum.  
  
A, Carpogonium, c, with its trichogyne t.

B, Apex of the trichogyne with the spermatium, s, attached.  
  
The first exact knowledge as to the origin of the ascocarp was the work of Stahl on Collema in 1877. He showed that the archicarp consisted of two parts, a lower coiled portion, the ascogonium, and an upper portion, the trichogyne, which projected from the thallus. Only when a spermatium was found attached to the trichogyne did the further development of the ascogonium take place. From these observations he drew the natural conclusion that the spermatium was a male, sexual cell. This view was hotly contested by many workers and it was sought to explain the trichogyne—without much success—as a respiratory organ, or as a boring organ which made a way for the developing apothecium. It was not till 1898, however, that Stahl’s work received confirmation and addition at the hands of Baur (fig. 16). The latter showed that in Collema crispum there are two kinds of thalli, one with numerous apothecia, the other quite sterile or bearing only a few. The sterile thalli possessed no spermogonia, but were found to show sometimes as many as 1000 archicarps with trichogynes; yet none or very few came to maturity. The fertile thalli were shown to bear either spermogonia or to be in immediate connexion with spermogonia-bearing thalli. Furthermore Baur showed that after the fusion of the spermatium with the trichogyne the transverse walls of that organ became perforated. There was thus very strong circumstantial evidence in favour of fertilization, although the male nucleus was not traced. The further work of Baur, and that of Darbishire, Funfstuck and Lindau, have shown that in a number of other cases trichogynes are present. Thus ascogonia with trichogynes have been observed in Endocarpon, Collema, Pertusaria, Lecanora, Gyrophora, Parmelia, Ramalina, Physcia, Anaptychia and Cladonia. In Nephroma, Peltigera, Peltidea and Solorina a cogonia without trichogynes have been observed. In Collema and a form like Xanthoria parietina it is probable that actual fertilization takes place, and possibly also in some of the other forms. It is probable, however, that in the majority of cases the ascogonia develop without normal fertilization, as is necessarily the case where the ascogonia have no trichogynes or the spermatia are absent. In these cases we should expect to find some reduced process of fertilization similar to that of Humaria granulata among the ordinary Ascomycetes, where in the absence of the antheridia the female nuclei fuse in pairs. In other lichens we should expect to find the ascogenous hyphae arising directly from the vegetative hyphae as in Humaria rutilans among the ordinary fungi, where the process is associated with the fusion of vegetative nuclei. It is possible that Solorina saccata belongs to this class. Cytological details of nuclear behaviour among the lichens are, however, difficult to obtain owing to the slow growth of these forms and the often refractory nature of the material in the matter of preparation for microscopical examination.

Ejection of Spores.—The spores are ejected from the apothecia and perithecia as in the fungi by forcible ejaculation from the asci. In the majority of forms it is clear that the soredia rather than the ascospore must play the more important part in lichen distribution as the development of the ordinary spores is dependent on their finding the proper alga on the substratum on which they happen to fall. In a number of forms (Endocarpon pusillum, Stigmaatonima cataleptum, various species of Staurothele), however, there is a special arrangement by which the spores are, on ejection, associated with gonidia. In these forms gonidia are found in connexion with the young fruit; such algal cells undergo numerous divisions becoming very small in size and penetrating into the hymenium among the asci and paraphyses. When the spores are thrown out some of these hymenial gonidia, as they are called, are carried with them. When the spores germinate the germ-tubes surround the algal cells, which now increase in size and become the normal gonidia of the thallus.

Basidiolichens.  
  
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From Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer.  
Fig. 17.—Cora pavonia. A, Viewed from above; B, From below; hym, hymenium. (Nat. size.)  
  
As is clear from the above, nearly all the lichens are produced by the association of an ascomycetous fungus with algae. For some obscure reason the Basidiomycetes do not readily form lichens, so that only a few forms are known in which the fungal element is a member of this family. The two best-known genera are Cora and Dictyonema; Corella, whose hymenium is unknown, is also placed here by Wainio. The so-called Gasterolichens, Trichocoma and Emericella, have been shown to be merely ascomycetous fungi. Clavaria mucida, however, has apparently some claims to be considered as a Basidiolichen, since the base of the fruit body and the thallus from which it arises, according to Coker, always shows a mixture of hyphae and algae.

The best-known species is Cora pavonia, which is found in tropical regions growing on the bare earth and on trees; the gonidia belong to the genus Chroococcus while the fungus belongs, apparently, to the Thelephoreae (see Fungi). This lichen seems unique in the fact that the fungal element is also found growing and fruiting entirely devoid of algae, while in the ascolichens the fungus portion seems to have become so specialized to its symbiotic mode of life that it is never found growing independently.

The genus Dictyonema has gonidia belonging to the blue-green alga, Scytonema. When the fungus predominates in the thallus it has a bracket-like mode of growth and is found projecting from the branches of trees with the hymenium on the under side. When the alga is predominant it forms felted patches on the bark of trees, the Laudatea form. It is said that the fungus of Cora pavonia and of Dictyonema is identical, the difference being in the nature of the alga.

Mode of Life.

Lichens are found growing in various situations such as bare earth, the bark of trees, dead wood, the surface of stones and rocks, where they have little competition to fear from ordinary plants. As is well known, the lichens are often found in the most exposed and arid situations; in the extreme polar regions these plants are practically the only vegetable forms of life. They owe their capacity to live under the most inhospitable conditions to the dual nature of the organism, and to their capacity to withstand extremes of heat, cold and drought without destruction. On a bare rocky surface a fungus would die from want of organic substance and an alga from drought and want of mineral substances. The lichen, however, is able to grow as the alga supplies organic food material and the fungus has developed a battery of acids (see below) which enable it actually to dissolve the most resistant rocks. It is owing to the power of disintegrating by both mechanical and chemical means the rocks on which they are growing that lichens play such an important part in soil-production. The resistance of lichens is extraordinary; they may be cooled to very low temperatures and heated to high temperatures without being killed. They may be dried so thoroughly that they can easily be reduced to powder yet their vitality is not destroyed but only suspended; on being supplied with water they absorb it rapidly by their general surface and renew their activity. The life of many lichens thus consists of alternating periods of activity when moisture is plentiful, and completely suspended animation under conditions of dryness. Though so little sensitive to drought and extremes of temperature lichens appear to be very easily affected by the presence in the air of noxious substances such as are found in large cities or manufacturing towns. In such districts lichen vegetation is entirely or almost entirely absent. The growth of lichens is extremely slow and many of them take years before they arrive at a spore-bearing stage. Xanthoria parietina has been known to grow for forty-five years before bearing apothecia. This slowness of growth is associated with great length of life and it is probable that individuals found growing on hard mountain rocks or on the trunks of aged trees are many hundreds of years old. It is possible that specimens of such long-lived species as Lecidea geographica actually outrival in longevity the oldest trees.

Relation of Fungus and Alga.

The relation of the two constituents of the lichen have been briefly stated in the beginning of this article. The relation of the fungus to the alga, though it may be described in general terms as one of symbiosis, partakes also somewhat of the nature of parasitism. The algal cells are usually controlled in their growth by the hyphae and are prevented from forming zoospores, and in some cases, as already described, the algal cells are killed sooner or later by the fungus. The fungus seems, on the other hand, to stimulate the algal cells to special development, for those in the lichen are larger than those in the free state, but this is not necessarily adverse to the idea of parasitism, for it is well known that an increase in the size of the cells of the host is often the result of the attacks of parasitic fungi. It must be borne in mind that the exact nutritive relations of the two constituents of the lichen have not been completely elucidated, and that it is very difficult to draw the line between symbiosis and parasitism. The lichen algae are not alone in their specialization to the symbiotic (or parasitic) mode of life, for, as stated earlier, the fungus appear in the majority of cases to have completely lost the power of independent development since with very rare exceptions they are not found alone. They also differ very markedly from free living fungi in their chemical reactions.

Chemistry of Lichens.

The chemistry of lichens is very complex, not yet fully investigated and can only be very briefly dealt with here. The wall of the hyphae of the fungus give in the young state the ordinary reactions of cellulose but older material shows somewhat different reactions, similar to those of the so-called fungus-cellulose. In many lichen-fungi the wall shows various chemical modifications. In numerous lichens, e.g. Cetraria islandica, the wall contains Lichenin (C6H10O5), a gummy substance which swells in cold water and dissolves in hot. Besides this substance, a very similar one, Isolichenin, is also found which is distinguished from lichenin by the fact that it dissolves in cold water and turns blue under the reaction of Iodine. Calcium oxalate is a very common substance, especially in crustaceous lichens; fatty oil in the form of drops or as an infiltration in the membrane is also common; it sometimes occurs in special cells and in extreme cases may represent 90% of the dry substance as in Verrucaria calciseda, Biatora immersa.

Colouring Matters.—Many lichens, as is well known, exhibit a vivid colouring which is usually due to the incrustation of the hyphae with crystalline excretory products. These excretory products have usually an acid nature and hence are generally known as lichen-acids. A large number of these acids, which are mostly benzene derivatives, have been isolated and more or less closely investigated. They are characterized by their insolubility or very slight solubility in water; as examples may be mentioned erythrinic acid in Roccella and Lecanora; evernic acid in species of Evernia, Ramalina and Cladonia; lecanoric acid in Lecanora, Gyrophora. The so-called chrysophanic acid found in Xanthoria (Physcia) parietina is not an acid but a quinone and is better termed physcion.

Colour Reactions of Lichens.—The classification of lichens is unique in the fact that chemical colour reactions are used by many lichenologists in the discrimination of species, and these reactions are included in the specific diagnoses. The substances used as tests in these reactions are caustic potash and calcium hypochlorite; the former being the substance dissolved in an equal weight of water and the latter a saturated extract of bleaching powder in water. These substances are represented by lichenologists by the signs K and CaCl respectively, and the presence or absence of the colour reactions are represented thus, K+, CaCl+, or K−, CaCl−. If the cortical layer should exhibit positive reaction and the medulla of the same species a negative reaction with both reagents, the result is represented thus, K±CaCl±. If a reaction is only produced after the consecutive addition of the two reagents, this is symbolized by K(CaCl)+. A solution of iodine is also used as a test owing to the blue or wine-red colour which the thallus, hymenium or spores may give with this reagent. The objection to the case of these colour reactions is due to the indefinite nature of the reaction and the doubt as to the constant presence of a definite chemical compound in a given species. A yellow colour with caustic potash solution is produced not only by atranoric acid but also by evernic acid, thamnolic acid, &c. Again in the case of Xanthoria parietina vulpinic acid is only to be found in young thalli growing on sandstone; in older forms or in those growing on another substratum it is not to be detected. A similar relation between oil formation and the nature of the substratum has been observed in many lichens. Considerations such as these should make one very wary in placing reliance on these colour reactions for the purposes of classification.

Economic Uses of Lichens.

In the arts, as food and as medicine, many lichens 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 dyes known in 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. Montaguei, R. tinctoria), which yield a rich purple dye; it once fetched a high price in the market. Of considerable value is the “perelle” prepared from Lecanora parella, and 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 indigo-blue, 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. amphalodes, Umbilicaria pustulata and several species of Gyrophora, Urceolaria scruposa, all of which are more or less employed as domestic dyes. Yellow dyes, again, are derived from Chlorea vulpina, Platysma juniperinum, Parmelia caperata and P. conspersa, Physcia flavicans, 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 argyracea), are rich in colouring 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 prunastri 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 Evernia furfuracea and Cladonia 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 starchy 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 Cladonia rangiferina 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 in. thick over large tracts of country in the form of small irregular lumps of a greyish or white colour. 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 Cladonia rangiferina 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, Usnea barbata, Ramalina farinacea, Evernia prunastri, Cetraria ìslandica, Sticla pulmonaria, Parmelia saxatilis, Xanthoria 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. No lichen is known to be possessed of any poisonous properties to man, although Chlorea vulpina is believed by the Swedes to be so. Zukal has considered that the lichen acids protect the lichen from the attacks of animals; the experiments of Zopf, however, have cast doubt on this; certainly lichens containing very bitter acids are eaten by mites though some of the acids appear to be poisonous to frogs.

Classification.

The dual nature of the lichen thallus introduces at the outset a classificatory difficulty. Theoretically the lichens may be classified on the basis of their algal constituent, on the basis of their fungal constituent, or they may be classified as if they were homogeneous organisms. The first of these systems is impracticable owing to the absence of algal reproductive organs and the similarity of the algal cells (gonidia) in a large number of different forms. The second system is the most obvious one, since the fungus is the dominant partner and produces reproductive organs. The third system was that of Nylander and his followers, who did not accept the Schwenderian doctrine of duality. In actual practice the difference between the second and third methods is not very great since the fungus is the producer of the reproductive organs and generally the main constituent. Most systems agree in deriving the major divisions from the characters of the reproductive organs (perithecia, apothecia, or basidiospore bearing fructification), while the characters of the algal cells and those of the thallus generally are used for the minor divisions. The difference between the various systems lies in the relative importance given to the reproductive characters on the one hand and the vegetative characters on the other. In the system (1854-1855) of Nylander the greater weight is given to the latter, while in more modern systems the former characters receive the more attention.

A brief outline of a system of classification, mainly that of Zahlbruckner as given in Engler and Prantl’s Pflanzenfamilien, is outlined below.

There are two main divisions of lichens, Ascolichenes and Basidiolichenes, according to the nature of the fungal element, whether an ascomycete or basidiomycete. The Ascolichenes are again divided into Pyrenocarpeae or Pyrenolichenes and Gymnocarpeae or Discolichenes; the first having an ascocarp of the nature of a perithecium, the second bearing their ascospores in an open apothecium.

Pyrenolichenes

Series I. Perithecium simple not divided.

a. With Pleurococcus or Palmella gonidia. Moriolaceae, Verrucariaceae, Pyrenothamnaceae.

b. With Chroolepus gonidia. Pyrenulaceae, Paratheliaceae.

c. With Phyllactidium or Cephaleurus gonidia. Strigulaceae.

d. With Nostoc or Scytonema gonidia. Pyrenidiaceae.

Series II. Perithecia divided or imperfectly divided by cross-walls. Mycoporaceae with Palmella or Chroolepus gonidia.

Discolichenes

Series I. Coniocarpineae. The paraphyses branch and form a network (capillitium) over the asci, the capillitium and ejected spores forming a long persistent powdery mass (mazaedium).

Caliciaceae, Cypheliaceae, Sphaerophoraceae.

Series II. Graphidineae. Apothecia seldom round, usually elongated-ellipsoidal, no capillitium.

Arthoniaceae, Graphidiaceae, Roccellaceae.

Series III. Cyclocarpineae, Apothecium usually circular, no capillitium.

A. Spores usually two-celled, either with a strongly thickened cross-wall often perforated by a narrow canal or with cross-wall only slightly thickened. In the first case the spores are usually colourless, the second case always brown. Buelliaceae, Physciaceae.

B. Spores unicellular, parallel-multicellular or muriform, usually colourless, cross-walls usually thin.

α Thallus in moist state more or less gelatinous. Gonidia always belonging to the Cyanophyceae, Lichinaceae, Ephebaceae, Collemaceae, Pyrenopsidaceae.

β Thallus not gelatinous. Coenogoniaceae, Lecideaceae, Cladoniaceae, Lecanoraceae, Pertusariaceae, Peltigeraceae, Stictaceae, Pannariaceae, Gyrophoraceae, Parmeliaceae, Cladoniaceae, Usneaceae.

Basidiolichenes (Hymenolichenes)

Cora, Dictyonema (incl. Laudatea), Corella (doubtfully placed here as the hymenium is unknown).

Habitats and Distribution of Lichens.

1\. Habitats.—These are extremely varied, and comprise 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 corticolous 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.g. Usnea, Alectoria jubata); while a few occur more especially on trees by roadsides (e.g. Physcia parietina and Ph. pulverulenta). In connexion with corticolous 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 saxicolous lichens, which occur on rocks and stones, they may be divided into two sections, viz. calcicolous and calcifugous. To the former belong such as are found on calcareous and cretaceous rocks, and the mortar of walls (e.g. Lecanora calcarea, Lecidea calcivora and several Verrucariae), while all other saxicolous 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 calcicolous lichens (e.g. Lecanora Prevostii, Lecidea calcivora) have the power of forming minute cavities in the rock, in which they are partially buried. (c) With respect to terrestrial species, some prefer peaty soil (e.g. Cladonia, Lecidea decolorans), others calcareous soil (e.g. Lecanora crassa, Lecidea decipiens), others sandy 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) Muscicolous lichens again are such as are most frequently met with on decayed mosses and Jungermannia, 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.g. Lecidea, Bouteillei, Strigula).

Sometimes 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. 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 saxicolous 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 capacity for retaining heat and moisture. As a rule lichens grow commonly in 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, Verrucaria 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.g. Habrothallus parmeliarum). A few, generally known as erratic species, have been met with growing unattached to any substratum (e.g. Parmella revoluta, var. concentrica, Lecanora esculenta); but it can hardly be that these are really free ab initio (vide Crombie in Journ. 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.

2\. Distribution.—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 more especially by climatic 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 arctic and antarctic seas 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 snowline (e.g. Lecidea geographica). 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 terrestrial species of Cladonia and Stereocaulon are seen in the greatest luxuriance and abundance spreading over extensive tracts almost to the entire exclusion of other vegetation. The lichen flora of temperate regions again is essentially distinguished from the preceding by the frequency of corticolous 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 Pyrenocarpei. Some lichens, especially saxicolous ones, seem to be cosmopolitan (e.g. Lecanora subfusca, Cladonia 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 climatic 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 available limits.

Bibliography.—General: Engler and Prantl, Die natürlichen Pflanzenfamilien, Teil I, Abt. 1 * where full literature will be found up to 1898. M. Funfstuck, “Der gegenwärtige Stand der Flechtenkunde,” Refer. Generalvers. d. deut. bot. Ges. (1902). Dual Nature: J. Baranetzky, “Beiträge zur Kenntnis des selbstständigen Lebens der Flechtengonidien,” Prings. Jahrb. f. wiss. Bot. vii. (1869); E. Bornet, “Recherches sur les gonidies des lichens,” Ann. de sci. nat. bot., 5 sér. n. 17 (1873); G. Bonnier, “Recherches sur la synthèse des lichens,” Ann. de sci. nat. bot., 7 sér. n. 9 (1889); A. Famintzin and J. Baranetzky, “Zur Entwicklungsgeschichte der Gonidien u. Zoosporenbildung der Lichenen,” Bot. Zeit. (1867, p. 189, 1868, p. 169); S. Schwendener, Die Algentypen der Flechtengonidien (Basel, 1869); A. Möller, Über die Kultur flechtenbildender Ascomyceten ohne Algen. (Münster, 1887). Sexuality: E. Stahl, Beiträge zur Entwickelungsgeschichte der Flechten (Leipzig, 1877); G. Lindau, Über Anlage und Entwickelung einiger Flechtenapothecien (Flora, 1888); E. Baur, “Zur Frage nach der Sexualität der Collemaceae,” Ber. d. deut. bot. Ges. (1898); “Über Anlage und Entwicklung einiger Flechtenapothecien” (Flora, Bd. 88, 1901); “Untersuchungen über die Entwicklungsgeschichte der Flechtenapothecien,” Bot. Zeit. (1904); O. V. Darbishire, “Über die Apothecium-entwickelung der Flechte, Physcia pulverulenta,” Nyl. Prings. Jahrb. (Bd. 34, 1900). Chemistry.—W. Zopf, “Vergleichende Produkte,” Beitr. z. bot. Centralbl. (Bd. 14, 1903); Die Flechtenstoffe (Jena, 1907).

(J. M. C; V. H. B.)

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1 The thalline margin (margo thallinus) is the projecting edge of a special layer of thallus, the amphithecium, round the actual apothecium; the proper margin (margo proprius) is the projecting edge of the apothecium itself.