Lichens

The Cocciferae represent a phylum parallel in development with the Ochrophaeae. The species have perhaps most affinity with the Clausae, the vegetative thallus—both the squamules and the podetia—being very much alike in several species. Wainio distinguishes two groups based on a difference of colour in the squamules, glaucous green in one case, yellowish in the other.

6. Causes of Variation. External causes of variation in Cladonia are chiefly humidity and light, excess or lack of either effecting changes which may have become fixed and hereditary. Minor changes directly traceable to these influences are also frequent, viz. size of podetia, proliferation and the production more or less of soredia or of squamules on the podetia, though only in connection with species in which these variations are already an acquired character. The squamules on the podetium more or less repeat the form of the basal squamules.

7. Podetial Development and Spore-dissemination. In a recent paper by Hans Sättler[1010] the problem of podetial development in Cladonia is viewed from a different standpoint. He holds that as the podetia are apothecial stalks, their service to the plant consists in the raising of the mature fruit in order to secure a wide distribution of the spores, and that changes in the form of the podetium are therefore but new adaptations for the more efficient discharge of this function.

Following out this idea he regards as the more primitive forms those in which both the spermogonia, as male reproductive bodies, and the carpogonia occur on the primary thallus, ascogonia and trichogynes being formed before the podetium emerges from the thallus. Fertilization thus must take place at a very early period, though the ultimate fruiting stage may be long delayed. Sättler considers that any doubt as to actual fertilization is without bearing on the question, as sexuality he holds must have originally existed and must have directed the course of evolution in the reproductive bodies. In this primitive group, called by him the “Floerkeana” group, the podetia are always short and simple, they are terminated by the apothecium and no scyphi are formed (Cl. Floerkeana, Cl. leptophylla, Cl. cariosa, Cl. caespiticia, Cl. papillaria, etc.).

In his second or “pyxidata” group, he places those species in which the apothecia are borne at the edge of a scyphus. That structure he follows Wainio in regarding as a morphological reaction on the failure of the first formed apical apothecium: it is, he adds, a new thallus in the form of a spreading cup and bears, as did the primary thallus, both the female primordia and the spermogonia. In some species, such as Cl. foliacea, there may be either scyphous or ascyphous podetia, and spermogonia normally accompany the carpogonium appearing accordingly along with it either on the squamule or on the scyphus.

As the pointed podetia are the more primitive, Sättler points out that they may reappear as retrogressive structures, and have so appeared in the “pyxidata” group in such species as Cl. fimbriata. He refers to Wainio’s statement that the abortion of the apothecium being a retrogressive anomaly, while scyphus formation is an evolutionary advance, the scyphiferous species present the singular case, “that a progressive transmutation induced by a retrogressive anomaly has become constant.”

His third group includes those forms that grow in crowded tufts or swards such as Cl. rangiferina, Cl. furcata, Cl. gracilis, etc. They originate, as did the pyxidata group, in some Floerkeana-like form, but in the “rangiferina” group instead of cup-formation there is extensive branching. In the closely packed phalanx of branches water is retained as in similar growths of mosses, and moist conditions necessary for fertilization are thus secured as efficiently as by the water-holding scyphus.

Sättler in his argument has passed over many important points. Above all he ignores the fact that whatever may have been the original nature and function of the podetium, it has now become a thalline structure and provides for the vegetative life of the plant, and that it is in its thalline condition that the many variations have been formed; the scyphus is not, as he contends, a new thallus, it is only an extension of thalline characters already acquired.

8. Pilophorus, Stereocaulon and Argopsis. These closely related genera are classified with Cladonia as they share with it the twofold thallus and the lecideine apothecia. The origin of the podetium being different they may be held to constitute a phylum apart, which has however taken origin also from some Biatora form.

The primary thallus is crustaceous or minutely squamulose and the podetia of Pilophorus, which are short and unbranched (or very sparingly branched), are beset with thalline granules. The podetia of Stereocaulon and Argopsis are copiously branched and are more or less thickly covered with minute variously divided leaflets. Cephalodia containing blue-green algae occur on the podetia of these latter genera; in Pilophorus they are intermixed with the primary thallus.

The tissue systems are less advanced in these genera than in Cladonia: there is no cortex present either in Pilophorus or in Argopsis or in some species of Stereocaulon, though in others a gelatinous amorphous layer covers the podetia and also the stalk leaflets. The stalks are filled with loose hyphae in the centre.

BB. Lecanorales. This second group of Cyclocarpineae is distinguished by the marginate apothecium, a thalline layer providing a protecting amphithecium. The lecanorine apothecium is of a more or less soft and waxy consistency, and though the disc is sometimes almost black, neither hypothecium nor parathecium is carbonaceous as in Lecidea. The affinity of Lecanora is with sect. Biatora, and development must have been from a biatorine form with a persistent thallus. The margin or amphithecium varies in thickness: in some species it is but scanty and soon excluded by the over-topping growth of the disc, so that a zone of gonidia underlying the hypothecium is often the only evidence of gonidial intrusion left in fully formed fruits.

The marginate apothecium has appeared once and again as we have seen. It is probable however that its first development was in this group of lichens, and even here there may have been more than one origin as there is certainly more than one phylum.

aa. Course of Development. At the base of the series, the thallus is of the crustaceous type somewhat similar to that of Lecidea, but there are none of the very simple primitive forms. Lecanora must have originated when the crustaceous lecideine thallus was already well established. Its affinity is with Lecidea and not with any fungus: where the thallus is evanescent or scanty, its lack is due to retrogressive rather than to primitive characters.

bb. Lecanoraceae. A number of genera have arisen in this large family, but they are distinguished mainly if not entirely by spore characters, and by some systematists have all been included in the one genus Lecanora, since the changes have taken place within the developing apothecium.

There is one genus, Harpidium, which is based on thalline characters, represented by one species, H. rutilans, common enough on the Continent, but not yet found in our country. It has a thin crustaceous homoiomerous thallus, the component hyphae of which are divided into short cells closely packed together and forming a kind of cellular tissue in which the algae are interspersed. The dorsiventral stratose arrangement prevails however in the other genera and a more or less amorphous “decomposed” cortex is frequently present. The medulla rests on the substratum.

With the stouter thallus, there is slightly more variety of crustaceous form than in Lecideaceae: there occurs occasionally an outgrowth of the thalline granules as in Haematomma ventosum which marks the beginning of fruticulose structure. Of a more advanced structure is the thallus of Lecanora esculenta, a desert lichen which becomes detached and erratic, and which in some of its forms is almost coralline, owing to the apical growth of the original granules or branches: a more or less radiate arrangement of the tissues is thus acquired.

The squamulose type is well represented in Lecanora, and the species with that form of thallus have frequently been placed in a separate genus, Squamaria. These squamules are never very large; they possess an upper, somewhat amorphous, cortex; the medulla rests on the substratum, except in such a species as Lecanora lentigera, where they are free, a sort of fibrous cortex being formed of hyphae which grow in a direction parallel with the surface. In none of them are rhizinae developed.

cc. Parmeliaceae. The chief advance, apart from size, of the squamulose to the foliose type is the acquirement of a lower cortex along with definite organs of attachment which in Parmeliaceae are invariably rhizoidal and are composed of compact strands of hyphae extending from the cells of the lower cortex.

In the genus Parmelia rhizinae are almost a constant character, though in a few species, such as Parmelia physodes, they are scanty or practically absent. It is not possible, however, to consider that these species form a lower group, as in other respects they are highly evolved, and rhizinae may be found at points on the lower surface where there is irritation by friction. Soredia and isidia occur frequently and, in several species, almost entirely replace reproduction by spores. In one or two northern or Alpine species, P. stygia and P. pubescens, the lobes are linear or almost filamentous. They are retained in Parmelia because the apothecia are superficial on the fronds which are partly dorsiventral, and because rhizinae have occasionally been found. Some of the Parmeliae attain to a considerable size; growth is centrifugal and long continued.

Two monotypic genera classified under Parmeliaceae, Physcidia and Heterodea, are of considerable interest as they indicate the bases of parallel development in Parmelia and Cetraria. The former, a small lichen, is corticate only on the upper surface, and without rhizinae; and from the description, the cortex is of a fastigiate character. The solitary species grows on bark in Cuba; it is related to Parmelia, as the apothecia are superficial on the lobes. The second, Heterodea Mülleri, a soil-lichen from Australasia, is more akin to Cetraria in that the apothecia are terminal. The upper surface is corticate with marginal cilia, the lower surface naked or only protected by a weft of brownish hyphae amongst which cyphellae are formed; pseudocyphellae appear in Cetraria.

The genus Cetraria contains very highly developed thalline forms, either horizontal (subgenus Platysma), or upright (Eucetraria). Rhizinae are scanty or absent, but marginal cilia in some upright species act as haptera. Cetraria aculeata is truly fruticose with a radiate structure.

An extraordinary development of the under cortex characterizes the genera Anzia[1011] and Pannoparmelia: rhizinae-like strands formed from the cortical cells branch and anastomose with others till a wide mesh of a spongy nature is formed. They are mostly tropical or subtropical or Australasian, and possibly the spongy mass may be of service in retaining moisture. A species of Anzia has been recorded by Darbishire[1012] from Tierra del Fuego.

dd. Usneaceae. As we have seen, the change to fruticose structure has arisen as an ultimate development in a number of groups; it reaches however its highest and most varied form in this family. Not only are there strap-shaped thalli, but a new form, the filamentous and pendulous, appears; it attains to a great length, and is fitted to withstand severe strain. The various adaptations of structure in these two types of thallus have already been described[1013].

In Parmelia itself there are indications of this line of development in P. stygia, with short stiff upright branching fronds, and in P. pubescens, with its tufts of filaments, but these two species are more or less dorsiventral in structure and do not rise from the substratum. In Cetraria also there is a tendency towards upright growth and in C. aculeata even to radiate structure. But advance in these directions has stopped short, the true line of evolution passing through species like Parmelia physodes with raised, and in some varieties, tubular fronds, and the somewhat similar species P. Kamtschadalis with straggling strap-like lobes, to Evernia. That genus is a true link between foliose and fruticose forms and has been classified now with one series, now with the other.

In Evernia furfuracea, the lobes are free from the substratum except when friction causes the development of a hold-fast and the branching out of new lobes from that point. It is however dorsiventral in structure, the under surface is black and the gonidial zone lies under the upper cortex. Evernia prunastri is white below and is more fruticose in habit, the long fronds all rising from one base. They are thin and limp, no strengthening tissue has been evolved, and they tend to lie over on one side; both surfaces are corticate and gonidia sometimes travel round the edge, becoming frequently lodged here and there along the under side.

The extreme of strap-shaped fruticose development is reached in the genus Ramalina. In less advanced species such as R. evernioides there is a thin flat expansion anchored to the substratum at one point and alike on both surfaces. In R. fraxinea the fronds may reach considerable width (var. ampliata), but in that and in most species there is a provision of sclerotic strands to support and strengthen the fronds. One of those best fitted to resist bending strains is R. scopulorum (siliquosa) which grows by preference on sea-cliffs and safely withstands the maximum of exposure to wind or weather.

The filamentous structure appears abruptly, unless we consider it as foreshadowed by Parmelia pubescens. The base is secured by strong sheaths of enduring character; tensile strains are provided for either by a chondroid axis, as in Usnea, or by cortical development, as in Alectoria; the former method of securing strength seems to be the most advantageous to the plant as a whole, since it leaves the outer structures more free to develop, and there is therefore in Usnea a greater variety of branching and greater growth in length, which are less possible with the thickened cortex of Alectoria.

ee. Physciaceae. There remains still an important phylum of Lecanorales well defined by the polarilocular spores[1014]. It also arises from a Biatora species and forms a parallel development. Even in this phylum there are two series: one with colourless spores and mostly yellow or reddish either in thallus or apothecium, and the other with brown spores and with cinereous-grey or brown thalli. The dark spores are in many of the species typically polarilocular, though in some the median septum is not very wide and no canal is visible. Practically all of the lighter coloured forms contain parietin either in thallus or apothecia or in both; it is absent in the dark-spored series.

Among the lighter coloured forms it is difficult to decide which of these two striking characteristics developed first, the acid or the peculiar spore. Probably the acid has the priority: there is one common rock lichen in this country, Placodium rupestre (Lecanora irrubata), which gives a strong red acid reaction with potash, but in which the spores are still simple, and the fruit structure in the biatorine stage. Another species, Pl. luteoalbum, with a purplish reaction in the fruit only, shows septate spores but with only a rather narrow septum. The development continues through biatorine forms to lecanorine with a fully formed thalline margin. Among these latter we encounter Pl. nivale which is well provided with acid but in which the spores have become long and fusiform with little trace of the polar cells or central canal. We must allow here also for reversions, and wanderings from the straight road.

From crustaceous the advance is normal and simple to squamulose forms which in this phylum maintain a stiff regularity of thalline outline termed “effigurate”; the squamules, developing from the centre, extend outwards in a radiate-stellate manner. There are also foliose thalli in the genus Xanthoria and fruticose in Teloschistes. The cortex in the former horizontal genus is of plectenchyma, and no peculiar structures have emerged. In Teloschistes the cortex is of compact parallel hyphae (fibrous) which form the strengthening structure of the narrow compressed fronds (T. flavicans).

In the brown-spored series there is a considerable number of species that are crustaceous united in the genus Rinodina, all of which have marginate apothecia. One of them, Rinodina oreina, approaches in thalline structure the effigurate forms of Placodium; while in R. isidioides, a rare British species, there is an isidioid squamulose development.

Among foliose genera, the tropical genus Pyxine is peculiar in its almost lecideine fruit, a few gonidia occurring only in the early stages; its affinity with Physcia holds, however, through the one-septate brown spores with very thick walls and the reduced lumen of the cells. The more simple type of fruit may be merely retrogressive.

Physcia, the remaining genus, is mainly foliose and with dorsiventral thallus. A few species have straggling semi-upright fronds and these have sometimes been placed in a separate genus Anaptychia. Only one “Anaptychia,” Ph. intricata, has a radiate structure with fibrous cortex all round; in the others the upper cortex alone is fibrous—of long parallel hyphae—but that character appears in nearly every one of the horizontal species as well, sometimes in the upper, sometimes in the lower cortex.

In Physcia the horizontal thallus is of smaller dimensions than in Parmelia, and never becomes so free from the substratum: it is attached by rhizinae and soredia appear frequently. Very often the circular effigurate type of development prevails.

It is difficult to trace with any certainty the origin of this series of the phylum. Some workers have associated it with the purely lecideine genus, Buellia, but the brown septate spores of the latter are of simple structure, though occasionally approaching the Rinodina type. There are also differences in the thallus, that of Buellia, especially when it is saxicolous, inclining to Rhizocarpon in form. It is more consistent with the outer and inner structure to derive Rinodina from some crustaceous Placodium form with a marginate apothecium, therefore from a form of fairly advanced development. As the parietin content disappeared—perhaps from the preponderance of other acids—the colouration changed and the spores became dark-coloured.

Many genera and even families, such as Thelotremaceae, etc., have necessarily been omitted from this survey of phylogeny in lichens, but the tracing of the main lines of development has indicated the course of evolution, and has demonstrated not only the close affinity between the members of this polyphyletic class of plants, as shown in the constantly recurring thalline types, but it has proved the extraordinary vigour gained by both the component organisms through the symbiotic association.

The principal phyla[1015], developing on somewhat parallel lines, are given in the appended table:

Archilichens

SCHEME OF SUGGESTED PROGRESSION IN LICHEN STRUCTURE

CHAPTER VIII
SYSTEMATIC

I. CLASSIFICATION

A. Work of successive Systematists

Since the time when lichens were first recognized as a separate class—as members of the genus Lichen by Tournefort[1016] or as “Musco-fungi” by Morison[1017],—many schemes of classification have been outlined, and the history of the science of lichenology, as we have seen, is a record of attempts to understand their puzzling structure, and to express that understanding by relating them to each other and to allied classes of plants. The great diversity of opinion in regard to their affinities is directly due to their composite nature.

a. Dillenius and Linnaeus. The first systematists were chiefly impressed by their likeness to mosses, hepatics or algae. Dillenius[1018] in the Historia Muscorum grouped them under the moss genera:—IV. Usnea, V. Coralloides and VI. Lichenoides. Linnaeus[1019] classified them among algae under the general name Lichen, dividing them into eight orders based on thalline characters in all but one instance, the second order being distinguished from the first by bearing scutellae. The British botanists of the latter part of the eighteenth century—Hudson, Lightfoot and others—were content to follow Linnaeus and in general adopted his arrangement.

b. Acharius. Early in the nineteenth century Acharius, the Swedish Lichenologist, worked a revolution in the classification of lichens. He gave first place to the form of the thallus, but he also noted the fundamental differences in fruit-formation: his new system appeared in the Methodus Lichenum[1020] with an introduction explaining the terms he had introduced, many of them in use to this day.

Diagnoses of twenty-three genera are given with their included species. The work was further extended and emended in Lichenographia Universalis[1021] and in the Synopsis Lichenum[1022]. In his final arrangement the family “Lichenes” is divided into four classes, three of which are characterized solely by apothecial characters; the fourth class has no apothecia. They are as follows:

• Class I. Idiothalami with three orders, Homogenei, Heterogenei and Hyperogenei: the apothecia differ in texture and colouration from the thallus: Lecidea, Opegrapha, Gyrophora, etc.
• Class II. Coenothalami, with three orders, Phymaloidei, Discoidei and Cephaloidei. The apothecia are partly formed from the thallus: Lecanora, Parmelia, etc. The Pyrenolichens are also included by him in this class, because “the thallus surrounds and is concrete with the partly or wholly immersed apothecia.”
• Class III. Homothalami with two orders, Scutellati and Peltati. The apothecia are formed from the cortical and medullary tissue of the thallus: Ramalina, Usnea, Collema, etc.
• Class IV. Athalami, with but one sterile genus, Lepraria.

The orders are thus based on the form of the fruit; the genera in the Synopsis number 41. Large genera such as Lecanora with 132 species are divided into sections, many of which have in turn been established as genera, by S. F. Gray in 1821, and later by other systematists.

The Synopsis was the text-book adopted by succeeding botanists for some 40 years with slight alterations in the arrangement of classes, genera, etc.

Wallroth[1023] and Meyer[1024] followed with their studies on the lichen thallus, and Wallroth’s division into “Homoiomerous” and “Heteromerous” was accepted as a useful guide in the maze of forms, representing as it did a great natural distinction.

c. Schaerer. This valiant lichenologist worked continuously during the first half of the nineteenth century, but with very partial use of the microscope. His last publication in 1850, an Enumeration of Swiss Lichens, was the final declaration of the older school that relied on field characters. His classification is as follows:

• Class I. Lichenes Discoidei, with ten orders from Usneacei to Graphidei; fruits open.
• Class II. Lichenes Capitati, with three orders: Calicioidei, Sphaerophorei and Cladoniacei; fruits stalked.
• Class III. Lichenes Verrucarioidei, with three orders: Verrucarii, Pertusarii and Endocarpei: fruits closed.

An “Appendix” contains descriptions of Crustacei and Fruticulosi, all sterile forms, except Coniocarpon and Arthonia, which seem out of place, and finally a “Corollarium” of gelatinous lichens all classified under one genus Collema.

d. Massalongo and Koerber. As a result of their microscopic studies, these two workers proposed many changes based on fruit and spore characters, and Koerber in the Systema Lichenum Germaniae (1855) gave expression to these views in his classification. He also made use of Wallroth’s distinctions of “homoiomerous” and “heteromerous,” thus dividing lichens at the outset into those mostly with blue-green and those with bright-green gonidia.

The following is the main outline of Koerber’s classification:

• Series I. Lichenes Heteromerici.
• Order I. Lich. Thamnoblasti (fruticose).
• Order II. Lich. Phylloblasti (foliose).
• Order III. Lich. Kryoblasti (crustaceous).
• Series II. Lichenes Homoeomerici.
• Order IV. Lich. Gelatinosi.
• Order V. Lich. Byssacei.

With the exception of Order V all are subdivided into two sections, “gymnocarpi” with open fruits and “angiocarpi” with closed fruits, a distinction that had long been recognized both in lichens and in fungi.

e. Nylander. The above writers had been concerned with the interrelationships of lichens; Nylander, who was now coming forward as a lichenologist of note, gave a new turn to the study by dwelling on their relation to other classes of plants. Without for a moment conceding that they were either algal or fungal, he yet insisted on their remarkable affinity to algae on the one hand, and to fungi on the other, and he sought to make evident this double connection by his very ingenious scheme of classification[1025]. He began with what we may call “algal lichens,” those associated with blue-green gonidia in the family “Collemacei”; he continued the series to the most highly evolved foliose forms and then wound up with those that are most akin to fungi, that is, those with least apparent thalline formation—according to him—the “Pyrenocarpei.”

In his scheme, which is the one followed by Leighton and Crombie, the “family” represents the highest division; series, tribe, genus and species come next in order. We have thus:

• Fam. I. Collemacei.
• Fam. II. Myriangiacei (now reckoned among fungi).
• Fam. III. Lichenacei.

This last family, which includes the great bulk of lichens, is divided into the following series: I. Epiconiodei; II. Cladoniodei; III. Ramalodei; IV. Phyllodei; V. Placodei; VI. Pyrenodei. It is an ascending series up to the Phyllodei, or foliaceous lichens, which he considers higher in development than the fruticose or filamentous Ramalodei. The Placodei include four tribes on a descending scale, the Lecanorei, Lecidinei, Xylographidei and Graphidei. The classification is almost wholly based on thalline form, except for the Pyrenodei in which are represented genera with closed fruits, there being one tribe only, the Pyrenocarpei.

Nylander claims however to have had regard equally to the reproductive system and was the first to give importance to the spermogonia. The classification is coherent and easy to follow, though, like all classifications based on imperfect knowledge, it is not a little artificial; also while magnifying the significance of spermogonia and spermatia, he overlooked the much more important characters of the ascospores.

f. Müller(-Argau). In preparing his lists of Genevan lichens (1862), Müller realized that Nylander’s series was unnatural, and he found as he studied more deeply that lichens must be ranged in parallel or convergent but detached groups. He recognized three main groups:

• 1. Eulichens, divided into Capitularieae, Discocarpeae and Verrucaroideae.
• 2. Epiconiaceae.
• 3. Collemaceae.

He suggested that, in relation to other plants, Eulichens approach Pezizae, Hysteriaceae and Sphaeriaceae; Epiconiaceae have affinity with Lycoperdaceae, while Collemaceae are allied to the algal family Nostocaceae. These three groups of Eulichens, he held, advanced on somewhat parallel lines, but reached a very varied development, the Discocarpeae attaining the highest stage of thalline form. Müller accepted as characters of generic importance the form and structure of the fruiting body, the presence or absence of paraphyses, and the septation, colour, etc. of the spores.

A few years later (1867) the composite nature of the lichen thallus was announced by Schwendener, and, after some time, was acknowledged by most botanists to be in accordance with the facts of nature. Any system of classification, therefore, that claims to be a natural one, must, while following as far as possible the line of plant development, take into account the double origin of lichens both from algae and fungi, the essential unity and coherence of the class being however proved by the recurring similarity between the thalline types of the different phyla. As Müller had surmised: “they are a series of parallel detached though convergent groups.”

g. Reinke. The arrangement of Ascolichens on these lines was first seriously studied by Reinke[1026], and his conclusions, which are embodied[1027] in the Lichens of Schleswig-Holstein, have been largely accepted by succeeding workers. He recognizes three great subclasses: 1. Coniocarpi; 2. Discocarpi; 3. Pyrenocarpi.

The Coniocarpi are a group apart, but as their fruit is at first entirely closed—at least in some of the genera—the more natural position for them is between Discocarpi and Pyrenocarpi. It is in the arrangement of the Discocarpi that variation occurs. Reinke’s arrangement of orders and families in that sub-class is as follows:

Subclass 2. Discocarpi.

• Order I. GRAMMOPHORI: Fam. Graphidacei and Xylographacei.
• Order II. LECIDEALES: Fam. Gyalectacei, Lecideacei, Umbilicariaceiand Cladoniacei.
• Order III. PARMELIALES: Fam. Urceolariacei, Pertusariacei, Parmeliacei,Physciacei, Teloschistacei and Acarosporacei.
• Order IV. CYANOPHILI: Fam. Lichinacei, Ephebacei, Pannariacei,Stictacei, Peltigeracei, Collemacei and Omphalariacei.

The orders represent generally the principal phyla or groups, the families subordinate parallel phyla within the orders. The first three orders are stages of advance as regards fruit development; the Cyanophili are a group apart.

Wainio[1028] rendered great service to Phylogeny in his elaborate work on Cladoniaceae, the most complicated of all the lichen phyla. He also drew up a scheme of arrangement in his work on Brazil Lichens[1029]. There is in it some divergence from Reinke’s arrangement, as he tends to give more importance to the thallus than to fruit characters as a guide. He places, for instance, Gyrophorei beside Parmelei and at a long distance from his Lecidei. The Cyanophili group of families he has interpolated between Buelliae (Physciaceae) and Lecideae. Many workers approve of Wainio’s classification but it presents some difficult problems.

h. Zahlbruckner. The systematist of greatest weight in recent times is A. Zahlbruckner, who is responsible for the systematic account of lichens in Engler and Prantl’s Natürlichen Pflanzenfamilien. It is difficult to express the very great service he has rendered to Lichenology, in that and other world-wide studies of lichens. The sketch of lichen phylogeny as given in the present volume owes a great deal to the sound and clear guidance of his work, though his conclusions may not always have been accepted. The classification in the Pflanzenfamilien is the one now generally followed.

The class Lichenes is divided by Zahlbruckner[1030] into two subclasses, I. Ascolichens and II. Hymenolichens. He gives a third class, Gasterolichens[1031], but as it was founded on error[1032], it need not concern us here. The Ascolichens are by far the more important. These are subdivided into:

• Series 1. PYRENOCARPEAE, with perithecial fruits.
• Series 2. GYMNOCARPEAE, with apothecial fruits.

These are again broken up into families, and in the arrangement and sequence of the families Zahlbruckner indicates his view of development and relationship. They occur in the following order:

Series 1. PYRENOCARPEAE

Series 2. GYMNOCARPEAE

• Subseries 1. Coniocarpineae, with subperithecial fruits.
• Subseries 2. Graphidineae, with elongate, narrow fruits.
• Subseries 3. Cyclocarpineae, with round open fruits.

Subseries 1. CONIOCARPINEAE

This is a well-defined group, peculiar in the disappearance of the asci at an early stage so that the spores lie like a powder in the globose partly closed fruits. Algal cells, bright-green; Protococcaceae. There are only three families:

Subseries 2. GRAPHIDINEAE

This subseries comes next in the form of fruit development; generally the apothecia are elongate, with a narrow slit-like opening, so that a transverse section shows almost a perithecial outline. Algal cells are mostly Trentepohlia.

Subseries 3. CYCLOCARPINEAE

A large and very varied group! In most of the families the algal cells are bright-green (Chlorophyceae), in some they are blue-green (Cyanophyceae), these latter corresponding to Reinke’s order Cyanophili. The apothecia, as the name implies, are round and open; the “Cyanophili” have been placed by Zahlbruckner after those families in which the apothecium has no thalline margin. They form a phylum distinct from those that precede and those that follow.

The first family of the Cyclocarpineae, the Lecanactidaceae, is often placed under Graphidineae; in any case it forms a link between the two subseries.

1. Lecideine group (apothecia without a thalline margin).

2. Cyanophili group.

In this group the classification depends almost entirely on the nature of the algal constituents. The apothecia are in most genera provided with a thalline margin.

a. More or less gelatinous when moist.

b. Not gelatinous when moist.

3. Lecanorine group (apothecia with a thalline margin).

The remaining families have all bright-green gonidia and nearly always apothecia with a thalline margin. The group includes several distinct phyla:

Subclass 2. Hymenolichens.

There are only three closely related genera of Hymenolichens, Cora, Corella and Dictyonema with Chroococcus or Scytonema algae.

There is reason to dissent from the arrangement in one or two instances which will be pointed out in the following examination of families and genera.

B. Families and Genera of Ascolichens

The necessity for a well-reasoned and well-arranged system of classification is self-evident: without a working knowledge of the plants that are the subject of study no progress can be made. The recognition of plants as isolated individuals is not sufficient, it must be possible to place them in relation to others; hence the importance of a natural system. In identifying species artificial aids, such as habitat and substratum, are also often of great value, and a good working system should take account of all characteristics.

Lichen development is the result of two organisms mutually affecting each other, but as the fungus provides the reproductive system, it is the dominant partner: the main lines of classification are necessarily determined by fruit characters. The algae occupy a subsidiary position, but they also are of importance in shaping the form and structure of the thallus. The different phyla are often determined by the presence of some particular alga; it is in the delimitation of families that the algal influence is of most effect.

Zahlbruckner’s system gives due weight to the inheritance from both fungus and alga with, however, the fungus as the chief factor in development, and as his work is certain to be generally followed by modern lichenologists, it is the one of most immediate interest. His scheme has been accepted in the following more detailed account of families and genera, and for the benefit of home workers those that have not so far been recorded from the British Isles have been marked with an asterisk.

It cannot be affirmed that nomenclature is as yet firmly established in lichenology. Both on historical grounds and on those of convenience, the subject is one of extreme importance, and interest in it is one of the main avenues by which we secure continuity with the past, and by which we are able to realize not only the difficulty, but the romance of pioneer work. Besides, there can be no exchange of opinion between students nor assured knowledge of plants, until the names given to them are beyond dispute. According to the ruling of the Brussels Botanical Congress in 1910, Linnaeus’s[1033] list of lichens in the Species Plantarum has been selected as the basis of nomenclature, but since his day many new families, genera and species have been described and often insufficiently delimited. It is not easy to decide between priority, which appeals to the historical sense, and recent use which is the plea of convenience. Here also it seems there can be no rigid decision; the one aim should be to arrive at a conclusion satisfactory to all, and accepted by all.

In the following necessarily brief account of families and genera, the “spermogonia” or “pycnidia” have in most cases been left out of account, as in many instances they vary within the family and occasionally even within the genus. Their taxonomic value is not without importance, but, in the general systematic arrangement, they are only subsidiary characters. An account of them has already been given, and for more detailed statements the student is referred to purely systematic works.

There are two main types of spore production in the “pycnidia” which have been shortly described by Steiner[1034] as “exobasidial” and “endobasidial.” In the former the sporophores are simple or branched filaments, at the apices of which a short process grows out and buds off a pycnidiospore; in the latter the spores are budded directly from cells lining the walls or filling the cavity of the pycnidium. The exobasidial type is more simply rendered in the following pages by “acrogenous,” the endobasidial by “pleurogenous” spore production. In many cases the “spermogonia” or “pycnidia” are still imperfectly known. In designating the gonidial algae, the more comprehensive Protococcaceae has been substituted for Protococcus, as in many cases the alga is probably not Protococcus as now understood, but some other genus of the family[1035].

Subclass I. ASCOLICHENS

Series I. PYRENOCARPINEAE

It is on mycological grounds that Pyrenocarpineae are placed at the base of lichen classification. There is no evidence that the series was first in time.

I. Moriolaceae

This family was described by Norman[1036] in 1872 from specimens collected by himself in Norway or in the Tyrol, on soil or more frequently on trees. There seems to have been no further record, and Zahlbruckner, while accepting the family, suggests that an examination or revision may be necessary.

The thallus is crustaceous. The algal cells, Protococcaceae, occur either in groups (sometimes stalked) surrounded by a plectenchymatous wall and called by Norman “goniocysts,” or they form nests in the thallus termed “nuclei” which are surrounded by a double wall of plectenchyma, colourless in the interior and brown outside. Norman invented the term “Allelositismus,” which may be rendered “mutualism,” to indicate this peculiar form of thallus. The species of Spheconisca are fairly numerous on poplars, willows and conifers:

II. Epigloeaceae

The family consists of but one genus and one species, Epigloea bactrospora, and, according to Zahlbruckner, further examination is necessary to make certain as to the lichenoid nature of the plant.

Zukal[1038] found the perithecia scattered over the leaves of mosses, and he alleges that hyphae connected with the perithecium were closely associated with the alga, Palmella botryoides, and were causing it no harm. Along with the perithecia he also found minute pycnidia. The “thallus” is of a gelatinous nature and homoiomerous in structure; the perithecia are soft and clear-coloured with many-spored asci and colourless one-septate spores.

The small globose pycnidia contain simple sporophores and acrogenous straight or slightly bent rod-like spores.

III. Verrucariaceae

In all the genera of this family the thallus is crustaceous, and, with very few exceptions, the species are saxicolous or terricolous. The thallus is variable within the crustaceous limits, and may be superficial and very conspicuous, almost imperceptible, or wholly immersed in the substratum. The algal cells are Protococcaceae, and in two of the genera the green cells penetrate the hymenium and grow in rows alongside of the asci. The perithecia are small roundish structures scattered over the thallus, the base immersed, but the upper portion generally projecting. An outer dark-coloured wall surrounds the whole perithecium (entire) or only the upper exposed portion (dimidiate); it opens above by a pore or ostiole more or less prominent.

In some of the genera the paraphyses become dissolved at an early stage, and somewhat similar filaments near the ostiole, termed periphyses, aid in the expulsion of the spores. The spores vary in septation, colour and size, and these variations have served to delimit the genera which have been formed from the original very large genus Verrucaria. The ascus may be 1-, 2-, 4- or 8-spored. In only one genus is it many-spored (Trimmatothele).

The genera are as follows:

IV. Dermatocarpaceae

In this family there is a much more advanced thalline development—generally squamulose or with some degree of foliose structure, though in the genus Endocarpon, some of the species are little more than crustaceous. The gonidia are bright-green Protococcaceae (according to Chodat, Coccobotrys in Dermatocarpon). In Endocarpon they appear in the hymenium.

The least developed in structure is Normandina: the thallus of the single species consists of delicate shell-like squamules which are non-corticate above and below. In the other genera there is a cortex of plectenchyma.

The perithecia are almost wholly immersed, and open above by a straight ostiole. The fructification of Dacampia is considered by some lichenologists to be only a parasite on the white thickish squamulose thallus with which it is associated.

V. Pyrenothamniaceae

Thallus more or less fruticose and corticate on both surfaces. Algal cells Protococcaceae.

Only two genera are included in this family: Nylanderiella with one species from New Zealand, with a small laciniate thallus up to 15 mm. in height, partly upright, partly decumbent, and attached to the substratum by basal rhizinae; the other small genus, Pyrenothamnia, belongs to N. America; the thallus has a short rounded stalk which expands above to an irregular frond. The perithecia are immersed in the fronds.

VI. Mastoideaceae

A family containing one genus and one species, with a wide distribution, having been found in Siberia, on the Antarctic continent (Graham’s Land), as also in Tierra del Fuego, South Georgia, South Shetland Islands and Kerguelen. The thallus is foliose, of small thin lobes, and without rhizinae. Algal cells Prasiola[1040]. The perithecia are globose and partly project from the thallus; the asci are 8-spored; the paraphyses are mucilaginous and partly dissolving.

VII. Pyrenulaceae

This family of crustaceous lichens differs from Verrucariaceae chiefly in the gonidium which is a species of Trentepohlia. Genera and species are largely corticolous and the thallus is inconspicuous, often developing within the substratum. The perithecia, like those of Verrucariae, are immersed or partly emergent and have an entire or dimidiate outer wall. They are scattered over the thallus except in Anthracothecium where they are often coalescent. This genus is tropical or subtropical except for one species which inhabits S.W. Ireland.

Paraphyses are variable, and in some species tend to disappear, but do not dissolve in mucilage. The spores are generally colourless, only in one monotypic genus, Coccotrema, are they simple. The cells into which the spore is divided differ in form according to the genus.

VIII. Paratheliaceae

This family is peculiar in that the perithecia open by a somewhat elongate ostiole that slants at an oblique angle. The algal cells are Trentepohlia. Genera and species are endemic in tropical or subtropical regions of the Western hemisphere, though a species of Pleurotrema has been found in subantarctic America. They are corticolous and the thallus is either superficial or embedded. The genera are arranged according to spore characters:

IX. Trypetheliaceae

This and the following two families are distinguished by the pseudostroma or compound fruit, a character rare among lichens, though the true stroma is frequent in Pyrenomycetes in such genera as Dothidea, Valsa, etc. The genera are crustaceous and corticolous and occur with few exceptions in tropical or subtropical regions, mostly in the Western Hemisphere. Several grow on officinal bark (Cinchona, etc.). Algal cells are Trentepohlia. As in many tropical lichens, the spores are large. The genera are based chiefly on spore characters, on septation, and on the form of the spore cells:

X. Astrotheliaceae