Though lichens are very old members of the vegetable kingdom, as symbiotic plants they yet date necessarily from a time subsequent to the evolution of their component symbionts. Phylogeny of lichens begins with symbiosis.

The algae, which belong to those families of Chlorophyceae and Myxophyceae that live on dry land, had become aerial before their association with fungi to form lichens. They must have been as fully developed then as now, since it is possible to refer them to the genus or sometimes even to the species of free-living forms. The fungus hyphae have combined with a considerable number of different algae, so that, even as regards the algal symbiont, lichens are truly polyphyletic in origin.

The fungus is, however, the dominant partner, and the principal line of development must be traced through it, as it provides the reproductive organs of the plant. Representatives of two great groups of fungi are associated with lichens: Basidiomycetes, found in only a few genera, and Ascomycetes which form with the various algae the great bulk of lichen families. In respect of their fungal constituents lichens are also polyphyletic, and more especially in the Ascolichens which can be traced back to several starting points. But though lichens have no common origin, the manner of life is common to them all and has influenced them all in certain directions: they are fitted for a much longer existence than that of the fungi from which they started; and both the thallus and the fruiting bodies—at least in the sub-class Ascolichens—can persist through great climatic changes, and can pass unharmed through prolonged periods of latent or suspended vitality.

Another striking note of similarity that runs through the members of this sub-class, with perhaps the exception of the gelatinous lichens, is the formation of lichen-acids which are excreted by the fungus. These substances are peculiar to lichens and go far to mark their autonomy. The production of the acids and the many changes evolved in the vegetative thallus suggest the great antiquity of lichens.

B. Algal Ancestors

It is unnecessary to look far for the algae as they have persisted through the ages in the same form both without and within the lichen thallus. By many early lichenologists the free-living algae, similar in type to lichen algae, were even supposed to be lichen gonidia in a depauperate condition and were, for that reason, termed by Wallroth “unfortunate brood-cells.” In the condition of symbiosis they may be considerably modified, but they revert to their normal form, and resume their normal life-history of spore production, etc., under suitable and free culture. The different algae taking part in lichen-formation have been treated in an earlier chapter[980].

C. Fungal Ancestors

a. Hymenolichens. The problem of the fungal origin in this sub-class is comparatively simple. It contains but three genera of tropical lichens which are all associated with Myxophyceae, and the fungus in them, to judge from the form and habit of the plants, is a member of the Thelephoraceae. It may be that Hymenolichens are of comparatively recent origin and that the fungi belonging to the Basidiomycetes had, in the course of time, become less labile and less capable of originating a new method of existence. Whatever the reason, they lag immeasurably behind Ascomycetes in the formation of lichens.

b. Ascolichens. Lichens are again polyphyletic within this sub-class. The main groups from which they are derived are evident. Whether there has been a series of origins within the different groups or a development from one starting point in each it would be difficult to determine. In any case great changes have taken place after symbiosis became established.

The main divisions within the Ascolichens are related to fungi thus:

II. THE REPRODUCTIVE ORGANS

A. Theories of Descent in Ascolichens

It has been suggested that ascomycetous fungi, from which Ascolichens are directly derived, are allied to the Florideae, owing to the appearance of a trichogyne in the carpogonium of both groups. That organ in the red seaweeds is a long delicate cell in direct communication with the egg-cell of the carpogonium. It is a structure adapted to totally submerged conditions, and fitted to attach the floating spermatia.

In fungi there is also a structure considered as a trichogyne[981], which, in the Laboulbeniales, is a free, simple or branching organ. There is no other instance of any similar emergent cell or cells connected with the ascogonium of the Ascomycetes, though the term has been applied in these fungi to certain short hyphal branches from the ascogonium which remain embedded in the tissue. In the Ascomycetes examined all traces of emergent receptive organs, if they ever existed, have now disappeared; in some few there are possible internal survivals which never reach the surface.

In Ascolichens, on the contrary, the “trichogyne,” a septate hyphal branch extending upwards from the ascogonium, and generally reaching the open, has been demonstrated in all the different groups except, as yet, in the Coniocarpineae which have not been investigated. Its presence is a strong point in the argument of those who believe in the Floridean ancestry of the Ascomycetes. It should be clearly borne in mind that Ascolichens are evolved from the Ascomycetes: these latter stand between them and any more remote ancestry.

In the Ascomycetes, there is a recognized progression of development in the form of the sporophore from the closed perithecium of the Pyrenomycetes and possibly through the Hysteriaceae, which are partially closed, to the open ascocarp of the Discomycetes. If the fungal and lichenoid “trichogyne” is homologous with the carpogonial organ in the Florideae, then it must have been retained in all the groups of Ascomycetes as an emergent structure, and as such passed on from them to their lichen derivatives. Has that organ then disappeared from fungi since symbiosis began? There is no trace of it now, except as already stated in Laboulbeniales with which lichens are unconnected.

Were Ascolichens monophyletic in origin, one could more easily suppose that both the fungal and lichen series might have started at some early stage from a common fungal ancestor possessing a well-developed trichogyne which has persisted in lichens, but has been reduced to insignificance in fungi, while fruit development proceeded on parallel lines in both. There is no evidence that such progression has taken place among lichens; the theory of a polyphyletic origin for the different series seems to be unassailable. At the same time, there is no evidence to show in which series symbiosis started first.

It is more reasonable to accept the polyphyletic origin, as outlined above, from forms that had already lost the trichogyne, if they ever really possessed it, and to regard the lichen trichogyne as a new organ developing in lichens in response to some requirement of the deep-seated ascogonium. Its sexual function still awaits satisfactory proof, and it is wiser to withhold judgment as to the service it renders to the developing fruit.

B. Relation of Lichens to Fungi

a. Pyrenocarpineae. In Phycolichens (containing blue-green gonidia) and especially in the gelatinous forms, fructification is nearly always a more or less open apothecium. The general absence of the perithecial type is doubtless due to the gelatinous consistency of the vegetative structure; it is by the aid of moisture that the hymenial elements become turgid enough to secure the ejection of the spores through the narrow ostiole of the perithecium, and this process would be frustrated were the surrounding and enveloping thallus also gelatinous. There is only one minutely foliose or fruticose gelatinous family, the Pyrenidiaceae, in which Pyrenomycetes are established, and the gonidia, even though blue-green, have lost the gelatinous sheath and do not swell up.

In Archilichens (with bright-green gonidia), perithecial fruits occur frequently; they are nearly always simple and solitary; in only a few families with a few representatives, is there any approach to the stroma formation so marked among fungi. The single perithecium is generally semi-immersed in the thallus. It may be completely surrounded by a hyphal “entire” wall, either soft and waxy or dark coloured and somewhat carbonaceous. In numerous species the outer protective wall covers only the upper portion that projects beyond the thallus, and such a perithecium is described as “dimidiate,” a type of fruit occurring in several genera, though rare among fungi.

As to internal structure, there is a dissolution and disappearance of the paraphyses in some genera, their protective function not being so necessary in closed fruits, a character paralleled in fungi. There is a great variety of spore changes, from being minute, simple and colourless, to varied septation, general increase in size, and brown colouration. The different types may be traced to fungal ancestors with somewhat similar spores, but more generally they have developed within the lichen series. From the life of the individual it is possible to follow the course of evolution, and the spores of all species begin as simple, colourless bodies; in some genera they remain so, in others they undergo more or less change before reaching the final stage of colour or septation that marks the mature condition.

As regards direct fungal ancestors, the Pyrenocarpineae, with solitary perithecia, are nearest in fruit structure to the Mycosphaerellaceae, in which family are included several fungus genera that are parasitic on lichens such as Ticothecium, MÜllerella, etc. In that family occurs also the genus Stigmatea, in which the perithecia in form and structure are very similar to dimidiate Verrucariae.

Zahlbruckner[982] has suggested as the starting point for the Verrucariaceae the fungus genus Verrucula. It was established by Steiner[983] to include two species, V. cahirensis and V. aegyptica, their perithecia being exactly similar to those of Verrucaria[984] in which genus they were originally placed. Both are parasitic on species of Caloplaca (Placodium). The former, on C. gilvella, transforms the host thallus to the appearance of a minutely lobed Placodium; the latter occupies an island-like area in the centre of the thallus of Caloplaca interveniens, and gives it, with its accompanying parasite, the character of an Endopyrenium (Dermatocarpon), while the rest of the thallus is normal and fertile.

Zahlbruckner may have argued rightly, but it is also possible to regard these rare desert species as reversions from an originally symbiotic to a purely parasitic condition. Reinke came to the conclusion that if a parasitic species were derived directly from a lichen type, then it must still rank as a lichen, a view that has a direct bearing on the question. The parallel family of Pyrenulaceae which have Trentepohlia gonidia is considered by Zahlbruckner to have originated from the fungus genus Didymella.

Compound or stromatoid fructifications occur once and again in lichen families; but, according to Wainio[985], there is no true stroma formation, only a pseudostroma resulting from adhesions and agglomerations of the thalline envelopes or from cohesions of the margins of developing fruit bodies. These pseudostromata are present in the genera Chiodecton and Glyphis (Graphidineae) and in Trypethelium, Mycoporium, etc. (Pyrenocarpineae). This view of the nature of the compound fruits is strengthened, as Wainio points out, by the presence in certain species of single apothecia or perithecia on the same specimen as the stromatoid fruits.

b. Coniocarpineae. This subseries is entirely isolated. Its peculiarity lies in the character of the mature fruit in which the spores, owing to the early breaking down of the asci, lie as a loose mass in the hymenium, while dispersal is delayed for an indefinite time. This type of fruit, termed a mazaedium by Acharius, is in the form of a stalked or sessile roundish head—the capitulum—closed at first and only half-open at maturity rarely, as in Cyphelium, an exposed disc. There is a suggestion, but only a suggestion, of a similar fructification in the tropical fungus Camillea in which there is sometimes a stalk with one or more perithecia at the tip, and in some species early disintegration of the asci, leaving spore masses[986]. But neither in fungi nor in other lichens is there any obvious connection with Coniocarpineae. In some of the genera the fungus alone forms the stalk and the wall of the capitulum; in others the thallus shares in the fruit-formation growing around it as an amphithecium.

The semi-closed fruits point to their affinity with Pyrenolichens, though they are more advanced than these judging from the thalline wall that is present in some genera and also from the half-open disc at maturity. The latter feature has influenced some systematists to classify the whole subseries among Cyclocarpineae. The thallus, as in Sphaerophorus, reaches a high degree of fruticose development; in other genera it is crustaceous without any formation of cortex, while in several genera or species it is non-existent, the fruits being parasites on the thalli of other lichens or saprophytes on dead wood, humus, etc. These latter—both parasites and saprophytes—are included by Rehm[987] and others among fungi, which has involved the breaking up of this very distinctive series. Rehm has thus published as Discomycetes the lichen genera Sphinctrina, Cyphelium, Coniocybe, Acolium, Calicium and Stenocybe, since some or all of their species are regarded by him as fungi.

Reinke[988] in his lichen studies states that it might not be impossible for a saprophytic fungus to be derived from a crustaceous lichen—a case of reversion—but that no such instance was then known. More exact studies[989] of parasymbiosis and antagonistic symbiosis have shown the wide range of possible life-conditions, and such a reversion does not seem improbable. We must also bear in mind that in suitable cultures, lichen hyphae can be grown without gonidia: they develop in that case as saprophytes.

On Reinke’s[988] view, however, that these saprophytic species, belonging to different genera in the Coniocarpineae, are true fungi, they would represent the direct and closely related ancestors of the corresponding lichen genera, giving a polyphyletic origin within this group. As fungus genera he has united them in Protocaliciaceae, and the representatives among fungi he distinguishes, as does Wainio[990], under such names as Mycocalicium and Mycoconiocybe.

If we might consider the saprophytic forms as also retrogressive lichens, a monophyletic origin from some remote fungal ancestor would prove a more satisfactory solution of the inheritance problem. This view is even supported by a comparison Reinke himself has drawn between the development of the fructification in Mycocalicium parietinum, a saprophyte, and in his view a fungus, and Chaenotheca chrysocephala, a closely allied lichen. Both grow on old timber. In the former (the fungus), the mycelium pervades the outer weathered wood-cells, and the fruit stalk rises from a clump of brownish hyphae; there is no trace of gonidia. Chaenotheca chrysocephala differs in the presence of gonidia which are associated with the mycelium in scattered granular warts; but the fruit stalk here also rises directly from the mycelium between the granules. The presence of a lichen thallus chiefly differentiates between the two plants, and this thallus is not a casual or recent association; it is constant and of great antiquity as it is richly provided with lichen-acids.

Reinke has indicated the course of evolution within the series but that is on the lines of thalline development and will be considered later.

c. Graphidineae. This series contains a considerable variety of lichen forms, but all possess to a more or less marked degree the linear form of fructification termed a “lirella” which has only a slit-like opening. There is a tendency to round discoid fruits in the Roccellae and also in the Arthoniae; the apothecia of the latter, called by early lichenologists “ardellae,” are without margins. In nearly all there is a formation of carbonaceous black tissue either in the hypothecium or in the proper margins. In some of them the paraphyses are branched and dark at the tips, the branches interlocking to form a strong protective epithecium. There are, however, constant exceptions, in some particular, to any generalization in genera and in species. MÜller-Argau’s[991] pronouncement might be held to have special reference to Graphidineae: “that in any genus, species or groups of species are to be found which outwardly shew something that is peculiar, though of slight importance.” The most constant type of gonidium is Trentepohlia, but Palmella and Phycopeltis occasionally occur. The spores are various in colour and form; they are rarely simple.

The genus Arthonia is derived from a member of the Patellariaceae, from which family many of the Discomycetes have arisen. The course of development does not follow from a closed to an open fruit; the apothecium is open from the first, and growth proceeds from the centre outwards, the fertile cells gradually pushing aside the sterile tissue of the exterior. The affinity of Xylographa (with Palmella gonidia) is to be found in Stictis in the fungal family Stictidaceae, the apothecia of Stictis being at first closed, then open, and with a thick margin; Xylographa has a more elongate lirella fruit, though otherwise very similar, and has a very reduced thallus. Rehm[992] has classified Xylographa as a fungus.

The genera with linear apothecia are closely connected with Hysteriaceae, and evidently inherit their fruit form severally from that family. There is thus ample evidence of polyphyletic descent in the series. Stromatoid fruits occur in Chiodectonaceae, with deeply sunk, almost closed disc, but they have evidently evolved within the series, possibly from a dividing up of the lirellae.

In Graphidineae there are also forms, more especially in Arthoniaceae, on the border line between lichens and fungi: those with gonidia being classified as lichens, those without gonidia having been placed in corresponding genera of fungi. These latter athalline species live as parasites or saprophytes.

The larger number of genera have a poorly developed thallus; in many of them it is embedded within the outer periderm-cells of trees, and is known as “hypophloeodal.” But in some families, such as Roccellaceae, the thallus attains a very advanced form and a very high production of acids.

The conception of Graphidineae as a whole is puzzling, but one or other characteristic has brought the various members within the series. It is in this respect an epitome of the lichen class of which the different groups, with all their various origins and affinities, yet form a distinct and well-defined section of the vegetable kingdom.

d. Cyclocarpineae. This is by far the largest series of lichens. The genera are associated with algae belonging both to the Myxophyceae and the Chlorophyceae, and from the many different combinations are produced great variations in the form of the vegetative body. The fruit is an emergent, round or roundish disc or open apothecium in all the members of the series except Pertusariaceae, where it is partially immersed in thalline “warts.” In its most primitive form, described as “biatorine” or “lecideine,” it may be soft and waxy (Biatora) or hard and carbonaceous (Lecidea), in the latter the paraphyses being mostly coloured at the tips; these are either simple or but sparingly branched, so that the epithecium is a comparatively slight structure. The outer sterile tissue forms a protective wall or “proper margin” which may be entirely pushed aside, but generally persists as a distinct rim round the disc.

A great advance within the series arose when the gonidial elements of the thallus took part in fruit-formation. In that case not only is the hymenium generally subtended by a layer of algae, but thalline tissue containing algae grows up around the fruit, and forms a second wall or thalline margin. This type of apothecium, termed “lecanorine,” is thus intimately associated with the assimilating tissue and food supply, and it gains in capacity of ascus renewal and of long duration. This development from non-marginate to marginate ascomata is necessarily an accompaniment of symbiosis.

There is no doubt that the Cyclocarpineae derive from some simple form or forms of Discomycete in the Patellariaceae. The relationship between that family and the lower Lecideae is very close. Rehm[993] finds the direct ancestors of Lecidea itself in the fungus genus, Patinella, in which the apothecia are truly lecideine in character—open, flat and slightly margined, the hypothecium nearly always dark-coloured and the paraphyses branched, septate, clavate and coloured at the tips, forming a dark epithecium. More definitely still he describes Patinella atroviridis, a new species he discovered, as in all respects a Lecidea, but without gonidia.

In the crustaceous Lecideaceae, a number of genera have been delimited on spore characters—colourless or brown, and simple or variously septate. In Patellariaceae as described by Rehm are included a number of fungus genera which correspond to these lichen genera. Only two of them—Patinella and Patellaria—are saprophytic; in all the other genera of the family, the species with very few exceptions are parasitic on lichens: they are parasymbionts sharing the algal food supply; in any case, they thrive on a symbiotic thallus.

Rehm unhesitatingly derives the corresponding lichen genera from these fungi. He takes no account of the difficulty that if these parasitic (or saprophytic) fungi are primitive, they have yet appeared either later in time than the lichens on which they exist, or else in the course of ages they have entirely changed their substratum.

He has traced, for instance, the lichen, Buellia, to a saprophytic fungus species, Karschia lignyota, to a genus therefore in which most of the species are parasitic on lichens and have generally been classified as parasitic lichens. There is no advance in apothecial characters from the fungus, Karschia, to Buellia, merely the change to symbiosis. It therefore seems more in accordance with facts to regard Buellia as a genus evolved within the lichen series from Patinella through Lecidea, and to accept these species of Karschia on the border line as parasitic, or even as saprophytic, reversions from the lichen status. We may add that while these brown-spored lichens are fairly abundant, the corresponding athalline or fungus forms are comparatively few in number, which is exactly what might be expected from plants with a reversionary history.

Occasionally in biatorine or lecideine species with a slight thalline development all traces of the thallus disappear after the fructification has reached maturity. The apothecia, if on wood or humus, appear to be saprophytic and would at first sight be classified as fungi. They have undoubtedly retained the capacity to live at certain stages, or in certain conditions, as saprophytes.

The thallus disappears also in some species of the crustaceous genera that possess apothecia with a thalline margin, and the fruits may be left stranded and solitary on the normal substratum, or on some neighbouring lichen thallus where they are more or less parasitic; but as the thalline margin persists, there has been no question as to their nature and affinity.

Rehm suggests that many species now included among lichens may be ultimately proved to be fungi; but it is equally possible that the reverse may be the case, as for instance Bacidia flavovirescens, held by Rehm and others to be a parasitic fungus species, but since proved by Tobler[994] to be a true lichen.

A note by Lightfoot[995], one of our old-time botanists who gave lichens a considerable place in his Flora, foreshadows the theory of evolution by gradual advance, and his views offer a suggestive commentary on the subject under discussion. He was debating the systematic position of the maritime lichen genus Lichina, considered then a kind of Fucus, and had observed its similarity with true lichens. “The cavity,” he writes, “at the top of the fructification (in Lichina) is a proof how nearly this species of Fucus is related to the scutellated lichens. Nature disdains to be limited to the systematic rules of human invention. She never makes any sudden starts from one class or genus to another, but is regularly progressive in all her works, uniting the various links in the chain of beings by insensible connexions.”

III. THE THALLUS

A. General Outline of Development

a. Preliminary Considerations. The evolution of lichens, as such, has reference mainly to the thallus. Certain developments of the fructification are evident, but the changes in the reproductive organs have not kept pace with those of the vegetative structures: the highest type of fruit, for instance, the apothecium with a thalline margin, occurs in genera and species with a very primitive vegetative structure as well as in those that have attained higher development.

Lichens are polyphyletic as regards their algal, as well as their fungal, ancestors, so that it is impossible to indicate a straight line of progression, but there is a general process of thalline development which appears once and again in the different phyla. That process, from simpler to more complicated forms, follows on two lines: on the one there is the endeavour to increase the assimilating surface, on the other the tendency to free the plant from the substratum. In both, the aim has been the same, to secure more favourable conditions for assimilation and aeration. Changes in structure have been already described[996], and it is only needful to indicate here the main lines of evolution.

b. Course of Evolution in Hymenolichens. There is but little trace of development in these lichens. The fungus has retained more or less the form of the ancestral Thelephora which has a wide-spreading superficial basidiosporous hymenium. Three genera have been recognized, the differences between them being due to the position within the thallus, and the form of the Scytonema that constitutes the gonidium. The highest stage of development and of outward form is reached in Cora, in which the gonidial zone is central in the tissue and is bounded above and below by strata of hyphae.

c. Course of Evolution in Ascolichens. It is in the association with Ascomycetes that evolution and adaptation have had full scope. In that sub-class there are four constantly recurring and well-marked stages of thalline development. (1) The earliest, most primitive stage, is the crustaceous: at first an accretion of separate granules which may finally be united into a continuous crust with a protective covering of thick-walled amorphous hyphae forming a “decomposed” cortex. The extension of a granule by growth in one direction upwards and outwards gives detachment from the substratum, and originates (2) the squamule which is, however, often of primitive structure and attached to the support, like the granule, by the medullary hyphae. Further growth of the squamule results in (3) the foliose thallus with all the adaptations of structure peculiar to that form. In all of these, the principal area of growth is round the free edges of the thallus. A greater change takes place in the advance to (4) the fruticose type in which the more active growing tissue is restricted to the apex, and in which the frond or filament adheres at one point only to the support, a new series of strengthening and other structures being evolved at the same time.

The lichen fungi associate, as has been already stated, with two different types of algae: those combined with the Myxophyceae have been designated Phycolichenes, those with Chlorophyceae as Archilichenes. The latter predominate, not only in the number of lichens, but also in the more varied advance of the thallus, although, in many instances, genera and species of both series may be closely related.

B. Comparative Antiquity of Algal Symbionts

One of the first questions of inheritance concerns the comparative antiquity of the two gonidial series: with which kind of alga did the fungus first form the symbiotic relationship? No assistance in solving the problem is afforded by the type of fructification. The fungus in Archilichens is frequently one of the more primitive Pyrenomycetes, though more often a Discomycete, while in Phycolichens Pyrenomycetes are very rare. There is, as already stated, no correlation of advance between the fruit and the thallus, as the most highly evolved apothecia with well-formed thalline margins are constantly combined with thalli of low type.

Forssell[997] gave considerable attention to the question of antiquity in his study of gelatinous crustaceous lichens in the family Pyrenopsidaceae, termed by him Gloeolichens, and he came to the conclusion that Archilichens represented the older combination, Phycolichens being comparatively young.

His view is based on a study of the development of certain lichen fungi that seem able to adapt themselves to either kind of algal symbiont. He found[997] in Euopsis (Pyrenopsis) granatina, one of the Pyrenopsidaceae, that certain portions of the thallus contained blue-green algae, while others contained Palmella, and that these latter, though retrograde in development, might become fertile. The granules with blue-green gonidia were stronger, more healthy and capable of displacing those with Palmella, but not of bearing apothecia, though spermogonia were embedded in them—a first step, according to Forssell, towards the formation of apothecia. These granules, not having reached a fruiting stage, were reckoned to be of a more recent type than those associated with Palmella. In other instances, however, the line of evolution has been undoubtedly from blue-green to more highly evolved bright-green thalli.

The striking case of similarity between Psoroma hypnorum (bright-green) and Pannaria rubiginosa (blue-green) may also be adduced. Forssell considers that Psoroma is the more ancient form, but as the fungus is adapted to associate with either kind of alga, the type of squamules forming the thallus may be gradually transformed by the substitution of blue-green for the earlier bright-green—the Pannaria superseding the Psoroma. There is a close resemblance in the fructification—that is of the fungus—in these two different lichens.

Hue[998] shares Forssell’s opinion as to the greater antiquity of the bright-green gonidia and cites the case of Solorina crocea. In that lichen there is a layer of bright-green gonidia in the usual dorsiventral position, below the upper cortex. Below this zone there is a second formed entirely of blue-green cells. Hue proved by his study of development in Solorina that the bright-green were the normal gonidia of the thallus, and were the only ones present in the growing peripheral areas; the blue-green were a later addition, and appeared first in small groups at some distance from the edge of the lobes.

The whole subject of cephalodia-development[999] has a bearing on this question. These bodies always contain blue-green algae, and are always associated with Archilichens. Mostly they occur as excrescences, as in Stereocaulon and in Peltigera. The fungus of the host-lichen though normally adapted to bright-green algae has the added capacity of forming later a symbiosis with the blue-green. This tendency generally pervades a whole genus or family, the members of which, as in Peltigeraceae, are too closely related to allow as a rule of separate classification even when the algae are totally distinct.

C. Evolution of Phycolichens

The association of lichen-forming fungi with blue-green algae may have taken place later in time, or may have been less successful than with the bright-green: they are fewer in number, and the blue-green type of thallus is less highly evolved, though examples of very considerable development are to be found in such genera as Peltigera, Sticta or Nephromium.

a. Gloeolichens. Among crustaceous forms the thallus is generally elementary, more especially in the Gloeolichens (Pyrenopsidaceae). The algae of that family, Gloeocapsa, Xanthocapsa or Chroococcus, are furnished with broad gelatinous sheaths which, in the lichenoid state, are penetrated and traversed by the fungal filaments, a branch hypha generally touching with its tip the algal cell-wall. Under the influence of symbiosis, the algal masses become firmer and more compact, without much alteration in form; algae entirely free from hyphae are often intermingled with the others. Even among Gloeolichens there are signs of advancing development both in the internal structure and in outward form. Lobes free from the substratum, though very minute, appear in the genus Paulia, the single species of which comes from Polynesia. Much larger lobes are characteristic of Thyrea, a Mediterranean and American genus. The fruticose type, with upright fronds of minute size, also appears in our native genus Synalissa. It is still more marked in the coralloid thalli of Peccania and Phleopeccania. In most of these genera there is also a distinct tendency to differentiation of tissues, with the gonidia congregating towards the better lighted surfaces. The only cortex formation occurs in the crustaceous genus Forssellia in which, according to Zahlbruckner[1000], it is plectenchymatous above, the thallus being attached below by hyphae penetrating the substratum. In another genus, Anema[1001], which is minutely lobate-crustaceous, the internal hyphae form a cellular network in which the algae are immeshed. As regards algal symbionts, the members of this family are polyphyletic in origin.

b. Ephebaceae and Collemaceae. In Ephebaceae the algae are tufted and filamentous, Scytonema, Stigonema or Rivularia, the trichomes of which are surrounded by a common gelatinous sheath. The hyphae travel in the sheath alongside the cell-rows, and the symbiotic plant retains the tufted form of the alga as in Lichina with Rivularia, Leptogidium with Scytonema, and Ephebe with Stigonema. The last named lichen forms a tangle of intricate branching filaments about an inch or more in length. The fruticose habit in these plants is an algal characteristic; it has not been acquired as a result of symbiosis, and does not signify any advance in evolution.

A plectenchymatous cortex marks some progress here also in Leptodendriscum, Leptogidium and Polychidium, all of which are associated with Scytonema. These genera may well be derived from an elementary form such as Thermutis. They differ from each other in spore characters, etc., Polychidium being the most highly developed with its cortex of two cell-rows and with two-celled spores.

Nostoc forms the gonidium of Collemaceae. In its free state it is extremely gelatinous and transmits that character more or less to the lichen. In the crustaceous genus Physma, which forms the base of the Collema group or phylum, there is but little difference in form between the thalline warts of the lichen crust and the original small Nostoc colonies such as are to be found on damp mosses, etc.

In Collema itself, the less advanced species are scarcely more than crusts, though the more developed show considerable diversity of lobes, either short and pulpy, or spreading out in a thin membrane. The Nostoc chains pervade the homoiomerous thallus, but in some species they lie more towards the upper surface. There is no cortex, though once and again plectenchyma appears in the apothecial margin, both in this genus and in Leprocollema which is purely crustaceous.

Leptogium is a higher type than Collema, the thallus being distinguished by its cellular cortex. The tips of the hyphae, lying close together at the surface, are cut off by one or more septa, giving a one- or several-celled cortical layer. The species though generally homoiomerous are of thinner texture and are less gelatinous than those of Collema.

c. Pyrenidiaceae. This small family of pyrenocarpous Phycolichens may be considered here though its affinity, through the form of the fruiting body, is with Archilichens. The gonidia are species of Nostoc, Scytonema and Stigonema. There are only five genera; one of these, Eolichen, contains three species, the others are monotypic.

The crustaceous genera have a non-corticate thallus, but an advance to lobate form takes place in Placothelium, an African genus. The two genera that show most development are both British: Coriscium (Normandina), which is lobate, heteromerous and corticate—though always sterile—and Pyrenidium which is fruticose in habit; the latter is associated with Nostoc and forms a minute sward of upright fronds, corticate all round; the perithecium is provided with an entire wall and is immersed in the thallus.

If the thallus alone were under consideration these lichens would rank with Pannariaceae.

d. Heppiaceae and Pannariaceae. The next stage in the development of Phycolichens takes place through the algae, Scytonema and Nostoc, losing not only their gelatinous sheaths, but also, to a large extent, their characteristic forms. Chains of cells can frequently be observed, but accurate and certain identification of the algal genus is only possible by making separate cultures of the gonidia.

Scytonema forms the gonidium of the squamulose Heppiaceae consisting of the single genus Heppia. The ground tissue of the species is either wholly of plectenchyma with algae in the interstices, or the centre is occupied by a narrow medulla of loose filaments.

In the allied family Pannariaceae, a number of genera contain Scytonema or Nostoc, while two, Psoroma and Psoromaria, have bright-green gonidia. The thallus varies from crustaceous or minutely squamulose, to lobes of fair dimension in Parmeliella and in Hydrothyria venosa, an aquatic lichen. Plectenchyma appears in the upper cortex of both of these, and in the proper margin of the apothecia, while the under surface is frequently provided with rhizoidal filaments.

These two families form a transition between the gelatinous, and mostly homoiomerous thallus, and the more developed entirely heteromerous thallus of much more advanced structure. The fructification in all of them, gelatinous and non-gelatinous, is a more or less open apothecium, sometimes immarginate, and biatorine or lecideine, but often, even in species nearly related to these, it is lecanorine with a thalline amphithecium. Rarely are the sporiferous bodies sunk in the tissue, with a pseudo-perithecium, as in Phylliscum. It would be difficult to trace advance in all this group on the lines of fruit development. The two genera with bright-green gonidia, Psoroma and Psoromaria, have been included in Pannariaceae owing to the very close affinity of Psoroma hypnorum with Pannaria rubiginosa; they are alike in every respect except in their gonidia. Psoromaria is exactly like Psoroma, but with immarginate biatorine apothecia, representing therefore a lower development in that respect.

These lichens not only mark the transition from gelatinous to non-gelatinous forms, but in some of them there is an interchange of gonidia. The progression in the phylum or phyla has evidently been from blue-green up to some highly evolved forms with bright-green algae, though there may have been, at the beginning, a substitution of blue-green in place of earlier bright-green algae, Phycolichens usurping as it were the Archilichen condition.

e. Peltigeraceae and Stictaceae. The two families just examined marked a great advance which culminated in the lobate aquatic lichen Hydrothyria. This lichen, as Sturgis pointed out, shows affinity with other Pannariaceae in the structure of the single large-celled cortical layer as well as with species of Nephroma (Peltigeraceae). A still closer affinity may be traced with Peltigera in the presence in both plants of veins on the under surface. The capacity of Peltigera species to grow in damp situations may also be inherited from a form like the submerged Hydrothyria. In both families there are transitions from blue-green to bright-green gonidia, or vice versa, in related species. Thus in Peltigeraceae we find Peltigera containing Nostoc in the gonidial zone, with Peltidea which may be regarded as a separate genus, or more naturally as a section of Peltigera; it contains bright-green gonidia, but has cephalodia containing Nostoc associated with its thallus.

The genus Nephroma is similarly divided into species with a bright-green gonidial zone, chiefly Arctic or Antarctic in distribution, and species with Nostoc (subgenus Nephromium) more numerous and more widely distributed.

Peltigera and Nephroma are also closely related in the character of the fructification. It is a flat non-marginate disc borne on the edge of the thallus: in Peltigera on the upper surface, in Nephroma on the under surface. The remaining genus Solorina contains normally a layer of bright-green algae, but, along with these, there are always present more or fewer Nostoc cells, either in a thin layer as in S. crocea or as cephalodia in others, while, in three species the algae are altogether blue-green.

The members of the Peltigeraceae have a thick upper cortex of plectenchyma and in some cases strengthening veins, and long rhizinae on the lower side. Some of the species attain a large size, and, in some, soredia are formed, an evidence of advance, this being a peculiarly lichenoid form of reproduction.

The Stictaceae form a parallel but more highly organized family, which also includes closely related bright-green and blue-green series. They are all dorsiventral, but they are mostly attached by a single hold-fast and the lobes in some species suggest the fruticose type in their long narrow form. A wide cortex of plectenchyma protects both the upper and the lower surface and a felt of hairs replaces the rhizinae of other foliose lichens. In the genus Sticta (including the section Stictina) special aeration organs, cyphellae or pseudocyphellae, are provided; in Lobaria these are replaced by naked areas which serve the same purpose.

Nylander[1002] regarded the Stictaceae as the most highly developed of all lichens, and they easily take a high place among dorsiventral forms, but it is generally conceded that the fruticose type is the more highly organized. In any case they are the highest reach of the phylum or phyla that started with Pyrenopsidaceae and Collemaceae; the lowly gelatinous thalli changing to more elaborate structures with the abandonment of the gelatinous algal sheath, as in the Pannariaceae, and with the replacement of blue-green by bright-green gonidia. Reinke[1003], considers the Stictaceae as evolved from the Pannariaceae more directly from the genus Massalongia. Their relationship is certainly with Pannariaceae and Peltigeraceae rather than with Parmeliaceae; these latter, as we shall see, belong to a wholly different series.

D. Evolution of Archilichens

The study of Archilichens as of Phycolichens is complicated by the many different kinds of fungi and algae that have entered into combination; but the two principal types of algae are the single-celled Protococcus group and the filamentous Trentepohlia: as before only the broad lines of thalline development will be traced.

The elementary forms in the different series are of the simplest type—a somewhat fortuitous association of alga and fungus, which in time bears the lichen fructification. It has been stated that the greatest advance of all took place with the formation of a cortex over the primitive granule, followed by a restricted area of growth outward or upward which resulted finally in the foliose and fruticose thalli. Guidance in following the course of evolution is afforded by the character of the fructification, which generally shows some great similarity of type throughout the different phyla, and remains fairly constant during the many changes of thalline evolution. Development starting from one or many origins advances point by point in a series of parallel lines.

a. Thallus of Pyrenocarpineae. In this series there are two families of algae that function as gonidia: Protococcaceae, consisting of single cells, and Trentepohliaceae, filamentous. Phyllactidium (Cephaleuros) appears in a single genus, Strigula, a tropical epiphytic lichen.

Associated with these types of algae are a large number of genera and species of an elementary character, without any differentiation of tissue. In many instances the thallus is partly or wholly embedded in the substratum.

Squamulose or foliose forms make their appearance in Dermatocarpaceae: in Normandina the delicate shell-like squamules are non-corticate, but in other genera, Endocarpon, Placidiopsis, etc., the squamules are corticate and of firmer texture, while in Dermatocarpon, foliose fronds of considerable size are formed. The perithecial fruits are embedded in the upper surface.

In only one extremely rare lichen, Pyrenothamnia Spraguei (N. America), is there fruticose development: the thallus, round and stalk-like at the base, branches above into broader more leaf-like expansions.

b. Thallus of Coniocarpineae. At the base of this series are genera and species that are extremely elementary as regards thalline formation, with others that are saprophytic and parasitic. The simplest type of thallus occurs in Caliciaceae, a spreading mycelium with associated algae (Protococcaceae) collected in small scattered granules, resembling somewhat a collection of loose soredia. The species grow mostly on old wood, trunks of trees, etc. In Calicium (Chaenotheca) chrysocephalum as described by Neubner[1004] the first thallus formation begins with these scattered minute granules; gradually they increase in size and number till a thick granular coating of the substratum arises, but no cortex is formed and there is no differentiation of tissue.

The genus Cyphelium (Cypheliaceae) is considered by Reinke to be more highly developed, inasmuch as the thalline granules, though non-corticate, are more extended horizontally, and, in vertical section, show a distinct differentiation into gonidial zone and medulla. The sessile fruit also takes origin from the thallus, and is surrounded by a thalline amphithecium, or rather it remains embedded in the thalline granule. A closely allied tropical genus Pyrgillus has reached a somewhat similar stage of development, but with a more coherent homogeneous thallus, while in Tylophoron, also tropical or subtropical, the fruit is raised above the crustaceous thallus but is thickly surrounded by a thalline margin. The alga of that genus is Trentepohlia, a rare constituent of Coniocarpineae.

A much more advanced formation appears in the remaining family Sphaerophoraceae. In Calycidium, a monotypic New Zealand genus, the thallus consists of minute squamules, dorsiventral in structure but with a tendency to vertical growth, the upper surface is corticate and the mazaedial apothecia—always open—are situated on the margins. Tholurna dissimilis, (Scandinavian) still more highly developed, has two kinds of rather small fronds corticate on both surfaces, the one horizontal in growth, crenulate in outline, and sterile, the other vertical, about 2 mm. in height, hollow and terminating in a papilla in which is seated the apothecium.

Two other monotypic subtropical genera form a connecting link with the more highly evolved forms. In the first, Acroscyphus sphaerophoroides, the fronds are somewhat similar to the fertile ones of Tholurna, but they possess a solid central strand and the apical mazaedium is less enveloped by the thallus. The other, Pleurocybe madagascarea, has narrow flattish branching fronds about 3 cm. in height, hollow in the centre and corticate with marginal or surface fruits.

The third genus, Sphaerophorus, is cosmopolitan; three of the species are British and are fairly common on moorlands, etc. They are fruticose in habit, being composed of congregate upright branching stalks, either round or slightly compressed and varying in height from about 1 to 8 cm. The structure is radiate with a well-developed outer cortex, and a central strand which gives strength to the somewhat slender stalks. The fruits are lodged in the swollen tips and are at first enclosed; later, the covering thallus splits irregularly and exposes the hymenium.

Coniocarpineae comprise only a comparatively small number of genera and species, but the series is of unusual interest as being extremely well defined by the fruit-formation and as representing all the various stages of thalline development from the primitive crustaceous to the highly evolved fruticose type. With the primitive thallus is associated a wholly fungal fruit, both stalk and capitulum, which in the higher forms is surrounded and protected by the thallus. Lichen-acids are freely produced even in crustaceous forms, and they, along with the high stage of development reached, testify to the great antiquity of the series.

c. Thallus of Graphidineae. As formerly understood, this series included only crustaceous forms with an extremely simple development of thallus, fungi and algae—whether Palmellaceae, etc., or more frequently Trentepohliaceae—growing side by side either superficially or embedded in tree or rock, the presence of the vegetative body being often signalled only by a deeper colouration of the substratum. The researches of Almquist, and more recently of Reinke and Darbishire, have enlarged our conception of the series, and the families Dirinaceae and Roccellaceae are now classified in Graphidineae.

Arthoniaceae, Graphidaceae and Chiodectonaceae are all wholly crustaceous. The first thalline advance takes place in Dirinaceae with two allied genera, Dirina and Dirinastrum. Though the thallus is still crustaceous, it is of considerable thickness, with differentiation of tissues: on the lower side there is a loosely filamentous medulla from which hyphae pierce the substratum and secure attachment. Trentepohlia gonidia lie in a zone above the medulla, and the upper cortex is formed of regular palisade hyphae forming a “fastigiate cortex.” It is the constant presence of Trentepohlia algae as well as the tendency to ellipsoid or lirellate fruits that have influenced the inclusion of Dirinaceae and Roccellaceae in the series.

The thallus of Dirinaceae is crustaceous, while the genera of Roccellaceae are mostly of an advanced fruticose type, though in one, Roccellina, there is a crustaceous thallus with an upright portion consisting of short swollen podetia-like structures with apothecia at the tips; and in another, Roccellographa, the fronds broaden to leafy expansions. They are nearly all rock-dwellers, often inhabiting wind-swept maritime coasts, and a strong basal sheath has been evolved to strengthen their foothold. In some genera the sheath contains gonidia; in others the tissue is wholly of hyphae—in nearly every case it is protected by a cortex.

In the upright fronds the structure is radiate: generally a rather loose strand of hyphae more or less parallel with the long axis of the plant forms a central medulla. The gonidia lie outside the medulla and just within the outer cortex. The latter, in a few genera, is fibrous, the parallel hyphae being very closely compacted; but in most members of the family the fastigiate type prevails, as in the allied family Dirinaceae.

d. Thallus of Cyclocarpineae. This is by far the largest and most varied series of Archilichens. It is derived, as regards the fungal constituent, from the Discomycetes, but in these fungi, the vegetative or mycelial body gives no aid to the classification which depends wholly on apothecial characters. In the symbiotic condition, on the contrary, the thallus becomes of extreme importance in the determination of families, genera and species. There has been within the series a great development both of apothecial and of thalline characters in parallel lines or phyla.

AA. Lecideales. The type of fruit nearest to fungi in form and origin occurs in the Lecideales. It is an open disc developed from the fungal symbiont alone, the alga taking no part. There are several phyla to be considered.

aa. Coenogoniaceae. There are two types of gonidial algae in this family, and both are filamentous forms, Trentepohlia in Coenogonium and Cladophora in Racodium. The resulting lichens retain the slender thread-like form of the algae, their cells being thinly invested by the hyphae and both symbionts growing apically. The thalline filaments are generally very sparingly branched and grow radially side by side in a loose flat expansion attached at one side by a sheath, or the strands spread irregularly over the substratum. Plectenchyma appears in the apothecial margin in Coenogonium. Fruiting bodies are unknown in Racodium.

Coenogoniaceae are a group apart and of slight development, only the one kind of thallus appearing; the form is moulded on that of the gonidium, and is, as Reinke[1005] remarks, perfectly adapted to receive the maximum of illumination and aeration.

bb. Lecideaceae and Gyrophoraceae. The origin of this thalline phylum is distinct from that of the previous family, being associated with a different type of gonidium, the single-celled alga of the Protococcaceae.

The more elementary species are of extremely simple structure as exemplified in such species as Lecidea (Biatora) uliginosa or Lecidea granulosa. These lichens grow on humus-soil and the thallus consists of a spreading mycelium or hypothallus with more or less scattered thalline granules containing gonidia, but without any defined structure. The first advance takes place in the aggregation and consolidation of such thalline granules and the massing of the gonidia towards the light, thus substituting the heteromerous for the homoiomerous arrangement of the tissues. The various characters of thickness, areolation, colour, etc. of the thallus are constant and are expressed in specific diagnoses. Frequently an amorphous cortex of swollen hyphae provides a smooth upper surface and forms a protective covering for such long-lived species as Rhizocarpon geographicum, etc.

The squamulose thallus is well represented in this phylum. The squamules vary in size and texture but are mostly rather thick and stiff. In Lecidea ostreata they rise from the substratum in serried rows forming a dense sward; in L. decipiens, also a British species, the squamules are still larger, and more horizontal in direction; they are thick and firm and the upper cortex is a plectenchyma of cells with swollen walls. Solitary hyphae from the medulla pass downwards into the support.

Changes in spore characters also arise in these different thalline series, as for instance in genera such as Biatorina and Buellia, the one with colourless, the other with brown, two-celled spores. These variations, along with changes in the thallus, are of specific or generic importance following the significance accorded to the various characters.

In one lichen of the series, the monotypic Brazilian genus Sphaerophoropsis stereocauloides, the thallus is described by Wainio[1006] as consisting of minute clavate stalks of interwoven thick-walled hyphae, with gelatinous algae, like Gloeocapsa, interspersed in groups, though with a tendency to congregate towards the outer surface.

The highest development along this line of advance is to be found in the Gyrophoraceae, a family of lichens with a varied foliose character and dark lecideine apothecia. The thallus may be monophyllous and of fairly large dimensions or polyphyllous; it is mostly anchored by a central stout hold-fast and both surfaces are thickly corticate with a layer of plectenchyma; the under surface is mostly bare, but may be densely covered with rhizina-like strands of dark hyphae. They are all northern species and rock-dwellers exposed to severe extremes of illumination and temperature, but well protected by the thick cortex and the dark colouration common to them all.

cc. Cladoniaceae. This last phylum of Lecideales is the most interesting as it is the most complicated. It possesses a primary, generally sterile, thallus which is dorsiventral and crustaceous, squamulose or in some instances almost foliaceous, along with a secondary thallus of upright radiate structure and of very varied form, known as the podetium which bears at the summit the fertile organs.

A double thallus has been suggested in the spreading base, containing gonidia, of some radiate lichens such as Roccella, but the upright portion of such lichens, though analogous, is not homologous with that of Cladoniaceae.

The algal cells of the family belong to the Protococcaceae. Blue-green algae are associated in the cephalodia of Pilophorus and Stereocaulon. The primary thallus is a feature of all the members, though sometimes very slight and very short-lived, as in Stereocaulon or in the section Cladina of the genus Cladonia. Where the primary thallus is most largely developed, the secondary (the podetium) is less prominent.

This secondary thallus originates in two different ways: (1) the primary granule may grow upward, the whole of the tissues taking part in the new development; or (2) the origin may be endogenous and proceed from the hyphae only of the gonidial zone: these push upwards in a compact fascicle, as in the apothecial development of Lecidea, but instead of spreading outward on reaching the surface, they continue to grow in a vertical direction and form the podetium. In origin this is an apothecial stalk, but generally it is clothed with gonidial tissue. The gonidia may travel upwards from the base or they may possibly be wind borne from the open. The podetium thus takes on an assimilative function and is a secondary thallus.

The same type of apothecium is common to all the genera; the spores are colourless and mostly simple, but there are also changes in form and septation not commensurate with thalline advance, as has been already noted. Thus in Gomphillus, with primitive thallus and podetium, the spores are long and narrow with about 100 divisions.

1. Origin of Cladonia. There is no difficulty in deriving Cladoniaceae from Lecidea, or, more exactly, from some crustaceous species of the section Biatora in which the apothecia—as in Cladoniaceae—are waxy and more or less light-coloured and without a thalline margin. In only a very few isolated instances has a thalline margin grown round the Cladonia fruit.

There are ten genera included in the Cladoniaceae, of which five are British. Considerable study has been devoted to the elucidation of developmental problems within the family by various workers, more especially in the large and varied genus Cladonia which is complicated by the presence of the two thalli. The family is monophyletic in origin, though many subordinate phyla appear later.

2. Evolution of the Primary Thallus. At the base of the series we find here also an elementary granular thallus which appears in some species of most of the genera. In Gomphillus, a monospecific British genus, the granules have coalesced into a continuous mucilaginous membrane. In Baeomyces, though mostly crustaceous, there is an advance to the squamulose type in B. placophyllus, and in two Brazilian species described by Wainio, one of which, owing to the form of the fronds, has been placed in a separate genus Heteromyces. The primary thallus becomes almost foliose also in Gymnoderma coccocarpum from the Himalayas, with dorsiventral stratose arrangement of the tissues, but without rhizinae. The greatest diversity is however to be found in Cladonia where granular, squamulose and almost foliose thalli occur. The various tissue formations have already been described[1007].

3. Evolution of the Secondary Thallus. Most of the interest centres round the development and function of the podetium. In several genera the primordium is homologous with that of an apothecium; its elongation to an apothecial stalk is associated with delayed fructification, and though it has taken on the function of the vegetative thallus, the purpose of elongation has doubtless been to secure good light conditions for the fruit, and to facilitate a wide distribution of spores: therefore, not only in development but in function, its chief importance though now assimilative was originally reproductive. The vegetative development of the podetium is correlated with the reduction of the primary thallus which in many species bears little relation in size or persistence to the structure produced from it, as, for instance, in Cladonia rangiferina where the ground thallus is of the scantiest and very soon disappears, while the podetial thallus continues to grow indefinitely and to considerable size.

4. Course of Podetial Development. In Baeomyces the podetial primordium is wholly endogenous in some species, but in others the outer cortical layer of the primary thallus as well as the gonidial hyphae take part in the formation of the new structure which, in that case, is simply a vertical extension of the primary granule. This type of podetium—called by Wainio[1008] a pseudopodetium—also recurs in Pilophorus and in Stereocaulon. To emphasize the distinction of origin it has been proposed to classify these two latter genera in a separate family, but in that case it would be necessary to break up the genus Baeomyces. We may assume that the endogenous origin of the “apothecial stalk” is the more primitive, as it occurs in the most primitive lecideine lichens, whereas a vertical thallus is always an advanced stage of vegetative development.

Podetia are essentially secondary structures, and they are associated both with crustaceous and squamulose primary thalli. If monophyletic in origin their development must have taken place while the primary thallus was still in the crustaceous stage, and the inherited tendency to form podetia must then have persisted through the change to the squamulose type. In species such as Cl. caespiticia the presence of rudimentary podetia along with large squamules suggests a polyphyletic origin, but Wainio’s[1008] opinion is that such instances may show retrogression from an advanced podetial form, and that the evidence inclines to the monophyletic view of their origin.

The hollow centre of the podetium arises in the course of development and is common to nearly all advanced stages of growth. There are however some exceptions: in Glossodium aversum, a soil lichen from New Granada, and the only representative of the genus, a simple or rarely forked stalk about 2 cm. in height rises from a granular or minutely squamulose thallus. The apothecium occupies one side of the flattened and somewhat wider apex. There is no external cortex and the central tissue is of loose hyphae. In Thysanothecium Hookeri, also a monotypic genus from Australia, the podetia are about the same height, but, though round at the base, they broaden upwards into a leaf-like expansion. The central tissue below is of loose hyphae, but compact strands occur above, where the apothecium spreads over the upper side. The under surface is sterile and is traversed by nerve-like strands of hyphae.

5. Variation in Cladonia. It is in this genus that most variation is to be found. Characters of importance and persistence have arisen by which secondary phyla may be traced within the genus: these are mainly (1) the relative development of the horizontal and vertical structures, (2) formation of the scyphus and branching of the podetium, with (3) differences in colour both in the vegetative thallus and in the apothecia.

Wainio has indicated the course of evolution on the following lines: (1) the crustaceous thallus is monophyletic in origin and here as elsewhere precedes the squamulose. The latter he considers to be also monophyletic, though at more than one point the more advanced and larger foliose forms have appeared: (2) the primitive podetium was subulate and unbranched, and the apex was occupied by the apothecium. Both scyphus and branching are later developments indicating progress. They are in both cases associated with fruit-formation—scyphi generally arising from abortive apothecia[1009], branching from aggregate apothecia. In forms such as Cl. fimbriata, where both scyphiferous and subulate sterile podetia are frequent, the latter (subspecies fibula) are retrogressive, and reproduce the ancestral pointed podetium. (3) In subgen. Cenomyce, with a squamulose primary thallus, there is a sharp division into two main phyla characterized by the colour of the apothecia, brown in Ochrophaeae—the colour being due to a pigment—and red in Cocciferae where the colouring substance is a lichen-acid, rhodocladonic acid. In the brown-fruited Ochrophaeae there are again several secondary phyla. Two of these are distinguished primarily by the character of the branching: (a) the Chasmariae in which two or several branches arise from the same level, entailing perforation of the axils (Cl. furcata, Cl. rangiformis, Cl. squamosa, etc.), the scyphi also are perforated. They are further characterized by peltate aggregate apothecia, this grouping of the apothecia according to Wainio being the primary cause of the complex branching, the several fruit stalks growing out as branches. The second group (b), the Clausae, are not perforated and the apothecia are simple and broad-based on the edge of the scyphus (Cl. pyxidata, Cl. fimbriata, etc.), or on the tips of the podetia (Cl. cariosa, Cl. leptophylla, etc.). A third very small group also of Clausae called (c) Foliosae has very large primary squamules and reduced podetia (Cl. foliacea, etc.), while finally (d) the Ochroleucae, none of which is British, have poorly developed squamules and variously formed yellowish podetia with pale-coloured apothecia.

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