cm. The structure is radiate with a well-developed outer cortex, and a

Chapter 185,788 wordsPublic domain

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

---------------+---------------+----------+------------+---------------- Phyla | Crustose |Squamulose| Foliose | Fruticose ---------------+---------------+----------+------------+---------------- | | | | Pyrenolichens | Verrucariaceae| Dermatocarpaceae | | | | | Coniocarpineae | Caliciaceae | Sphaerophoraceae |Sphaerophoraceae | | | | |{Arthoniaceae | | | Graphidineae |{Graphidaceae | | |Roccellaceae |{Dirinaceae | | | Cyclocarpineae | | | | | | | | |{ Lecideaceae Gyrophoraceae | |{ Coenogoniaceae | | Lecideales |{ (filamentous gonidia) | | |{ Cladoniaceae | | |{ (primary and secondary | | |{ thalli) | | | | | | Lecanorales | Lecanoraceae |Parmeliaceae|Usneaceae | | | | Polariloculares| | | | | | | | {Colourless | | | | { spores | Placodium |Xanthoria |Teloschistes {Brown spores |Rinodina, | |Physcia |Physcia | Pyxine | | | (Anaptychia) ---------------+--------------+-----------+------------+----------------

SCHEME OF SUGGESTED PROGRESSION IN LICHEN STRUCTURE

PYRENOCARPINEAE

PYRENOCARPEAE

CONIOCARPINEAE

CONIOCARPEAE

CYCLOCARPINEAE

PHYCOLICHENS (CYANOPHILI)

LECIDEALES LECANORALES POLARILOCULARES -----------^------------- Usneae | Stereocaulon Eucetraria Ramalina | | | Ochropheae Cocciferae | | | | | | | | +--------+--+ | | Evernia Teloschistes Physcia | | | | | sect. Sphaero- | | Cetraria Parmelia Xanthoria Anaptychia phoropsis | | (Platysma) physodes | | | | | | | | | Gyrophoraceae | Cladonia Pilophoron | Parme- | | Physcia | | | | | liaceae | | | | | +---+----+ | +----+ | | +-------+ Baeomyces Heterodea Physcidia {Euplacodium Rinodina | | +---------+ { | | {Sect. Psora Gonophillus {Lecanora sect. { | {Callopisma| { | | { Squamaria {Placodium | | {Sect. Eulecidea Sect. Biatora { | { | {Blastenia | | | { Lecanora Colourless Brown | | | spores spores | | | +------+---------+ | | Sect. Biatora Sect. Biatora | | | | +----------------+--------+---------+--------------------+ Lecidea (Protococcaceae)