Chapter VIII: Systematic (2)

Podetia simple, short, not widening upwards.
Podetial stalks naked.
Primary thallus thin, continuous 1. Gomphillus Nyl.
Primary thallus granular or
squamulose 2. Baeomyces Pers.
Primary thallus foliose.
Podetia superficial 3. *Heteromyces Müll.-Arg.
Podetia marginal 4. *Gymnoderma[1051] Nyl.
Podetial stalks granular, squamulose 5. Pilophorus Th. Fr.
Podetia short, widening upwards.
Podetia simple above, rarely
divided 6. *Glossodium Nyl.
Podetia lobed, leaf-like 7. *Thysanothecium Berk. &
Mont.
Podetia elongate, variously branched, or
scyphous and hollow 8. Cladonia Hill.
Podetia elongate, not scyphous, the stalks solid.
Spores elongate, septate 9. Stereocaulon Schreb.
Spores muriform 10. *Argopsis Th. Fr.

XXXIV. _GYROPHORACEAE_

A small family of foliose lichens allied to Lecideaceae by the character of the fruit—a superficial apothecium in the formation of which the gonidia take no share. There are only three genera, distinguished by differences in spore and other characters. _Dermatiscum_ has light-coloured thallus and fruits; of the two species, one occurs in Central Europe, the other in North America. _Umbilicaria_ and _Gyrophora_ are British; they are dark-coloured rock-lichens and are extremely abundant in Northern regions where they are known as “tripe de roche.” Algal cells Protococcaceae.

_Umbilicaria_, _Dermatiscum_, and some species of _Gyrophora_ are attached to the substratum by a central point. Other species of _Gyrophora_ are rhizinose. In all there is a cortex of plectenchyma above and below. In _Gyrophora_ the thallus may be monophyllous as in _Umbilicaria_, or polyphyllous and with or without rhizinae. New lobes frequently arise from protuberances or warts on the older parts of the thallus. At the periphery, in most species, growth is equal along the margins, in _G. erosa_[1052] the edge is formed of numerous anastomosing lobes with lateral branching, the whole forming a broadly meshed open network. Further back the tissues become continuous owing to the active growth of the lower tissue or hypothallus, which grows out from all sides and meets across the opening. The overlying layers, with gonidia, follow more slowly, but they also in time become continuous, so that the “erose” character persists only near the periphery. This forward growth of the lower thallus occurs in other species, though to a much less marked degree.

There is abundant detritus formation in this family; the outer layers of the cortex are continually being sloughed, the dead tissues lying on the upper surface as a dark gelatinous layer, continuous or in small patches. On the under surface the cast-off cortex gathers into a loose confused mass of dead tissues.

Asci 8-spored.
Spores mostly simple (disc gyrose) 1. Gyrophora Ach.
Spores 1-septate 2. *Dermatiscum Nyl.
Asci 1-2-spored.
Spores muriform 3. Umbilicaria Hoffm.

XXXV. _ACAROSPORACEAE_

Thallus foliose, squamulose or crustaceous, sometimes scarcely developed. Algal cells Protococcaceae.

Into this family Zahlbruckner has gathered the genera in which the asci are many-spored, as he considers that a character of great importance in determining relationship, but he has in doing so overlooked other very great differences. The fruit-bodies are round and completely enclosed in a thalline wall in _Thelocarpon_, which has however no perithecial wall. They have a proper margin only (lecideine) in _Biatorella_, and a thalline margin (lecanorine) in the remaining genera. In _Acarospora_ the apothecia are sunk in the thallus. Stirton’s genus _Cryptothecia_[1053] is allied to _Thelocarpon_ in the fruit-formation, but the basal thallus is well developed and the spores are few in number and variously divided.

Thallus none.
Apothecia (or perithecia) in thalline
warts 1. Thelocarpon Nyl.
Thallus crustaceous.
Apothecia lecideine; spores simple 2. Biatorella Th. Fr.
Apothecia lecanorine; spores septate 3. *Maronea Massal.
Thallus of small squamules 4. Acarospora Massal.
Thallus almost foliose, attached
centrally 5. *Glypholecia Nyl.

XXXVI. _EPHEBACEAE_

A family of very simple structure either filamentous, foliose or crustaceous. The algal cells which give a dark colour to the thallus are _Stigonema_ or _Scytonema_, members of the blue-green Myxophyceae, and consist of minute simple or branched filaments—single cell-rows in _Scytonema_, compound in _Stigonema_.

In some of the genera the lichen hyphae travel within the gelatinous sheath of the filaments, both algae and hyphae increasing by apical growth so that filaments many times the length of the alga are formed as in _Ephebe_. In others the filaments scarcely increase beyond the normal size of the alga as in _Thermutis (Gonionema)_; or the gelatinous algal cells may be distributed in a stratum of hyphae.

The apothecia are minute and almost closed; they may be embedded in swellings of the thallus, or are more or less superficial. The spores are rather small, colourless and simple or 1-septate.

The lichens of this family are rock-dwellers and are mostly to be found in hilly or Alpine regions. A tropical species, _Leptogidium dendriscum_, occurs in sterile condition in south-west Ireland. There are few species in any of the genera.

Algal cells _Scytonema_.
Thallus minutely fruticose,
non-corticate 1. Thermutis Fr.
Thallus minute, of felted filaments,
cortex one cell thick 2. *Leptodendriscum Wain.
Thallus of elongate filaments, cortex
of several cells 3. Leptogidium Nyl.
Thallus foliose or fruticose, cellular
throughout 4. Polychidium Ach.
Thallus crustaceous, non-corticate 5. Porocyphus Koerb.
Algal cells _Stigonema_.
Thallus minutely fruticose,
non-corticate 6. Spilonema Born.
Thallus of long branching filaments.
Spores septate; paraphyses wanting 7. Ephebe Fr.
Spores simple; paraphyses present 8. Ephebeia Nyl.
Thallus crustaceous; upper surface
non-corticate, lower surface
corticate 9. *Pterygiopsis Wain.

XXXVII. _PYRENOPSIDACEAE_

In this family are included gelatinous lichens of which the gonidium is a blue-green alga with a thick gelatinous coat, either _Gloeocapsa_ (including _Xanthocapsa_) or _Chroococcus_. In _Gloeocapsa_ and _Chroococcus_ the gelatinous envelope is often red, in _Xanthocapsa_ it is yellow, and these colours persist more or less in the lichens, especially in the outer layers.

The thallus is in many cases a formless gelatinous crust of hyphal filaments mingling with colonies of algal cells as in _Pyrenopsis_; but small fruticose tufts are characteristic of _Synalissa_, and larger foliose and fruticose thalli appear in some exotic genera. A plectenchymatous cortex is formed on the thallus of _Forssellia_, a crustaceous genus from Central Europe, with two species only; the whole thallus is built up of a kind of plectenchyma in some others, but in most of the genera there is no tissue formed.

The apothecia, as in Ephebaceae, are generally half-closed.

Thallus with _Gloeocapsa_ gonidia.
Thallus crustaceous.
Spores simple 1. Pyrenopsis Nyl.
Spores 1-septate 2. *Cryptothele Forss.
Thallus shortly fruticose 3. Synalissa Fr.
Thallus lobate, centrally attached 4. *Phylliscidium Forss.
Thallus with _Chroococcus_ gonidia.
Thallus crustaceous 5. Pyrenopsidium Forss.
Thallus lobate, centrally attached 6. *Phylliscum Nyl.
Thallus with _Xanthocapsa_ gonidia.
Thallus crustaceous.
Thallus non-corticate.
Spores simple.
Apothecia open, asci 8-spored 7. Psorotichia Forss.
Apothecia covered, asci
many-spored 8. *Gonohymenia Stein.
Spores 1-septate.
Apothecia closed 9. *Collemopsidium Nyl.
Thallus with plectenchymatous cortex 10. *Forssellia A. Zahlbr.
Thallus lobate, centrally attached.
Spores simple.
Thallus plectenchymatous
throughout 11. *Anema Nyl.
Thalline tissue of loose hyphae 12. *Thyrea Massal.
Cortex of upright parallel hyphae 13. *Jenmania Wächt.
Spores 1-septate.
Thalline tissue of loose hyphae 14. *Paulia Fée.
Thallus fruticose.
Thallus without a cortex 15. *Peccania Forss.
Thallus with cortex of parallel
hyphae 16. *Phloeopeccania Stein.

XXXVIII. _LICHINACEAE_

The only family of lichens associated with _Rivularia_ gonidia, the trichomes of which retain their filamentous form to some extent in the more highly developed genera; they lie parallel to the long axis of the squamule or of the frond except in _Lichinella_ in which genus they are vertical to the surface. The thallus may be crustaceous, or minutely foliose, or fruticose; in all cases it is dark-brown in colour, and the gelatinous character is evident in the moist condition. The best known British genus is _Lichina_ which grows on rocks by the sea.

The apothecia are more or less immersed in the tissue; in _Pterygium_ and _Steinera_ they are open and superficial (the latter monotypic genus confined to Kerguelen). They are also open in _Lichinella_ and _Homopsella_, both very rare genera. The spores are colourless and simple except in _Pterygium_ and _Steinera_ where they are elongate, and 1-3-septate.

Thallus crustaceous squamulose.
Apothecia immersed in thalline warts 1. *Calothricopsis Wain.
Apothecia superficial, with thalline
margin 2. *Steinera A. Zahlbr.
Apothecia superficial, without a
thalline margin 3. Pterygium Nyl.
Thallus of small fruticose fronds.
Gonidia occupying the central strand 4. *Lichinodium Nyl.
Gonidia not in the centre.
Apothecia immersed 5. Lichina Ag.
Apothecia superficial.
Paraphyses present 6. *Lichinella Nyl.
Paraphyses absent 7. *Homopsella Nyl.

XXXIX. _COLLEMACEAE_

The most important family of the gelatinous lichens and the most numerous. _Collema_ is historically interesting as having first suggested the composite thallus. Algal cells, _Nostoc_, which retain the chain-like form except in _Leprocollema_, a doubtful member of the family. The thallus varies from indeterminate crusts to lobes of considerable size; occasionally the lobes are narrow and erect, forming minute fruticose structures. In the more primitive genera the thallus is non-corticate, but in the more evolved, the apical cells of the hyphae coalesce to form a continuous cellular cortex, one or more cells thick, well marked in some species, in others rudimentary; the formation of plectenchyma also occurs occasionally in the apothecial tissues of some non-corticate species.

The apothecia are superficial except in _Pyrenocollema_, a monotypic genus of unknown locality. They are generally lecanorine, with gonidia entering into the formation of the apothecium: in some genera they are lecideine or biatorine, being formed of hyphae alone. The spores are colourless and vary in form, size and septation.

Apothecia immersed; spores fusiform,
1-septate 1. *Pyrenocollema Reinke.
Apothecia superficial.
Thallus without a cortex.
Spores simple, globose or ellipsoid.
Thallus crustaceous 2. *Leprocollema Wain.
Thallus largely squamulose-fruticose.
Apothecia lecideine
(dark-coloured) 3. *Leciophysma Th. Fr.
Apothecia lecanorine 4. Physma Massal.
Spores variously septate or muriform.
Apothecia biatorine
(light-coloured) 5. *Homothecium Mont.
Apothecia lecanorine 6. Collema Wigg.
Thallus with cortex of plectenchyma.
Spores simple.
Spores globose 7. Lemmopsis A. Zahlbr.
Spores ellipsoid, with thick
subverrucose wall 8. *Dichodium Nyl.
Spores vermiform, spirally curved 9. *Koerberia Massal.
Spores variously septate or muriform.
Apothecia biatorine
(light-coloured) 10. *Arctomia Th. Fr.
Apothecia lecanorine 11. Leptogium S. F. Gray.

XL. _HEPPIACEAE_

A family belonging to the “blue-green” series as it is associated with a gelatinous alga, _Scytonema_, but is of almost entirely cellular structure and is non-gelatinous. The thallus is squamulose or minutely foliose, or is formed of narrow almost fruticose lobes; the apothecia are semi-immersed; the asci are 4-many-spored.

_Heppia_ is a wide-spread genus both in northern and tropical regions with about forty species that live on soil or rock. So far, no representative has been recorded in our Islands.

Spores simple, colourless, globose or
ellipsoid 1. *Heppia Naeg.
Spores muriform, colourless, ellipsoid 2. *Amphidium[1054] Nyl.

XLI. _PANNARIACEAE_

The members of this family are also non-gelatinous, though for the most part associated with blue-green gelatinous algae, _Nostoc_ or _Scytonema_. The gonidia are bright-green in the genera _Psoroma_ and _Psoromaria_, the former often included under _Lecanora_, but too closely resembling _Pannaria_ to be dissociated from that genus.

The thallus varies from being crustaceous to squamulose or foliose, and has a cortex of plectenchyma on the upper and sometimes also on the lower surface. The apothecia are superficial or lateral and with or without a thalline margin (lecanorine or biatorine), the spores are colourless.

Zahlbruckner has included _Hydrothyria_ in this family. It is a monotypic aquatic genus found in North America and very closely allied to _Peltigera_. The British species of the genus, familiarly known as _Coccocarpia_, have been placed under _Parmeliella_, the former name being restricted to the tropical or subtropical species first assigned to _Coccocarpia_ and distinguished by the cortex, the hyphae forming it lying parallel with the surface though forming a regular plectenchyma.

An Antarctic lichen _Thelidea corrugata_ with _Palmella_ gonidia is doubtfully included: the thallus is foliose, the apothecia biatorine with colourless 1-septate spores.

Thallus with bright-green gonidia.
With _Palmella_ 1. *Thelidea Hue.
With Protococcaceae.
Apothecia non-marginate (biatorine) 2. *Psoromaria Nyl.
Apothecia marginate 3. Psoroma Nyl.
Thallus with _Scytonema_ gonidia.
Apothecia marginate, spores 1-septate 4. Massalongia Koerb.
Apothecia non-marginate; spores simple.
Upper surface smooth 5. *Coccocarpia Pers.
Upper surface felted 6. *Erioderma Fée.
Thallus with _Nostoc_ gonidia.
Apothecia marginate; spores simple 7. Pannaria Del.
Apothecia non-marginate; spores various.
Thallus crustaceous or minutely
squamulose 8. Placynthium Ach.
Thallus squamulose, cortex
indistinct 9. *Lepidocollema Wain.
Thallus squamulose or foliose,
cortex cellular 10. Parmeliella Müll.-Arg.
Thallus foliose, thin veined
below 11. *Hydrothyria Russ.

XLII. _STICTACEAE_

Thallus foliose, mostly horizontal, with a plectenchymatous cortex on both surfaces, a tomentum of hair-like hyphae taking the place of rhizinae on the lower surface. Algal cells Protococcaceae or _Nostoc_. Cephalodia and cyphellae or pseudocyphellae often present. Apothecia superficial or lateral; spores colourless or brown, variously septate.

The highly organized cortex and the presence of aeration organs—cyphellae or pseudocyphellae—which are almost solely confined to the genus _Sticta_ give this family a high position as regards vegetative development. The two genera are of wide distribution, but _Sticta_ is more abundant in the Southern Hemisphere. _Lobaria pulmonaria_ is one of our largest lichens.

Under surface dotted with cyphellae
or pseudocyphellae 1. Sticta Schreb.
Under surface without these organs 2. Lobaria Schreb.

XLIII. _PELTIGERACEAE_

A family of heteromerous foliose lichens containing in some instances blue-green (_Nostoc_), in others bright-green (Protococcaceae) gonidia, and thus representing a transition between these two series. They have large or small lobes and grow on the ground or on trees.

Cephalodia, either ectotrophic (Peltidea) or endotrophic (Solorina), occur in the family and further exemplify the capacity of the fungus hyphae to combine with different types of algae.

The upper surface is a wide cortex of plectenchyma, which in some forms (_Nephromium_) is continued below. In the non-corticate under surface of _Peltigera_, the lower hyphae grow out in hairs or rhizinae, very frequently brown in colour. Intercalary growth of the upper tissues stretches the thallus and tears apart the lower under surface so that the hair-bearing areas become a network of veins, with the white exposed medulla between. In _Peltigera canina_ there is further growth and branching of the hyphae in the veins, adding to the bulk of the interlacing ridges.

From all other foliose lichens Peltigeraceae are distinguished by the flat wholly appressed or peltate apothecia without a thalline margin which arise mostly on the upper surface, but in _Nephromium_ on the extreme margin of the under surface, the tip of the fertile lobe in that case is turned back as the apothecium matures, so that the fruit eventually faces the light. In _Nephroma_ has been included _Eunephroma_ with bright-green gonidia and _Nephromium_ with blue-green.

Bitter[1055] has recorded the finding of apothecia on the under surface of _Peltigera malacea_ and not at the margin, as in _Nephromium_. The plant was otherwise normal and healthy. _Solorinella_, from Central Europe and _Asteristion_ from Ceylon are monotypic genera with poorly developed thalli.

Thallus poorly developed.
Asci 6-8-spored; spores 3-5-septate 1. *Asteristion Leight.
Asci many-spored; spores 1-septate 2. *Solorinella Anzi.
Thallus generally well developed.
Apothecia superficial, sunk in the
thallus 3. Solorina Ach.
Apothecia terminal on upper surface
of lobes 4. Peltigera Willd.
Apothecia terminal on lower surface
of lobes 5. Nephroma Ach.

XLIV. _PERTUSARIACEAE_

Thallus crustaceous, often rather thick and with an amorphous cortex on the upper surface. Algal cells Protococcaceae. Apothecia solitary or several immersed in thalline warts, generally with a narrow opening which barely exposes the disc, and which in one genus, _Perforaria_, is so small as almost to constitute a perithecium; spores are often very large and with thick walls; some if not all are multinucleate and germinate at many points.

In the form of the fruit, this family stands between Pyrenocarpeae and Gymnocarpeae, though more akin to the latter. _Perforaria_, with two species, belongs to New Zealand and Japan. _Pertusaria_ has a world-wide distribution, and _Varicellaria_, a monotypic genus, with a very large two-celled spore, is an Alpine plant, recorded from Europe and from Antarctic America.

Spores simple.
Apothecia with pore-like opening 1. *Perforaria Müll.-Arg.
Apothecia with a wider opening 2. Pertusaria DC.
Spores 1-septate 3. Varicellaria Nyl.

XLV. _LECANORACEAE_

Thallus mostly crustaceous, occasionally squamulose or very rarely minutely fruticulose. The squamulose thallus is corticate above, the under surface appressed and attached to the substratum by penetrating hyphae, often effigurate at the circumference. Algal cells Protococcaceae. Apothecia well distinguished by the thalline margin; spores colourless, simple or variously septate or muriform.

_Lecanora_, _Ochrolechia_, _Lecania_, _Haematomma_ and _Phlyctis_ are cosmopolitan genera, some of them with a very large number of species; the other genera are more restricted in distribution and generally with few species.

The genus _Candelariella_ is of uncertain position; the spores are 8 or many in the ascus and are simple or 1-septate, and not unfrequently become polarilocular as in Caloplacaceae, but there is no parietin present.

Algae distributed through the thallus.
Spores simple 1. *Harpidium Koerb.
Algae restricted to a definite zone.
Spores simple.
Thallus grey, white or yellowish.
Spores rather small 2. Lecanora Ach.
Spores large 3. Ochrolechia Massal.
Thallus bright yellow.
Spores simple or 1-septate 4. Candelariella Müll.-Arg.
Spores 1-septate (rarely pluri-septate).
Paraphyses free.
Thallus squamulose, effigurate 5. Placolecania Zahlbr.
Thallus crustaceous.
Apothecial disc brownish 6. Lecania Zahlbr.
Apothecial disc flesh-coloured 7. Icmadophila Trevis.
Paraphyses branched, intricate 8. *Calenia Müll.-Arg.
Spores elongate, pluri-septate.
Apothecia superficial 9. Haematomma Massal.
Apothecia immersed.
Paraphyses free 10. *Phlyctella Müll.-Arg.
Paraphyses branched, intricate 11. *Phlyctidia Müll.-Arg.
Spores muriform.
Apothecia superficial 12. *Myxodictyon Massal.
Apothecia immersed 13. Phlyctis Wallr.

XLVI. PARMELIACEAE

A very familiar family of foliose lichens. Genera and species are dorsiventral and stratose in structure, though some _Cetrariae_ are fruticose in habit. Algal cells are Protococcaceae; in _Physcidia_ they are _Palmellae_. In every case the upper surface of the thallus is corticate and generally of plectenchyma, the lower being somewhat similar, but in _Heterodea_ and _Physcidia_, monotypic Australasian genera, the upper cortex is of branching hyphae parallel with the surface, the lower surface being non-corticate.

The _Parmeliae_ are mostly provided with abundant rhizinae; in _Cetrariae_ and _Nephromopsis_ these are very sparingly present, while in _Anzia_ (including _Pannoparmelia_) the medulla passes into a wide net-like structure of anastomosing hyphae.

In _Heterodea_, cyphellae occur on the under surface as in Stictaceae; and in _Cetraria islandica_ bare patches have been described as pseudocyphellae. The latter lichen is one of the few that are of value as human food. Special aeration structures are present on the upper cortex of _Parmelia aspidota_.

Thallus non-corticate below.
Apothecia terminal 1. *Heterodea Nyl.
Apothecia superficial 2. *Physcidia Tuck.
Thallus spongy below 3. *Anzia Stizenb.
Thallus corticate below.
Asci poly-spored 4. Candelaria Massal.
Asci 8-spored.
Spermatia acrogenous 5. Parmeliopsis Nyl.
Spermatia pleurogenous.
Apothecia superficial 6. Parmelia Ach.
Apothecia lateral.
Apothecia on upper surface 7. Cetraria Ach.
Apothecia on lower surface 8. *Nephromopsis Müll.-Arg.

XLVII. _USNEACEAE_

This also is a familiar family of lichens, _Usnea barbata_ the “bearded moss” being one of the first lichens noted and chronicled. Algal cells Protococcaceae. Structure radiate, the upright or pendulous habit characteristic of the family securing all-round illumination. Special adaptations of the cortex or of the internal tissues have been evolved to strengthen the thallus against the strains incidental to their habit of growth as they are attached in nearly all cases by one point only, by a special sheath, or by penetrating hold-fasts.

Apothecia are superficial or marginal and sometimes shortly stalked; spores are simple or variously septate.

_Ramalina_ and _Usnea_, the most numerous, are cosmopolitan genera; _Alectoria_ inhabits northern or hilly regions.

The genus _Evernia_, also cosmopolitan, represents a transition between foliose and fruticose types; the fronds of the two species, though strap-shaped and generally upright, are dorsiventral and stratose, the gonidia for the most part lying beneath one surface; the other (lower) surface is either white or very dark-coloured. _Everniopsis_, formed of thin branching strap-shaped fronds, is also dorsiventral.

A number of genera, _Thamnolia_, _Siphula_, etc. are of podetia-like structure, generally growing in swards. Several of them have been classified with _Cladoniae_, but they lack the double thallus. One of these, _Endocena_, a sterile monotypic Patagonian lichen, with stiff hollow coralloid fronds, was classified by Hue[1056] along with _Siphula_; recently he has transferred it to his family Polycaulionaceae[1057] based on _Polycauliona regale_ (_Placodium frustulosum_ Darbish.), and allied to _Placodium_ Sect. _Thamnoma_[1058]. In recent studies Hue has laid most stress on thalline characters. He places the new family between “Ramalinaceae” and “Alectoriaceae.” _Dactylina arctica_ is a common Arctic soil-lichen.

Thallus strap-shaped.
Structure dorsiventral.
Greyish-green above 1. Evernia Ach.
Whitish-yellow above 2. *Everniopsis Nyl.
Structure radiate alike on both surfaces.
Fronds grey; medulla of loose hyphae 3. Ramalina Ach.
Fronds yellow; medulla traversed by
strands 4. *Letharia A. Zahlbr.
Thallus filamentous.
Medulla a strong “chondroid” strand 5. Usnea Dill.
Medulla of loose hyphae.
Spores simple 6. Alectoria Ach.
Spores muriform, brown 7. *Oropogon Fr.
Thallus of upright podetia-like fronds.
Fronds rather long (about two inches),
tapering, white 8. Thamnolia Ach. (Cerania
S. F. Gray).
Fronds shorter, blunt.
Medulla solid 9. *Siphula Fr.
Medulla partly or entirely hollow.
Fronds swollen and tall (about two
inches) 10. *Dactylina Nyl.
Fronds coralloid, entangled 11. *Endocena Cromb.
Fronds short, upright 12. *Dufourea Nyl.

XLVIII. _CALOPLACACEAE_

In this family Zahlbruckner has included the squamulose or crustaceous lichens with colourless polarilocular spores, relegating those with more highly developed thallus or with brown spores to other families. He has also substituted the name _Caloplaca_ for the older _Placodium_, the latter being, as he considers, less well defined.

Algal cells are Protococcaceae. The thallus is mostly light-coloured, generally some shade of yellow, and, with few exceptions, contains parietin, which gives a purple colour on the application of potash. The squamulose forms are closely appressed to the substratum, and have often a definite rounded outline (effigurate). The spores have a thick median septum with a loculus at each end and a connecting canal[1059].

In _Blastenia_ the outer thalline margin is obscure or absent—though gonidia are frequently present below the hymenium. Caloplacaceae occur all over the globe; they are among the most brilliantly coloured of all lichens. _Polycauliona_ Hue[1060] possibly belongs here: though based on thalline rather than on spore characters, one species at least has polarilocular spores.

Apothecia with a distinct thalline margin 1. Caloplaca Th. Fr.
Apothecia without a thalline margin 2. Blastenia Th. Fr.

XLIX. _TELOSCHISTACEAE_

Polarilocular colourless spores are the distinguishing feature of this family as of the Caloplacaceae. Algal cells Protococcaceae. The thallus of Teloschistaceae is more highly developed, being either foliose or fruticose, though never attaining to very large dimensions. The cortex of _Xanthoria_ (foliose) is plectenchymatous, that of _Teloschistes_ (fruticose) is fibrous. The species of both genera are yellow or greenish-yellow due to the presence of the lichen-acid parietin.

Both genera have a wide distribution over the globe, more especially in maritime regions.

Thallus foliose 1. Xanthoria Th. Fr.
Thallus fruticose 2. Teloschistes Norm.

L. _BUELLIACEAE_

A family of crustaceous lichens distinguished by the brown two-celled spores. Algal cells Protococcaceae. Zahlbruckner has included here _Buellia_ and _Rinodina_; the former with a distinctly lecideine fruit and with thinly septate spores; the latter lecanorine and with spores of the polarilocular type, with a very wide central septum pierced in most of the species by a canal which may or may not traverse the middle lamella of the wall. _Rinodina_ is closely allied to Physciaceae, while _Buellia_ has more affinity with Lecideaceae and is near to _Rhizocarpon_.

Both genera are of world-wide distribution.

Apothecia lecideine, without a thalline
margin 1. Buellia De Not.
Apothecia lecanorine, with a thalline
margin 2. Rinodina Massal.

LI. _PHYSCIACEAE_

Thallus foliose or partly fruticose, and generally attached by rhizinae. Algal cells Protococcaceae. The spores resemble those of _Rinodina_, dark-coloured with a thick septum and reduced cell-lumina. As in that species there may be a second septum in each cell, giving a 3-septate spore; but that is rare.

_Pyxine_, a tropical or subtropical genus, is lecanorine only in the very early stages; it soon loses the thalline margin. _Anaptychia_ is differentiated from _Physcia_ by the subfruticose habit, though the species are nearly all dorsiventral in structure, only a few of them being truly radiate and corticate on both surfaces. The upper cortex of _Anaptychia_ is fibrous, but that character appears also in most species of _Physcia_ either on the upper or the lower side. _Physcia_ and _Anaptychia_ are widely distributed.

Thalline margin absent in apothecia 1. *Pyxine Nyl.
Thalline margin present in apothecia.
Thallus foliose 2. Physcia Schreb.
Thallus fruticose 3. Anaptychia Koerb.

C. *HYMENOLICHENS

Fungus a Basidiomycete, akin to _Thelephora_. Algal cells _Scytonema_ or _Chroococcus_. Thallus crustaceous, squamulose or foliose. Spores colourless, produced on basidia, on the under surface of the free thallus.

The Hymenolichens[1061] are few in number and are endemic in tropical or warm countries. They inhabit soil or trees.

Thallus of extended lobes.
Gonidia near the upper surface 1. *Dictyonema Zahlbr.
Gonidia in centre of tissue 2. *Cora Fr.
Thallus squamulose, irregular 3. *Corella Wain.

II. NUMBER AND DISTRIBUTION OF LICHENS

1. ESTIMATES OF NUMBER

Calculations have been made and published, once and again, as to the number of lichen species occurring over the globe or in definite areas. In 1898 Fünfstück stated that about 20,000 different species had been described, but as many of them had been proved to be synonyms, and since many must rank as forms or varieties, the number of well-authenticated species did not then, according to his estimate, exceed 4000. Many additional genera and species have, however, been discovered since then. In Engler and Prantl’s _Pflanzenfamilien_, over 50 families and nearly 300 genera find a place, but even in these larger groupings opinions differ as to the limits both of genera and families, and lichenologists would not all accept the arrangement given in that volume.

Fünfstück has reckoned that of his estimated 4000, about 1500 are European and of these at least 1200 occur in Germany. Probably this is too low an estimate for that large country. Leighton in 1879 listed, in his _British Lichen Flora_, 1710 in all, and, as the compilation includes varieties, it cannot be considered as very far astray. On comparing it with Olivier’s[1062] recent statistics of lichens, we find that of the larger fruticose and foliose species, 310 are recognized by him for the whole of Europe, 206 of these occurring in the British Isles. Leighton’s estimate of similar species is about 145, without including varieties now reckoned as good species. In a more circumscribed area, Th. Fries[1063] described for Spitzbergen about 210 different lichens, a number that closely approximates to the 206 recent records by Darbishire[1064] for the same area.

A general idea of the comparative numbers of the different types of lichens may be gathered from Hue’s compilation of exotic lichens[1065], examined or described by Nylander, and now in the Paris herbarium. There are 135 genera with 3686 species. Of these, about 829 belong to the larger foliose and fruticose lichens (including _Cladoniae_); the remaining 2857 belong to the smaller kinds, most of them crustaceous.

2. GEOGRAPHICAL DISTRIBUTION

A. GENERAL SURVEY

The larger foliose and fruticose lichens are now fairly well known and described for Europe, and the knowledge of lichens in other continents is gradually increasing. It is the smaller crustaceous forms that baffle the investigator. The distribution of all lichens over the surface of the earth is controlled by two principal factors, climate and substratum; for although lichens as a rule require only support, they are most of them restricted to one or another particular substratum, either organic or inorganic. As organisms which develop slowly, they require an unchanging substratum, and as sun-plants they avoid deeply shaded woodlands: their occurrence thus depends to a large extent on the configuration and general vegetation of the country.

Though so numerous and so widely distributed, lichens have not evolved that great variety of families and genera characteristic of the allied fungi and algae. They conform to a few leading types of structure, and thus the Orders and Families are comparatively few, and more or less universal. They are most of them undoubtedly very old plants and were probably wide-spread before continents and climates had attained their present stability. Arnold[1066] indeed considers that a large part of the present-day lichens were almost certainly already evolved at the end of the Tertiary period, and that they originated in a warm or probably subtropical climate. As proof of this he cites such genera as _Graphis_, _Thelotrema_ and _Arthonia_[1067] which are numerous in the tropics though rare in the colder European countries; and he sees further proof in the fact that many fruticose and gelatinous lichens do not occur further north than the forest belt, though they are adapted to cold conditions. Several genera that are abundant in the tropics are represented outside these regions by only one or few species, as for instance _Conotrema urceolatum_ and _Bombyliospora incana_.

During the Ice age of the Quaternary period, not many new species can have arisen, and such forms as were not killed off must have been driven towards the south. As the ice retreated the valleys were again stocked with southern forms, and northern species were left behind on mountain tops all over the globe.

In examining therefore the distribution of lichens, it will be found that the distinction between different countries is relative, certain families being more or less abundant in some regions than others, but, in general, nearly all being represented. Certain species are universal, where similar conditions prevail. This is especially true of those species adapted to extreme cold, as that condition, normal in polar regions, recurs even on the equator if the mountains reach the limit of perpetual snow; the vertical distribution thus follows on the lines of the horizontal.

In all the temperate countries we find practically the same families, with some few exceptions; there is naturally more diversity of genera and species. Genera that are limited in locality consist, as a rule, of one or few species. In this category, however, are not included the tropical families or genera which may be very rich in species: these are adapted to extreme conditions of heat and often of moisture, and cannot exist outside tropical or subtropical regions, extreme heat being more restricted as to geographical position than extreme cold.

In the study of distribution the question which arises as to the place of origin of such widely distributed plants is one that is difficult to solve. Wainio[1068] has attempted the task in regard to _Cladonia_, one of the most unstable genera, the variations of form, which are dependent on external circumstances, being numerous and often bewildering. In his fine monograph of the genus, 132 species are described and 25 of these are cosmopolitan.

The distribution of Phanerogams is connected, as Wainio points out, with causes anterior to the present geological era, but this cannot be the case in a genus so labile and probably so recent as _Cladonia_, though some of the species have existed long enough to spread and establish themselves from pole to pole. Endemic species, or those that are confined to a comparatively limited area, are easily traced to their place of origin, that being generally the locality where they are found in most abundance, and as a general rule in the centre of that area, though there may be exceptions: a plant for instance that originated on a mountain would migrate only in one direction—towards the regions of greater cold.

The difficulty of determining the primitive stations of cosmopolitan, or of widely spread, species is much greater, but generally they also may be referred to their area of greatest abundance. Thus a species may occur frequently in one continent and but rarely in another, even where the conditions of climate, etc., are largely comparable. It may therefore be inferred that the plant has not yet reached the full extent of possible distribution in the less frequented area. As examples of this, Wainio cites, among other instances, _Cladonia papillaria_, which has a very wide distribution in Europe, but, as yet, has been found only in the eastern parts of North America; and _Cl. pycnoclada_, a plant which braves the climate of Cape Horn and the Falkland Islands, but has not travelled northward beyond temperate North America: the southern origin of that species is thus plainly indicated. Wainio also finds that evidence of the primitive locality of a very widely spread species may be obtained by observing the locality of species derived from it, which are as yet of limited distribution; presumably these arose in the ancestral place of origin, though this indication is not always to be relied on. If, however, the ancestral plant has given rise to several of these rarer related species, those of them that are most closely allied to the primitive plant would be found near to it in the original locality.

A detailed account of species distribution according to these indications is given by Wainio and is full of interest. No such attempt has been made to deal with any other group, and the distribution of genera and species can only be suggested. An exhaustive comparison of the lichens of different regions is beyond the purpose of our study and is indeed impossible as, except in some limited areas, or for certain species, the occurrence and distribution are not fully known. It is in any case only tentatively that genera or species can be described as local or rare, until diligent search has been made for them over a wider field. The study of lichens from a floristic point of view lags behind that of most other groups of plants. The larger lichen forms have received more attention, as they are more evident and more easily collected; but the more minute species are not easily detected, and, as they are largely inseparable from their substratum of rocks, or trees, etc., on which they grow, they are often difficult to collect. They are also in many instances so indefinite, or so alike in outward form, that they are liable to be overlooked, only a microscopic examination revealing the differences in fruit and vegetative structure.

Though much remains to be done, still enough is known to make the geographical distribution of lichens a subject of extreme interest. It will be found most instructive to follow the usual lines of treatment, which give the three great divisions: the Polar, the Temperate and the Tropical regions of the globe.

B. LICHENS OF POLAR REGIONS

Strictly speaking, this section should include only lichens growing within the Polar Circles; but in practice the lichens of the whole of Greenland and those of Iceland are included in the Arctic series, as are those of Alaska: the latitudinal line of demarcation is not closely adhered to. With the northern lichens may also be considered those of the Antarctic continent, as well as those of the islands just outside the Antarctic Circle, the South Shetlands, South Orkneys, Tierra del Fuego, South Georgia and the Falkland Islands. During the Glacial period, the polar forms must have spread with the advancing cold; as the snow and ice retreated, these forms have been left, as already stated, on the higher colder grounds, and representatives of polar species are thus to be found very far from their original haunts. There are few exclusively boreal genera: the same types occur at the Poles as in the higher subtemperate zones. One of the most definitely polar species, for instance, _Usnea_ (_Neuropogon_) _melaxantha_ grows in the whole Arctic zone, and, in the Antarctic, is more luxuriant than any other lichen, but it has also been recorded from the Andes in Chili, Bolivia and Peru, and from New Zealand (South Island).

Cold winds are a great feature of both poles, and the lichens that by structure or habit can withstand these are the most numerous; those that have a stout cortical layer are able to resist the low temperatures, or those that grow in tufts and thus secure mutual protection. In Arctic and Subarctic regions, 495 lichens have been recorded, most of them crustaceous. Among the larger forms the most frequently met are certain species of _Peltigera_, _Parmelia_, _Gyrophora_, _Cetraria_, _Cladonia_, _Stereocaulon_ and _Alectoria_. Among smaller species _Lecanora tartarea_ spreads everywhere, especially over other vegetation, _Lecanora varia_ reaches the farthest limits to which wood, on which it grows, has drifted, and several species of _Placodium_ occur constantly, though not in such great abundance. Over the rocks spread also many crustaceous Lecideaceae too numerous to mention, one of the most striking being the cosmopolitan _Rhizocarpon geographicum_.

Wainio[1069] has described the lichens collected by Almquist at Pitlekai in N.E. Siberia just on the borders of the Arctic Circle, and he gives a vivid account of the general topography. The snow lies on the ground till June and falls again in September, but many lichens succeed in growing and fruiting. It is a region of tundra and sand, strewn more or less with stones. Most of the sand is bare of all vegetation; but where mosses, etc., have gained a footing, there are also a fair number of lichens: _Lecanora tartarea_, _Psoroma hypnorum_, with _Lecideae_, _Parmeliae_, _Cladoniae_, _Stereocaulon alpinum_, _Solorina crocea_, _Sphaerophorus globosus_, _Alectoria nigricans_ and _Gyrophora proboscidea_. Some granite rocks in that neighbourhood rise to a height of 200 ft., and though bare of vegetation on the north side, yet, in sheltered nooks, several species are to be found. Stunted bushes of willow grow here and there, and on these occur always the same species: _Placodium ferrugineum_, _Rinodina archaea_, _Buellia myriocarpa_ and _Arthopyrenia punctiformis_. Some species such as _Sphaerophorus globosus_, _Dactylina arctica_ (a purely Arctic genus and species) and _Thamnolia vermicularis_ are so abundant that they bulk as largely as other better represented genera such as _Cladoniae_, _Lecanorae_ or _Lecideae_. On the soil, _Lecanorae_ cover the largest areas.

Wainio determined a large number of lichens with many new species, but the region is colder than that of Lappland, and trees with tree-lichens are absent, with the exception of those given above. In Arctic Siberia, Elenkin[1070] discovered a new lichen _Placodium subfruticulosum_ which scarcely differs from Darbishire’s[1071] Antarctic species _Pl. fruticulosum_ (or _P. regale_); both are distinguished by the fruticose growth of the thallus, for which reason Hue[1072] placed them in a new genus, _Polycauliona_.

The Antarctic Zone and the neighbouring lands are less hospitable to plant life than the northern regions, and there is practically no accumulation of detritus. Collections have been made by explorers, and several lists have been published which include a marvellous number of species common to both Poles, if the subantarctic lands are included in the survey. An analytic study of the various lists has been published by Darbishire[1073]. He recognizes 106 true Antarctic lichens half of which are Arctic as well. The greater number are crustaceous and are plants common also to other lands though a certain number are endemic. The most abundant genera in species as well as individuals are _Lecidea_ and _Lecanora_. Several bright yellow species of _Placodium_—_Pl. elegans_, _Pl. murorum_, etc., are there as at the North Pole. Among the larger forms, _Parmeliae_, _Cetrariae_, and _Cladoniae_ are fairly numerous; _Usneae_ and _Ramalinae_ rather uncommon, while members of the Stictaceae are much more abundant than in the North. The common species of _Peltigera_ also occur in Antarctica, though _P. aphthosa_ and _P. venosa_ are wanting; both of these latter are boreal species. Darbishire adds that lichens have so great a capacity to withstand cold, that they are only checked by the snow covering, and were bare rocks to be found at the South Pole, he is sure lichens would take possession of them. The most southerly point at which any plant has been found is 78° South latitude and 162° East longitude, in which locality the lichen _Lecanora subfusca_ was collected by members of Scott’s Antarctic expedition (1901-1904) at a height of 5000 ft.

A somewhat different view of the Antarctic lichen flora is indicated by Hue[1072] in his account of the plants brought back by the second French Antarctic Expedition. The collection was an extremely favourable and important one: great blocks of stone with their communities of lichens were secured, and these blocks were entirely covered, the crustaceous species, especially, spreading over every inch of space.

Hue determined 126 species, but as 15 of these came from the Magellan regions only 111 were truly Antarctic. Of these 90 are new species, 29 of them belonging to the genus _Buellia_. Hue considers, therefore, that in Antarctica there is a flora that, with the exception of cosmopolitan species, is different from every other, and is special to these southern regions. Darbishire himself described 34 new Antarctic species, but only 10 of these are from true Antarctica; the others were collected in South Georgia, the Falkland Islands or Tierra del Fuego. Even though many species are endemic in the south, the fact remains that a remarkable number of lichens which occur intermediately on mountain summits are common to both Polar areas.

C. LICHENS OF THE TEMPERATE ZONES

Regions outside the Polar Circles which enjoy, on the whole, cool moist climates, are specially favourable to lichen growth, and the recorded numbers are very large. The European countries are naturally those in which the lichen flora is best known. Whereas polar and high Alpine species are stunted in growth and often sterile, those in milder localities grow and fruit well, and the more highly developed species are more frequent. _Parmeliae_, _Nephromae_, _Usneae_ and _Ramalinae_ become prominent, especially in the more northern districts. Many Arctic plants are represented on the higher altitudes. A comparison has been made between the lichens of Greenland and those of Germany: of 286 species recorded for the former country, 213 have been found in Germany, the largest number of species common to both countries being crustaceous. Lindsay[1074] considered that Greenland lichens were even more akin to those of Scandinavia.

There is an astonishing similarity of lichens in the Temperate Zone all round the world. Commenting on a list of Chicago lichens by Calkins[1075], Hue[1076] pointed out that with the exception of a few endemic species they resemble those of Normandy. The same result appears in Bruce Fink’s[1077] careful compilation of Minnesota lichens, which may be accepted as typical of the Eastern and Middle States of North Temperate America. The genera from that region number nearly 70, and only two of these, _Omphalaria_ and _Heppia_, are absent from our British Flora. The species naturally present much greater diversity. Very few Graphideae are reported. In other States of North America there occurs the singular aquatic lichen, _Hydrothyria venosa_, nearly akin to _Peltigera_.

If we contrast American lichens with these collected in South Siberia near Lake Baikal[1078], we recognize there also the influence of temperate conditions. Several species of _Usnea_ are listed, _U. barbata_, _U. florida_, _U. hirta_ and _U. longissima_, all of them also American forms, _U. longissima_ having been found in Wisconsin. _Xanthoria parietina_, an almost cosmopolitan lichen, is absent from this district, and is not recorded from Minnesota. The opinion[1079] in America is that it is a maritime species: Tuckerman gives its habitat as “the neighbourhood of a great water,” and reports it from near Lake Superior. In our country it grows at a good distance from the sea, in Yorkshire dales, etc., but all our counties would rank as maritime in the American sense. _Lecanora tartarea_ which is rare in Minnesota is also absent from the Lake Baikal region. It occurs frequently both in Arctic and in Antarctic regions, and is probably also somewhat maritime in habitat. Many of the _Parmeliae_, _Nephromiae_ and _Peltigerae_, common to all northern temperate climes, are Siberian as are also _Cladoniae_ and many crustaceous species. There is only one _Sticta_, _St. Wrightii_, a Japanese lichen, recorded by Wainio from this Siberian locality.

A marked difference as regards species is noted between the Flora of Minnesota and that of California. Herre[1080] has directed attention to the great similarity between the lichens of the latter state and those of Europe: many European species occur along the coast and nowhere else in America so far as is yet known; as examples he cites, among others, _Calicium hyperellum_, _Lecidea quernea_, _L. aromatica_, _Gyrophora polyrhiza_, _Pertusaria amara_, _Roccella fuciformis_, _R. fucoides_ and _R. tinctoria_. The Scandinavian lichen, _Letharia vulpina_, grows abundantly there and fruits freely; it is very rare in other parts of America. Herre found, however, no specimens of _Cladonia rangiferina_, _Cl. alpestris_ or _Cl. sylvatica_, nor any species of _Graphis_; he is unable to explain these anomalies in distribution, but he considers that the cool equable climate is largely responsible: it is so much more like that of the milder countries of Europe than of the states east of the Sierra Nevada. His contention is supported by a consideration of Japanese lichens. With a somewhat similar climate there is a great preponderance of European forms. Out of 382 species determined by Nylander[1081], 209 were European. There were 17 _Graphideae_, 31 _Parmeliae_, and 23 _Cladoniae_, all of the last named being European. These results of Nylander’s accord well with a short list of 30 species from Japan compiled by Müller[1082] at an earlier date. They were chiefly crustaceous tree-lichens; but the _Cladoniae_ recorded are the familiar British species _Cl. fimbriata_, _Cl. pyxidata_ and _Cl. verticillata_.

With the Japanese Flora may be compared a list[1083] of Maingay’s lichens from China, 35 in all. _Collema limosum_, the only representative of Collemaceae in the list, is European, as are the two species of _Ramalina_, _R. gracilenta_ and _R. pollinaria_; four species of _Physcia_ are European, the remaining _Ph. picta_ being a common tropical or subtropical plant. _Lecanora saxicola_, _L. cinerea_, _Placodium callopismum_ and _Pl. citrinum_ are cosmopolitan, other _Lecanorae_ and most of the _Lecideae_ are new. _Graphis scripta_, _Opegrapha subsiderella_ and _Arthonia cinnabarina_—the few Graphideae collected—are more or less familiar home plants. Among the Pyrenocarpei, _Verrucaria_ (_Pyrenula_) _nitida_ occurs; it is a widely distributed tree-lichen.

It is unnecessary to describe in detail the British lichens. Some districts have been thoroughly worked, others have barely been touched. The flora as a whole is of a western European type showing the influence of the Gulf Stream, though there is also a representative boreal growth on the moorlands and higher hills, especially in Scotland. Such species as _Parmelia pubescens_, _P. stygia_ and _P. alpicola_ recall the Arctic Circle while _Alectoriae_, _Cetrariae_ and _Gyrophorae_ represent affinity with the colder temperate zone.

In the southern counties such species as _Sticta aurata_, _S. damaecornis_, _Phaeographis Lyellii_ and _Lecanora_ (_Lecania_) _holophaea_ belong to the flora of the Atlantic seaboard, while in S.W. Ireland the tropical genera _Leptogidium_ and _Anthracothecium_ are each represented by a single species. The tropical or subtropical genus _Coenogonium_ occurs in Great Britain and in Germany, with one sterile species, _C. ebeneum_. _Enterographa crassa_ is another of our common western lichens which however has travelled eastwards as far as Wiesbaden. _Roccella_ is essentially a maritime genus of warm climates: two species, _R. fuciformis_ and _R. fucoides_, grow on our south and west coasts. The famous _R. tinctoria_ is a Mediterranean plant, though it is recorded also from a number of localities outside that region and has been collected in Australia.

In the temperate zones of the southern hemisphere are situated the great narrowing projections of South Africa and South America with Australia and New Zealand. As we have seen, the Antarctic flora prevails more or less in the extreme southern part of America, and the similarity between the lichens of that country and those of New Zealand is very striking, especially in the fruticulose forms. There is a very abundant flora in the New Zealand islands with their cool moist climate and high mountains. Churchill Babington[1084] described the collections made by Hooker. Stirton[1085] added many species, among others _Calycidium cuneatum_, evidently endemic. Later, Nylander[1086] published the species already known, and Hellbom[1087] followed with an account of New Zealand lichens based on Berggsen’s collections; many more must be still undiscovered. Especially noticeable as compared with the north, are the numbers of Stictaceae which reach their highest development of species and individuals in Australasia. They are as numerous and as prominent as are Gyrophoraceae in the north. A genus of Parmeliaceae, _Hetorodea_, which, like the _Stictae_, bears cyphellae on the lower surface, is peculiar to Australia.

A warm current from the tropical Pacific Ocean passes southwards along the East Coast of Australia, and Wilson[1088] has traced its influence on the lichens of Australia and Tasmania to which countries a few tropical species of _Graphis_, _Chiodecton_ and _Trypethelium_ have migrated. Various unusual types are to be found there also: the beautiful _Cladonia retepora_ (Fig. 71), which spreads over the ground in cushion-like growths, with the genera _Thysanothecium_ and _Neophyllis_, genera of Cladoniaceae endemic in these regions.

The continent of Africa on the north and east is in so close connection with Europe and Asia that little peculiarity in the flora could be expected. In comparing small representative collections of lichens, 37 species from Egypt and 20 from Palestine, Müller[1089] found that there was a great affinity between these two countries. Of the Palestine species, eight were cosmopolitan; among the crustaceous genera, _Lecanorae_ were the most numerous. There was no record of new genera.

The vast African continent—more especially the central region—has been but little explored in a lichenological sense; but in 1895 Stizenberger[1090] listed all of the species known, amounting to 1593, and new plants and new records have been added since that day. The familiar genera are well represented, _Nephromium_, _Xanthoria_, _Physcia_, _Parmelia_, _Ramalina_ and _Roccella_, some of them by large and handsome species. In the Sahara Steiner[1091] found that genera with blue-green algae such as the Gloeolichens were particularly abundant; _Heppia_ and _Endocarpon_ were also frequent. Algeria has a Mediterranean Flora rather than tropical or subtropical. Flagey[1092] records no species of _Graphis_ for the province of Constantine, and only 22 species of other Graphideae. Most of the 519 lichens listed by him there are crustaceous species. South America stretches from the Tropics in the north to Antarctica in the south. Tropical conditions prevail over the central countries and tropical tree-lichens, Graphidaceae, Thelotremaceae, etc. are frequent; further West, on the Pacific slopes, _Usneae_ and _Ramalinae_ hang in great festoons from the branches, while the foliose _Parmeliae_ and _Stictae_ grow to a large size on the trunks of the trees.

Wainio’s[1093] _Lichens du Brésil_ is one of the classic systematic books and embodies the writer’s views on lichen classification. There are no new families recorded though a number of genera and many species are new, and, so far as is yet known, these are endemic. Many of our common forms are absent; thus _Peltigera_ is represented by three species only, _P. leptoderma_, _P. spuriella_ and _P. Americana_, the two latter being new species. _Sticta_ (including _Stictina_) includes only five species, and _Coenogonium_ three. There are 39 species of _Parmelia_ with 33 of _Lecanora_ and 68 of _Lecidea_, many of them new species.

D. LICHENS OF TROPICAL REGIONS

In the tropics lichens come under the influence of many climates: on the high mountains there is a region of perpetual snow, lower down a gradual change to temperate and finally to tropical conditions of extreme heat, and, in some instances, extreme moisture. There is thus a bewildering variety of forms. By “tropical” however the warmer climate is always implied. Several families and genera seem to flourish best in these warm moist conditions and our familiar species grow there to a large size. Among crustaceous families Thelotremaceae and Graphidaceae are especially abundant, and probably originated there. In the old comprehensive genus _Graphis_, 300 species were recorded from the tropics. It should be borne in mind that _Trentepohlia_, the alga that forms the gonidia of these lichens, is very abundant in the tropics. _Coenogonium_, a genus containing about twelve species and also associated with _Trentepohlia_, is scarcely found in Europe, except one sterile species, _C. ebeneum_. Other species of the genus have been recorded as far north as Algeria in the Eastern Hemisphere and Louisiana in the Western, while one species, _C. implexum_, occurs in the southern temperate zone in Australia and New Zealand.

Of exclusively tropical lichens, the Hymenolichens are the most noteworthy. They include three genera, _Cora_, _Corella_ and _Dictyonema_, the few species of which grow on trees or on the ground both in eastern and western tropical countries.

Other tropical or subtropical forms are _Oropogon loxensis_, similar to _Alectoria_ in form and habit, but with one brown muriform spore in the ascus; it is only found in tropical or subtropical lands. _Physcidia Wrightii_ (Parmeliaceae) is exclusively a Cuban lichen. Several small genera of Pyrenopsidaceae such as _Jenmania_ (British Guiana), _Paulia_ (Polynesia) and _Phloeopeccania_ (South Arabia) seem to be confined to very hot localities. On the other hand Collemaceae are rare: Wainio records from Brazil only four species of _Collema_, with nine of _Leptogium_.

Among Pyrenolichens, Paratheliaceae, Mycoporaceae and Astrotheliaceae are almost exclusively of tropical distribution, and finally the leaf lichens with very few exceptions. These follow the leaf algae, _Mycoidea_, _Phycopeltis_, etc., which are so abundant on the coriaceous long-lived green leaves of a number of tropical Phanerogams. All the Strigulaceae are epiphytic lichens. _Phyllophthalmaria_ (Thelotremaceae) is also a leaf genus; one of the species, _Ph. coccinea_, has beautiful carmine-red apothecia. The genera of the tropical family Ectolechiaceae also inhabit leaves, but they are associated with Protococcaceae; one of the genera _Sporopodium_[1094] is remarkable as having hymenial gonidia. Though tropical in the main, epiphyllous lichens may spread to the regions beyond: _Sporopodium Caucasium_ and a sterile _Strigula_ were found by Elenkin and Woronichin[1095] on leaves of _Buxus sempervirens_ in the Caucasus, well outside the tropics.

_Pilocarpon_, an epiphytic genus, is associated with Protococcaceae; one of the species, _P. leucoblepharum_, spreads from the bark to the leaves of pine-trees; it is widely distributed and has also been reported in the Caucasus[1096]. _Chrysothrix_, in which the gonidia belong to the algal genus _Palmella_, grows on Cactus spines in Chili, and may also rank as a subtropical epiphyllous lichen.

A series of lichens from the warm temperate region of Transcaucasia investigated by Steiner[1097] were found to be very similar to those of Central Europe. Lecanoraceae were, however, more abundant than Lecideaceae and Verrucariaceae were comparatively rare.