Cambridge Botanical Handbooks

Edited by A. C. Seward and A. G. Tansley

LICHENS

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LICHENS

BY
ANNIE LORRAIN SMITH, F.L.S.
ACTING ASSISTANT, BOTANICAL DEPARTMENT, BRITISH MUSEUM

CAMBRIDGE:
AT THE UNIVERSITY PRESS
1921

PRINTED IN GREAT BRITAIN

PREFACE

The publication of this volume has been delayed owing to war conditions, but the delay is the less to be regretted in that it has allowed the inclusion of recent work on the subject. Much of the subject-matter is of common knowledge to lichenologists, but in the co-ordination and arrangement of the facts the original papers are cited throughout. The method has somewhat burdened the pages with citations, but it is hoped that, as a book of reference, its value has been enhanced thereby. The Glossary includes terms used in lichenology, or those with a special lichenological meaning. The Bibliography refers only to works consulted in the preparation of this volume. To save space, etc., the titles of books and papers quoted in the text are generally translated and curtailed: full citations will be found in the Bibliography. Subject-matter has been omitted from the index: references of importance will be found in the Table of Contents or in the Glossary.

I would record my thanks to those who have generously helped me during the preparation of the volume: to Lady Muriel Percy for taking notes of spore production, and to Dr Cavers for the loan of reprints. Prof. Potter and Dr Somerville Hastings placed at my disposal their photographs of the living plants. Free use has been made of published text-figures which are duly acknowledged.

I have throughout had the inestimable advantage of being able to consult freely the library and herbarium of the British Museum, and have thus been able to verify references to plants as well as to literature. A special debt of gratitude is due to my colleagues Mr Gepp and Mr Ramsbottom for their unfailing assistance and advice.

A. L. S.

London, February, 1920

CONTENTS

GLOSSARY

Acrogenous, borne at the tips of hyphae; [see spermatium], [312].

Allelositismus, Norman’s term to describe the thallus of Moriolaceae (mutualism), [313].

Amorphous cortex, formed of indistinct hyphae with thickened walls; [cf. decomposed cortex].

Amphithecium, thalline margin of the apothecium, [157].

Antagonistic symbiosis, hurtful parasitism of one lichen on another, [261 et seq.]

Apothecium, open or disc-shaped fructification, [11], [156] et passim. Veiled apothecium, [169]. Closed or open at first, [182].

Archilichens, lichens in which the gonidia are bright green (Chlorophyceae), [52], [55] et passim.

Ardella, the small spot-like apothecium of Arthoniaceae, [158].

Areola (areolate), small space marked out by lines or chinks on the surface of the thallus, [73] et passim.

Arthrosterigma, septate tissue-like sterigma (spermatiophore), [197].

Ascogonium, the cell or cells that produce ascogenous hyphae, [180 et seq.]

Ascolichens, lichens in which the fungus is an Ascomycete, [159], [173] et passim.

Ascus, enlarged cell in which a definite number of spores (usually 8) are developed; [cf. theca], [157], [184].

Ascyphous, podetia without scyphi, [119] et passim.

Biatorine, apothecia that are soft or waxy, and often brightly coloured, as in Biatora, [158].

Blasteniospore, [see polarilocular spore].

Byssoid, slender, thread-like, as in the old genus Byssus.

Campylidium, supposed new type of fructification in lichens, [191].

Capitulum, the globose apical apothecium of Coniocarpineae; [cf. mazaedium], [319].

Carpogonium, primordial stage of fructification, [160], [164] et passim.

Cephalodium, irregular outgrowth from the thallus enclosing mostly blue-green algae; or intruded packet of algae within the thallus, [11], [133] et passim.

Chrondroid, hard and tough like cartilage, a term applied to strengthening strands of hyphae, [104], [114].

Chroolepoid, like the genus Chroolepis (Trentepohlia).

Chrysogonidia, yellow algal cells (Trentepohlia).

Cilium, hair-like outgrowth from surface or margin of thallus, or margin of apothecium, [91].

Consortium (consortism), mutual association of fungus and alga (Reinke); also termed “mutualism,” [31], [313].

Corticolous, living on the bark of trees, [363].

Crustaceous, crust-like closely adhering thallus, [70-79].

Cyphella, minute cup-like depression on the under surface of the thallus (Sticta, etc.), [11], [126].

Decomposed, term applied to cortex formed of gelatinous indistinct hyphae (amorphous), [73-81] et passim, [357].

Determinate, thallus with a definite outline, [72].

Dimidiate, term applied to the perithecium, when the outer wall covers only the upper portion, [159].

Discoid, disc-like, an open rounded apothecium, [156].

Discolichens, in which the fructification is an apothecium, [160 et seq.]

Dual hypothesis, the theory of two organisms present in the lichen thallus, [27 et seq.]

Effigurate, having a distinct form or figure; [cf. placodioid], [80], [201].

Endobasidial, Steiner’s term for sporophore with a secondary sporiferous branch, [200].

Endogenous, produced internally, as spores in an ascus, [179]; [see also under thallus].

Endolithic, embedded in the rock, [75].

Endosaprophytism, term used by Elenkin for destruction of the algal contents by enzymes of the fungus, [36].

Entire, term applied to the perithecium when completely surrounded by an outer wall, [159].

Epilithic, growing on the rock surface, [70].

Epiphloeodal, thallus growing on the surface of the bark, [77].

Epiphyllous, growing on leaves, [363].

Epithecium, upper layer of thecium (hymenium), [158].

Erratic lichens, unattached and drifting, [259].

Exobasidial, Steiner’s term for sporophore without a secondary sporiferous branch, [200].

Exogenous, produced externally, as spores on tips of hyphae; [see also under thallus].

Fastigiate cortex, formed of clustered parallel hyphal branches vertical to long axis of thallus, [82].

Fat-cells, specialized hyphal cells containing fat or oil, [75], [215] et passim.

Fibrous cortex, formed of hyphae parallel with long axis of thallus, [82].

Filamentous, slender thallus with radiate structure, [101 et seq.]

Foliose, lichens with a leafy form and stratose in structure, [82-97].

Foveolae, Foveolate, pitted, [373].

Fruticose, upright or pendulous thallus, with radiate structure, [101 et seq.]

Fulcrum, term used by Steiner for sporophore, [200].

Gloeolichens, lichens in which the gonidia are Gloeocapsa or Chroococcus, [284], [373], [389].

Gonidium, the algal constituent of the lichen thallus, [20-45] et passim.

Gonimium, blue-green algal cell (Myxophyceae), constituent of the lichen thallus, [52].

Goniocysts, nests of gonidia in Moriolaceae, [313].

Gyrose, curved backward and forward, furrowed fruit of Gyrophora, [184].

Hapteron, aerial organ of attachment, [94], [122].

Haustorium, outgrowth or branch of a hypha serving as an organ of suction, [32].

Helotism, state of servitude, term used to denote the relation of alga to fungus in lichen organization, [38], [40].

Heteromerous, fungal and algal constituents of the thallus in definite strata, [13], [68], [305] et passim.

Hold-fast, rooting organ of thallus, [109], [122] et passim.

Homobium, interdependent association of fungus and alga, [31].

Homoiomerous, fungal and algal constituents more or less mixed in the thallus, [13], [68], [305] et passim.

Hymenial gonidia, algal cells in the hymenium, [30], [314], [315], [327].

Hymenium, apothecial tissue consisting of asci and paraphyses; [cf. thecium], [157].

Hymenolichens, lichens of which the fungal constituent is a Hymenomycete, [152-154], [342].

Hypophloeodal, thallus growing within the bark, [78], [364].

Hypothallus, first growth of hyphae (proto- or pro-thallus) persisting as hyphal growth at base or margin of the thallus, [70], [257] et passim.

Hypothecium, layer below the thecium (hymenium), [157].

Intricate cortex, composed of hyphae densely interwoven but not coalescent, [83].

Isidium, coral-like outgrowth on the lichen thallus, [149-151].

Lecanorine, apothecium with a thalline margin as in Lecanora, [158].

Lecideine, apothecium usually dark-coloured or carbonaceous and without a thalline margin, [158].

Leprose, mealy or scurfy, like the old form genera, Lepra, Lepraria, [191].

Lichen-acids, organic acids peculiar to lichens, [221 et seq.]

Lignicolous, living on wood or trees, [366].

Lirella, long narrow apothecium of Graphideae, [158].

Mazaedium, fructification of Coniocarpineae, the spores lying as a powdery mass in the capitulum, [176].

Medulla, the loose hyphal layer in the interior of the thallus, [88] et passim.

Meristematic, term applied by Wainio to growing hyphae, [48].

Microgonidia, term applied by Minks to minute greenish bodies in lichen hyphae, [26].

Multi-septate, term applied to spores with numerous transverse septa, [316 et seq.]

Murali-divided, Muriform, term applied to spores divided like the masonry of a wall, [187].

Oidium, reproductive cell formed by the breaking up of the hyphae, [189].

Oil-cell, hyphal cell containing fat globules, [215].

Orculiform, [see polarilocular].

Orthidium, supposed new type of fructification in lichens, [192].

Palisade-cells, the terminal cells of the hyphae forming the fastigiate cortex, [82], [83].

Panniform, having a felted or matted appearance, [260].

Paraphysis, sterile filament in the hymenium, [157].

Parasymbiosis, associated harmless but not mutually useful growth of two organisms, [263].

Parathecium, hyphal layer round the apothecium, [157].

Peltate, term applied to orbicular and horizontal apothecia in the form of a shield, [336].

Perithecium, roundish fructification usually with an apical opening (ostiole) containing ascospores, [158] et passim.

Pervious, referring to scyphi with an opening at the base (Perviae), [118].

Phycolichens, lichens in which the gonidia are blue-green (Myxophyceae), [52] et passim.

Placodioid, thallus with a squamulose determinate outline, generally orbicular; [cf. effigurate], [80].

Placodiomorph, [see polarilocular].

Plectenchyma (Plectenchymatous), pseudoparenchyma of fungi and lichens, [66] et passim.

Pleurogenous, borne laterally on hyphal cells; [see spermatium], [312].

Pluri-septate, term applied to spores with several transverse septa, [321 et seq.]

Podetium, stalk-like secondary thallus of Cladoniaceae, [114], [293 et seq.]

Polarilocular, Polaribilocular, two-celled spores with thick median wall traversed by a connecting tube, [188], [340-341].

Polytomous, arising of several branches of the podetium from one level, [118].

Proper margin, the hyphal margin surrounding the apothecium, [157].

Prothallus, Protothallus, first stages of hyphal growth; [cf. hypothallus], [71].

Pycnidiospores, stylospores borne in pycnidia, [198] et passim.

Pycnidium, roundish fructification, usually with an opening at the apex, containing sporophores and stylospores; [cf. spermogonium], [192 et seq.]

Pyrenolichens, in which the fructification is a closed perithecium, [173] et passim.

Radiate thallus, the tissues radiate from a centre, [98 et seq.]

Rhagadiose, deeply chinked, [74]; [cf. rimose].

Rhizina, attaching “rootlet,” [92-94].

Rimose, Rimulose, cleft or chinked into areolae, [73].

Rimose-diffract, widely cracked or chinked, [74].

Scutellate, shaped like a platter, [156].

Scyphus, cup-like dilatation of the podetium, [111], [117].

Signature, a term in ancient medicine to signify the resemblance of a plant to any part of the human body, [406], [409].

Soralium, group of soredia surrounded by a definite margin, [144].

Soredium, minute separable particle arising from the gonidial tissue of the thallus, and consisting of algae and hyphae, [141].

Spermatium, spore-like body borne in the spermogonium, regarded as a non-motile male cell or as a pycnidiospore, [201].

Spermogonium, roundish closed receptacle containing spermatia, [192].

Sphaeroid-cell, swollen hyphal cell, containing fat globules, [215].

Squamule, a small thalline lobe or scale, [74] et passim.

Sterigma, Nylander’s term for the spermatiophore, [197].

Stratose thallus, where the tissues are in horizontal layers, [70].

Stratum, a layer of tissue in the thallus, [70].

Symbiont, one of two dissimilar organisms living together, [32].

Symbiosis, a living together of dissimilar organisms, also termed commensalism, [31, 32 et seq.]

Tegulicolous, living on tiles, [369].

Terebrator, boring apparatus, term used by Lindau for the lichen “trichogyne,” [179].

Thalline margin, an apothecial margin formed of and usually coloured like the thallus; [cf. amphithecium].

Thallus, vegetative body or soma of the lichen plant, [11], [421]. Endogenous thallus in which the alga predominates, [68]. Exogenous thallus in which the fungus predominates, [69].

Theca, enlarged cell containing spores; [cf. ascus].

Thecium, layer of tissue in the apothecium consisting of asci and paraphyses; [cf. hymenium], [157].

Trichogyne, prolongation of the egg-cell in Florideae which acts as a receptive tube; septate hypha in lichens arising from the ascogonium, [160], [177-181], [273].

Woronin’s hypha, a coiled hypha occurring in the centre of the fruit primordium, [159], [163].

ERRATA

Transcriber’s Note: The errata have been corrected.

INTRODUCTION

Lichens are, with few exceptions, perennial aerial plants of somewhat lowly organization. In the form of spreading encrustations, horizontal leafy expansions, of upright strap-shaped fronds or of pendulous filaments, they take possession of the tree-trunks, palings, walls, rocks or even soil that afford them a suitable and stable foothold. The vegetative body, or thallus, which may be extremely long-lived, is of varying colour, white, yellow, brown, grey or black. The great majority of lichens are Ascolichens and reproduction is by ascospores produced in open or closed fruits (apothecia or perithecia) which often differ in colour from the thallus. There are a few Hymenolichens which form basidiospores. Vegetative reproduction by soredia is frequent.

Lichens abound everywhere, from the sea-shore to the tops of high mountains, where indeed the covering of perpetual snow is the only barrier to their advance; but owing to their slow growth and long duration, they are more seriously affected than are the higher plants by chemical or other atmospheric impurities and they are killed out by the smoke of large towns: only a few species are able to persist in somewhat depauperate form in or near the great centres of population or of industry.

The distinguishing feature of lichens is their composite nature: they consist of two distinct and dissimilar organisms, a fungus and an alga, which, in the lichen thallus, are associated in some kind of symbiotic union, each symbiont contributing in varying degree to the common support: it is a more or less unique and not unsuccessful venture in plant-life. The algae—Chlorophyceae or Myxophyceae—that become lichen symbionts or “gonidia” are of simple structure, and, in a free condition, are generally to be found in or near localities that are also the customary habitats of lichens. The fungus is the predominant partner in the alliance as it forms the fruiting bodies. It belongs to the Ascomycetes[1], except in a few tropical lichens (Hymenolichens), in which the fungus is a Basidiomycete. These two types of plants (algae and fungi) belonging severally to many different genera and species have developed in their associated life this new lichen organism, different from themselves as well as from all other plants, not only morphologically but physiologically. Thus there has arisen a distinct class, with families, genera and species, which through all their varying forms retain the characteristics peculiar to lichens.

In the absence of any “visible” seed, there was much speculation in early days as to the genesis of all the lower plants and many opinions were hazarded as to their origin. Luyken[2], for instance, thought that lichens were compounded of air and moisture. Hornschuch[3] traced their origin to a vegetable infusorium, Monas Lens, which became transformed to green matter and was further developed by the continued action of light and air, not only to lichens, but to algae and mosses, the type of plant finally evolved being determined by the varying atmospheric influences along with the chemical nature of the substratum. An account[4] is published of Nees von Esenbeck, on a botanical excursion, pointing out to his students the green substance, Lepraria botryoides, which covered the lower reaches of walls and rocks, while higher up it assumed the grey lichen hue. This afforded him sufficient proof that the green matter in that dry situation changed to lichens, just as in water it changed to algae. An adverse criticism by Dillenius[5] on a description of a lichen fructification is not inappropriate to those early theorists: “Ex quo apparet, quantum videre possint homines, si imaginatione polleant.”

A constant subject of speculation and of controversy was the origin of the green cells, so dissimilar to the general texture of the thallus. It was thought finally to have been established beyond dispute that they were formed directly from the colourless hyphae and, as a corollary, Protococcus and other algal cells living in the open were considered to be escaped gonidia or, as Wallroth[6] termed them, “unfortunate brood-cells,” his view being that they were the reproductive organs of the lichen plant that had failed to develop.

It was a step forward in the right direction when lichens were regarded as transformed algae, among others by Agardh[7], who believed that he had followed the change from Nostoc lichenoides to the lichen Collema limosum. Thenceforward their double resemblance, on the one hand to algae, on the other to fungi, was acknowledged, and influenced strongly the trend of study and investigation.

The announcement[8] by Schwendener[9] of the dual hypothesis solved the problem for most students, though the relation between the two symbionts is still a subject of controversy. The explanation given by Schwendener, and still held by some[10], that lichens were merely fungi parasitic on algae, was indeed a very inadequate conception of the lichen plant, and it was hotly contested by various lichenologists. Lauder Lindsay[11] dismissed the theory as “merely the most recent instance of German transcendentalism applied to the Lichens.” Earlier still, Nylander[12], in a paper dealing with cephalodia and their peculiar gonidia, had denounced it: “Locum sic suum dignum occupat algolichenomachia inter historias ridiculas, quae hodie haud paucae circa lichenes, majore imaginatione quam scientia, enarrantur.” He never changed his attitude and Crombie[13], wholly agreeing with his estimate of these “absurd tales,” translates a much later pronouncement by him[14]: “All these allegations belong to inept Schwendenerism and scarcely deserve even to be reviewed or castigated so puerile are they—the offspring of inexperience and of a light imagination. No true science there.” Crombie[15] himself in a first paper on this subject declared that “the new theory would necessitate their degradation from the position they have so long held as an independent class.” He scornfully rejected the whole subject as “a Romance of Lichenology, or the unnatural union between a captive Algal damsel and a tyrant Fungal master.” The nearest approach to any concession on the algal question occurs in a translation by Crombie[16] of one of Nylander’s papers. It is stated there that a saxicolous alga (Gongrosira Kütz.) had been found bearing the apothecia of Lecidea herbidula n. sp. Nylander adds: “This algological genus is one which readily passes into lichens.” At a later date, Crombie[17] was even more comprehensively contemptuous and wrote: “whether viewed anatomically or biologically, analytically or synthetically, it is instead of being true science, only the Romance of Lichenology.” These views were shared by many continental lichenologists and were indeed, as already stated, justified to a considerable extent: it was impossible to regard such a large and distinctive class of plants as merely fungi parasitic on the lower algae.

Controversy about lichens never dies down, and that view of their parasitic nature has been freshly promulgated among others by the American lichenologist Bruce Fink[18]. The genetic origin of the gonidia has also been restated by Elfving[19]: the various theories and views are discussed fully in the chapter on the lichen plant.

Much of the interest in lichens has centred round their symbiotic growth. No theory of simple parasitism can explain the association of the two plants: if one of the symbionts is withdrawn—either fungus or alga—the lichen as such ceases to exist. Together they form a healthy unit capable of development and change: a basis for progress along new lines. Permanent characters have been formed which are transmitted just as in other units of organic life.

A new view of the association has been advanced by F. and Mme Moreau[20]. They hold that the most characteristic lichen structures—more particularly the cortex—have been induced by the action of the alga on the fungus. The larger part of the thallus might therefore be regarded as equivalent to a gall: “it is a cecidium, an algal cecidium, a generalized biomorphogenesis.”

The morphological characters of lichens are of exceptional interest, conditioned as they are by the interaction of the two symbionts, and new structures have been evolved by the fungus which provides the general tissue system. Lichens are plants of physiological symbiotic origin, and that aspect of their life-history has been steadily kept in view in this work. There are many new requirements which have had to be met by the lichen hyphae, and the differences between them and the true fungal hyphae have been considered, as these are manifested in the internal economy of the compound plant, and in its reaction to external influences such as light, heat, moisture, etc.

The pioneers of botanical science were of necessity occupied almost exclusively with collecting and describing plants. As the number of known lichens gradually accumulated, affinities were recognized and more or less successful efforts were made to tabulate them in classes, orders, etc. It was a marvellous power of observation that enabled the early workers to arrange the first schemes of classification. Increasing knowledge aided by improved microscopes has necessitated changes, but the old fundamental “genus” Lichen is practically equivalent to the Class Lichenes.

The study of lichens has been a slow and gradual process, with a continual conflict of opinion as to the meaning of these puzzling plants—their structure, reproduction, manner of subsistence and classification as well as their relation to other plants. It has been found desirable to treat these different subjects from a historical aspect, as only thus can a true understanding be gained, or a true judgment formed as to the present condition of the science. It is the story of the evolution of lichenology as well as of lichens that has yielded so much of interest and importance.

The lichenologist may claim several advantages in the study of his subject: the abundant material almost everywhere to hand in country districts, the ease with which the plants are preserved, and, not least, the interest excited by the changes and variations induced by growth conditions; there are a whole series of problems and puzzles barely touched on as yet that are waiting to be solved.

In field work, it is important to note accurately and carefully the nature of the substratum as well as the locality. Crustaceous species should be gathered if possible along with part of the wood or rock to which they are attached; if they are scraped off, the pieces may be reassembled on gummed paper, but that is less satisfactory. The larger forms are more easily secured; they should be damped and then pressed before being laid away: the process flattens them, but it saves them from the risk of being crushed and broken, as when dry they are somewhat brittle. Moistening with water will largely restore their original form. All parts of the lichen, both thallus and fruit, can be examined with ease at any time as they do not sensibly alter in the herbarium, though they lose to some extent their colouring: the blue-grey forms, for instance, often become a uniform dingy brownish-grey.

Microscopic examination in the determination of species is necessary in many instances, but that disability—if it ranks as such—is shared by other cryptogams, and may possibly be considered an inducement rather than a deterrent to the study of lichens. For temporary examination of microscopic preparations, the normal condition is best observed by mounting them in water. If the plants are old and dry, the addition of a drop or two of potash—or ammonia—solution is often helpful in clearing the membranes of the cells and in restoring the shrivelled spores and paraphyses to their natural forms and dimensions.

If serial microtome sections are desired, more elaborate methods are required. For this purpose Peirce[21] has recommended that “when dealing with plants that are dry but still alive, the material should be thoroughly wetted and kept moist for two days, then killed and fixed in a saturated solution of corrosive sublimate in thirty-five per cent. alcohol.” The solution should be used hot: the usual methods of dehydrating and embedding in paraffin are then employed with extra precautions on account of the extremely brittle nature of lichens.

Another method that also gave good results has been proposed by French[22]: “first the lichen is put into 95 per cent. alcohol for 24 hours, then into thin celloidin and thick celloidin 24 hours each. After this the specimens are embedded in thick celloidin which is hardened in 70 per cent. alcohol for 24 hours and then cut.” French advises staining with borax carmine: it colours the fungal part pale carmine and the algal cells a greenish-red shade.

Modern research methods of work are generally described in full in the publications that are discussed in the following chapters. The student is referred to these original papers for information as to fixing, embedding, staining, etc.

Great use has been made of reagents in determining lichen species. They are extremely helpful and often give the clinching decision when morphological characters are obscure, especially if the plant has been much altered by the environment. It must be borne in mind, however, that a species is a morphological rather than a physiological unit, and it is not the structures but the cell-products that are affected by reagents. Those most commonly in use are saturated solutions of potash and of bleaching-powder (calcium hypochlorite). The former is cited in text-books as KOH or simply as K, the latter as CaCl or C. The C solution deteriorates quickly and must, therefore, be frequently renewed to produce the required reaction, i.e. some change of colour. These two reagents are used singly or, if conjointly, K followed by C. The significance of the colour changes has been considered in the discussion on lichen-acids.

Iodine is generally cited in connection with its staining effect on the hymenium of the fruit; the blue colour produced is, however, more general than was at one time supposed and is not peculiar to lichens; the asci of many fungi react similarly though to a less extent. The medullary hyphae in certain species also stain blue with iodine.

CHAPTER I
HISTORY OF LICHENOLOGY

A. Introductory

The term “lichen” is a word of Greek origin used by Theophrastus in his History of Plants to signify a superficial growth on the bark of olive-trees. The name was given in the early days of botanical study not to lichens, as we understand them, but to hepatics of the Marchantia type. Lichens themselves were generally described along with various other somewhat similar plants as “Muscus” (Moss) by the older writers, and more definitely as “Musco-fungus” by Morison[23]. In a botanical work published in 1700 by Tournefort[24] all the members of the vegetable kingdom then known were for the first time classified in genera, and the genus Lichen was reserved for the plants that have been so designated since that time, though Dillenius[25] in his works preferred the adjectival name Lichenoides.

A painstaking historical account of lichens up to the beginning of modern lichenology has been written by Krempelhuber[26], a German lichenologist. He has grouped the data compiled by him into a series of Periods, each one marked by some great advance in knowledge of the subject, though, as we shall see, the advance from period to period has been continuous and gradual. While following generally on the lines laid down by Krempelhuber, it will be possible to cite only the more prominent writers and it will be of much interest to British readers to note especially the work of our own botanists.

Krempelhuber’s periods are as follows:

I. From the earliest times to the end of the seventeenth century.

II. Dating from the arrangement of plants into classes called genera by Tournefort in 1694 to 1729.

III. From Micheli’s division of lichens into different orders in 1729 to 1780.

IV. The definite and reasoned establishment of lichen genera based on the structure of thallus and fruit by Weber in 1780 to 1803.

V. The arrangement of all known lichens under their respective genera by Acharius in 1803 to 1846.

VI. The recognition of spore characters in classification by De Notaris in 1846 to 1867.

A seventh period which includes modern lichenology, and which dates after the publication of Krempelhuber’s History, was ushered in by Schwendener’s announcement in 1867 of the hypothesis as to the dual nature of the lichen thallus. Schwendener’s theory gave a new impulse to the study of lichens and strongly influenced all succeeding investigations.

B. Period I. Previous to 1694

Our examination of lichen literature takes us back to Theophrastus, the disciple of Plato and Aristotle, who lived from 371 to 284 B.C., and who wrote a History of Plants, one of the earliest known treatises on Botany. Among the plants described by Theophrastus, there are evidently two lichens, one of which is either an Usnea or an Alectoria, and the other certainly Roccella tinctoria, the last-named an important economic plant likely to be well known for its valuable dyeing properties. The same or somewhat similar lichens are also probably alluded to by the Greek physician Dioscorides, in his work on Materia Medica, A.D. 68. About the same time Pliny the elder, who was a soldier and traveller as well as a voluminous writer, mentions them in his Natural History which was completed in 77 A.D.

During the centuries that followed, there was little study of Natural History, and, in any case, lichens were then and for a long time after considered to be of too little economic value to receive much attention.

In the sixteenth century there was a great awakening of scientific interest all over Europe, and, after the printing-press had come into general use, a number of books bearing on Botany were published. It will be necessary to chronicle only those that made distinct contributions to the knowledge of lichens.

The study of plants was at first entirely from a medical standpoint and one of the first works, and the first book on Natural History, printed in England, was the Grete Herball[27]. It was translated from a French work, Hortus sanitatis, and published by Peter Treveris in Southwark. One of the herbs recommended for various ailments is “Muscus arborum,” the tree-moss (Usnea). A somewhat crude figure accompanies the text.

Ruel[28] of Soissons in France, Dorstenius[29], Camerarius[30] and Tabernaemontanus[31] in Germany followed with works on medical or economic botany and they described, in addition to the tree-moss, several species of reputed value in the art of healing now known as Sticta (Lobaria) pulmonaria, Lobaria laetevirens, Cladonia pyxidata, Evernia prunastri and Cetraria islandica. Meanwhile L’Obel[32], a Fleming, who spent the latter part of his life in England and is said to have had charge of a physic garden at Hackney, was appointed botanist to James I. He published at Antwerp a large series of engravings of plants, and added a species of Ramalina to the growing list of recognized lichens. Dodoens[33], also a Fleming, records not only the Usnea of trees, but a smaller and more slender black form which is easily identifiable as Alectoria jubata. He also figures Lichen pulmonaria and gives the recipe for its use.

The best-known botanical book published at that time, however, is the Herball of John Gerard[34] of London, Master in Chirurgerie, who had a garden in Holborn. He recommends as medicinally valuable not only Usnea, but also Cladonia pyxidata, for which he coined the name “cuppe- or chalice-moss.” About the same time Schwenckfeld[35] recorded, among plants discovered by him in Silesia, lichens now familiar as Alectoria jubata, Cladonia rangiferina and a species of Peltigera.

Among the more important botanical writers of the seventeenth century may be cited Colonna[36] and Bauhin[37]. The former, an Italian, contributes, in his Ecphrasis, descriptions and figures of three additional species easily recognized as Physcia ciliaris, Xanthoria parietina and Ramalina calicaris. Kaspar Bauhin, a professor in Basle, who was one of the most advanced of the older botanists, was the first to use a binomial nomenclature for some of his plants. He gives a list in his Pinax of the lichens with which he was acquainted, one of them, Cladonia fimbriata, being a new plant.

John Parkinson’s[38] Herball is well known to English students; he adds one new species for England, Lobaria pulmonaria, already recorded on the Continent. Parkinson was an apothecary in London and held the office of the King’s Herbarist; his garden was situated in Long Acre. How’s[39] Phytographia is notable as being the first account of British plants compiled without reference to their healing properties. Five of the plants described by him are lichen species: “Lichen arborum sive pulmonaria” (Lobaria pulmonaria), “Lichen petraeus tinctorius” (Roccella), “Muscus arboreus” (Usnea), “Corallina montana” (Cladonia rangiferina) and “Muscus pixoides” (Cladonia). Several other British species were added by Merrett[40], who records in his Pinax, “Muscus arboreus umbilicatus” (Physcia ciliaris), “Muscus aureus tenuissimus” (Teloschistes flavicans), “Muscus caule rigido” (Alectoria) and “Lichen petraeus purpureus” (Parmelia omphalodes), the last-named, a rock lichen, being used, he tells us, for dyeing in Lancashire.

Merret or Merrett was librarian to the Royal College of Physicians. His Pinax was undertaken to replace How’s Phytographia published sixteen years previously and then already out of print. Merrett’s work was issued in 1666, but the first impression was destroyed in the great fire of London and most of the copies now extant are dated 1667. He arranged the species of plants in alphabetical order, but as the work was not critical it fell into disuse, being superseded by John Ray’s Catalogus and Synopsis. To Robert Plot[41] we owe the earliest record of Cladonia coccifera which had hitherto escaped notice; it was described and figured as a new and rare plant in the Natural History of Staffordshire[41]. Plot was the first Custos of Ashmole’s Museum in Oxford and he was also the first to prepare a County Natural History.

The greatest advance during this first period was made by Robert Morison[42], a Scotsman from Aberdeen. He studied medicine at Angers in France, superintended the Duke of Orleans’ garden at Blois, and finally, after his return to this country in 1669, became Keeper of the botanic garden at Oxford. In the third volume of his great work[42] on Oxford plants, which was not issued till after his death, the lichens are put in a separate group—“Musco-fungus”—and classified with some other plants under “Plantae Heteroclitae.” The publication of the volume projects into the next historical period.

Long before this date John Ray had begun to study and publish books on Botany. His Catalogue of English Plants[43] is considered to have commenced a new era in the study of the science. The Catalogue was followed by the History of Plants[44], and later by a Synopsis of British Plants[45], and in all of these books lichens find a place. Two editions of the Synopsis appeared during Ray’s lifetime, and to the second there is added an Appendix contributed by Samuel Doody which is entirely devoted to Cryptogamic plants, including not a few lichens—still called “Mosses”—discovered for the first time. Doody, himself an apothecary, took charge of the garden of the Apothecaries’ Society at Chelsea, but his chief interest was Cryptogamic Botany, a branch of the subject but little regarded before his day. Pulteney wrote of him as the “Dillenius of his time.”

Among Doody’s associates were the Rev. Adam Buddle, James Petiver and William Sherard. Buddle was primarily a collector and his herbarium is incorporated in the Sloane Herbarium at the British Museum. It contains lichens from all parts of the world, many of them contributed by Doody, Sherard and Petiver. Only a few of them bear British localities: several are from Hampstead where Buddle had a church.

The Society of Apothecaries had been founded in 1617 and the members acquired land on the river-front at Chelsea, which was extended later and made into a Physick Garden. James Petiver[46] was one of the first Demonstrators of Plants to the Society in connection with the garden, and one of his duties was to conduct the annual herborizing tours of the apprentices in search of plants. He thus collected a large herbarium on the annual excursions, as well as on shorter visits to the more immediate neighbourhood of London. He wrote many tracts on Natural History subjects, and in these some lichens are included. He was one of the best known of Ray’s correspondents, and owing to his connection with the Physic Garden received plants from naturalists in foreign countries.

Sherard, another of Doody’s friends, had studied abroad under Tournefort and was full of enthusiasm for Natural Science. It was he who brought Dillenius to England and finally nominated him for the position of the first Sherardian Professor of Botany at Oxford. Another well-known contemporary botanist was Leonard Plukenet[47] who had a botanical garden at Old Palace Yard, Westminster. He wrote several botanical works in which lichens are included.

Morison is the only one of all the botanists of the time who recognized lichens as a group distinct from mosses, algae or liverworts, and even he had very vague ideas as to their development. Malpighi[48] had noted the presence of soredia on the thallus of some species, and regarded them as seeds. Porta[49], a Neapolitan, has been quoted by Krempelhuber as probably the first to discover and place on record the direct growth of lichen fronds from green matter on the trunks of trees.

C. Period II. 1694-1729

The second Period is ushered in with the publication of a French work, Les Élémens de Botanique by Tournefort[50], who was one of the greatest botanists of the time. His object was—“to facilitate the knowledge of plants and to disentangle a science which had been neglected because it was found to be full of confusion and obscurity.” Up to this date all plants were classified or listed as individual species. It was Tournefort who first arranged them in groups which he designated “genera” and he gave a careful diagnosis of each genus.

Les Élémens was successful enough to warrant the publication a few years later of a larger Latin edition entitled Institutiones[51] and thus fitted for a wider circulation. Under the genus Lichen, he included plants “lacking flowers but with a true cup-shaped shallow fruit, with very minute pollen or seed which appeared to be subrotund under the microscope.” Not only the description but the figures prove that he was dealing with ascospores and not merely soredia, though under Lichen along with true members of the “genus” he has placed a Marchantia, the moss Splachnum and a fern. A few lichens were placed by him in another genus Coralloides.

Tournefort’s system was of great service in promoting the study of Botany: his method of classification was at once adopted by the German writer Rupp[52] who published a Flora of plants from Jena. Among these plants are included twenty-five species of lichens, several of which he considered new discoveries, no fewer than five being some form of Lichen gelatinosus (Collema). Buxbaum[53], in his enumeration of plants from Halle, finds place for forty-nine lichen species, with, in addition, eleven species of Coralloides; and Vaillant[54] in listing the plants that grew in the neighbourhood of Paris gives thirty-three species for the genus Lichen of which a large number are figured, among them species of Ramalina, Parmelia, Cladonia, etc.

In England, however, Dillenius[55], who at this time brought out a third edition of Ray’s Synopsis and some years later his own Historia Muscorum, still described most of his lichens as “Lichenoides” or “Coralloides”; and no other work of note was published in our country until after the Linnaean system of classification and of nomenclature was introduced.

D. Period III. 1729-1780

Lichens were henceforth regarded as a distinct genus or section of plants. Micheli[56], an Italian botanist, Keeper of the Grand Duke’s Gardens in Florence, realized the desirability of still further delimitation, and he broke up Tournefort’s large comprehensive genera into numerical Orders. In the genus Lichen, he found occasion for 38 of these Orders, determined mainly by the character of the thallus, and the position on it of apothecia and soredia. He enumerates the species, many of them new discoveries, though not all of them recognizable now. His great work on Plants is enriched by a series of beautiful figures. It was published in 1729 and marks the beginning of a new Period—a new outlook on botanical science. Micheli regarded the apothecia of lichens as “floral receptacles,” and the soredia as the seed, because he had himself followed the development of lichen fronds from soredia.

The next writer of distinction is the afore-mentioned Dillen or Dillenius. He was a native of Darmstadt and began his scientific career in the University of Giessen. His first published work[57] was an account of plants that were to be found near Giessen in the different months of the year. Mosses and lichens he has assigned to December and January. Sherard induced him to come to England in 1721, and at first engaged his services in arranging the large collections of plants which he, Sherard, had brought from Smyrna or acquired from other sources.

Three years after his arrival Dillenius had prepared the third edition of Ray’s Synopsis for the press, but without putting his name on the title-page[58]. Sherard explained, in a letter to Dr Richardson of Bierly in Yorkshire, that “our people can’t agree about an editor, they are unwilling a foreigner should put his name to it.” Dillenius, who was quite aware of the prejudice against aliens, himself writes also to Dr Richardson: “there being some apprehension (me being a foreigner) of making natives uneasy if I should publicate it in my name.” Lichens were already engaging his attention, and descriptions of 91 species were added to Ray’s work. So well did this edition meet the requirements of the age, that the Synopsis remained the text-book of British Botany until the publication of Hudson’s Flora Anglica in 1762.

William Sherard died in 1728. He left his books and plates to the University of Oxford with a sum of money to endow a Professorship of Botany. In his will he had nominated Dr Dillenius for the post. The great German botanist was accordingly appointed and became the first Sherardian Professor of Botany, though he did not remove to Oxford till 1734. The following years were devoted by him to the preparation of Historia Muscorum, which was finally published in 1741. It includes an account of the then known liverworts, mosses and lichens. The latter—still considered by Dillenius as belonging to mosses—were grouped under three genera, Usnea, Coralloides and Lichenoides. The descriptions and figures are excellent, and his notes on occasional lichen characteristics and on localities are full of interest. His lichen herbarium, which still exists at Oxford, mounted with the utmost care and neatness, has been critically examined by Nylander and Crombie[59] and many of the species identified.

Dillenius was ignorant of, or rejected, Micheli’s method of classification, adopting instead the form of the thallus as a guide to relationship. He also differed from him in his views as to propagation, regarding the soredia as the pollen of the lichen, and the apothecia as the seed-vessels, or even in certain cases as young plants.

Shortly after the publication of Dillenius’ Historia, appeared Haller’s[60] Systematic and Descriptive list of plants indigenous to Switzerland. The lichens are described as without visible leaves or stamens but with “corpuscula” instead of flowers and leaves. He arranged his lichen species, 160 in all, under seven different Orders: 1. “Lichenes Corniculati and Pyxidati”; 2. “L. Coralloidei”; 3. “L. Fruticosi”; 4. “L. Pulmonarii”; 5. “L. Crustacei” (with flower-shields); 6. “L. Scutellis” (with shields but with little or no thallus); and 7. “L. Crustacei” (without shields).

This period extends till near the end of the eighteenth century, and thus includes within its scope the foundation of the binomial system of naming plants established by Linnaeus[61]. The renowned Swedish botanist rather scorned lichens as “rustici pauperrimi,” happily translated by Schneider[62] as the “poor trash of vegetation,” but he named and listed about 80 species. He divided his solitary genus Lichen into sections: 1. “Leprosi tuberculati”; 2. “Leprosi scutellati”; 3. “Imbricati”; 4. “Foliacei”; 5. “Coriacei”; 6. “Scyphiferi”; 7. “Filamentosi.” By this ordered sequence Linnaeus showed his appreciation of development, beginning, as he does, with the leprose crustaceous thallus and continuing up to the most highly organized filamentous forms. He and his followers still included the genus Lichen among Algae.

A voluminous History of Plants had been published in 1751 by Sir John Hill[63], the first superintendent to be appointed to the Royal Gardens, Kew. In the History lichens are included under the Class “Mosses,” and are divided into several vaguely limited “genera”—Usnea, tree mosses, consisting of filaments only; Platysma, flat branched tree mosses, such as lung-wort; Cladonia, the orchil and coralline mosses, such as Cladonia furcata; Pyxidium, the cup-mosses; and Placodium, the crustaceous, friable or gelatinous forms. A number of plants are somewhat obscurely described under each genus. Not only were these new Lichen genera suggested by him, but among his plants are such binomials as Usnea compressa, Platysma corniculatum, Cladonia furcata and Cladonia tophacea; other lichens are trinomial or are indicated, in the way then customary, by a whole sentence. Hill’s studies embraced a wide variety of subjects; he had flashes of insight, but not enough concentration to make an effective application of his ideas. In his Flora Britannica[64], which was compiled after the publication of Linnaeus’s Species Plantarum, he abandoned his own arrangement in favour of the one introduced by Linnaeus and accepted again the single genus Lichen.

Sir William Watson[65], a London apothecary and physician of scientific repute at this period, proposed a rearrangement and some alteration of Linnaeus’s sections. He had failed to grasp the principle of development, but he gives a good general account of the various groups. Watson was the progenitor of those who decry the makers and multipliers of species. So in regard to Micheli, who had increased the number to “298,” he writes: “it is to be regretted, that so indefatigable an author, one whose genius particularly led him to scrutinize the minuter subjects of the science, should have been so solicitous to increase the number of species under all his genera: an error this, which tends to great confusion and embarrassment, and must retard the progress and real improvement of the botanic science.” Linnaeus however in redressing the balance earned his full approbation: “He has so far retrenched the genus (Lichen) that in his general enumeration of plants he recounts only 80 species belonging to it.”

Linnaeus’s binomial system was almost at once adopted by the whole botanical world and the discovery and tabulation of lichens as well as of other plants proceeded apace. Scopoli’s[66] Flora Carniolica, for instance, published in 1760, still adhered to the old descriptive method of nomenclature, but a second edition, issued twelve years later, is based on the new system: it includes 54 lichen species.

About this time Adanson[67] proposed a new classification of plants, dividing them into families, and these again into sections and genera. He transferred the lichens to the Family “Fungi,” and one of his sections contains a number of lichen genera, the names of these being culled from previous workers, Dillenius, Hill, etc. A few new ones are added by himself, and one of them, Graphis, still ranks as a good genus.

In England, Hudson[68], who was an apothecary and became sub-librarian of the British Museum, followed Linnaeus both in the first and later editions of the Flora Anglica. He records 102 lichen species. Withering[69] was also engaged, about this time, in compiling his Arrangement of Plants. He translated Linnaeus’s term “Algae” into the English word “Thongs,” the lichens being designated as “Cupthongs.” In later editions, he simply classifies lichens as such. Lightfoot[70], whose descriptive and economic notes are full of interest, records 103 lichens in the Flora Scotica, and Dickson[71] shortly after published a number of species from Scotland, some of them hitherto undescribed. Dickson was a nurseryman who settled in London, and his avocations kept him in touch with plant-lovers and with travellers in many lands.

E. Period IV. 1780-1803

The inevitable next advance was made by Weber[72] who at the time was a Professor at Kiel. In a first work dealing with lichens he had followed Linnaeus; then he published a new method of classification in which the lichens are considered as an independent Order of Cryptogamia, and that Order, called “Aspidoferae,” he subdivided into genera. His ideas had been partly anticipated by Hill and by Adanson, but the work of Weber indicates a more correct view of the nature of lichens. He established eight fairly well-marked genera, viz. Verrucaria, Tubercularia, Sphaerocephalum and Placodium, which were based on fruit-characters, the thallus being crustaceous and rather insignificant, and a second group Lichen, Collema, Cladonia and Usnea, in which the thallus ranked first in importance. Though Weber’s scheme was published in 1780, it did not at first secure much attention. The great authority of Linnaeus dominated so strongly the botany of the period that for a long time no change was welcomed or even tolerated.

In our own country Relhan at Cambridge and Sibthorp[73] at Oxford were making extensive studies of plants. The latter was content to follow Linnaeus in his treatment of lichens. Relhan[74] also grouped his lichens under one genus though, in a second edition of his Flora, he broke away from the Linnaean tradition and adopted the classification of Acharius.

Extensive contributions to the knowledge of English plants generally were made by Sir James Edward Smith[75] who, in 1788, founded the Linnean Society of London of which he was President until his death in 1828. He began his great work, English Botany, in 1790 with James Sowerby as artist. Smith’s and Sowerby’s part of the work came to an end in 1814; but a supplement was begun in 1831 by Hooker who had the assistance of Sowerby’s sons in preparing the drawings. Nearly all the lichens recorded by Smith are published simply as Lichen, and his Botany thus belongs to the period under discussion, though in time it stretches far beyond.

Continental lichenologists had been more receptive to new ideas, and other genera were gradually added to Weber’s list, notably by Hoffmann[76] and Persoon[77].

For a long time little was known of the lichens of other than European countries. Buxbaum[78] in the East, Petiver[79] and Hans Sloane[80] in the West made the first exotic records. The latter notes how frequently lichens grew on the imported Jesuit’s bark, and he quaintly suggests in regard to some of these species that they may be identical with the “hyssop that springeth out of the wall.” It was not however till towards the end of the eighteenth century that much attention was given to foreign lichens, when Swartz[81] in the West Indies and Desfontaines[82] in N. Africa collected and recorded a fair number. Swartz describes about twenty species collected on his journey through the West Indian Islands (1783-87).

Interest was also growing in other aspects of lichenology. Georgi[83], a Russian Professor, was the first to make a chemical analysis of lichens. He experimented on some of the larger forms and extracted and examined the mucilaginous contents of Ramalina farinacea, Platysma glaucum, Lobaria pulmonaria, etc., which he collected from birch and pine trees. About this time also the French scientists Willomet[84], Amoreux and Hoffmann jointly published theses setting forth the economic value of such lichens as were used in the arts, as food, or as medicine.

F. Period V. 1803-1846

The fine constructive work of Acharius appropriately begins a new era in the history of lichenology. Previous writers had indeed included lichens in their survey of plants, but always as a somewhat side issue. Acharius made them a subject of special study, and by his scientific system of classification raised them to the rank of the other great classes of plants.

Acharius was a country doctor at Wadstena on Lake Mälar in Sweden, as he himself calls it, “the country of lichens.” He was attracted to the study of them by their singular mode of growth and organization, both of thallus and reproductive organs, for which reason he finally judged that lichens should be considered as a distinct Order of Cryptogamia.

In his first tentative work[85] he had followed his great compatriot Linnaeus, classifying all the species known to him under the one genus Lichen, though he had progressed so far as to divide the unwieldy Genus into Families and these again into Tribes, these latter having each a tribal designation such as Verrucaria, Opegrapha, etc. He established in all twenty-eight tribes which, at a later stage, he transformed into genera after the example of Weber.

Acharius, from the beginning of his work, had allowed great importance to the structure of the apothecia as a diagnostic character though scarcely recognizing them as true fruits. He gave expression to his more mature views first in the Methodus Lichenum[86], then subsequently in the larger Lichenographia Universalia[87]. In the latter work there are forty-one genera arranged under different divisions; the species are given short and succinct descriptions, with habitat, locality and synonymy. No material alteration was made in the Synopsis Lichenum[88], a more condensed work which he published a few years later.

The Cryptogamia are divided by Acharius into six “Families,” one of which, “Lichenes,” is distinguished, he finds, by two methods of propagation: by propagula (soredia) and by spores produced in apothecia. He divides the family into classes characterized solely by fruit characters, and these again into orders, genera and species, of which diagnoses are given. With fuller knowledge many changes and rearrangements have been found necessary in the application and extension of the system, but that in no way detracts from the value of the work as a whole.

In addition to founding a scientific classification, Acharius invented a terminology for the structures peculiar to lichens. We owe to him the names and descriptions of “thallus,” “podetium,” “apothecium,” “perithecium,” “soredium,” “cyphella” and “cephalodium,” the last word however with a different meaning from the one now given to it. He proposed several others, some of which are redundant or have fallen into disuse, but many of his terms as we see have stood the test of time and have been found of service in allied branches of botany.

Lichens were studied with great zest by the men of that day. Hue[89] recalls a rather startling incident in this connection: Wahlberg, it is said, had informed Dufour that he had sent a large collection of lichens from Spain to Acharius who was so excited on receiving them, that he fell ill and died in a few days (Aug. 14th, 1819). Dufour, however, had added the comment that the illness and death might after all be merely a coincidence.

Among contemporary botanists, we find that De Candolle[90] in the volume he contributed to Lamarck’s French Flora, quotes only from the earlier work of Acharius. He had probably not then seen the Methodus, as he uses none of the new terms; the lichens of the volume are arranged under genera which are based more or less on the position of the apothecia on the thallus. Flörke[91], the next writer of consequence, frankly accepts the terminology and the new view of classification, though differing on some minor points.

Two lists of lichens, neither of particular note, were published at this time in our country: one by Hugh Davies[92] for Wales, which adheres to the Linnaean system, and the other by Forster[93] of lichens round Tonbridge. Though Forster adopts the genera of Acharius, he includes lichens among algae. A more important publication was S. F. Gray’s[94] Natural Arrangement of British Plants. Gray, who was a druggist in Walsall and afterwards a lecturer on botany in London, was only nominally[95] the author, as it was mainly the work of his son John Edward Gray[96], sometime Keeper of Zoology in the British Museum. Gray was the first to apply the principles of the Natural System of classification to British plants, but the work was opposed by British botanists of his day. The years following the French Revolution and the Napoleonic wars were full of bitter feeling and of prejudice, and anything emanating, as did the Natural System, from France was rejected as unworthy of consideration.

In the Natural Arrangement, Gray followed Acharius in his treatment of lichens; but whereas Acharius, though here and there confusing fungus species with lichens, had been clear-sighted enough to avoid all intermixture of fungus genera, with the exception of one only, the sterile genus Rhizomorpha, Gray had allowed the interpolation of several, such as Hysterium, Xylaria, Hypoxylon, etc. He had also raised many of Acharius’s subgenera and divisions to the rank of genera, thus largely increasing their number. This oversplitting of well-defined genera has somewhat weakened Gray’s work and he has not received from later writers the attention he deserves.

The lichens of Hooker’s[97] Flora Scotica, which is synchronous with Gray’s work, number 195 species, an increase of about 90 for Scotland since the publication of Lightfoot’s Flora more than 40 years before. Hooker also followed Acharius in his classification of lichens both in the Flora Scotica and in the Supplement to English Botany[98], which was undertaken by the younger Sowerbys and himself. To that work Borrer (1781-1862), a keen lichenologist, supplied many new and rare lichens collected mostly in Sussex.

It is a matter of regret that Greville should have so entirely ignored lichens in his great work on Scottish Cryptogams[99]. The two species of Lichina are the only ones he figured, and these he took to be algae. He[100] was well acquainted with lichens, for in the Flora Edinensis he lists 128 species for the Edinburgh district, arranging the genera under “Lichenes” with the exception of Opegrapha and Verrucaria which are placed with the fungus genus Poronia in “Hypoxyla.” Though he cites the publications of Acharius, he does not employ his scientific terms, possibly because he was writing his diagnoses in English. Two other British works of this time still remain to be chronicled: Hooker’s[101] contributions to Smith’s English Flora and Taylor’s[102] work on lichens in Mackay’s Flora Hibernica. Through these the knowledge of the subject was very largely extended in our country.

The classification of lichens and their place in the vegetable kingdom were now firmly established on the lines laid down by Acharius. Fries[103] in his important work Lichenographia Europaea more or less followed his distinguished countryman. The uncertainty as to the position and relationship of lichens had rendered the task of systematic arrangement one of peculiar difficulty and had unduly absorbed attention; but now that a satisfactory order had been established in the chaos of forms, the way was clear for other aspects of the study. Several writers expressed their views by suggesting somewhat different methods of classification, others wrote monographs of separate groups, or genera. Fée[104] published an Essay on the Cryptogams (mostly lichens) that grew on officinal exotic barks; Flörke[105] took up the difficult genus Cladonia; Wallroth[106] also wrote on Cladonia; Delise[107] on Sticta, and Chevalier[108] published a long and elaborate account of Graphideae.

Wallroth and Meyer at this time published, simultaneously, important studies on the general morphology and physiology of lichens. Wallroth[109] had contemplated an even larger work on the Natural History of Lichens, but only two of the volumes reached publication. In the first of these he devoted much attention to the “gonidia” or “brood-cells” and established the distinction between the heteromerous and homoiomerous distribution of green cells within the thallus; he also describes with great detail the “morphosis” and “metamorphosis” of the vegetative body. In the second volume he discusses their physiology—the contents and products of the thallus, colouring, nutrition, season of development, etc.—and finally the pathology of these organisms. He made no great use of the compound microscope, and his studies were confined to phenomena that could be observed with a single lens.

Meyer’s[110] work contains a still more exact study of the anatomy and physiology of lichens; he also devotes many passages to an account of their metamorphoses, pointing out that species alter so much in varying conditions, that the same one at different stages may be placed even in different genera; he however carries his theory of metamorphosis too far and unites together widely separated plants. Meyer was the first to describe the growth of the lichen from spores, though his description is somewhat confused. Possibly the honour of having first observed their germination should be given to a later botanist, Holle[111]. The works of both Wallroth and Meyer enjoyed a great and well-merited reputation: they were standard books of consultation for many years. Koerber[112], who devoted a long treatise to the study of gonidia, confirmed Wallroth’s theories: he considered at that time that the gonidia in the soredial condition were organs of propagation.

Mention should be made here of the many able and keen collectors who, in the latter half of the eighteenth century and the beginning of the nineteenth, did so much to further the knowledge of lichens in the British Isles. Among the earliest of these naturalists are Richard Pulteney (1730-1801), whose collection of plants, now in the herbarium of the British Museum, includes many lichens, and Hugh Davies (1739-1821), a clergyman whose Welsh plants also form part of the Museum collection. The Rev. John Harriman (1760-1831) sent many rare plants from Egglestone in Durham to the editors of English Botany and among them were not a few lichens. Edward Forster (1765-1849) lived in Essex and collected in that county, more especially in and near Epping Forest, and another East country botanist, Dawson Turner (1775-1858), though chiefly known as an algologist, gave considerable attention to lichens. In Scotland the two most active workers were Charles Lyell (1767-1849), of Kinnordy in Forfarshire, and George Don (1798-1856), also a Forfar man. Don was a gardener and became eventually a foreman at the Chelsea Physic Garden. Sir Thomas Gage of Hengrave Hall (1781-1823) botanized chiefly in his own county of Suffolk; but most of his lichens were collected in South Ireland and are incorporated in the herbarium of the British Museum. Miss Hutchins also collected in Ireland and sent her plants for inclusion in English Botany. But in later years, the principal lichenologist connected with that great undertaking was W. Borrer, who spent his life in Sussex: he not only supplied a large number of specimens to the authors, but he himself discovered and described many new lichens.

American lichenologists were also extremely active all through this period. The comparatively few lichens of Michaux’s[113] Flora grouped under “Lichenaceae” were collected in such widely separated regions as Carolina and Canada. A few years later Mühlenberg[114] included no fewer than 184 species in his Catalogue of North American Plants. Torrey[115] and Halsey[116] botanized over a limited area near New York, and the latter, who devoted himself more especially to lichens, succeeded in recording 176 different forms, old and new. These two botanists were both indebted for help in their work to Schweinitz, a Moravian brother, who moved from one country to another, working and publishing, now in America and now in Europe. His name is however chiefly associated with fungi. Later American lichenology is nobly represented by Tuckerman[117] who issued his first work on lichens in 1839, and who continued for many years to devote himself to the subject. He followed at first the classification and nomenclature that had been adopted by Fée, but as time went on he associated himself with all that was best and most enlightened in the growing science.

Travellers and explorers in those days of high adventure were constantly sending their specimens to European botanists for examination and determination, and the knowledge of exotic lichens as of other classes of plants grew with opportunity. Among the principal home workers in foreign material, at this time, may be cited Fée[118] who described a very large series on officinal barks (Cinchona, etc.) so largely coming into use as medicines; he also took charge of the lichens in Martius’s[119] Flora of Brazil. Montagne[120] named large collections, notably those of Leprieur collected in Guiana, and Hooker[121] and Walker Arnott determined the plants collected during Captain Beechey’s voyage, which included lichens from many different regions.

G. Period VI. 1846-1867

The last work of importance, in which microscopic characters were ignored, was the Enumeratio critica Lichenum Europaeum by Schaerer[122], a veteran lichenologist, who rather sadly realized at the end the limitations of that work, as he asks the reader to accept it “such as it is.” Many years previously, Eschweiler[123] in his Systema and Fée[124] in his account of Cryptogams on Officinal Bark, had given particular attention to the internal structure as well as to the outward form of the lichen fructification. Fée, more especially, had described and figured a large number of spores; but neither writer had done more than suggest their value as a guide in the determination of genera and species.

It was an Italian botanist, Giuseppe de Notaris[125], a Professor in Florence, who took up the work where Fée had left it. His comparative studies of both vegetative and reproductive organs convinced him of the great importance of spore characters in classification, the spore being, as he rightly decided, the highest and ultimate product of the lichen plant. In his microscopic examination of the various recognized genera, he found that while, in some genera, the spores conformed to one distinct type, in others their diversities in form, septation or colour gave a decisive reason for the establishment of new genera, while minor differences in size, etc. of the spores proved to be of great value in distinguishing species. The spore standard thus marks a new departure in lichenology. De Notaris published the results of his researches in a fragment of a projected larger work that was never completed. Though his views were overlooked for a time, they were at length fully recognized and further elaborated by Massalongo[126] in Italy, by Norman[127] in Norway, by Koerber[128] in Germany and by Mudd[129] in our own country. Massalongo had drawn up the scheme of a great Scolia Lichenographica, but like de Notaris, he was only able to publish a part. After twelve years of ill-health, in which he struggled to continue his work, he died at the early age of 36.

Lindsay[130], Mudd and Leighton[131] were at this time devoting great attention to British lichens. Lauder Lindsay’s Popular History of British Lichens, with its coloured plates and its descriptive and economic account of these plants has enabled many to acquire a wide knowledge of the group. Mudd’s Manual, a more complete and extremely valuable contribution to the subject, followed entirely on the lines of Massalongo’s work. From his large experience in the examination of lichens he came to the conclusion that: “Of all organs furnished by a given group of plants, none offer so many real, constant and physiological characters as the spores of lichens, for the formation of a simple and natural classification.”

Meanwhile, a contemporary writer, William Nylander, was rising into fame. He was born at Uleaborg in Finland[132] in 1822 and became interested in lichens very early in his career. His first post was the professorship of botany at Helsingfors; but in 1863 he gave up his chair and removed to Paris where he remained, except for short absences, until his death. One of his excursions brought him to London in 1857 to examine Hooker’s herbarium. He devoted his whole life to the study of lichens, and from 1852, the date of his first lichen publication, which is an account of the lichens of Helsingfors, to the end of his life he poured out a constant succession of books or papers, most of them in Latin. One of his earliest works was an Essay on Classification[133] which he elaborated later, but which in its main features he never altered. He relied, in his system, on the structure and form of thallus, gonidia and fructifications, more especially on those of the spermogonia (pycnidia), but he rejected ascospore characters except so far as they were of use in the diagnosis of species. He failed by being too isolated and by his unwillingness to recognize results obtained by other workers. In 1866 he had discovered the staining reactions of potash and hypochlorite of lime on certain thalli, and though these are at times unreliable owing to growth conditions, etc., they have generally been of real service. Nylander, however, never admitted any criticism of his methods; his opinions once stated were never revised. He rejected absolutely the theory of the dual nature of lichens propounded by Schwendener without seriously examining the question, and regarded as personal enemies those who dared to differ from him. The last years of his life were passed in complete solitude. He died in March 1899.

Owing to the very inadequate powers of magnification at the service of scientific workers, the study of lichens as of other plants was for long restricted to the collecting, examining and classifying of specimens according to their macroscopic characters; the microscopic details observed were isolated and unreliable except to some extent for spore characters. Special interest is therefore attached to the various schemes of classification, as each new one proposed reflects to a large extent the condition of scientific knowledge of the time, and generally marks an advance. It was the improvement of the microscope from a scientific toy to an instrument of research that opened up new fields of observation and gave a new impetus to the study of a group of plants that had proved a puzzle from the earliest times.

Tulasne was one of the pioneers in microscopic botany. He made a methodical study of a large series of lichens[134] and traced their development, so far as he was able, from the spore onwards. He gave special attention to the form and function of spermogonia and spermatia, and his work is enriched by beautiful figures of microscopic detail. Lauder Lindsay[135] also published an elaborate treatise on spermogonia, on their occurrence in the lichen kingdom and on their form and structure. The paper embodies the results of wide microscopic research and is a mine of information regarding these bodies.

Much interesting work was contributed at this time by Itzigsohn[136], Speerschneider[137], Sachs[138], Thwaites[139], and others. They devoted their researches to some particular aspect of lichen development and their several contributions are discussed elsewhere in this work.

Schwendener[140] followed with a systematic study of the minute anatomy of many of the larger lichen genera. His work is extremely important in itself and still more so as it gradually revealed to him the composite character of the thallus.

Several important monographs date from this period: Leighton[141] reviewed all the British “Angiocarpous” lichens with special reference to their “sporidia” though without treating these as of generic value. He followed up this monograph by two others, on the Graphideae[142] and the Umbilicarieae[143], and Mudd[144] published a careful study of the British Cladoniae. On the Continent Th. Fries[145] issued a revision of Stereocaulon and Pilophoron and other writers contributed work on smaller groups.

H. Period VII. 1867 and after