Lichens

According to Schwendener[485], the formation of soredia is due to increased and almost abnormal activity of division in the gonidial cell; the hyphal filament attached to it also becomes active and sends out branches from the cell immediately below the point of contact which force their way between the newly divided gonidia and finally surround them. A soredial “head” of smaller or larger size is thus gradually built up on the stalk filament or filaments, and is ultimately detached by the breaking down of the slender support.

a. Scattered Soredia. The simplest example of soredial formation may be seen on the bark of trees or on palings when the green coating of algal cells is gradually assuming a greyish hue caused by the invasion of hyphal lichenoid growth. This condition is generally referred to as “leprose” and has even been classified as a distinct genus, Lepra or Lepraria. Somewhat similar soredial growth is also associated with many species of Cladonia, the turfy soil in the neighbourhood of the upright podetia being often powdered with white granules. Such soredia are especially abundant in that genus, so much so, that Meyer[486], Krabbe[487] and others have maintained that the spores take little part in the propagation of species. The under side of the primary thallus, but more frequently the upright podetia, are often covered with a coating of soredia, either finely furfuraceous, or of larger growth and coarsely granular, the size of the soredia depending on the number of gonidia enclosed in each “head.”

Soredia are only occasionally present on the apothecial margins: the rather swollen rims in Lobaria scrobiculata are sometimes powdery-grey, and Bitter[488] has observed soredia, or rather soralia, on the apothecial margins of Parmelia vittata; they are very rare, however, and are probably to be explained by excess of moisture in the surroundings.

b. Isidial Soredia. In a few lichens soredia arise by the breaking down of the cortex at the tips of the thalline outgrowths termed “isidia.” In Parmelia verruculifera, for instance, where the coralloid isidia grow in closely packed groups or warts, the upper part of the isidium frequently becomes soredial. In that lichen the younger parts of the upper cortex bear hairs or trichomes, and the individual soredia are also adorned with hairs. The somewhat short warted isidia of P. subaurifera may become entirely sorediose, and in P. farinacea the whole thallus is covered with isidia transformed into soralia. The transformation is constant and is a distinct specific character. Bitter[488] considers that it proves that no sharp distinction exists between isidia and soralia, at least in their initial stages.

c. Soredia as Buds. Schwendener[489] has described soredia in the genus Usnea which give rise to new branches. Many of the species in that genus are plentifully sprinkled with the white powdery bodies. A short way back from the apex of the filament the separate soredia show a tendency to apical growth and might be regarded as groups of young plants still attached to the parent branch. One of these developing more quickly pushes the others aside and by continued growth fills up the soredial opening in the cortex with a plug of tissue; finally it forms a complete lateral branch. Schwendener calls them “soredial” branches (Fig. 82) to distinguish them from the others formed in the course of the normal development.

B. Soralia

In lichens of foliose and fruticose structure, and in a few crustaceous forms, the soredia are massed together into the compact bodies called soralia, and thus are confined to certain areas of the plant surface. The simpler soralia arise from the gonidial zone below the cortex by the active division of some of the algal cells. The hyphae, interlaced with the green cells, are thin-walled and are, as stated by Wainio[490], still in a meristematic condition; they are thus able readily to branch and to form new filaments which clasp the continually multiplying gonidia. This growth is in an upward or outward direction away from the medulla, and strong mechanical pressure is exerted by the increasing tissue on the overlying cortical layers. Finally the soredia force their way through to the surface at definite points. The cortex is thrown back and forms a margin round the soralium, though shreds of epidermal tissue remain for a time mixed with the powdery granules.

a. Form and Occurrence of Soralia. The term “soralium” was first applied only to the highly developed soredial structures considered by Acharius to be secondary apothecia; it is now employed for any circumscribed group of soredia.[491] The soralia vary in size and form and in position, according to the species on which they occur; these characters are constant enough to be of considerable diagnostic value. Within the single genus Parmelia, they are to be found as small round dots sprinkled over the surface of P. dubia; as elongate furrows irregularly placed on P. sulcata; as pearly excrescences at or near the margins of P. perlata, and as swollen tubercles at the tips of the lobes of P. physodes (Fig. 83). Their development is strongly influenced and furthered by shade and moisture, and, given such conditions in excess, they may coalesce and cover large patches of the thallus with a powdery coating, though only in those species that would have borne soredia in fairly normal conditions.

Soralia of definite form are of rather rare occurrence in crustaceous lichens, with the exception of the Pertusariaceae, where they are frequent, and some species of Lecanora and Placodium. They are known in only two hypophloeodal (subcortical) lichens, Arthonia pruinosa and Xylographa spilomatica. Among squamulose thalli they are typical of some Cladoniae, and also of Lecidea (Psora) ostreata, where they are produced on the upper surface towards the apex of the squamule.

b. Position of soraliferous Lobes. According to observations made by Bitter[492], the occurrence of soralia on one lobe or another may depend to a considerable extent on the orientation of the thallus. He cites the variability in habit of the familiar lichen, Parmelia physodes and its various forms, which grow on trees or on soil. In the horizontal thalli there is much less tendency to soredial formation, and the soredia that arise are generally confined to branching lobes on the older parts of the thallus.

That type of growth is in marked contrast with the thallus obliged to take a vertical direction as on a tree. In such a case the lobes, growing downward from the point of origin, form soralia at their tips at an early stage (Fig. 84). The lateral lobes, and especially those that lie close to the substratum, are the next to become soraliate. Similar observations have been made on the soraliferous lobes of Cetraria pinastri. The cause is probably due to the greater excess of moisture draining downwards to the lower parts of the thallus. The lobes that bear the soralia are generally narrower than the others and are very frequently raised from contact with the substratum. They tend to grow out from the thallus in an upright direction and then to turn backwards at the tip, so that the opening of the soralium is directed downwards. Bitter says that the cause of this change in direction is not clear, though possibly on teleological reasoning it is of advantage that the opening of the soralium should be protected from direct rainfall. The opening lies midway between the upper and lower cortex, and the upper tissue in these capitate soralia continues to grow and to form an arched helmet or hood-covering which serves further to protect the soralium.

Similar soralia are characteristic of Physcia hispida (Ph. stellaris subsp. tenella), the apical helmet being a specially pronounced feature of that species, though, as Lesdain[493] has pointed out, the hooded structures are primarily the work of insects. In vertical substrata they occur on the lower lobes of the plant.

Apical soralia are rare in fruticose lichens, but in an Alpine variety of Ramalina minuscula they are formed at the tips of the fronds and are protected by an extension of the upper cortical tissues. Another instance occurs in a Ramalina from New Granada referred by Nylander to R. calicaris var. farinacea: it presents a striking example of the helmet tip.

c. Deep-seated Soralia. In the cases already described Schwendener[494] and Nilson[495] held that the algae gave the first impulse to the formation of the soredia; but in the Pertusariaceae[496], a family of crustaceous lichens, there has been evolved a type of endogenous soralium which originates with the medullary hyphae. In these, special hyphae rise from a weft of filaments situated just above the lowest layer of the thallus at the base of the medulla, the weft being distinguished from the surrounding tissue by staining blue with iodine. A loose strand of hyphae staining the usual yellow colour rises from the surface of the “blue” weft and, traversing the medullary tissue, surrounds the gonidia on the under side of the gonidial zone. The hyphae continue to grow upward, pushing aside both the upper gonidial zone and the cortex, and carrying with them the algal cells first encountered. When the summit is reached, there follows a very active growth of both gonidia and hyphae. Each separate soredium so produced consists finally of five to ten algal cells surrounded by hyphae and measures 8 µ to 13 µ in diameter. The cortex forms a well-defined wall or margin round the mass of soredia.

A slightly different development is found in Lecanora tartarea, one of the “crottle” lichens, which has been placed by Darbishire in Pertusariaceae. The hyphae destined to form soredia also start from the weft of tissue at the base of the thallus, but they simply grow through the gonidial zone instead of pushing it aside.

In his examination of Pertusariaceae Darbishire found that the apothecia also originated from a similar deeply seated blue-staining tissue, and he concluded that the soralia represented abortive apothecia and really corresponded to Acharius’s “apothecia of the second order.” His conclusion as to the homology of these two organs is disputed by Bitter[497], who considers that the common point of origin is explained by the equal demand of the hyphae in both cases for special nutrition, and by the need of mechanical support at the base to enable the hyphae to reach the surface and to thrust back the cortex without deviating from their upward course through the tissues.

C. Dispersal and Germination of Soredia

Soredia become free by the breaking down of the hyphal stalks at the septa or otherwise. They are widely dispersed by wind or water and soon make their appearance on any suitable exposed soil. Krabbe[498] has stated that, in many cases, the loosely attached soredia coating some of the Cladonia podetia are of external origin, carried thither by the air-currents. Insects too aid in the work of dissemination: Darbishire[499] has told us how he watched small mites and other insects moving about over the soralia of Pertusaria amara and becoming completely powdered by the white granules.

Darbishire[499] also gives an account of his experiments in the culture of soredia. He sowed them on poplar wood about the beginning of February in suitable conditions of moisture, etc. Long hyphal threads were at once produced from the filaments surrounding the gonidia, and gonidia that had become free were seen to divide repeatedly. Towards the end of August of the same year a few soredia had increased in size to about 450µ in diameter, and were transferred to elm bark. By September they had further increased to a diameter of 520µ, and the gonidia showed a tendency towards aggregation. No further differentiation or growth was noted.

More success attended Tobler’s[500] attempt to cultivate the soredia of Cladonia sp. He sowed them on soil kept suitably moist in a pot and after about nine months he obtained fully formed squamules, at first only an isolated one or two, but later a plentiful crop all over the surface of the soil. Tobler also adds that soredia taken from a Cladonia, that had been kept for about half a year in a dry room, grew when sown on a damp substratum. The algae however had suffered more or less from the prolonged desiccation, and some of them failed to develop.

A suggestion has been made by Bitter[501] that a hybrid plant might result from the intermingling of soredia from the thallus of allied lichens. He proposed the theory to explain the great similarity between plants of Parmelia physodes and P. tubulosa growing in close proximity. There is no proof that such mingling of the fungal elements ever takes place.

D. Evolution of Soredia

Soredia have been compared to the gemmae of the Bryophytes and also to the slips and cuttings of the higher plants. There is a certain analogy between all forms of vegetative reproduction, but soredia are peculiar in that they include two dissimilar organisms. In the lichen kingdom there has been evolved this new form of propagation in order to secure the continuance of the composite life, and, in a number of species, it has almost entirely superseded the somewhat uncertain method of spore germination inherited from the fungal ancestor, but which leaves more or less to chance the encounter with the algal symbiont.

From a phylogenetic point of view we should regard the sorediate lichens as the more highly evolved, and those which have no soredia as phylogenetically young, though, as Lindau[502] has pointed out, soredia are all comparatively recent. They probably did not appear until lichens had reached a more or less advanced stage of development, and, considering the polyphyletic origin of lichens, they must have arisen at more than one point, and probably at first in circumstances where the formation of apothecia was hindered by prolonged conditions of shade and moisture.

That soredia are ontogenetic in character, and not, as Nilson[503] has asserted, accidental products of excessively moist conditions is further proved by the non-sorediate character of those species of crustaceous lichens belonging to Lecanora, Verrucaria, etc. that are frequently immersed in water. Bitter[504] found that the soredia occurring on Peltigera spuria were not formed on the lobes which were more constantly moist, nor at the edges where the cortex was thinnest: they always emerged on young parts of the thallus a short way back from the edge.

Bitter[504] points out that in extremely unfavourable circumstances—in the polluted atmosphere near towns, or in persistent shade—lichens, that would otherwise form a normal thallus, remain in a backward sorediose state. He considers, however, that many of these formless crusts are autonomous growths with specific morphological and chemical peculiarities. They hold these outposts of lichen vegetation and are not found growing in any other localities. The proof would be to transport them to more favourable conditions, and watch development.

4. ISIDIA

A. Form and Structure of Isidia

Many lichens are rough and scabrous on the surface, with minute simple or divided coral-like outgrowths of the same texture as the underlying thallus, though sometimes they are darker in colour as in Evernia furfuracea. They always contain gonidia and are covered by a cortex continuous with that of the thallus.

This very marked condition was considered by Acharius[505] as of generic importance and the genus, Isidium, was established by him, with the diagnostic characters: “branchlets produced on the surface, or coralloid, simple and branched.” In the genus were included the more densely isidioid states of various crustaceous species such as Isidium corallinum and I. Westringii, both of which are varieties of Pertusariae. Fries[506], with his accustomed insight, recognized them as only growth forms. The genus was however still accepted in English Floras[507] as late as 1833, though we find it dropped by Taylor[508] in the Flora Hibernica a few years later.

The development of the isidial outgrowth has been described by Rosendahl[509] in several species of Parmelia. In one of them, P. papulosa, which has a cortical layer one cell thick, the isidium begins as a small swelling or wart on the upper surface of the thallus. At that stage the cells of the cortex have already lost their normal arrangement and show irregular division. They divide still further, as gonidia and hyphae push their way up. The full-grown isidia in this species are cylindrical or clavate, simple or branched. They are peculiar in that they bear laterally here and there minute rhizoids, a development not recorded in any other isidia. The inner tissue accords with that of the normal thallus and there is a clearly marked cortex, gonidial zone and pith. A somewhat analogous development takes place in the isidia of Parmelia proboscidea; in that lichen they are mostly prolonged into a dark-coloured cilium.

In Parmelia scortea the cortex is several cells thick, and the outermost rows are compressed and dead in the older parts of the thallus; but here also the first appearance of the isidium is in the form of a minute wart. The lower layers (4 to 6) of living cortical cells divide actively; the gonidia also share in the new growth, and the protuberance thus formed pushes off the outer dead cortex and emerges as an isidium (Fig. 85). They are always rather stouter in form than those of P. papulosa and may be simple or branched. The gonidia in this case do not form a definite zone, but are scattered through the pith of the isidium.

Here also should be included the coralloid branching isidia that adorn the upper surface and margins of the thallus of Umbilicaria pustulata. They begin as small tufts of somewhat cylindrical bodies, but they sometimes broaden out to almost leafy expansions with crisp edges. Most frequently they are situated on the bulging pustules where intercalary growth is active. Owing to their continued development on these areas, the tissue becomes slack, and the centre of the isidial tuft may fall out, leaving a hole in the thallus which becomes still more open by the tension of thalline expansion. New isidia sprout from the edges of the wound and the process may again be repeated. It has been asserted that these structures are only formed on injured parts of the thallus—something like gall-formations—but Bitter[510] has proved that the wound is first occasioned by the isidial growth weakening the thallus.

B. Origin and Function of Isidia

Nilson[511] (later Kajanus[512]) insists that isidia and soredia are both products of excessive moisture. He argues that lichen species, in the course of their development, have become adapted to a certain degree of humidity, and, if the optimum is passed, the new conditions entail a change in the growth of the plant. The gonidia are stimulated to increased growth, and the mechanical pressure exerted by the multiplying cells either results in the emergence of isidial structures where the cortex is unbroken, or, if the cortex is weaker and easily bursts, in the formation of soralia.

This view can hardly be accepted; isidia as well as soredia are typical of certain species and are produced regularly and normally in ordinary conditions; both of them are often present on the same thallus. It is not denied, however, that their development in certain instances is furthered by increased shade or moisture. In Evernia furfuracea isidia are more freely produced on the older more shaded parts of the thallus. Zopf[513] has described such an instance in Evernia olivetorina (E. furfuracea), which grew in the high Alps on pine trees, and which was much more isidiose when it grew on the outer ends of the branches, where dew, rain or snow had more direct influence. He[514] quotes other examples occurring in forms of E. furfuracea which grew on the branches of pines, larches, etc. in a damp locality in S. Tyrol. The thalli hung in great abundance on each side of the branches, and were invariably more isidiose near the tips, because evidently the water or snow trickled down and was retained longer there than at the base.

Bitter[515] has given a striking instance of shade influence in Umbilicaria. He found that some boulders on which the lichen grew freely had become covered over with fallen pine needles. The result was at first an enormous increase of the coralline isidia, though finally the lichen was killed by the want of light.

Isidia are primarily of service to the plant in increasing the assimilating surface. Occasionally they grow out into new thallus lobes. The more slender are easily rubbed off, and, when scattered, become efficient organs of propagation. This view of their function is emphasized by Bitter who points out that both in Evernia furfuracea and in Umbilicaria pustulata other organs of reproduction are rare or absent. Zopf[513] found new plants of Evernia furfuracea beginning to grow on the trunk of a tree lower down than an old isidiose specimen. They had developed from isidia which had been detached and washed down by rain.

VI. HYMENOLICHENS

A. Supposed Affinity with other Plants

Lichens in which the fungal elements belong to the Hymenomycetes are confined to three tropical genera. They are associated with blue-green algae and are most nearly related to the Thelephoraceae among fungi. The spores are borne, as in that family, on basidia.

The best known Hymenolichen, Cora Pavonia (Fig. 86), was discovered by Swartz[516] during his travels in the W. Indies (1785-87) growing on trees in the mountains of Jamaica, and the new plant was recorded by him as Ulva montana. Gmelin[517] also included it in Ulva in close association with Ulva (Padina) Pavonia, but that classification was shortly after disputed by Woodward[518] who thought its affinity was more nearly with the fungi and suggested that it should be made the type of a new genus near to Boletus (Polystictus) versicolor. Fries[519] in due time made the new genus Cora, though he included it among algae; finally Nylander[520] established the lichenoid character of the thallus and transferred it to the Lecanorei.

It was made the subject of more exact investigation by Mattirolo[521] who recognized its affinity with Thelephora, a genus of Hymenomycetes. Later Johow[522] went to the West Indies and studied the Hymenolichens in their native home. The genera and species described by Johow have been reduced to Cora and Dictyonema; a new genus Corella has since been added by Wainio[523].

Johow found that Cora grew on the mountains usually from 1000 to 2000 ft. above sea-level. As it requires for its development a cool damp climate with strong though indirect illumination, it is found neither in sunny situations nor in the depths of dark woods. It grows most freely in diffuse light, on the lower trunks and branches of trees in open situations, but high up on the stem where the vegetation is more dense. It stands out from the tree like a small thin bracket fungus, one specimen placed above another, with a dimidiate growth similar to that of Polystictus versicolor. Both surfaces are marked by concentric zones which give it an appearance somewhat like Padina Pavonia. These zones indicate unequal intercalary growth both above and below. The whole plant is blue-green when wet, greyish-white when dry, and of a thin membranaceous consistency.

B. Structure of Thallus

There is no proper cortex in any of the genera, but in Cora there is a fastigiate branching of the hyphae in parallel lines towards the upper surface; just at the outside they turn and lie in a horizontal direction, and, as the branching becomes more profuse, a rather compact cover is formed. The gonidia, which consist of blue-green Chroococcus cells, lie at the base of the upward branches and they are surrounded with thin-walled short-celled hyphae closely interwoven into a kind of cellular tissue. The medulla of loose hyphae passes over to the lower cortex, also of more or less loose filaments. The outermost cells of the latter very frequently grow out into short jagged or crenate processes (Fig. 87).

In Corella, the mature lichen is squamulose or consists of small lobes; in Dictyonema there is a rather flat dimidiate expansion; in both the alga is Scytonema, the trichomes of which largely retain their form and are surrounded by parallel growths of branching hyphae. The whole tissue is loose and spongy.

Corella spreads over soil on a white hypothallus without rhizinae. In the other two genera which live on soil, or more frequently on trees, there is a rather extensive formation of hold-fast tissue. When the dimidiate thallus grows on a rough bark, rhizoidal strands of hyphae travel over it and penetrate between the cracks; if the bark is smooth, there is a more continuous weft of hyphae. In both cases a spongy cushion of filamentous tissue develops at the base of the lichen between the tree and the bracket thallus. There is also in both genera an encrusting form which Johow regarded as representing a distinct genus Laudatea, but which Möller found to be merely a growth stage. Möller[524] judged from that and from other characteristics that the same fungus enters into the composition of both Cora and Dictyonema and that only the algal constituents are different.

C. Sporiferous Tissues

As in Hymenomycetes, the spores of Hymenolichens are exogenous, and are borne at the tips of basidia which in these lichens are produced on the under surface of the thallus. In Cora the fertile filaments may form a continuous series of basidia over the surface, but generally they grow out in separate though crowded tufts. As these tufts broaden outwards, they tend to unite at the free edges, and may finally present a continuous hymenial layer. Each basidium bears four sterigmata and spores (Fig. 87 e); paraphyses exactly similar to the basidia are abundant in the hymenium. In Dictyonema the hymenium is less regular, but otherwise it resembles that of Cora. No hymenium has as yet been observed in Corella; it includes, so far as known, one species, C. brasiliensis, which spreads over soil or rocks.

CHAPTER IV
REPRODUCTION

1. REPRODUCTION BY ASCOSPORES

A. Historical Survey

The earliest observations as to the propagation of lichens were made by Malpighi[525] who recorded the presence of soredia on the lichen plant and noted their function as reproductive bodies. He was followed after a considerable interval by Tournefort[526] who placed lichens in a class apart owing to the form of the fruit: “This fruit,” he writes, “is a species of bason or cup which seems to take the place of seeds in these kinds of plants.” He figures Ramalina fraxinea and Physcia ciliaris, both well fruited specimens, and he represents the “minute dust” contained in the fruits as subrotund in form. The spores of Physcia ciliaris are of a large size and dark in colour and were undoubtedly seen by Tournefort. Morison[527], in his History of Oxford Plants, published very shortly after, dismissed Tournefort’s “seeds” as being too minute to be of any practical interest.

Micheli[528], with truer scientific insight, made the fruiting organs the subject of special study. He decided that the apothecia were floral receptacles, receptacula florum, and that the spores were the “flowers” of the lichen. He has figured them in a vertical series in situ, in a section of the disc of Solorina saccata[529] and also in a species of Pertusaria[529], in both of which plants the ascospores are unusually large. He adds that he had not so far seen the “semina.”

Micheli’s views were not shared by his immediate successors. Dillenius[530] scarcely believed that the spores could be “flowers” and, in any case, he concluded that they were too minute to be of any real significance in the life of the plant.

Linnaeus[531], and after him Necker[532], Scopoli[533] and others describe the apothecia as the male, the soredia as the female organs of lichens. These old time botanists worked with very low powers of magnification, and easily went astray in the interpretation of imperfectly seen phenomena.

Koelreuter[534], a Professor of Natural History in Carlsruhe, who published a work on The discovered Secret of Cryptogams, next hazarded the opinion that the seeds of lichens originated from the substance of the pith, and that the overlying cortical layer supplied the fertilizing sap. Hoffmann[535] devoted a great deal of attention to lichen fructification and he also thought that fertilization must take place within the tissue of the lichens. He regarded the soredia as the true seeds, while allowing that a second series of seeds might be contained in the scutellae (apothecia).

A distinct advance was made by Hedwig[536], a Professor of Botany in Leipzig, towards the end of the eighteenth century. He followed Tournefort in selecting Physcia ciliaris for research, and in that plant he describes and figures not only the apothecia with the dark-coloured septate spores, but also the pycnidia or spermogonia which he regarded as male organs. The soredia, typically represented and figured by him on Parmelia physodes, he judged to be “male flowers of a different type.”

Acharius[537] did not add much to the knowledge of reproduction in lichens, though he takes ample note of the various fruiting structures for which he invented the terms apothecia, perithecia and soredia. Under still another term gongyli he included not only spores, but the spore guttulae as well as the gonidia or cells forming the soredia.

Hornschuch[538] of Greifswald described the propagation of the lower lichens as being solely by means of a germinating “powder”; the more highly organized forms were provided with receptacles or apothecia containing spores which he considered as analogous to flowers rather than to fruits. The important contributions to Lichenology of Wallroth[539] and Meyer[540] close this period of uncertainty: the former deals almost exclusively with the form and character of the vegetative thallus and the function of the “reproductive gonidia.” Meyer, a less prolix writer, very clearly states that the method of reproduction is twofold: by spores produced in fruits, or by the germinating granules of the soredia.

B. Forms of Reproductive Organs

From the time of Tournefort, considerable attention had been given to the various forms of scutellae, tuberculae, etc., as characters of diagnostic importance. Sprengel[541] grouped these bodies finally into nine different types with appropriate names which have now been mostly superseded by the comprehensive terms, apothecia and perithecia. A general classification on the lines of fruit development was established by Luyken[542], who, following Persoon’s[543] classification of fungi, and thus recognizing their affinity, summed up all known lichens as Gymnocarpeae with open fruits, and Angiocarpeae with closed fruits.

a. Apothecia. As in discomycetous fungi, the lichen apothecium is in the form of an open concave or convex disc, but generally of rather small size, rarely more than 1 cm. in diameter (Fig. 88); there is no development in lichen fruits equal to the cup-like ascomata of the larger Pezizae. In most cases the lichen apothecium retains its vitality as a spore-bearing organ for a considerable period, sometimes for several years, and it is strengthened and protected by one or more external margins of sterile tissue. Immediately surrounding the fertile disc there is a compact wall of interwoven hyphae. In some of the shorter-lived soft fruits, as in Biatora, this hyphal margin may be thin, and may gradually be pushed aside as the disc develops and becomes convex, but generally it forms a prominent rim round the disc and may be tough or even horny, and often hard and carbonaceous. This wall, which is present, to some extent, in nearly all lichens, is described as the “proper margin.” A second “thalline margin” containing gonidia is present in many genera[544]: it is a structure peculiar to the lichen apothecium and forms the amphithecium.

At the base of the apothecium there is a weft of light- or dark-coloured hyphae called the hypothecium, which is continued up and round the sides as the parathecium merging into the “proper margin.” It forms the lining of a cup-shaped hollow which is filled by the paraphyses, which are upright closely packed thread-like hyphae, and by the spore-containing asci or thecae, these together constituting the thecium or hymenium. The paraphyses are very numerous as compared with the asci; they are simple or branched, frequently septate, especially towards the apex, and mostly slender, varying in width from 1-4µ, though Hue describes paraphyses in Aspicilia atroviolacea as 8-12µ thick. They may be thread-like throughout their length, or they may widen towards the tips which are not infrequently coloured. Small apical cells are often abstricted and lie loose on the epithecium, giving at times a pruinose or powdered character to the disc. In some genera there is a profuse branching of the paraphyses to form a dense protective epithecium over the surface of the hymenium as in the genus Arthonia.

The apothecia may be sessile and closely adnate to or even sunk in the thallus, or they may be shortly stalked. The thalline margin shares generally the characters of the thallus; the disc is mostly of a firm consistency and is light or dark in colour according to genus or species; most frequently it is some shade of brown. Marginate apothecia, i.e. those with a thalline margin, are often referred to as “lecanorine,” that being a distinctive feature of the genus Lecanora. In the immarginate series, with a proper margin only, the texture may be soft and waxy, termed “biatorine” as in Biatora; or hard and carbonaceous as in the genus Lecidea, and is then described as “lecideine.”

In the subseries Graphidineae, the apothecium has the form of a very flat, roundish or irregular body entirely without a margin, called an “ardella” as in Arthonia; or more generally it is an elongate narrow “lirella,” in which the disc is a mere slit between two dark-coloured proper margins. The hypothecium of the lirellae is sometimes much reduced and in that case the hymenium rests directly on a thin layer above the thalline tissue as in Graphis elegans (Fig. 89).