Lichens

b. Development of Cyphellae. The cortex of both surfaces in the thallus of Sticta is a several-layered plectenchyma of thick-walled closely packed cells, the outer layer growing out into hairs on the under surface of most of the species. Where either cyphellae or pseudocyphellae occur, a more or less open channel is formed between the exterior and the internal tissues of the lichen. In the case of the cyphellae, the medullary hyphae which line the cup are divided into short roundish cells with comparatively thin walls (Fig. 73). They form a tissue sharply differentiated from the loose hyphae that occupy the medulla. The rounded cells tend to lie in vertical rows, though the arrangement in fully formed cyphellae is generally somewhat irregular. The terminal empty cells are loosely attached and as they are eventually abstricted and strewn over the inside of the cup they give to it the characteristic white powdery appearance.

According to Schwendener[434] development begins by an exuberant growth of the medulla which raises and finally bursts the cortex; prominent cyphellae have been thus formed in Sticta damaecornis (Fig. 73). In other species the swelling is less noticeable or entirely absent. The opening of the cup measures usually about 1/2 mm. across, but it may stretch to a greater width.

c. Pseudocyphellae. In these no margin is formed, the cortex is simply burst by the protruding filaments which are of the same colour—yellow or white—as the medullary hyphae. They vary in size, from a minute point up to 4 mm. in diameter.

d. Occurrence and Distribution. The genus Sticta is divided into two sections: (1) Eusticta in which the gonidia are bright-green algae, and (2) Stictina in which they are blue-green. Cyphellae and pseudocyphellae are fairly evenly distributed between the sections; they never occur together. Stizenberger[435] found that 36 species of the section Eusticta were cyphellate, while in 43 species pseudocyphellae were formed. In the section Stictina there were 38 of the former and only 31 of the latter type. Both sections of the genus are widely distributed in all countries, but they are most abundant south of the equator, reaching their highest development in Australia and New Zealand.

In the British Isles Sticta is rather poorly represented as follows:

Structures resembling cyphellae, with an overarching rim, are sprinkled over the brown under surface of the Australian lichen, Heterodea Mülleri; the thallus is without a lower cortex, the medulla being protected by thickly woven hyphae. Heterodea was at one time included among Stictaceae, though now it is classified under Parmeliaceae. Pseudocyphellae are also present on the non-corticate under surface of Nephromium tomentosum, where they occur as little white pustules among the brown hairs; and the white impressed spots on the under surface of Cetraria islandica and allied species, first determined as air pores by Zukal[436], have also been described by Wainio[437] as pseudocyphellae.

There seems no doubt that the chief function of these various structures is, as Schwendener[438] suggested, to allow a free passage of air to the assimilating gonidial zone. Jatta[439] considers them to be analogous to the lenticels of higher plants and of service in the interchange of gases—expelling carbonic acid and receiving oxygen from the outer atmosphere. It is remarkable that such serviceable organs should have been evolved in so few lichens.

B. Breathing-Pores

a. Definite Breathing-Pores. The cyphellae and pseudocyphellae described above are confined to the under surface of the thallus in those lichens where they occur. Distinct breathing-pores of a totally different structure are present on the upper surface of the tree-lichen, Parmelia aspidota (P. exasperata), one of the brown-coloured species. They are somewhat thickly scattered as isidia- or cone-like warts over the lichen thallus (Fig. 74) and give it the characteristically rough or “exasperate” character. They are direct outgrowths from the thallus, and Zukal[440], who discovered their peculiar nature and function, describes them as being filled with a hyphal tissue, with abundant air-spaces, and in direct communication with the medulla; gonidia, if present, are confined to the basal part. The cortex covering these minute cones, he further states, is very thin on the top, or often wanting, so that a true pore is formed which, however, is only opened after the cortex elsewhere has become thick and horny. Rosendahl[441], who has re-examined these “breathing-pores,” finds that in the early stage of their growth, near the margin or younger portion of the thallus, they are entirely covered by the cortex. Later, the hyphae at the top become looser and more frequently septate, and a fine network of anastomosing and intricate filaments takes the place of the closely cohering cortical cells. These hyphae are divided into shorter cells, but do not otherwise differ from those of the medulla. Rosendahl was unable to detect an open pore at any stage, though he entirely agrees with Zukal as to the breathing function of these structures. The gonidia of the immediately underlying zone are sparsely arranged and a few of them are found in the lower half of the cone; the hyphae of the medulla can be traced up to the apex. Zukal[442] claims to have found breathing-pores in Cornicularia (Parmelia) tristis and in several other Parmeliae, notably in Parmelia stygia. The thallus of the latter species has minute holes or openings in the upper cortex, but they are without any definite form and may be only fortuitous.

Zukal[442] published drawings of channels of looser tissue between the exterior and the pith in Oropogon Loxensis and in Usnea barbata. He considered them to be of definite service in aeration. The fronds of Ramalina dilacerata by stretching develop a series of elongate holes. Reinke[443] found openings in Ramalina Eckloni which pierced to the centre of the thallus, and Darbishire[444] has figured a break in the frond of another species, R. fraxinea (Fig. 75 A), which he has designated as a breathing-pore. Finally Brandt[445], in his careful study of the anatomy of Ramalinae, has described as breathing-pores certain open areas usually of ellipsoid form in the compact cortex of several species: in R. strepsilis (Fig. 75 B) and R. Landroensis, and in the British species, R. siliquosa and R. fraxinea. These openings are however mostly rare and difficult to find or to distinguish from holes that may be due to any accident in the life of the lichen. It is noteworthy that Rosendahl found no further examples of breathing-pores in the brown Parmeliae that he examined in such detail. No other organs specially adapted for aeration of the thallus have been discovered.

b. Other openings in the Thallus. Lobaria is the only genus of Stictaceae in which neither cyphellae nor pseudocyphellae are formed; but in two species, L. scrobiculata and L. pulmonaria, the lower surface is marked with oblong or angular bare convex patches, much larger than cyphellae. They are exposed portions of the medulla, which at these spots has been denuded of the covering cortex. Corresponding with these bare spots there is a pitting of the upper surface.

A somewhat similar but reversed structure characterizes Umbilicaria pustulata, which as the name implies is distinguished by the presence of pustules, ellipsoid swellings above, with a reticulation of cavities below. Bitter[446] in this instance has proved that they are due to disconnected centres of intercalary growth which are more vigorous on the upper surface and give rise to cracks in the less active tissue beneath. These cracks gradually become enlarged; they are, as it were, accidental in origin but are doubtless of considerable service in aeration.

In some Parmeliae there are constantly formed minute round holes, either right through the apothecia (P. cetrata, etc.), or through the thallus (P. pertusa). Minute holes are also present in the under cortex of Parmelia vittata and of P. enteromorpha, species of the subgenus Hypogymnia. Nylander[447], who first drew attention to these holes of the lower cortex, described them as arising at the forking of two lobes; but though they do occur in that position, they as frequently bear no relation to the branching. Bitter’s[448] opinion is that they arise by the decay of the cortical tissues in very limited areas, from some unknown cause, and that the holes that pierce right through the thallus in other species may be similarly explained.

Still other minute openings into the thallus occur in Parmelia vittata, P. obscurata and P. farinacea var. obscurascens. In the two latter the openings like pin-holes are terminal on the lobes and are situated exactly on the apex, between the pith and the gonidial zone; sometimes several holes can be detected on the end of one lobe. Further growth in length is checked by these holes. They appear more frequently on the darker, better illuminated plants. In Parmelia vittata the terminal holes are at the end of excessively minute adventitious branches which arise below the gonidial zone on the margin of the primary lobes. All these terminal holes are directed upwards and are visible from above.

Bitter does not attribute any physiological significance to these very definite openings in the thallus. It has been generally assumed that they aid in the aeration of the thallus; it is also possible that they may be of service in absorption, and they might even be regarded as open water conductors.

C. General Aeration of the Thallus

Definite structures adapted to secure the aeration of the thallus in a limited number of lichens have been described above. These are the breathing-pores of Parmelia exasperata and the cyphellae and pseudocyphellae of the Stictaceae, with which also may be perhaps included the circumscribed breaks in the under cortex in some members of that family.

Though lichens are composed of two actively growing organisms, the symbiotic plant increases very slowly. The absorption of water and mineral salts must in many instances be of the scantiest and the formation of carbohydrates by the deep-seated chlorophyll cells of correspondingly small amount. Active aeration seems therefore uncalled for though by no means excluded, and there are many indirect channels by which air can penetrate to the deeper tissues.

In crustaceous forms, whether corticate or not, the thallus is often deeply seamed and cracked into areolae, and thus is easily pervious to water and air. The growing edges and growing points are also everywhere more or less loose and open to the atmosphere. In the larger foliose and fruticose lichens, the soredia that burst an opening in the thallus, and the cracks that are so frequent a feature of the upper cortex, all permit of gaseous interchange. The apical growing point of fruticose lichens is thin and porous, and in many of them the ribs and veins of their channelled surfaces entail a straining of the cortical tissue that results in the formation of thinner permeable areas. Zukal[449] devoted special attention to the question of aeration, and he finds evidence of air-passages through empty spermogonia and through the small round holes that are constant in the upper surface of certain foliose species. He claims also to have proved a system of air-canals right through the thallus of the gelatinous Collemaceae. Though his proof in this instance is somewhat unconvincing, he establishes the abundant presence of air in the massively developed hypothecium of Collema fruits. He found that the carpogonial complex of hyphae was always well supplied with air, and that caused him to view with favour the suggestion that the function of the trichogyne is to provide an air-passage. In foliose lichens, the under surface is frequently non-corticate, in whole or in part; or the cortex becomes seamed and scarred with increasing expansion, the growth in the lower layers failing to keep pace with that of the overlying tissues, as in Umbilicaria pustulata.

It is unquestionable that the interior of the thallus of most lichens contains abundant empty spaces between the loose-lying hyphae, and that these spaces are filled with air.

2. CEPHALODIA

A. Historical and Descriptive

The term “cephalodium” was first used by Acharius[450] to designate certain globose apothecia (pycnidia). At a later date he applied it to the peculiar outgrowths that grow on the thallus of Peltigera aphthosa, already described by earlier writers, along with other similar structures, as “corpuscula,” “maculae,” etc. The term is now restricted to those purely vegetative gall-like growths which are in organic connection with the thallus of the lichen, but which contain one or more algae of a different type from the one present in the gonidial zone. They are mostly rather small structures, and they take various forms according to the lichen species on which they occur. They are only found on thalli in which the gonidia are bright-green algae (Chlorophyceae) and, with a few exceptions, they contain only blue-green (Myxophyceae). Cephalodia with bright-green algae were found by Hue[451] on two Parmeliae from Chili, in addition to the usual blue-green forms; the one contained Urococcus, the other Gloeocystis. Several with both types of algae were detected also by Hue[451] within the thallus of Aspicilia spp.

Flörke[452] in his account of German lichens described the cephalodia that grow on the podetia of Stereocaulon as fungoid bodies, “corpuscula fungosa.” Wallroth[453], who had made a special study of lichen gonidia, finally established that the distinguishing feature of the cephalodia was their gonidia which differed in colour from those of the normal gonidial zone. He considered that the outgrowths were a result of changes that had arisen in the epidermal tissues of the lichens, and, to avoid using a name of mixed import such as “cephalodia,” he proposed a new designation, calling them “phymata” or warts.

Further descriptions of cephalodia were given by Th. M. Fries[454] in his Monograph of Stereocaulon and Pilophorus; but the greatest advance in the exact knowledge of these bodies is due to Forssell[455] who made a comprehensive examination of the various types, examples of which occurred, he found, in connection with about 100 different lichens. Though fairly constant for the different species, they are not universally so, and are sometimes very rare even when present, and then difficult to find. A striking instance of variability in their occurrence is recorded for Ricasolia amplissima (Lobaria laciniata) (Fig. 76). The cephalodia of that species are prominent upright branching structures which grow in crowded tufts irregularly scattered over the surface. They are an unfailing and conspicuous specific character of the lichens in Europe, but are entirely wanting in North American specimens.

As cephalodia contain rather dark-coloured, blue-green algae, they are nearly always noticeably darker than the thalli on which they grow, varying from yellowish-red or brown in those of Lecanora gelida to pale-coloured in Lecidea consentiens[456], a darker red in Lecidea panaeola and various shades of green, grey or brown in Stereocaulon, Lobaria (Ricasolia), etc. They form either flat expansions of varying size on the upper surface of the thallus, rounded or wrinkled wart-like growths, or upright branching structures. On the lower surface, where they are not unfrequent, they take the form of small brown nodules or swellings. In a number of species packets of blue-green algae surrounded by hyphae are found embedded in the thallus, either in the pith or immediately under the cortex. They are of the same nature as the superficial excrescences and are also regarded as cephalodia.

B. Classification

Forssell has drawn up a classification of these structures, as follows:

I. Cephalodia vera.

1. Cephalodia epigena (including perigena) developed on the upper outer surface of the thallus, which are tuberculose, lobulate, clavate or branched in form. These are generally corticate structures.

2. Cephalodia hypogena which are developed on the under surface of the thallus; they are termed “thalloid” if they are entirely superficial, and “immersed” when they are enclosed within the tissues. They are non-corticate though surrounded by a weft of hyphae. Forssell further includes here certain placodioid (lobate), granuliform and fruticose forms which develop on the hypothallus of the lichen, and gradually push their way up either through the host thallus, or, as in Lecidea panaeola, between the thalline granules.

Nylander[457] arranged the cephalodia known to him in three groups: (1) Ceph. epigena, (2) Ceph. hypogena and (3) Ceph. endogena. Schneider[458] still more simply and practically describes them as Ectotrophic (external), and Endotrophic (internal).

II. Pseudocephalodia.

These are a small and doubtful group of cephalodia which are apparently in very slight connection with the host thallus, and show a tendency to independent growth. They occur as small scales on Solorina bispora[459] and S. spongiosa and also on Lecidea pallida. Forssell has suggested that the cephalodia of Psoroma hypnorum and of Lecidea panaeola might also be included under this head.

Forssell and others have found and described cephalodia in the following families and genera:

Sphaerophoraceae.

Lecideaceae.

Cladoniaceae.

Pannariaceae.

Peltigeraceae.

Stictaceae.

Lecanoraceae.

Physciaceae.

C. Algae that form Cephalodia

The algae of the cephalodia belong mostly to genera that form the normal gonidia of other lichens. They are:

Stigonema,—in Lecanora gelida, Stereocaulon, Pilophorus robustus, and Lecidea pelobotrya.

Scytonema,—a rare constituent of cephalodia.

Nostoc,—the most frequent gonidium of cephalodia. It occurs in those of the genera Sticta, Lobaria, Peltigera, Nephroma, Solorina and Psoroma; occasionally in Stereocaulon and in Lecidea pallida.

Lyngbya and Rivularia,—rarely present, the latter in Sticta oregana[462].

Chroococcus and Gloeocapsa,—also very rare.

Scytonema, Chroococcus, Gloeocapsa and Lyngbya are generally found in combination with some other cephalodia-building alga, though Nylander[463] found Scytonema alone in the lobulate cephalodia of Sphaerophorus stereocauloides, a New Zealand lichen, and the only species of that genus in which cephalodia are developed; and Hue[460] records Gloeocapsa as forming internal cephalodia in two species of Aspicilia. Bornet[464] found Lyngbya associated with Scytonema in the cephalodia of Stereocaulon ramulosum, and, in the same lichen, Forssell[465] found, in the several cephalodia of one specimen, Nostoc, Scytonema, and Lyngbya, while, in those of another, Scytonema and Stigonema were present. In the latter instance these algae were living free on the podetium. Forssell[465] also determined two different algae, Gloeocapsa magma and Chroococcus turgidus, present in a cephalodium on Lecidea panaeola var. elegans.

As a general rule only one kind of alga enters into the formation of the cephalodia of any species or genus. A form of Nostoc, for instance, is invariably the gonidial constituent of these bodies in the genera, Lobaria, Sticta, etc. In other lichens different blue-green algae, as noted above, may occupy the cephalodia even on the same specimen. Forssell finds alternative algae occurring in the cephalodia of:

Lecanora gelida and Lecidea illita contain either Stigonema or Nostoc;

Lecidea panaeola, with Gloeocapsa, Stigonema or Chroococcus;

Lecidea pelobotrya, with Stigonema or Nostoc;

Pilophorus robustus, with Gloeocapsa, Stigonema, or Nostoc.

Riddle[466] has employed cephalodia with their enclosed algae as diagnostic characters in the genus Stereocaulon. When the alga is Stigonema, as in S. paschale, etc., the cephalodia are generally very conspicuous, grey in colour, spherical, wrinkled or folded, though sometimes black and fibrillose (S. denudatum). Those containing Nostoc are, on the contrary, minute and are coloured verdigris-green (S. tomentosum and S. alpinum).

Instances are recorded of algal colonies adhering to, and even penetrating, the thallus of lichens, but as they never enter into relationship with the lichen hyphae, they are antagonistic rather than symbiotic and have no relation to cephalodia.

D. Development of Cephalodia

a. Ectotrophic. Among the most familiar examples of external cephalodia are the small rather dark-coloured warts or swellings that are scattered irregularly over the surface of Peltigera (Peltidea) aphthosa. This lichen has a grey foliose thallus of rather large sparingly divided lobes; it spreads about a hand-breadth or more over the surface of the ground in moist upland localities. The specific name “aphthosa” was given by Linnaeus to the plant on account of the supposed resemblance of the dotted thallus to the infantile ailment of “thrush.” Babikoff[467] has published an account of the formation and development of these Peltidea cephalodia. He determined the algae contained in them to be Nostoc by isolating and growing them on moist sterilized soil. He observed that the smaller, and presumably younger, excrescences were near the edges of the lobes. The cortical cells in that position grow out into fine septate hairs that are really the ends of growing hyphae. Among the hairs were scattered minute colonies of Nostoc cells lying loose or so closely adhering to the hairs as to be undetachable (Fig. 78 A). In older stages the hairs, evidently stimulated by contact with the Nostoc, had increased in size and sent out branches, some of which penetrated the gelatinous algal colony; others, spreading over its surface, gradually formed a cortex continuous with that of the thallus. The alga also increased, and the structure assumed a rounded or lentiform shape. The thalline cortex immediately below broke down, and the underlying gonidial zone almost wholly died off and became absorbed. The hyphae of the cephalodium had meanwhile penetrated downwards as root-like filaments, those of the thallus growing upwards into the new overlying tissue (Fig. 78 B). The foreign alga has been described as parasitic, as it draws from the lichen hyphae the necessary inorganic food material; but it might equally well be considered as a captive pressed into the service of the lichen to aid in the work of assimilation or as a willing associate giving and receiving mutual benefit.

Th. M. Fries[468] had previously described the development of the cephalodia in Stereocaulon but failed to find the earliest stages. He concluded from his observations that parasitic algae were common in the cortical layer of the lichens, but only rarely formed the “monstrous growths” called cephalodia.

b. Endotrophic. Winter[469] examined the later stages of internal cephalodine formation in a species of Sticta. The alga, probably a species of Rivularia, which gives origin to the cephalodia, may be situated immediately below the upper cortex, in the medullary layer close to the gonidial zone, or between the pith and the under cortex. The protuberance caused by the increasing tissue, which also contains the invading alga, arises accordingly either on the upper or the lower surface. In some cases it was found that the normal gonidial layer had been pushed up by the protruding cephalodium and lay like a cap over the top. The cephalodia described by Winter are endogenous in origin, though the mature body finally emerges from the interior and becomes either epigenous or hypogenous. Schneider[470] has followed the development of a somewhat similar endotrophic or endogenous type in Sticta oregana due also to the presence of a species of Rivularia. How the alga attained its position in the medulla of the thallus was not observed.

Both the algal cells of internal cephalodia and the hyphae in contact with them increase vigorously, and the newly formed tissue curving upwards or downwards appears on the outside as a swelling or nodule varying in size from that of a pin-head to a pea. On the upper surface the gonidial zone partly encroaches on the nodule, but the foreign alga remains in the centre of the structure well separated from the thalline gonidia by a layer of hyphae. The group is internally divided into small nests of dark-green algae surrounded by strands of hyphae (Fig. 79). The swellings, when they occur on the lower surface of the lichen, correspond to those of the upper in general structure, but there is no intermixture of thalline gonidia. That Nostoc cells can grow and retain the power to form chlorophyll in adverse conditions was proved by Etard and Bouilhac[471] who made a culture of the alga on artificial media in the dark, when there was formed a green pigment of chlorophyll nature.

Endotrophic cephalodia occur in many groups of lichens. Hue[472] states that he found them in twelve species of Aspicilia. As packets of blue-green algae they are a constant feature in the thallus of Solorinae. The species of that genus grow on mossy soil in damp places, and must come frequently in contact with Nostoc colonies. In Solorina crocea an interrupted band of blue-green algae lies below the normal gonidial zone and sometimes replaces it—a connecting structure between cephalodia and a true gonidial zone.

c. Pseudocephalodia. Under this section have been classified those cephalodia that are almost independent of the lichen thallus though to some extent organically connected with it, as for instance that of Lecidea panaeola which originate on the hypothallus of the lichen and maintain their position between the crustaceous granules.

The cephalodia of Lecanora gelida, as described by Sernander[473], might also be included here. He watched their development in their native habitat, an exposed rock-surface which was richly covered with the lichen in all stages of growth. Two kinds of thallus, the one containing blue-green algae (Chroococcus), the other bright-green, were observed on the rock in close proximity. At the point of contact, growth ceased, but the thallus with bright-green algae, being the more vigorous, was able to spread round and underneath the other and so gradually to transform it to a superficial flat cephalodium. All such thalli encountered by the dominant lichen were successively surrounded in the same way. The cephalodium, growing more slowly, sent root-like hyphae into the tissue of the underlying lichen, and the two organisms thus became organically connected. Sernander considers that the two algae are antagonistic to each other, but that the hyphae can combine with either.

The pseudocephalodia of Usnea species are abortive apothecia; they are surrounded at the base by the gonidial zone and cortex of the thallus, and they contain no foreign gonidia.

E. Autosymbiotic Cephalodia

Bitter[474] has thus designated small scales, like miniature thalli, that develop constantly on the upper cortex of Peltigera lepidophora, a small lichen not uncommon in Finland, and first recorded by Wainio as a variety of Peltigera canina. The alga contained in the scales is a blue-green Nostoc similar to the gonidia of the thallus. Bitter[475] described the development as similar to that of the cephalodia of Peltigera aphthosa, but the outgrowths, being lobate in form, are less firmly attached and thus easily become separated and dispersed; as the gonidia are identical with those of the parent thallus they act as vegetative organs of reproduction.

Bitter’s work has been criticized by Linkola[476] who claims to have discovered by means of very thin microtome sections that there is a genetic connection between the scales and the underlying thallus, not only with the hyphae, as in true cephalodia, but with the algae as well, so that these outgrowths should be regarded as isidia.

In the earliest stages, according to Linkola, a small group of algae may be observed in the cortical tissue of the Peltigera apart from the gonidial zone and near the upper surface. Gradually a protruding head is formed which is at first covered over with a brown cortical layer one cell thick. The head increases and becomes more lobate in form, being attached to the thallus at the base by a very narrow neck and more loosely at other parts of the scale. In older scales, the gonidia are entirely separated from those of the thallus, and a dark-brown cortex several cells in thickness covers over the top and sides; there is a colourless layer of plectenchyma beneath. At this advanced stage the scales are almost completely superficial and correspond with the cephaloidal rather than with the isidial type of formation. The algae even in the very early stages are distinct from the gonidial zone and the whole development, if isidial, must be considered as somewhat abnormal.

3. SOREDIA

A. Structure and Origin of Soredia

Soredia are minute separable parts of the lichen thallus, and are composed of one or more gonidia which are clasped and surrounded by the lichen hyphae (Fig. 80). They occur on the surface or margins of the thallus of a fairly large number of lichens either in a powdery excrescence or in a pustule-like body comprehensively termed a “soralium” (Fig. 81). The soralia vary in form and dimensions according to the species. Each individual soredium is capable of developing into a new plant; it is a form of vegetative reproduction characteristic of lichens.

Acharius[477] gave the name “soredia” to the powdery bodies with reference to their propagating function; he also interpreted the soredium as an “apothecium of the second order.” But long before his time they had been observed and commented on by succeeding botanists: first by Malpighi[478] who judged them to be seeds, he having seen them develop new plants; by Micheli[479] who however distinguished between the true fruit and those seeds; and by Linnaeus[480] who considered them to be the female organs of the plant, the apothecia being, as he then thought, the male organs. Hedwig[481], on the other hand, regarded the apothecia as the seed receptacles and the soredia as male bodies. Sprengel’s[482] statement that they were “a subtile germinating powder mixed with delicate hair-like threads which take the place of seeds” established finally their true function. Wallroth[483], who was the first really to investigate their structure and their relation to the parent plant, recognized them as of the same type as the “brood-cells” or gonidia; and as the latter, he found, could become free from the thallus and form a green layer on trees, walls, etc., in shady situations, so the soredia also could become free, though for a time they remained attached to the lichen and were covered by a veil, i.e. by the surrounding hyphal filaments. Koerber[484] also gave much careful study to soredia, their nature and function. As propagating organs he found they were of more importance than spores, especially in the larger lichens.