Lichens

CHAPTER II
CONSTITUENTS OF THE LICHEN THALLUS

I. LICHEN GONIDIA

The thallus or vegetative body of lichens differs from that of other green plants in the sharp distinction both of form and colour between the assimilative cells and the colourless tissues, and in the relative positions these occupy within the thallus: in the greater number of lichen species the green chlorophyll cells are confined to a narrow zone or band some way beneath and parallel with the surface (Fig. 1); in a minority of genera they are distributed through the entire thallus (Fig. 2); but in all cases the tissues remain distinct. The green zone can be easily demonstrated in any of the larger lichens by scaling off the outer surface cells, or by making a vertical section through the thallus. The colourless cells penetrate to some extent among the green cells; they also form the whole of the cortical and medullary tissues.

These two different elements we now know to consist of two distinct organisms, a fungus and an alga. The green algal cells were at one time considered to be reproductive bodies, and were called “gonidia,” a term still in use though its significance has changed.

1. GONIDIA IN RELATION TO THE THALLUS

A. Historical account of Lichen Gonidia

There have been few subjects of botanical investigation that have roused so much speculation and such prolonged controversy as the question of these constituents of the lichen plant. The green cells and the colourless filaments which together form the vegetative structure are so markedly dissimilar, that constant attempts have been made to explain the problem of their origin and function, and thereby to establish satisfactorily the relationship of lichens to other members of the Plant Kingdom.

In gelatinous lichens, represented by Collema, of which several species are common in damp places and grow on trees or walls or on the ground, the chains of green cells interspersed through the thallus have long been recognized as comparable with the filaments of Nostoc, a blue-green gelatinous alga, conspicuous in wet weather in the same localities as those inhabited by Collema. So among early systematists, we find Ventenat[152] classifying the few lichens with which he was acquainted under algae and hazarding the statement that a gelatinous lichen such as Collema was only a Nostoc changed in form. Some years later Cassini[153] in an account of Nostoc expressed a somewhat similar view, though with a difference: he suggested that Nostoc was but a monstrous form of Collema, his argument being that, as the latter bore the fruit, it was the normal and perfect condition of the plant. A few years later Agardh[154] claimed to have observed the metamorphosis of Nostoc up to the fertile stage of a lichen, Collema limosum. But long before this date, Scopoli[155] had demonstrated a green colouring substance in non-gelatinous lichens by rubbing a crustaceous or leprose thallus between the fingers; and Persoon[156] made use of this green colour characteristic of lichen crusts to differentiate these plants from fungi. Sprengel[157] went a step further in exactly describing the green tissue as forming a definite layer below the upper cortex of foliaceous lichens.

The first clear description and delimitation of the different elements composing the lichen thallus was, however, given by Wallroth[158]. He drew attention to the great similarity between the colourless filaments of the lichen and the hyphae of fungi. The green globose cells in the chlorophyllaceous lichens he interpreted as brood-cells or gonidia, regarding them as organs of reproduction collected into a “stratum gonimon.” To the same author we owe the terms “homoiomerous” and “heteromerous,” which he coined to describe the arrangement of these green cells in the tissue of the thallus. In the former case the gonidia are distributed equally through the structure; in the latter they are confined to a distinct zone.

Wallroth’s terminology and his views of the function of the gonidia were accepted as the true explanation for many years, the opinion that they were solely reproductive bodies being entirely in accordance with the well-known part played by soredia in the propagation of lichens—and soredia always include one or more green cells.

B. Gonidia contrasted with Algae

In describing the gonidia of the Graphideae Wallroth[159] had pointed out their affinity with the filaments of Chroolepus (Trentepohlia) umbrina. He considered these and other green algae when growing loose on the trunks of trees to be but “unfortunate brood-cells” which had become free and, though capable of growth and increase, were unable to form again a lichen plant.

Further observations on gonidia were made by E. Fries[160]: he found that the green cells escaped from the lichen matrix and produced new individuals; and also that the whole thallus in moist localities might become dissolved into the alga known as Protococcus viridis; but, he continues, “though these Protococcus cells multiplied exceedingly, they never could rise again to the perfect lichen.” Kützing[161], in a later account of Protococcus viridis, also recognized its affinity with lichens; he stated that he could testify from observation that, according to the amount of moisture present, it would develop, either in excessive moisture to a filamentous alga, or in drier conditions “to lichens such as Lecanora subfusca or Xanthoria parietina.”

A British botanist, G. H. K. Thwaites[162], at one time superintendent of the botanical garden at Peradeniya in Ceylon, published a notable paper on lichen gonidia in which he pointed out that as in Collema the green constituents of the thallus resembled the chains of Nostoc, so in the non-gelatinous lichens, the green globose cells were comparable or identical with Pleurococcus, and Thwaites further observed that they increased by division within the lichen thallus. He insisted too that in no instance were gonidia reproductive organs: they were essential component parts of the vegetative body and necessary to the life of the plant. In a further paper on Chroolepus ebeneus Ag., a plant consisting of slender dark-coloured felted filaments, he described these filaments as being composed of a central strand which closely resembled the alga Chroolepus, and of a surrounding sheath of dark-coloured cells (Fig. 3): “occasionally,” he writes, “the internal filament protrudes beyond the investing sheath, and may then be seen to consist of oblong cells containing the peculiar reddish oily-looking endochrome of Chroolepus.” Thwaites placed this puzzling plant in a new genus, Cystocoleus, at the same time pointing out its affinity with the lichen genus Coenogonium. The plant is now known as Coenogonium ebeneum. Thwaites was on the threshold of the discovery as to the true nature of the relationship between the central filament and the investing sheath, but he failed to take the next forward step.

Very shortly after, Von Flotow[163] published his views on some other lichen gonidia. He had come to the conclusion that the various species of the alga, Gloeocapsa, so frequently found in damp places, among mosses and lichens, were merely growth stages of the gonidia of Ephebe pubescens, and bore the same relation to Ephebe as did Lepra viridis (Protococcus) to Parmelia. The gonidium of Ephebe is the gelatinous filamentous blue-green alga Stigonema (Fig. 4), and the separate cells are not unlike those of Gloeocapsa. Flotow had also demonstrated that the same type of gonidium was enclosed in the cephalodia of Stereocaulon. Sachs[164], too, gave evidence as to the close connection between Nostoc and Collema. He had observed numerous small clumps of the alga growing in proximity to equally abundant thalli of Collema, with every stage of development represented from one to the other. He found cases where the gelatinous coils of Nostoc chains were penetrated by fine colourless filaments “as if invaded by a parasitic fungus.” Later these threads were seen to be attached to some cell of the Nostoc trichome. Sachs concluded, however, from very careful examination at the time, that the colourless filaments were produced by the green cells. As growth proceeded, the coloured Nostoc chains became massed towards the upper surface, while the colourless filaments tended to occupy the lower part of the thallus. He calculated that during the summer season the metamorphosis from Nostoc to a fertile Collema thallus took from three to four months. He judged that in favourable conditions the change would inevitably take place, though if there should be too great moisture no Collema would be formed. His study of Cladonia was less successful as he mistook some colonies of Gloeocapsa for a growth condition of Cladonia gonidia, an error corrected later by Itzigsohn[165].

But before this date Itzigsohn[166] had published a paper setting forth his views on thallus formation, which marked a distinct advance. He did not hazard any theory as to the origin of gonidia, but he had observed spermatia growing, much as did the cells of Oscillaria: by increase in length, and, by subsequent branching, filaments were formed which surrounded the green cells; these latter had meanwhile multiplied by repeated division till finally a complete thallus was built up, the filamentous tissue being derived from the spermatia, while the green layer came from the original gonidium. In contrasting the development with that of Collema, he represents Nostoc as a sterile product of a lichen and, like the gonidia of other lichens, only able to form a lichen thallus when it encounters the fructifying spermatia.

Braxton Hicks[167], a London doctor, some time later, made experiments with Chroococcus algae which grew in plenty on the bark of trees, and followed their development into a lichen thallus. He further claimed to have observed a Chlorococcus, which was associated with a Cladonia, divide and form a Palmella stage.

C. Culture Experiments with the Lichen Thallus

It had been repeatedly stated that the gonidia might become independent of the thallus, but absolute proof was wanting until Speerschneider[168], who had turned his attention to the subject, made direct culture experiments and was able to follow the liberation of the green cells. He took a thinnish section of the thallus of Hagenia (Physcia) ciliaris, and, by keeping it moist, he was able to observe that the gonidial cells increased by division; the moist condition at the same time caused the colourless filaments to die away. This method of investigation was to lead to further results. It was resorted to by Famintzin and Baranetzky[169] who made cultures of gonidia extracted from three different lichens, Physcia (Xanthoria) parietina, Evernia furfuracea and Cladonia sp. They were able to observe the growth and division of the green cells and, in addition, the formation of zoospores. They recognized the development as entirely identical with that of the unicellular green alga, Cystococcus humicola Naeg. Baranetzky[170] continued the experiments and made cultures of the blue-green gonidia of Peltigera canina and of Collema pulposum. In both instances he succeeded in isolating them from the thallus and in growing them in moist air as separate organisms. He adds that “many forms reckoned as algae, may be considered as vegetating lichen gonidia such as Cystococcus, Polycoccus, Nostoc, etc.” Meanwhile Itzigsohn[171] had further demonstrated by similar culture experiments that the gonidia of Peltigera canina corresponded with the algae known as Gloeocapsa monococca Kütz., and as Polycoccus punctiformis Kütz.

D. Theories as to the Origin of Gonidia

Though the relationship between the gonidia within the thallus and free-living algal organisms seemed to be proved beyond dispute, the manner in which gonidia first originated had not yet been discovered. Bayrhoffer[172] attacked this problem in a study of foliose and other lichens. According to his observations, certain colourless cells or filaments, belonging to the “gonimic” layer, grew in a downward direction and formed at their tips a faintly yellowish-green cell; it gradually enlarged and was at length thrown off as a free globose gonidium, which represented the female cell. Other filaments from the “lower fibrous layer” of the thallus at the same time grew upwards and from them were given off somewhat similar gonidia which functioned as male cells. His observations and deductions were fanciful, but it must be remembered that the attachment between hypha and alga in lichens is in many cases so close as to appear genetic, and also it often happens that as the gonidium multiplies it becomes free from the hypha.

In his Mémoire sur les Lichens, Tulasne[173] described the colourless filaments as being fungal in appearance. The green cells he recognized as organs of nutrition, and once and again in his paper he states that they arose directly by a sort of budding process from the medullary or cortical filaments, either laterally or at the apex. This apparently reasonable view of their origin was confirmed by other writers on the subject: by Speerschneider[174] in his account of the anatomy of Usnea barbata, by de Bary[175], and by Schwendener[176] in their earlier writings. But even while de Bary accepted the hyphal origin of the gonidia, he noted[177] that, accompanying Opegrapha atra and other Graphideae, on the bark were to be found free Chroolepus cells similar to the gonidia in the lichen thallus. He added that gonidia of certain other lichens in no way differed from Protococcus cells; and as for the gelatinous lichens he declared that “either they were the perfect fruiting form of Nostocaceae and Chroococcaceae—hitherto looked on as algae—or that these same Nostocaceae and Chroococcaceae are algae which take the form of Collema, Ephebe, etc., when attacked by an ascomycetous fungus.”

All these investigators, and other lichenologists such as Nylander[178], still regarded the free-living organisms identified by them as similar to the green cells of the thallus, as only lichen gonidia escaped from the matrix and vegetating in an independent condition.

The old controversy has in recent years been unexpectedly reopened by Elfving[179] who has sought again to prove the genetic origin of the green cells. His method has been to examine a large series of lichens by making sections of the growing areas, and he claims to have observed in every case the hyphal origin of the gonidia: not only of Cystococcus but also of Trentepohlia, Stigonema and Nostoc. In the case of Cystococcus, the gonidium, he says, arises by the swelling of the terminal cell of the hypha to a globose form, and by the gradual transformation of the contents to a chlorophyll-green colour, with power of assimilation. In the case of filamentous gonidia such as Trentepohlia, the hyphal cells destined to become gonidia are intercalary. In Peltigera the cells of the meristematic plectenchyma become transformed to blue-green Nostoc cells.

A study was also made by him of the formation of cephalodia[180], the gonidia of which differ from those of the “host” thallus. In Peltigera aphthosa he claims to have traced the development of these bodies to the branching and mingling of the external hairs which, in the end, form a ball of interwoven hyphae. The central cells of the ball are then gradually differentiated into Nostoc cells, which increase to form the familiar chains. Elfving allows that the gonidia mainly increase by division within the thallus, and that they also may escape and live as free organisms. His views are unsupported by direct culture experiments which are the real proof of the composite nature of the thallus.

E. Microgonidia

Another attempt to establish a genetic origin for lichen gonidia was made by Minks[181]. He had found in his examination of Leptogium myochroum that the protoplasmic contents of the hyphae broke up into a regular series of globular corpuscles which had a greenish appearance. These minute bodies, called by him microgonidia, were, he states, at first few in number, but gradually they increased and were eventually set free by the mucilaginous degeneration of the cell wall. As free thalline gonidia, they increased in size and rapidly multiplied by division. Minks was at first enthusiastically supported by Müller[182] who had found from his own observations that microgonidia might be present in any of the lichen hyphae and in any part of the thallus, even in the germinating tube of the lichen spore, and was in that case most easily seen when the spores germinated within the ascus. He argued that as spores originated within the ascus, so microgonidia were developed within the hyphae. Minks’s theories were however not generally accepted and were at last wholly discredited by Zukal[183] who was able to prove that the greenish bodies were contracted portions of protoplasm in hyphae that suffered from a lowered supply of moisture, the green colour not being due to any colouring substance, but to light effect on the proteins—an outcome of special conditions in the vegetative life of the plant. Darbishire[184] criticized Minks’s whole work with great care and he has arrived at the conclusion that the microgonidium may be dismissed as a totally mistaken conception.

F. Composite Nature of Thallus

Schwendener[185] meanwhile was engaged on his study of lichen anatomy. Though at first he adhered to the then accepted view of the genetic connection between hyphae and gonidia, his continued examination of the vegetative development led him to publish a short paper[186] in which he announced his opinion that the various blue-green and green gonidia were really algae and that the complete lichen in all cases represented a fungus living parasitically on an alga: in Ephebe, for example, the alga was a form of Stigonema, in the Collemaceae it was a species of Nostoc. In those lichens enclosing bright green cells, the gonidia were identical with Cystococcus humicola, while in Graphideae the brightly coloured filamentous cells were those of Chroolepus (Trentepohlia). This statement he repeated in an appendix to the larger work on lichens[187] and again in the following year[188] when he described more fully the different gonidial algae and the changes produced in their structure and habit by the action of the parasite: “though eventually the alga is destroyed,” he writes, “it is at first excited to more vigorous growth by contact with the fungus, and in the course of generations may become changed beyond recognition both in size and form.” In support of his theory of the composite constitution of the thallus, Schwendener pointed out the wide distribution and frequent occurrence in nature of the algae that become transformed to lichen gonidia. He claimed as further proof of the presence of two distinct organisms that, while the colourless filaments react in the same way as fungi on the application of iodine, the gonidia take the stain of algal membranes.

G. Synthetic Cultures

Schwendener’s “dual hypothesis,” as it was termed, excited great interest and no little controversy, the reasons for and against being debated with considerable heat. Rees[189] was the first who attempted to put the matter to the proof by making synthetic cultures. For this purpose he took spores from the apothecium of a Collema and sowed them on pure cultures of Nostoc, and as a result obtained the formation of a lichen thallus, though he did not succeed in producing any fructification. He observed further that the hyphal filaments from the germinating spore died off when no Nostoc was forthcoming.

Bornet[190] followed with his record of successful cultures. He selected for experiment the spores of Physcia (Xanthoria) parietina and was able to show that hyphae produced from the germinating spore adhered to the free-growing cells of Protococcus[191] viridis and formed the early stages of a lichen thallus. Woronin[192] contributed his observations on the gonidia of Parmelia (Physcia) pulverulenta which he isolated from the thallus and cultivated in pure water. He confirmed the occurrence of cell division in the gonidia and also the formation of zoospores, these again forming new colonies of algae identical in all respects with the thalline gonidia. He was able to see the germinating tube from a lichen spore attach itself to a gonidium, though he failed in his attempts to induce further growth. In our own country Archer[193] welcomed the new views on lichens, and attempted cultures but with very little success. Further synthetic cultures were made by Bornet[194], Treub[195] and Borzi[196] with a series of lichen spores. They also were able to observe the first stages of the thallus. Borzi observed spores of Physcia (Xanthoria) parietina scattered among Protococcus cells on the branch of a tree. The spores had germinated and the first branching hyphae had already begun to encircle the algae.

Additional evidence in favour of the theory of the independent origin of the colourless filaments and the green cells was furnished by Stahl’s[197] research on hymenial gonidia in Endocarpon (Fig. 5). By making synthetic cultures of the mature spores with these bodies, he was able to observe not only the germination of the spores and the attachment of the filaments to the gonidia (Fig. 6), but also the gradual building up of a complete lichen thallus to the formation of perithecia and spores.

Some years later Bonnier[198] made an interesting series of synthetic cultures between the spores of lichens germinated in carefully sterilized conditions, and algae taken from the open (Figs. 7 and 8). Separate control cultures of spores and algae were carried on at the same time, with the result that in one case lichen hyphae alone, in the other algae were produced. The various lichen spores with which he experimented were sown in association with the following algae:

(1) Protococcus.

Pure synthetic cultures of Physcia (Xanthoria) parietina were begun in August 1884 on fragments of bark. In October 1886 the thallus was several centimetres in diameter, and some of the lobes were fruited.

Physcia stellaris was also grown on bark; in one case both thallus and apothecia were developed.

Parmelia acetabulum, another corticolous species, formed only a minute thallus about 5 mm. in diameter, but entirely identical with normally growing specimens.

(2) Pleurococcus.

Lecanora (Rinodina) sophodes, sown on rock in 1883, reached in 1886 a diameter of 13 mm. with fully developed apothecia.

Lecanora ferruginea and L. subfusca after three years’ culture formed sterile thalli only.

Lecanora coilocarpa in four years, and L. caesio-rufa in three years formed very small thalli without fructification.

(3) Trentepohlia (Chroolepus).

Opegrapha vulgata in two years had developed thallus and apothecia. The control culture of the spores formed, as in nature, a considerable felt of mycelium in the interstices of the bark, but no pycnidia or apothecia.

Graphis elegans. Only the beginning of a differentiated thallus was obtained with this species.

Verrucaria muralis (?)[199] gave in less than a year a completely developed thallus.

Bonnier also attempted cultures with species of Collema and Ephebe, but was unsuccessful in inducing the formation of a lichen plant.

H. Hymenial Gonidia

Reference has already been made to the minute green cells which were originally described by Nylander[200] as occurring in the perithecia of a few Pyrenolichens as free gonidia, i.e. unentangled with lichen hyphae. Fuisting[201] found them in the perithecium of Polyblastia (Staurothele) catalepta at a very early stage of its development when the perithecial tissues were newly differentiated from those of the surrounding thallus. The gonidia enclosed in the perithecium differed in no wise from those of the thallus: they had become mechanically enclosed in the new tissue; and while those in the outer compact layers died off, those in the centre of the structure, where a hollow space arises, were subject to very active division, becoming smaller in the process and finally filling the cavity. Winter’s[202] researches on similar lichens confirmed Fuisting’s conclusions: he described them as similar to the thalline gonidia but lighter in colour and of smaller size, measuring frequently only 2·3 µ in diameter, though this size increased to about 7 µ when cultivated outside the perithecium.

Stahl[203] sufficiently demonstrated the importance of these gonidia in supplying the germinating spores with the necessary algae. They come to lie in vertical rows between the asci and, owing to pressure, assume an elongate form[204] (Figs. 5 and 6). They have been seen in very few lichens, in Endocarpon and Staurothele, both rather small genera of Pyrenolichens, and, so far as is known, in two Discolichens, Lecidea phylliscocarpa and L. phyllocaris, the latter recorded from Brazil by Wainio[205], and, on account of the inclusion of gonidia in the hymenium, placed by him in a section, Gonothecium.

I. Nature of Association between Alga and Fungus

a. Consortium and Symbiosis. These cultures had established convincingly the composite nature of the lichen thallus, and Schwendener’s opinion, that the relationship between the two organisms was some varying degree of parasitism, was at first unhesitatingly accepted by most botanists. Reinke[206] was the first to point out the insufficiency of this view to explain the long continued healthy life of both constituents, a condition so different from all known instances of the disturbing or fatal parasitism of one individual on another. He recognized in the association a state of mutual growth and interdependence, that had resulted in the production of an entirely new type of plant, and he suggested Consortium as a truer description of the connection between the fungus and the alga. This term had originally been coined by his friend Grisebach in a paper[206] describing the presence of actively growing Nostoc algae in healthy Gunnera stems; and Reinke compared that apparently harmless association with the similar phenomenon in the lichen thallus. The comparison was emphasized by him in a later paper[207] on the same subject, in which he ascribes to each “consort” its function in the composite plant, and declares that if such a mutual life of Alga and Ascomycete is to be regarded as one of parasitism, it must be considered as reciprocal parasitism; and he insists that “much more appropriate for this form of organic life is the conception and title of Consortium.” In a special work on lichens, Reinke[208] further elaborated his theory of the physiological activity and mutual service of the two organisms forming the consortium.

Frank[209] suggested the term Homobium as appropriate, but it is faulty inasmuch as it expresses a relationship of complete interdependence, and it has been proved that the fungus partly, and the alga entirely, have the power of free growth.

A wider currency was given to this view of a mutually advantageous growth by de Bary[210]. He followed Reinke in refusing to accept as satisfactory the theory of simple parasitism, and adduced the evident healthy life of the algal cells—the alleged victims of the fungus—as incompatible with the parasitic condition. He proposed the happily descriptive designation of a Symbiosis or conjoint life which was mostly though not always, nor in equal degree, beneficial to each of the partners or symbionts.

b. Different Forms of Association. The type of association between the two symbionts varies in different lichens. Bornet[211], in describing the development of the thallus in certain members of the Collemaceae, found that though as a rule the two elements of the thallus, as in some species of Collema itself, persisted intact side by side, there was in other members of the genus an occasional parasitism: short branches from the main hyphae applied themselves by their tips to some cell of the Nostoc chain (Fig. 9). The cell thus seized upon began to increase in size, and the plasma became granular and gathered at the side furthest away from the point of attachment. Finally the contents were used up, and nothing was left but an empty membrane adhering to the fungus hypha. In another species the hypha penetrated the cell. These instances of parasitism are most readily seen towards the edge of the thallus where growth is more active; towards the centre the attached cells have become absorbed, and only the shortened broken chains attest their disappearance. The other cells of the chains remain uninjured.

In Synalissa, a small shrubby gelatinous genus, the hypha, as described by Bornet and by Hedlund[212], pierces the outer wall of the gelatinous alga (Gloeocapsa) and swells inside to a somewhat globose haustorium which rests in a depression of the plasma (Fig. 10). The alga, though evidently undamaged, is excited to a division which takes place on a plane that passes through the haustorium; the two daughter-cells then separate, and in so doing free themselves from the hypha.

Hedlund followed the process of association between the two organisms in the lichens Micarea (Biatorina) prasina and M. denigrata (Biatorina synothea), crustaceous species which inhabit trunks of trees or palings. In these the alga, one of the Chlorophyceae, has assumed the character of a Gloeocapsa but on cultivation it was found to belong to the genus Gloeocystis. The cells are globose and rather small; they increase by the division of the contents into two or at most four portions which become rounded off and covered with a membrane before they become free from the mother-cell. The lichen hypha, on contact with any one of the green cells, bores through the outer membrane and swells within to a haustorium, as in the gonidia of Synalissa.

Penetrating haustoria were demonstrated by Peirce[213] in his study of the gonidia of Ramalina reticulata. In the first stage the tip of a hypha had pierced the outer wall of the alga, causing the protoplasm to contract away from the point of contact (Fig. 11). More advanced stages showed the extension of the haustorium into the centre of the cell, and, finally, the complete disappearance of the contents. In many cases it was found that penetration equally with clasping of the alga by the filament sets up an irritation which induces cell-division, and the alga, as in Synalissa, thus becomes free from the fungus. Hue[214] has recorded instances of penetration in an Antarctic species, Physcia puncticulata. It is easy, he says, to see the tips of the hyphae pierce the sheath of the gonidium and penetrate to the nucleus.

Lindau[215] has described the association between fungus and alga in Pertusaria and other crustaceous forms as one of contact only (Fig. 12). He found that the cell-membrane of the two adhering organisms was unbroken. Occasionally the algal cell showed a slight indentation, but was otherwise unchanged. The hyphal branch was somewhat swollen at the tip where it touched the alga, and the wall was slightly thinner. The attachment between the two cells was so close, however, that pressure on the cover-glass failed to separate them.

Generally the hypha simply surrounds the gonidium with clasping branches. Many algae also lie free in the gonidial zone, and Peirce[216] claims that these are larger, more deeply coloured and in every way healthier looking than those in the grasp of the fungus. He ignores, however, the case of the soredial algae which though very closely invested by the fungus are yet entirely healthy, since on their future increase depends in many cases the reproduction of new individual lichens.

In a recent study of a crustaceous sandstone lichen, “Caloplaca pyracea,” Claassen[217] has sought to prove a case of pure parasitism. The rock was at first covered with the green cells of Cystococcus sp. Later there appeared greyish-white patches on the green, representing the invasion of the lichen fungus. These patches increased centrifugally, leaving in time a bare patch in the centre of growth which was again colonized by the green alga. The lichen fruited abundantly, but wherever it encroached the green cells were more or less destroyed. The true explanation seems to be that the green cells were absorbed into the lichen thallus, though enough of them persisted to start new colonies on any bare piece of the stone. In the same way large patches of Trentepohlia aurea have been observed to be gradually invaded by the dark coloured hyphae of Coenogonium ebeneum. In time the whole of the alga is absorbed and nothing is to be seen but the dark felted lichen. The free alga as such disappears, but it is hardly correct to describe the process as one of destruction.

This algal genus Trentepohlia (Chroolepus) forms the gonidia of the Graphideae, Roccelleae, etc. It is a filamentous aerial alga which increases by apical growth. In the Graphideae, many of which grow on trees beneath the outer bark (hypophloeodal), the association between the two symbionts may be of the simplest character, but was considered by Frank[218] to be of an advanced type. According to his observations and to those of Lindau[219], the fungal hyphae penetrate first between the cells of the periderm. The alga, frequently Trentepohlia umbrina, tends to grow down into any cracks of the surface. It goes more deeply in when preceded by the hyphae. In some species both organisms maintain their independent growth, though each shows increased vigour when it comes into contact with the other. In some instances the cells of the alga are clasped by the fungus which causes the disintegration of the filament. The cells lose their bright yellow or reddish colour and are changed in appearance to greenish lichen gonidia; but no penetration by haustoria has ever been observed in Trentepohlia.

Bachmann’s[220] study of a similar gonidium in a calcicolous species of Opegrapha confirms Frank’s results. The algae had pierced not only between the looser lime granules but also through a crystal of calcium carbonate, and occupied nests scooped out in the rock by means of acid formed and excreted by their filaments. When association took place with the fungus, the algal cells were more restricted to a gonidial zone; but some of the cells, having been pushed aside by the hyphae, had started new centres of gonidia. On contact with the hyphae there was a tendency to bud out in a yeast-like growth.

In the thallus of the Roccelleae, the algal filament, also a Trentepohlia, is broken up into separate cells, but in the Coenogoniaceae, whether the gonidium be a Cladophora as in Racodium, or a Trentepohlia as in Coenogonium, the filaments remain intact and are invested more or less closely by the hyphae.