Chapter IV: Reproduction (2)

In the Laboulbeniaceae, a numerous and very peculiar series of Ascomycetes that live on insects, there are, in nearly all of the reproductive bodies, a carpogonial cell, a trichophoric cell and a trichogyne. The last-named organ is in some genera a simple continuous cell, in others it is septate and branched, occasionally it is absent[634]. The male cells are spermatia of two kinds, exogenous or endogenous, and the plants are monoecious or dioecious. Laboulbeniaceae have no connection with lichens. Faull[635], a recent worker on the group, states that though he observed spermatia attached to the trichogynes, he was not able to demonstrate copulation (possibly owing to over-staining), nor could he trace any migration of the nucleus through the trichophoric cell down to the carpogonial cell. In two species of _Laboulbenia_ that he examined there were no antheridia, and the egg-cell acquired its second nucleus from the neighbouring trichophoric cell. These conjugate nuclei divided simultaneously and the two daughter nuclei passed on to the ascus and fused, as in other Ascomycetes, to form the definitive nucleus.

Convincing evidence as to the importance of the trichogyne in fungi was supposed, until lately, to be afforded by the presence and functional activity of that organ associated with spermogonia in a few Pyrenomycetes—in _Poronia_, _Gnomonia_ and _Polystigma_. _Poronia_ was examined by M. Dawson[636] who found that a trichogyne-like filament distinct from the vegetative hyphae rose from the neighbourhood of the ascogonial cells. It took an upward course towards the exterior, but there was no indication that it was ever receptive. In _Gnomonia erythrostoma_ and in _Polystigma rubrum_ spermogonia with spermatia—presumably male organs—are produced in abundance shortly before the ascosporous fruit is developed. The spermatia in both cases exhibit the characters of male cells, _i.e._ very little cytoplasm and a comparatively large nucleus that occupies most of the cell cavity, along with complete incapacity to germinate. Brooks[637] found in _Gnomonia_ that tufts of the so-called trichogynes originated near the ascogonial cells, but they were “mere continuations of ordinary vegetative hyphae belonging to the coil.” They are septate and reach the surface, and the tip-cell is longer than the others as in the lichen trichogyne.

A somewhat similar arrangement is present in _Polystigma_, in which Blackman and Welsford[638] have proved that the filaments, considered as trichogynes by previous workers, are merely vegetative hyphae. A trichogyne-like structure is also present in _Capnodium_, one of the more primitive Pyrenomycetes, but it has no sexual significance.

Lindau[639] in his paper on _Gyrophora_ suggested that the trichogyne in lichens acted as a “terebrator” or boring apparatus, of service to the deeply immersed carpogonium in enabling it to reach the surface. Van Tieghem[640] explained its presence on physiological grounds as necessary for respiration, a view also favoured by Zukal[641], while Wainio[642] and Steiner[643] see in it only an “end-hypha,” the vigorous growth of which is due to its connection with the well-nourished cells of the ascogonium.

Lindau’s view has been rejected by succeeding writers: as has been already stated, it is the paraphyses that usually open the way outward for the apothecium. Van Tieghem’s theory has been considered more worthy of attention and both Dawson and Brooks incline to think that the projecting filaments described above may perform some service in respiration, even though primarily they may have functioned as sexual receptive organs.

There is very little support to be drawn from fungi for the theory that the presence of a trichogyne necessarily entails fertilization by spermatia. Lichens in this connection must be judged as a class apart.

It has perhaps been too lightly assumed that the trichogyne in lichens indicates some relationship with the Florideae[644]. Such a view might be possible if we could regard lichens and Florideae as derived from some common remote ancestor, though even then the difference in spore production—in one case exogenous, and in the other in asci and therefore endogenous—would be a strong argument against their affinity. But all the evidence goes to prove that lichens are late derivatives of fungi and have originated from them at different points. Fungi are interposed between lichens and any other ancestors, and inherited characters must have been transmitted through them. F. Bachmann’s suggestion[645] that _Collema pulposum_ should be regarded “as a link between aquatic red algae and terrestrial ascomycetes such as _Pyronema_ and the mildews” cannot therefore be accepted. It seems more probable that the lichen trichogyne is a new structure evolved in response to some physiological requirement—either sexual or metabolic—of the deeply embedded fruit primordium.

_b._ THE ASCOGONIUM. In fungi there is usually one cell forming the ascogonium, a coenogamete, which after fertilization produces ascogenous hyphae. There are exceptions, such as Cutting[646] found in _Ascophanus carneus_, in which it is composed of several cells in open contact by the formation of wide secondary pores in the cell-walls. In lichens the ascogonium is divided into a varying number of uninucleate cells. Darbishire[647] (in _Physcia_) and Baur[648] (in _Anaptychia_) have described an opening between the different cells, after presumed fertilization, that might perhaps constitute a coenogamete. Ascogenous hyphae arise from all, or nearly all the cells, whether fertilized by spermatia or not, and asci continue to be formed over a long period of time. There may even be regeneration of the entire fruiting body as described in _Graphis elegans_ and in _Pertusaria_, apparently without renewed fertilization.

Spermogonia (or pycnidia) and the ascosporous fruits generally grow on the same thallus, though not unfrequently only one of the two kinds is present. As the spermogonia appear first, while the apothecia or perithecia are still in the initial stages, that sequence of development seems to add support to the view that their function is primarily sexual; but it is equally valid as a proof of their pycnidial nature since the corresponding bodies in fungi precede the more perfect ascosporous fruits in the life-cycle.

The differences in fertility between the two kinds of thallus in _Collema crispum_ may be recalled[649]. Baur considered that development of the carpogonia was dependant on the presence of spermatia: a strong argument for the necessity of fertilization by these. The conditions in _Parmelia acetabulum_, also recorded by Baur, lend themselves less easily to any conclusion. On the thallus of that species the spermogonia and carpogonia present are out of all proportion to the very few apothecia that are ultimately formed. Though Baur suggested that cross-fertilization might be necessary, he admits that the development may be vegetative and so uninfluenced by the presence or absence of spermatia.

It is the very frequent occurrence of the trichogyne as an integral part of the carpogonium that constitutes the strongest argument for fertilization by spermatia. There is a possibility that such an organ may have been universal at one time both in fungi and in lichens, and that it has mostly degenerated through loss of function in the former, as it has disappeared in many instances in lichens. Again, there is but a scanty and vestigial record of spermogonia in Ascomycetes. They may have died out, or they may have developed into the asexual pycnidia which are associated with so many species. If we take that view we may trace the same tendency in lichens, as for instance in the capacity of various spermatia to germinate, though in lichen spermogonia there has been apparently less change from the more primitive condition. It is also possible that some process of nuclear fusion, or more probably of conjugation, takes place in the ascogonial cells, and that in the latter case the only fusion, as in some (or most) fungi, is between the two nuclei in the ascus.

If it be conceded that fully developed carpogonia with emergent trichogynes, accompanied by spermogonia and spermatia, represent fertilization, or the probability of fertilization, then the process may be assumed to take place in a fairly large and widely distributed series of lichens. Copulation between the spermatium and the trichogyne has been seen by Stahl[650], Baur[651] and by F. Bachmann[652] in _Collema_. In _Physcia pulverulenta_ Darbishire[653] could not prove copulation in the earlier stages, but he found what he took to be the remains of emptied spermatia adhering to the tips of old trichogynes. Changes in the trichogyne following on presumed copulation have been demonstrated by several workers in the Collemaceae, and open communication as a result of fertilization between the cells of the ascogonium has been described in two species. This coenocytic condition of the ascogonium (or archicarp), considered by Darbishire and others as an evidence of fertilization, has been demonstrated by Fitzpatrick[654] in the fungus _Rhizina undulata_. The walls between the cells of the archicarp in that Ascomycete became more or less open, so that the ascogenous hyphae growing from the central cells were able easily to draw nutrition from the whole coenocyte, but no process of fertilization in _Rhizina_ preceded the breaking down of the septa and no fusion of nuclei was observed until the stage of ascus formation.

The real distinction between fertile and vegetative hyphae lies, according to Harper[655], in the relative size of the nuclei. F. Bachmann speaks of one large nucleus in the spermatium of _Collema pulposum_ which would indicate sexual function. There is however very little nuclear history of lichens known at any stage until the beginning of ascus formation, when fusion of two nuclei certainly take place as in fungi to form the definitive nucleus of the ascus.

The whole matter may be summed up in Fünfstück’s[656] statement that: “though research has proved as very probable that fertilization takes place, it is an undoubted fact that no one has observed any such process.”

F. FINAL STAGES OF APOTHECIAL DEVELOPMENT

The emergence of the lichen apothecium from the thallus, and the form it takes, are of special interest, as, though it is essentially fungal in structure, it is subject to various modifications entailed by symbiosis.

_a._ OPEN OR CLOSED APOTHECIA. Schwendener[657] drew attention to two types of apothecia directly influenced by the thallus: those that are closed at first and only open gradually, and those which are, as he says, open from the first. The former occur in genera and species in which the thallus has a stoutish cortex, as, for instance, in _Lobaria_ where the young fructification has all the appearance of an opening perithecium. The open apothecia (_primitus aperta_) are found in non-corticate lichens, in which case the pioneer paraphyses arrive at the surface easily and without any converging growth. Similar apothecia are borne directly on the hypothallus at the periphery, or between the thalline areolae, and they are also characteristic of thin or slender thalli as in _Coenogonium_.

In both types of apothecium, the paraphyses pierce the cortex (Fig. 100) and secure the emergence of the developing ascomata.

_b._ EMERGENCE OF THE ASCOCARP. Hue[658] has taken up this subject in recent years and has described the process by which the vegetative hyphae surrounding the fruit primordium, excited to active growth by contact with the generative system, take part in the later stages of fruit formation. The primordium generally occupies a position near to, or just within, the upper medulla, and the hyphae in contact with it soon begin to branch freely in a vertical direction, surrounding the developing fruit and carrying it upwards generally to a superficial position. The different methods of the final emergence give two very distinct types of mature apothecium: the _lecideine_ in which the gonidial zone takes no part in the upward growth, and the _lecanorine_ into which the gonidia enter as an integral part.

In the lecideine series (Fig. 101) the encircling hyphae from the upper medulla rise as a compact column through the gonidial zone to the surface of the thallus; they then spread radially before curving up to form the outer wall or “proper margin” round the spore-bearing disc. The branching of the hyphae is fastigiate with compact shorter branches at the exterior. In such an apothecium gonidia are absent both below the hypothecium and in the margins.

In lecanorine development the ascending hyphae from the medulla, in some cases, carry with them algal cells which multiply and spread as a second gonidial layer under the hypothecium (Fig. 102). These hyphae may also spread in a radial direction while still within the thallus and give rise to an “immersed” apothecium which is lecanorine as it encloses gonidia within its special tissues, for example, in _Acarospora_ and _Solorina_. But in most cases the lecanorine fruit is superficial and not unfrequently it is raised on a short stalk (_Usnea_, etc.); both the primary gonidial zone of the thallus and the outer cortex are associated with the medullary column of hyphae from the first and grow up along with it, thus providing the outer part of the apothecium, an additional “thalline margin” continuous with the thallus itself. It is an advanced type of development peculiar to lichens, and it provides for fertility of long continuance which is in striking contrast with the fugitive ascocarps of the Discomycetes.

The distinction between lecideine and lecanorine apothecia is of great value in classification, but it is not always easily demonstrable; it is occasionally necessary to examine the early stages, as in the more advanced the thalline margin may be pushed aside by the turgid disc and become practically obliterated.

The “proper margin” reaches its highest development in the lecideine and graphideine types. It is less prominent or often almost entirely replaced when the thalline margin is superadded, except in genera such as _Thelotrema_ and _Diploschistes_ which have distinct “double margins.”

There is an unusual type of apothecium in the genus _Gyrophora_. The fruit is lecideine, the thalline gonidia taking no part in the development. The growth of the initial ascogenous tissue, according to Lindau[659], is constantly towards the periphery of the disc so that a weak spot arises in the centre which is promptly filled by a vigorous sterile growth of paraphyses. This process is repeated from new centres again and again, resulting in the irregularly concentric lines of sterile and fertile areas of the “gyrose” fruit (Fig. 103). The paraphyses soon become black at the tips. Asci are not formed until the ascogenous layer has acquired a certain degree of stability, and spores are accordingly present only in advanced stages of growth.

G. LICHEN ASCI AND SPORES

_a_. HISTORICAL. The presence of spores, as such, in the lichen fruit was first established by Hedwig[660] in _Anaptychia_ (_Physcia_) _ciliaris_. He rightly judged the minute bodies to be the “semina” of the plant. In that species they are fairly large, measuring about 50µ, long and 24µ thick, and as they are very dark in colour when mature, they stand out conspicuously from the surrounding colourless tissue of the hymenium. Acharius[661] also took note of these “semina” and happily replaced the term by that of “spores.” They may be produced, he states, in a compact nucleus (_Sphaerophoron_), in a naked disc (_Calicium_), or they may be embedded in the disc (_Opegrapha_ and _Lecidea_). Sprengel[662] opined that the spores—which he figures—were true seeds, though he allows that there had been no record of their development into new plants. Luyken[663] made a further contribution to the subject by dividing lichens into gymnocarpous and angiocarpous forms, according as the spores, enclosed in cells or vesicles (thecae), were borne in an open disc or in a closed perithecium.

In his _Systema_ of lichen genera Eschweiler[664], some years later, described and figured the spores as “thecae” enclosed in cylindrical asci. Fée[665] in contemporary works gave special prominence to the colour and form of the spores in all the lichens dealt with.

_b_. DEVELOPMENT OF THE ASCUS. The first attempt to trace the origin and development of lichen asci and spores was made by Mohl[666]. He describes the mother-cell (the ascus) as filled at first with a clouded granular substance changing later into a definite number—usually eight—of simple or septate spores. Dangeard[667] included the lichens _Borrera_ (_Physcia_) _ciliaris_ and _Endocarpon_ (_Dermatocarpon_) _miniatam_ among the plants that he studied for ascus and spore development. He found that in lichens, as in fungi, the ascus arose usually from the penultimate cell of a crooked hypha (Fig. 104) and that it contained at first two nuclei derived from adjoining cells. These nuclei are similar in size to those of the vegetative hyphae, and in each there is a large nucleolus with chromatin material massed on one side. Fusion takes place, as in fungi, between the two nuclei, and the secondary or definitive nucleus thus formed divides successively to form the eight spore-nuclei. Baur[668] and Nienburg[669] have confirmed Dangeard’s results as regards lichens, and René Maire[670] has also contributed important cytological details on the development of the spores. In _Anaptychia_ (_Physcia_) _ciliaris_ he found that the fused nucleus became larger and that a synapsis stage supervened during which the long slender chromatin filaments became paired, and at the same time shorter and thicker. The nuclear membrane disappeared as the chromatin filaments were united in masses joined together by linin threads which also disappeared later. At the most advanced stage observed by Maire there was visible a nucleolus embedded in a condensed plasma and surrounded by eight medianly constricted filaments destined to form the equatorial plate. A few isolated observations were also made on the cytology of the ascus in _Peltigera canina_, in which lichen the preceding ascogonial development is wholly vegetative. The secondary nucleus was seen to contain a chromatin mass and a large nucleolus; in addition two angular bodies of uncertain signification were associated with the nucleolus, each with a central vacuole. The nucleolus disappeared in the prophase of the first division and four double chromosomes were then plainly visible. The succeeding phases of the first and the second nuclear division were not seen, but in the prophase of the third it was possible to distinguish four chromatin masses outside the nucleolus. The slow growth of the lichen plant renders continuous observation extremely difficult.

F. Bachmann[671] was able to make important cytological observations in her study of _Collema pulposum_. As regards the vegetative and ascogonial nuclei, five or perhaps six chromosomes appeared on the spindle when the nucleus divided. In the asci, the usual paired nuclei were present in the early stages and did not fuse until the ascus had elongated considerably. After fusion the definitive nucleus enlarged with the growth of the ascus and did not divide until the ascus had attained full size. The nucleolus was large, and usually excentric, and there were at first a number of chromatin masses on an irregular spirem. In synapsis the spirem was drawn into a compact mass, but after synapsis, “the chromatin is again in the form of a knotty spirem.” In late prophases the chromosomes, small ovoid bodies, were scattered on the spindle; later they were aggregated in the centre, and, in the early metaphase, about twelve were counted now split longitudinally. There were thus twice as many chromosomes in the first division in the ascus as in nuclear divisions of the vegetative hyphae. F. Bachmann failed to see the second division; there were at least five chromosomes in the third division.

Considerable importance is given to the number of the chromosomes in the successive divisions in the ascus since they are considered to be proof of a previous double fusion—in the ascogonium and again in the ascus—necessitating, therefore, a double reduction division to arrive at the gametophytic or vegetative number of five or six chromosomes in the third division in the ascus. There have been too few observations to draw any general conclusions.

_c._ DEVELOPMENT OF SPORES. The spore wall begins to form, as in Ascomycetes, at the apex of the nucleus with the curving over of the astral threads, the nucleus at that stage presenting the figure of a flask the neck of which is occupied by the centrosome. The final spore-nucleus, as observed by Maire, divides once again in _Anaptychia_ and division is followed by the formation of a median septum, the mature spore being two-celled. In _Peltigera_ the spore is at first ovoid, but both nucleus and spore gradually elongate. The fully formed spore is narrowly fusiform and by repeated nuclear division and subsequent cross-septation it becomes 4- or even 5-6-celled.

The spores of lichens are wholly fungoid, and, in many cases, form a parallel series with the spores of the Ascomycetes. Markings of the epispore, such as reticulations, spines, etc., are rarely present (_Solorina spongiosa_), though thickening of the wall occurs in many species (_Pertusariae_, etc.), a peculiarity which was first pointed out by Mohl[672] who contrasted the spore walls with the delicate membranes of other lichen cells. Some spores, described as “halonate,” have an outer gelatinous covering which probably prevents the spore from drying up and thus prolongs the period of possible germination. Both asci and spores are, as a rule, more sparingly produced than in fungi; in many instances some or all of the spores in the ascus are imperfectly formed, and the full complement is frequently lacking, possibly owing to some occurrence of adverse conditions during the long slow development of the apothecium. In the larger number of genera and species the spores are small bodies, but in some, as for instance in the _Pertusariae_ and in some _Pyrenocarpeae_, they exceed in size all known fungus spores. In _Varicellaria microsticta_, a rare crustaceous lichen of high mountains, the solitary 1-septate spore measures up to 350 µ, in length and 115 µ in width. Most spores contain reserve material in the form of fat, etc., many are dark-coloured; Zukal[673] has suggested that the colour may be protective.

Their ejection from the ascus at maturity is caused by the twofold pressure of the paraphyses and the marginal hyphae on the addition of moisture. The spores may be shot up at least 1 cm. from the disc[674].

_d._ SPORE GERMINATION. Meyer[675] was the first who cultivated lichen spores and the dendritic formation which he obtained by growing them on a smooth surface was undoubtedly the prothallus (or hypothallus) of the lichen. Actual germination was however not observed till Holle[676] in 1846 watched and figured the process as it occurs in _Physcia ciliaris_.

Spores divided by transverse septa into two or more cells, as well as those that have become “muriform” by transverse and longitudinal septation, may germinate from each cell.

_e._ MULTINUCLEATE SPORES. These spores, which are all very large, occur in several genera or subgenera: in _Lecidea_ subg. _Mycoblastus_ (Fig. 105), _Lecanora_ subg. _Ochrolechia_ and in Pertusariaceae. Tulasne[677] in his experiments with germinating spores found that in _Lecanora parella (Ochrolechia pallescens?)_ germinating tubes were produced all over the surface of the spore (Fig. 106). De Bary[678] verified his observations in that and other species and added considerable detail: about twenty-four hours after sowing spores of _Ochrolechia pallescens_, numerous little warts arose on the surface of the spore which gradually grew out into delicate hyphae. All these spores contain fat globules and finely granular protoplasm with a very large number of minute nuclei; the presence of the latter has been demonstrated by Haberlandt[679] and later by Zopf[680] who reckoned about 200 to 300 in the spore of _Mycoblastus sanguinarius_. These nuclei had continued to multiply during the ripening of the spore while it was still contained in the ascus[680]. Owing to the presence of the large fat globules the plasma is confined to an external layer close to the spore wall; the nuclei are embedded in the plasma and are connected by strands of protoplasm. The epispore in some of these large spores is extremely developed: in some _Pertusariae_ it measures 4-5 µ in thickness.

_f._ POLARIBILOCULAR SPORES. The most peculiar of all lichen spores are those termed _polaribilocular_—signifying a two-celled spore of which the median septum has become so thickened that the cell-cavities with their contents are relegated to the two poles of the spore, an open canal frequently connecting the two cell-spaces (Fig. 107). Other terms have been suggested and used by various writers to describe this unusual character such as blasteniospore[681], orculiform[682] and placodiomorph[683] or more simply polarilocular.

The polarilocular colourless spore is found in a connected series of lichens—crustaceous, foliose and fruticose (_Placodium_, _Xanthoria_, _Teloschistes_). In another series with a darker thallus (_Rinodina and Physcia_) the spore is brown-coloured, and the median septum cuts across the plasma-connection. In other respects the brown spore is similar to the colourless one and possesses a thickened wall with reduced cell-cavities.

The method of cell-division in these spores resembles that known as “cleavage by constriction,” in which the cross wall arises by an ingrowth from all sides of the cell; in time the centre is reached and the wall is complete, or an open pore is left between the divided cells. Cell “cleavage” occurs frequently among Thallophytes, though it is unknown among the higher plants. Among Algae it is the normal form of cell-division in _Cladophora_ and also in _Spirogyra_, though in the latter the wall passes right across and cuts through the connecting plasma threads. Harper[684] found “cleavage by constriction” in two instances among fungi: the conidia of _Erysiphe_ and the gametes of _Sporodinia_ are cut off by a septum which originates as a circular ingrowth of the outer wall, comparable, he considers, with the cell-division of _Cladophora_.

The development of the thickened wall of polarilocular spores has been studied by Hue[685], who contends however that there is no true septation in the colourless spores so long as the central canal remains open. According to his observations the wall of the young spore is formed of a thin tegument, everywhere equal in thickness, and consisting of concentric layers. This tegument becomes continually thicker at the equator of the spore by the addition of new layers from the interior, and the protoplasmic contents are compressed into a gradually diminishing space. In the end the wall almost touches at the centre, and the spore consists of two polar cell-cavities with a narrow open passage between. A median line pierced by the canal is frequently seen. In a few species there is a second constriction cleavage and the spore becomes quadrilocular.

Hue insists that this spore should be regarded as only one-celled; for though the walls may touch at the centre, he says they never coalesce. He has unfortunately given no cytological observations as to whether the spore is uni- or binucleate.

In _Xanthoria parietina_, one of the species with characteristic polaribilocular spores, germination, it would seem, takes place mostly at one end only of the spore, though a germinating tube issues at both ends frequently enough to suggest that the spore is binucleate and two-celled. The absence of germination from one or other of the cells only may probably be due to the drain on their small resources. Hue has cited the rarity of such instances of double germination in support of his view of the one-celled nature of the spore. He instances that out of fifteen spores, Tulasne[686] has figured only three that have germinated at each end; Bornet[687] figures one in seven with the double germination and Bonnier[688] one in sixteen spores.

Further evidence is wanted as to the nuclear history of these hyaline spores. In the case of the brown spores, which show the same thickening of the wall and restricted cell-cavity, though with a distinct median septum, nuclear division was observed by René Maire[689] before septation in one such species, _Anaptychia ciliaris_.

II. SECONDARY SPORES

A. REPRODUCTION BY OIDIA

In certain conditions of nutrition, fungal hyphae break up into separate cells, each of which functions as a reproductive _conidium_ or _oidium_, which on germination forms new hyphae. Neubner[690] has demonstrated a similar process in the hyphae of the Caliciaceae and compares it with the oidial formation described by Brefeld[691] in the Basidiomycetes.

The thallus of this family of lichens is granular or furfuraceous; it never goes beyond the _Lepra_ stage of development[692]. In some species it is scanty, in others it is abundant and spreads over large areas of the trunks of old trees. It is only when growth is especially luxuriant that oidia are formed. Neubner was able to recognize the oidial condition by the more opaque appearance of the granules, and under the microscope he observed the hyphae surrounding the gonidia gradually fall away and break up into minute cylindrical cells somewhat like spermatia in size and form. There was no question of abnormal or unhealthy conditions, as the oidia were formed in a freely fruiting thallus.

The gonidia associated with the oidial hyphae also showed unusual vitality and active division took place as they were set free by the breaking up of the encircling hyphae. The germination of the oidia provides an abundance of hyphal filaments for the rapidly increasing algal cells, and there follows a wide-spread development of the lichen thallus.

Oidial formation has not been observed in any other family of lichens.

B. REPRODUCTION BY CONIDIA

_a._ INSTANCES OF CONIDIAL FORMATION. It is remarkable that the type of asexual reproduction so abundantly represented in fungi by the large and varied group of the Hyphomycetes is practically absent in lichens. An exception is to be found in a minute gelatinous lichen that grows on soil. It was discovered by Bornet[693] and called by him _Arnoldia_ (_Physma_) _minutula_. From the thallus rise up simple or sparingly branched colourless conidiophores which bear at the tips globose brown conidia (Fig. 108). Bornet[694] obtained these conidia by keeping very thin sections of the thallus in a drop of water[693].

Yet another instance of conidial growth is given by Steiner[695]. He had observed that the apothecia on plants of _Caloplaca aurantia_ var. _callopisma_ Stein. differed from those of normal appearance in the warted unevenness of the disc and also in being more swollen and convex, the thalline margin being almost obliterated. He found, on microscopical examination, that the hymenium was occupied by paraphyses and by occasional asci, the latter seldom containing spores, and being usually more or less collapsed. The component parts of the apothecium were entirely normal and healthy, but the paraphyses and the few asci were crushed aside by the intrusion of numerous slender unbranched septate conidiophores. Several of these might spring from one base and the hypha from which they originated could be traced some distance into the ascogenous layer, though a connection with that cell-system could not be demonstrated. While still embedded in the hymenium, an ellipsoid or obovate swelling began to form at the apex of the conidiophore; it became separated from the stalk by a septum and later divided into a two-celled conidium. The conidiophore increased in length by intercalary growth and finally emerged above the disc; the mature conidium was pyriform and measured 15-20 µ × 9-11 µ.

Steiner regarded these conidia as entirely abnormal; pycnidia with stylospores are unknown in the genus and they were not, he alleges, the product of any parasitic growth.

_b._ COMPARISON WITH HYPHOMYCETES. The conidial form of fructification in fungi, known as a Hyphomycete, is generally a stage in the life-cycle of some Ascomycete; it represents the rapid summer form of asexual reproduction. The ascospore of the resting fruit-form in many species germinates on any suitable matrix and may at once produce conidiophores and conidia, which in turn germinate, and either continue the conidial generation or proceed to the formation of the perfect fruiting form with asci and ascospores.

Such a form of transient reproduction is almost impossible in lichens, as the hypha produced by the germinating lichen ascospore has little vitality without the algal symbiont. In natural conditions development practically ceases in the absence of symbiosis. When union between the symbionts takes place, and growth becomes active, thallus construction at once commences. But in certain conditions of shade and moisture, only the rudiments of a lichen thallus are formed, known as a leprose or sorediose condition. Soredia also arise in the normal life of many lichens. As the individual granules or soredia may each give rise to a complete lichen plant, they may well be considered as replacing the lost conidial fructification.

C. CAMPYLIDIUM AND ORTHIDIUM

Müller[696] has described under the name _Campylidium_ a supposed new type of asexual fructification which he found on the thallus of tropical species of _Gyalecta_, _Lopadium_, etc., and which he considered analogous to pycnidia and spermogonia. Wainio[697] has however recognized the cup-like structure as a fungus, _Cyphella aeruginascens_ Karst., which grows on the bark of trees and occasionally is parasitic on the crustaceous thallus of lichens. Wainio has also identified the plant, _Lecidea irregularis_, first described by Fée[698], as also synonymous with the fungus.

Another name _Orthidium_ was proposed by Müller[699] for a type of fructification he found in Brazil which he contrasts or associates with _Campylidium_. It has an open marginate disc with sporophores bearing acrogenous spores. He found it growing in connection with a thin lichen thallus on leaves and considered it to be a form of lichen reproduction. Possibly _Orthidium_ is also a _Cyphella_.

III. SPERMOGONIA OR PYCNIDIA

A. HISTORICAL ACCOUNT OF SPERMOGONIA

The name spermogonium was given by Tulasne[700] to the “punctiform conceptacles” that are so plentifully produced on many lichen thalli, on the assumption that they were the male organs of the plant, and that the spore-like bodies borne in them were non-motile male cells or spermatia.

The first record of their association with lichens was made by Dillenius[701], who indicates the presence of black tubercles on the thallus of _Physcia ciliaris_. He figures them also on several species of _Cladonia_, on _Ramalina_ and on _Dermatocarpon_, but without any suggestion as to their function. Hedwig’s[702] study of the reproductive organs of the Linnaean Cryptogams included lichens. He examined _Physcia ciliaris_, a species that not only is quite common but is generally found in a fruiting condition and with very prominent spermogonia, and has been therefore a favourite lichen for purposes of examination and study. Hedwig describes and figures not only the apothecia but also those other bodies which he designates as “punctula mascula,” or again as “puncta floris masculi.” In his later work he gives a drawing of _Lichen_ (_Gyrophora_) _proboscideus_, with two of the spermogonia in section.

Acharius[703] included them among the lichen structures which he called “cephalodia”: he described them as very minute tubercles rising up from the substance of the thallus and projecting somewhat above it. He also figures a section through two “cephalodia” of _Physcia ciliaris_. Fries[704] looked on them as being mostly “anamorphoses of apothecia, the presence of abortive fruits transforming the angiocarpous lichen to the appearance of a gymnocarpous form.” Wallroth[705] assigned the small black fruits to the comprehensive fungus genus _Sphaeria_ or classified lichens bearing spermogonia only as distinct genera and species (_Pyrenothea_ and _Thrombium_). Later students of lichens—Schaerer[706], Flotow[707], and others—accepted Wallroth’s interpretation of their relation to the thallus, or they ignored them altogether in their descriptions of species.

B. SPERMOGONIA AS MALE ORGANS

Interest in these minute “tubercles” and their enclosed “corpuscles” was revived by Itzigsohn[708] who examined them with an improved microscope. He macerated in water during a few days that part of the thallus on which they were developed, and, at the end of the time, discovered that the solution contained large numbers of motile bodies which he naturally took to be the corpuscles from the broken down tubercles. He claimed to have established their function as male motile cells or spermatozoa. The discovery seemed not only to prove their sexual nature, but to link up the reproduction of lichens with that of the higher cryptogams. The tubercles in which the “spermatozoa” were produced he designated as antheridia. More prolonged maceration of the tissue to the very verge of decay yielded still larger numbers of the “spermatozoa” which we now recognize to have been motile bacilli.

Tulasne[709] next took up the subject, and failing to find the motile cells, he wrongly insisted that Itzigsohn had been misled by mere Brownian movement, but at the same time he accepted the theory that the minute conceptacles were spermogonia or male organs of lichens. He also pointed out that their constant occurrence on the thallus of practically every species of lichen, and their definite form, though with considerable variation, rendered it impossible to regard them as accidental or of no importance to the life of the plant. He compared them with fungal pycnidia such as _Phyllosticta_ or _Septoria_ which outwardly they resembled, but whereas the pycnidial spores germinated freely, the spermatia of the spermogonia, as far as his experience went, were incapable of germination.

C. OCCURRENCE AND DISTRIBUTION

_a._ RELATION TO THALLUS AND APOTHECIA. We owe to Tulasne[710] the first comparative study of lichen spermogonia. He described not only their outward form, but their minute structure, in a considerable number of representative species. A few years later Lindsay[711] published a memoir dealing with the spermogonia of the larger foliose and fruticose lichens, and, in a second paper, he embodied the results of his study of an equally extensive selection of crustaceous species. Lindsay’s work is unfortunately somewhat damaged by faulty determination of the lichens he examined, and by lack of the necessary discrimination between one thallus and another of associated and intermingled species. Both memoirs contain, however, much valuable information as to the forms of spermogonia, with their spermatiophores and spermatia, and as to their distribution over the lichen thallus.

Though spermogonia are mostly found associated with apothecia, yet in some lichens, such as _Cerania_ (_Thamnolia_) _vermicularis_, they are the only sporiferous organs known. Not unfrequently crustaceous thalli bear spermogonia only, and in some _Cladoniae_, more especially in ascyphous species, spermogonia are produced abundantly at the tips of the podetial branches (Fig. 109), while apothecia are exceedingly rare. Usually they occur in scattered or crowded groups, more rarely they are solitary. Very often they are developed and the contents dispersed before the apothecia reach the surface of the thallus; hence the difficulty in relating these organisms, since the mature apothecium is mostly of extreme importance in determining the species.

In a very large number of lichens, both crustaceous and foliose, the spermogonia are scattered over the entire thallus (Fig. 110), covering it more or less thickly with minute black dots, as in _Parmelia conspersa_. In other instances, they are to some extent confined to the peripheral areas as in _Parmelia physodes_; or they occur on the extreme edge of the thallus as in the crustaceous species _Lecanora glaucoma_ (_sordida_). In _Pyrenula nitida_ they grow on the marginal hypothallus, usually on the dark line of demarcation between two thalli.

They tend to congregate on, and indeed are practically restricted to the better lighted portions of the thallus. On the fronds of foliose forms, they appear, for instance, on the swollen pustules of _Umbilicaria pustulata_, while in _Lobaria pulmonaria_, they are mostly lodged in the ridges that surround the depressions in the thallus. In _Parmelia conspersa_, _Urceolaria_ (_Diploschistes_) _scruposa_ and some others, they occasionally invade the margins of the apothecium or even the apothecial disc as in _Lichina_. Forssell[712] found that a spermogonium had developed among cells of _Gloeocapsa_ that covered the disc of a spent apothecium of _Pyrenopsis haematopis_.

In fruticose lichens such as _Usnea_, _Ramalina_, etc. they occur near the apex of the fronds, and in _Cladonia_ they occupy the tips of the ascyphous podetia or the margins of the scyphi. In some _Cladoniae_, however, spermogonia are produced on the basal squamules, more rarely on the squamules that clothe the podetia.

_b._ FORM AND SIZE. Spermogonia are specifically constant in form, the same type being found on the same lichen species all over the globe. The larger number are entirely immersed and are ovoid or roundish (Fig. 111 A) or occasionally somewhat flattened bodies (_Nephromium laevigatum_), or again, but more rarely, they are irregular in outline with an infolding of the walls that gives the interior a chambered form (Fig. 111 B) (_Lichina pygmaea_); but all of these are only visible as minute points on the thallus.

A second series, also immersed, are borne in small protuberances of the thallus. These very prominent forms are rarely found in crustaceous lichens, but they are characteristic of such well-known species as _Ramalina fraxinea_, _Xanthoria parietina_, _Ricasolia amplissima_, _Baeomyces roseus_, etc. Other spermogonia project slightly above the level of the thallus, as in _Cladonia papillaria_ and _Lecidea lurida_; while in a few instances they are practically free, these last strikingly exemplified in _Cetraria islandica_ where they occupy the small projections or cilia (Fig. 112) that fringe the margins of the lobes; they are free also in most species of _Cladonia_.

In size they vary from such minute bodies as those in _Parmelia exasperata_ which measure 25-35 µ in diam., up to nearly 1 mm. in Lobaria _laetevirens_. As a rule, they range from about 150 µ to 400 µ across the widest part, and are generally rather longer than broad. They open above by a small slit or pore called the ostiole about 20 µ to 100 µ wide which is frequently dark in colour. In one instance, in _Icmadophila aeruginosa_, Nienburg[713] has described a spermogonium with a wide opening, the spermatiophores being massed in palisade formation along the bottom of a cup-like structure.

_c._ COLOUR OF SPERMOGONIA. Though usually the ostiole is visible as a darker point than the surrounding tissue, spermogonia are often difficult to locate unless the thallus is first wetted, when they become visible to slight magnification. They appear as black points in many _Parmeliae_, _Physciae_, _Roccellae_, etc., though even in these cases they are often brown when moistened. They are distinctly brown in some _Cladoniae_, in _Nephromium_, and in some _Physciae_; orange-red or yellow in _Placodium_ and concolorous with the thallus in _Usnea_, _Ramalina_, _Stereocaulon_, etc.

D. STRUCTURE

_a._ ORIGIN AND GROWTH. The spermogonia (or pycnidia) of lichens when mature are more or less hollow structures provided with a distinct wall or “perithecium,” sometimes only one cell thick and then not easily demonstrable, as in _Physcia speciosa_, _Opegrapha vulgata_, _Pyrenula nitida_, etc. More generally the “perithecium” is composed of a layer of several cells with stoutish walls which are sometimes colourless, but usually some shade of yellow to dark-brown, with a darker ostiole. The latter, a small slit or pore, arises by the breaking down of some of the cells at the apex. After the expulsion of the spermatia, a new tissue is formed which completely blocks up the empty spermogonium. In filamentous lichens such as _Usnea_ a dangerous local weakening of the thallus is thus avoided.

Spermogonia originate from hyphae in or near the gonidial zone. The earliest stages have not been seen, but Möller[714] noted as the first recognizable appearance or primordium of the “pycnidia” in cultures of _Calicium trachelinum_ a ball or coil of delicate yellowish-coloured hyphae. At a more advanced stage the sporophores (or spermatiophores) could be traced as outgrowths from the peripheral hyphae, directed in palisade formation towards the centre of the hyphal coil about 20-30 µ long and very slender and colourless. They begin to bud off spermatia almost immediately, as it has been found that these are present in abundance while the developing spermogonium is still wholly immersed in the thallus. Meanwhile there is gradually formed on the outside a layer of plectenchyma which forms the wall. Additional spermatiophores arise from the wall tissue and push their way inwards between the ranks of the first formed series. The spermogonium slowly enlarges and stretches and as the spermatiophores do not grow any longer a central hollow arises which becomes packed with spermatia (or spores) before the ostiole is open.

A somewhat similar process of development is described by Sturgis[715] in the spermogonia of _Ricasolia amplissima_, in which species the primordium arises by a profuse branching of the medullary hyphae in certain areas close to the gonidial zone. The cells of these branching hyphae are filled with oily matter and gradually they build up a dense, somewhat cylindrical body which narrows above to a neck-like form. The growth is upwards through the gonidial layer, and the structure widens to a more spherical outline. It finally reaches the outer cortex when some of the apical cell membranes are absorbed and a minute pore is formed. The central part becomes hollow, also by absorption, and the space thus left is lined and almost filled with multicellular branches of the hyphae forming the wall; from the cells of this new tissue the spermatia are abstricted.

_b._ FORMS AND TYPES OF SPERMATIOPHORES. The variations in form of the fertile hyphae in the spermogonium were first pointed out by Nylander[716] who described them as sterigmata[717]. He considered the differences in branching, etc. as of high diagnostic value, dividing them into two groups: simple “sterigmata” (or spermatiophores), with non-septate hyphae, and arthrosterigmata, with jointed or septate hyphae.

Simple “sterigmata” comprise those in which the spore or spermatium is borne at the end of a secondary branch or sterigma, the latter having arisen from a cell of the upright spermatiophore or from a simple basal cell. The arthrosterigmata consist of “short cells almost as broad as they are long, much pressed together, and appearing almost agglutinate especially toward the base; they fill almost the whole cavity of the spermogonium.” The arthrosterigmata may grow out into the centre of the cavity as a single cell-row, as a loose branching network, or, as in _Endocarpon_, they may form a tissue filling the whole interior. Each cell of this tissue that borders on a cavity may bud off a spermatium either directly or from the end of a short process.

The most important contributions on the subject of spermogonia in recent years are those of Glück[718] and Steiner[719]. Glück, who insisted on the “pycnidial” non-sexual character of the organs, recognized eight types of “sporophores” differing in the complexity of their branching or in the form of the “spores” (Fig. 113 A):

1. The _Peltigera_ type: the sporophores consist of a basal cell bearing one or more long sterigmata and rather stoutish ellipsoid spores. (These are true pycnidia.)

2. The _Psora_ type: a more elongate simple sporophore with sterigmata and oblong spores.

3. The _Cladonia_ type: a branching sporophore, each branch with sterigmata and oblong spores.

4. The _Squamaria_ type (called by Glück _Placodium_): also a branching sporophore but with long sickle-like bent spores.

5. The _Parmelia_ type: a more complicated system of branching and anastomosing of the sporophores, with oblong spores.

6, and 7. The _Sticta_ and _Physcia_ types: in both of these the sporophores are multi-septate; they consist of a series of radiately arranged hyphae rising from a basal tissue all round the pycnidium. They anastomose to form a network and bud off “spermatia” from the free cells or rather from minute sterigmata. In the _Physcia_ type there is more general anastomosis of the sporophores and frequently masses of sterile cells along with the fertile members occupy the centre of the pycnidium. The spermatia of these and the following _Endocarpon_ type are short cylindrical bodies (Fig. 113 B).

8. _Endocarpon_ type: the pycnidium is filled by a tissue of short broad cells, with irregular hollow spaces lined by fertile cells similar to those of the _Sticta_ and _Physcia_ types.

The three last named types of sporophores represent Nylander’s section of arthrosterigmata. Steiner has followed Nylander in also arranging the various forms into two leading groups. The first, characterized by the secondary branch or “sterigma,” he designates “exobasidial”; the second, comprising the three last types in which the spores are borne directly on the cells of the sporophore or on very short processes, he describes as “endobasidial.” Steiner also introduces a new term, _fulcrum_, for the sporophore.

The pycnidia in which these different sporophores occur are not, as a rule, characteristic of one family. _Peltigera_ type is found only in one family and the Cladonia type is fairly constant in _Cladoniae_, but “_Psora_” pycnidia are found on very varying lichens among the Lecideaceae, Verrucariaceae and others. The _Squamaria_ type with long bent spores is found not only in _Squamaria_ (Glück’s _Placodium_) but also in _Lecidea_, _Roccella_, _Pyrenula_, etc. _Parmelia_ type is characteristic of many _Parmeliae_ and also of species of _Evernia_, _Alectoria_, _Platysma_ and _Cetraria_. The _Sticta_ type occurs in _Gyrophora_, _Umbilicaria_, _Nephromium_ and _Lecanora_ as well as in _Sticta_ and in one species at least of _Collema_. To the _Physcia_ type belong the pycnidia of most _Physciaceae_ and of various _Parmeliae_, and to the closely related _Endocarpon_ type the pycnidia of _Endocarpon_ and of _Xanthoria parietina_.

_c._ PERIPHYSES AND STERILE FILAMENTS. In a few species, _Roccella tinctoria_, _Pertusaria globulifera_, etc., short one-celled sterile hyphae are formed within the spermogonium near the ostiole, towards which they converge. They correspond to the periphyses in the perithecia of some Pyrenolichens, Verrucaria, etc. (described by Gibelli[720] as spermatiophores); they are also present in some of the Pyrenomycetes (_Sordaria_, etc.), and in many cases replace the paraphyses in function when these have broken down. Sterile hyphae also occur, towards the base, mingled with the fertile spermatiophores (Fig. 114). These latter were first described and figured by Tulasne[721] in the spermogonia of _Ramalina fraxinea_ as stoutish branching filaments, rising from the same base as the spermatiophores but much longer, and frequently anastomosing with each other. They have been noted also in _Usnea barbata_ and in several species of _Parmelia_, and have been compared by Nylander[722] to paraphyses. They are usually colourless, but, in the _Parmeliae_, are often brownish and thus easily distinguished from the spermatiophores. It has been stated that these filaments are sometimes fertile. Similar sterile hyphae have been recorded in the pycnidia of fungi, in _Sporocladus_ (_Hendersonia_) _lichenicola_ (Sphaeropsideae) by Corda[723] who described them as paraphyses, and also in _Steganosporium cellulosum_ (Melanconieae). These observations have been confirmed by Allescher[724] in his recent work on _Fungi Imperfecti_. Keiszler[725] has described a _Phoma_-like, pycnidium parasitic on the leprose thallus of _Haematomma elatinum_. It contains short slender sporophores and, mixed with these, long branched sterile hyphae which reach to the ostiole and evidently function as paraphyses, though Keiszler suggests that they may be a second form of sporophore that has become sterile. On account of their presence he placed the fungus in a new genus _Lichenophoma_.

E. SPERMATIA OR PYCNIDIOSPORES

_a._ ORIGIN AND FORM OF SPERMATIA. Lichen spermatia arise at the tips of the sterigmata either through simple abstriction or by budding. In the former case—as in the _Squamaria_ type—a delicate cross-wall is formed by which the spermatium is separated off. When they arise by budding, there is first a small clavate sac-like swelling of the end of the short process or sterigma which gradually grows out into a spermatium on a very narrow base. This latter formation occurs in the _Sticta_, _Physcia_ and _Endocarpon_ types.

Nylander[726] has distinguished the following forms of spermatia:

1. Ob-clavate, the broad end attached to the sterigma as in _Usneae_, _Cetraria glauca_ and _C. juniperina_.

2. Acicular and minute but slightly swollen at each end, somewhat dumb-bell like, in _Cetraria nivalis_, _C. cucullata_, _Alectoria_, _Evernia_ and some _Parmeliae_, frequently borne on “arthrosterigmata.”

3. Acicular, cylindrical and straight, the most common form; these occur in most of the _Lecanorae_, _Cladoniae_, _Lecideae_, Graphideae, Pyrenocarpeae and occasionally they are budded off from arthrosterigmata.

4. Acicular, cylindrical, bent; sometimes these are very long, measuring up to 40 µ; they are found in various _Lecideae_, _Lecanorae_, Graphideae, Pyrenocarpeae, and also in _Roccella_, _Pilophorus_ and species of _Stereocaulon_.

5. Ellipsoid or oblong and generally very minute; they are borne on simple sterigmata and are characteristic of the genera _Calicium_, _Chaenotheca_, _Lichina_, _Ephebe_, of the small genus _Glypholecia_ and of a few species of _Lecanora_ and _Lecidea_.

In many instances there is more or less variation of form and of size in the species or even in the individual. There are no spherical spermatia.

_b._ SIZE AND STRUCTURE. The shortest spermatia in any of our British lichens are those of _Lichina pygmaea_ which are about 1·4 µ in length and the longest are those of _Lecanora crassa_ which measure up to 39 µ. In width they vary from about 0·5 µ to 2 µ. The mature spermogonium is filled with spermatia and, generally, with a mass of mucilage that swells with moisture and secures their expulsion.

The spermatia of lichens are colourless and are provided with a cell-wall and a nucleus. The presence of a nucleus was demonstrated by Möller[727] in the spermatia of _Calicium parietinum_, _Opegrapha atra_, _Collema microphyllum_, _C. pulposum_ and _C. Hildenbrandii_, and by Istvanffi[728] in those of _Buellia punctiformis_ (_B. myriocarpa_), _Opegrapha subsiderella_, _Collema Hildenbrandii_, _Calicium trachelinum_, _Pertusaria communis_ and _Arthonia communis_ (_A. astroidea_). Istvanffi made use of fresh material, fixing the spermatia with osmic acid, and in all of these very minute bodies he demonstrated the presence of a nucleus which stained readily with haematoxylin and which he has figured in the spermatia of _Buellia punctiformis_ as an extremely small dot-like structure in the centre of the cell. On germination, as in the cell-multiplication of other plants, the nucleus leads the way. Germination is preceded by nuclear division, and each new hyphal cell of the growing mycelium receives a nucleus.