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.
Fig. 109. Cladonia furcata Schrad. Branched podetium with spermogonia at the tips (after Krabbe).
Fig. 110. Physcia hispida Tuckerm. Ciliate frond. a, spermogonia; b, apothecia. × ca. 5 (after Lindsay).
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.
Fig. 111. Immersed spermogonia. A, globose in Parmelia acetabulum Dub. × 600; B, with infolded walls in Lecidea (Psora) testacea Ach. × 144 (after Glück).
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.
Fig. 112. Free spermogonia in spinous cilia of Cetraria islandica Ach. A, part of frond; B, cilia. × 10.
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.
Fig. 113 A. Types of lichen “sporophores” and pycnidiospores. 1, Peltigera rufescens Hoffm. × 910; 2,Lecidea (Psora) testacea Ach. × 1200; 3,Cladonia cariosa Spreng. × 1000; 4, Pyrenula nitida Ach. × 1130; 5, Parmelia tristis Nyl. × 700; 6, Lobaria pulmonaria Hoffm. × 1000 (after Glück).
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]).
Fig. 113 B. 7, Physcia ciliaris DC. × 600; 8, Endocarpon sp. × 600 (after Glück).
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.
Fig. 114. Sterile filaments in spermogonia of Lecidea fuscoatra Ach. much magnified (after Lindsay).
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.
c. Germination of Spermatia (pycnidiospores). The strongest argument in favour of regarding the spermatia of lichens as male cells had always been the impossibility of inducing their germination. That difficulty had at length been overcome by Möller[727] who cultivated them in artificial solutions, and by that means obtained germination in nine different lichen species. He therefore rejected the commonly employed terms spermatia and spermogonia and substituted pycnoconidium and pycnidia. Pycnidiospore has been however preferred as more in accordance with modern fungal terminology. His first experiment was with the “spermatia” of Buellia punctiformis (B. myriocarpa) which measure about 8-10 µ in length and about 3 µ in width, and are borne directly on the septate spermatiophores (arthrosterigmata). In a culture drop, the spore had swelled to about double its size by the second or third day, and germination had taken place at both ends, the membrane of the spore being continuous with that of the germinating tube. In a short time cross septa were formed in the hyphae which at first were very close to each other. While apical growth advanced these first formed cells increased in width to twice the original size and, in consequence, became slightly constricted at the septa. In fourteen days a circular patch of mycelium had been formed about 280 µ in diameter. The development exactly resembled that obtained from the ascospores of the same species grown in the absence of gonidia. The largest thallus obtained in either case was about 2 mm. in diameter after three months’ growth. The older hyphae had a tendency to become brownish in colour; those at the periphery remained colourless. In Opegrapha subsiderella the development, though equally successful, was very much slower. The pycnidiospores (or spermatia) have the form of minute bent rods measuring 5·7 µ × 1·5 µ. Each end of the spore produced slender hyphae about the fifth or sixth day after sowing. In four weeks, the whole length of the filament with the spore in the middle was 300 µ. In four months a patch of mycelium was formed 2 mm. in diameter. Growth was even more sluggish with the pycnidiospores of Opegrapha atra. In that species they are rod-shaped and 5-6 µ long. Germination took place on the fifth or sixth day and in fourteen days a germination tube was produced about five times the length of the spore. In four weeks the first branching was noticed and was followed by a second branching in the seventh week. In three months the mycelial growth measured 200-300 µ across.
Germination was also observed in a species of Arthonia, the spores of which had begun to grow while still in the pycnidium. The most complete results were obtained in species of Calicium: in C. parietinum the spores, which are ovoid, slightly bent, and brownish in colour, swelled to an almost globose shape and then germinated by a minute point at the junction of spore and sterigma, and also at the opposite end; very rarely a third germinating tube was formed. Growth was fairly rapid, so that in four weeks there was a loose felt of mycelium measuring about 2 cm. × 1 cm. and 1 mm. in depth. Parallel cultures were carried out with the ascospores and the results in both cases were the same; in five or six weeks small black points appeared, which gradually developed to pycnidia with mature pycnidiospores from which further cultures were obtained.
On C. trachelinum, which has a thin greyish-white thallus spreading over old trunks of trees, the pycnidia are usually abundant. Lindsay had noted two different kinds and his observation was confirmed by Möller. The spores in one pycnidium are ovoid, measuring 2·5-3 µ × 1·5-2 µ; in the other rarer form, they are rod-shaped and 5-7 µ long. In the artificial cultures they both swelled, the rod-like spores to double their width before germination, and sometimes several tubes were put forth. Growth was slow, but of exactly the same kind from these two types of spores as from the ascospores. At the end of the second month pycnidia appeared on all the cultures, in each case producing the ovoid type of spore.
In a second paper Möller[729] recorded the partially successful germination of the “spermatia” of Collema (Leptogium) microphyllum, the species in which Stahl had demonstrated sexual reproduction. Growth was extraordinarily slow: after a month in the culture solution the first swelling of the spermatium prior to germination took place, and some time later small processes were formed in two or three directions. In the fourth month a branched filament was formed.
Möller’s experiments with ascospores and pycnidiospores were primarily undertaken to prove that the lichen hyphae were purely fungal and parasitic on the algae. A series of cultures were made by Hedlund[730] in order to demonstrate that the pycnidiospores were asexual reproductive bodies; they were grown in association with the lichen alga and their germination was followed up to the subsequent formation of a lichen thallus.
d. Variation in Pycnidia. On the thallus of Catillaria denigrata (Biatorina synothea) Hedlund found that there were constantly present two types of pycnidia: the one with short slightly bent spores 4-8 µ × 1·5 µ, the other with much longer bent spores 10-20 µ × 1·5 µ; there were numerous transition forms between the two kinds of spores. Germination took place by the prolongation of the spore; the hypha produced became septate and branches were soon formed. Hedlund found that frequently germination had already begun in the spores expelled from the spermogonium. In newly formed thalline areolae it was possible to trace back the mycelium to innumerable germinating spores of both types, long and short.
Lindsay had recorded more than one form of spermogonium on the same lichen thallus, the spermatia varying considerably in size; but he was most probably dealing with the mixed growth of more than one species. The observations of Möller and Hedlund on this point are more exact, but the limits of variation would very well include the two forms found by Möller in Calicium trachelinum; and in the different pycnidia of Catillaria denigrata Hedlund not only observed transition stages between the two kinds of spores, but the longer pycnidiospores, as he himself allows, indicated the elongation prior to germination: there is no good evidence of more than one form in any species.
F. Pycnidia with Macrospores
Tulasne[731] records the presence on the lichen thallus of “pycnidia” as well as of “spermogonia”; the former producing stylospores, larger bodies than spermatia, occasionally septate and containing oil-drops or guttulae. These spores are pyriform or ovoid in shape and are always borne at the tips of simple sporophores. He compared the pycnidia with the fungus genera Cytospora, Septoria, etc. As a rule they occur on lichens with a poorly developed thallus, on some species of Lecanora, Lecanactis, Calicium, Porina, in the family Strigulaceae and in Peltigera.
There is no morphological difference between pycnidia and spermogonia except that the spermatia of the latter are narrower; but the difference is so slight that, as Steiner has pointed out, these organs found on Lecanora piniperda, L. Sambuci and L. effusa have been described at one time as containing microconidia (spermatia), at another macroconidia (stylospores). He also regards as macrospores those of the pycnidia of Calicium trachelinum which Möller was able to germinate so successfully, and all the more so as they were brownish in colour, true microspores or spermatia being colourless.
Müller[732] has recorded some observations on the pycnidia and stylospores of the Strigulaceae, a family of tropical lichens inhabiting the leaves of the higher plants. On the thallus of Strigula elegans var. tremula from Madagascar and from India, he found pycnidia with stylospores of abnormal dimensions measuring 18-26 µ in length and 3 µ in width, and with 1 to 7 cross septa. In Strigula complanata var. genuina the stylospores were 2-8-septate and varied from 7-65 µ, in length, some of the spores being thus ten times longer than others, while the width remained the same. Müller considers that in these cases the stylospore has already grown to a septate hypha while in the pycnidium. As in the pycnidiospores, described later by Hedlund, the spores had germinated by increase in length followed by septation.
The spermogonia of Strigula, which are exactly similar to the pycnidia in size and structure, produce spermatia, measuring about 3 µ × 2 µ, and it is suggested by Müller that the stylospores may represent merely an advanced stage of development of these spermatia. Both organs were constantly associated on the same thallus; but whereas the spermogonia were abundant on the younger part of the thallus at the periphery, they were almost entirely replaced by pycnidia on the older portions near the centre, only a very few spermogonia (presumably younger pycnidial stages) being found in that region.
Lindsay[733] has described a great many different lichen pycnidia, but in many instances he must have been dealing with species of the “Fungi imperfecti” that were growing in association with the scattered granules of crustaceous lichens. There are many fungi—Discomycetes and Pyrenomycetes—parasitic on lichen thalli, and he has, in some cases, undoubtedly been describing their secondary pycnidial form of fruit, which indeed may appear far more frequently than the more perfect ascigerous form, and might easily be mistaken for the pycnidial fructification of the lichen.
G. General Survey
a. Sexual or Asexual. It has been necessary to give the preceding detailed account of these various structures—pycnidia or spermogonia—in view of the extreme importance attached to them as the possible male organs of the lichen plant, and, in giving the results obtained by different workers, the terminology employed by each one has been adopted as far as possible: those who consider them to be sexual structures call them spermogonia; those who refuse to accept that view write of them as pycnidia.
Tulasne, Nylander and others unhesitatingly accepted them as male organs without any knowledge of the female cell or of any method of fertilization. Stahl’s discovery of the trichogyne seemed to settle the whole question; but though he had evidence of copulation between the spermatium and the receptive cell or trichogyne he had no real record of any sexual process.
Many modern lichenologists reject the view that they are sexual; they regard them as secondary organs of fructification analogous to the pycnidia so abundant in the related groups of fungi. One would naturally expect these pycnidia to reappear in lichens, and it might be considered somewhat arbitrary to classify pycnidia in Sphaeropsideae as asexual reproductive organs, and then to regard the very similar structures in lichens as sexual spermogonia. It has also been pointed out that when undoubted pycnidia do occur on the lichen thallus, as in Calicium, Strigula, Peltigera, etc., they in no way differ from structures regarded as spermogonia except in the size of the pycnidiospores—and, even among these, there are transition forms. The different types of spermatia can be paralleled among the fungal pycnidiospores and the same is also true as regards the sporophores generally. Those described as arthrosterigmata by Nylander—as endosporous by Steiner—were supposed to be peculiar to lichens; but recently Laubert[734] has described a fungal pycnidium which grew on the trunk of an apple tree and in which the spores are not borne on upright sporophores but are budded off from the cells of the plectenchyma lining the pycnidium. It may be that future research will discover other such instances, though that type of sporophore is evidently of very rare occurrence among fungi.
b. Comparison with Fungi. The most obvious spermogonia among fungi with which to compare those of lichens occur in the Uredineae where they are associated with the life-cycle of a large number of rust species. They are small flask-shaped structures very much like the simpler forms of pycnidia and they produce innumerable spermatia which are budded off from the tips of simple spermatiophores. The mature spermatium has a delicate cell-wall and contains a thin layer of cytoplasm with a dense nucleus which occupies almost the whole cavity, cytological characters which, as Blackman[735] has pointed out, are characteristic of male cells and are not found in any asexual reproductive spores. If we accept Istvanffi’s[736] description and figures of the lichen spermatia as correct, their structure is wholly different: there being a very small nucleus in the centre of the cell comparable in size with those of the vegetative hyphae ([Fig. 115]).
Fig. 115. a, spermatia; b, hypha produced from spermatium of Buellia punctiformis Th. Fr. × 950 (after Istvanffi).
Lichen “spermatia” also differ very strikingly from the male cells of any given group of plants in their very great diversity of form and size; but the chief argument adduced by the opponents of the sexual theory is the capacity of germination that has been proved to exist in a fair number of species. It is true that germination has been induced in the spermatia of the Uredines by several research workers—by Plowright[737], Sappin-Trouffy[738] and by Brefeld[739]—who employed artificial nutritive solutions (sugar or honey), but the results obtained were not much more than the budding process of yeast cells. Brefeld also succeeded in germinating the “spermatia” of a pyrenomycetous fungus, Polystigma rubrum, one of the germinating tubes reaching a length four times that of the spore; but it is now known that all of these fungal spermatia are non-functional, either sexually or asexually, and degenerate soon after their expulsion, or even while still in the spermogonium.
c. Influence of Symbiosis. In any consideration of lichens it is constantly necessary to hark back to their origin as symbiotic organisms, and to bear in mind the influence of the composite life on their development. After germination from the spore, the lichen hypha is so dependant on its association with the alga, that, in natural conditions, though it persists without the gonidia for a time, it attains to only a rather feeble growth of mycelial filaments. In nutritive cultures, as Möller has proved, the absence of the alga is partly compensated by the artificial food supply, and a scanty thalline growth is formed up to the stage of pycnidial fruits. Not only in pycnidia but in all the fruiting bodies of lichens, symbiosis has entailed a distinct retrogression in the reproductive importance of the spores, as compared with fungi.
In Ascomycetes, the asci constitute the overwhelming bulk of the hymenium; in most lichens, there are serried ranks of paraphyses with comparatively few asci, and the spores are often imperfectly developed. It would not therefore be surprising if the bodies claimed by Möller and others as pycnidiospores had also lost even to a considerable extent their reproductive capacity.
d. Value in Diagnosis. Lichen spermogonia have once and again been found of value in deciding the affinity of related plants, and though there are a number of lichens in which we have no record of their occurrence, they are so constant in others, that they cannot be ignored in any true estimation of species. Nylander laid undue stress on spermogonial characters, considering them of almost higher diagnostic value than the much more important ascosporous fruit. They are, after all, subsidiary organs, and often—especially in crustaceous species—they are absent, or their relation to the species under examination is doubtful.