Fruit Bodies.—We find two chief types of fruit bodies in the lichens, the perithecium and apothecium; the first when the fungal element is a member of the Pyrenomycetes division of the Ascomycetes, the second when the fungus belongs to the Discomycetes division. In the two genera of lichens—the Basidiolichens—in which the fungus is a member of the Basidiomycetes, we have the fructification characteristic of that class of fungi: these are dealt with separately. The perithecium is very constant in form and since the gonidia take no part in the formation of this organ or that of the apothecium it has the general structure characteristic of that division of fungi. The apothecia, though of the normal fungal type and usually disk-shaped, are somewhat more variable, and since the variations are of value in classification some more details may be added.
| After Schwendener, from De Bary’s Vergleichende Morphologie und Biologie der Pilze Mycetozoen und Bacterien, by permission of Wilhelm Engelmann. |
| Fig. 12.—Usnea barbata. (Mag. more than 500 times.) |
| c, An isolated mature soredium, with an algal cell (Pleurococcus) in the envelope or hyphae. d, Another with several algal cells in optical longitudinal section. e, f, Two soredia in the act of germinating; the hyphal envelope has grown out below into rhizoid branches, and above shows already the structure of the apex of the thallus (see fig 9). |
They present various shapes, of which the following are the principal: (a) peltate, which are large, rounded, without any distinct thalline margin[1] (e.g. Usnea, Peltigera); (b) lecanorine, or scutelliform, which are orbicular and surrounded by a distinct, more or less prominent thalline margin (e.g. Parmelia, Lecanora), having sometimes also in addition a proper one¹ (e.g. Thelotrema, Urceolaria); (c) lecideine, or patelliform, which are typically orbicular, with only a proper margin (e.g. Lecidea), sometimes obsolete, and which are occasionally irregular in shape, angular or flexuose (e.g. Lecidea jurana, L. myrmecina), or complicated and gyrose (e.g. Gyrophora), and even stipitate (e.g. Baeomyces); (d) lirelliform, which are of very irregular figure, elongated, branched or flexuose, with only a proper margin (e.g. Xylographa, Graphis, &c.) or none (e.g. some Arthoniae), and often very variable even in the same species. In colour the apothecia are extremely variable, and it is but rarely that they are the same colour as the thallus (e.g. Usnea, Ramalina). Usually they are of a different colour, and may be black, brown, yellowish, or also less frequently rose-coloured, rusty-red, orange-reddish, saffron, or of various intermediate shades. Occasionally in the same species their colour is very variable (e.g. Lecanora metaboloides, Lecidea decolorans), while sometimes they are white or glaucous, rarely greenish, pruinose. Lecideine apothecia, which are not black, but otherwise variously coloured, are termed biatorine.
 | |
| After Tulasne, from De Bary’s Vergleichende Morphologie undBiologie der Pilze, Mycetozoen und Bacterien, by permission of WilhelmEngelmann. | |
| Fig. 13.—A, B, Gyrophora cylindrica. (A mag. 90,B 390 times, C highly magnified.) | |
A, A vertical median section through a spermogonium imbedded in the thallus. o, Upper rind. u, Under rind. m, Medullary layer of the thallus. B, Portion of a very thin section from the base of the spermogonium. | w, Its wall from which proceed sterigmata with rod-like spermatia (s). m, Medullary hyphae of the thallus. C, Cladonia novae Angliae, Delise; sterigmata with spermatia from the spermogonium. |
The two principal parts of which an apothecium consists are the hypothecium and the hymenium, or thecium. The hypothecium is the basal part of the apothecium on which the hymenium is borne; the latter consists of asci (thecae) with ascospores, and paraphyses. The paraphyses (which may be absent entirely in the Pyrenolichens) are erect, colourless filaments which are usually dilated and coloured at the apex; the apices are usually cemented together into a definite layer, the epithecium (fig. 14). The spores themselves may be unicellular without a septum or multicellular with one or more septa. Sometimes the two cavities are restricted to the two ends of the spore, the polari-bilocular type and the two loculi may be united by a narrow channel (fig. 15). At other times the spores are divided by both transverse and longitudinal septa producing the muriform (murali-divided) spore so called from the resemblance of the individual chambers to the stones in a wall. The very large single spores of Pertusaria have been shown to contain numerous nuclei and when they germinate develop a large number of germ tubes.
| After Darbishire, from Berichte der deutschen botanischen Gesellschaft, by permission of Borntraeger & Co. |
| Fig. 14.—Diagram showing Apothecium in Section and surrounding Portion of Thallus, and special terms used to designate these parts. |
Development of the Ascocarps.—As the remarks on the nature of the spermatia show, the question of the sexuality of the lichens has been hotly disputed in common with that of the rest of the Ascomycetes. As indicated above, the weight of evidence seems to favour what has been put forward in the case of the non-lichen-forming fungi (see [Fungi]), that in some cases the ascogonia develop as a result of a previous fertilization by spermatia, in other cases the ascogonia develop without such a union, while in still other cases the reduction goes still farther and the ascogenous hyphae instead of developing from the ascogonia are derived directly from the vegetative hyphae.
| Fig. 15.—Vertical Section of Apothecium of Xanthoria parietina. |
| a, Paraphyses. b, Asci (thecae) with bilocular spores. c, Hypothecium. |
| After E. Baur, from Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer. |
| Fig. 16.—Collema crispum. |
| A, Carpogonium, c, with its trichogyne t. B, Apex of the trichogyne with the spermatium, s, attached. |
The first exact knowledge as to the origin of the ascocarp was the work of Stahl on Collema in 1877. He showed that the archicarp consisted of two parts, a lower coiled portion, the ascogonium, and an upper portion, the trichogyne, which projected from the thallus. Only when a spermatium was found attached to the trichogyne did the further development of the ascogonium take place. From these observations he drew the natural conclusion that the spermatium was a male, sexual cell. This view was hotly contested by many workers and it was sought to explain the trichogyne—without much success—as a respiratory organ, or as a boring organ which made a way for the developing apothecium. It was not till 1898, however, that Stahl’s work received confirmation and addition at the hands of Baur (fig. 16). The latter showed that in Collema crispum there are two kinds of thalli, one with numerous apothecia, the other quite sterile or bearing only a few. The sterile thalli possessed no spermogonia, but were found to show sometimes as many as 1000 archicarps with trichogynes; yet none or very few came to maturity. The fertile thalli were shown to bear either spermogonia or to be in immediate connexion with spermogonia-bearing thalli. Furthermore Baur showed that after the fusion of the spermatium with the trichogyne the transverse walls of that organ became perforated. There was thus very strong circumstantial evidence in favour of fertilization, although the male nucleus was not traced. The further work of Baur, and that of Darbishire, Funfstuck and Lindau, have shown that in a number of other cases trichogynes are present. Thus ascogonia with trichogynes have been observed in Endocarpon, Collema, Pertusaria, Lecanora, Gyrophora, Parmelia, Ramalina, Physcia, Anaptychia and Cladonia. In Nephroma, Peltigera, Peltidea and Solorina a cogonia without trichogynes have been observed. In Collema and a form like Xanthoria parietina it is probable that actual fertilization takes place, and possibly also in some of the other forms. It is probable, however, that in the majority of cases the ascogonia develop without normal fertilization, as is necessarily the case where the ascogonia have no trichogynes or the spermatia are absent. In these cases we should expect to find some reduced process of fertilization similar to that of Humaria granulata among the ordinary Ascomycetes, where in the absence of the antheridia the female nuclei fuse in pairs. In other lichens we should expect to find the ascogenous hyphae arising directly from the vegetative hyphae as in Humaria rutilans among the ordinary fungi, where the process is associated with the fusion of vegetative nuclei. It is possible that Solorina saccata belongs to this class. Cytological details of nuclear behaviour among the lichens are, however, difficult to obtain owing to the slow growth of these forms and the often refractory nature of the material in the matter of preparation for microscopical examination.