Until comparatively recent times the lichens were considered as a group of simple organisms on a level with algae and fungi. The green (or blue-green) cells were termed gonidia by Wallroth, who looked upon them as asexual reproductive cells, but when it was later realized that they were not reproductive elements they were considered as mere outgrowths of the hyphae of the thallus which had developed chlorophyll. In 1865 De Bary suggested the possibility that such lichens as Collema, Ephebe, &c., arose as a result of the attack of parasitic Ascomycetes upon the algae, Nostoc, Chroococcus, &c. In 1867 the observations of Famintzin and Baranetzky showed that the gonidia, in certain cases, were able to live outside the lichen-thallus, and in the case of Physcia, Evernia and Cladonia were able to form zoospores. Baranetzky therefore concluded that a certain number, if not all of the so-called algae were nothing more than free living lichen-gonidia. In 1869 Schwendener put forward the really illuminating view—exactly opposite to that of Baranetzky—that the gonidia in all cases were algae which had been attacked by parasitic fungi. Although Schwendener supported this view of the “dual” nature of lichens by very strong evidence and identified the more common lichen-gonidia with known free-living algae, yet the theory was received with a storm of opposition by nearly all lichenologists. These workers were unable to consider with equanimity the loss of the autonomy of their group and its reduction to the level of a special division of the fungi. The observations of Schwendener, however, received ample support from Bornet’s (1873) examination of 60 genera. He investigated the exact relation of fungus and alga and showed that the same alga is able to combine with a number of different fungi to form lichens; thus Chroolepus umbrinus is found as the gonidia of 13 different lichen genera.

The view of the dual nature of lichens had hitherto been based on analysis; the final proof of this view was now supplied by the actual synthesis of a lichen from fungal and algal constituents. Rees in 1871 produced the sterile thallus of a Collema from its constituents; later Stahl did the same for three species. Later Bonnier (1886) succeeded in producing fertile thalli by sowing lichen spores and the appropriate algae upon sterile glass plates or portions of bark, and growing them in sterilized air (fig. 1). Möller also in 1887 succeeded in growing small lichen-thalli without their algal constituent (gonidia) on nutritive solutions; in the case of Calicium pycnidia were actually produced under these conditions.

The thallus or body of the lichen is of very different form in different genera. In the simplest filamentous lichens (e.g. Ephebe pubescens) the form of thallus is the form of the filamentous alga which is merely surrounded by the fungal hyphae (fig. 2). The next simplest forms are gelatinous lichens (e.g. Collemaceae); in these the algae are Chroococcaceae and Nostocaceae, and the fungus makes its way into the gelatinous membranes of the algal cells and ramifies there (fig. 3). We can distinguish this class of forms as lichens with a homoiomerous thallus, i.e. one in which the alga and fungus are equally distributed. The majority of the lichens, however, possess a stratified thallus in which the gonidia are found as a definite layer or layers embedded in a pseudo-parenchymatous mass of fungal hyphae, i.e. they are heteromerous (figs. 8 and 9). Obviously these two conditions may merge into one another, and the distinction is not of classificatory value.

After Bonnier, from v. Tavel.   From Strasburger’s Lehrbuch der Botanik, bypermission of Gustav Fischer.
Fig. 1.—Xanthoria parietina. By the fusion of the hyphae in themiddle of the mycelium a pseudo-parenchymatous cortical layerhas begun to form.

1, Germinating ascospore (sp)with branching germ-tubeapplied to the Cystococcuscells (a).

2, Thallus in process of formation.

sp, Two ascospores.

p, Cystococcus cells.

In external form the heteromerous thallus presents the following modifications. (a) The foliaceous (leaf-like) thallus, which may be either peltate, i.e. rounded and entire, as in Umbilicaria, &c., or variously lobed and laciniated, as in Sticta, Parmelia, Cetraria (fig. 4), &c. This is the highest type of its development, and is sometimes very considerably expanded. (b) The fruticose thallus may be either erect, becoming pendulous, as in Usnea (fig. 5), Ramalina, &c., or prostrate, as in Alectoria jubata, var. chalybeiformis. It is usually divided into branches and branchlets, bearing some resemblance to a miniature shrub. An erect cylindrical thallus terminated by the fruit is termed a podetium, as in Cladonia (fig. 7). (c) The crustaceous thallus, which is the most common of all, forms a mere crust on the substratum, varying in thickness, and may be squamose (in Squamaria), radiate (in Placodium), areolate, granulose or pulverulent (in various Lecanorae and Lecideae). (d) The hypophloeodal thallus is often concealed beneath the bark of trees (as in some Verrucariae and Arthoniae), or enters into the fibres of wood (as in Xylographa and Agyrium), being indicated externally only by a very thin film (figs. 3, 4, 5, 6, 7 and 8). In colour also the thallus externally is very variable. In the dry and more typical state it is most frequently white or whitish, and almost as often greyish or greyish glaucous. Less commonly it is of different shades of brown, red, yellow and black. In the moist state of the thallus these colours are much less apparent, as the textures then become more or less translucent, and the thallus usually prevents the greenish colour of the gonidia (e.g. Parmelia Borreri, Peltidea aphthosa, Umbilicaria pustulata and pulverulent Lecideae).

The thallus may be free upon the surface of the substratum (e.g. Collema) or may be fixed more or less closely to it by special hyphae or rhizoids. These may penetrate but slightly into the substratum, but the connexion established may be so close that it is impossible to remove the thallus from the substratum without injury (e.g. Physcia, Placodium). In some cases the rhizoids are united together into larger strands, the rhizines.

The typical heteromerous thallus shows on section a peripheral, thin and therefore transparent, layer, the cortical layer, and centrally a mass of denser tissue the so-called medullary layer, between these two layers is the algal zone or gonidial layer (figs. 8 and 9).

The term epithallus is sometimes applied to the superficial dense portion of the cortical layer and the term hypothallus to the layer, when specially modified, in immediate contact with the substratum; the hypothallus is usually dark or blackish. The cylindrical branches of the fruticose forms are usually radially symmetrical, but the flattened branches of these forms and also the thalli of the foliaceous form show a difference in the cortex of the upper and lower side. The cortical layer is usually more developed on the side towards the light, while in many lichens this is the only side provided with a cortical layer. The podetia of some species of Cladonia possess no cortical layer at all. The surface of the thallus often exhibits outgrowths in the form of warts, hairs, &c. The medullary layer, which usually forms the main part of the thallus, is distinguished from the cortical layer by its looser consistence and the presence in it of numerous, large, air-containing spaces.

After Sachs, from De Bary’s Vergleichende Morphologie und Biologie der Pilze, Mycetozoen und Bacterien, by permission of Wilhelm Engelmann. Fig. 3.—Section of Homoiomerous Thallus of Collema conglomeratum, with Nostoc threads scattered among the hyphae.
Fig. 2.—Ephebe pubescens, Fr. A branched filiform thallus of Stigonema with the hyphae of the fungus growing through its gelatinous membranes. Extremity of a branch of the thallus with a young lateral branch a; h, hyphae; g, cells of the alga; gs, the apex of the thallus.

Gonidia.—It has been made clear above that the gonidia are nothing more than algal cells, which have been ensnared by fungal hyphae and made to develop in captivity (fig. 1). Funfstuck gives ten free living algae which have been identified as the gonidia of lichens. Pleurococcus (Cystococcus) humicola in the majority of lichens, e.g. Usnea, Cladonia, Physcia, Parmelia, Calicium, many species of Lecidea, &c., Trentepohlia (Chroolepus) umbrina in many species of Verrucaria, Graphidieae and Lecidea; Palmella botryoides in Epigloea; Pleurococcus vulgaris in Acarospora, Dermatocarpon, Catillaria; Dactylococcus infusionum in Solorina, Nephromia; Nostoc lichenoides in most of the Collemaceae; Rivularia rutida in Omphalaria; Lichina, &c., Polycoccus punctiformis in Peltigera, Pannaria and Stictina; Gloeocapsa polydermatica in Baeomyces and Omphalaria; Sirosiphon pulvinatus in Ephebe pubescens. The majority of lichens are confined to one particular kind of gonidium (i.e. species of alga) but a few forms are known (Lecanora granatina, Solorina crocea) which make use of more than one kind in their development. In the case of Solorina, for example, the principal alga is a green alga, one of the Palmellaceae, but Nostoc (a blue-green alga) is also found playing a subsidiary part as gonidia. In L. granatina the primary alga is Pleurococcus, the secondary, Gleococapsa.