Page:EB1911 - Volume 16.djvu/599

 into one another, and the distinction is not of classificatory value.

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.

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