Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/573

LICHENS 553

2. The internal structure of the thallus presents two principal modifications, viz., the stratified thallus, having its different elements (hyphæ and gonidia) arranged in layers, and the unstratified thallus, in which these different elements are confused in a homogeneous tissue.

Fig 1. – Section of Stratified Thallus of Ricasolia herbacea. a, cortical stratum, b, gonidial stratum; c, medullary stratum.

A. The stratified thallus. – On making a vertical section this is seen in a foliaceous lichen to consist of three layers constituting a cortical, a gonidial, and a medullary system, to which in the case of many crustaceous lichens is to be added a fourth, viz., a hypothalline stratum.

(a) The cortical stratum occupies the whole of the external surface of filamentose and fruticulose lichens, both the upper and under surfaces of some foliaceous and squamulose species, while it is found only on the upper surface of crustaceous lichens. It usually consists of a colourless cellular tissue, in which the cellules are closely compacted and form a pseudo-parenchyma. Its most superficial portion, termed by Nylander the epithallus, a sort of cuticle, is amorphous, often more indurated and coloured. In some lichens (e.g., Collema) it is the only portion of the cortex present, while in pulverulent crustaceous thalli it is entirely wanting, (b) The gonidial stratum is situated immediately beneath the cortical stratum, and consists usually of greenish spherical cellules, or of granules destitute of a cellular membrane. It is not always continuous, but is often interrupted, the gonidia occurring in dissociated masses. Sometimes it is situated on the upper part of the medullary stratum, in which case the gonidia are arranged either between or amongst its exterior elements. In general its limits may readily be distinguished from the others by its peculiar colour. Various important matters relating to the gonidia will more appropriately be afterwards discussed at length, (c) The medullary stratum is more variable in its constituent elements, but, being always colourless, is easily recognized. It presents the three following principal modifications, (α) The woolly medulla consists of simple or branched filaments, which in foliaceous species are loosely intersected and entangled, and in fruticulose species are more or less conglutinated, assuming n longitudinal direction, and constituting, as in Usnea, a kind of solid axis for the support of the thallus. (β) The cretaceous medulla occurs only in crustaceous lichens, and is generally characterized by its tartareous appearance. It is more compact than the preceding, and consists for the most part of molecular granulations often intermixed with octahedral crystals of lime, and presenting but few traces of filamentose elements. (γ) The cellulose medulla consists of a tissue of angular, rounded, or oblong cellules containing gonidia in their interior or in their interstices (e.g., Pannaria, Endocarpon). In some species (e.g., Verrucaria fuscula) the cellules have a tendency to reunite into filaments and then to separate again into rows of cellules. (d) The hypothalline stratum is the inferior one of the thallus and that upon which the other strata are developed, though it is not always visible, and is sometimes entirely wanting. It usually presents itself under a twofold aspect, viz., the hypothallus and rhizinæ. (α) The hypothallus proper, which is immediately developed upon the prothallus (i.e., the filaments of the germinating spore), is a horizontal stratum consisting of interlacing filaments or of elongated, short, or rounded cellules, and is sometimes of a white or whitish colour, but usually dark or blackish. In many crustaceous lichens it is represented only by a black or dark-coloured border limiting the thallus (e.g. Lecidea geographica, &c.). (β) The rhizinæ consist of vertical rhizoid fibrillæ, usually branching and tufted at their extremities, blackish or greyish in colour, rarely white, which occur on the lower surface of foliaceous lichens. They consist of several filamentose elements which are most frequently articulated and agglutinated (e.g., Parmelia, Physcia), or sometimes simple and then always articulated (e.g., Sticta). It is to be observed that the hypothallus and the rhizinæ serve merely as bases of attachment for the lichens to the substratum, and do not in any way aid in its nutrition.

B. The unstratified thallus. – This occurs amongst the Lichenacci (which, however, are most frequently stratified as above), and in various species belonging to the inferior genera, which have a pulverulent or hypophlœodal thallus. In these the constituent elements are more or less mixed together, though the gonidial stratum generally remains distinct, and is often visible when the others are absent. It is, however, the families of the Byssacei and Collemacei that are more especially characterized by an unstratified thallus. Here the cortical stratum is chiefly represented by a greenish (in Collema'), rarely brown (in Synalissa, &c.). non-cellular epithallus, or in others (Leptogium) by a thin stratum of angulose cellules distinct from the other elements of the thallus. The gonidial granules are also disposed in a different manner to those of the Lichenacei. In the majority of the Collemacei they are strung together moniliformly, and distributed without order in a gelatinous pellucid substance; while sometimes they are agglomerated into small groups, and situated for the most part next to the epithallus. In Ephebacei they are not moniliformly arranged, but are tunicated or involved in a gelatinous cellulose stratum. The rest of the thallus consists of the medullary system (except in Ephebacei, in which there is no medulla), and is composed of tubular or hollow filaments, with roundish cavities containing the gonidial granules, and imbedded in the gelatinous substance, which very readily imbibes water There are a few lichens in which there is no trace whatever of stratification, as the genus Cœnogonium, in which the entire thallus is composed of filamentose membranous elements, and the peculiar family of the Myriangiacei (doubtfully, however, referable to lichens), in which it is equally cellulose throughout.

Fig 2. Section of Unstratified Tha]lus of Collema conglomeratum, with Moniliform Gonimia scattered amongst the Hyphal Filaments.

In addition to the hyphal and gonidial anatomical elements which thus enter into the structure of the thallus, there is another to be noticed, which, however, is to be regarded rather as an immediate principle. This is the molecular granulations, which are extremely small and (in form) irregular corpuscles, 0.001-0.002 millim. in diameter, and visible only when very highly magnified (300-400 diameters). They occur in all parts of the thallus, especially in the younger cellules, from the epithallus to the hypothallus, being especially abundant in the medulla of crustaceous species. In the epithallus they are variously coloured according to the colours which it presents, but in all other parts they are colourless. They occur also in the apothcca, in the epithecium, the thecæ, and the spores, and constitute the famous "micro- gonidia" of Dr Minks. By the application of sulphuric acid many of them are transformed into small acicular crystals, and in the spores they are frequently agitated by a Brownian movement.

We may here also, in connexion with the vegetative system of lichens, refer to certain peculiar excrescences which are sometimes presented by the upper or under surface of the thallus. Of these the principal are the following. (1) Soredia are pulverulent eruptions on the cortical stratum, varying in form, being rounded or diffuse, and either are scattered upon the upper surface of the thallus or border its margins. They are of a lighter colour than the thallus, and consist of a mass of gonidia and of molecular granulations intermingled with filamentose elements. They occur in many fruticulose, foliaceous, and crustaceous lichens, and their protrusion through the cortical stratum is owing most probably to an excessive development of the gonidial element. Occasionally also they appear on the disk of apothecia (in Pertusaria), which they render abortive, and in this case constitute the pseudo-genus Variolaria of older authors. When detached from the thallus they are capable under certain favourable circumstances of giving rise to new plants, and thus act the part of bulbils in the Phanerogamia. It is no doubt by their means that many species which are never found in a fertile state (e.g., Thamnolia vermicularis) are propagated. (2) Cuphellæ are small, urceolate, pale exeavations which occur abundantly on the under surface of many species of Stictei. They are generally naked, but are often also pulverulent or sorediiferous,