LICHENS 553 2. The internal structure of the thallus presents two principal modifications, viz., the stratified thallus, having its different elements o. (hyphse and gonidia) ar ranged in layers, and the unstratified thallus, in which these different elements are confused in a homogeneous tissue. A. The stratified thallus. On making a vertical sec tion this is seen in a folia ceous lichen to consist of three layers constituting a cortical, a gonidial, and a medullary system, to which in the case of many crusta ceous lichens is to be added a fourth, viz. , a hypothalline stratum. (a) The cortical stratum occupies the whole of the ex ternal surface of filamen tose and fruticulose lichens, Loth the upper and under Lection of Stratified Thallus surfaces of some foliaceous . T and squamulose species, , .,., , .,, while it is found only on the stra 1 tl " n > b f mdial stiatum ; C upper surface of crustaceous medullary stratum, 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 ejnthallus, 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 con tinuous, 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 colour less, is easily recognized. It presents the three following princi pal modifications, (a) The icoolly medulla consists of simple or branched filaments, which in foliaceous species are loosely intersected and entangled, and in fruticulose species are more or less congluti- nated, assuming n longitudinal direction, and constituting, as in Usnca, a kind of solid axis for the support of the thallus. () 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. (y) 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., Vcrrucaria fuscula) the cellules have a tendency to reunite into filaments and then to separate again into rows of cellules. (<^) The hypothaVine 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 rhizinse. (a) The hypothallus proper, which is immediately developed upon the prothallus (i.e., the filaments of the germinating spore), is a hori zontal 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. Leeidea gcographica, &c.). () The rhizinae consist of vertical rhizoid fibrillse, usually branching and tufted at their extremities, blackish or greyish in colour, rarely white, which occur on tlu lower surface of foliaceous lichens. They consist of several filamentose elements which are most frequently articulated and agglutinated (e.g., Purmdia, Pliyscia), or sometimes simple and then always articulated (e.g., Sticta). It is to be observed that the hypothallus and the rhizime 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 thaUitx. This occurs amongst the Lichcnacci (which, however, are most frequently stratified as above), and in various species belonging to the inferior genera, which have a pul verulent or hypophlceodal 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 Collcmacei 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 (Leptogittm) by a thin stratum of angulose cellules dis tinct 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 CoUc-:iiacei they are strung together mouili- formly, and distributed without order in a gela tinous pellucid sub stance ; while sometimes they are agglomerated into small groups, and situated for the most part next to the epithal lus. In Ephtbacci they are not moniliformly arranged, but are tuui- cated or involved in a gelatinous cellulose stra tum. The rest of the thallus consists of the medullary system (ex cept in Ephcbacei, in which there is no me dulla), and is composed of tubular or hollow filaments, with roundish cavities containing the gonidial granules, and imbedded in the gela tinous substance, which very readily imbibes water There are a few lichens in which there FlG - 2. -Section of Unstratified Tha]lus of is no trace whatever of Collema conglomeratum, with Momlitorm stratification, as the Gonimia scattered amongst the Hyphal genus Ccenogoniiim, in Filaments. 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. 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, 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 the} 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 theca?, 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 some times presented by the upper or under surface of the thallus. Of these the principal are the following. (1) Sorcdia are pulveru lent 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 clement. Occasionally also they appear on the disk of apothecia (in Fcrtusaria), 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 vcrmicularis) are propagated. (2) Ciiphcllx are small, urceolate, pale excavations which occur abundantly on the under surface of many species of Stictci.^ They are generally naked, but are often also pulverulent or sorediiferous, XIV. 70
