Wainio has indicated the course of evolution on the following lines: (1) the crustaceous thallus is monophyletic in origin and here as elsewhere precedes the squamulose. The latter he considers to be also monophyletic, though at more than one point the more advanced and larger foliose forms have appeared: (2) the primitive podetium was subulate and unbranched, and the apex was occupied by the apothecium. Both scyphus and branching are later developments indicating progress. They are in both cases associated with fruit-formation—scyphi generally arising from abortive apothecia[1009], branching from aggregate apothecia. In forms such as Cl. fimbriata, where both scyphiferous and subulate sterile podetia are frequent, the latter (subspecies fibula) are retrogressive, and reproduce the ancestral pointed podetium. (3) In subgen. Cenomyce, with a squamulose primary thallus, there is a sharp division into two main phyla characterized by the colour of the apothecia, brown in Ochrophaeae—the colour being due to a pigment—and red in Cocciferae where the colouring substance is a lichen-acid, rhodocladonic acid. In the brown-fruited Ochrophaeae there are again several secondary phyla. Two of these are distinguished primarily by the character of the branching: (a) the Chasmariae in which two or several branches arise from the same level, entailing perforation of the axils (Cl. furcata, Cl. rangiformis, Cl. squamosa, etc.), the scyphi also are perforated. They are further characterized by peltate aggregate apothecia, this grouping of the apothecia according to Wainio being the primary cause of the complex branching, the several fruit stalks growing out as branches. The second group (b), the Clausae, are not perforated and the apothecia are simple and broad-based on the edge of the scyphus (Cl. pyxidata, Cl. fimbriata, etc.), or on the tips of the podetia (Cl. cariosa, Cl. leptophylla, etc.). A third very small group also of Clausae called (c) Foliosae has very large primary squamules and reduced podetia (Cl. foliacea, etc.), while finally (d) the Ochroleucae, none of which is British, have poorly developed squamules and variously formed yellowish podetia with pale-coloured apothecia.
The Cocciferae represent a phylum parallel in development with the Ochrophaeae. The species have perhaps most affinity with the Clausae, the vegetative thallus—both the squamules and the podetia—being very much alike in several species. Wainio distinguishes two groups based on a difference of colour in the squamules, glaucous green in one case, yellowish in the other.
6. Causes of Variation. External causes of variation in Cladonia are chiefly humidity and light, excess or lack of either effecting changes which may have become fixed and hereditary. Minor changes directly traceable to these influences are also frequent, viz. size of podetia, proliferation and the production more or less of soredia or of squamules on the podetia, though only in connection with species in which these variations are already an acquired character. The squamules on the podetium more or less repeat the form of the basal squamules.
7. Podetial Development and Spore-dissemination. In a recent paper by Hans Sättler[1010] the problem of podetial development in Cladonia is viewed from a different standpoint. He holds that as the podetia are apothecial stalks, their service to the plant consists in the raising of the mature fruit in order to secure a wide distribution of the spores, and that changes in the form of the podetium are therefore but new adaptations for the more efficient discharge of this function.
Following out this idea he regards as the more primitive forms those in which both the spermogonia, as male reproductive bodies, and the carpogonia occur on the primary thallus, ascogonia and trichogynes being formed before the podetium emerges from the thallus. Fertilization thus must take place at a very early period, though the ultimate fruiting stage may be long delayed. Sättler considers that any doubt as to actual fertilization is without bearing on the question, as sexuality he holds must have originally existed and must have directed the course of evolution in the reproductive bodies. In this primitive group, called by him the “Floerkeana” group, the podetia are always short and simple, they are terminated by the apothecium and no scyphi are formed (Cl. Floerkeana, Cl. leptophylla, Cl. cariosa, Cl. caespiticia, Cl. papillaria, etc.).
In his second or “pyxidata” group, he places those species in which the apothecia are borne at the edge of a scyphus. That structure he follows Wainio in regarding as a morphological reaction on the failure of the first formed apical apothecium: it is, he adds, a new thallus in the form of a spreading cup and bears, as did the primary thallus, both the female primordia and the spermogonia. In some species, such as Cl. foliacea, there may be either scyphous or ascyphous podetia, and spermogonia normally accompany the carpogonium appearing accordingly along with it either on the squamule or on the scyphus.
As the pointed podetia are the more primitive, Sättler points out that they may reappear as retrogressive structures, and have so appeared in the “pyxidata” group in such species as Cl. fimbriata. He refers to Wainio’s statement that the abortion of the apothecium being a retrogressive anomaly, while scyphus formation is an evolutionary advance, the scyphiferous species present the singular case, “that a progressive transmutation induced by a retrogressive anomaly has become constant.”
His third group includes those forms that grow in crowded tufts or swards such as Cl. rangiferina, Cl. furcata, Cl. gracilis, etc. They originate, as did the pyxidata group, in some Floerkeana-like form, but in the “rangiferina” group instead of cup-formation there is extensive branching. In the closely packed phalanx of branches water is retained as in similar growths of mosses, and moist conditions necessary for fertilization are thus secured as efficiently as by the water-holding scyphus.
Sättler in his argument has passed over many important points. Above all he ignores the fact that whatever may have been the original nature and function of the podetium, it has now become a thalline structure and provides for the vegetative life of the plant, and that it is in its thalline condition that the many variations have been formed; the scyphus is not, as he contends, a new thallus, it is only an extension of thalline characters already acquired.