THE PODETIUM OF CLADONIA.
This paper is in reality a criticism of Krabbe'st monograph of the genus Cladonia. Krabbe maintains that the podetia belong to and are a part of the spore-bearing organs (apothecia). His conclusions are based upon the observation of the morphological characters and the mode of development which, according to Reinke and other modern biologists, are not sufficient. Nor can the mor phological characters or structure be deduced from its develop ment. Krabbe assumes that since the podetia of Cladonia origi nate in a manner similar to the apothecia of Ranialina, Parmelia, Lecanora, etc., they must be apothecial structures The entire discussion depends upon the points of view. Krabbe, having con sidered the podetia from the morphological and developmental point of view, naturally comes to different conclusions from Reinke, who views the same structures from the physiological standpoint. It is an excellent example illustrating the necessity of combining morphology and physiology. Reinke very aptly states that if we allow morphology and development to dominate our investiga tions we may come to the absurd conclusion that the floral leaves are foliage-leaves and, vice versa, that the foliage-leaves are floral leaves. Extending the illustration we might conclude that mosses are liverworts, and that fungi are algae, etc. Reinke be lieves that the development of an organ should be represented as describing a curve which extends from the beginning to the point of maximum development of that organ. All intermediary stages of development are to be considered, but the special importance is to be attached to the fully developed structure. According to this view thallus, foliage-leaf, thorn, tendril, sepal, petal, stamen, pistil, should be represented as terminating at different points of the curve. With these and similar introductory considerations, the author finally enters upon the discussion of the podetia of Cla donia.
Reinke contents himself with de Bary's definition of the lichen thallus ; de Bary designates it as the vegetative organ bearing the apothecia, spermagonia, and in certain instances also pycnidia. This rather negative definition is accepted by most modern lichen ologists who define the vegetative organ as the structure whose prime function is to take up and assimilate food-substances. Reinke also accepts, or at least expresses his disinclination to change, de Bary's definition of the reproductive organs ; that is, the reproductive organs are the apothecia and spermagonia. If
the experiments and observations of Stahl* and Sturgist prove correct it would be more correct to speak of the corpogone and spermagone as the reproductive organs ; but since it is highly pro bable that the spermagonia are merely parasitic fungi, or since their true nature is as yet problematical, we have no scientific au thority to designate them as reproductive organs. This difference of opinion does not have any important bearing upon the question under consideration. It is, however, evident that Reinke con siders the spermagonia as reproductive organs.
Reinke recognizes two portions of the C/adonia-thallus: the horizontal portion which is also called primary thallus or proto thallus, and the vertical portion, or the podetium of lichenolo gists. The term thallus is quite generally applied to the horizontal or primary thallus, while the term podetium is retained to desig nate the vertical thallus. In all instances the podetium takes its origin from the gonidial (algal) zone of the horizontal thallus. Whether the beginnings of the podetium are due to a sexual act or not, is not definitely known. Krabbe and Reinke are inclined to believe that it is nonsexual. Reinke, however, hints at a pos sible sexual act represented by a fusion of the nuclei of two neigh boring hyphal cells.
The primary axis of the podetium is negatively geotropic, thus differing essentially from the transversely geotropic dorsi ventral horizontal structure of the primary thallus. If the po detia branch, it is found that the negative geotropism decreases with the number of branchings. In cross section the podetium is seen to have a radial structure, hollow, bearing a gonidial zone just within the outermost protective (cortical) covering. The assimi lative surface of the podetium is usually much greater than that of the thallus-lobe, from which it took its origin. Both Krabbe and Schwendener* look upon the two distinctive forms of thallus as a species of alternation of generation. Wainio j believes that the apothecial stalk is converted into a vertical thallus by a form of metamorphosis. A similar view is held by Reinke ; that is, the podetium was originally an apothecial stalk (perhaps comparable to that of the Caliciaceae and of Baeomyces) which finally became metamorphosed into a true thallus. This metamorphosis was of such a nature that the increase in size and assimilative function of the podetium corresponded to a decrease in size and function of the primary thallus.