THE SYSTEMATIC POSITION OF LICHENS.
This paper is a critical review of lichenology since the time of Schwendener's epoch-making investigations.t The author credits De Bary with first having indicated the dual nature of lichens by demonstrating that species of Nostoc and Chroococcus may be con verted into gelatinous lichens upon becoming penetrated with the hyphae of certain parasitic Ascomycetes. It must be remembered that as late as 1863 Schwendener still believed that the gonidia (algae) were developed upon lateral and terminal branches of the hyphael: De Bary made his discovery in 1865. In 1868§ Schwendener accepted this view and added that all lichens were the result of the union of an alga with some fungus. As the re sult of his investigations Schwendener concludes that lichens should no longer be considered as a distinct class, but as fungi parasitic upon algae (gonidia). He states : In development the vegetative organs and the organs of reproduction of lichens are in all respects similar to those of Ascomycctcs." Reinke has always agreed with Schwendener in the belief that the gonidia of lichens were true algae and the hyphae true fungi. Their opinions differed, however, as to the relationship of the two organisms. Reinke was not at all willing to look upon it as a form of ordinary parasitism. During the summer of 1872, in verbal communication with Griese bach, Reinke pointed out that the relationship indicated a mu tual benefit. While the alga supplied the fungus with assimilated food-substances, the latter supplied the alga with water, nitrogen, phosphorus, sulphur and mineral salts. The relation of the fungus and alga in the lichen was comparable to the relation of the root and the leaves of a tree. In order to distinguish this form of re lationship (symbiosis) from parasitism (antagonistic symbiosis) it was necessary to introduce a new term. Reinke and Griscbach finally hit upon the word " consortism " as being especially appro priate. Reinke gives conclusive evidence that he has not been duly credited with first having pointed out the true conditions met with in the lichen thallus. This credit is given to de Bary, who is generally supposed to have been the first to point out and explain the phenomenon of " symbiosis "* as it occurs in lichens. The author had, however, previously explained this condition in three different publications. From these it is evident that De Bary was not the discoverer of the phenomenon now known as mutualistic symbiosis, and that the term consortism antedates that of sym biosis. Reinke explains his views with regard to the lichen-thallus more fully as follows : " We have, therefore, in the thallus of lichens, a consortism, the components of which form a unit, a mor phological individual, somewhat as the different tissues in a higher plant unite to form the individual. The fact that the alga can ex ist independently is dependent upon its ability to assimulate car bon. In the state of consortism, at least in the heteromerous thallus, the alga is nourished by the enclosing fungus ; that is it receives from the hyphae the necessary minerals, nitrogen, oxygen, hydrogen and water. The alga in return supplies the fungus with the essential carbon compounds. From this it is evident that the components (alga and fungus) are biologically associated, mutu ally dependent upon each other, for the formation of the organic substances required for the upbuilding of the common body." The above is certainly conclusive evidence that Reinke had a cor rect view of the mutualistic relationship of alga and fungus in the lichen-thallus and furthermore that he was convinced that the lichen formed an autonomous structure.
The author expresses it as his opinion that the fungi of the higher Ascolichenes no longer exist independently, perhaps never so existed. The alga is, however, still able to lead an independent
existence. It is also evident that there are free algae closely re lated, if not identical with the gonidia of lichens. This fact is of great importance in the consideration of the phylogenetic devel opment of lichens. It is practically impossible to determine what free fungi are identical with the fungi of lichens. All investigators in this line have met with very unsatisfactory results, a thing natur ally to be expected. According to Tavel* the fungi of the Colle maceae, Arthoniae and Lecideaceae are more or less closely re lated to the Patellariaceae. The relationship of the fungi of the Pertusariae, Lecanareae, Pannarieae, Umbilicarieae, Peltidiaceae, Parmeliaceae, Usneaceae, Cladoniaceae, Sphaerophoreae, Ephebeae and Lichineae is practically unknown. The Verrucarieae, De campieae and Endocarpieae are related to the Amphisphaeriae, Sphaerelloideae. Such uncertainty is certainly very unsatisfactory Although there may be algae, such as humkola, Plenrococcus vulgaris, Nostoc lickenoia'es, etc., which only await the opportunity to unite with some fungus to form a lichen, yet it is evident that no true Ascomycete has the power to enter into such a union. There is great uncertainty as to the exact method by which the first lichen or lichens were formed. The author makes the following hypothetical assumption : " In the beginning several lichens were formed by the union of true fungi with algae. Ac cording to 111611ert such a process is now going on in the case of Cora pavonia and the related forms of Diayonema and Lauda/ea. The gelatinous lichens are very likely the oldest forms of the Ascolichenes. Such a gelatinous lichen took its origin as the re sult of the parasitic union of a fungus and a spherical colony of Nostoc lichenonies: The question of the origin of the fungus coin cides with the question of the phylogenetic origin of parasitic fungi in general and need not be further discussed. The fungus which at first behaved like a true parasite (antagonistic symbiosis) took its entire food-supply from the nostoc. The condition of mutualism (consartism) was a phylogenetic product ; perhaps due to natural selection or other formative causes resulting from the union of alga and fungus. From this proto-Collema other gelatinous lichens were evolved ; finally also such with hetero merous thallus. It is probable that in the course of phylogenetic processes the developing spores of gelatinous lichens acquired the ability to enter into a mutually symbiotic union with other algae. A series phylogenetically derived from Coacma may have taken an upward course, that is, proceeded from the lower to the higher, from the simpler to the more complex. As an example we may cite the natural series CoIlema, Leptogirtm, itydrothyria, PcItigera. If Stictina is derived from Peltigera, Sticta, which is evidently Stictina with bright green algae, would also be included in the series. Every phylogenetically derived lichen-type constituted the beginning of a new series which may have proceeded upward or downward; that is in the direction of either higher or lower forms. In certain cases it is difficult to decide whether given lichens form the beginnings of a series or whether they are degenerate forms. Among such doubtful forms are included Biatora uhkinosa, Thelidium minzt iulum, and species of Biteilia and Arthonia. Many of these plants live parasitically upon other lichens, and no doubt bear a relation to these similar to the relation of Cucuta and Monotropa to the chlorophyll-bearing genera of the same family. The above sum mary leads to the conclusion that there is a natural system of lichens distinct from that of fungi.