CHAPTER IV.

LICHENES.

General Characteristics-Development of Mutualism or Commensalism -The Schwendenerian View of the Nature of Lichens-Views of Opponents of Schwendener's Ideas-Probable Origin of Mutualism -Do Lichens constitute a Terminal Group ?—Fossil Lichens.

UNTIL recently the group of Thallogens or Cellular Plants was considered to consist of three typical sections, Alga, Fungi, and Lichenes. The two former still retain their individuality, but it has been conclusively shown that the lichens are compound organisms; in other words, every lichen consists of two elements, a fungus and an alga. It has been already stated that numerous species of fungi are parasitic on various phanerogamic plants, but in most such instances the fungus acts as a true parasite, the benefit being entirely one-sided, all in favour of the parasite and to the decided disadvantage of the host, but even in this connection there are differences of degree. In extreme cases the host is killed by the parasite, whereas in others the host appears to suffer but little inconvenience from the presence of the fungus, due to a partial readjustment of the mode of life of parasite and host, and mutual adaptation to the new conditions of existence. This condition of things has in lichens attained to the point where the two

plants-alga and lichen-mutually benefit by their close mechanical combination, and appear to form a single individual; but in reality the two components are not organically connected. The hyphæ of the fungus clasp the algal portion of the fungus, but do not penetrate the cells, so that an interchange of food materials is enabled to pass from one to the other. The algal portion of the lichen, possessing chlorophyll, assimilates carbonic dioxide and forms organic carbon compounds, while the mycelium of the fungal portion absorbs the required mineral substances in solution from the substratum. This perfect balance and division of labour between two originally distinct plants to form a third possessing marked peculiarities of its own, with a corresponding loss of the individuality of its components, suggests to the mind the analogy with the formation of a chemical compound, and probably the same law is a prominent factor in both instances. This condition of things is expressed by the various terms, commensalism, mutualism, symbiosis, etc. Schwendener was the first to point out the true nature of lichens, hence his statements are spoken of by opponents as the "Schwendenerian theory," although the term theory is no longer applicable, as his statements have been repeatedly corroborated experimentally. Bornet followed by showing that in numerous instances the so-called gonidia of lichens-the algal portion-could with certainty be referred to known species of algæ, and also succeeded in producing a lichen synthetically, by sowing the spores of the lichen called Parmelia parietina with Protococcus. Quite recently Bonnier has published the results of prolonged investigations on the nature of lichens, and furnishes a long list of species produced artificially, by sowing the spores of lichens with species of

algæ corresponding to those met with in the same species of lichens growing naturally. The fruit of lichens is always formed entirely by the fungal element, which in the great majority of species belongs to the Ascomycetes. The opponents of Schwendener's views include some of the leading lichenologists of the present day, who have devoted

Fig. 25.-Transverse section of the thallus of a lichen (Sticta fuliginosa). 00, cortex of the upper surface; u, under surface; mm, interlaced hyphæ forming the medullary layer; gg, gonidia; rr, root-like outgrowths or rhizoids of the under surface (x 500). (From Prantl, after Sachs.)

themselves more especially to the systematic side of the study, and who never suspected otherwise than that lichens were autonomous, and this belief has grown into a con

viction which they consider so self-evident as to require no demonstration.

The lichens may be considered as a terminal group; yet, although at present the complete mutualism necessary to form a lichen exists only between certain algæ and fungi, what has been said respecting the relative degrees of parasitism between fungi and phanerogams, it is not improbable that eventually mutualism may be attained in some instances in this direction also; and it has also been shown that the spores of fungi germinate readily and form the incipient stage of a lichen on the protonema of mosses, but owing to lack of mutualism the perfect lichen is not evolved; in other words, the fungal element is as yet parasitic on the protonema. In all probability the mutualism of the components of lichens originated as fungi parasitic on algæ, examples of which are not wanting at the present day.

Fossil Lichens.-The lichens as a group were probably differentiated at a much later geological period than the fungi. Their fossil remains are first met with in Tertiary formations, and all the species belong to genera that exist at the present day, as Parmelia, Lecidea, Cladonia, Ramalina, etc.