LICHENS are, with few exceptions, land plants of simple structure. They grow almost everywhere, spreading over soil, rocks, the trunks, branches and leaves of trees, etc., as flat crusts, leafy expansions, shrub-like tufts or pendulous filaments in va rious colour shades of white, grey, yellow, brown or almost black. The term lichen, a word of Greek origin, was first definitely given to lichens as we know them by Tournefort (1700).
Lichens are of unusual interest in that the vegetative body or thallus is a composite plant formed by the interdependent growth of unicellular or filamentous green or blue green algae Myxophyceae or Chlorophya ceae (fig. I), with the filaments (hyphae) of one of the higher fungi—Ascomycetes or, in one or two genera only, Basidiomy cetes. On this basis of combination or sym biosis there has been evolved a great series of distinctive plants, capable of vigorous life and of reproduction from generation to generation. Phycolichens signify those that contain blue-green, Archilichens those witl_ bright-green algae, designated as lichen gonidia. The fungus is the dominant part ner as it provides the fruiting bodies.
For long it was ac cepted that the green bodies in the lichen plant were cells budded off from the col ourless hyphae that gradually acquired a green colour. It was known that minute portions of a lichen plant—the soredia—each composed of a few green cells with en tangled colourless filaments were agents of propagation. Wallroth (1835), for that reason, coined for the green cells the term gonidia to signify their reproductive function (fig. 2). In most lichens there is a gonidial zone near the surface and to that he gave the name stratum gonimon. In a lesser number the gonidia are dis tributed through the thallus (fig. 3). These two types he dis tinguished as heteromerous with distinctive layers, and homoimer OUS where there is no such diversity.
The belief in the genetic origin of the green cells within the thallus held sway for many years, though observations of a disturb ing character were not lacking. Agardh (1821) had suggested that they were transformed algae as he had followed the develop ment of the blue-green alga, Nostoc, to the complete thallus of the lichen, Collema. The view gradually gained ground that the bright-green gonidia of many lichens were comparable to the alga Protococcus. The explanation given was that these free grow ing algae were lichen gonidia es caped from the thallus that had continued independent growth.
Wallroth spoke of them as "un fortunate brood cells" that could not again form a lichen plant.
Finally in 1867 Schwendener pub lished his bold theory that lichen gonidia were true algae impris oned and parasitized by fungal.
hyphae. The statement was wel comed by many as enlightening and convincing. Others, among whom were the renowned Finnish lichenologist, W. Nylander and the British J. M. Crombie, scorn fully rejected the new view. The theory was, however, successfully tested by cultures of lichen spores with free-growing algae—first by Rees (1871), then by Bornet (1872) and others who followed the development from spore to fruiting stage, a slow growth of several years' duration.
The relation between the two organisms was re garded at first as a parasitism of the fungus on the alga, or as helotism. Reinke (1873) pointed out the insufficiency of a con dition of parasitism to explain the healthy lichen, and he there fore proposed the term consortium as a truer conception. A few years later de Bary (1873) suggested symbiosis as an adequate term and it is now generally accepted as a mutual symbiosis. This view has been confirmed by culture experiments. In general the alga supplies carbohydrates by photosynthesis, the fungus provides salts and water storage. Symbiosis in lichens is a fairly stable life-balance which may tip, however, to the detriment of one or other of the organisms : there are instances, perhaps more fre quent than we have supposed, of gonidia perishing in the grip of the fungi, but there are also cases where owing to some unfavour able condition, the fungus has succumbed while the algae increased enormously. There is no doubt as to the normal healthy condition of the thallus and of both symbionts. The interest in lichen go nidia has of late centred in the globose bright-green alga for many years considered to be a species of Protococcus, but that alga multiplies by cell division and is now recognized as the gonid ium of only a few lichens. The ordinary lichen gonidium was found by Paulson and Somerville Hastings (192o) to have a mas sive parietal chromatophore, and to multiply freely and abun dantly in the thallus by the free cell formation of aplanospores. The season of greatest increase was from February to April, or after heavy rain following a season of drought ; zoospores were not seen in the gonidial state. The sporulating gonidia were most abundant in the actively growing regions. More recently Puymaly (1924) has proposed a new genus, Trebouxia, for the alga without and within the lichen thallus. He describes it, however, as possess ing a massive stellate chromatophore. In view of Paulson's ob servations, again renewed, it is impossible to regard the gonidium chromatophore as of stellate form.