James Morrison Crombie and Vernon Herbert Blackman

“ The soredia are the most successful method of reproduction in lichens, for not only are some forms nearly always without spore-formation and in others the spores largely abortive, but in all cases the spore represents only the fungal component of the thallus, and its success in the development of a new lichen-thallus depends on the chance meeting, at the time of germination, with the appropriate algal component. ”

“ In arctic regions lichens form by far the largest portion of the vegetation, occurring everywhere on the ground and on rocks, and fruiting freely; while terrestrial species of Cladonia and Stereocaulon are seen in the greatest luxuriance and abundance spreading over extensive tracts almost to the entire exclusion of other vegetation. The lichen flora of temperate regions again is essentially distinguished from the preceding by the frequency of corticolous species belonging to Lecanora, Lecidea and Graphidei. ”

“ The dual nature of the lichen thallus introduces at the outset a classificatory difficulty. Theoretically the lichens may be classified on the basis of their algal constituent, on the basis of their fungal constituent, or they may be classified as if they were homogeneous organisms. The first of these systems is impracticable owing to the absence of algal reproductive organs and the similarity of the algal cells (gonidia) in a large number of different forms. The second system is the most obvious one, since the fungus is the dominant partner and produces reproductive organs. ”

“ Thus in the case of saxicolous lichens the mineralogical character of the rock has of itself little or no influence upon lichen growth, which is influenced more especially and directly by their physical properties, such as their capacity for retaining heat and moisture. As a rule lichens grow commonly in open exposed habitats, though some are found only or chiefly in shady situations ”

“ The relation of the fungus to the alga, though it may be described in general terms as one of symbiosis, partakes also somewhat of the nature of parasitism. The algal cells are usually controlled in their growth by the hyphae and are prevented from forming zoospores, and in some cases, as already described, the algal cells are killed sooner or later by the fungus. ”

“ The majority of lichens are confined to one particular kind of gonidium (i.e. species of alga) but a few forms are known (Lecanora granatina, Solorina crocea) which make use of more than one kind in their development. In the case of Solorina, for example, the principal alga is a green alga, one of the Palmellaceae, but Nostoc (a blue-green alga) is also found playing a subsidiary part as gonidia. In L. granatina the primary alga is Pleurococcus, the secondary, Gleococapsa. ”

“ The growth of lichens is extremely slow and many of them take years before they arrive at a spore-bearing stage. Xanthoria parietina has been known to grow for forty-five years before bearing apothecia. This slowness of growth is associated with great length of life and it is probable that individuals found growing on hard mountain rocks or on the trunks of aged trees are many hundreds of years old. It is possible that specimens of such long-lived species as Lecidea geographica actually outrival in longevity the oldest trees.
Relation of Fungus and Alga. ”

“ The question is one very difficult to settle owing to the fact that the majority of spermatia appear to be functionless. In favour of the conidial view is the fact that in the case of Collema and a few other forms the spermatia have been made to germinate in artificial cultures, and in the case of Calicium parietinum Möfler succeeded in producing a spermogonia bearing thallus from a spermatium. For the germination of the spermatia in nature there is only the observation of Hedlund, that in Catillaria denigrata and C. prasena a thallus may be derived from the spermatia under natural conditions. ”

“ Lecanora subfusca, Cladonia pyxidata) ; and others, not strictly cosmopolitan, have been observed in regions widely apart. A considerable number of species, European and exotic, seem to be endemic, but further research will no doubt show that most of them occur in other climatic regions similar to those in which they have hitherto alone been detected. To give any detailed account, however, of the distribution of the different genera (not to speak of that of individual species) of lichens would necessarily far exceed available limits. ”

“ When the fungus predominates in the thallus it has a bracket-like mode of growth and is found projecting from the branches of trees with the hymenium on the under side. When the alga is predominant it forms felted patches on the bark of trees, the Laudatea form. It is said that the fungus of Cora pavonia and of Dictyonema is identical, the difference being in the nature of the alga.
Mode of Life.
Lichens are found growing in various situations such as bare earth, the bark of trees, dead wood, the surface of stones and rocks, where they have little competition to fear from ordinary plants. ”

“ A large number of these acids, which are mostly benzene derivatives, have been isolated and more or less closely investigated. They are characterized by their insolubility or very slight solubility in water; as examples may be mentioned erythrinic acid in Roccella and Lecanora; evernic acid in species of Evernia, Ramalina and Cladonia; lecanoric acid in Lecanora, Gyrophora. The so-called chrysophanic acid found in Xanthoria (Physcia) parietina is not an acid but a quinone and is better termed physcion. ”

“ The most satisfactory view in the present state of our knowledge seems to be that the spermatia are male cells which, while retaining their fertilizing action in a few cases are now mainly functionless. ”

“ The cortical layer is usually more developed on the side towards the light, while in many lichens this is the only side provided with a cortical layer. The podetia of some species of Cladonia possess no cortical layer at all. ”

“ As is well known, the lichens are often found in the most exposed and arid situations; in the extreme polar regions these plants are practically the only vegetable forms of life. They owe their capacity to live under the most inhospitable conditions to the dual nature of the organism, and to their capacity to withstand extremes of heat, cold and drought without destruction. ”

“ Medicinal Lichens.—During the middle ages, and even in some quarters to a much later period, lichens were extensively used in medicine in various European countries. Many species had a great repute as demulcents, febrifuges, astringents, tonics, purgatives and anthelmintics. The chief of those employed for one or other, and in some cases for several, of these purposes were Cladonia pyxidata, Usnea barbata, Ramalina farinacea, Evernia prunastri, Cetraria ìslandica, Sticla pulmonaria, Parmelia saxatilis, Xanthoria parietina and Pertusaria amara. ”

“ If the cortical layer should exhibit positive reaction and the medulla of the same species a negative reaction with both reagents, the result is represented thus, K±CaCl±. If a reaction is only produced after the consecutive addition of the two reagents, this is symbolized by K (CaCl) +. A solution of iodine is also used as a test owing to the blue or wine-red colour which the thallus, hymenium or spores may give with this reagent. ”

“ Many species also prefer growing in moist places by streams, lakes and the sea, though very few are normally and probably none entirely, aquatic, being always at certain seasons exposed for a longer or shorter period to the atmosphere (e.g. Lichina, Leptogium rivulare, Endocarpon fluviatile, Verrucaria maura) . ”

“ The female sexual organs, the ascogonia, would thus in the majority of cases develop by the aid of some reduced sexual process or the ascocarps be developed without relation to sexual organs. A further argument in support of this view is that it is in complete agreement with what we know of the sexuality of the ordinary, free-living ascomycetes, where we find both normal and reduced forms (see Fungi) .
Fruit Bodies.—We find two chief types of fruit bodies in the lichens, the perithecium and apothecium ”

“ They may be dried so thoroughly that they can easily be reduced to powder yet their vitality is not destroyed but only suspended; on being supplied with water they absorb it rapidly by their general surface and renew their activity. The life of many lichens thus consists of alternating periods of activity when moisture is plentiful, and completely suspended animation under conditions of dryness. ”

“ Collemaceae) ; in these the algae are Chroococcaceae and Nostocaceae, and the fungus makes its way into the gelatinous membranes of the algal cells and ramifies there (fig. 3) . We can distinguish this class of forms as lichens with a homoiomerous thallus, i.e. one in which the alga and fungus are equally distributed. The majority of the lichens, however, possess a stratified thallus in which the gonidia are found as a definite layer or layers embedded in a pseudo-parenchymatous mass of fungal hyphae, i.e. they are heteromerous (figs. 8 and 9) . ”

“ There was thus very strong circumstantial evidence in favour of fertilization, although the male nucleus was not traced. The further work of Baur, and that of Darbishire, Funfstuck and Lindau, have shown that in a number of other cases trichogynes are present. Thus ascogonia with trichogynes have been observed in Endocarpon, Collema, Pertusaria, Lecanora, Gyrophora, Parmelia, Ramalina, Physcia, Anaptychia and Cladonia. ”

“ Cladonia, Lecidea decolorans) , others calcareous soil (e.g. Lecanora crassa, Lecidea decipiens) , others sandy soil or hardened mud (e.g. Collema limosum, Peltidea venosa) ; while many may be found growing on all kinds of soil, from the sands of the sea-shore to the granitic detritus of lofty mountains, with the exception of course of cultivated ground, there being no agrarian lichens. (d) Muscicolous lichens again are such as are most frequently met with on decayed mosses and Jungermannia, whether on the ground, trees or rocks (e.g. Leptogium muscicola, Gomphillus calicioides) . ”

“ Apothecia seldom round, usually elongated-ellipsoidal, no capillitium.
Arthoniaceae, Graphidiaceae, Roccellaceae. Series III. Cyclocarpineae, Apothecium usually circular, no capillitium.
A. Spores usually two-celled, either with a strongly thickened cross-wall often perforated by a narrow canal or with cross-wall only slightly thickened. In the first case the spores are usually colourless, the second case always brown. Buelliaceae, Physciaceae. B. Spores unicellular, parallel-multicellular or muriform, usually colourless, cross-walls usually thin. ”

“ Laudatea) , Corella (doubtfully placed here as the hymenium is unknown) .
Habitats and Distribution of Lichens.
1. Habitats.—These are extremely varied, and comprise a great number of very different substrata. Chiefly, however, they are the bark of trees, rocks, the ground, mosses and, rarely, perennial leaves. (a) With respect to corticolous lichens, some prefer the rugged bark of old trees (e.g. Ramalina, Parmelia, Stictei) and others the smooth bark of young trees and shrubs (e.g. Graphidei and some Lecideae) . Many are found principally in large forests (e.g. Usnea, Alectoria jubata) ”