Though lichens are composed of two actively growing organisms, the symbiotic plant increases very slowly. The absorption of water and mineral salts must in many instances be of the scantiest and the formation of carbohydrates by the deep-seated chlorophyll cells of correspondingly small amount. Active aeration seems therefore uncalled for though by no means excluded, and there are many indirect channels by which air can penetrate to the deeper tissues.
In crustaceous forms, whether corticate or not, the thallus is often deeply seamed and cracked into areolae, and thus is easily pervious to water and air. The growing edges and growing points are also everywhere more or less loose and open to the atmosphere. In the larger foliose and fruticose lichens, the soredia that burst an opening in the thallus, and the cracks that are so frequent a feature of the upper cortex, all permit of gaseous interchange. The apical growing point of fruticose lichens is thin and porous, and in many of them the ribs and veins of their channelled surfaces entail a straining of the cortical tissue that results in the formation of thinner permeable areas. Zukal[449] devoted special attention to the question of aeration, and he finds evidence of air-passages through empty spermogonia and through the small round holes that are constant in the upper surface of certain foliose species. He claims also to have proved a system of air-canals right through the thallus of the gelatinous Collemaceae. Though his proof in this instance is somewhat unconvincing, he establishes the abundant presence of air in the massively developed hypothecium of Collema fruits. He found that the carpogonial complex of hyphae was always well supplied with air, and that caused him to view with favour the suggestion that the function of the trichogyne is to provide an air-passage. In foliose lichens, the under surface is frequently non-corticate, in whole or in part; or the cortex becomes seamed and scarred with increasing expansion, the growth in the lower layers failing to keep pace with that of the overlying tissues, as in Umbilicaria pustulata.
It is unquestionable that the interior of the thallus of most lichens contains abundant empty spaces between the loose-lying hyphae, and that these spaces are filled with air.
2. CEPHALODIA
A. Historical and Descriptive
The term “cephalodium” was first used by Acharius[450] to designate certain globose apothecia (pycnidia). At a later date he applied it to the peculiar outgrowths that grow on the thallus of Peltigera aphthosa, already described by earlier writers, along with other similar structures, as “corpuscula,” “maculae,” etc. The term is now restricted to those purely vegetative gall-like growths which are in organic connection with the thallus of the lichen, but which contain one or more algae of a different type from the one present in the gonidial zone. They are mostly rather small structures, and they take various forms according to the lichen species on which they occur. They are only found on thalli in which the gonidia are bright-green algae (Chlorophyceae) and, with a few exceptions, they contain only blue-green (Myxophyceae). Cephalodia with bright-green algae were found by Hue[451] on two Parmeliae from Chili, in addition to the usual blue-green forms; the one contained Urococcus, the other Gloeocystis. Several with both types of algae were detected also by Hue[451] within the thallus of Aspicilia spp.
Flörke[452] in his account of German lichens described the cephalodia that grow on the podetia of Stereocaulon as fungoid bodies, “corpuscula fungosa.” Wallroth[453], who had made a special study of lichen gonidia, finally established that the distinguishing feature of the cephalodia was their gonidia which differed in colour from those of the normal gonidial zone. He considered that the outgrowths were a result of changes that had arisen in the epidermal tissues of the lichens, and, to avoid using a name of mixed import such as “cephalodia,” he proposed a new designation, calling them “phymata” or warts.
Further descriptions of cephalodia were given by Th. M. Fries[454] in his Monograph of Stereocaulon and Pilophorus; but the greatest advance in the exact knowledge of these bodies is due to Forssell[455] who made a comprehensive examination of the various types, examples of which occurred, he found, in connection with about 100 different lichens. Though fairly constant for the different species, they are not universally so, and are sometimes very rare even when present, and then difficult to find. A striking instance of variability in their occurrence is recorded for Ricasolia amplissima (Lobaria laciniata) ([Fig. 76]). The cephalodia of that species are prominent upright branching structures which grow in crowded tufts irregularly scattered over the surface. They are an unfailing and conspicuous specific character of the lichens in Europe, but are entirely wanting in North American specimens.