The relation of the lichen thallus to the environment is obscure. Of all macroscopic plants lichens resist drying-out the most suc cessfully. With respect to the temperature ex tremes which they can endure, they are sur passed only by the bacteria. Many lichens withstand temperatures greater than 65° C.. and nearly all are able to resist the intense cold of Arctic and Alpine winters, with minima of —40° to —60° C. Lichens exhibit very different sensibility to light: the majority of them grow in the fdllest sunlight, while some, Evernia, Usnea, Peltigera, Graphis, etc., are adapted to more or less intense shade. In Alpine regions especially, orange and yellow thalli occur almost exclusively on the under, or shaded, side of rocks. This fact is explained by Zukal's re searches, in which he found that the algal layer was most highly developed under a cortex orange or yellow in color, these colors being most penetrable by the rays active in carbon assimilation. The color of the cortex is also thought to be a protection against excessive illumination, though this explanation can scarcely hold for those lichens in which the lower cortex is highly colored. According to Schwendener, the growth of the thallus is largely intercalary, marginal or apical growth being relatively insignificant. In lichens with yellow-green alga, the growth of the thallus is determined by the fungus, and the development of the algal layer takes place subsequently. In this process, the alga and hypha show a tend ency to aggregate into tubercles, which modify the surface of the thallus. Somewhat similar sculpturings are produced by tensions in the growing thallus, especially by the alternation of wet and dry periods. In nearly all the gelati nous lichens, and particularly those parasitic on filamentous alga, growth is controlled by the alga, and the fungus has little or no influence upon it.
The propagation of lichens occurs ordinarily by means of soredia. It may take place also by fragments of the thallus, whether abstricted naturally or cut off by accident. The propaga tive value of the pycnidium (spermogonium) in nature is unknown. The conidia have been germinated in cultures with difficulty, and at present there is no evidence that they grow more readily under normal conditions. Repro duction is a characteristic feature of the lichens: a few species produce apothecia rarely, while in certain sterile forms they are never de veloped. It is still an open question whether the apothecium is the result of fertilization. Some investigators have thought to demonstrate the presence of a carpogone and trichogyne, and to follow the development of a spore fruit, which results from the contact of spermatium and trichogyne. The germination of many spermatia points strongly to the conclusion that these are mere conidia and not male sexual cells. No fusion of sexual cells has yet been seen in lichens, and, until this is seen, it is impossible to settle the question of their sexual nature. The apothecium arises usually from
certain more or less distinctly twisted hyphae lying between the medulla and the algal layer. Sometimes the point of origin is just beneath the cortex, especially. near the margin, and in crustose forms it is in the hyphal layer just above the substratum. The development is essentially the same as in the other cup-fungi; the hyphal fundament increases in size, and be comes differentiated above into two sorts of threads. The first to grow up are the para physes, in the centre of which push up the club shaped branches, which become the asci. Spore formation in the ascus follows the method typical of all sac-fungi. The escape of the spores takes place through a terminal rift in the ascus or by the breaking up of the latter. The mature spores germinate readily under the proper conditions, usually sending out a single filament from each cell ; large spores, however such as those found in Megalospora and Pertusaria, produce many germinating filaments. The young mycelium is capable of slight de velopment only, unless it comes in contact with the proper alga, when it grows at once into the thallus. The uncertainty that the spores will germinate in a place where the proper host occurs makes multiplication by spores much less sure than by soredia. In a few cases (Endocarpon, Staurothele, etc.), this disad vantage has been overcome by the develop ment of alga in the thecium between the para physes and spore-sacs (hymenial gonidia). These are ejected with the spores, and, clinging to the latter, furnish a certain substratum for the germination of the spores.
Origin and Classification.— Lichens are sac-fungi and rod-fungi which show more or less similarity in their vegetative body because of their parasitism upon certain The clew to their origin and relationship is to be found in the inherited reproductive organ, the sporocarp, and not in the thallus. From this standpoint, the group is highly artificial, con taining representatives of two distinct classes of fungi, the Ascomycetes and the Basidiomy cetes. The basidiolichens are a small group, containing but a few genera: the ascolichens consist of the representatives of several un related families. It is evident that the lichens have not arisen from a single point, as members or offshoots of one line of development, but that they have originated at several widely separated points. They are of multiple origin; that is, they are polyphyletic. In ascolichens, the form of the sporocarp indicates the main places of origin: the Verrucariacear, with perithecia, are Pyrenomycetales; the Graphi daces% which show the hysterothecium, be long to the Hysteriales; the remaining families, Caliciacece, Collemacece, Parmeliacece, etc., be long to the Pezizales.
The following synopsis will indicate the re lationship and limits of the various families of lichens: Class Ascomycetem: fruit a sporocarp, • spores borne in sacs (asci).