Fig. 138.—Lecanora subfusca: a the bark on which it is situated; l the thallus; s the ascocarp; s’ an ascocarp.
Fig. 139.—Graphis (two species).
Fig. 140.—Pertusaria communis.
The Lichens, like other Ascomycetes, have very variously constructed ascospores (Fig. [137]), which are enclosed in asci (Fig. [136]), usually surrounded by paraphyses attached together. Furthermore they possess pycnidia (Fig. [141]) containing numerous microconidia. These were formerly considered as organs of fructification, and were termed “spermatia,” and the pycnidia, “spermogonia.” Alfr. Möller proved, in 1887, that the microconidia are able to germinate and produce a mycelium with new conidia, just as in other Ascomycetes.
Vegetative Reproduction takes place by soredia, which to the naked eye appear as whitish powder on the surface of the thallus. They are small round bodies, formed by one or a group of gonidia, which are surrounded by a mass of felted hyphæ. After the rupture of the cortex they are set free, and readily carried by the wind to other places, where under favourable circumstances they establish a new thallus.
Fig. 141.—A A portion of the thallus of Parmelia parietina with ascocarps (a) and pycnidia (b). B A portion of the thallus of Cetraria islandica with pycnidia at the end of small lobes. C A lobe with pycnidia and ejected microconidia. (Magnified).
Geographical Distribution. The Lichens are the most hardy plants, and are the first to appear on hitherto bare rocks which they gradually disintegrate, and hence prepare the way for the growth of other plants. They are to be found from the Polar regions to the Equator; from the highest snow-free mountain-peaks down to the level of the sea; on the stems of trees; on rocks, soil, some even on inundated places; on stones in woodland streams, and on beaches; but they are never found upon rotten organic remains. Some grow gregariously in enormous masses, and form wide-stretching carpets, e.g. Reindeer Moss (Cladonia rangiferina), species of Cetraria and other fruticose Lichens.