Ejection of Spores.—The spores are ejected from the apothecia and perithecia as in the fungi by forcible ejaculation from the asci. In the majority of forms it is clear that the soredia rather than the ascospore must play the more important part in lichen distribution as the development of the ordinary spores is dependent on their finding the proper alga on the substratum on which they happen to fall. In a number of forms (Endocarpon pusillum, Stigmaatonima cataleptum, various species of Staurothele), however, there is a special arrangement by which the spores are, on ejection, associated with gonidia. In these forms gonidia are found in connexion with the young fruit; such algal cells undergo numerous divisions becoming very small in size and penetrating into the hymenium among the asci and paraphyses. When the spores are thrown out some of these hymenial gonidia, as they are called, are carried with them. When the spores germinate the germ-tubes surround the algal cells, which now increase in size and become the normal gonidia of the thallus.
Basidiolichens.
| From Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer. |
| Fig. 17.—Cora pavonia. A, Viewed from above; B, From below; hym, hymenium. (Nat. size.) |
As is clear from the above, nearly all the lichens are produced by the association of an ascomycetous fungus with algae. For some obscure reason the Basidiomycetes do not readily form lichens, so that only a few forms are known in which the fungal element is a member of this family. The two best-known genera are Cora and Dictyonema; Corella, whose hymenium is unknown, is also placed here by Wainio. The so-called Gasterolichens, Trichocoma and Emericella, have been shown to be merely ascomycetous fungi. Clavaria mucida, however, has apparently some claims to be considered as a Basidiolichen, since the base of the fruit body and the thallus from which it arises, according to Coker, always shows a mixture of hyphae and algae.
The best-known species is Cora pavonia, which is found in tropical regions growing on the bare earth and on trees; the gonidia belong to the genus Chroococcus while the fungus belongs, apparently, to the Thelephoreae (see [Fungi]). This lichen seems unique in the fact that the fungal element is also found growing and fruiting entirely devoid of algae, while in the ascolichens the fungus portion seems to have become so specialized to its symbiotic mode of life that it is never found growing independently.
The genus Dictyonema has gonidia belonging to the blue-green alga, Scytonema. 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. 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. On a bare rocky surface a fungus would die from want of organic substance and an alga from drought and want of mineral substances. The lichen, however, is able to grow as the alga supplies organic food material and the fungus has developed a battery of acids (see below) which enable it actually to dissolve the most resistant rocks. It is owing to the power of disintegrating by both mechanical and chemical means the rocks on which they are growing that lichens play such an important part in soil-production. The resistance of lichens is extraordinary; they may be cooled to very low temperatures and heated to high temperatures without being killed. 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. Though so little sensitive to drought and extremes of temperature lichens appear to be very easily affected by the presence in the air of noxious substances such as are found in large cities or manufacturing towns. In such districts lichen vegetation is entirely or almost entirely absent. 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 relation of the two constituents of the lichen have been briefly stated in the beginning of this article. 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 fungus seems, on the other hand, to stimulate the algal cells to special development, for those in the lichen are larger than those in the free state, but this is not necessarily adverse to the idea of parasitism, for it is well known that an increase in the size of the cells of the host is often the result of the attacks of parasitic fungi. It must be borne in mind that the exact nutritive relations of the two constituents of the lichen have not been completely elucidated, and that it is very difficult to draw the line between symbiosis and parasitism. The lichen algae are not alone in their specialization to the symbiotic (or parasitic) mode of life, for, as stated earlier, the fungus appear in the majority of cases to have completely lost the power of independent development since with very rare exceptions they are not found alone. They also differ very markedly from free living fungi in their chemical reactions.