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LICHENS 557

account of the investigations of these authors, and the arguments by which they endeavour to support the hypothesis, would exceed the limits of the present article, even were all other matters relating to lichens to be excluded. Suffice it to state briefly that, according to Schwendenerismn, a lichen is not an individual plant, but rather a community made up of two different kinds of individuals belonging to two distinct classes of cryptogams, viz., a master-fungus and colonies of algal slaves, which it has sought out, caught hold of, and retains in perpetual captivity in order to provide it with nourish ment. To such a singular theory, which from its plausibility has met with considerable support in certain quarters, various a priori objections of great validity may be taken. Amongst others which have been adduced two may especially be noticed, having reference to the nature of this alleged parasitism, and the situations in which lichens are found. (1) The parasitism described is of a kind un known in the vegetable kingdom, inasmuch as the host (the Algæ), instead of suffering any injury, only flourishes the more vigorously. Moreover, the algal slaves being entirely enclosed in the master- fungus, can evidently supply no nourishment to it whatever, while direct observation shows that it is through the surface of the thallus of the lichen that nourishment is conveyed to the gonidial stratum, where the active life chiefly has its seat. (2) As is well known, lichens shun such habitats as are most frequented by algæ and fungi, and occur in situations where neither of these are seen. Where then are the algal colonies which, according to the hypothesis, the fungus goes forth in quest of, and "presses into its service"? Either of these arguments is sufficient to throw more than doubt upon Schwendenerism. At the same time, as will be perceived, the origin of the gonidia in the thallus remains to be accounted for, in order that the hypothesis may be utterly subverted. It might at first sight be supposed that this was a matter of no very great diffi culty, since by cultivating lichens from the spores it would readily become apparent. All attempts, however, in this direction (and many such have recently been made) have, owing to the peculiar character and conditions of lichen-growth, confessedly proved futile, and the experiments instituted have been productive of no definite results. This is not altogether to be regretted, since in nature itself, when lichens occur on certain substrata, we have in various instances the whole process of the evolution of their vegetative system placed before our eyes, from the first germinations of the spores to the for mation of the per- feet thallus. It is to Nylander, whose services here as elsewhere in liche- nological science – structural, physio logical, and syste matic – are so valu able, that we owe the first clear en unciation as to the origin of the gonidia in the lichen-thallus. This, though pre viously indicated by him, as we shall presently see, was at greater length and still more de finitely stated in several important papers in the Flora, viz., "De gonidiis et eorum formis animadversiones" (1877, No. 23), "Cirea lichenes vitricolas notula" (1879, No. 19), and "De hypothallo notula" (1879, No. 36). In these he incontrovertibly demonstrates that the gonidia originate, not from the filaments themselves, but in the cellules of the first cortical glonuTules which are produced upon the young hypothallus. This may very easily be studied in the earlier stages of development of crustaceous lichens growing on pure quartz rocks (e.g., Lecidea geographica), on the smooth bark of young trees (e.g., Lecanora subfusca), and more especially on the surface of old glass (e.g., Lecanora galactina and Lecidea alboatra). In the case of the young thalli of Lecanora cinerea this was sufficiently well de lineated by the celebrated Tulasne in 1852, the year subsequent to the promulgation of the Bayrhofferian theory, in his elaborate "Mémoire sur les Lichens," pl. iii. f. 3. Afterwards, as the evolu-

tion of the cortical stratum advances, its lower portion is resorbed, and the gonidia there become free, giving rise to the gonidial stratum, to the hyphæ in which they are not adnate, as has been represented, but only adherent by means of the gelatin which pene trates all the elements of the lichen. Often also growing gonidia, young and adult, may be observed in the pseudo-parenchymatous cortical cellules in lichens which in this respect are best adapted for examination, such as Umbilicaria (vide Nylander in Flora, 1875, p. 303), Physcia pulverulenta, Psoroma hypnorum, &c. Similarly, with respect to the origin of the gonimia, Nylander observes (Flora, 1868, p. 353) that the isidia in the Collemacci (more especially in Collema) "show very clearly under the microscope the entire his tory of the evolution of the thallus from its first origin from a cellule containing a single gonimium to a minute true nostoe, and ultimately to the perfect texture of a Collema." With these facts before us, to which various others might be added, such as the entire absence of any algals on pure substrata, where lichen growth presents itself in all its stages, we are compelled to come to the con clusion that the gonidia constitute a true organic system in lichens, and in nature are nowhere seen outside the thallus. Consequently those free algal forms, such as Protococcus, &c., regarded as the free gonidia of lichens, are true algæ, there being indeed a parallel ism between them but no identity. On the other hand, as to the alleged identity of the lichen-hypha with a fungus-mycelium, it is to be observed that the two are totally different in their nature. The hyphæ of lichens (myelohyphæ of Nylander) are rigid, elastic, containing lichenine, not becoming putrid by maceration, with no faculty of penetrating or involving, while the hyphse of fungi are caducous, soft, flexile, with thin walls, &c. Hence, as there is no algal in the lichen, so neither is there any fungus, though there is a parallelism between the fructification of lichens and the asciferous section of fungi (Crombie in Pop. Sc. Rev., 1874).

Fig. 6. – Hypothalline condition of Lecanora cinerea, showing the origin of the first Cortical Gonidiogenous Cellules. (After Tulasne.)

Nutrition and Life of Lichens.

As already intimated, lichens derive their nourishment directly from the atmosphere, in the shape of rain (or dew) with the materials contained in it. Here, as elsewhere, water is the condition of life, and through its medium is conveyed to them the nutrient substances requisite for their existence and growth, from the clouds, from rivers, and, in the case of maritime species, even from the sea. Where, however, the atmosphere is impregnated with smoke, soot, or other deleterious ingredients, lichens will not grow nor flourish. Hence in our larger cities, or even in smaller manufacturing towns, scarcely any lichen vegetation, or none whatever, is seen. Even in their more immediate suburban districts they occur only in a gonidial or rudimentary state, constituting the pseudo-genus Lepraria of the older botanists, and increasing through long periods by bisection, but never developing into perfect plants. Indeed it is now a well-known fact that their fully developed condition is a sure indication of the purity of the air and the salubrity of the districts in which they occur. It has sometimes been stated that they draw some portion at least of their nutriment from the substratum to which they are affixed. For this, however, their structure is by no means well adapted, and such inorganic substances as iron and lime, which enter into their composition, are only as if mechanically derived in solution from the substratum. This in very many instances, e.g., bare quartzose rocks, dead sapless wood, and pure naked glass, can evidently supply no nutriment whatever. Moreover, in the case of crustaceous species, such as Lecanora tartarea, &c., and also of terricole fruticulose species, such as Cetraria islandica, &., the portion of the thallus next to the substratum is dead, so that no nutrient substances can be conveyed through it to the upper layers of the thallus. A very simple, but at the same time convincing, illustration of this is adduced by Nylander. "By immersing," he says, "any fruticulose thallus, such as Usnea, by the base in water, it remains entirely dry (with the exception of the part submersed), but if water be poured over the other portions, it quickly absorbs it, softens, and revives." The same thing may be seen in nature itself, in the case of such species as Cladina rangiferina, Alectoria ochroleuca, Plalysma nivale, &c., growing on temporarily wet substrata,