558 LICHENS
when the base of their thalli is soft and moist, and all the rest dry and rigid. It cannot therefore be doubted that the nutritive elements contained in the rain or other water are conveyed to the lichen through the surface of the thallus. It is in the superficial parts also, as Nylander has well observed (in Flora, 1874, No. 4), that "the active life has its seat, chiefly around the gonidia, manifesting itself in the putting forth of young parts (lobes, laciniæ, branches, isidia), and in the functions of the apothecia and the spermogones, so that the nourishing humours necessary for all the actions of life are especially and directly poured upon these." The vital activities, however, in lichens thus nourished are, as might be expected from the nature of the source whence their nutriment is derived, very intermittent, and in dry weather cease to operate, and become entirely dormant. Hence their life, unlike that of all other plants, is twofold, viz., one active, in which when moistened all the vegetative and reproductive functions are at work, and the other passive, in which when dry these functions are completely in abeyance. For such a peculiar duplex existence, at one time vegetating, at another lethargic, their organization in all its parts, gonidial and otherwise, is admirably adapted. More especially is this the case with respect to the lichenine found in their textures, which, being readily dried and as readily moistened, enables them to resist with impunity the greatest extremes of temperature, alternate periods of drought and wet, the scorching heat of the sun, the vehemence of stormy winds, and the nipping frosts of winter.
In this fitful and abnormal life of lichens we have the explanation in a great measure of their almost indefinite duration of existence. It is well known that they are perennial plants in the widest sense of the term, and that, though in the earlier stages of their existence their growth is comparatively rapid, yet this becomes extremely slow when they arrive at a certain age. The time required for the development of even the most rapidly growing species may be calculated by the appearance of such of these as are met with on gravestones, mortar of houses, stone walls, wooden palings, and such like, the date of whose erection is known. Amongst other instances which have come under the present writer's own observation may be adduced the case of Physcia parietina, growing in fair quantity on the stones of a granite wall built in 1836 in a maritime district where the plant in extremely abundant, and where atmospherical and other conditions are well suited for its growth. In a recent visit to the spot it was found that, although the thallus is now well developed, no fructification whatever is visible, though traces of spermogones are beginning to appear, so that in the space of forty-five years this plant has not yet attained full maturity. But slow as is the growth of lichens after a certain stage of their development, their tenacity of life is very remarkable, as might a priori be inferred from their capacity of enduring without injury the greatest extremes of temperature and of hygrometrical conditions. It is on record that, after the lapse of nearly half a century, the same specimen on the same spot of the same tree has been observed without any change in its condition. On this point also E. Fries (in L. E., p. xlv.) notices that certain species such as Physcia ciliaris, kept in houses for upwards of a year, revive when again exposed to the influences of the atmosphere, – an observation which in the case of Cladina rangiferina similarly kept for a still longer period the present writer can fully corroborate. Endowed then with this singular intermittent vitality, we can easily understand how many individuals which occur on hard mountainous rocks or on the trunks of aged trees in ancient forests are in all probability many hundreds of years old. Nor does age seem in any way to weaken their fecundity, even when the thallus has apparently ceased to grow. This, as observed by Nylander (in Syn., p. 5), is shown from the circumstance that were it otherwise "the already old fruits would be destitute of spores, which is never the case," unless in plants of some lower tribes, e.g., Graphidei and Verrucariæ, in which the thallus is but sparingly gonidiose, and the life consequently is shorter. In other instances the central portion of the thallus sometimes normally perishes in old plants, as in Pterygium centrifugum, Collema melænum, Parmelia centrifuga, and P. saxatilis, leaving only peripherical circles, in which, however, the life of the individual still continues for ages. In fact, "the life of lichens bears in itself no cause of death, and is only to be ended by external injuries" (E. Fries, L. E., loc. cit.), or by the alteration of climatic and atmospherical conditions. Hence the assumption is not unwarrantable that individuals of such confessedly long-lived species as Lecidea geographica, growing on rocks upon the summits of lofty mountains, date from more than "fabulous epochs," and probably outrival in longevity the ages assigned to the oldest trees on the surface of the globe.
Chemistry of Lichens, and Chemical Reactions.
Chemistry of Lichens. – This is still but little understood, notwithstanding that the subject has been more or less investigated by authors. Their examinations, however, have been too limited and desultory to enable us to give any detailed account of the different principles which enter into the composition of the lichen tissues. Moreover, with respect to those species which have been more particularly analysed, they have sometimes employed not only the same terms in different senses, but also different terms to denote the same substance. There can, however, be no doubt that the chemical composition of lichens not only produces great modifications in their form, but also considerable diversities in their properties.
The principal substance which occurs in lichens, especially in such as are foliaceous and fruticulose, is lichenine – a special kind of gelatin peculiar to them. It is intermediate in character between dextrin and starch, and very eagerly imbibes water, though if boiled in water it is dissolved and lost. Starch also very rarely occurs in large lenticular grains scattered in the tissues. In crustaceous lichens oxalate of lime predominates, and forms a large proportion of the thallus, e.g., 65 per cent. in Lecanora esculenta according to Gœbel. Chlorophyll and its modification phycochrome are found in lichens only in relatively small quantities, as is also the case with most of the other principles they present. These are enumerated by Nylander (in Syn., p. 51) according to the affinities they bear, as follows: – (1) phosphate of lime, sea-salt, manganese, iron; (2) picrolichenine, variolarine, orceine, cetrarine, inuline, erythrine, roccelline, picroerythrine; (3) gyrophoric acid, parellic acid, usneic acid, orceic acid, erythrinic acid; (4) sugar not crystallizable, oil, waxy matter, resinous matter. Several others, especially phosphates and acids, have been enumerated by authors, which in some instances have not been corroborated, and in others are doubtfully distinct. In illustration of the manner in which several of these principles enter into the composition of lichens, we may adduce the analyses given in one or two species which have been more fully subjected to experiment. In Cetraria islandica there have been detected starch, including lichenine and inuline, to the extent of 80 per cent.; gummy and waxy matters; a bitter principle, cetraric acid; a fatty principle, lichestearic acid; fumaric acid, which exists also in Fumaria officinalis; gallic acid, the astringent principle of galls; uncrystallizable sugar; various salts, such as bitartrate of potash, and tartrate and phosphate of lime. Again, in Physcia parietina there have been found several alimentary principles, as gliadine, starch, sugar, gum; several medicinal substances, as resin, bitter matter, gallic acid, and a peculiar ethereal oil; a yellow colouring matter called chrysophanic acid; also wax, stearine, and some salts, as carbonate of lime. The ash of lichens is said to constitute from 8 to 10 per cent. of their bulk, and consists of various earthy bases, such as potash, soda, lime, magnesia, alumina, silica, manganese, and peroxide of iron, in combination with various acids, such as carbonic, phosphoric, sulphuric, and hydrochloric. The whole subject, however, of the chemistry of lichens requires to be investigated in a more extensive and methodical manner than has hitherto been done either by chemists or lichenists.
