SECTION IV.
LICHENS.
Lichens are essentially air plants, being nourished, like the Algæ, by the medium in which they grow. They vary from a pulverulent or dry papillose crust, to a leathery or horny expansion, and even acquire an erect stem. They are independent of the matrix to which they are attached. Hence they spread their coloured frond, or thallus, in circular or indefinite patches on old walls, the tiles of houses, stones, and rocks. They appear in large expansions of red, golden yellow, grey or white, on barren heaths, under plantations, and on the stems of aged trees; while others of them often hang from the branches like long shaggy grey hair, and many form forests of miniature bushes on the northern plains. The lichen is the last trace of vegetation on the tops of the mountains, and on the arctic deserts. Some lichens are patient of severe cold, yet in general they prefer heat and moderate warmth, and they love bright light so much, that they are usually barren, or else yield little fruit under shade. Though differing greatly from fungi in slowness of growth, length of life, and the power of forming chlorophyll, they resemble them in having a mycelium in their youth, and in their ascigerous fructification. A perfect lichen without an ascus would be an anomaly, for the asci contain the true fruit, associated with vertical threads or elongated cells called paraphyses, which sometimes bear secondary spores on their summit. The asci with their paraphyses inclosed in vessels of various shapes, called perithecia, are aggregated in discs or shields, which form projections on the surface of the plant. Some of these discs are closed, and give egress to the spores through a fracture or pore on their surface; others are open cups of various forms, either with stalks or sessile on the frond, and through these the spores have egress. Hence the whole order of lichens is naturally divided into two groups, according as their discs are open (Gymnocarpei), or closed (Angiocarpei), the first being incomparably the most important. [Fig. 37] shows open, and sections of closed cups, or perithecia.
Fig. 37. Lichens:—a, Trypethelium Sprengelii, pustules, with sporidia; b, Verrucaria variolosa, section of perithecium with sporidium; c, Endocarpon lacteum, thallus with section and fruit; d, Stegobolus Berkeleianus, portion of plant with ascus, and sporidia.
The highest type of horizontal lichens, of which [fig. 37] b is a perpendicular section, has a firm, spreading, superficial crust or surface, formed of oblong coloured cells, or of coloured filaments closely aggregated, and which covers two distinct layers of cellular tissue. In the layer immediately below the surface, the cells are globular and of a paler colour; the second layer, or marrow, which is the origin of fructification, consists of lax, detached, branching rows of elongated cells and gelatinous matter. These branches of cells spring up from a fourth layer, which is the base of the plant. It is of a strong, firm, and tough nature, composed of interlaced filaments and is often ribbed on the under-side. White fibres fix the base of the plant to the surface on which it is spread; they are the remains of a mycelium or matted mass of fibres, from whence lichens spring, which vanishes when the plant is full grown.
Fig. 38. a, Sporopodium Leprieurii, ascus; b, Coccocarpia smaragdina, section; c, Lecanora affinis, section.
Globular bodies of vegetable green called gonidia, like those at the base of [fig. 39] a, are arranged in regular parallel rows, and placed between the surface and the base of the lichen, as in [fig. 38] b and c; they are said to proceed from the medullary layer, though Mr. Berkeley has seen them springing from the threads of the mycelium of Parmelia parietina. The gonidia find their way to the air through rents in the surface of the plant, and are washed off by the rains, after losing a little of their green colour. When they germinate they only produce a facsimile of the mother plant, as buds do in the highest classes. But according to the microscopic observations of M. Tulasne, the true fruit differs little if at all from the asci-bearing fungi. Through the open discs of the higher lichens, sporidia are discharged from perpendicular septate asci which, with their paraphyses, are imbedded in the substance below. The asci are formed by the elongation of some of the cells of that layer into cylindrical septate vessels, generally containing from four to eight sporidia, the ultimate result of as many free cells. In whatever part of a lichen the perithecia may be placed, the asci and paraphyses invariably originate in the medullary layer.
Fig. 39. a, Paulia perforata, gonidia, paraphysis, and asci; b, Calicium tympanellum, perithecium and sporidia; c, Graphis Leprevostei, with excipulum, asci, and sporidium.