Lichenaceæ.
The lichens are a family of autonomous plants, an intermediary group of algals or cellular cryptogams, drawing their nourishment from the air through their whole surface medium, and propagating by spores usually enclosed in asci, and always having green gonidia in their thallus. Their gonidia, bright coloured globular cells, form layers under the cortical covering of the thallus, and generally develop in the form of incrustations, which cover stones, wood, and the bark of trees, or penetrate into the lamellæ of the epidermis of woody plants. The gonidia of lichens partake of both the character of vegetative and reproductive cells.
The thallus in the fructicose group attaches itself by a narrow base, growing in the form of a miniature shrub. Another group is met with in a slimy condition—the gelatinous lichens. These species, for the most part, furnished dyes before the discovery of the coal-tar dyes. In many of the Palmella cruenta, commonly found growing on the walls and roofs of houses, a colourless acid liquid is found, which, on being treated with alkali, produces a bright yellow colour; and another, Avernia vulpina, furnishes a brown dye; the Rocella fuciformis and R. tinctoria yield the purple dye substance known as orchil, or archil, from which the useful blue paper of the chemist for testing acidity is manufactured. Usnic acid, combined with green and yellow resins, seems to be more or less a constituent of many lichens.
A vertical section of Palmella stellata is given in [Plate I]., No. 26, in which the emission of the ripe spores of the lichens is seen to be not unlike that which takes place in some of the fungi, Pezizæ, Sphæriæ, &c. If a portion of the thallus be moistened and placed in a common phial, with the apotheca turned toward one side, in a few hours the opposite surface of the glass will be found covered with patches of spores, easily perceptible by their colour; or if placed on a moistened surface, and one of the usual glass slips laid over it, the latter will be covered in a short time. As to the powers of dissemination of these lowly organised plants, an observation led to the conclusion that the gonidia of lichens have greater powers in this direction than was formerly supposed. It is found that by placing a clean sheet of glass in the open air during a fall of snow, and receiving the melting water in a tube or bottle, quantities of what has been looked upon as a “unicellular plant” can be taken, the cells of which may be kept in a dormant condition for a long time during cold weather, but upon the return of spring warmth and moisture they begin to increase, by a process of subdivision, into two, four or eight portions; these soon assume a rounded form, and burst the parent cell-wall open; these secondary cells then begin to divide and subdivide again, and the process may go on without much variation for a long time. The phenomena described may be watched by taking a portion of the bark of a tree on which Chlorococcus has been deposited, and placing it under a glass to keep it in a moderately moist atmosphere; the only difference being a change in colour, caused by the growth of the fibres, as may be seen on microscopical examination. “And this,” says Dr. Hicks, who first observed this phenomenon in plant life, “is an instructive point, because it will be found that the colour varies notably according to the lichen prevalent in its neighbourhood.”[59] He believes there can be no doubt that what has been called Chlorococcus is nothing more than the gonidia of a lichen; and that under suitable conditions, chiefly drought and warmth, the gonidium often throws out from its external envelope a small fibre, which, adhering and branching, forms a “soridium.” “The soridia remain dormant for a very long time, and do not develop into thalli unless in a favourable situation, in some cases it may be for years. It will be perceived that the soridium contains all the elements of a thallus in miniature; in fact, a thallus does frequently arise from one alone, and the fibres of neighbouring soridia interlace; thus a thallus is matured very rapidly. This is one of the causes of the variation of appearance so common in many species of lichens, more readily seen towards the centre of the parent thallus. When the gonidia remain attached to the parent thallus, the circumstances are, of course, more favourable, and they develop into secondary thalli, attached more or less to the older one, which, in many instances, decays beneath them. This process being continued year after year gives an apparent thickness and spongy appearance to the lichen, and is the principal cause of the various modifications in the external aspect of the lichens which caused them formerly to be misunderstood and wrongly classified.”[60]
The erratic lichens are found among the genus Palmella, some of which grow among boulders of the primary and metamorphic formations, curled up into a ball, and only fixed to their matrix by a slender thread. The globular Lecanora esculenta will at times suddenly cover large tracts of country in Persia and Tartary, where it is eaten by the cattle. During a scarcity of food a shower of these lichens, Mr. Berkeley tells us, fell at Erzeroum, and saved the cattle from starvation.[61]
Another group of the Palmella, or Peltigeri, so named from the target-like discs on their surface, spread their foliaceous fronds over the ground, and as the fruit is marginal, it gives the thallus a digitate appearance. These are often spotted over by a little red fungus. The Lecidinei contains numerous species of the most varied habits, and always crustaceous, and so closely adherent to the hard rocks and stones on which they grow, that at length they disintegrate them. From this low species a higher form arises, with erect branching stems, and clothed with foliaceous, brightly-coloured scales.
The Coccocarpei is mainly distinguished by having orbicular discs entirely deprived of the cortical envelope called an excipulum. The discs spring at once from the medullary stratum, and contain asci and sporidia similar to those of the minute fungi Sphæriæ. Some of the lichens are themselves parasitic, and begin existence under the thick skin of the leaves of tropical plants, and spread encrusting thallus over their surface, the excipulum and perithecia being black; but in most cases these are beautifully sculptured, and are interesting objects for the microscope. Indeed, the sphere-bearing lichens, with upright stems bearing globular fruit at the extremity of their branches, are at first indicated by a swelling, but in time the outer layer bursts and exposes sporidia, which are beautiful objects under the microscope on account of their spherical form and more or less deep blue tint. Humble and lowly as lichens may appear to be, they have been divided into fifty-eight or more genera and 2,500 species. The brothers Tulasne, De Bary, the Rev. Mr. Berkeley, and others, devoted great attention to the peculiarities of their structure and natural history.
Hepaticæ.—An intermediary group of much interest to the microscopist are the Hepaticæ (liverworts). These are found growing on damp rocks in the neighbourhood of springs and dripping banks. The scale-moss, the Marchantia polymorphia ([Fig. 299]), may be taken as typical of this little group, with its gemmiparous conceptacles and lobed receptacles, bearing archegones on transparent glass-like fruit stalks, carrying on their summits either round shield-like discs or radiating bodies with a striking resemblance to a wheel without its tyre.
Fig. 299.—Marchantia polymorphia.
The liverworts are closely allied to the mosses, and as much difficulty was experienced in dividing the two, Hooker placed the whole under one genus, the Jungermannia. More recently, however, they have been divided into those with a stem and leaves confluent in a frond, Marchantia; those with stem and leaves distinct, Jungermannia; and those with a solitary capsule, filiform, bivalved, stalked, with a free central placentation, Anthocotaceæ. Some botanists have further divided them, but they are all extensively propagated by gemmæ.
The fronds carry the male organs, or antherids, and the disc, in the first instance, bears the female organs, or archegones, and after a time gives place to the sporanges, or spore cases. It is these bodies which are of so much interest to microscopists; if the plant is brought into a warm room, they suddenly burst open with some violence the moment a drop of water is applied to them, and the sporanges are dispersed in a small cloud of brownish dust. If this dust is examined under a medium power, it is seen to consist of a number of chain-like bodies, somewhat like the spring of a small watch; and if the process of bursting be closely watched, these minute springs will be found twisting and curling about in every direction. The structure of the frond itself will be seen to be interesting when cut in the vertical direction and placed under the microscope.
Fig. 300.—Gemmiparous conceptacle of Marchantia polymorphia, expanding and rising from the surface of a frond. In the interior are seen gopidial gemmæ already detached by the splitting of the epiderm.
The gemmæ of Marchantia polymorphia are produced in elegant membranous cups, with a toothed margin growing on the upper surface of the frond, especially in very damp courtyards between the stones, or near running water, where its lobed fronds are found covering extensive tracts of moist soil. At the period of fructification the fronds send up stalks, which carry at their summit round shield-like radiating discs, which bear upon their surface a number of little open basket-shaped “conceptacles.” These again expand into singularly graceful cups (as in [Fig. 300]), and are found in all stages of development. When mature, the basket contains a number of little green round or oblong discs, each composed of two or more layers of cells; the wall itself being surmounted by a glistening fringe of teeth, whose edges are themselves regularly fringed with minute outgrowths. The cup seems to be formed by a development of the superior epidermis, which is raised up, and finally bursts and spreads out, laying bare the seeds.
The archegones of Marchantia are very curious bodies, while the elater and spores are even still more so. These are elongated cells, each containing a double spiral fibre coiled up in the interior. It is the elasticity of this which tears apart the cell-membrane, and sends forth the spores with a jerk, and thus assists in their dispersion. Marchantia is the type of the malloid Hepaticæ.