The higher groups have a distinct upright stem with symmetrical leaves, which leaves, however, in the lower genera, are merely fleshy fronds without veins, of numerous and sometimes grotesque forms, while in the higher, they are generally oval, and disposed in spiral imbricated rows on each side of the stem and branches, every leaf overlapping part of that which is adjacent to it like a row of tiles. They are disposed after two distinctly different plans. In one, the leaves are arranged in a spiral, which turns from left to right; consequently, the anterior border of each inferior leaf is covered by the posterior border of that immediately above it. This constitutes the succubous group of frondose Jungermannieæ, which embraces five tribes. In the second case, the spiral of the leaves turns from right to left, and the anterior border of each inferior leaf covers the posterior border of that immediately above it. This constitutes the incubous group embracing three tribes of the frondose Jungermannieæ. Besides the imbricated spiral leaves, there are small leaves alternating with them, and applied directly on the stem between them.
The male fruit of the Scale Mosses consists of antheridia, which form ovoid masses of cells variously disposed; in each of these cells, there is a filiform spermatozoid coiled in a circle in active motion, which is maintained after leaving the cell. The female fruit consists of archegonia, containing embryo cells, which ultimately develope into sporangia; but the plants arrive at their full development before the archegonia appear. The same plant may produce more than one crop of these objects, which are occasionally abortive. The stalk of the sporangium is first surrounded by the sac of the archegonium, then by an involucre or circle of bracts, placed in a rosette of leaves. The sporangia contain cells which ultimately become spores mixed with long cells containing elaters, that is, elastic spiral filaments. When the fruit is ripe, the sporangia split into four equal parts, which form a cross on the top of their stems, and the elaters spring out of their cells, and disperse the spores.
The plants of this order are also reproduced by gonidia, but in such numerous genera and species the arrangement of the reproductive bodies and their minute details are exceedingly varied. They are chiefly distinguished by the insertion of the fruit, and the form of the different organs which surround it. They are pretty little plants, occasionally of a bright green, but oftener inclined to red, purple, and chocolate colour; a few are fragrant, but they are of no known utility. They are found in shady woods and moist situations, throughout all regions of the globe, but are most abundant in damp, tropical forests.
In all the families of the leafy Cryptogamia as well as in the Hepaticæ, antheridia exist; they differ much in form and structure, but they collectively agree in developing in the interior of delicately walled cells, an amorphous substance, coloured yellow by iodine, in place of which, at the epoch of maturation, spermatozoids appear, thick at one end and running to a very fine point at the other, and displaying several spiral convolutions. When rolled up like a watch-spring the motion is more or less rotary, but if it be coiled in the form of a corkscrew, the movement is at the same time advancing. The thin end of the filament always goes first both within the cell and after it comes out.[[67]] The whole structure of the Hepaticæ is full of objects of interest to the microscopic observer.
SECTION VII.
MUSCI, OR MOSSES.
Mosses approach the higher classes of vegetable life in having roots, and a more or less upright stem or axis of growth. Like all other plants, they are chiefly formed of cellular tissue, yet, in the stems, there is an indication of a separation between the bark and pith by the intervention of a circle of elongated cells approaching to woody fibre, which passing into the branches and leaves form a kind of midrib, either extending to the extremity of the leaf or not. The delicate little leaves, which are arranged with great regularity, consist of a single or double layer of cells united by their flattened sides, and rarely exhibiting any epiderm or skin. Green is the prevailing colour in this order of beautiful little plants, but, when exposed to the sun and much moisture, they sometimes become red.
The urn-like vessels ([fig. 47] B), containing the spores, are either terminal or lateral on the stem of the moss, and rise on a slender stalk from a rosette of narrow leaves possessing a skin perforated by stomata or breathing pores of simple structure. This is the ultimate result of fructification; for at a very early period, while the rosettes of narrow leaves are merely buds, they contain antheridia and archegonia, the latter or female organs being either inclosed in the same bud, or in different buds placed on the same or on different plants; but however that may be, the buds are invariably placed at the base of the leaves, close to the axis of the plant.
When a male bud is developed, the antheridia are found to be globular, ovoid, or elongated hyaline bodies, set in a cluster of hairs and paraphyses, or sterile cellular filaments. [Fig. 48] represents the antheridia of Polytrichum commune, with the paraphyses and hairs, from the microscopic observations of Dr. Carpenter. The central antheridium is discharging its spermatozoids; the one on the right is empty, that on the left immature.
Fig. 47. A, plant of Funaria hygrometrica:—f, leaves; u, urns or capsules; S, seta or foot-stalk; o, operculum; c, calyptra. B, sporangia of Encalypta vulgaris:—u, urns; o, opercula; c, calyptra; p, peristome; s, seta.