The Characeæ may also be artificially reproduced by cuttings; while under favourable circumstances they are reproduced by nodular bodies rich in starch grains, which arise from the creeping root, and also by clusters of cells called bulbils filled with starch grains, which arise from a modification of the nodes.
The Charas, properly so called, are monœcious or diœcious, more or less opaque and brittle, their many-jointed tubular stems bearing whorls of long slender awl-shaped branchlets. The fruit, accompanied by a cluster of short bracts or ramuli, is placed at intervals in the hollow side of the branchlets, one at each joint. In the bristly Chara the awl-shaped branchlets are simple, pointed at the extremity, and composed of about seven joints, with a whorl of from four to seven short bracts, and having fruit at each articulation. Many of the species of Characeæ are thickly incrusted with lime, and the whole order is exceedingly fetid.
Fig. 42. Antheridia of Chara fragilis:—D, E, F, successive stages of formation of spermatozoids in the linear cells of the antheridial filaments; G, escape of mature spermatozoids, which are shown detached at H.
SECTION VI.
HEPATICÆ, OR LIVERWORTS.
The Hepaticæ are the small herbaceous plants, which constitute the three distinct natural orders called respectively Ricciaceæ, Marchantiaceæ, and Jungermanniaceæ. They are distinguished primarily by the first having the sporangia valveless, without elaters; by the second having dependent valvate or irregularly bursting sporangia; and by the third having the sporangia valvate and erect. Both the latter, moreover, have the spores mixed with elaters.
The Ricciaceæ, popularly called Crystalworts, which form the lowest grade of Hepaticæ, are inconspicuous plants, growing in mud, or floating on water. They have spreading, horizontal fronds, of a delicate cellular structure and of indefinite form. Their fructification consists of valveless spherical sporangia, or spore sacs, imbedded in the frond; cells are formed within these sporangia, each of which cells is divided into four parts, which become spores, and, when ripe, the surface of the sporangia is fractured to give them egress. In some species there are many air passages in the cavities where the sporangia are produced. The under-surface of the frond is often beset with scales. The genus Riella differs from all the other genera in having an upright branchless stem with a distinct wing or limb forming a continuous spiral round it. In the male plant, the edge of the frond bears the antheridia containing spermatozoids; in the female plant the sporangia spring from the stem; they have a separate envelope, and the spores are echinate.
Fig. 43. Marchantia polymorpha:—a, gemmiparous conceptacles; b, lobed receptacles bearing pistillidia.
The Marchantiaceæ rank as higher forms of Hepaticæ. In the Marchantia polymorpha ([figs. 43], [44]), which may be taken as an illustration of the group, the structure of the horizontal frond is complicated, for, besides the colourless transparent skin, there are three distinct layers, the uppermost of which consists of cells filled with green matter, the lowermost (or base of the frond) being formed of close-set cells full of very solid matter, while between the two there is a cavity filled with air and loosely branching filaments, which spring from the base, and consist of green cells fixed end to end, as in [fig. 44] B. The surface of the plant appears to be smooth and shining, but when magnified it is found to be marked with numerous narrow elevated green bands, crossing one another diagonally so as to mark out the surface into a number of small lozenge-shaped divisions ([fig. 44] A). These elevated bands are merely the tops of very solid walls which descend perpendicularly to the base of the plant, consequently they divide the internal air chamber into a number of lozenge-shaped compartments, each of which communicates with the external atmosphere by means of chimneys opening in the centre of each compartment on the surface of the plant. These chimneys are so constructed of four or five superposed rings of cells (see [fig. 44] B), that by the expansion and contraction of the cells of the lowermost ring, more or less air can be admitted into the cavity below. This is a very beautiful instance of the contractile vital energy acting for the production of motion, its object being to supply air, so essential to the health of all plants. White filaments from the base at once fix the Marchantia to the earth, and supply it with food.