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. This is the youthful state of the Marchantia polymorpha, but after a time green points appear from under little reddish scales on the surface, and these are developed into stalks an inch or less in height, which terminate differently, some in lobed shields, others in spoked whorls, like a carriage wheel without the rim (fig. 43). The lobed shields are rather concave and covered with little elevations, in each of which there is a flask-shaped cavity with a long neck opening on the surface of the shield. In all of these hollows there is a mass of cells full of an amorphous substance, which is changed into spermatozoids, having the form of delicate spiral filaments, thicker at one end, and furnished with two cilia, with which they revolve in a spiral within their cell. At last they emerge from it, and come through the neck of the flask-shaped hollow to the surface of the shield. [Illustration: Fig. 44. Marchantia polymorpha:—A, portion of frond seen from above, showing lozenge-shaped divisions (_a_), with central stomata (_b_); B, vertical section, showing the layers of tissue, and one of the stomata (_g_).] [Illustration: Fig. 45. Marchantia polymorpha:—archegonia.] [Illustration: Fig. 46. Elater and spores of Marchantia.]