The liverworts are peculiar flat green plants usually found on wet cliffs and in other moist, shady places. They frequently occur in greenhouses where the soil is kept constantly wet. One of the commonest liverworts is _Marchantia polymorpha_, two plants of which are shown in Figs. 288, 289. The plant consists of a ribbon-like thallus that creeps along the ground, becoming repeatedly forked as it grows. The end of each branch is always conspicuously notched. There is a prominent midrib extending along the centre of each branch of the thallus. On the under side of the thallus, especially along the midrib, there are numerous rhizoids which serve the purpose of roots, absorbing nourishment from the earth and holding the plant in its place. The upper surface of the thallus is divided into minute rhombic areas that can be seen with the naked eye. Each of these areas is perforated by a small breathing pore or stomate that leads into a cavity just beneath the epidermis. This space is surrounded by chlorophyll-bearing cells, some of which stand in rows from the bottom of the cavity (Fig. 290). The delicate assimilating tissue is thus brought in close communication with the outer air through the pore in the thick, protecting epidermis. [Illustration: FIG. 290.—SECTION OF THALLUS OF MARCHANTIA. Stomate at _a_.] At various points on the midrib are little cups containing small green bodies. These bodies are buds or _gemmæ_ which are outgrowths from the cells at the bottom of the cup. They become loosened and are then dispersed by the rain to other places, where they take root and grow into new plants. The most striking organs on the thallus of marchantia are the peculiar stalked bodies shown in Figs. 288, 289. These are termed _archegoniophores_ and _antheridiophores_ or _receptacles_. Their structure and function are very interesting, but their parts are so minute that they can be studied only with the aid of a microscope magnifying from 100 to 400 times. Enlarged drawings will guide the pupil. [Illustration: FIG. 291.—SECTION THROUGH ANTHERIDIOPHORE OF MARCHANTIA, showing antheridia. One antheridium more magnified.] The _antheridiophores_ are fleshy, lobed disks borne on short stalks (Fig. 291). The upper surface of the disk shows openings scarcely visible to the naked eye. However, a section of the disk, such as is drawn in Fig. 291, shows that the pores lead into oblong cavities in the receptacle. From the base of each cavity there arises a thick, club-shaped body, the _antheridium_. Within the antheridium are formed many sperm-cells which are capable of swimming about in water by means of long lashes or cilia attached to them. When the antheridium is mature, it bursts and allows the ciliated sperm cells to escape. [Illustration: FIG. 292.—ARCHEGONIUM OF MARCHANTIA.]