[Illustration: FIG. 197.--_Marchantia polymorpha. A_ Female plant (nat. size): _a_ and _b_ are archegoniophores in various stages of development; _s_ cupules with gemmæ (see page 183). _B_ An archegoniophore seen from below, the short-stalked sporangia are seen placed in 8–10 double rows. _C_ Male plant, with a young and an older antheridiophore. _D_ Antheridiophore halved vertically to show the antheridia (_h_); _m_ the aperture of the pits in which they are sunk--the older ones to the left, the younger to the right.] Order 2. =Corsiniaceæ.= (Not native). Intermediate forms between the preceding and the following order. In internal and external structure mainly resembling the Marchantiaceæ. _Corsinia_; _Boschia_. Order 3. =Marchantiaceæ=, are large, fleshy forms. The surface of the thallus is divided into small rhombic areas, in the centre of each of which is found a large, peculiarly constructed stoma (Fig. 197 _A_); beneath each of these a large air-cavity is to be found. From the floor of the air-cavity a number of alga-like cells project into it; these contain chlorophyll and are therefore the assimilating cells. The antheridia and archegonia are each found aggregated on specially formed branches (somewhat resembling Mushrooms) projecting from the surface of the thallus. The antheridia are developed on the upper surface (Fig. 197 _C_, _D_) and the archegonia on the lower (Fig. 197 _A_, _B_), near the centrally-placed stalk. _Marchantia polymorpha_ is diœcious (Fig. 197), and very common on damp places. _Lunularia_ (South Europe), frequently found on flower-pots in conservatories; _Preissia_, _Fegatella_, _Reboulia_, _Targionia_. Family 2. =Anthoceroteæ.= These have an entirely leafless, fleshy, flat, and irregularly-shaped thallus. In its intercellular chambers Nostoc-colonies are often found, which have forced their way through the stomata situated on the under side. The antheridia and archegonia arise from the cells lying inside the thallus. The capsule resembles a long, thin pod; it has two valves and a columella. _Anthoceros_ (_A. lævis_, Fig. 194, and _punctatus_). Family 3. =Jungermannieæ.= Some forms in this family have a thallus in which leaf-like structures are found (_Blasia_), while in others (_e.g. Metzgeria_, _Pellia_, _Aneura_) they are entirely absent. The majority, however, have round, thick stems, bearing dorsally two rows of leaves, and one row ventrally. Some of these have the leaves “underlying” (Fig. 195), while in others (Fig. 198) they are “overlying.” (See Figs. 195, 198, with explanation). The sporangia are spherical, stalked, and situated singly on the apex of the branches, and open by four valves (in _Sphærocarpus_ they are indehiscent).