JUNGERMANNIACEiE 129 



leafy, creeping forms, the leaves being composed of a 

 single layer of cells, and the cells of both leaves and 

 stems are of a similar kind. The sporogonium is charac- 

 teristic; it consists of a long stalk surmounted by a 

 spherical capsule, and the latter, in order to discharge 

 the spores, always splits into four valves, which, to- 

 gether with the elaters, are hygroscopic, and by their 

 movements assist in scattering the spores. The ger- 

 minating spore may produce a young gametophyte 

 directly; but in some thalloid forms, and in all the 

 foliose species, the first product from the spore is a 

 protonema (Gr. protos, first; nema, thread) — a stage of 

 development strongly marked in the Mosses, which will 

 shortly receive our attention. The species of the Order 

 are usually small in size; they grow on moist earth, 

 on the bark of trees, and on stones; in the tropics they 

 occur on the living leaves of other plants. 



Pellia epiphylla is an abundant species of the Order, 

 and a good example of the thalloid form. It grows in 

 many situations — on walls, by the side of springs, on 

 the banks of streams, in damp woods, and in other 

 places where moisture abounds. The thallus is lobed 

 and branched, and the plants occur in large patches. 

 Fig. 41 gives a general view of a single plant, and 

 Fig. 42 shows a part of a thallus with antheridia and 

 rhizoids, or root-hairs. Metzgeria is also thalloid, the 

 thallus being ribbon-like and branched. M. furcata, 

 the Forked Liverwort, is common in Britain, growing 

 on rocks, but more abundantly on tree-trunks, where 

 it works its way in and out of crevices, clinging firmly 

 to the bark; its forked ribbons are not more than a 

 millimetre in breadth; they display a distinct midrib. 



17 



