[Illustration: Fig. 53. Archegonium of Pteris serrulata:—A, as seen from above; B, side view, showing at A the cavity containing the germ cell; at B, the walls of the archegonium made up of four layers of cells; and at C, the spermatozoids within the cavity.] It is evident that there is an essential difference between the archegonium of the Marchantia, and that of the ferns. The archegonium of the Marchantia merely produces a sporangium, while that of the ferns produces a new plant, and is therefore, in some sense, analogous to the seed of one of the higher vascular plants, though there is no affinity between the two. The roots of ferns, like those of forest trees, are delicate fibres, which descend either from a woody stem called a rhizome, from _rhiza_, a root, or from a caudex, so named from the Latin, _caudex_, a trunk. Thus the stems of ferns are of two distinct types. The rhizome generally grows horizontally, and creeps along the surface of the soil or rock, or tree trunk to which it is affixed, but it is sometimes subterranean. In this, the growing point is in advance of the fronds, which appear at intervals along the exposed side or sides, and after they have reached maturity drop off, leaving a clean scar or cicatrix. The caudex varies from the size of a small wire-like thread to the size of a tree trunk. It is sometimes elongated, as in the superfine wiry thread-like stems of Hymenophyllum, the scandent ivy-like rooting stems of Stenochlæna, and the subterranean, horizontal, widely-creeping stems of Pteris aquilina, all of which develop single fronds at intervals, but fronds which adhere permanently to the stem, and which do not fall off, leaving a scar, as do the fronds borne by the rhizome. The more frequent form of caudex, however, is that of a short stocky stem producing fronds on all sides from its apex: very frequently this is a globose or oblong mass growing erect, yet scarcely reaching above the surface of the soil; but sometimes it is more lengthened, showing a more or less elevated stem, or turning sideways, and taking a decumbent position, the young fronds, however, always rising from its apex. It is on the latter plan, by a continuous erect and upward growth, that the trunks of tree ferns, which rise sometimes sixty or eighty feet high, are formed. The common Lady Fern, Athyrium Filix-fœmina, furnishes an example of the ordinary herbaceous caudex; as also does the Male Fern, Lastrea Filix-mas. In the Hart’s-tongue, or Scolopendrium, the caudex is generally compact, and increases by the formation of new crowns or centres of growth around the older one, till the whole becomes an almost spherical mass of considerable size.