The prothallus.—The new plant, which is produced when a fern spore germinates, is called a prothallus (Fig. 147, A). It is a flat, filmy little plant, of the form which is generally called heart-shaped. It has neither stem nor roots, but, as it contains green colouring matter like that of leaves, and as it puts out on its lower surface little hairs (rh) which take up watery solutions from the soil, it is in no danger of starvation, and is quite capable of taking care of itself and leading an independent existence. This tiny plant (a large fern prothallus is perhaps half the size of a 3d. piece), in contrast with its parent, produces sexual organs. Some of these organs (an) give rise to male cells and others (ar) to female cells. The male cells are excessively small, and can only be seen by high powers of the microscope. When they are ripe they swim about in a drop of rain or dew, as if they were little animals, and find their way to the female cells, which they fertilise. The embryo which results from the union grows up (Fig. 147, B) into an ordinary fern plant, one being borne by each prothallus. In its young stages it is parasitic on the prothallus, i.e. it depends entirely upon the prothallus for its nutrition; but it soon develops a little leaf (b) and a root (w), and henceforth feeds itself. When the young fern is well established in [Pg 189] independent life, the prothallus shrivels up and dies. The subsequently-formed roots of the fern are not branches of the primary root, but spring from the bases of the leaves. Alternation of generations.—In the life-history of the fern there are thus two very different generations. The first generation—the ordinary fern, which is non-sexual—produces, by means of spores, the sexual generation, called the prothallus. The prothallus gives rise, in its turn, by a sexual process, to the obvious fern plant. Each generation therefore resembles, not its parent, but its grandparent. Fig. 148.—Cross section of underground stem (rhizome) of Bracken Fern. s, Conducting strands; l, lp, strengthening tissue; R, ground-tissue; e, skin. (× 7.) Fig. 149.—Leaf of the Bracken Fern. (× ₁₂.)