=562.= On the other hand the sexual phase of the ferns (gametophyte), while it still is capable of leading an independent existence, is short-lived (with very few exceptions). It is also much smaller than most of the liverworts and mosses, especially as compared with the size of the spore-bearing phase. The gametophyte phase or stage of the plants, then, is decreasing in size and durance as the sporophyte stage is increasing. We shall be interested to see if this holds good of the fern allies, that is of the plants which belong to the same group as the ferns. And as we come later to take up the study of the higher plants we must bear in mind to carry on this comparison, and see if this progression on the one hand of the sporophyte continues, and if the retrogression of the gametophyte continues also. [Illustration: Fig. 313. Embryo of fern (Adiantum concinnum) still surrounded by the archegonium, which has grown in size, forming the “calyptra.” _L_, leaf; _S_, stem; _R_, root; _F_, foot.] [Illustration: Fig. 314. Young plant of Pteris serrulata still attached to prothallium.] [Illustration: Fig. 315. Two embryos from one prothallium of Adiantum cuneatum.] CHAPTER XXVIII. DIMORPHISM OF FERNS. =563.= In comparing the different members of the leaf series there are often striking illustrations of the transition from one form to another, as we have noted in the case of the trillium flower. This occurs in many other flowers, and in some, as in the water-lily, these transformations are always present, here showing a transition from the petals to the stamens. In the bud-scales of many plants, as in the butternut, walnut, currant, etc., there are striking gradations between the form of the simple bud-scales and the form of the leaf. Some of the most interesting of these transformations are found in the dimorphic ferns. =564. Dimorphism in the leaves of ferns.=—In the common polypody fern, the maidenhair, and in many other ferns, all the leaves are of the same form. That is, there is no difference between the fertile leaf and the sterile leaf. On the other hand, in the case of the Christmas fern we have seen that the fertile leaves are slightly different from the sterile leaves, the former having shorter pinnæ on the upper half of the leaf. The fertile pinnæ are here the shorter ones, and perform but little of the function of carbon conversion. This function is chiefly performed by the sterile leaves and by the sterile portions of the fertile leaves. This is a short step toward the division of labor between the two kinds of leaves, one performing chiefly the labor of carbon conversion, the other chiefly the labor of bearing the fruit. [Illustration: Fig. 316. Sensitive fern; normal condition of vegetative leaves and sporophylls.]