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.
Fig. 316.
Sensitive fern; normal condition of
vegetative leaves and sporophylls.
565. The sensitive fern.—This division of labor is carried to an extreme extent in the case of some ferns. Some of our native ferns are examples of this interesting relation between the leaves like the common sensitive fern (Onoclea sensibilis) and the ostrich fern (O. struthiopteris) and the cinnamon-fern (Osmunda cinnamomea). The sensitive fern is here shown in [fig. 316]. The sterile leaves are large, broadly expanded, and pinnate, the pinnæ being quite large. The fertile leaves are shown also in the figure, and at first one would not take them for leaves at all. But if we examine them carefully we see that the general plan of the leaf is the same: the two rows of pinnæ which are here much shorter than in the sterile leaf, and the pinnules, or smaller divisions of the pinnæ, are inrolled into little spherical masses which lie close on the side of the pinnæ. If we unroll one of these pinnules we find that there are several fruit dots within protected by this roll. In fact when the spores are mature these pinnules open somewhat, so that the spores may be disseminated.