Fig. 70.—Forms of ferns. A, grape fern (Botrychium), × ½. x, fertile part of the leaf. B, sporangia of Botrychium, × 3. C, flowering fern (Osmunda). x, spore-bearing leaflets, × ½. D, a sporangium of Osmunda, × 25. r, ring. E, Polypodium, × 1. F, brake (Pteris), × 1. G, shield fern (Aspidium), × 2. H, spleen-wort (Asplenium), × 2. I, ostrich fern (Onoclea), × 1. J, the same, with the incurved edges of the leaflet partially raised so as to show the masses of sporangia beneath, × 2.
In the true ferns (Filices), the sporangia resemble those already described, arising in all (unless possibly Osmunda) from a single epidermal cell.
One group, the water ferns (Rhizocarpeæ), produce two kinds of spores, large and small. The former produce male, the latter female prothallia. In both cases the prothallium is small, and often scarcely protrudes beyond the spore, and may be reduced to a single archegonium or antheridium ([Fig. 71], B, C) with only one or two cells representing the vegetative cells of the prothallium (v). The water ferns are all aquatic or semi-aquatic plants, few in number and scarce or local in their distribution. The commonest are those of the genus Marsilia ([Fig. 71], A), looking like a four-leaved clover. Others (Salvinia, Azolla) are floating forms ([Fig. 71], D).
Fig. 71.—A, Marsilia, one of the Rhizocarpeæ (after Underwood). sp. the “fruits” containing the sporangia. B, a small spore of Pilularia, with the ripe antheridium protruding, × 180. C, male prothallium removed from the spore, × 180. D, Azolla (after Sprague), × 1.
Of the true ferns there are a number of families distinguished mainly by the position of the sporangia, as well as by some differences in their structure. Of our common ferns, those differing most widely from the types are the flowering ferns (Osmunda), shown in [Figure 70], C, D. In these the sporangia are large and the ring (r) rudimentary. The leaflets bearing the sporangia are more or less contracted and covered completely with the sporangia, sometimes all the leaflets of the spore-bearing leaf being thus changed, sometimes only a few of them, as in the species figured.
Our other common ferns have the sporangia in groups (sori, sing. sorus) on the backs of the leaves. These sori are of different shape in different genera, and are usually protected by a delicate membranous covering (indusium). Illustrations of some of the commonest genera are shown in [Figure 70], E, J.