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entimetres in height. There are two genera in the United States, the true adder-tongues (_Ophioglossum_) and the grape ferns (_Botrychium_). They send up but one leaf each year, and this in fruiting specimens (Fig. 70, _A_) is divided into two portions, the spore bearing (_x_) and the green vegetative part. In _Botrychium_ the leaves are more or less deeply divided, and the sporangia distinct (Fig. 71, _B_). In _Ophioglossum_ the sterile division of the leaf is usually smooth and undivided, and the spore-bearing division forms a sort of spike, and the sporangia are much less distinct. The sporangia in both differ essentially from those of the true ferns in not being derived from a single epidermal cell, but are developed in part from the ground tissue of the leaf. [Illustration: FIG. 70.--Forms of ferns. _A_, grape fern (_Botrychium_), x 1/2. _x_, fertile part of the leaf. _B_, sporangia of _Botrychium_, x 3. _C_, flowering fern (_Osmunda_). _x_, spore-bearing leaflets, x 1/2. _D_, a sporangium of _Osmunda_, x 25. _r_, ring. _E_, _Polypodium_, x 1. _F_, brake (_Pteris_), x 1. _G_, shield fern (_Aspidium_), x 2. _H_, spleen-wort (_Asplenium_), x 2. _I_, ostrich fern (_Onoclea_), x 1. _J_, the same, with the incurved edges of the leaflet partially raised so as to show the masses of sporangia beneath, x 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 (_Rhizocarpeae_), 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_). [Illustration: FIG. 71.--_A_, _Marsilia_, one of the _Rhizocarpeae_ (after Underwood). _sp._ the "fruits" containing the sporangia. _B_, a small spore of _Pilularia_, with the ripe antheridium protruding, x 180. _C_, male prothallium removed from the spore, x 180. _D_, _Azolla_ (after Sprague), x 1.]
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