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.]