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_.

CLASS II.--HORSE-TAILS (_Equisetinae_).

The second class of the pteridophytes includes the horse-tails
(_Equisetinae_) of which all living forms belong to a single genus
(_Equisetum_). Formerly they were much more numerous than at present,
remains of many different forms being especially abundant in the coal
formations.

[Illustration: FIG. 72.--_A_, spore-bearing stem of the field
horse-tail (_Equisetum_), x 1. _x_, the spore-bearing cone. _B_,
sterile stem of the same, x 1/2. _C_, underground stem, with tubers
(_o_), x 1/2. _D_, cross-section of an aerial stem, x 5. _f.b._
fibro-vascular bundle. _E_, a single fibro-vascular bundle, x 150.
_tr._ vessels. _F_, a single leaf from the cone, x 5. _G_, the same
cut lengthwise, through a spore sac (_sp._), x 5. _H_, a spore, x 50.
_I_, the same, moistened so that the elaters are coiled up, x 150.
_J_, a male prothallium, x 50. _an._ an antheridium. _K_,
spermatozoids, x 300.]

One of the commonest forms is the field horse-tail (_Equisetum
arvense_), a very abundant and widely distributed species. It grows in
low, moist ground, and is often found in great abundance growing in
the sand or gravel used as "ballast" for railway tracks.

The plant sends up branches of two kinds from a creeping underground
stem that may reach a length of a metre or more. This stem (Fig. 72,
_C_) is distinctly jointed, bearing at each joint a toothed sheath,
best seen in the younger portions, as they are apt to be destroyed in
the older parts. Sometimes attached to this are small tubers (_o_)
which are