for which Arber has proposed the collective
name Primofilices); the best known of these are referred to the family
Botryopterideae, consisting of plants of small or moderate dimensions,
with, on the whole, a simple anatomical structure, in certain cases
actually simpler than that of any recent Ferns. On the other hand the
sporangia of these plants were usually borne on special fertile fronds,
a mark of rather high differentiation. This group goes back to the
Devonian and includes some of the earliest types of Fern with which we
are acquainted. It is probable that the Primofilices (though not the
particular family Botryopterideae) represent the stock from which the
various families of modern Ferns, already developed in the Mesozoic
period, may have sprung.

None of the early Ferns show any clear approach to other classes of
Vascular Cryptogams; so far as the fossil record affords any evidence,
Ferns have always been plants with relatively large and usually compound
leaves. There is no indication of their derivation from a microphyllous
ancestry, though, as we shall see, there is some slight evidence for the
converse hypothesis. Whatever the origin of the Ferns may have been it
is hidden in the older rocks.

It has, however, been held that certain other Cryptogamic phyla had
a common origin with the Ferns. The Equisetales are at present a
well-defined group; even in the rich Palaeozoic floras the habit,
anatomy and reproductive characters usually render the members of this
class unmistakable, in spite of the great development and stature which
they then attained. It is interesting, however, to find that in the
oldest known representatives of the Equisetales the leaves were highly
developed and dichotomously divided, thus differing greatly from the
mere scale-leaves of the recent Horsetails, or even from the simple
linear leaves of the later Calamites. The early members of the class, in
their forked leaves, and in anatomical characters, show an approximation
to the Sphenophyllales, which are chiefly represented by the large genus
Sphenophyllum, ranging through the Palaeozoic from the Middle Devonian
onwards. These were plants with rather slender, ribbed stems, bearing
whorls of wedge-shaped or deeply forked leaves, six being the typical
number in each whorl. From their weak habit it has been conjectured,
with much probability, that they may have been climbing plants, like the
scrambling Bedstraws of our hedgerows. The ana