of the
Pterides,--Equiseta and Lycopodia,--plants which, in most of our modern
treatises on the ferns proper, take their place as the fern allies. (See
Fig. 148.) Let us place these along two of the sides of a pentagon,--the
Lycopodia on the right side of the ferns, the Equiseta on the left;
further, let us occupy the two remaining sides of the figure by the
Coniferae and the Cycadaceae,--placing the Coniferae on the side next the
Lycopodia, and the Cycadaceae, as the last added keystone of the
erection, between these and the Equiseta. And now, let us consider how
very curious the links are which give a wonderful unity to the whole. We
still find great difficulty in distinguishing between the foliage of
some of even the existing club mosses and the conifers; and the ancient
Lepidodendra are very generally recognized as of a type intermediate
between the two. Similar intermediate types, exemplified by extinct
families, united the conifers and the ferns. The analogy of _Kirchneria_
with the _Thinnfeldia_, says Dr. Braun, is very remarkable,
notwithstanding that the former is a fern, and that the latter is ranked
among conifers. The points of resemblance borne by the conifers to the
huge Equiseta of the Oolitic period seem to have been equally striking.
The pores which traverse longitudinally the channelled grooves by which
the stems of our recent Equiseta are so delicately fluted, are said
considerably more to resemble the discs of pines and araucarians than
ordinary stomata. Mr. Francis does not hesitate to say, in his work on
British Ferns, that the relation of this special family to the Coniferae
is so strong, both in external and internal structure, that it is not
without some hesitation he places them among the fern allies; and it has
been ascertained by Mr. Dawes, in his researches regarding the calamite,
that in its internal structure this apparent representative of Equiseta
in the earlier ages of the world united "a network of quadrangular
tissue similar to that of Coniferae to other quadrangular cells arranged
in perpendicular series," like the cells of plants of a humbler order.
The relations of the Cycadacean order to ferns on the one hand, and to
the Coniferae on the other, are equally well marked. As in the ferns, the
venation of its fronds is circinate, or scroll-like,--they have in
several respects a resembling structure,--in at least one recent species
they have a nearly identical form; and fronds of this fe