h isto r y of BRITISH FERNS.
bursts the case irregularly, and the seeds or spores, in the
shape of fine dust, almost invisible, become dispersed.
This is what occurs in the majority of the native species ; in
Trichomanes and the llymenophyllums, however, the elastic
band is horizontal or oblique; and in Osmunda, Botrychium,
and Ophioglossum, the spore-cases are two-valved, and destitute
of the elastic ring. _
In a considerable proportion of the known species of
Terns, and in the majority of those^ which are natives of
Great Britain, the sori are covered in the earlier stages o
growth by what is commonly called the mdusium, which
Ts mostly a thin transparent membranous scale of the same
o-eneral form as the sorus itself, at first completely covering
or enclosing the young seed-cases. Eventually, however,
by their growth, its margins are disrupted, and it is cast oft,
frequently even before the maturity of the seeds. Some
«species, however, never bear any indusium, and its presence
OT absence is consequently one of the technical points by
which the large body of Terns are divided into groups of
manageable extent. In some Terns the indusium, or cover,
or at least what is considered analogous to it, is cup-shaped,
containing the seed-cases; but this form is of very rare
occurrence among the native species, and exists only m
Trichomanes and the llymenophyllums.
THE STRUCTURE OF FERNS. 19
Taking now a retrospective glance, we have seen 'that
the Terns are, as regards external structure, flowerless
plants, having erect or creeping stems, which bear the leaflike
fronds; and on some part of the surface of the latter,
usually the lower side, but sometimes the margin, are borne
the clusters of seeds, which, in the majority of the native
species, are, when young, furnished with a membranous
scale-like cover.
The subject of internal structure, or anatomy, is foreign
to the purposes of this volume. We may, however, mention
in general terms, that the Terns belong to the lowest
group of vegetation, which is especially remarkable for its
loose and often succulent texture, owing to the absence, or
nearly so, of those tissues which give firmness and elasticity
to the higher orders of plants. The Terns, however,
are the highest members of this group, and hence we find
them possessing, to some extent, both woody and vascular
tissue,—matters which, together with cellular tissue, the
soft loose material above mentioned, maybe found explained
in any elementary book on physiological botany.