This class, independent of extinct forms, comprises the most highly
developed of the Gymnosperms, partly from the circumstance that a
perianth of 2–4 members encloses the _terminally placed ovule_, which
is provided with one, or (in _Gnetum_) two, integuments, and partly
owing to the fact that the wood has true vessels. There is only one
order.
[Illustration: FIG. 275.--_Welwitschia mirabilis_ (considerably
reduced). The horizontal lines indicate the surface of the soil.]
Order. =Gnetaceæ.= The three known genera differ very much in
appearance. _Welwitschia mirabilis_ (from the deserts of South
Western Africa) is the oldest (?) genus now living. It resembles
a giant radish, in that the hypocotyl is the only part of the
main axis of the stem which becomes developed. It attains a
circumference of upwards of four metres with a length of 1/2½-⅔
of a metre. It bears _only_ two oblong, leathery leaves (Fig.
275) which are torn into segments at the apex and lie on the
surface of the soil; these are the two first foliage-leaves
which succeed the cotyledons, and they are remarkable for
their enormous length (upwards of two metres) as well as for
their long duration, living as long as the plant itself. In
their axils are situated the 4-rowed, spike-like male and
scarlet-coloured female cones, upon dichotomous branches. The
perianth consists in the ♂ of 2 alternating pairs of leaves,
the inner ones of which are slightly united. The andrœcium
likewise consists of 2 whorls: the external (transverse) with
2, the internal with 4 stamens; the lower halves of the 6
filaments uniting to form a cup. Each of the terminal anthers
corresponds to a sorus of 3 sporangia, the sporangia being fused
together, and opening at the top by _one_ three-rayed cleft.
In the centre of the ♂-flower there is a sterile ovule. In the
♀-flower a perianth of two connate leaves is present.--_Ephedra_
(desert plants, especially in the Mediterranean and W. Asia)
at first sight resembles an _Equisetum_; the stems are thin,
long-jointed, and the leaves opposite, small, and united into
a bidentate sheath; ♂-perianth of two connate leaves (median
leaves); 2–8 stamens united into a column. Each anther is formed
of 2 sporangia (is bilocular). ♀ mainly, as in _Welwitschia_.
The seeds are surrounded by the perianth which finally becomes
red and fleshy. There are 30 species.--_Gnetum_ has opposite,
lanceolate, pinnately-veined, leathery leaves. They are mostly
climbers (Lianas) from Tropical Asia and America. The ♂-flowers
have a tubular perianth, (formed from two median leaves) which
surrounds a centrally-placed filament, bearing 2 anthers. In
the ♀-flower there is a similar perianth, surrounding an ovule
provided with 2 integuments. The perianth becomes fleshy and
envelops the hard seed. 20 species.
From the circumstance of _Welwitschia_ having ♂ flowers
which, besides stamens, possess also a rudiment of an ovule,
Celakovsky draws the inference that the earliest Gymnosperms
had hermaphrodite flowers which from this structure became
differentiated entirely into ♂-and ♀-flowers, with the exception
of _Welwitschia_ only, in which this differentiation was only
carried out in the ♀-flower. This theory has so far been
scarcely proved.
=Fossil Gymnosperms.=
The earliest continental plants which are known belong to the
CORDAITACEÆ, a group of plants which existed as early as the
Silurian period; they were Gymnosperms, but it has not yet been
determined whether they were Cycads or Conifers. The CYCADS,
even in the Coal period, were scarce; they attained their
fullest development in Jurassic and Cretaceous periods, during
which they were rich in species and genera, and extended as far
as the Polar regions. In addition to these, Taxaceæ, Abietaceæ,
and Taxodiaceæ appeared in the Carboniferous period. The TAXACEÆ
appear to have attained their culmination in the Jurassic and
Cretaceous periods; _Ginkgo_ appears in the Rhætic; _Torreya_,
in the Cretaceous; _Taxus_ and _Podocarpus_ in the Tertiary
periods. The ABIETACEÆ also appear in the Carboniferous;
_Pinus_ was first known with certainty in the English Weald
and in the Cretaceous; almost all other contemporary genera
are represented in this latter period. The ARAUCARIACEÆ first
appear, with certainty, in the Jurassic. The TAXODIACEÆ may
be traced back as far as the Carboniferous (?); _Sequoia_ is
first found in the lowest Cretaceous, at that period it spread
throughout the entire Arctic zone, and being represented by a
large number of species, formed an essential part of the forest
vegetation. _Sequoia_ played a similar part in the Tertiary
period. The CUPRESSACEÆ are first known with certainty in the
Jurassic, but they appeared more frequently and numerously in
the Tertiary period, in which most of the present living genera
were to be found. The GNETACEÆ, according to a theory advanced
by Renault were represented in the Coal period by the genus
_Stephanospermum_, which had four ovules enclosed by an envelope.