The following characters should be added to those already given on page
2:--
The Gymnosperms comprise only trees or shrubs. The flowers are always
_unisexual_ and destitute of perianth (except _Gnetaceæ_); the female
plant of _Cycas_ is the only one which has no flower. The MALE FLOWERS
are constructed on the same type as the cones of the Horsetails and
Club-Mosses, and are _most frequently long shoots_ (Figs. 243, 258, 260
_A_, 267 _J_) bearing a number of spiral or verticillate stamens. The
FEMALE FLOWERS are of a more varied structure (see the orders). The
OVULE _is orthotropous_ (except _Podocarpus_ which is anatropous) and
projects from the carpel uprightly, inverted, or horizontally; it has
usually _only one integument_ (compare however Taxaceæ) which proceeds
from the upper part of the nucellus, so that the embryo-sac in part is
placed below the integuments (Figs. 251, 264). The drop of mucilage
which catches the pollen-grains dries up and draws the pollen-grain
through the micropyle to a space just above the nucellus--_the
pollen-chamber_--in which the germination of the pollen-grain commences.
In each seed, only one of the many embryos which are formed proceeds to
its full development. The seed is always _endospermous_, and the embryo
has one, two, or a whorl of several cotyledons. A vigorous primary
root is developed on germination. THE VASCULAR BUNDLES in the stem are
arranged in a ring, and _increase in thickness_ takes place by a closed
cambium-ring which forms bast (_phlœem_) externally, and wood (_xylem_)
internally with distinct annual rings, _as in the Dicotyledons_. Only
certain of the Cycadeæ deviate from this arrangement. The _secondary
wood_ is very uniform, as it is almost exclusively _formed of
tracheides_ with bordered pits, but _true vessels are wanting_; this
also indicates a relationship with the Pteridophyta (see page 202).
The Gymnosperms are biologically lower than the Angiosperms; they are
wind-fertilised, and without extra floral-nectaries.
This Division embraces three classes: CYCADEÆ, CONIFERÆ, And GNETEÆ. It
is no doubt monophyletic, and has taken its origin from heterosporous
Ferns, now extinct, most nearly related to the Ophioglossaceæ and
Marattiaceæ. The Cycadeæ appear to be the oldest class. The Coniferæ
are related to these through Ginkgo. The Gnetaceæ are more isolated.
The Division is not continued into the higher Flowering-plants; it has
evidently attained its highest development, and is now in a retrograde
condition. The similarity which has often been pointed out between
certain Coniferæ and Lycopodinæ is only in analogous resemblances, and
does not entitle one to suppose that there is a nearer relationship, or
that the former take their origin from the latter.