Trees with verticillate, scale-like leaves forming sheaths at the
nodes. Monœcious. Flowers unisexual. ♂-flowers in catkins; ♀ in short
spikes. _Pollen-tube entering the ovule at the chalaza_, and not
through the micropyle. Ovary 1-seeded, unilocular. Carpels uniting into
a multiple fruit. Only one order.
Order. =Casuarinaceæ.= Trees (30 species), from Australia and certain
parts of S.E. Asia, with peculiar, equisetum-like appearance. The
leaves are verticillate, scale-like and united into sheaths. The
internodes are furrowed. Branching verticillate. The unisexual flowers
are situated in catkins or short spikes. The ♂-flower has a central
stamen, surrounded by 2 median, scale-like perianth-leaves and 2
lateral bracteoles. The ♀-flower has a 1-chambered ovary (2 ascending,
orthotropous ovules), no perianth, but 2 large, lateral bracteoles
which finally become woody and form two valves, between which the
nut-like fruit is situated. The multiple-fruits therefore resemble
small cones.--_Casuarina equisetifolia_, cultivated, gives “iron-wood.”
[The Casuarinas differ from the ordinary Dicotyledons in many
important respects which may be briefly summarised thus:--The
bicarpellate ♀-flower has a well-pronounced stylar-cylinder
terminated by two stigmas, but the cavity of the ovary closes
very soon after its formation, and in it are developed two
parietal ovules; these are united by a bridge of cellulose to
the stylar-cylinder or summit of the ovary, and hence the ovules
are connected with the walls of the ovary by the bridge (above),
as well as by the funicle (below). The archespore is developed
from the hypodermal cells at the summit of the nucellus,
two primordial mother-cells are first formed and from these
by tangential divisions a central cylindrical mass of cells
(sporogenous-tissue) is produced which is surrounded by tapetal
cells. The cells of the sporogenous tissue correspond to the
mother-cells of the embryo-sac of other Angiosperms; they divide
transversely and from 16–20 macrospores are formed together with
inactive cells which are not crushed together as in the case
of other Phanerogams. The sexual apparatus is developed from a
single cell, but the number of cells composing this apparatus
is subject to variation, the oosphere being accompanied by one
or two neighbouring cells which resemble canal-cells rather
than synergidæ. The sexual apparatus is found in the majority
of the macrospores, but in most of these it remains as a number
of naked cells; while in the fertile macrospores the cells
are invested by walls of cellulose (usually only one fertile
macrospore is found in each ovule). Antipodal cells are never
developed. The macrospores elongate considerably towards the
chalaza, into which some penetrate. The pollen-tube traverses
the stylar-cylinder and enters the ovules at the chalaza, its
passage through the tissue of the nucellus being assisted by
the prolongation of the macrospores. About the centre of the
nucellus the pollen-tube is ruptured; the apical portion which
alone takes part in the fertilisation being firmly attached to
the macrospore. Although the actual impregnation has not been
observed, Treub considers that the endosperm begins to be formed
before fertilisation.]