[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.]
Family 3. Quercifloræ.
Trees and shrubs with small, unisexual, monœcious flowers, having no perianth or a simple inconspicuous one. The ♂ and ♀ flowers are very different and generally placed in separate inflorescences. The ♂-flowers are most often adnate to the bracts. The stamens are placed opposite the perianth-leaves, when they are present in equal numbers. The ♀-flower is naked, or has a superior perianth. The ovary at the base is 2- or 3-(-6) locular with 1 or 2 pendulous ovules in each loculus, only one of which is developed; the fruit is a one-seeded nut; endosperm absent; embryo straight. The inflorescences, which are either compound and mixed (small dichasia in spikes) or simple, are here also termed catkins; but, strictly speaking, this term is applied to the ♂-inflorescences only. In all Quercifloræ the leaves are scattered (usually in 2 rows) simple, and penninerved, and with deciduous stipules.
It is worthy of remark that in Betulaceæ, Corylaceæ and Quercus the ovules, and to some extent the loculi of the ovary are not developed till after pollination, so that the development of the pollen-tube proceeds very slowly. The smallness of the flowers, the absence of honey, the dryness and lightness of the pollen, the size of the stigma and the abundance of hairs found on many stigmas are all adaptations for wind-pollination. It is also an advantage that the flowers are generally pollinated before the foliage-leaves are developed, thus preventing the pollen being entangled by the leaves.
The two orders Betulaceæ and Corylaceæ mentioned here are by other authors united into one order. [It is doubtful whether these two should be retained in the family Quercifloræ, as recent researches (p. [273]) have shown that they differ from the Cupuliferæ in many important points, and agree with the Casuarinas in the fact that the pollen-tube enters the ovule through the chalaza.]
Order 1. Betulaceæ (Birches). Monœcious, with thick, cylindrical, compound ♂ and ♀ inflorescences (2- or 3-flowered dichasia in a spike with spirally-placed floral-leaves) (Figs. [324], [326], [328]). When the perianth in the ♂-flower is completely developed, it is composed of 4 somewhat united leaves, which are placed opposite the 4 stamens (Figs. [325], [326] A). The female flowers are naked; the ovary is bilocular, with two styles and one pendulous ovule in each loculus. The subtending floral-leaves unite with the bracteoles and form a 3–5-lobed cover-scale, which is not attached to the fruit (Figs. [325] D, [326] B). Fruit a nut without cupule (see Corylaceæ and Cupuliferæ). In the bud the leaves are flat. The stipules are deciduous. On germination the cotyledons are raised above the ground. Terminal buds are only found on old Alder trees; the Birch has sympodial branches.
Fig. 324.—Alnus glutinosus. Branch of Alder with ♂-(n) and ♀-(m) catkins: k bud; b fruit-bearing catkin (“cone.”)
Alnus (Alder) (Figs. [324–326]). In the majority of species the ♂-and ♀-catkins are both developed in the year previous to their flowering, and pass the winter naked and bloom before the leaves expand. ♂-flower: 4 stamens. ♀-flower: the 5-lobed cover-scales of the ♀-catkin are woody and remain attached to the axis, so that the entire catkin when ripe resembles a small cone (Fig. [324] b). Each cover-scale supports two winged or wingless nuts. In the native species of Alder the buds are stalked (Fig. [324] k). The bud-scales are formed by the stipules of the lowest leaves.
Betula (Birch). The ♂-catkins, in the native species, appear in autumn, the ♀-catkins in the flowering year on leaf-bearing, short-lived shoots. ♂-flowers: 2 stamens, divided (Fig. [328] A). The 3-lobed cover-scales (Fig. [327] a) of the ♀-catkin are detached from the axis; each cover-scale supports 3 broadly winged nuts (b). The stem has cork with annual rings. The young twigs and leaves have aromatic resin glands.