Fig. 540.—Flower of Aristolochia clematitis (long. sect.). A Before pollination, and B after: n stigma; a anthers; t an insect; kf ovary.
Order 1. Aristolochiaceæ. The majority are perennial herbs or twining shrubs, whose stalked, simple, and generally more or less cordate or reniform leaves are borne in 2 rows and are exstipulate. The flowers are hermaphrodite, epigynous, regular or zygomorphic; perianth-leaves united, simple but most frequently petaloid and 3-merous; 6 or 12 (in Thottea as many as 36) stamens with extrorse anthers. The ovary is more or less completely 4–6-locular with ovules attached in the inner angles of the loculi (Fig. [540] kf). The style is short, and has a large, radiating stigma (Fig. [540] n). Fruit a capsule. Seeds rich in endosperm.
Asarum europæum. Each shoot has 2 reniform foliage-leaves, between which the terminal flower is borne (the rhizome becomes a sympodium by development of the bud in the axil of the upper foliage-leaf). The flower is regular and has a bell-shaped perianth with 3 outer valvate, and 3 inner small segments (which may be wanting). 12 (2 × 6) free, extrorse stamens, 6 carpels.—Aristolochia clematitis (Birth-wort) has an erect, unbranched stem, bearing many flowers in the leaf-axils, in a zig-zag row (accessory buds in a unipared scorpioid cyme). The flowers are zygomorphic (Fig. [540]), formed by 3 alternating, 6-merous whorls. The perianth has a lower, much-distended part (k), succeeded by a narrow, bent tube (r), which passes over into an oblique, almost tongue-like projection (6 vascular bundles indicate that the number 6 is prevalent here, as in Asarum); 6 stamens (Fig. [540] a), with the dorsal portion turned upwards, are united with the short style to form a stylar column; they are placed quite beneath the 6 commissural stigmatic rays, which arch over them as short, thick lobes. Protogynous; Pollination is effected in Arist. clematitis by small flies; these enter the erect unfertilised flower through the tube (Fig. [540] A, l) without being prevented by the stiff, downwardly-turned hairs which line the tube and prevent their escape; they find the stigma (n) fully developed, and may pollinate it with the pollen they have brought with them. The stigmas then straighten and wither (B, n), the anthers open, and the flies may again be covered with pollen; but the hairs which blocked up the tube do not wither until the anthers have shed their pollen, and only then allow the imprisoned flies to escape and effect cross-pollination. Prior to pollination, the flowers stand erect, but after this has taken place they become pendulous, and the perianth soon withers.—A. sipho (Pipe-flower), another species, is a climber, and often grown in gardens; it has only one row of accessory buds in the leaf-axils.—200 species; chiefly in S. Am. Officinal: the rhizome of Aristolochia serpentaria (N. Am.).
Fig. 541.—A fruit of Myzodendron brachystachyum (slightly mag.) germinating on a branch.
Order 2. Santalaceæ. Parasites containing chlorophyll, which, by the help of peculiar organs of suction (haustoria) on their roots, live principally on the roots of other plants. Some are herbs, others under-shrubs. The regular, most frequently ☿-flowers have a simple perianth, which is gamophyllous, 3- or 5 partite with the segments valvate in the bud, and a corresponding number of stamens opposite the perianth-leaves. In the inferior ovary there is a free, centrally placed, often long and curved placenta with three ovules (one opposite each carpel); these are naked, or in any case have an extremely insignificant integument. Fruit a nut or drupe. Seed without testa. Endosperm fleshy. 225 species; chiefly in the Tropics.—Thesium, a native, is a herb with scattered, linear leaves and small 5-merous flowers (P5, A5, G3) in erect racemes; the subtending bracts are displaced on the flower-stalks. Fruit a nut.—Osyris (diœcious shrub; 3-merous flowers) is another European genus.—Santalum album, which grows in E. Ind., yields the valuable, scented Sandalwood, the oil of which is used medicinally.—Quinchamalium.
Myzodendron is a reduced form of the Santalaceæ; the ♂-flowers are without perianth; the perianth of the ♀-flower is 3-merous. About 7 species; S. Am.; parasitic on a Beech (Nothofagus). The fruit has 3 feathery brushes, alternating with the lobes of the stigma, which serve as flying organs and to attach the fruits to a branch (Fig. [541]), the brushes twining round as soon as they come in contact with it. There is only 1 seed in the fruit, which germinates by a long, negatively heliotropic hypocotyl, and is attached by a radicle modified into an haustorium.
Order 3. Loranthaceæ (Mistletoes). Plants containing chlorophyll which are parasites on trees, and most frequently have opposite, simple, entire leaves and regular, epigynous, often unisexual, 2- or 3-merous flowers, with single or double perianth. Stamens equal in number and opposite to the perianth-leaves, free, or in varying degrees united to one another. The inferior ovary is constructed as in the Santalaceæ, the ovules being situated on a low, free, centrally-placed placenta, but the placenta and ovules unite with the wall of the ovary into one connected, parenchymatous mass, in which the embryo-sacs are imbedded. Only 1 (less frequently 2–3) of the 1–6 embryo-sacs is fertile. The number of the carpels however varies. The fruit is a 1-seeded berry, whose inner layer is changed into a tough slimy mass (bird-lime), which serves to attach the fruits to other plants.
The two groups, Loranthoideæ and Viscoideæ, are distinguished by the fact that the former has a distinct “calyculus,” i.e. an entire or lobed, or dentate swelling on the receptacle below the perianth. The majority of the Loranthoideæ have a petaloid perianth; in all the Viscoideæ, on the other hand, it is sepaloid.
