Fig. 473.—Seed in longitudinal section.
Æsculus (Horse-Chestnut). Trees with opposite, digitate, dentate leaves without stipules; the inflorescence is composed of unipared scorpioid cymes arranged in a pyramidal panicle (termed a thyrsus). The flowers are irregular, with an oblique plane of symmetry (through the 4th sepal, Fig. [471]); there are 5 sepals, 5 free petals, of which the one lying between S3 and S5 is the smallest (see Fig. [471]) and may be absent; stamens 7 (5 + 2), three being suppressed; gynœceum simple, 3-carpellary and 3-locular, with single style; of the two ovules one is ascending, the other descending (Fig. [472]).—The fruit is a 3-valvate, sometimes spiny, capsule, with loculicidal dehiscence, the seed having a large hilum, a curved embryo without endosperm and united cotyledons (the radicle lies in a fold of the testa, Fig. [473]). Æ. hippocastanum (Greece, Asia), introduced into cultivation about 300 years ago; the majority of the other species, e.g. Æ. pavia, etc., several of which are frequently cultivated in gardens, are from N. America. The flower of the Horse-Chestnut is adapted for bees, whose abdomen touches the anthers or style when visiting the flower. The flowers are protogynous.
The other Sapindaceæ have most frequently 4 sepals, 8 stamens, various fruits (septicidal capsule, nuts with or without wings, schizocarp), etc. Serjania, Cardiospermum, Sapindus, Koelreuteria, etc. (about 118 genera, 970 species). The seeds of Paullinia sorbilis contain caffeine, and are used as “Pasta guaranà,” in the North Western Brazils in the manufacture of a common drink. Nephelium (or Euphoria) litchi (with edible aril), and other species, from Asia.
Fig. 474.—Samara of Acer platanoides.
Order 4. Aceraceæ. This order is so closely allied to the Sapindaceæ, that some authorities have classed it with them. The main difference is in the regularity of the flowers, and the 2-merous gynœceum (in abnormal cases several carpels occur).—They are trees, and, like the Horse-Chestnuts, have opposite leaves without stipules; in Acer the leaves are palminerved, but imparipinnate in Negundo, a plant frequently cultivated in gardens. The flowers are often unisexual, polygamous (some species have ☿-, ♂-and ♀-flowers); sepals 5, petals 5 free, stamens 8 (that is, 5 + 5, but the two median ones are absent) inside a large disc. Fruit a samara (schizocarp) with 2 winged, nut-like fruitlets (Fig. [474]). In each of the 2 loculi of the ovary are 2 ovules. Embryo curved, with thin, folded cotyledons. Endosperm absent.—The inflorescences are racemes with a more or less elongated main axis and terminal flower (which sometimes has 10 stamens); when the lateral branches are developed they are similar to the main axis. In some species both corolla and petal-stamens are suppressed. Acer is pollinated by insects, Negundo by the wind.—88 species; North Temperate zone. Acer in the Tertiary from the Oligocene. The following are native plants: Maple (Acer campestre), Sycamore (A. pseudoplatanus, doubtful native). Important as avenue trees and timber. Sugar is obtained from the spring sap of the Sugar Maple (N. Am.).
Order 5. Malpighiaceæ. A tropical (especially American) order closely related to the Aceraceæ, having often the same form of fruit (but 3-partite). Some species are lianes with anomalous stem-structure. Leaves opposite. The flowers are regular or obliquely zygomorphic (the plane of symmetry passing through sepal 3), with S5, P5, A5 + 5, G3; 1 pendulous ovule in each loculus. Important characteristics for identification are the numerous grandular structures on the sepals. Peculiar 2-spined hairs are found in some. Malpighia, Bunchosia, Galphimia, Tetrapterys, Heteropterys, etc.—About 600 species.
Order 6. Erythroxylaceæ. Sepals 5, petals 5 (with a ligular corona), 10 stamens in one bundle. Gynœceum 3-locular. Fruit a drupe. Tropical (especially American) trees and shrubs, the Coca-plant (Erythroxylon coca) being best known. Its leaves are considered by the inhabitants of Chile and Peru to be one of the indispensable necessaries of life; they are chewed, and possess intoxicating, exhilarating properties, and contain the alkaloid cocaine, which is frequently employed as a local anæsthetic.—103 species; chiefly in America.
Order 7. Vochysiaceæ. Trees; Trop. Am. 1 stamen.—140 species.