Order 5. Rhamnaceæ. The stamens are placed opposite the petals as in the Ampelidaceæ (Fig. [482]), but the flowers are much more perigynous or entirely epigynous. The trees and shrubs belonging to this order have simple, most frequently penninerved leaves with stipules; frequently thorny (modified branches). The flowers are inconspicuous, sometimes unisexual (Fig. [482]), and have 5 (-4) sepals, petals, stamens, and generally 3 (2–5) carpels. The calyx has valvate æstivation. The petals are very small (generally less than the sepals), often spoon-like, hollow, and embracing the stamens; a disc covers the inner surface of the thalamus or the base of the style in the epigynous flower; gynœceum simple, with one style and one erect ovule in each loculus. The fruit is most frequently a drupe. The embryo is large, often green or yellow, with endosperm.
Rhamnus (Buckthorn) has a juicy drupe with 3 (2–4) stones, surrounded at the base by the persistent portion of the receptacle; the disc is thin. R. cathartica (common Buckthorn): diœcious, with opposite, serrate leaves. R. frangula (Alder Buckthorn): flowers ☿, with scattered, entire leaves.—Ceanothus (N. Am., with richly-flowered inflorescences and a fruit closely resembling that of the Euphorbias). Phylica, Pomaderris (Austr., fruit a capsule). Zizyphus, Paliurus, Colletia (S. Am.) are thorny shrubs; C. spinosa has thorny shoots with small, caducous leaves; the seedling has normal foliage-leaves. Others climb by tendrils as in the Ampelidaceæ, e.g. Gouania.
Fig. 482.—Rhamnus cathartica: A long-styled ♂-flower; pet petals; B short-styled ♂-flower; C long-styled ♀-flower; D short-styled ♀-flower (after Darwin).
475 species, 40 genera; chiefly in temperate and tropical climes. Some are medicinal plants, the bark and fruit having purgative properties (the bark of Rhamnus frangula and “Cascara Sagrada” from the bark of R. purshiana are officinal). The fruits and seeds of others are edible, for example, the fruits of Zizyphus lotus, Z. vulgaris, Z. spina Christi, etc. Green and yellow dyes are obtained from the fruit of R. cathartica, infectoria and others (Avignon grain). Ceanothus-, Rhamnus- and evergreen Phylica-species are ornamental shrubs.
Family 18. Thymelæinæ.
Exclusively trees or shrubs with simple, entire, scattered leaves without stipules. They have a strongly perigynous, regular, 4-merous flower. The receptacle (often coloured) envelopes a simple gynœceum formed of 1 carpel and with, in most cases, 1 ovule, bearing on its edge 4 (or 5) petaloid sepals and, but rarely at the same time, small, scale-like petals. The corolla is most frequently entirely wanting (and hence these plants were formerly reckoned among the Monochlamydeæ); frequently only one of the 2 whorls of stamens, which are situated on the inner side of the edge of the receptacle, is developed. The fruit is most frequently a 1-seeded berry or drupe, or a nut which may be falsely berry-like, the partly persistent receptacle being fleshy and enveloping it.
This family appears the most nearly allied to the Frangulinæ, especially the Rhamnaceæ, and may be considered as a further development of these in the direction of the petaloid development of the receptacle and reduction of the corolla and gynœceum, which in this instance only consists of one carpel. Another deviation is that both the whorls of stamens are present, while one of these is always wanting in Frangulinæ. They also appear to be related to the Lauraceæ (see page [391]).
Order 1. Thymelæaceæ. The flowers are most frequently ☿ (Fig. [483]). The receptacle is high, generally tubular, coloured, and bears on its edge the 4-(or 5)-merous calyx, with imbricate æstivation. The corolla is wanting or is represented by small scales. The stamens are situated on the inside of the receptacle, and number 4 + 4 (or 5 + 5); stigma capitate. 1 pendulous ovule (Fig. [483] B), the radicle pointing upwards. The fruit is most frequently a berry. A disc is sometimes developed. Endosperm wanting or very slight.
