1911 Encyclopædia Britannica/Saxifragaceae

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SAXIFRAGACEAE, in botany, a small natural order of Dicotyledons belonging to the sub-class Polypetalae and containing 27 genera with about 350 species distributed through the Arctic and north temperate zone, often alpine. It is represented in Britain by its largest genus Saxifrage (q.v.), Chrysosplenium (golden saxifrage) and Parnassia (grass of Parnassus). The plants are herbs, generally with scattered exstipulate leaves with a broad leaf-base. The small flowers are generally arranged in cymose inflorescences and are bisexual, regular and hypogynous, perigynous or more frequently more or less epigynous, this variation in the relative position of the ovary occurring in one and the same genus Saxifraga (fig. 1). The flowers are 5-merous, more rarely 4-merous, having 5 (or 4) sepals, 5 (or 4) free petals, two 5- or 4-merous whorls of free stamens which are obdiplostemonous, i.e. those of the outer whorl are opposite to the petals, and two carpels (see fig. 2). The carpels are sometimes free, more generally united at the base, or sometimes completely joined to form a one- or two-chambered ovary with two free styles. The fruit is a many-seeded capsule.

Fig. 1. — Saxifraga umbrosa, London Pride, about half natural size, 1, Flower enlarged. 2, Vertical section of ovary with sepals, more enlarged.

Fig. 2. — Diagram of a saxifrage (Saxifraga tridactylites). The calyx and corolla each consist of five parts, there are ten stamens in two series, and a pistil of two carpels.

More than half the species (200) are contained in the genus Saxifrage (q.v.). Chrysosplenium, with 39 species, two of which are British, has a very similar distribution. The North American genus Heuchera has sometimes apetalous flowers. Astilbe has 6 species in temperate Asia and north-eastern North America; A. japonica is commonly grown in the spring as a pot-plant, and often misnamed Spiraea.

The order is frequently much extended to include other groups of genera differing in habit and more or less in general conformation from those to which the order is here confined, and which are then regarded as forming one of several tribes. Among these is the order Ribesiaceae, comprising one single genus Ribes, to which belong the gooseberry (R. Grossularia) and currants of gardens. These are shrubs with racemes of flowers which have only one whorl of stamens (isostemonous), an inferior unilocular ovary with two parietal placentas, and fruit a berry. Another is the Hydrangeaceae, to which belong Hydrangea (q.v.), Deutzia and Philadelphus, all well-known garden plants; P. coronarius is the so-called Syringa or mock-orange. They are shrubs or trees with simple generally opposite leaves, 5-merous flowers with epigynous stamens and a 3- to 5-locular ovary.

Escallonia, which represents a small group of genera with leathery gland-dotted leaves, is also often included.