1911 Encyclopædia Britannica/Geraniaceae
GERANIACEAE, in botany, a small but very widely distributed natural order of Dicotyledons belonging to the subclass Polypetalae, containing about 360 species in 11 genera. It is represented in Britain by two genera, Geranium (crane’s-bill) and Erodium (stork’s-bill), to which belong nearly two-thirds of the total number of species. The plants are mostly herbs, rarely becoming shrubby, with generally simple glandular hairs on the stem and leaves. The opposite or alternate leaves have a pair of small stipules at the base of the stalk and a palminerved blade. The flowers, which are generally arranged in a cymose inflorescence, are hermaphrodite, hypogynous, and, except in Pelargonium, regular. The parts are arranged in fives. There are five free sepals, overlapping in the bud, and, alternating with these, five free petals. In Pelargonium the flower is zygomorphic with a spurred posterior sepal and the petals differing in size or shape. In Geranium the stamens are obdiplostemonous, i.e. an outer whorl of five opposite the petals alternates with an inner whorl of five opposite the sepals; at the base of each of the antisepalous stamens is a honey-gland. In Erodium the members of the outer whorl are reduced to scale-like structures (staminodes), and in Pelargonium from two to seven only are fertile. There is no satisfactory explanation of this break in the regular alternation of successive whorls; the outer whorl of stamens arises in course of development before the inner, so that there is no question of subsequent displacement. There are five, or sometimes fewer, carpels, which unite to form an ovary with as many chambers, in each of which are one or two, rarely more, pendulous anatropous ovules, attached to the central column in such a way that the micropyle points outwards and the raphe is turned towards the placenta. The long beak-like style divides at the top into a corresponding number of slender stigmas.
|Meadow Crane’s-bill, Geranium pratense. (After Curtis, Flora Londinensis.)|
| 1, Flower after removal of petals.
2, Fruit after splitting. 1 and 2
about natural size.
|3, Floral diagram, the dots|
opposite the inner stamens
The larger-flowered species of Geranium are markedly protandrous, the outer stamens, inner stamens and stigmas becoming functional in succession. For instance, in meadow crane’s-bill G. pratense, each whorl of stamens ripens in turn, becoming erect and shedding their pollen; as the anthers wither the filaments bend outwards, and when all the anthers have diverged the stigmas become mature and ready for pollination. By this arrangement self-pollination is prevented and cross-pollination ensured by the visits of bees which come for the honey secreted by the glands at the base of the inner stamens.
In species with smaller and less conspicuous flowers, such as G. molle, the flowers of which are only 13 to 12 in. in diameter, self-pollination is rendered possible, since the divisions of the stigma begin to separate before the outer stamens have shed all their pollen; the nearness of the stigmas to the dehiscing anthers favours self-pollination.
In the ripe fruit the carpels separate into five one-seeded portions (cocci), which break away from the central column, either rolling elastically outwards and upwards or becoming spirally twisted. In most species of Geranium the cocci split open on the inside and the seeds are shot out by the elastic uptwisting (fig. 1); in Erodium and Pelargonium each coccus remains closed, and the long twisted upper portion separates from the central column, forming an awn, the distribution of which is favoured by the presence of bristles or hairs. The embryo generally fills the seed, and the cotyledons are rolled or folded on each other.
Geranium is the most widely distributed genus; it has 160 species and is spread over all temperate regions with a few species in the tropics. Three British species—G. sylvaticum, G. pratense and G. Robertianum (herb-Robert)—reach the arctic zone, while G. patagonicum and G. magellanicum are found in the antarctic. Erodium contains 50 species (three are British), most of which are confined to the Mediterranean region and west Asia, though others occur in America, in South Africa and West Australia. Pelargonium, with 175 species, has its centre in South Africa; the well-known garden and greenhouse “geraniums” are species of Pelargonium (see Geranium).