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Encyclopedia Britannica - Main :: PRE-PYR |
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PRIMULACEAE , in botany, an order of Gamopetalous Dicotyledons belonging to the series Primulales and containing 28 genera with about 350 species. It is cosmopolitan in distribution, but the majority of the species are confined to the temperate and colder parts of the northern hemisphere and many are arctic or alpine. Eight genera are represented in the British flora.The plants are herbs, sometimes annual as in pimpernel (Anagallis arvensis) (fig. 1), but generally perennial as in Primula, (After Wossidlo. From Strasburger's Lehrbuch der Bolanik, by permission of Gustav Fischer.) 1, Flowering branch. 3, Capsule. 2, A flower cut through longi- 4, Seed. tudinally, showing the central 2, 3, 4, Enlarged. placenta
where the plant persists by means of a sympodial rhizome, or in Cyclamen by means of a tuber formed from the swollen hypocotyl. The leaves form a radical rosette as in Primula (primrose, cowslip, &c.), or there is a well-developed aerial stem which is erect, as in species of Lysimachia, or creeping, as in Lysimachia Nummularia (creeping jenny or money-wort). Hottonia (water violet) is a floating water plant with submerged leaves cut into fine linear segments. The leaves are generally simple, often with a toothed margin; their arrangement is alternate, opposite or whorled, all three forms occurring in one and the same genus Lysimachia. The flowers
flowers
fives
spreading, as in primrose, or reflexed, as in Diagram of a typical Cyclamen; in Soldanella it is bell-shaped; in sower of Pruuula- Lysimachia the tube is often very short, the ceae petals appearing almost free; in Glaux the petals are absent. The five stamens spring from the corolla-tube end areopposite to its lobes; this anomalous position is generally explained by assuming that an outer whorl of stamens opposite the sepals has disappeared, though sometimes represented by scales as in Samolus and Soldanella. Another explanation is based on the late
appearance of the petals in the floral development and their origin from the backs of the primordia of the stamens; it is then assumed that three alternating whorls only are present, namely, sepals, stamens bearing petal-like dorsal outgrowths, and carpels. The superior ovaryhalf-inferior in Samolusbears a simple style ending in a capitate entire stigma, and contains a free-central placenta
Cross pollination is often favoured by dimorphism of the flower, as shown in species of Primula (fig. 3). The two forms have long and short styles repectively, the stamens occupying corresponding positions half-way down or at the mouth of the corolla-tube; the long-styled flowers have smaller pollen -grains, which correspond with smaller stigmatic papillae on the short styles.The order is divided into five tribes by characters based on differences in position of the ovuleswhich are generally semianatropous so that the seed is peltate with the hilum in the centre on one side (or ventral), but sometimes, as in Hottonia and (From Strasburger's Lehrbuch der Botanik.) L, Long-styled flowers. P, Pollen grains, and N, stig-K, Short-styled flowers. matic papillae of long-styled G, Style. form. S, Anthers. p, n, Ditto of short-styled form. (P, N, p, n.) Samolus, anatropous with the hilum basaltogether with the method of dehiscence of the capsule and the relative position of the ovary. The chief
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