1911 Encyclopædia Britannica/Primulaceae

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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 Botanik, by permission of Gustav Fischer.)

Fig. 1.—Anagallis arvensis (pimpernel).
1, Flowering branch. 3, Capsule.
2, A flower cut through longitudinally,   4, Seed.
  showing the central placenta. 2, 3, 4, Enlarged.

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 are solitary' in the leaf-axils as in pimpernel, money-wort, &c., or umbelled as in primrose, where the umbel is sessile, and cowslip, where it is stalked, or in racemes or spikes as in species of Lysimachia. Each flower is subtended by a bract, but there are no bracteoles, and corresponding with the absence of the latter the two first

developed sepals stand right and left (fig. 2).

The flowers are hermaphrodite and regular with parts in fives (pentamerous) throughout, though exceptions from the pentamerous arrangement occur. The sepals are leafy and persistent; the corolla is generally divided into a longer or shorter tube and a limb which is spreading, as in primrose, or reflexed, as in Cyclamen; in Soldanella it is bell-shaped; in Lysimachia the tube is often very short, the petals appearing almost free; in Glaux the petals are absent. The five stamens spring from the corolla-tube and are

Diagram of a typical flower of Primulaceae. opposite 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 primordial 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 ovary-half-inferior in Samolus—bears a simple style ending in a capitate entire stigma, and contains a free-central placenta bearing generally a large number of ovules, which are exceptional in the group Gamopetalae in having two integuments. The fruit is a capsule dehiscing by 5 sometimes 10 teeth or valves, or sometimes transversely (a pyxidium) as in Anagallis.

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 respectively, 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.

(From Strasburger’s Lehrbuch der Botanik.)

Fig. 3.—Primula sinensis.
L, Long-styled flowers. P, Pollen grains, and N, stigmatic
K, Short-styled flowers.   papillae of long-styled form.
G, Style. p, n, Ditto of short-styled form.
S, Anthers. (P, N, p, n.)

The order is divided into five tribes by characters based on differences in position of the ovules—which are generally semi-anatropous so that the seed is peltate with the hilum in the centre on one side (or ventral), but sometimes, as in Hottania and Samolus, anatropous with the hilum basal—together with the method of dehiscence of the capsule and the relative position of the ovary. The chief British genera are Primula, including P. vulgaris, primrose, P. veris, cowslip, P. elatior, oxlip, and the small alpine species P. farinosa, with mealy leaves; Lysimachia, loose strife, including L. Nummularia, money-wort; Anagallis, pimpernel; and Hottonia, water violet.