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Encyclopedia Britannica - Main :: GAG-GEO |
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GENTIANACEAE (the gentian family) , in botany, an order of Dicotyledons belonging to the sub-class Sympetalae or Gamopetalae, and containing about 750 species in 64 genera. It has a world-wide distribution, and representatives adapted to very various conditions, including, for instance, alpine plants, like the true gentians (Gentiana), meadow plants such as the British Chlora perfoliata (yellow-wort) or Erythraea Centaurium(centaury), marsh plants such as Menyanthes trifoliata (bog-bean), floating water plants such as Limnanthemum, or steppe and sea- coast
The leaves are in decussating pairs (that is, each pair is in a plane at right angles to the previous or succeeding pair), except in Menyanthes and a few allied aquatic or marsh genera, where they are alternate or radical. Several genera, chiefly American, are saprophytes, forming slender low-growing herbs, containing little or no chlorophyll and with leaves reduced to scales; such are Voyria and Leiphaimos, mainly tropical American. The inflorescence is generally cymose, often dichasial, recalling that of Caryophyllaceae, the lateral branches often becoming monochasial; it is sometimes reduced to a few flowers
flowers
great
the right in the bud; the throat is often fimbriate or bears scales. The stamens, as many as, and alternating with, the corolla-segments, are inserted at very different heights on the corolla-tube; the filaments are slender, the anthers are usually attached dorsally, are versatile, and dehisce by two longitudinal slits; after escape of the pollen they some-times become spirally twisted as inErythraza. Dimorphic flowers are frequent, as in the bog-bean (Menyanthes). There is considerable variation in the size, shape and external markings of the pollen grains, and a division of the order into tribes and sub-tribes based primarily on pollen characters has been proposed. The form of the honey-secreting developments of the disk at the base of the ovary also shows considerable variety. The superior ovary is generally one-chambered, withCentral figure and figs
Curtis , Flora Londinensis. two variously de-Gentiana Amarella. veloped parietal pla- centas, which occa- I, A small form, natural size. sionally meet, form- 2, Calyx and protruding style. ing two chambers; 3, Corolla, laid open. the ovules are gener- 4, Capsule, bursting into two valves, and ally very numerous showing the seeds attached to their and anatropous or margins. half - anatropous in 5, Floral diagram. form. The style, which varies much in length, is simple, with an undivided or bilobed or bipartite stigma. The fruit is generally a membranous or leathery capsule, splitting septicidally into two valves; the seeds are small and numerous, and contain a small embryo in a copious endosperm. The brilliant colour of the flowers, often occurring in large numbers (as in the alpine gentians), the presence of honey-glands and the frequency of dimorphy and dichogamy, are adaptations for pollination by insect
insect
bright blue corolla, is visited by humble bees; and G. verna, with a still longer narrower tube, is visited by Lepidoptera.Gentiana, the largest genus, contains nearly three hundred species, distributed over Europe (including arctic), five being British, the mountains of Asia, south
east
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