Online Encyclopedia

MALVACEAE

Online Encyclopedia
Originally appearing in Volume V17, Page 518 of the 1911 Encyclopedia Britannica.
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MALVACEAE, in botany, an order of Dicotyledons belonging to the series Columniferae, to which belong also the orders Tiliaceae (containing Tilia, the lime-tree), Bombaceae (containing Adansonia, the baobab), Sterculiaceae (containing Theobroma, cocoa, and Colo, cola-nut). It contains 39 genera with about 300 species, and occurs in all regions except the coldest, the number of species increasing as we approach the tropics. It is represented in Britain by three genera: Malva, mallow; Althaea, marsh-mallow; and Lavatera, tree-mallow. The plants are herbs, as in the British mallows, or, in the warmer parts of the earth, shrubs or trees. The leaves are alternate and often palmately lobed or divided; the stipules generally fall early. The leaves and young shoots often bear stellate hairs and the tissues contain mucilage-sacs. The regular, hermaphrodite, often showy flowers are borne in the leaf-axils, soli- tary or in fasicles, or form more or less complicated cymose arrangements. An epicalyx (see MALLOW, figs. 3, 4), formed by a whorl of three or more bracteoles is generally present just beneath the calyx; sometimes, as in Abutilon, it is absent. The parts of the flowers are typically a, Stamens. b, Bract. in fives (fig. 1); the five sepals, g, Pistil of carpels. which have a valvate aestivation, i, Epicalyx, formed from an in-are succeeded by five often large volucre of bracteoles. showy petals which are twisted p ' Petals. s, Sepals. in the bud; they are free to the base, where they are attached to the staminal tube and fall with it when the flower withers. The very numerous stamens are regarded as arising from the branching of a whorl of five opposite the petals; they are united into a tube at the base, and bear kidney-shaped one-celled anthers which open by a slit across the top (fig. 2). The large spherical pollen-grains are cov- ered with spines. The carpels are one to numerous; when five in number, as in Abutilon, they are opposite the petals, or, as in Hibiscus, opposite the sepals. In the British genera and many others they are numerous, forming a whorl round the top 1, Anther. of the axis in the centre of the 2 Pollen grain of Hollyhock (Althaea rosea) enlarged. The pollen grain bears numerous spines, the dark spots indicate thin places in the extine. b flower, the united styles rising from the centre and bearing a corresponding number of stigmatic branches. In Malope the numerous carpels are arranged one above the other in vertical rows. One or more anatropous ovules are attached to the inner angle of each carpel; they are generally ascending but sometimes pendulous or horizontal; the position may vary, as in Abutilon, in one and the same carpel. The flowers are proterandrous; when the flower opens the unripe stigmas are hidden in the staminal tube and the anthers occupy the centre of the flower; as the anthers dehisce the filaments bend backwards and finally the ripe stigmas spread in the centre. Pollination is effected by insects which visit the flower for the honey, which is secreted in pits one between the base of each petal and is protected from rain by hairs on the lower margin of the petals. In small pale-flowered forms, like Malva rotundifolia, which attract few insects, self-pollination has been observed, the style-arms twisting to bring the stigmatic surfaces into contact with the anthers. Except in Malvaviscus which has a berry, the fruits are dry. In Malva (see MALLOW) and allied genera they form one-seeded schizocarps separating from the persistent central column and from each other. In Hibiscus and Gossypium (cotton-plant, q.v.), the fruit is a capsule splitting loculicidally. Distribution of the seeds is sometimes aided by hooked outgrowths on the wall of the schizocarp, or by a hairy covering on the seed, an extreme case of which is the cotton-plant where the seed is buried in a mass of long tangled hairs—the cotton. The embryo is generally large with much-folded cotyledons and a small amount of endosperm. The largest genus, Hibiscus, contains 150 species, which are widely distributed chiefly in the tropics; H. rosasinensis is a well-known greenhouse plant. Abutilon (q.v.) contains 8o species, mainly tropical; Lavatera, with 20 species, is chiefly Mediterranean; Althaea has about 15 species in temperate and warm regions, A. rosea being the hollyhock (q.v.); Malva has about 30 species in the north-temperate zone. Several genera are largely or exclusively American.
End of Article: MALVACEAE
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