CONVOLVULACEAE, a botanical natural order belonging to the series Tubiflorae of the sympetalous group of Dicotyledons. It contains about 40 genera with more than 1000 species, and is found in all parts of the world except the coldest, but is especially well developed in tropical Asia and tropical America. The most characteristic members of the order are twining plants with generally smooth heart-shaped leaves and large showy white or purple flowers, as, for instance, the greater bindweed of English hedges, Calystegia sepium, and many species of the genus Ipomaea, the largest of the order, including the “convolvulus major” of gardens, and morning glory. The creeping or trailing type is a common one, as in the English bindweed (Convolvulus arvensis), which has also a tendency to climb, and Calystegia Soldanella, the sea-bindweed, the long creeping stem of which forms a sand-binder on English seashores; a widespread and efficient tropical sand-binder is Ipomaea Pes-Caprae. One of the commonest tropical weeds, Evolvulus alsinoides, has slender, long-trailing stems with small leaves and flowers. In hot dry districts such as Arabia and north-east tropical Africa, genera have been developed with a low, much-branched, dense, shrubby habit, with small hairy leaves and very small flowers. An exceptional type in the order is represented by Humbertia, a native of Madagascar, which forms a large tree. The dodder (q.v.) is a genus (Cuscuta) of leafless parasites with slender thread-like twining stems. The flowers stand singly in the leaf-axils or form few or many flowered cymose inflorescences; the flowers are sometimes crowded into small heads. The bracts are usually scale-like, but sometimes foliaceous, as for instance in Calystegia, where they are large and envelop the calyx.
The parts of the flower are in fives in calyx, corolla and stamens, followed by two carpels which unite to form a superior ovary. The sepals, which are generally free, show much variation in size, shape and covering, and afford valuable characters for the distinction of genera or sub-genera. The corolla is generally funnel-shaped, more rarely bell-shaped or tubular; the outer face is often marked out in longitudinal areas, five well-defined areas tapering from base to apex, and marked with longitudinal striae corresponding to the middle of the petals, and alternating with five non-striated weaker triangular areas; in the bud the latter are folded inwards, the stronger areas being exposed and showing a twist to the right. The slender filaments of the stamens vary widely, often in the same flower; the anthers are linear to ovate in shape, attached at the back to the filament, and open lengthwise. Some importance attaches to the form of the pollen grains; the two principal forms are ellipsoidal with longitudinal bands forming the Convolvulus-type, and a spherical form with a spiny surface known as the Ipomaea-type. The ovary is generally two-chambered, with two inverted ovules standing side by side at the inner angle of each chamber. The style is simple or branched, and the stigma is linear, capitate or globose in form; these variations afford means for distinguishing the different genera. The fruit is usually a capsule opening by valves; the seeds, where four are developed, are each shaped like the quadrant of a sphere; the seed-coat is smooth, or sometimes warty or hairy; the embryo is large with generally broad, folded, notched or bilobed cotyledons surrounded by a horny endosperm. Cuscuta has a thread-like, spirally twisted embryo with no trace of cotyledons.
|Convolvulus sepium, slightly reduced.|
|1. Flower cut vertically.||4. Embryo taken out of seed.|
|2. Fruit, slightly reduced.||5. Horizontal plan of arrangement of flower.|
|3. Seed cut lengthwise showing embryo.|
The large showy flowers are visited by insects for the honey which is secreted by a ring-like disk below the ovary; large-flowered species of Ipomaea with narrow tubes are adapted for the visits of honey-seeking birds.
The largest genus, Ipomaea, has about 400 species distributed throughout the warmer parts of the earth. Convolvulus has about 150 to 200 species, mainly in temperate climates; the genus is principally developed in the Mediterranean area and western Asia. Cuscuta contains nearly 100 species in the warmer and temperate regions; two are native in Britain.
The tubers of Ipomaea Batatas are rich in starch and sugar, and, as the “sweet potato,” form one of the most widely distributed foods in the warmer parts of the earth. Several members of the order are used medicinally for the strong purging properties of the milky juice (latex) which they contain; scammony is the dried latex from the underground stem of Convolvulus Scammonia, a native of the Levant, while jalap is the product of the tubercles of Exogonium Purga, a native of Mexico. Species of Ipomaea (morning glory), Convolvulus and Calystegia are cultivated as ornamental plants. Convolvulus arvensis (bindweed) is a pest in fields and gardens on account of its wide-spreading underground stem, and many of the dodders (Cuscuta) cause damage to crops.