4000 species, chiefly tropical, but spreading over the whole earth with the exception of the arctic and cold alpine zones. They are represented in Britain by the spurges (Euphorbia, q.v.) (fig. 1) and dog’s mercury (Mercurialis) (fig. 2), which are herbaceous plants, but the greater number are woody plants and often trees. The large genus Euphorbia shows great variety in habit; many species, like the English spurges, are annual herbs, others form bushes, while in the desert regions of tropical Africa and the Canary Islands species occur resembling cacti, having thick fleshy stems and leaves reduced to spines. Another large genus, Phyllanthus, contains small annual herbs as well as trees, while in some species the leaves are reduced to scales, and the branches are flattened, forming phylloclades. The leaves also show great variety in form and arrangement, being simple and entire as in the English spurges, or deeply cut as in Ricinus (castor-oil) (fig. 3), and Manihot or sometimes palmately compound (Hevea). The majority contain a milky juice or latex in their tissues which exudes on cutting or bruising. In Hevea, Manihot and others the latex yields caoutchouc. The flowers are unisexual; male and female flowers are borne on the same, as in the spurges (fig. 1), or on different plants, as in dog’s mercury (fig. 2). Their arrangement shows considerable variation, but the flowers are generally grouped in crowded definite partial inflorescences, which are themselves arranged in spikes or stand in the axils of the upper leaves. These partial inflorescences are generally unisexual, the male often containing numerous flowers while the female flowers are solitary. The partial inflorescence (cyathium) of Euphorbia (fig. 1) resembles superficially a hermaphrodite flower. It contains a central terminal flower, consisting of a naked pistil; below this are borne four or five bracts which unite to form a cup-shaped involucre resembling a calyx; each of these bracts subtends a small cyme of male flowers each consisting only of one stamen. Between the segments of the cup are large oval or crescent-shaped glands which are often brightly coloured, forming petal-like structures.
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| Fig. 1. | |
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1. Shoot of Euphorbia hypericifolia, 2. A partial inflorescence, cyathium, 3. A similar one at a later stage, 4. A cyathium without petaloid 5. A similar one at a later stage 6. An anther dehiscing. 7. Fruit dehiscing and exposingone of the three seeds. |
8. Seed. 9. Seed cut lengthwise exposing 10. Diagram of the inflorescence of b, Bract subtending the central a′b′, Bracteoles of the first order subtending the tertiary cyathia III. |
| In the central cyathium I. are shown the details of the arrangement of the male flowers in monochasial cymes, m, and the central female flower, f. | |
The form of the flower shows great variety. The most complete type occurs in Wielandia, a shrub from the Seychelles Islands, in which the flowers have their parts in fives, a calyx and corolla being succeeded in the male flower by 5 stamens, in the female by 5 carpels. Generally, however, only 3 carpels are present, as in Euphorbia; Mercurialis (fig. 2) has minute apetalous flowers with 3 sepals, followed in the male by 8 to 20 stamens, in the female by a bicarpellary pistil. In the large tropical genus Croton a pentamerous calyx and corolla are generally present, the stamens are often very numerous, and the female flower has three carpels. In Manihot, a large tropical American genus to which belongs the manioc or cassava (M. utilissima), the calyx is often large and petaloid. In a great many genera the corolla is absent. The most reduced type of flower is that described in Euphorbia, where the male consists of one stamen separated from its pedicel by a joint, and the female of a naked tricarpellary pistil. The stamens are sometimes more or less united (monadelphous), and in castor-oil (Ricinus) (fig. 3) are much branched. The ovary generally contains three chambers, and bears three simple or more often bipartite styles; each chamber contains one or two pendulous ovules, which generally
