Euphorbiaceae

Euphorbiaceae, in botany, a large natural order of flowering plants, containing more than 220 genera with about 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.  
  
1\. Shoot of Euphorbia hypericifolia, about ½ nat. size.

2\. A partial inflorescence, cyathium, bearing the petaloid glands.

3\. A similar one at a later stage, cut open to show the single-stamened (monandrous) male flowers and the central long-stalked female flower.

4\. A cyathium without petaloid glandular appendages.

5\. A similar one at a later stage with nearly ripe fruit.

6\. An anther dehiscing.

7\. Fruit dehiscing and exposing one of the three seeds.

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8\. Seed.

9\. Seed cut lengthwise exposing the embryo.

10\. Diagram of the inflorescence of Euphorbia, illustrating the dichasial cymose arrangement of the ultimate branches.

b, Bract subtending the central terminal cyathium I.

a′b′, Bracteoles of the first order subtending the secondary cyathia II.

a″b″, Bracteoles of the second 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 bear a cap-like outgrowth or caruncle, which persists in the seed (well shown in castor oil, fig. 3).  
  
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Fig. 2.—Dog’s Mercury (Mercurialis perennis).  
  
1\. Male plant.

2\. Female plant; 1⁄3 nat. size.

3\. Female flower.

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4\. Male flower.

5\. Fruit beginning to split open.

6\. Seed cut lengthwise showing the embryo.  
  
As the stamens and pistil are borne by different flowers, cross-fertilization is necessary. In Mercurialis and others with inconspicuous flowers pollination is effected by the wind, but in many cases insects are attracted to the flower by the highly-coloured bracts, as in many Euphorbias and Dalechampia, or by the coloured calyx as in Manihot; the presence of honey is also frequently an attraction, as in the honey-glands on the bracts of the cyathium of Euphorbia. The fruit is generally a capsule which splits into three divisions (cocci), separating from the central column, and splitting lengthwise into two valves. In the mancinil (Hippomane mancinella) of Central America the fruit is a drupe like a plum, and in some genera berries occur. In the sandbox tree (Hura crepitans) of tropical America the ovary consists of numerous carpels, and forms when mature a capsule which splits with great violence and a loud report into a number of woody cocci. The seeds contain abundant endosperm and a large straight or bent embryo.  
  
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From Bentley and Trimen’s Medicinal Plants, by permission of J. & A. Churchill.  
Fig. 3.—Castor Oil (Ricinus communis). End of shoot with flower-spike; about 1⁄3 nat. size.  
  
1\. Section of male flower, about nat. size.

2\. Group of stamens.

3\. Fruit.

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4\. Seed.

5 and 6. Vertical and transverse sections of seed showing embryo in position.  
  
Several members of the order are of economic importance. Manihot utilissima, manioc or cassava (q.v.), is one of the most important tropical food-plants, its thick tuberous root being rich in starch; it is the source of Brazilian arrowroot. Caoutchouc or india-rubber is obtained from species of Hevea, Mabea, Manihot and Sapium. Castor oil (q.v.) is obtained from the seeds of Ricinus communis. The seeds of Aleurites moluccana and Sapium sebiferum also yield oil. Resin is obtained from species of Croton and Euphorbia. Many of the species are poisonous; e.g. the South African Toxicodendron is one of the most poisonous plants known. Many, such as Euphorbia, Mercurialis, Croton, Jatropha, Tragia, have been, or still are, used as medicines. Species of Codiaeum (q.v.), Croton, Euphorbia, Phyllanthus, Jatropha and others are used as ornamental plants in gardens.

The box (Buxus) and a few allied genera which were formerly included in Euphorbiaceae are now generally regarded as forming a distinct order—Buxaceae, differing from Euphorbiaceae in the position of the ovule in the ovary-chamber and in the manner of splitting of the fruit.

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