Herbs or shrubs or trees of exceedingly various habit; juice milky, acrid. Leaves alternate, rarely opposite, often stipulate. Flowers usually small, unisexual (in Euphorbia reduced to single naked stamens surrounding a solitary pistil and enclosed within a calyx page 627like involucre). Perianth generally simple and calycine, but often wanting, rarely double, the inner of 4–5 minute petals. Stamens 1 to many; anthers 2-celled. Ovary superior, of 3 (rarely 2 or more than 3) united carpels; styles as many as the carpels, free or united, entire or divided; ovules 1 or 2 to each carpel, pendulous from the inner angle of the cell. Fruit either a capsule of 2-valved 1–2-seeded cocci separating from a persistent axis, or a 1–3-celled drupe, or of 1 or more combined nuts. Seed laterally attached at or above the middle of the cell; embryo straight, in the axis of fleshy albumen, cotyledons flat, radicle superior.

A large order, of about 200 genera and 3000 species, most abundant in the tropics, rare in very cold climates. Many species are poisonous, and a considerable number yield medicinal products, as castor-oil, croton-oil, gum euphorbium, &c. Others afford a wholesome food, as the manioc and tapioca. Of the 4 genera found in New Zealand, one (Euphorbia) has a worldwide distribution; another (Poranthera) is found elsewhere only in Australia. The two remaining (Aleurites and Homalanthus) have their headquarters in the Pacific islands, but extend northwards to China and the Malay Archipelago.

* Flowers without a perianth, several males and one female in a cup-shaped calyx-like involucre	1. EuphoRbia.
** Flowers provided with a perianth.
Low-growing herbs. Flowers in terminal racemes or heads. Anthers opening by pores	2. Poranthera.
Trees with digitately lobed or veined leaves. Flowers in terminal cymes. Fruit large, somewhat fleshy, with 1–3 large oily seeds	3. Aleurites.
Trees. Flowers in slender racemes; males numerous, females few at the base of the raceme. Fruit capsular	4. Homalanthus.