Flora of Thailand

Euphorbiaceae

 

Characters

 

Goto on this page

Monoecy / Dioecy

Glands

Hairs

Leaves

Inflorescences

Fruits

Seeds and seed appendages

 

Some characters have their own terminology within the Euphorbiaceae. The most important ones (in bold), mainly used in the keys, are discussed here.

 

Monoecy / Dioecy

The flowers in Euphorbiaceae are unisexual (with a few exceptions, see Drypetes, though, probably, the flowers are functionally unisexual), either staminate (male: stamens present, pistil absent or reduced) or pistillate (female: stamens absent or reduced, pistil present).

    - Dioecious: whole plant with only flowers of one gender, other sex absent.

    - Monoecious: both sexes on same plant, either developing at different times, in different inflorescences or in different places in the same inflorescence (usually pistillate flowers beneath).

 

Glands

Come in a wide variety:

    - seldom glandular hairs: glands on top of a hair-like structure (plants sticky to the touch), e.g. Jatropha gossypiifolia.

    - stipules (pair of small, usually triangular structures at base of leaf petiole) may have glands along the margin or underneath (Macaranga).

    - at leaf blade insertion: usually raised extrafloral nectaries (the stipellae, small stipule-like orangs in some Alchornea species, are probably derived from these nectaries).

    - on the leaf surface: usually flat, circular to oval areas, also extrafloral nectaries, often termed macular glands.

    - teeth of the leaf blade usually end in a gland or possess a gland on the lower surface.

    - glandular scales (whitish, yellowish, orangish or reddish dots) are present on the lower leaf surface and in the inflorescences of species of Mallotus, and in all species of Macaranga, Hymenocardia, and Pantadenia.

    - in the inflorescences the bracts often possess extrafloral nectaries (which may transform into small bracteoles with some bracts).

    - in the flowers: basal sepal glands (some Alchornea spp.), tip of stamen connective (e.g., Melanolepis), on the discs, stigma tips (Hymenocardia).

 

Hairs

    - usually the hairs are simple and can be quite dense. This may conceal the next class:

    - stellate hairs. These hairs are usually groups of simple hairs together (no stalk cell), but they can also be real stellate hairs (similar but on a stalk).

    - in few genera (e.g. Croton) the stellate hairs may transform into lepidote hairs (flat, horizontal, hairs act as rays with some tissue in between), which may finally be scale hairs (shields on a stalk, e.g. Homonoia).

 

Leaves

    - Most leaves in the Euphorbiaceae are simple, meaning that there is a single blade on a petiole.

    - Sometimes the leaves show lobes (e.g., species in Baliospermum, Codiaeum, Mallotus), then the margin shows extensions, often supported by a nerve.

    - Seldom the leaves are deeply divided for more than half the blade, it is termed pinnatifid when the blade is divided for more than twothirds.

    - The leaves of Euphorbiaceae are seldom compound, composed of completely divided leaflets, in Bischofia, one species in Dalechampia, and in Hevea the leaves are trifoliolate, three leaflets originating from the same petiole (a leaf is defined as having a bud in the petiole axil, the buds are absent in the leaflet axils, but present in their joint petiole).

 

Inflorescences

great variety, mainly derived from a cymose type:

    The staminate flowers are usually several at every/each node in the inflorescence, the pistillate flowers usually single on every/each node.

    The inflorescences are usually axillary (originating from axil of leaves) or terminal (end of branch developed into inflorescence), sometimes ramiflorous (along branches) to cauliflorous (along trunk of tree).

    - Racemes (flowers pedicelled) or spikes (flowers sessile) are the most common types, a single branch with flowers per node, terminal bud not developed into a flower; often several racemes or spikes together in the same axil.

    - Cymes (terminal flower developed first, usually in center of inflorescence, followed by the two side-flowers, then the side-flowers of these, etc.).

    - Panicles (branching inflorescences). It may be racemes that branch (compound racemes) or the branches are arranged as in a raceme but per noder there are cymes (inflorescence called a thyrse or thyrsoid).

    - Umbels (flowers arranged like an umbrella, developing from end of peduncle).

    - Heads (umbels with (sub)sessile flowers): group of staminate flowers with a single pistillate flower at end of pdeduncle.

    - Fascicles (the inflorescence is absent, single or group of flowers developing from axil, if flowering is prolonged a short raceme may develop).

    - Cyathia (condensed inflorescences, usually subtended by several bracts, usually with 1-5 glands (shape and colour important), petaloid appendages, a single to a few single pistillate flowers in the center and surrounded by monochasial (cymes developing on one side) branches with staminate flowers. Found in Euphorbia and Pedilanthus (e.g., here staminate flowers reduced to a single stamen and soon caducous). The inflorescence of Dalechampia resembles this too.

 

Fruits

transitions between the different types exist (e.g., fleshy, late (= tardily) opening capsules)

    -Capsule (dry fruit, opening = dehiscing). This type of fruit is quite typical for most Euphorbiaceae (the only character unifying the family s.l.). The capsules usually open septicidally (along septum, the separation between locules) and loculicidally (along suture over locule) and the fruit parts (cocci, corresponding to the number of locules) fall off, together with the seeds (this can be somewhat explosive) leaving the persistent columella. This type of capsule is often called a rhegma or schizocarp. Sometimes the septicidal or loculicidal dehiscence is partial and bivalved cocci (V-split parts) result.

    - Drupe (fleshy fruit, not opening). The three layers of the fruit are the skin (exocarp), a fleshy, often edible middle part (mesocarp) and a very hard inner kernel (endocarp, enclosing the seed).

    - Berry (fleshy fruit with seeds not in a hard endocarp but in fruit flesh). Rather rare in Euphorbiaceae (e.g., some Baccaurea species).

 

Seeds and seed appendages

    - Many species have seeds without any fleshy appendage, but sometimes the seeds show the same kind of pattern found on camouflage suits, which is often termed marbled.

    - Quite often a fleshy appendage is present near the attachment of the funicle (the hilum), this is called a caruncle.

    - The outer seed layer may be completely fleshy, a sarcotesta.

    - The seed can also be partly or completely covered with a separate fleshy layer, loose from the seed coat, this is an aril.