Euphorbiaceae
Euphorbiaceae, the spurge family, is a huge and extraordinarily diverse family of eudicots in the order Malpighiales (Rosid clade), containing about 220 genera and over 6,000 species. Cosmopolitan but most diverse in the tropics, the family includes herbs, shrubs, trees, vines, and remarkably convergent stem succulents resembling cacti. Key unifying features include strictly unisexual flowers (plants monoecious or dioecious) and a superior ovary typically composed of 3 carpels that matures into a schizocarpic capsule splitting into 3 single-seeded segments. Many members possess milky or colored latex, which is often caustic or poisonous. The giant genus Euphorbia is further characterized by its unique cyathium inflorescence.
Overview
The Spurge family is one of the largest families of flowering plants, exhibiting tremendous morphological and ecological diversity. Its members range from tiny annual weeds to large rainforest trees, desert succulents, and aquatic herbs. This diversity is now understood within a more narrowly defined Euphorbiaceae (sensu stricto), as molecular phylogenetics led to the removal of several large groups (e.g., Phyllanthaceae, Picrodendraceae) into separate families. Still, the remaining core family is vast and widespread across the globe, particularly abundant in tropical regions.
Economically, Euphorbiaceae is highly significant. Natural rubber is derived from the latex of Hevea brasiliensis. Cassava or manioc (Manihot esculenta) provides staple starchy roots for hundreds of millions in the tropics (requires processing to remove cyanogenic glycosides). Castor oil, used industrially and medicinally, comes from the seeds of Ricinus communis (the seeds themselves contain the deadly toxin ricin). Tung oil comes from Vernicia species. Countless species are grown as ornamentals, including the iconic Poinsettia (Euphorbia pulcherrima) with its colorful bracts, various succulent spurges mimicking cacti, crotons (Codiaeum variegatum) with colorful leaves, and Acalypha species (e.g., chenille plant). However, the family also includes numerous noxious weeds (Euphorbia, Croton) and highly poisonous plants.
The presence of milky latex (often white, sometimes clear or colored) is common but not universal; it frequently contains toxic compounds like diterpene esters. A unique feature of the largest genus, Euphorbia (spurges, >2000 species), is the highly reduced and specialized inflorescence called a cyathium, which superficially resembles a single flower but contains multiple unisexual flowers within a cup-like structure. In Oklahoma (current location as of March 27, 2025), the family is well-represented by numerous native and weedy spurges (Euphorbia spp.), crotons or doveweeds (Croton spp.), three-seeded mercuries (Acalypha spp.), and the stinging noseburns (Tragia spp.).
Quick Facts
- Scientific Name: Euphorbiaceae Juss. (sensu stricto)
- Common Name: Spurge family
- Number of Genera: Approximately 218-240
- Number of Species: Approximately 6,000-6,500
- Distribution: Cosmopolitan, most diverse in tropical regions.
- Evolutionary Group: Eudicots - Rosids (Fabids)
- Order: Malpighiales
Key Characteristics
Growth Form and Habit
Extremely diverse: annual or perennial herbs, shrubs, trees, succulent stem/root forms (often strikingly convergent with cacti), and vines. A key feature in many (but not all) species is the presence of milky or colored latex, which is often caustic, irritant, or poisonous.
Leaves
Leaves are usually alternate and spiral, but sometimes opposite or whorled. They are typically simple, although sometimes deeply lobed or palmately compound (e.g., Manihot, Ricinus). Leaf margins vary from entire to toothed. Stipules are usually present, but can be highly modified into glands, hairs, or spines, or sometimes absent or deciduous. Paired nectar glands (extrafloral nectaries) are sometimes found at the leaf base or on the petiole.
Inflorescence
Inflorescence structure is highly variable. Flowers can be arranged in cymes, racemes, panicles, spikes, or head-like clusters. A highly specialized type found only in genus Euphorbia and close relatives is the cyathium:
- A cup-shaped structure (involucre) formed from fused bracts.
- Often bears prominent glands on its rim (marginal glands), which may have colorful petal-like appendages.
- Inside the involucre are several highly reduced male flowers, each consisting of only a single stamen on a short stalk.
- A single, central, typically stalked female flower, consisting only of a pistil (3-carpellate ovary), emerges from the center of the cyathium.
Unique cyathium inflorescence of Euphorbia: Cup-like involucre with marginal glands enclosing reduced male flowers (single stamens) and a single central female flower (pistil).
Flowers
Flowers are always unisexual (plants monoecious or dioecious). Flower structure varies greatly from relatively 'complete' (though usually apetalous) to extremely reduced.
- Perianth: Highly variable. Petals are almost always absent (apetalous). Sepals may be present (typically 3-6, small, often greenish) or absent entirely (aperianthous, as in the individual flowers within a cyathium).
- Androecium (Male Flowers): Stamens range from 1 (in cyathia) to numerous (>100). Filaments can be free or fused (monadelphous).
- Gynoecium (Female Flowers): Ovary is superior, typically composed of 3 fused carpels forming 3 locules. Placentation is axile, with typically 1 (sometimes 2) ovule per locule, usually pendulous. Styles are typically 3, often deeply bifid (forked) or branched.
Fruits and Seeds
The characteristic fruit is typically a schizocarpic capsule (often called a regma) derived from the 3-carpellate ovary. At maturity, it usually splits elastically or explosively into three single-seeded segments (mericarps or cocci), often leaving behind a persistent central column. Less commonly, the fruit is a drupe or berry. Seeds often possess a fleshy or oily appendage called a caruncle near the attachment point (hilum), which may attract ants for dispersal. Endosperm is usually present and oily.
Typical Euphorbiaceae fruit: a 3-lobed schizocarpic capsule splitting into segments, often leaving a central column. Seed often has a caruncle.
Chemical Characteristics
The family is known for diverse and often potent secondary metabolites. Latex contains various compounds, including complex diterpene esters (e.g., phorbol esters in Euphorbia and related genera, known skin/eye irritants and tumor promoters), triterpenes (source of rubber in Hevea), and sometimes alkaloids. Cyanogenic glycosides are common (e.g., in Manihot). Extremely toxic proteins like ricin occur in Ricinus seeds. Glucosinolates (mustard oils) and iridoids are absent.
Identification Basics
Identifying Euphorbiaceae often relies on checking for milky latex, unisexual flowers, the characteristic 3-parted schizocarpic fruit, or the unique cyathium structure of Euphorbia.
Primary Identification Features
- Milky or Colored Latex (Often Present): Check broken stems or leaves (use caution, often irritant).
- Unisexual Flowers: Flowers strictly male or female (plants monoecious or dioecious).
- Superior Ovary, Typically 3-Carpellate: Ovary positioned above other floral parts, usually with 3 locules and 3 (often bifid) styles.
- Schizocarpic Capsule (Typically): Fruit usually a 3-lobed capsule splitting into 1-seeded segments.
- OR Cyathium Inflorescence: Cup-like structure containing reduced unisexual flowers (diagnostic for Euphorbia).
- Leaves Usually Alternate and Simple: Often with stipules (may be small/modified).
Secondary Identification Features
- Presence/Type of Stipules: Glandular, spiny, or typical.
- Leaf Shape/Compoundness: Simple vs. palmately compound (Manihot, Ricinus).
- Flower Details (Non-Cyathium): Presence/absence of sepals, number/fusion of stamens.
- Cyathium Details: Shape of involucre, number/shape of glands and appendages.
- Succulent Habit: Cactus-like forms (check for latex/cyathia).
- Seed Caruncle: Presence of appendage on seed.
Seasonal Identification Tips
- Growing Season (Warm Months): Latex is present. Flowers and fruits develop, crucial for ID. Many are weedy annuals prominent in summer/fall.
- Year-round: Woody or succulent forms retain structure. Persistent dried capsules might be found.
- (Oklahoma Context: Many native and weedy Euphorbia (spurges), Croton (doveweeds), and Acalypha (copperleafs) flower and fruit throughout the summer and fall. Look for latex and fruit characteristics).
Common Confusion Points
- Cactaceae (Cactus Family): Succulent stem convergence. Key differences: Cacti lack milky latex (usually watery sap); have areoles (specialized spine-bearing buds); flowers bisexual with inferior ovary; fruit a berry.
- Apocynaceae (Dogbane Family): Often have milky latex. Key differences: Leaves usually opposite; flowers usually bisexual, 5-merous with distinct corolla/stamens (or column/pollinia); fruit typically paired follicles or drupes.
- Moraceae (Fig Family): Woody plants often with milky latex, alternate simple leaves. Key differences: Stipule scar usually encircles twig; flowers tiny, unisexual, aggregated into complex structures (syconia, heads, catkins); fruit a multiple.
- Malvaceae (Mallow Family): Herbs/shrubs often with alternate simple leaves (often palmately lobed) and stellate hairs, superior ovary, capsule/schizocarp fruit. Key differences: Usually mucilaginous sap (not milky latex); flowers bisexual with 5 distinct petals and stamens fused into a column.
- Phyllanthaceae: Formerly included in Euphorbiaceae. Similarities exist, but often lack latex, typically have 2 ovules per locule (vs. 1), and different fruit dehiscence/seed details (molecular data separates them).
Look for: Milky Latex + Unisexual Flowers + Superior 3-Carpellate Ovary + Schizocarpic Capsule. If these fit, it's likely Euphorbiaceae. If flowers are in cyathia, it's Euphorbia.
Field Guide Quick Reference (Euphorbiaceae s.s.)
Look For:
- Habit diverse (herb, shrub, tree, succulent)
- Milky or colored latex (often)
- Leaves usually alternate, simple, stipulate
- Flowers ALWAYS unisexual
- Perianth reduced or absent (apetalous)
- Ovary superior, typically 3-carpellate/locular
- Fruit usually a 3-lobed schizocarpic capsule
- Seeds often with caruncle
Key Variations:
- Presence of Cyathia (Euphorbia)
- Succulent vs. Non-succulent habit
- Latex presence/color
- Stamen number (1 to many)
- Leaf shape (simple, lobed, compound)
Representative Genera and Species
The Spurge family contains plants ranging from economically vital crops and popular ornamentals to noxious weeds and dangerous toxins.
Euphorbia pulcherrima
Poinsettia
Native to Mexico, this shrub is globally popular as a Christmas ornamental. The actual flowers are tiny cyathia clustered at the center; the large, showy "petals" are brightly colored red, pink, or white leaves (bracts) subtending the inflorescence. Contains white latex.
Euphorbia marginata
Snow-on-the-mountain
An annual herb native to the Great Plains, including Oklahoma, often found in prairies and disturbed areas. Known for its striking appearance: upper leaves and bracts subtending the cyathia have prominent white margins, creating a showy display. Contains milky latex.
Ricinus communis
Castor Bean / Castor Oil Plant
A large annual herb or small tree, likely native to NE Africa/Middle East, now pantropical and grown ornamentally (can be weedy). Features large, alternate, palmately lobed leaves. Flowers apetalous, unisexual (monoecious) in terminal panicles (females above males). Fruit a spiny schizocarpic capsule. Seeds are source of castor oil but also contain the deadly poison ricin.
Manihot esculenta
Cassava / Manioc / Tapioca
A shrubby perennial native to South America, cultivated pantropically for its starchy tuberous roots, a staple food for millions. Leaves alternate, deeply palmately lobed. Flowers unisexual, apetalous. Roots contain cyanogenic glycosides and must be properly processed before consumption.
Hevea brasiliensis
Rubber Tree / Pará Rubber Tree
A large tree native to the Amazon basin, the primary source of natural rubber. Features alternate, trifoliolate leaves. Produces abundant white latex in specialized vessels (laticifers) in the bark, which is harvested by tapping. Flowers small, unisexual, in panicles.
Croton species (e.g., C. glandulosus)
Crotons / Doveweeds
A very large genus of herbs, shrubs, and trees, common in tropics and subtropics, several native/weedy in Oklahoma. Often characterized by stellate (star-shaped) or lepidote (scale-like) hairs, sometimes aromatic. Flowers unisexual, usually apetalous (sometimes petals present), borne in racemes or spikes. C. glandulosus is a common annual weed with small glands at leaf base.
Acalypha species (e.g., A. ostryifolia)
Copperleafs
Genus of herbs or shrubs, several native or weedy in Oklahoma. Lack milky latex. Monoecious; flowers apetalous. Male flowers tiny in catkin-like spikes. Female flowers subtended by a conspicuous, often large and leaf-like or deeply lobed bract, which persists in fruit. A. ostryifolia is a common annual weed.
Phylogeny and Classification
Euphorbiaceae belongs to the enormous and diverse order Malpighiales, situated within the Fabid (Eurosid I) clade of Rosids (Core Eudicots). Molecular phylogenetics has led to a significant refinement of the family's boundaries. The traditional, very broad concept of Euphorbiaceae was found to be polyphyletic. Several distinct lineages, such as Phyllanthaceae, Picrodendraceae, Putranjivaceae, Pandaceae, and Peraceae, have been removed and recognized as separate families, sometimes placed in different orders. Euphorbiaceae sensu stricto (in the strict sense), as defined by APG, represents a large, well-supported monophyletic group, although internal relationships are still being resolved.
Within Malpighiales, Euphorbiaceae s.s. is part of a large clade, but its precise sister group is not always consistently resolved; potential relatives include families like Rafflesiaceae (giant parasitic flowers, surprisingly nested within Euphorbiaceae s.l. by some analyses, though relationship complex) and others within the vast Malpighiales radiation (which also includes Salicaceae, Violaceae, Passifloraceae, Rhizophoraceae, Linaceae, Erythroxylaceae, etc.). The family showcases immense evolutionary diversification in morphology (especially habit, including convergence with cacti), biochemistry (latex, diterpenes, ricin), and reproductive strategies (unisexual flowers, specialized cyathium).
Position in Plant Phylogeny
- Kingdom: Plantae
- Clade: Angiosperms (Flowering plants)
- Clade: Eudicots
- Clade: Rosids
- Clade: Fabids (Eurosids I)
- Order: Malpighiales
- Family: Euphorbiaceae (sensu stricto)
Evolutionary Significance
Euphorbiaceae is a prime example of explosive evolutionary radiation, resulting in tremendous diversity in form, function, and ecological niche occupation, particularly in tropical regions. Key evolutionary themes include the development of diverse latex systems containing potent chemical defenses, remarkable convergence with cacti in succulent lineages adapting to arid environments, and the highly specialized cyathium inflorescence in Euphorbia – a unique evolutionary solution for packaging tiny unisexual flowers into functional pollination units. The family's economic impact highlights the evolutionary success of its chemical and structural innovations exploited by humans.
