The Rosidae
Family Overview - The Euphorbiales
Euphorbiaceae - the Spurge Family

Diversity:  One of the largest (ca. 300 genera with over 7,500 species) most diverse dicot families.  It includes a wide range of flowering plant structural and life-cycle types, i.e. herbs - including cactus-like stem succulents, shrubs, and trees.  Much of this range of variation is found within the type genus, Euphorbia, which is one of the largest (over 2,000 species) of angiosperm genera.  Natural rubber is derived from Hevea brasiliensis and Manihot esculenta is an important food crop of the tropics (cassava, tapioca).

Distribution:  Worldwide with centers of diversity in both temperate and tropical areas.  We have 19 genera with 154 species (5th largest family) in Texas with quite a few endemics and a listed endangered species (Manihot walkerae).

Floral structure:

 Significant features:  As indicated by Hevea, latex production is a major feature of the family and a useful field character.  Latex usually signals toxicity and this is true for this family.  The castor bean (Ricinus communis) produces a protein (ricin) believed to be the most toxic natural product. Two floral features provide good recognition characters for the family; unisexual flowers and a tricarpellate gynoecium with each carpel containing a single seed.  The latter produces a three-lobed ovary and fruit that, when present on a pistillate flower, is a very good key character.  The seed often shows a fleshy outgrowth from the integuments - known as a caruncle - that often functions as an inducement for animal (usually ants) dispersal.  This is a good family to be able to recognize on sight, i.e., before you are forced to use a key to families.  This is especially true if species of its largest genus - Euphorbia - are encountered.  As indicated by the distinct 'Euphorbia' floral formula above, this genus has followed an evolutionary path that we will encounter again with the largest and most specialized dicots; the Asteraceae.  It has involved extreme reduction of the flower BUT these reduced flowers have become organized in an inflorescence, known as a cyathium (overview), that appears to be single flower.

Euphorbia (overview)

The white structures pictured here are not petals and, in fact, not of floral origin.  These cyathia are formed within a cup-like structure, possibly derived from connation of an involucre of 4 to 5 bracts.  Secretory structures or 'glands' are positioned on the rim of this cup and, in most species, petaloid extensions of these glands - known as appendages - extend outward to form a corolla-like whorl.  The imperfect flowers are positioned within the cup and emerge at anthesis (uppermost cyathium here) to mimic the gynoecium (actually a single, tricarpellate, pistillate flower) and androecium (actually clusters of staminate flowers that are each composed of just the pedicel and a single stamen) of a single flower.  The pistillate flower of the lowermost cyathium (annotated here) shows the typical three-lobed ovary that - in combination with latex production - provides key characters that allow those with an 'eye' of this genus to avoid the key.
 
 
 

Euphorbia antisyphilitica - a perennial stem succulent of the Texas Big Bend with cyathia (right)

Succulent Euphorbia - see an overview

 
Euphorbia pulcherrima - poinsettia- with showy bracts subtending the central cluster of cyathia. The poinsettia cyathium (right) has only a single gland (yellow with nectar drop) with no appendages

Euphorbia bicolor - 'snow on the prairie' found locally at open sites, often remnant prairies (especially along rt. 21 to Austin) which, like E. pulcherrima (above) features showy bracts subtending clusters of cyathia:
 
 

Cyathia/bracts
Plants
Maturing Pistillate Flower

non - Euphorbia


Ricinus communis  - castor bean - a monoecious (far right) annual with carunculate (seee Elaiosome) seeds (right) and palmately veined/lobed leaves (left)

Cnidoscolus texanus - 'bull nettle':
 

Plant
Stinging trichomes
staminate flower
pistillate flower

More information on the Euphorbiaceae


Return to Lecture Notes, the Biology 301 homepage, or the Rosidae page   
20 Oct 2010