Taxonomy of Flowering Plants - LECTURE NOTES - Fall, 1998
Hugh D. Wilson - rm. 306, Butler Hall
The Asteridae
Family Overview - The Gentianales
Asclepiadaceae - the Milkweed Family
Diversity:  Mostly perennial herbs, including many scandent types and some stem succulents similar to those found in the Cactaceae and Euphorbia, with some shrubs and, rarely, trees.  The family consists of over 250 genera and ca. 2,000 species.  These include the 'carrion flower' (Stapelia), a African stem succulent that produces fly-pollinated flowers that - in terms of both structure and smell - mimic decaying meat, and ornamentals like Hoya (wax plant) and Ceropegia (rosary plant).

Distribution:  Worldwide, but most diverse in tropical and subtropical areas.  With 5 genera and 58 species in Texas, including several endemics.

Floral structure:

flowers include a 5th whorl - or corona - of various forms
 
 Significant features:  Vegetative markers for the Milkweed family include, as indicated by the local name, latex production in combination with opposite leaves that tend to be both simple and entire. The floral structure is distinctive and highly specialized to 'manage' pollination by specific pollen vectors using features that similar to those found in the orchids (Liliopsida - Orchidaceae).  Asclepiad flowers are atypical in several respects and, as a result, key identification is difficult, i.e., a good family for sight recognition.  They include an extra floral whorl or nectar-producing structures, known as the corona, that are positioned above the corolla.  The androecium is not evident as a floral whorl.  The 5 stamens are adnate to the gynoecium to form a single structure, known as a gynostegium.  The gynoecium consists of two nearly distinct carpels with ovaries and styles separate and distinct but united by a single massive stigma.  Anthers, usually attached to the stigma, produce paired sacs of pollen called pollinia.  Pollinia from adjacent anthers are connected by 'translator arms' which both attach to a 'gland' producing a single unit, the pollinarium, which is carried by the vector during pollination.
Floral Structure - Asclepias
 
flower-overview
gynoecium 
anther
pollinarium
 

Each of the two ovaries of the gynoecium are the distinct product of a single carpel.  Each has a single
locule with numerous marginal ovules. The fruit is a follicle and the seeds usually are usually comose.
 
 

Asclepias seeds
Asclepias fruit with latex
Ceropegia fruits
 
 Asclepias viridis: (petals green, corona reddish color)
 
 
plant
flower-front 
flower-side
flower-section
 
 
More information on the Asclepiadaceae
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