The Asteridae
Family Overview - Asterales

Asteraceae (Compositae) - the Sunflower Family

Diversity: Mostly herbs, but many shrubs, and few arborescent types.  Given the size of this family (over 1,600 genera and ca. 23, 000 species) variation includes essentially all adaptive structural types (annuals, perennials, stem succulents, aquatics, epiphytes, etc.), although scandent types are rare.  The most important domesticated taxon is Helianthus annuus (sunflower-harvest = achenes - Radiate Inflorescence), but the family also includes lettuce (Lactuca - harvest=shoot/leaves - Ligulate Inflorescence) and the globe artichoke (Cynara - harvest = immature capitulum - Discoid Inflorescence), and many ornamentals.

Distribution:  Worldwide and well expressed as a common element of just about all global ecosystems although, in contrast with many large flowering plant families, biodiversity in tropical forests is relatively low.  Maximum diversity is associated with open, often temperate areas, i.e., prairie, step, savannah, etc.  The most diverse family in the Texas flora with 204 genera, 713 species, and 327 infraspecific taxa.

Floral structure:

Significant features:  As indicated above, structural diversity precludes any useful vegetative 'tag' for this family.  Its primary key characteristic is the involucrate pseudanthium (flower-like inflorescence = head or capitulum or 'composite' structure) subtended by bracts (phyllaries) and this feature allows, in most cases, relatively easy family recognition.   Flowers of the Asteraceae either lack a calyx or, in most cases, this basal perianth whorl is modified to form a highly variable series (hair-like or capillary appendages, awns, scales, etc.), known as the pappus positioned at the base of the corolla.  The corolla can either be actinomorphic or zygomorphic and, in contrast to most flowering plant families, both types can occur on the same plant and within the same inflorescence.  While various forms of zygomorphic corollas are produced, the most common type involves connation of the five petals along their margins to produce an elongate strap-shaped corolla.  With few exceptions, the androecium of the Asteraceae is a single unit formed via connation of the five stamens at the anthers (synantherous) with the filaments free from one another but attached to the corolla (epipetalous).  The most stable floral element is the gynoecium which is bicarpellate, syncarpous, uniloculate, and uniovulate.  The fruit is an achene or, to use a term often applied to achenes of the Asteraceae, cypsela.  Individual flowers (commonly termed 'florets') are attached to a common receptacle and, in some cases, each floret is associated by a small bract (chaff) and the receptacle is 'chaffy'. See an overview of terminology for the family.


Common floret types:
 



 
 
 




Common inflorescence types:
Radiate - mix of disc (center) and ray (periphery)


 
 

  Helianthus annuus

Coreopsis grandiflora
Bidens laevis 
Baileya multiradiata
Erigeron philadelphicus 
Ligulate - ligulate florets only, no disc florets
Cichorium intibus
Hieracium aurantiacum 
Lygodesmia texana
Discoid - disc florets only, no ray florets
Liatris squarrosa
Palafoxia rosea 
Marshallia caespitosa
Cirsium sp.

 

More information on the Asteraceae or Compositae


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