Asteraceae Page

Taxonomy of Flowering Plants - LECTURE NOTES - Fall, 1998 Hugh D. Wilson - rm. 306, Butler Hall 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 (1, 100 genera and ca. 20, 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), but the family also includes lettuce (Lactuca - harvest=shoot/leaves) and the globe artichoke (Cynara - harvest = immature capitulum), 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 159 genera and over 600 species.

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'.

Common floret types: