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Asteraceae or Compositae (sunflower family)

Mark A. Dimmitt

The sunflower family is stunningly successful. It is the largest plant family, or the second largest after orchids, depending on the criteria used, with over 20,000 species occupying almost all of the world’s habitats except underwater. The genus Senecio (groundsel) alone has 1000 species. So many variations within a single group make composites the bane of many botanists seeking to identify the species. Because of this and the great preponderance of yellow flowers, unidentified plants are often semi-affectionately dubbed “DYCs” (damn yellow composites). This term has been adopted by some Mexican botanists as “PCAs” (pinchi compuestas amarillas). The family is well represented in the Sonoran Desert, constituting, for example, 16% of the Tucson Mountains’ flora (105 species and subspecies).

The flowers, also called florets, are nearly always clustered into heads, with each subtended by a whorl or whorls of modified leaves called bracts (the involucre). There are two general forms of flowers. A disk flower, in its most complete form, has 5 petals fused into a tube, with a tube of 5 fused anthers inside the petal tube, and a 2-lobed stigma exserted through the anther tube. A ray flower (a “petal” of a daisy) is similar, except that some of the fused petals extend on one side into a flat strap-like ligule. Flowers may be unisexual or sterile, lacking either or both “male” and “female” sexual parts. Each functionally “female” flower, whether ray or disk, has a single inferior ovary with a single ovule. If the ovule is fertilized, it will develop into a single seed in a special fruit called an achene. Each head may have only ray flowers or disk flowers, or both.