capitulum (head) |
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Flowers in the Family Asteraceae are borne in a compact influorescence called a capitulum or head which superficially resembles a single flower. The individual flowers, sometimes referred to as "florets", share a compound receptacle (base) and are sessile (stalkless) or nearly sessile. There are two types of flowers: straplike ray flowers and tubular disk flowers. Many species have both types with the ray flowers around the perimeter and the disk flowers in the center (e.g., Coreopsis shown below, daisies). Some species have only ray flowers (e.g., dandelions, chicory), others only disk flowers (e.g., thistles). The receptacle is commonly surrounded by a whorl of bracts (modified leaves) called an involucre which superficially resembles the sepals making up the corolla of a simple flower.
A capitulum could be determinate (centremost flowers mature first) or indeterminate (outermost flowers mature first) according to the taxa.
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 From Gray's Lessons in Botany and Vegetable Physiology 1 :FIG. 219. Head of Flowers (the so-called "compound flower") of Coreopsis, divided lengthwise.FIG 220. A slice of Fig. 219, more enlarged, with one tubular perfect flower (a) left standing on the receptacle, with its bractlet or chaff (b), one ligulate, neutral ray-flower (c), and part of another; d, section of bracts or leaves of the involucre.  |
1. Asa Gray. 1880. Gray's Lessons in Botany and Vegetable Physiology
Vision, Blakeman, Taylor & Co., New York and Chicago, 1880.