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Asteraceae
| The Asteraceae are herbs, shrubs, or less commonly trees and are arguably the largest family of flowering plants, comprising about 1,100 genera and 20,000 species that are characterized by having the flowers reduced and organized into an involucrate pseudanthium in the form of a head or capitulum. The leaves are alternate, opposite, or less commonly whorled, and range from simple to pinnately or palmately compound; stipules are absent. Subtending and often partly enclosing the florets of the head is one or more series of usually green, free or variously connate bracts called involucral bracts or phyllaries. Another kind of bract called a receptacular bract or chaff may be associated with each disk floret throughout the head. The flowers are of two basic types: those with tubular actinomorphic corollas and those with strap-shaped or radiate zygomorphic corollas, often within the same head. Either type may be bisexual or unisexual. Where both types are found in a single head, the central flowers have tubular, usually 4-5- lobed corollas, and generally are bisexual, and the peripheral flowers have strap-shaped corollas generally with 3 distal teeth, and are usually female. Sometimes the heads lack ray flowers and are said to be discoid, consisting of only bisexual florets with tubular corollas. So-called disciform heads have bisexual central disk flowers surrounded by female flowers that have a very slender tube and an extremely suppressed or obsolete ligule. Ligulate heads consist only of bisexual florets with corollas of the ligulate or strap-shaped type but with generally 5 rather than 3 distal teeth. In another variation some or all of the florets in the head have 2-lipped corollas. In all cases the calyx is absent or so highly modified as hairs, bristles or scales on the ovary summit that it is given the alternative name of pappus. The corolla is sympetalous with mostly 3-5 lobes. The androecium nearly always consists of 4 or 5 stamens that are united by their anthers and are adnate to the corolla tube or epigynous zone, alternate with the lobes. The gynoecium consists of a single compound pistil of 2 carpels, a single 2-cleft style, and an inferior ovary with one locule and one basal ovule. During maturation of a flower, the style grows through the anther column, and as it does, hairs on the outer surface of the closed style lobes brush the pollen that is released into the anther column to the distal opening where it is available for biotic pollinators. A nectary in the form of a scale or small cup is commonly found alongside or around the base of the style. The fruit is an achene which may have a persistent pappus that commonly functions in fruit dispersal. |
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