ASTERACEAE (COMPOSITAE) (Sunflower Family)

Plants annual or perennial herbs (shrubs or less commonly trees), rarely scrambling or climbing. Stems branched or unbranched, rarely twining. Leaves alternate, opposite, or less commonly whorled, occasionally all or mostly basal, variously simple or compound, petiolate or sessile, rarely perfoliate, lacking stipules. Inflorescences terminal and/or axillary, composed of heads, these 1 to many per stem, sessile or stalked, solitary or more commonly in secondary inflorescences of clusters, spikes, racemes, or panicles. Heads composed of a flat to conical or rarely concave receptacle surrounded by an involucre of bracts and with few to many dense florets, these sessile or very short stalked on the receptacle, sometimes with subtending receptacular (chaffy) bracts or hairs. Involucral bracts few to many, of various shapes and sizes, arranged in 1 to several series, rarely the inner series fused into a tube or cup. Flowers within a head (collectively referred to as florets) all perfect or sometimes some or all pistillate or staminate (Missouri plants then completely or incompletely monoecious, elsewhere less commonly dioecious), epigynous, actinomorphic (disc florets) or zygomorphic (ray or ligulate florets). Calyx represented by a pappus of hairs, awns, and/or scales, this sometimes reduced to a low crown or lacking. Corollas (3–)5-lobed, the lobes sometimes difficult to interpret in ray and ligulate florets. Stamens (4)5, the filaments usually free but attached to the corolla tube, the anthers usually fused laterally into a tubular ring, dehiscing by vertical slits along the inner side. Pistil 1 per floret, composed of 2 fused carpels, but with only 1 locule and 1 ovule. Placentation basal. Style 1, forked toward the tip, each of the 2 branches with an elongate stigmatic band or pair of lines along the inner side toward the tip (the tip itself often nonstigmatic). Fruits achenes. About 1,535 genera, 23,000–32,000 species, cosmopolitan.

Specialists in the Asteraceae and Orchidaceae have competed for decades for the honor of claiming their group to be the largest family of flowering plants, but in recent years the orchidologists seem to be outpacing the asterologists in the race to describe new species. Regardless, the Asteraceae are one of the largest and most morphologically diverse plant families, found in nearly all habitats and having almost every type of growth form imaginable. Given its size, the family has only moderate economic importance. Numerous species are cultivated as ornamentals and specimen plants, but a number of other species are considered noxious weeds, both in agricultural areas and in natural plant communities. Although several species are used as foods and beverages, the composites overall are less important in this regard than some other relatively large families, such as the grasses and legumes. Sunflower (Helianthus annuus), which is grown for its edible seeds and the oil that they contain, has perhaps the greatest economic impact. Natural products produced by members of the family include pyrethrin insecticides (originally from Tanacetum), an alternative source of natural rubber (Parthenium), resins (several genera, but notably Grindelia), and various medicinal remedies. The windborne pollen grains of Ambrosia, Iva, and Xanthium are strong allergens, contributing heavily to the incidence of hay fever during the late summer and autumn.

The most distinctive feature of the family is the dense grouping of flowers (florets) into a head that functions much like an individual flower in most other plant families. Heads may contain from few to numerous florets. In a few genera, the head may contain only a single floret, and such reduced heads may even be grouped into secondary heads. The involucre associated with each head consists of bracts that are most often green and herbaceous, but they can be papery, leathery, or hardened (occasionally burlike) in a minority of genera. These involucral bracts may occur in a single series and be similar in shape and size, but more commonly they form 2 to several overlapping series, with the outermost bracts sometimes noticeably shorter or longer than the others. In a few groups, an inner series of bracts is fused into a cuplike or cylindrical structure, usually with shorter, free bracts below or outside these. The modified branch tip (receptacle) that forms the center of each head varies from elongate-cylindrical to cone-shaped in some of the coneflowers to hemispherical or nearly spherical in several genera, or strongly concave in a few species of cudweeds (Gamochaeta); however, in most species it is relatively flat to slightly convex (Pl. 289 e). Subtending each of the florets in some species is a scalelike bract (Pl. 289 a), in which case the receptacle is said to be chaffy (vs. naked). In a few groups, the floret bases are surrounded by short or long hairs. The bracts subtending individual florets are known as chaffy or receptacular bracts, in contrast to the involucral bracts that subtend an entire head. Chaffy bracts are scalelike and hard or papery in texture. In many cases, the best way to observe the shape of the receptacle and the presence of chaffy bracts is to section a head lengthwise (Pl. 289 e) and to examine the cut surface.

The florets themselves are quite variable and provide characters important in the delineation of tribes, genera, and species. Heads may contain all perfect florets, all staminate or pistillate florets, or a mixture of these types. In some species, some of the florets may be totally neutral, functioning only as a visual attractant to pollinators. When two different types of florets are present, these are usually distinguishable based on differences in size and shape of their component structures. Staminate florets almost always have a slender, nonfunctional ovary and may even produce a style that remains undivided at the tip. Pistillate florets do not produce staminodes.

In all Asteraceae, what one might think of as normal sepals are absent and instead the outermost floral whorl is known as a pappus. Most species have a pappus consisting of numerous capillary bristles (fine hairs), plumose (featherlike) bristles, 1 to several awns (stiff, stout, bristlelike structures visibly tapered from the base), few to several scales, or some combination of these structures. Rarely, the pappus is reduced to a low crown or rim of tissue or is totally absent.

The most visible floral variations are in the corolla are size and shape. Many members of the family produce actinomorphic florets referred to as disc florets. In these, the corolla is usually 5-lobed. It may be a slender tube (as in most thistles), or it may be shorter and differentiated into a narrow basal tube, expanded throat, and erect or spreading lobes. Occasionally, one of the sinuses between the lobes may be deeper than the others. Zygomorphic florets among Missouri genera occur in two distinct types. In the tribe Cichorieae, the florets of a head are all perfect and similar in size and shape, with a single long, strap-shaped lobe that usually has 5 teeth at the tip. Such ligulate florets are unique to this tribe. Ray florets, in contrast, occur in most other tribes and are always positioned along the periphery of a head that also contains a central group of disc florets. They may be pistillate (look for a divided style at flowering) or neutral. Heads with all-ligulate florets are referred to as ligulate, heads with only disc florets are called discoid, and heads with central disc florets and marginal ray florets are referred to as radiate.

Classification within the Asteraceae has remained controversial. Traditionally, the family was divided on a worldwide basis into two subfamilies, Lactucoideae (Liguliflorae) and Asteroideae (Tubuliflorae), with 13–19 total tribes mostly in the Asteroideae. Beginning in the 1970s, a lengthy series of studies by various authors based on different types of data began to dismantle the traditional approach, combining some tribes and segregating some genera into new tribes (for a review, see Bremer, 1994). Molecular studies of the family began with the work of Jansen and Palmer (1987), whose research suggested that the most primitive living Asteraceae were a small group of morphologically anomalous genera of shrubs growing in arid to seasonally dry portions of South America for which a third subfamilial name, Barnadesioideae, was proposed. Subsequent molecular and morphological analyses have broken up the traditional Lactucoideae (now called Cichorioideae) with its single tribe Cichorieae (sometimes referred to by the synonymous Lactuceae) into a series of groups recognized variously as tribes or subfamilies, none of which occur in Missouri except Cichorieae in the strict sense. These studies also have tended to group the traditional Vernonieae and Cynareae (now Cardueae) with the cichorioid groups. Perhaps the greatest disagreement over how many tribes to recognize has focused on the traditional tribes Eupatorieae, Helenieae, and Heliantheae, which various specialists have treated as a single tribe or a multitude, based primarily on molecular evidence (Baldwin et al., 2002). Different authors (Bremer, 1994; Panero and Funk, 2002) presently recognize 3–11 subfamilies and 17–35 total tribes worldwide. The classification of genera of Asteraceae into natural lineages is likely to continue to change as future research incorporates more species samples and additional data sets into the phylogenetic analyses.

The present treatment has borrowed shamelessly from the Flora of North America’s Asteraceae volumes (Flora of North America Editorial Committee, in press), which were under production at the time that the Missouri draft was being prepared. This classification is slightly more conservative than the last major familial summary (Bremer, 1994), wherein the authors of the chapter on Helenieae, Karis and Ryding, continued to accept this tribe while admitting that it was not a natural group. The present treatment does not, however, reflect the current trend toward recognition of many more tribal names to circumscribe all of the fully monophyletic lineages while also preserving all of the traditional tribal epithets (Baldwin et al., 2002; Panero and Funk, 2002). It has the practical advantages of deviating less from the traditional classification in Steyermark (1963) and that users of the present book will be able to locate general information on temperate North American composites more easily within the Flora of North America volumes. Also, many of the larger, traditional genera have been broken up and the tribal grouping accepted here continues to place the segregates relatively closely together in the work. Those interested in comparing the traditional tribal organization of Missouri Asteraceae in Steyermark’s (1963) treatment with that accepted here should note that: 1) Steyermark’s Inuleae have been split into three tribes, Gnaphalieae, Inuleae, and Plucheeae; 2) Cynareae are now known by the older name Cardueae; and 3) Steyermark’s Helenieae and Heliantheae are combined under the latter name.

Because the question of how many subfamilies should be formally recognized taxonomically remains controversial, it seems most useful to deal with the Missouri Asteraceae only at the tribal level. The following key to tribes of Missouri Asteraceae tends to favor relatively easily observed characters, but flowering heads must be present to determine most of the tribes. It draws inspiration mostly from keys in Steyermark (1963), Cronquist (1980), Gleason and Cronquist (1991), and Voss (1996).