PBIO 450 Lecture Notes

James L. Reveal

Norton-Brown Herbarium, University of Maryland
University of Maryland, College Park, MD 20742-5815


Selected Families of Angiosperms: Asteridae

Asterales Lindl., 1833

Cronquist (1988) defines the order to include only a single family, the Asteraceae. Except for the inclusion of Calyceraceae on occasions, and the recent views expressed by Thorne (1997) and by Bremer et al. (1998, 1999), the order is nearly always considered a monofamilial taxon. The ultimate origin of the order is to be sought near the dipsaccoid-like taxa of the Cornidae. Obviously there was some type of basal group that formed the basic expression of the modern Asteridae, and from this group two lines of evolution developed, one resulting in the modern Campanulanae and the second in the Asteranae. I can not concur with Small (New Phytologist Reprint No. 11. 1919) that the Asteraceae evolved from a lobelioid ancestor. The mere fact that both have a specialized pollen presentation mechanism is not sufficient to justify his conclusion as this ignores similar mechanisms found in the Calycerales as well as some members of the Rubiales. Cronquist (1981: 1018-1019, 1027-1028) is correct that one can not consider the Asterales to have evolved from the Calycerales, and he is equally correct in suggesting the Dipsacaceae can not be their point of origin. Even the Caprifoliaceae, he argues, are too advanced, and I think he is right in this conclusion as well. But interestingly, there is this (p. 1028): "The family that may be chemically most nearly like the Asteraceae is the Apiaceae, in the subclass Rosidae, but the morphological differences make any attempt to link these two families patently absurd."

Absurd, yes, patently so, maybe not. In establishing the Cornidae and rearranging portions of Cronquist's Rosidae and Asteridae, essentially all of the morphological, chemical and molecular objections disappear. Takhtajan's (1997: 418) suggestion that both Calyceraceae and Asteraceae "evidently share a common origin from some Goodeniaceae-like ancestor" points the way, but in my opinion not to some basal Campanulanae clade but rather to within the Cornidae itself and a branching event prior to the establishment of either Campanulanae or Asteranae. My basic proposal is that this point is somewhere after the demarcation of the Cornanae and around the point when the suborders Eucommianae, Aralianae and especially the Dipsacanae began to be established. Here we find the proper array of events for the Asteridae - alternate leaves, shrubby habit, a diverse array of potential chemical pathways, and a reduction and specialization of floral morphology. That pathway, as I visualize it, took its origin somewhere near the base of the Aralianae and Dipsacanae with an inital array of forms resulting in what is now the Asteridae.

As for the Asteraceae proper, they were initially woody shrubs with an array of protective chemicals to ward off herbivory coupled with a trend toward specialized floral morphology to promote survival in what I see as basically an arid environment. Like Cronquist (1981: 1026), I too feel chemistry is more important than morphology is defining and promoting the proliferation of early asteraceous genera and species. I therefore look upon the specialized pollen presentation mechanism in the family as just another of the many floral modifications and specializations associated with the initial establishment of the Asteridae, not just the Asteraceae, and it was the multitude of chemical defenses that allowed Asteraceae to more successful exploit ecological habits than any of the Campanulanae and most certainly the Calyceraceae.

The available rbcL data suggest the Calycerales are basal to the Asterales. Of the extant plants that have been examined to date, this is logical, both molecularly (Jansen et al., Syst. Bot. 16: 98-115. 1991) and anatomically (Harris, Bot. Rev. 61: 93-278. 1995). I do not suspect this will change with additional data. The two orders are so divergent from each other, and both (as the Asteranae) from the remainder of the Campanulanae, that any finer a resolution will be difficult.

Asteraceae Lindl., 1833

Herbs, subshrubs or subshrubs, occasionally lianas or small to medium-size trees, rarely epiphytes or tall trees; leaves alternate, occasionally opposite, infrequently verticillate, simple and entire to toothed or lobed, infrequently divided or compound, the stipules lacking; inflorescences arranged in dense, racemose heads (capitula) of 1-many or numerous, sessile flowers on a single receptacle nearly always subtended by 1-several series of bracts, the capitula sometime secondarily arranged in cymose heads with a series of additional subtending bracts; flowers mostly small, epigynous, either all alike (homogamous) or of two or more kinds (heterogamous) in a head, the homogamous ones usually bisexual, the heterogamous ones typically with an outer array of radiate female or neutral flowers and an inner group of bisexual or functionally male flowers, the sepals connate into a tube fused with the ovary with the free lobes modified into a pappus, this occasionally reduced or even obsolete, the petals 5, tubular or discoid and completely sympetalous, ligulate (or rayed) and zygomorphic with the ligule 3-5-toothed apically, or bilabiate, the petals rarely lacking entirely and then only in the outer female heterogramous flowers; stamens epipetalous, as many as and alternating with the corolla lobes, the filamens usually free and often with an annular or a slightly tubular nectary disk at the base, infrequently connate and tubular, typically with a well-defined appendage, this sometime becoming rather elaborate, the anthers connected into a tube with the pollen released to the inside and pushed out by the elongating style, (bi-) tetrasporangiate and 2-locular, dehiscing by longitudinal slits; gynoecium inferior, the carpels 2 and united to form a compound, unilocular ovary as a result of aborption, the style solitary and terminal, the stigma 2-lobed, the ovule 1, basal, anatropous, unitegmic and tenuinucellar; fruits an achene typically crowned with the persistent pappus of scales or bristles, rarely drupaceous with a fleshy pericarp, the pappus sometimes deciduous or rarely wanting, the embryo straight in scanty, oily endosperm; x= 9. CA(5) or CAp [CO(5) or COz(5)] A(5) GI(2). Nearly 1550 genera and some 24,000 species. Cosmopolitan but mainly in temperate and subtropical regions.

Few families (save Poaceae) have received as much attention, or are the subject of interest to so many botanists, as the Asteraceae (also known as the Compositae, a conserved, alternative name). As now understood, the Asteraceae are the largest family in terms of the number of genera and species, exceeding the Orchidaceae in both categories. The suprageneric taxonomy of the family is the current subject of numerous investigations, and while somewhat unsettled, a great deal is now known (see Bremer's book Asteraceae: Cladistics and Classification, 1994). The family is now divided into three subfamilies, the Barnadesioideae (nine genera and 90 species), the Carduoideae (including the Cichorioideae and Lactucoideae; 360 genera and perhaps 7000 species), and the Asteroideae (some 1175 genera and more than 17,000 species). The latter two subfamilies are divided into numerous tribes and subtribes. Takhtajan does not accept the Barnadesioideae.

Below are the arrangements adopted by Takhtajan and by Thorne. Authorships and dates of publications are based, for the most part, on my own recent work (Reveal, Compositae Newsl. 30: 29-45. 1997).

Takhtajan

Carduoideae Cass. ex Sweet (1826)
   Barnadesieae D. Don
   Mutisieae Cass. (1819)
   Echinopeae Cass. (1819)
   Carlineae Cass. (1819)
   Cardueae Cass. (1819)
   Vernonieae Cass. (1819)
   Liabeae H. Rob. & Brettell (1973)
   Arctotideae Cass. (1819)
   Eremothamneae
   Gundelieae DC. ex Lecoq & Juillet (1831)
   Cichorieae Lam. & DC. (1806)
Asteroideae (Cass.) Lindl. (1829)
   Inuleae Cass. (1819)
   Plucheae
   Gnaphalieae (Cass.) Lecoq & Juillet (1831)
   Astereae Cass. (1819)
   Anthemideae Cass. (1819)
   Ursinieae H. Rob. & Brettell (1973)
   Senecioneae Cass. (1819)
   Calenduleae Cass. (1819)
   Helenieae Lindl. (1829)
   Tageteae Cass. (1819)
   Coreopsideae Lindl. (1829)
   Heliantheae Cass. (1819)
   Eupatorieae Cass. (1819)

Thorne

Barnadesioideae Bremer & Jansen (1992)
Carduoideae Cass. ex Sweet (1826)
   Mutisieae Cass. (1819)
   Tarchonantheae Kostel. (1833)
   Cardueae Cass. (1819)
   Vernonieae Cass. (1819)
   Eremothamneae
   Liabeae H. Rob. & Brettell (1973)
   Cichorieae Lam. & DC. (1806)
   Arctotideae Cass. (1819)
Asteroideae (Cass.) Lindl. (1829)
   Astereae Cass. (1819)
   Anthemideae Cass. (1819)
   Inuleae Cass. (1819)
   Gnaphalieae (Cass.) Lecoq & Juillet (1831)
   Plucheae
   Senecioneae Cass. (1819)
   Calenduleae Cass. (1819)
   Eupatorieae Cass. (1819)
   Helenieae Lindl. (1829)
   Heliantheae Cass. (1819)
   Coreopsideae Lindl. (1829)
   Tageteae Cass. (1819)




As may be seen there is general agreement on the recognition of many names by these authors, but I have been told by several working on the family that this somewhat traditional view, even if largely that of Bremer (1992), is hardly stable and that significant changes are likely to occur as more and more molecular work is done. It will be interesting to follow the developments.

The Asteraceae are one of the more economically important of the vascular plant families. The family certainly harbors a multitude of foods, fibers, fuels, and fodder in additional to numerous ornamental and medicinal species. A few of the larger trees (such as Montanoa revealii) are cut for their timber. There is hardly an environment where members of the family are lacking although aquatic species are rare and tropical forest trees are few. Even so, as a source of major crops the Asteraceae do not rival the likes of Fabaceae or Poaceae, and as medicinals they can not match smaller families like Papaveraceae, Apocynaceae and Solanaceae to mention just three. As for weeds, few species that can rival some found in Asteraceae, the dandelion (Taraxacum officinale) coming instantly to mind.

All in all the Asteraceae are a source of great fascination for botanists and biologists. It would be hard to visualize the American West without sagebrush (Artemisia) or a garden without a host of asters and daisies and even black-eyed susans.

Asteridae


Alismatidae
Lecture Notes

Posted: 11 Feb 1998; last revised 14 Mar 1999