The Adaptive and Phylogenetic Significance of Receptacular Bracts in the Compositae

Abstract

Many morphological features of the heads of Compositae are systematically useful. Receptacular bracts (pales or paleae) are of particular significance taxonomically. These structures occur in nine of the 13recognized tribes (absent in Arctoteae, Calenduleae,Senecioneaeand Tageteae),with the greatest concentration in the Anthemideae, Heliantheae, and Inuleae. Because the Heliantheae contain the broadest diversity of pale types, the adaptive significance of these features within this tribe deserves special emphasis. Certain subtribes of the Heliantheae are characterized by distinctive types of pales, such as flattened with orange-brown lines in the Coreopsidinae, pales subtending only the outer series of disc florets in the Madiinae, or their complete absence in the Bahiinae. Of the five principal factors affecting the evolution offeatures of heads in the family, viz., protection, dispersal, pollination, breeding systems, and seed germination, the former two are most likely the most important selective forces in the evolution of pales. Dispersal functions of pales include attachment or close envelopment of the achene by the pales with removal of the entire unit by wind from the head, conduplicate pales serving as chutes for release of unattached achenes, and brightly colored, fleshy pales being attractive to animals. Protective functions occur against predators and environmental extremes, and occur in bud, anthesis, and mature fruiting stages. Pales function during anthesis to protect ovaries and achenes from apical and lateral insect attack. The particular variations of the pales depend on the associated structures of the heads and breeding systems. In a phylogenetic context, the large and sometimes foliose pales of the Heliantheae seem best interpreted as adaptations for protection of large achenes and not necessarily as reflections of a primitive condition within the family.

The Bracts of Saussurea velutina (Asteraceae) Protect Inflorescences from Fluctuating Weather at High Elevations of the Hengduan Mountains, Southwestern China

“…Two polygonaceous Himalayan plants, Rheum nobile and R. alexandrae (commonly known as Himalayan ''greenhouse'' plants), have large semitranslucent bracts covering their racemes (Ohba, 1988). Bracts occur on buds during anthesis and mature fruiting stages; may play an important role in dispersal, pollination, and seed germination (Stuessy and Spooner, 1988;Sun et al, 2008); and may also protect the inflorescences against predators and environmental extremes (Knutson, 1981;Stuessy and Liu, 1983;Evans, 1996). Based on measurements on a single inflorescence on R. nobile in Nepal, Omori and Ohba (1999) reported that bracts on the inflorescence elevated the temperature of floral tissues and had insulative effects.…”

Tribal and Subtribal Delimitation and Phylogeny of the Cardueae (Asteraceae): A Combined Nuclear and Chloroplast DNA Analysis

“…This structure has been the main reason for the segragation of Echinopsidinae as a different tribe by Wagenitz (1976), Dittrich (1977), and Petit (1988and Petit ( , 1997. Homologies of the synflorescence of Echinops L. and related genera are ex-tremely difficult to establish (Petit, 1988), as is usually the case with highly derived syncephalies (Stuessy and Spooner, 1988). A similar problem is posed by the complicated involucral structures of the Carlininae, which have been interpreted even as a third-order syncephalia in the genus Carlina L. (Meusel and Kohler, 1960;Meusel and Kä stner, 1994), an interpretation rejected by Petit (1988).…”

“…The receptable of Xeranthemum and related genera has long tapering scales, longer than florets, which have been interpreted usually as homologous to the receptacular bracts of Carlina (true bracts sensu Petit, 1988;Stuessy and Spooner, 1988) by most authors (Kruse and Meusel, 1972). This was the main reason for the inclusion of the Xeranthemum complex in the Carlininae (Bentham, 1873;Hoffmann, 1894;Dittrich, 1977;Bremer, 1994).…”

Flavonoids of the Sunflower Family (Asteraceae)

“…paleae). When present, these paleae also vary in size, shape, and ornamentation, which can have strong taxonomic value, as well as protective functions (Stuessy and Spooner, 1988).The point of attachment of florets in the head is the receptacle. This can be smooth with scarcely any notice of attachment of florets after they have matured and fallen, or, in other instances, it can be honeycombed, with the florets nestled into each little cubicle.…”

“…Cronquist (1955Cronquist ( , 1977 summarized the arguments for regarding Heliantheae as basal (founded in part on criteria articulated earlier by Bentham, 1873) including their woody nature (woodiness was largely believed as primitive and herbaceousness as derived, in part owing to gymnosperms and early angiosperm fossils being woody), their large and often leaf-like receptacular bracts (= paleae), and their often scaly pappus (i.e., more calyx-like). However, there were some who questioned this perspective (e.g., Stuessy and Spooner, 1988) on several grounds, such as that large paleae might simply be an adaptation for protection of larger achenes often found in the tribe, rather than representing ancestral modified bracts.Participants at the 1975 International Compositae conference in Reading, England, added many perspectives toward modifying the classification and phylogeny of the family. Because the conference was organized along existing tribal boundaries (i.e., chapters were devoted to morphology and chemistry of each of the tribes), no new framework of tribes is found in the published volumes (Heywood et aI., 1977).…”