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THE ADAPTIVE AND PHYLOGENETIC SIGNIFICANCE OF RECEPTACULAR BRACTS IN THE COMPOSITAE
Author(s) -
Stuessy Tod F.,
Spooner David M.
Publication year - 1988
Publication title -
taxon
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.819
H-Index - 81
eISSN - 1996-8175
pISSN - 0040-0262
DOI - 10.2307/1220938
Subject(s) - biology , bract , heliantheae , achene , botany , biological dispersal , tribe , inflorescence , population , asteraceae , demography , sociology , anthropology
Summary 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 13 recognized tribes (absent in Arctoteae, Calenduleae, Senecioneae and 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 of features 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.

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