AOBPreview originally published online on October 7, 2006
Annals of Botany 2006 98(6):1167-1178; doi:10.1093/aob/mcl209
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Elaborate Petals in Australian Spermacoce (Rubiaceae) Species: Morphology, Ontogeny and Function
1 Laboratory of Plant Systematics, Institute of Botany and Microbiology, Katholieke Universiteit Leuven Kasteelpark Arenberg 31, BE-3001 Leuven, Belgium
2 National Herbarium of the Netherlands, Leiden University Branch PO Box 9514, NL-2300 RA Leiden, The Netherlands
3 National Botanic Garden of Belgium, Domein van Bouchout BE-1860 Meise, Belgium
*For correspondence at: Royal Botanic Gardens, Kew, Seed Conservation Department, Wakehurst Place, Ardingly, West Sussex RH17 6TN, UK. E-mail e.vaes{at}rbgkew.org.uk
Received: 9 June 2006 Returned for revision: 26 July 2006 Accepted: 30 August 2006 Published electronically: 7 October 2006
• Background and Aims Australian Spermacoce species display various types of elaborate petals. Their precise morphology, ontogenetic origin, and function are hitherto unknown. The aim of the present paper is to unravel the development and nature of the diverse types of elaborate petals in Spermacoce through a floral ontogenetic study.
• Methods The floral ontogeny of six species characterized by different types of corolla appendages was studied by scanning electron microscopy and light microscopy. In order to elucidate the possible functions of the elaborate petals, field observations were conducted as well.
• Key Results Scanning electronmicrographs show that full-grown petals of Spermacoce lignosa, S. phaeosperma and S. redacta bear appendages on their ventral side. Despite their different appearance at anthesis, the appendages develop very similarly in all three species. They are initiated at the same developmental stage and are first visible as two arcs of primordia converging from the upper margins of the petal towards its midvein and downwards. In S. brevidens, S. caudata and S. erectiloba, the full-grown petals have two long, concave protuberances, which develop from the tissue at both sides of the petal's mid-vein. In these three species, initiation of appendages on the ventral side of the petals is also observed, but they are hardly visible on the mature petals. The two types of elaborate petals tightly enclose the anthers, both in bud and during most of the flowering period.
• Conclusions Among Australian Spermacoce species, two types of elaborate petals can be distinguished. The former hypothesis that the two types of elaborate petals are essentially homologous is here rejected. Field investigations point out that the elaborate petals might play a role in the pollination biology of the species.
Key words: Australia, corolla appendages, elaborate petals, floral ontogeny, pollination, Rubiaceae, Spermacoce brevidens, S. caudata, S. erectiloba, S. lignosa, S. phaeosperma, S. redacta
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