THE UNITED STAMENS OF Christian Puff, Anton Igersheim,RUBIACEAE. MORPHOLOGY, Ralf Buchner, and Ulrike Rohrhofer2ANATOMY; THEIR ROLE INPOLLINATION ECOLOGY'ABSTRACT Based on morphological and anatomical investigations of selected species of Argostemma, Steenisia, Strumpfia,Acranthera, Antherostele, and Posoqueria, and reports in the literature about several other taxa, the paper surveyshow unions in the androecium of Rubiaceae can corne about. In Argostemma, the formation of anther tubes is eitherby union of adjacent apical connective appendages and agglutination of neighboring pollen sac walls, or only by unionof connective appendages (with or without sterile remnants of pollen sacs); the anthers dehisce longitudinally and thesubepidermal cell layer (endothecium) has thickenings. Argostemma, moreover, has species without proper anthertubes in which the anthers (with apical pores and a pollen sac wall without thickenings in the subepidermal cell layer)are, nevertheless, variously grouped together. The anther tube of Steenisia cornes about by the interlocking ofepidermal layers of neighboring apical connective appendages and agglutinated cuticles. Strumpfia is unique in havinga special cell layer uniting ail anthers. The strictly cylindrical anther tube of Antherostele is, like in the Asteraceae,held together by agglutinated cuticles of neighboring pollen sacs; apical connective appendages are not involved inthe tube formation. Acranthera has an "anther-style and stigma complex" (stamens, united by apical connectiveappendages, are firmly connected with the stigma; descriptions and comments about the highly unusual stigmaticareas in this genus are also included). Posoqueria has an anther head, which is held together only by agglutinated,sterile appendages or extensions of the thecae that are produced both basally and apically. The unstable nature ofthe anther complex is a prerequisite for proper function of Posoqueria's unique "catapult mechanism" of the stamens,by which pollen is hurled at the pollinator. Argo'temma, Steenisia, Neurocalyx, and Strumpfia are believed to bebuzz pollinated. In taxa with longitudinal anther dehiscence (Argostemma in part, Steenisia, Neurocalyx), the apicallyconstricted anther tubes with porelike openings are thought to represent structures analogous to the poricidallydehiscent anthers of buzz-pollination systems. It is speculated that at least some Acranthera species may also be buzzpollinated; the windows formed by the elongated connective appendages above the longitudinally dehiscent anthersmight take over the function of pores. Agglutinations or fusion in the androecium, orunions by other means, are not confined to the"Synandrae" (= Asteridae sensu stricto) but alsooccur in various other families, including a numberof Rubiaceae. In one rubiaceous genus, there iseven an "anther-style and stigma complex." Thesephenomena are often imprecisely and vaguely de-scribed as "anther tube," "anther cone," "anthersconvergent," "anthers connivent," or "anthersconnate." One of the aims of the present paper is to showthe range of variation of the "synandry" syndromein the Rubiaceae, which apparently has evolvedindependently in various alliances. Detailed mor-phological and anatomical documentation, basedon the investigation of preserved material and sup-plemented by field observations, will be presentedfor selected taxa. The presence of united anthers ("united" is usedhere in a neutral and wide sense, i.e., presence ofagglutinations, proper fusion, or union by othermeans in the androecium), as well as shape, size,color, presentation, and orientation of the entireandroecium, appears to be intimately linked withpollination biological specializations.MATERIALS AND METHODS Detailed morphological and anatomical studieswere carried out on the following taxa and collec-tions (material preserved in 70% alcohol or FPA;vouchers deposited in WU, unless stated other-wise). Methods used for SEM (scanning electronmicroscope) investigations and microtome tech-niques are described in Puff et al. (1993) andIgersheim (1993a). **: carried out on reconsti-' This research was supported by the "Fonds zur F5irderung der Wissenschaftlichen Forschung" (Project P8260-BIO). Thanks are extended to W. Dvorak for help with the plates. 2 Ail: Institute of Botany, University of Vienna, Rennweg 14, A-1030 Wien, Austria. ANN. MISSOURI BOT. GARD. 82: 357-382. 1995.