Enigmatic floral structures in Alternanthera, Iresine, and Tidestromia (Gomphrenoideae, Amaranthaceae). A developmental homology assessment

Background and aims Amaranthaceae comprise the former Chenopodiaceae, Beta, and core-Amaranthaceae. Particularly in the Gomphrenoid clade, most species have flowers with an 'androecial tube'. On its rim and alternating with the stamens, so-called (pseudo-)staminodia occur. However, neither their origin nor function have been clarified. We aimed to determine the nature of both the 'androecial tube' and pseudostaminodia in species of three genera: Iresine, Alternanthera and Tidestromia. In addition, we also aimed to document the development of the gynoecium. Methods Scanning electron microscopic (SEM) and light microscopic (LM) observations of the floral development. Key results Five individual sepal primordia appear in quincuncial order, followed by the appearance of fiveindividual stamen primordia opposite them. At semi-maturity, the stamens are raised by the formation of a tube below them, with at its inside surface a glandular zone. Alternating with the stamens, appendages develop from individual primordia on the rim of the androecial tube. In bisexual/female flowers, from a primary gynoecium primordium, secondarily, an annular gynoecium wall primordium is formed, surrounding a central zone from which a single ovule develops. At maturity, the gynoecium stands on a gynophore. In Alternanthera microcephala, the gynophore may become large, sometimes developing appendages. Conclusions Our hypothesis that the androecial tube develops from an underlying annular intercalary meristem is supported by the absence of an annular androecium primordium and the late development of the androecial tube. The 'pseudostaminodes' are appendages of the androecial tube. Their often conspicuous appearance and the glandular inside surface of the androecial tube suggest an animal(insect)-based pollination syndrome. We consider the androecial tube, the staminal ring in flowers of Beta vulgaris and the 'fused flaments' in some Chenopodioideae as morphological homologues. We suggest that the development of the gynoecium wall might be decoupled from the development of the ovule. © 2014 Botanic Garden Meise and Royal Botanical Society of Belgium.