Extrafloral nectaries in Fabaceae: filling gaps in structural and anatomical diversity in the family

Abstract

Extrafloral nectaries (EFNs) are particularly common in Fabaceae, and their morphological diversity has proved taxonomically useful. However, their anatomy remains poorly investigated. In this study we analysed EFN anatomy in 12 species from ten genera of Fabaceae, using scanning electron microscopy and light microscopy. We found three main structural organizations of EFNs. (1) The ?embedded? EFNs (cryptic and externally non-visible) in Bauhinia forficata are secretory prickles in which nectar-secreting tissue is completely embedded within the prickle,representing a new kind of EFNs in plants. Among externally visible EFNs, (2) ?elevated? EFNs were found in seven species, including a Mimosa sp. belonging to a clade considered to lack EFNs, and (3) ?swollen-scar? EFNs derived from the abscission site of aborted buds were found in four species, including a new report in Galactia. In three species, we also found colleters anatomically similar to other colleters reported for Fabaceae. We discuss terminological issues in the (scarce) literature on EFN anatomy in Fabaceae, mainly due to incorrect interpretation of structures or misuse of existing terms.