Integrating morphology and anatomy to refine ligule classification in Poaceae

Ligules in the Poaceae are variable in morphology and organization and are potentially useful taxonomically. While membranous ligules have received more attention, other types of ligules are common. Here, we compare morphology and anatomy of the three ligule types (membranous, membranous-ciliate, fringe of hairs) among 18 species in eight of the 12 subfamilies of Poaceae. Mature leaves were examined under optical and scanning electron microscopy. A surprising 11 of 18 species (61%) have ligules that are different from descriptions in the literature. While anatomical diversity exists among all morphological types, including in the abaxial and marginal regions, number of parenchyma layers, presence of intercellular spaces, and presence of sclerenchyma and vascular tissues, species are very consistent in ligule type and structure. Two species, Streptochaeta spicata and Phyllostachys aurea, are discussed in detail due to their notable divergences from published descriptions. We describe the complexity of ligules in the Poaceae and suggest that integrative approaches, including ontogeny, should be used to improve our understanding of ligules, and how they may be used as characters to better understand evolution in the Poaceae.