| Summary: | Eriocaulaceae (Poales) differ from potentially related Xyridaceae in pattern of floral organ arrangement relative to subtending bract (with median sepal adaxial). Some Eriocaulaceae possess reduced and non-trimerous perianth, but developmental data are insufficient. We conducted a SEM investigation of flower development in three species of <i>Eriocaulon</i> to understand whether organ number and arrangement are stable in <i>E. redactum</i>, a species with a highly reduced calyx and reportedly missing corolla. Early flower development is similar in all three species. Male and female flowers are indistinguishable at early stages. Despite earlier reports, both floral types uniformly possess three congenitally united sepals and three petals in <i>E. redactum</i>. Petals and inner stamens develop from common primordia. We assume that scanning electron microscopy should be used in taxonomic accounts of <i>Eriocaulon</i> to assess organ number and arrangement. Two types of corolla reduction are found in Eriocaulaceae: suppression and complete loss of petals. Common petal–stamen primordia in <i>Eriocaulon</i> do not co-occur with delayed receptacle expansion as in other monocots but are associated with retarded petal growth. The ‘reverse’ flower orientation of <i>Eriocaulon</i> is probably due to strictly transversal lateral sepals. Gynoecium development indicates similarities of Eriocaulaceae with restiids and graminids rather than with Xyridaceae.
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