Abstract
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 Eriocaulon to understand whether organ number and arrangement are stable in E. redactum, a species with a highly reduced calyx and reportedly missing corolla of female flowers. 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 E. redactum. Petals and inner stamens develop from common primordia. We assume that scanning electron microscopy should be used in taxonomic accounts of Eriocaulon 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 Eriocaulon do not co-occur with delayed receptacle expansion as in other monocots but are associated with retarded petal growth. The ‘reverse’ flower orientation of Eriocaulon is probably due to strictly transversal lateral sepals. Gynoecium development indicates similarities of Eriocaulaceae with restiids and graminids rather than with Xyridaceae.
Highlights
The occurrence of trimerous pentacyclic flowers is a stable feature of many monocot lineages and one of key characteristics of monocots as a natural group [1,2,3,4]
We aimed to reveal whether this unusual gynoecium orientation is present in the Eriocaulon cinereum group and how stable is the pattern of gynoecium orientation here
Eriocaulon redactum stalked inflorescence (Figure 2) possesses a short and broad receptacle covered by spirally arranged involucral bracts followed by numerous flower-subtending bracts
Summary
The occurrence of trimerous pentacyclic flowers (with 3 + 3 perianth members, 3 + 3 stamens and 3 carpels) is a stable feature of many monocot lineages and one of key characteristics of monocots as a natural group [1,2,3,4]. Trimerous flowers of almost all monocots lacking floral prophylls (bracteoles) on lateral pedicels have a median outer whorl perianth member inserted abaxially and two other outer whorl perianth members in transverse-adaxial positions (Figure 1A). Exceptions from this rule are extremely rare and include the basal monocot family. Araceae are remarkable in the total absence of flower-subtending bract (that plays important morphogenetic roles in other monocots) and overall high variation of flower groundplan [6,12].
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