The conventional concept of an ‘undifferentiated perianth’, implying that all perianth organs of a flower are alike, obscures the fact that individual perianth organs are sometimes differentiated into sepaloid and petaloid regions, as in the early-divergent angiosperms Nuphar, Nymphaea, and Schisandra. In the waterlilies Nuphar and Nymphaea, sepaloid regions closely coincide with regions of the perianth that were exposed when the flower was in bud, whereas petaloid regions occur in covered regions, suggesting that their development is at least partly controlled by the environment of the developing tepal. Green and colourful areas differ from each other in trichome density and presence of papillae, features that often distinguish sepals and petals. Field experiments to test whether artificial exposure can induce sepalness in the inner tepals showed that development of sepaloid patches is initiated by exposure, at least in the waterlily species examined. Although light is an important environmental cue, other important factors include an absence of surface contact. Our interpretation contradicts the unspoken rule that ‘sepal’ and ‘petal’ must refer to whole organs. We propose a novel theory (the Mosaic theory), in which the distinction between sepalness and petalness evolved early in angiosperm history, but these features were not fixed to particular organs and were primarily environmentally controlled. At a later stage in angiosperm evolution, sepaloid and petaloid characteristics became fixed to whole organs in specific whorls, thus reducing or removing the need for environmental control in favour of fixed developmental control.
Perianth differentiation into distinct morphological whorls (sepals and petals) can be difficult to assess, particularly in early-divergent angiosperms. The perianth of members of Nymphaeales (Cabomba, Brasenia, Barclaya, Euryale, Nuphar, Nymphaea, Victoria, Ondinea) has been described as differentiated into sepals and petals, undifferentiated or presenting examples of both states. In this paper, we review perianth structure in Nymphaeales using morphology and ontogeny in order to determine whether their perianths are dimorphic.The results indicate that although there are differences between the outer and inner perianth series, the organs display few "typical" sepal-petal characteristics. Our observations of the perianth of Nuphar and Nymphaea, and reports on Euryale, show that sepaloid (green) and petaloid (yellow) areas can both occur on individual perianth organs, and that sepaloid areas occur mostly in the regions of the perianth organ that were exposed when the flower was in bud. SEM study revealed further morphological differences between the sepaloid and petaloid areas of Nuphar and Nymphaea and also between the exposed and covered regions of the perianth of Cabomba, Brasenia and Barclaya. We discuss the implications of these results with respect to current concepts of perianth differentiation.
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