Abstract
European species of Nuphar are amongthe most accessible members of the basal angiosperm grade, but detailed studies using scanning electron microscopy are lacking. We provide such data and discuss them in the evolutionary context. Dorsiventral monopodial rhizomes of Nuphar bear foliage leaves and non-axillary reproductive units (RUs) arranged in a Fibonacci spiral. The direction of the phyllotaxis spiral is established in seedlings apparently environmentally and maintained through all rhizome branching events. The RUs can be located on dorsal, ventral or lateral side of the rhizome. There is no seasonality in timing of their initiation. The RUs usually form pairs in positions N and N + 2 along the ontogenetic spiral. New rhizomes appear on lateral sides of the mother rhizome. A lateral rhizome is subtended by a foliage leaf (N) and is accompanied by a RU in the position N + 2. We hypothesize a two-step process of regulation of RU/branch initiation, with the second step possibly involving environmental factors such as gravitropism. Each RU has a short stalk, 1-2 scale-like phyllomes and a long-pedicellate flower. We support a theory that the flower is lateral to the RU axis. The five sepals initiate successively and form two whorls as 3 + 2. The sepal arrangement is not ‘intermediate’ between whorled and spiral. Mechanisms of phyllotaxis establishment differ between flowers and lateral rhizomes. Petal, stamen and carpel numbers are not precisely fixed. Petals are smaller than sepals and form a whorl. They appear first in the sectors of the outer whorl sepals. The stamen arrangement is whorled to chaotic. The merism of the androecium tends to be the same as in the corolla. Flowers with odd numbers of stamen orthostichies are found. These are interpreted as having a non-integer merism of the androecium (e.g., 14.5). Carpels form a whorl in N. lutea and normally alternate with inner whorl stamens. Sterile second whorl carpel(s) are found in some flowers of N. pumila.
Highlights
The question of the origin and early evolution of angiosperms and angiosperm flowers remains one of key problems of evolutionary botany (Bateman et al, 2006; Doyle, 2008, 2012; Friis et al, 2011; Herendeen et al, 2017; Wang, 2018; Coiro et al, 2019; Bateman, 2020)
Throughout the centuries of research in developmental plant morphology, European species Nuphar (Nymphaeaceae, Nymphaeales) have been among the most accessible plants currently recognized as members of the basal angiosperm grade
The one-flowered reproductive units (RUs) with longpedicellate flower as well as the long-petiolate foliage leaves with floating blades are spirally arranged along a massive thick creeping monopodial rhizome
Summary
The question of the origin and early evolution of angiosperms and angiosperm flowers remains one of key problems of evolutionary botany (Bateman et al, 2006; Doyle, 2008, 2012; Friis et al, 2011; Herendeen et al, 2017; Wang, 2018; Coiro et al, 2019; Bateman, 2020). In spite of the great amount of relevant publications and controversial morphological interpretations (Trecul, 1845; Raciborski, 1894a,b; Cutter, 1957a,b, 1958, 1959, 1961; Dormer and Cutter, 1959; Chassat, 1962; Moseley, 1965, 1972; Wolf, 1991; Igersheim and Endress, 1998; Endress, 2001; Schneider et al, 2003; Padgett, 2007; Endress and Doyle, 2009) a comprehensive developmental study of European species of Nuphar using scanning electron microscopy is lacking We are filling this gap and discuss the importance of Nuphar for understanding early evolution of angiosperms
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
