Notice: '?' represent unknown information

Aquatic herbs, internodes short, ?sympodial construction; mycorrhizae 0; starch grains compound; radicle aborts; root apex with secondary dermatogen, etc., epidermis derived from outer layer of cortex [?Hydatellaceae]; trichoblasts in vertical files, proximal cell smaller; root stele monarch; diaphragms in root aerenchyma; primary stem with scattered vascular bundles; cauline endodermis + (0); protoxylem lacunae +; vascular cambium 0; nodes?; articulated laticifers +; vascular bundles lacking associated sclerenchyma; aerenchyma common; 4-celled uniseriate secretory trichomes with a large terminal cell [= hydropoten]; stomata anomocytic; leaves spiral, ?lamina margins, leaf base broad; bracts 0; pollen boat-shaped, tectum continuous, infratectal space large, columellae conspicuous; ovule with semi-annular [hood-shaped] outer integument; first division of endosperm transverse, chalazal cell undivided; P persistent; fruit maturation underwater; seed operculate, endotegmen below operculum, hilum outside; seed coat exotestal, palisade/?exotestal cells low, thin-walled; endosperm scanty, [develops from micropylar cell], chalazal endosperm haustorium +, single-celled, perisperm +, starchy, precocious, cells multinucleate, embryo broad, cotyledons connate; intergenic inversion in chloroplast inverted repeat; germination hypogeal. - 3 families, 6 genera, 80 species.

Note: In all node characterizations, boldface denotes a possible apomorphy, (....) denotes a feature the exact status of which in the clade is uncertain, [....] includes explanatory material; other text lists features found pretty much throughout the clade. Note that the precise node to which many characters, particularly the more cryptic ones, should be assigned is unclear. This is partly because homoplasy is very common, in addition, basic information for all too many characters is very incomplete, frequently coming from taxa well embedded in the clade of interest and so making the position of any putative apomorphy uncertain. Then there are the not-so-trivial issues of how character states are delimited and ancestral states are reconstructed.

Age. The age of crown-group Nymphaeales is around 125 Ma (Magallan et al. (2015), (133.2-)126.7(-120.6) Ma (Iles et al. 2014), (160-)132(-114 Ma (Salomo et al. 2017), as little as (176.6-)97.7(-42.8) Ma (Zhou et al. 2014), or as much as ca 164 (Z. Wu et al. 2014), ca 188 Ma (Tank et al. 2015), or even ca 209 Ma (Foster et al. 2016a: q.v. for details).

The curious fossil Archaefructus, probably an aquatic plant and about 124 Ma old, has been linked with Hydatellaceae in morphological analyses (e.g. Doyle & Endress 2007, 2010; Doyle 2008b; Endress & Doyle 2009); see Sokoloff et al. (2011) for an evaluation. Although the two have very little in common in terms of overall appearance, Archaefructus may be another early aquatic angiosperm with very unconventional floral morphology.

Evolution: Divergence & Distribution. Cretaceous fossils assignable to Nymphaeaceae are quite common and these are discussed below under Cabombaceae and Nymphaeaceae. Indeed, it has been suggested that Nymphaeales were "the first globally diverse clade" (Borsch & Soltis 2008: p. 1051; see also Sender et al. 2010). Although looking only at extant taxa, their diversity may seem slight even if their morphology is difficult to interpret (see Crane & Friis 2020 for the way forward), Nymphaeales as a whole are aquatic herbs, again, the first such group represented by extant herbaceous hydrophytes, and some are very highly specialized.

Saarela et al. (2007) suggest a few additional possible synapomorphies for Nymphaeales, and Zeng et al. (2014) thought that having protoxylem lacunae and vascular bundles lacking associated sclerenchyma might be apomorphies. For the development of root hairs, see e.g. Clowes (2000) and Sokoloff et al. (2008a). The basic plant construction in Nymphaeales may be sympodial (see Kraehmer et al. 2023 and discussion). Borsch et al. (2007) outline the evolution of a number of floral characters within the order, M. L. Taylor et al. (2015) outline palynological variation in the context of its phylogeny, and Endress and Doyle (2015) look at floral morphology. Understanding where some of these features should be placed on the tree is complicated by the highly autapomorphic nature of Hydatella and some uncertainty over the position of Nuphar.

The development of perisperm (2n, maternal, c.f. endosperm, 2n, maternal/paternal) is an apomorphy for the order. Povilus et al. (2018 - read the details carefully) examined seed development in Nymphaea thermarum and interpreted perisperm development there as a way for the female parent to control resource allocation to the offspring. In general, the endosperm is slight if quite persistent and it probably functions as transfer tissue between embryo and perisperm (Friedman et al. 2012).

As might be expected, endomycorrhizae are at most uncommon in Nymphaeales (e.g. de Marins et al. 2009).

Chemistry, Morphology, etc.. It is possible that there are epidermal oil cells in Nymphaeaceae (Wilkinson 2006); do they contain ethereal oils? Hydrolysable tannins in this group (e.g. in Nuphar) are different to those found elsewhere (Gottlieb et al. 1993; Ishimatsu et al. 1989), although the chemistry of Hydatellaceae is poorly known.

A cauline endodermis is common in Nymphaeales although by no means universal; it may surround all or some of the individual vascular bundles (Seago 2020; C. Yang et al. 2019: Cabomba, all). Hydatellaceae (Trithuria filamentosa) are reported to have vessel elements with scalariform perforation plates throughout the plant (Cheadle & Kosokai 19), however, Carlquist and Schneider (2009) could not find them when they examined the original sections. For micromorphological details of tracheids, see e.g. Carlquist and Schneider (2009), E. L. Schneider et al. (2009), Schneider and Carlquist (2009a, b); details of the wall structure of tracheids, at least, in Cabombaceae and Nymphaeaceae are very distinctive. Roots of Cabombaceae and Nymphaeaceae may have vessels of a sort (Carlquist & Schneider 2009; Schneider & Carlquist 2009). The distinctive uniseriate trichomes found in all groups may secrete nectar or mucilage, or they may be involved in ion exchange (Vogel 1998a); Wilkinson (2006) calls the trichomes on the leaves, hydropotes.

The inner bracts found in some Hydatellaceae and the inner petals of Cabomba are notably slow in developing (Rudall et al. 2007). If the corolla represents sterilised stamens, as some think, having external staminodes will probably be another synapomorphy at least for [Nymphaeaceae + Cabombaceae]. For discussion about the presence of a granular infratectum in Nymphaeales, see M. L. Taylor et al. (2013, 2015, also 2014 for other characters); the infractectum is generally columellate, how obviously so depending on the thickness of the infratectal space. Some genera in all families have exotestal cells that are neither very tall nor much thickened (Hamann et al. 1979; Collinson 1980). Rudall et al. (2009b) discuss the distinctive single-celled chalazal endosperm haustorium, the presence of which is sometimes suggested by a space between the embryo plus endosperm and the perisperm. Baskin and Baskin (2018) discuss embryo morphology and seed germination.

Phylogeny. Hydatellaceae are sister to Xyridaceae in Stevenson et al. (2000; see also Stevenson & Loconte 1995); both have latrorse anthers and seeds with an operculum "stopper" that is tegmic in origin. Trithuria and Xyris appear as sister taxa (weak support) and in turn are sister to Mayaca (still weaker support), although other Xyridaceae are not immediately related in the phylogeny of Michelangeli et al. (2003). However, although Bremer (2002) noted that Mayacaceae and Hydatellaceae might be weakly associated with Xyridaceae or Eriocaulaceae, depending on what taxa were included in the analysis, there were a number of long branches in this area and he excluded the first two families from his final analysis, while Janssen and Bremer (2004) suggested that the association of Hydatellaceae with Mayacaceae was probably an artefact (see also Chase et al. 2006). Subsequent studies (Saarela et al. 2006, esp. 2007: several genes from two compartments, morphology; Friis & Crane 2007: commentary) placed Hydatellaceae firmly with Nymphaeales, and sister to [Cabombaceae + Nymphaeaceae]; the sequence that placed Hydatellaceae in Poales was a chimaeric pcr recombinant involving a grass and a moss.

Hydatellaceae aside, for a morphological phylogeny of [Cabombaceae + Nymphaeaceae], see D. W. Taylor (2008). Y.-L. Liu et al. (2005) provide an ITS phylogeny focussing on Nymphaeaceae, but with some at first sight rather surprising relationships - [Nuphar [Cabomba + Brasenia] [Nymphaea [Euryale + Victoria]]]. Nelumbo, which was included in the analysis, did at least stay outside this clade... However, this position of Nuphar is recovered in other analyses, too (e.g. Borsch et al. 2008; L?ne et al. 2007; D. W. Taylor & Gee 2014: some analyses; Gruenstaeudl et al. 2017) and so its position below as sister to Nymphaea, etc., may be provisional. He et al. (2018: chloroplast data) found that Nuphar was sister to Nymphaea and its associates, and with quite strong support; interestingly, a UPGMA dendrogram of RNA editing sites linked Nuphar with Cabombaceae. Moreover, in a renalaysis of the data in He et al. (2018), Gruenstaeudl (2019) found that how the data were partitioned affected the results, and individual gene trees also gave different results, thus almost as many supported a relationship of Cabombaceae with Nuphar as supported an association of Nuphar with Nymphaea and friends. H.-T. Li et al. (2019) found a [Nuphar + Nymphaea] association.

Previous Relationships. Many of the morphological features of Hydatellaceae that made it so different from other monocots are consistent with a position in Nymphaeales. Hamann (1998) had even noted that the antipodal cells were absent or degenerated early, and absence of these cells would be expected if Hydatellaceae were to be placed here - and Friedman (2008a) and Rudall et al. (2008) found that Hydatellaceae had the distinctive 4-celled embryo sac of other Nymphaeales and of Austrobaileyales.

Classification. Doweld (2022) placed members of Nymphaeales - extant and fossil - in two subclasses, four orders and six families; 32 fossil genera are recognized (over two thirds are in Nymphaeaceae), and one order and one family are based on fossils alone.