CLASS NYMPHAEOPSIDA (Brongniart 1843) & ANGIOSPERM ORIGINS
| EUKARYA> ARCHAEPLASTIDA> VIRIDIPLANTAE> STREPTOBIONTA> EMBRYOPHYTA> TRACHEOPHYTA> SPERMOPHYTA> ANGIOSPERMOPHYTA> NYMPHAEOPSIDA |
ANGIOSPERMOPHYTA LINKS
| Nymphaeopsida (nim-fe-OP-si-duh) is derived from two Greek roots which mean nymph (nymphe- νύμφη) and that which resembles (opsis -οψισ). Together, they mean that which resembles a nymph, a beautiful supernatural being of Greco-Roman mythology. The class name is derived from the genus Nymphaea, water lilies. Opsida, an ending derived from opsis, is the standard suffix for a plant class name. This name is my invention for a higher level taxon at the class-level. Clearly, these taxa are not monophyletic. |
INTRODUCTION TO THE CLASS NYMPHAEOPSIDA
The basal angiosperm families include a paraphyletic collection of taxa at the base of the angiosperm tree (See Figure 1). The group is referred to as the ANITA grade, a name that is derived from the first letter of five families: Amborellaceae, Nymphaeaceae, Illiciaceae, Trimeniaceae, and Austrobaileyaceae. Most of the plants in this group are shrubs and small trees, but one order, Nymphaeales, is aquatic. According to Judd et al. (2002), the primitive characters of this collection of taxa include the following:
- Carpel margins are sealed by a secretion and ascidiate (they develop like a tube).
- The stigma (text with tooltip) The apex of the style, usually enlarged, on which the pollen grains land and germinate. is elongate with a short style (text with tooltip) The elongated potion of a carpel that bears the stigma at its tip. .
- Stamens show poor differentiation between the filament (text with tooltip) A filament is a linear array of cells. In the Cyanobacteria, a filament is the linear array of cells (trichome) plus the surrounding mucilaginous sheath. and the anther (text with tooltip) An anther (n.) is a part of the stamen that produces pollen. .
- Pollen is monosulcate (text with tooltip) A pollen grain with one groove or furrow. .
- Generally the flowers are insect-pollinated, and actinomorphic (text with tooltip) It is derived from two Greek roots that mean ray of light (aktina-ακτίνα) and form (morphos- μορφή). This is an adjective that defines the structure of a flower according to its symmetry. An actinomorphic flower is radially symmetrical. That is, such a flower is divisible through the center of the flower in several or many longitudinal planes such that the halves form mirror images in each case. with free floral parts including a perianth (text with tooltip) A collective term for the outer, nonreproductive, parts of a flower, often differentiated into calyx and carolla. of tepals (text with tooltip) One of the petals or sepals of a flower in which the perianth segments closely resemble each other. .
- Usually the axes are without vessels (text with tooltip) (1) Vessels are special xylem cells that have a large diameter and can move larger amounts of water than the smaller tracheids. Vessels, though found in the gnetophytes, are characteristic of the flowering plants. (1) Vessels are special xylem cells that have a large diameter and can move larger amounts of water than the smaller tracheids. Vessels, though found in the gnetophytes, are characteristic of the flowering plants. .
 | FIGURE 1. CLADOGRAM OF THE BASAL ANGIOSPERM TAXA WITHIN THE CONTEXT OF THE FLOWERING PLANTS. The cladistic relationships between the basal orders (in the green box). Note that three are nested and Chloranthales is sister to the Magnoliids. Amborellales (with the single species Amborella trichopoda) is the sister to all extant angiosperms. The location and relative positions of the three basal families are consistent through almost all studies of the past 10 years (e.g. Mathews and Donoghue 2000, Soltis et al. 1999a, Hilu et al. 2003, Jansen et al. 2007, APG I 1998, APG II 2003, APG III 2009). The topology of this cladogram is derived from Moore et al. (2007) and APG III (2009). |
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| FIGURE 2. Amborella trichopoda; leaves and flowers of the most primitive living flowering plant. This plant was growing in the Florida International University Greenhouse. Image by Scott Zona and under Creative Commons License. | FIGURE 3. Illicium anisitum, Star Anise, leaves and flowers. Image by Kenpei and licensed through Creative Commons |
THE BASAL ANGIOSPERM FAMILIES (THE ANITA GRADE)
The most primitive of all living flowering plants known is Amborella trichopoda (Figure 2), a small woody shrub of New Caledonia, an island of the South Pacific near Australia. It is the only species in the Amborellales and has been consistently flagged as a sister to all other living flowering plant groups (e.g. Mathews and Donoghue 2000, Soltis et al. 1999a, Hilu et al. 2003, Jansen et al. 2007, APG I 1998, APG II 2003, APG III 2009).
Austrobaileyales is a basal order that contains three families. They are all shrubs and small trees that produce flowers similar to those of Nymphaeaceae except that the inner tepals are smaller and almost like the stamen segments. They occur mainly in the Asian and American tropics. One genus, Illicium (Figure 3), commonly called Star Anise, is sometimes used as an ornamental and as a spice.
The most widely distributed members of the Basal Families are in the Nymphaeales, which now includes three families of plants that are aquatic, usually in ponds and sheltered lake margins. APG II (2003) indicated that the Nymphaeales contained only one family; however, APG III (2007) bowed to the “general preference” to separate the Cabombaceae (Figure 4) from the Nymphaeaceae, waterlilies (Figures 5 and 6) and spatterdocks. Also, Friis and Crane (2007) and Saarela et al. (2007) provided convincing evidence that the grass-like plants in the Hydatellaceae (Figure 7) were not relatives of the grasses, but highly reduced aquatic plants that are sisters to Nymphaeaceae + Cabombaceae. Plants of the Nymphaeaceae occur globally although their diversity is not great (8 genera and 70 species, 40 of which are Nymphaea). The typical growth habit is a large fleshy rhizome with many scattered vascular bundles (atactostele?). Floating leaves emerge from the rhizomes on long, flexible petioles. The flowers are showy, actinomorphic, perfect and with multiple whorls of tepals, stamens and simple pistils. The stamens and stigmas are petaloid. Fruits vary from fleshy ( berries (text with tooltip) A fleshy indehiscent fruit with the seed or seeds immersed in pulp. ), to dry dehiscent ( capsules (text with tooltip) The sporangium of the sporophyte; elevated by the seta. ), to dry indehiscent ( nuts (text with tooltip) A dry, one-seeded, indehiscent fruit with a woody pericarp. ). Most of these plants do not make nectar but do produce copious amounts of pollen. Victoria and Nymphaea make special starchy structures to attract pollinators, especially beetles. Also, the flowers generate slightly elevated levels of heat. Victoria is one of the largest herbaceous plants with leaves more than 1.2m wide. Originally found in the Amazon River basin, Victoria species have been found as far south as northern Argentina. Most taxa of this family have floating leaves that are much smaller. Nymphaea is prized by water gardeners world-wide. Their flowers are large showy (Figure 2-B & C) and they are relatively easy to grow. The most common species, N. odorata, has been bred into many varieties. The Nymphaeaceae is considered to be one of the basal families (APG II 2003, APG III 2009) in the flowering plants. Although the vegetative morphology is highly derived, the floral characters are primitive (e.g. many floral segments, many tepals, actinomorphic flowers, petaloid stamens, and many separate or free pistils).
The order Chloranthales, had been difficult to characterize and place within a larger phylogenetic scheme (e.g. APG II). Like plants of the Austrobaileyales, the 24 species of the Chloranthaceae are small trees and shrubs which have distinctive serrated margins of their evergreen leaves (Figure 8). The flowers are small and reduced and the fruit is a small drupe (text with tooltip) A fleshy fruit containing one or more seeds, each with a stony endocarp. . Moore et al. (2007) established that the order was one of the basal groups of flowering plants and likely the sister group to the Magnoliids. That view was held by APG III (2009). Although, the group is not a member of the magnoliids, it clearly is not associated with the other three orders of the basal groups.
| ORIGIN OF THE ANGIOSPERMS The structures of the basal families provide some clues as to the origin of the flowering plants. Archaefructus (Figure 9), a genus of fossil plants collected in China (Sun et al. 2002, Ji et al. 2004), is consistent with many of the presumed primitive characters. Whether they came from the upper Jurassic or lower Cretaceous, the earliest known angiosperms were small, herbaceous aquatic plants that had highly dissected leaves, very much like Cabomba (Figure 4). The “flower” lacked petals and sepals and appeared to be a bisexual inflorescence (Figure 9). In addition, one segment on the fossil impressions reported by Ji et al. (2004) had two carpels associated with stamens, a “true bisexual flower”. Certainly, this was not the last word in the theories that abound in the origins of the flowering plants. Endress and Doyle (2009) present some of the problems in alignment between Archaefructus and the ANITA plants. At issue are concerns about how primitive the ANITA plants really are, and the assertion that their apparent primitive characters are reductions. In fact, Endress and Doyle (2009) conclude that the most basal flowering plants would be more like Ceratophyllum, itself a taxonomic nomad and included in this system as a separate class (the Basal Dicots = Ceratophyllidopsida). Philippe et al. (2008) explore the fossil record for remains of angiosperm wood during the early Cretaceous and infer that because the earliest angiosperms were not woody (or only weakly so), they remained minor components of the seed plant communities. The sudden radiation of flowering plants was associated. in part, with the production of woody stems. Field and Arens (2008) look to the ANITA group and compare members of the early angiosperm radiation to the living basal taxa with many positive correlations such as growth habit and cuticle. They suggest that the earliest flowering plants appeared in more upland habitats, some of which radiated into aquatic environments. So, although Archaefructus is one of the earliest flowering plants, it may just be because the plants grew in still water aquatic conditions in which fossilization would most likely occur. Either way, early angiosperms, similar to ANITA taxa, displayed characters like the production of net-veined leaves, lignified stems, and insect-pollinated flowers, a collective of characters that allowed them to explode in diversity by the middle Cretaceous. Other plants from the early Cretaceous have been unearthed in China. Dilcher et al. (2007) and Sun et al. (2008) describe Hyrcantha, a plant that seemed to be aquatic and possibly affiliated with the Eudicots. Some of the fossils that they present are in full fruit, which clearly are carpels with many seeds at the tips of the stems. They suggest that the fruits may have been held out of the water. Sun et al. (2011) report the occurrence of another fossil genus, Leefructus, which also seemed to be an aquatic plant and co-occurred with Archaefructus and Hyrcantha in sediments 122.9-124.4 mya. Features of Leefructus appear to affiliate it with the Rannunculales, an order of basal Eudicots. Also, tricolpate pollen, a defining feature of the Eudicots (also called the Tricolpates) had been found in strata dating to 127 mya. That they occur so early in the Cretaceous suggests that the flowering plants already were diversified. Sun et al. (2001) consider the possibility that the flowering plants appeared prior to the Cretaceous. Another equally contentious question is that of the group of seed plants from which the flowering plants emerged. The Anthophyte Hypothesis, the theory that gnetophytes are the living sisters of the flowering plants is supported by morphological, paleontological, and developmental studies (Friedman 1990a, 1990b, 1992, 1994; Doyle 2006; Hilton and Bateman 2006; and Tomescu 2008). However, molecular studies (see Chaw et al. 2000, Bowe et al. 2000, and Qiu et al. 2006 & 2007) almost always support a relationship between the gnetophytes and the Pinaceae (the Gne-Pine Hypothesis, also reviewed in the Gymnosperms page) and push the origin of the angiosperms to a group of pteridosperms. The Anthophyte Hypothesis will not go away, however, and is supported by a recent review by Rothwell et al. (2009) which draws upon new interpretations of the cycadeoids (also called Bennettiales). Frohlich and Chase (2007) dismiss the Anthophyte Hypothesis altogether and review genetic, developmental, and morphological studies which they think will produce more promising, testable theories. What almost everyone agrees upon is that the flowering plants emerged from a group of gymnosperms and that the flower evolved only once. Unfortunately, only four extant lineages (ginkgos, conifers, gnetophytes, and cycads) exist today, a pale reflection of the rich diversity that the fossil record presents. For a theory to be acceptable, it will have to be consistent with molecular/developmental evidence as well as morphological/paleontological evidence. So, the origin of flowering plants remains an “abominable mystery” (Darwin 1879 in a letter to J. D. Hooker, cited in Davies et al. 2004). |
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| FIGURE 4. Cabomba aquatica, an aquatic plant with finely-divided leaves. Image by Piotr Kuczynski and licensed through Creative Commons | FIGURE 5. Nymphaea odorata with its flower fully open. Note the petaloid yellow stamens. The variety is Peach Glow and is growing at the Brooklyn Botanical Gardens. Image by Ragesoss licensed by the Creative Commons. | FIGURE 7. Victoria cruziana is a very large relative of Nymphaea and grows in South America. Here, a woman is standing on a floating leaf outside of the Linnaean House of the Missouri Botanical Gardens in 1902. Image is in the Public Domain. | FIGURE 7. Trithura is grass-like aquatic plant recently placed in the Nymphaeales (APG III 2009). Image taken from Friis and Crane (2007). |
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| FIGURE 8. Sarcandra glabra, a typical member of the Chloranthaceae, is a small shrub with serrated, evergreen leaves. Image by Kenpei licensed by the Creative Commons. | FIGURE 9. A reconstruction of Archaefructus, an aquatic plant from the upper Jurassic or lower Cretaceous of China. Image from Ji et al. (2004). | FIGURE 10. A reconstruction of Hyrcantha, an aquatic plant from the upper lower Cretaceous of China. The pattern of fruits (the infructescence) shows branches terminating in two or three pods that seem to be fused at the base. Each fruit contains multiple seeds. Image from Dilcher et al. (2007). | FIGURE 11. A reconstruction of Leefructus, an aquatic plant bearing a capsule-like fruit from the upper lower Cretaceous of China. Image from Sun et al. (2011). |
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Proceedings of the National Academy of Sciences. U.S.A. 99:6848-6853. |
| This name is of our invention and does not imply that the groups included are monophyletic. |
| By Jack R. Holt. Last revised: 04/14/2019 |
