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The immense diversity of floral monosymmetry and asymmetry across angiosperms.


Floral symmetry, which has long fascinated botanists (Sprengel, 1793; de Candolle, 1813; Wydler, 1844), continues to attract different fields of research more than ever, such as (1) floral developmental genetics (Coen et al., 1995; Coen, 1996; Cubas et al., 1999b; Hileman et al., 2003; Busch & Zachgo, 2009; Jabbour et al., 2009b; Preston & Hileman, 2009; Specht & Bartlett, 2010; Zhang et al., 2010; Bartlett & Specht, 2011; Preston et al., 2011), (2) comparative morphology with a special focus on diversity (Endress, 1999, 2001 a, 2006, 2010, 2011 ; Tucker, 1999; Endress & Matthews, 2006), (3) pollination ecology (Dafni & Kevan, 1996; Neal et al., 1998; Giurfa et al., 1999; Lehrer, 1999; Jesson & Barrett, 2003; Kalisz et al., 2006), and (4) evolution (Donoghue et al., 1998; Ree & Donoghue, 1999; Citerne et al., 2010; Zhang et al., 2012). Monosymmetry and asymmetry are of special evolutionary significance. In contrast to my former syntheses on floral symmetry (Endress, 1999, 2001 a) this study goes more deeply into the diversity of monosymmetry and asymmetry expressions and more broadly into the systematic distribution.

The term monosymmetry is used here for flowers that have one symmetry plane (following Endress, 1999, 2001 a). Developmental geneticists have sometimes called this asymmetry (e.g., Luo et al., 1996, 1999; Cubas et al., 2001). However, asymmetric flowers--without a symmetry plane--are different from monosymmetric flowers. Both monosymmetric and asymmetric flowers are of great interest in their own right. But truly asymmetric flowers have been largely neglected in floral developmental genetics so far, in contrast to asymmetry in animals (Levin, 2005; Raya & Belmonte, 2006; Levin & Palmer, 2007). An exceptional enantiomorphy that has been studied genetically in plants (but not in flowers) are helically growing mutants of Arabidopsis (Hashimoto, 2002; Buschmann et al., 2004). It is to be expected that asymmetry in flowers will be tackled soon, too.

Monosymmetric flowers with their single symmetry plane can be described as having two mirror symmetrical halves (divided by the symmetry plane) or as having two unequal sides (divided by a plane at right angles to the symmetry plane). Because the symmetry plane is mostly perpendicular, I will call the former the left and fight half of the flower, and the latter the lower and upper side of the flower.

Monosymmetry and asymmetry are (immensely) diverse in two respects. First in structure, in the kinds of expression, and second in phylogeny, in the occurrence of these forms across the angiosperms. This paper will give a glimpse of both dimensions: the structural and the systematical and will also show some biological situations in which certain monosymmetry and asymmetry forms can be expected. For discussion of the distribution of different monosymmetry and asymmetry patterns primarily the classification in Stevens (2001 onwards) and APG (2009) is used.

Monosymmetric Flowers

If we think of monosymmetric flowers, some prominent large taxa come to mind, such as orchids, Lamiales, or papilionoids. They all have their own special clear-cut kinds of monosymmetry, if viewed in detail. However, if we consider monosymmetry across the entire angiosperms, we find an even greater diversity of forms. The situation is like an ocean of polysymmetry with a few large islands and a surprisingly great number of small islands of monosymmetry. Floral monosymmetry is an attractive topic for evo-devo studies. These studies have to date been concentrated on the big islands, whereas the small ones were not or rarely explored. The present study especially concentrates on the small islands to show what diversity of monosymmetry there really is. Thus the emphasis is not in how common monosymmetry is in the larger clades but whether it occurs at all in an order or family throughout the angiosperms and how it is expressed in morphology.

The attempt to consider the entire diversity is of much heuristic value to ask questions in evolutionary biology, and thus it is a contrast to the study of model species. Both approaches are complementary and therefore important. If we find certain patterns that appear again and again in unrelated groups, we should ask what this means in terms of evolution of developmental mechanisms and build our hypotheses to test based on this question.

Still 20 or 30 years ago there were only vague general ideas about evolution from polysymmetry to monosymmetry (e.g., Leppik, 1972). This is no longer the case, as now more or less detailed phylogenetic frameworks for the angiosperms and for many subgroups at various levels of angiosperm phylogeny (orders, families, genera) are available. They allow ever more precise reconstruction of where exactly and in which direction symmetry changes occurred (Ree & Donoghue, 1999). Specific case studies on secondary polysymmetry by pelorization are those on Linaria (Veroniaceae) by an epigenetic mutation (Cubas et al., 1999a), on some Gesneriaceae (Citerne et al., 2000), or on Cadia (Fabaceae) (Citerne et al., 2006).

A few angiosperm families or orders are prominent representatives of floral monosymmetry. Molecular developmental studies on monosymmetry have been carried out especially on such groups in eudicots, such as Fabaceae (Ree et al., 2004; Citerne et al., 2003, 2006; Feng et al., 2006; Wang et al., 2008), Lamiales (Cubas et al., 1999a, b; Moller et al., 1999; Citerne et al., 2000; Hileman & Baum, 2003; Reeves & Olmstead, 2003; Smith et al., 2004; Vincent & Coen, 2004; Aagaard et al., 2005; Davies et al., 2006; Baxter et al., 2007; Xiao & Wang, 2007; Du & Wang, 2008; Gao et al., 2008; Zhou et al., 2008), Asterales (Teeri et al., 2006; Broholm et al., 2008; Chapman et al., 2008; Kim et al., 2008), Dipsacales (Howarth & Donoghue, 2005, 2008). In contrast, in monocots, molecular developmental studies in large monosymmetric groups to date concentrated more on aspects of floral organ identity than symmetry, such as in Poaceae (Malcomber & Kellogg, 2004; Whipple & Schmidt, 2006), Orchidaceae (Tsai et al., 2008; Mondragon-Palomino & Theissen, 2009; Mondragon-Palomino et al., 2009), or Zingiberales (Specht et al., 2008; Bartlett & Specht, 2010, 2011). However, the present study does not especially focus on those large clades, but it aims to follow the occurrence of monosymmetry through all major angiosperm groups (orders and families; see also Endress, 2010, 2011).

Diverse Manifestations of Monosymmetry in Flowers

Monosymmetry has diverse expressions in flowers, which has been noticed since Wydler (1844). Floral monosymmetry is manifested in very different ways (Table 1). (1) Organ categories are affected in various combinations: all organs (in large groups with most conspicuous monosymmetry), or mainly calyx and corolla, or mainly only one organ category (calyx, corolla, androecium, or gynoecium). (2) Organs are affected to various degrees: by differential shape, curvature, reduction, loss, increase in number, or transfunctionalization (e.g., in stamens by heteranthery or in staminodes). (3) Monosymmetry by simplicity (when only a single organ of one kind is present, such as one stamen or one carpel). (4) Direction of monosymmetry (median, transverse, oblique) regarding the groundplan of ramification of the inflorescence. Thus, the notion of monosymmetry is somewhat vague and encompasses different kinds of forms, which may not be evolutionarily related.

Elaborate Monosymmetry. Elaborate monosymmetry especially occurs in flowers with highly synorganized organs (Endress, 2006). Classical cases of conspicuous monosymmetry are bilabiate flowers, which appear in two extreme forms: keel flowers and lip flowers, with the pollination organs more or less hidden in a container: either the keel (on the lower side) or the upper lip (on the upper side) (Endress, 1994; Westerkamp, 1997; Classen-Bockhoff et al., 2004; Classen-Bockhoff, 2007; Westerkamp & Classen-Bockhoff, 2007). Correspondingly, pollen transport is basically with the underside of the body in keel flowers and with the upper side in lip flowers (Fig. la, b).

In Lamiales, the largest angiosperm clade with almost exclusively monosymmetric flowers, corolla and androecium are greatly and differentially affected. The upper (posterior), median stamen of the five stamens is reduced, either present but not fertile or even absent, which was already noticed by de Candolle (1819). The paired stamens are much less affected. But in many taxa of different families, one of the two pairs is also reduced, either the upper or the lower (e.g., Endress, 1999). In other asterids (and many rosids) floral monosymmetry is commonly concentrated on or restricted to the perianth and less or not expressed in the androecium (e.g. Balsaminaceae, Apocynaceae, Rubiaceae, Asteraceae).

Sigmoidal Curvature of Organs. A widespread, much less complex monosymmetric architecture is by sigmoidal curvature of the pollination organs and often also the perianth either up-down-up or down-up-down (Fig. 2a, b). The curvature commonly develops late in floral development. These two forms correspond in the direction of curvature to those of the keel and lip flowers. There are also flowers in which the curvature of the stamens and the style are in opposite directions (e.g., Exacum: stamens: down-up-down, style: up-down-up).

This kind of monosymmetry is directly influenced by the position of the flower. It can be experimentally reversed by positioning the plant with flower buds upside down as shown in classical experiments with the clinostat by Vochting (1886), e.g. in Sprekelia (Amaryllidaceae). Such reversed flowers still have the stamens and style topographically up-down-up, but morphologically down-up-down. Plasticity in Adjustment of Flowers to Final Position. In many plants the flowers have a relatively constant position or direction at the time of anthesis. This direction is based on the position of the flower in the ramification system and thus on flower initiation and early development, and, even more, in final adjustments of the position before and during anthesis. Mechanisms for such late adjustment are bending or torsion of pedicels and peduncles, as shown for Aconitum, Antirrhinum, Scrophularia and Viola by Noll (1888) and later also for many other plants (Comehls, 1927; Zimmermann, 1933; Kaldewey, 1962; Huang et al., 2002). In this way monosymmetric flowers can be adjusted to a horizontal position with the symmetry plane perpendicular to the ground. In some cases, if a pedicel is lacking, bending or torsion can even take place in the middle of the flower (Lonicera; Noll, 1888), or in the inferior ovary ("resupination" in orchids). In orchid plants that are artificially reversed after flower resupination, the flowers attain again the right position (Noll, 1888). Such adjustment is not restricted to monosymmetric flowers, it also occurs in polysymmetric flowers that assume a constant final position. For example, in Ceropegia the corolla tube is bent (Bruyns, 1985). Narcissus is versatile as bending may involve only the pedicel or pedicel plus lowermost part of the flower (Church, 1908).

Minor Monosymmetry (Monosymmetry by Reduction or Simplicity). If a flower is reduced to one stamen or one carpel, it becomes strictly speaking monosymmetric (monosymmetry by simplicity or by reduction, passive monosymmetry). Even completely ascidiate carpels are monosymmetric (e.g., basalmost angiosperms; Endress, 1986b, 2005; or Araceae; Igersheim et al., 2001). Thus I have included also monocarpellate cases with an ascidiate carpel. Because such simplicity also occurs in some basal angiosperms (Chloranthaceae, Ceratophyllaceae, and the fossil Archaefructus) (e.g., Endress & Doyle, 2009), this kind of simple monosymmetry may have been present, besides polysymmetry, early in angiosperm evolution. In basal angiosperms it is safer to say monosymmetry by simplicity than by reduction. However, floral reduction can also have the opposite result: that monosymmetry becomes less strongly expressed, such as in some Amorpheae (Fabaceae) with reduced corolla (McMahon, 2005), or Apostasia (Orchidaceae) with reduced androecium (Kocyan & Endress, 2001b).

Minor monosymmetry is also present in flowers in which the floral whorls are not isomerous (or asymmetric if the symmetry plane is oblique). The most common cases are pentamerous flowers with a dimerous gynoecium (e.g. Saxifragaceae, Asteraceae), or pentamerous flowers with a trimerous gynoecium (e.g. Burseraceae, Cunoniaceae). I have not included these in the study, because they are much more frequent and widespread than those cases with a single stamen or carpel, and are thus too trivial.

Floral Organization and Architecture

Floral organisation (ground plan with initial disposition of young organs) and architecture (shape of the anthetic flower) are differentially affected by monosymmetry. Shallow (only architectural) monosymmetry is often present in a minority of taxa in largely polysymmetric groups (Meliaceae, Cleomaceae, Onagraceae). In contrast, organisational (or organisational plus architectural) monosymmetry often characterizes species-rich families or orders with complex monosymmetric flowers (Fabaceae, Lamiales, Orchidaceae, Zingiberales).

Such different degrees of monosymmetry were first discussed by Delpino (1887) who distinguished four levels of monosymmetry (zygomorphy): (1) very recent in evolutionary terms (deflection of styles and stigma), (2) recent, (3) old, and (4) very old (abortion of organs on one side). This discussion was taken up with new examples added by Robertson (1888).

Transient Early Floral Monosymmetry and Late Monosymmetry

There is also diversity in the timing of monosymmetry expression. Monosymmetry can have its strongest expression early in flower development or at anthesis or only in fruit (Table 2). Such changing or transient monosymmetry is poorly explored. This phenomenon may be especially interesting if it occurs in a polysymmetric group that is nested in a monosymmetric group.

A prominent example for such early monosymmetry in otherwise polysymmetric (disymmetric) flowers is Arabidopsis (Fig. 3a, b). In early development the lower side of the flower is more developed so that the flower bud is curved toward the inflorescence apex. Later in development this inequality disappears and anthetic flowers are no longer monosymmetric (see also Cubas et al., 2001). The same behavior is also present, e.g., in Siparuna (Siparunaceae; Endress, 1980), Achlys (Berberidaceae; Endress, 1989), Trochodendron (Trochodendraceae; Endress, 1986a), and Batis (Bataceae; Ronse De Craene, 2005). In contrast, in Euptelea (Eupteleaceae) (Endress, 1986a; Ren et al., 2007) and Hypoxis (Hypoxidaceae; Kocyan & Endress, 2001a) the upper side is more developed than the lower one.

The dimerous gynoecium in Apiaceae can have a superimposed pentamerous symmetry from the other floral whorls and thus be monosymmetric (instead of disymmetric). This monosymmetry is present in floral buds, and it may vanish during later flower and fruit development (Stegotaenia) or, in contrast, may become more pronounced (Polemanniopsis) (Liu et al., 2004). A similar kind of superimposed pentasymmetry is present in the bicarpellate Asclepiadeoideae (Apocynaceae) (Endress, 1994). In the likewise bicarpellate Catharanthus (Apocynaceae), thus with a basically disymmetric gynoecium, one of the two carpels is slightly larger in early development. This "leading" carpel induces meristematization of the epidermis of the smaller carpel for postgenital union of the carpel tips (unpublished results by Verbeke, reported by Mlot, 1998).

Tiarella and some Chrysosplenium species (Saxifragaceae) are of special interest because there are two changes in monosymmetry expression. Monosymmetry is strongest in early flower development (see also Endress, 1999) and at the mature fruit stage but least expressed at anthesis (see next chapter).

Floral Monosymmetry Associated with Special Biological Situations

Flower Direction and Expression of Monosymmetry--Differential Monosymmetry in Flat Inflorescences. Floral monosymmetry has long been recognized to have evolved in the interaction with pollinating hymenopters (Delpino, 1887; Robertson, 1888). There are also more specific conditions that favour monosymmetry or certain kinds of monosymmetry.

Flowers with elaborate monosymmetry are commonly horizontally directed, i.e., the longitudinal axis of the part of the flower that is visited by pollinators is parallel to the ground, thus conforming to the normal position (posture) of the pollinators (Sprengel, 1793; Ushimaru & Hyodo, 2005; Fenster et al., 2009). In contrast, in monosymmetric flowers that have a vertical upright position, monosymmetry is often only expressed in the perianth. Such flowers are often positioned at the periphery of umbel-like inflorescences (certain Brassicaceae, Rubiaceae, Asteraceae, Apiaceae, Dipsacaceae). In this second case, the entire inflorescence has a unified signalling effect with the radiating monosymmetric flowers at its periphery, and the flowers are less likely to be visited in the direction of their monosymmetry than the horizontally directed flowers. In such inflorescences polysymmetric and monosymmetric flowers tend to co-exist. And even more, different degrees of monosymmetry may be present in the same inflorescence. Thus there is differential monosymmetry. The peripheral most flowers are most strongly monosymmetric. The central flowers are the least strongly monosymmetric ones. There may be a fine gradation of morphs in the intermediate flowers. Such graded morphs occur in some Brassicaceae (Iberis) (Busch & Zachgo, 2007), Apiaceae (Orlaya) (Froebe, 1980; Classen-Bockhoff, 1992), Asteraceae (Hieracium), and Dipsacaceae (Scabiosa) (Kunze, 1976) (Fig. 4a-d). Such differential expression of monosymmetry with several flower morphs is being studied in Gerbera (Asteraceae), where a TCP domain transcription factor is responsible for this gradation (Broholm et al., 2008). Chapman et al. (2008) "identified ten members of the CYC/TBI family in sunflower, which is more than found in any other species investigated to date."

Buzz Pollination and Heteranthery. A conspicuous trend of change to monosymmetry is present in some buzz-pollinated groups. This is especially impressive when they are nested in a large clade of mainly polysymmetric flowers. Examples are Pseudocorchorus (Malvaceae; Bayer & Kubitzki, 2003), Exacum (Gentianaceae; Klackenberg, 2002), and Solanum sect. Androceras (Solanaceae; Whalen, 1978). This trend is omen further emphasized by the evolution of a division of function in stamens (heteranthery), with cryptic pollinating stamens and optically attractive feeding stamens (Vogel, 1978). If the flowers are monosymmetric, the enlarged feeding anthers in the median plane may especially contribute to pronounced monosymmetry, such as in Senna (Fabaceae; Marazzi & Endress, 2008), Solanum (Solanaceae; Whalen, 1978), Commelina (Commelinaceae; Faden, 1998), or Cyanella (Tecophilaeaceae; Simpson & Rudall, 1998).

Oil Flowers. Oil flowers tend to be more pronouncedly monosymmetric than their closest relatives that do not produce oil. In Cucurbitaceae, conspicuously monosymmetric flowers are restricted to the oil flowers of Momordica and Thladiantha (Vogel, 1990). The same applies for Krameria in Zygophyllales (Simpson, 1982). The oil-producing pseudanthia of species of Dalechampia (Euphorbiaceae) are strongly monosymmetric. The South American Malpighiaceae, which commonly have oil flowers, are slightly monosymmetric in contrast to the polysymmetric non-oil flower Malpighiaceae of the Old World (Davis & Anderson, 2010) and the polysymmetric sister family Elatinaceae (Davis & Chase, 2004; Davis et al., 2005). Some other groups with oil flowers are phylogenetically embedded in already strongly monosymmetric clades. The basis for this pronounced trend to monosymmetry in oil flowers may be that the pollinating bees take up oil from the flower with the two forelegs (Vogel, 1974), which is a more intimately "monosymmetric" activity than taking nectar with the proboscis.

Wind-Pollinated Flowers. In wind-pollinated plants there is a trend to reduction of floral organs in number, which may result in floral monosymmetry by simplicity or decay of elaborate monosymmetry (Preston et al., 2011). In a number of windpollinated groups among Fagales the flowers, especially the male flowers, are densely arranged in catkins and fill the (monosymmetric or even asymmetric) space created by neighboring bracts (e.g., Endress, 1967, 2008b).

Bird-Pollinated Flowers in Hawaii. In Hawaii some bird-pollinated flowers have evolved into monosymmetric shapes in clades with otherwise consistently polysymmetric flowers. The genus Geranium with more than 380 species world-wide has only one species with a monosymmetric corolla: the Hawaiian Geranium arboreum (Albers & Van der Walt, 2007). Likewise the genus Hibiscadelphus is--to my knowledge--the only genus in Malvaceae-Malvoideae with a consistently monosymmetric corolla and androecium. The conspicuous monosymmetry of these flowers may be functionally linked with the pronouncedly curved beaks of some Hawaiian flower-visiting birds.

Fruits with Rain-Dispersed Seeds. In some rain-dispersed plants of otherwise polysymmetric groups (with disymmetric gynoecium) the fruits (and the gynoecium at anthesis) are pronouncedly monosymmetric, such as Tiarella and some Chrysosplenium species of Saxifragaceae. The same applies for some species of Begonia (Begoniaceae, Cucurbitales) (Matthews & Endress, 2004; Yebbitt et al., 2006). In Tiarella the gynoecium is horizontally exposed with two broad, shovel-shaped carpels. When the seeds are mature, the upper, smaller carpel acts as a lid to cover the seeds lying on the lower carpel. Rain drops falling on the gynoecium cause the lower carpel to elastically move downwards. This movement opens the container with the seeds for a moment and sets free a small proportion of them (Savile, 1979); thus the fruit functions like a censer. In Chrysosplenium the seeds are also rain drop-dispersed but differently in detail. The gynoecium has an upright position but the two carpels commonly diverge at an angle of 180[degrees]. The two mature carpels are open and contain the seeds like a bowl. They can easily be washed out by rain drops (Savile, 1953). In most species both carpels are equally developed and the likewise bicarpellate gynoecium therefore disymmetric. However, in some Chinese species the two carpels are unequal (Pan & Ohba, 2001), thus the gynoecium is monosymmetric. One of the two carpels remains closed (personal observation), probably acting as a reservoir for seeds (the ovary is unilocular), which are thus dispersed more slowly than from a completely open fruit.

Systematic Distribution of Floral Monosymmetry Across Angiosperms

This chapter provides a survey of the widespread occurrence, almost ubiquity of monosymmetry in angiosperms, although with peaks in some groups and unequal distribution of different kinds of monosymmetry (Fig. 5). It appears that in many clades monosymmetry evolved multiple times.

Basal Angiosperms. Floral monosymmetry, although unusual, is not absent. It is mostly monosymmetry by simplicity, by the presence of only one carpel and/or only one stamen, such as in Hydatellaceae (Hamann, 1975; Saarela et al., 2007; Rudall et al., 2007), Trimeniaceae (Endress, 2001b), Chloranthaceae (Endress, 1987; Kong et al., 2002), Ceratophyllaceae (Endress, 2001b, 2004), some Winteraceae (Igersheim & Endress, 1997), Piperaceae (Tucker, 1984; Tucker et al., 1993), Degeneriaceae, Myristieaceae (Igersheim & Endress, 1997), Lauraceae (Endress, 1972), and Hernandiaceae (Endress & Lorence, 2004). Among Annonaceae flowers with one carpel have evolved several times as shown by their distribution in the phylogenetic tree by Doyle et al. (2004). Unique are Isolona and Monodora (Annonaceae) with a pluricarpellate monosymmetric gynoeciurn, which appears to begin development like a single carpel (Leins & Erbar, 1982) but is clearly pluricarpellate at anthesis (Deroin, 1985). The unusual case of Glossocalyx (Siparunaceae) with its onesided tongue is possibly not monosymmetry of the flower proper, but formed by the floral subtending bract which is fused with the flower (Staedler & Endress, 2009), comparable with similar flowers in Steganthera in the related Monimiaceae (Takeuchi, 2001). The most prominently monosymmetric group here is Aristolochia sensu lato (Aristolochiaceae), in which the single perianth whorl is greatly monosymmetric (Gonzalez & Stevenson, 2000).

Basal Eudicots. Elaborate monosymmetric flowers have evolved several times, at least once in Papaveraceae (Murbeck, 1912; Liden, 1993; Damerval & Nadot, 2007) and at least once in Ranunculaceae (Mair, 1977; Jabbour et al., 2009a), both families in Ranuneulales, and also in Sabiaceae (Ronse De Craene & Wanntorp, 2008), and more or less elaborate in Proteaceae (Proteales; Douglas, 1997; Weston, 2007). As in basal angiosperms, there are several cases of monosymmetry by simplicity with only one carpel, such as in Berberidaceae and some Ranunculaceae (Ranunculales), Proteaceae (Proteales), and Didymelaceae (von Balthazar et al., 2003); and there are also a few cases with one stamen as, e.g., Cissampelos (Menispermaceae) (Puri, 1978; Kessler, 1993) (Ranunculales), Placospermum (Proteaceae) (Douglas & Tucker, 1996), and Didymeles (Didymelaceae). Papaveraceae-Fumarioideae (Ranunculales) are twofold unusual because monosymmetry evolved from disymmetry and because the monosymmetry plane is transversal and not median. There is also fluctuation between disymmetry and monosymmetry, such as in Corydalis (Endress, 1999). In addition, in basal eudicots there are several completely perianthless genera, which have pronounced early monosymmetry. Either the abaxial side of the young flower is delayed (in Eupteleaceae, Ranuneulales) or the adaxial side (in Achlys of Berberidaceae, Ranunculales, and in Trochodendron of Trochodendraceae, Troehodendrales) (see above).

Core Eudicots

Gunnerales. Some Gunnera species (Gunneraceae) may have a single stamen and thus monosymmetry by simplicity (reduction) (Rutishauser et al., 2004; Wilkinson & Wanntorp, 2007).

Berberidopsidales. Monosymmetric flowers are lacking in Berberidopsidaceae (Ronse De Craene, 2004).

Dilleniales. Hibbertia (Dilleniaceae) has different kinds of monosymmetry: some species have a single carpel, some species have a monosymmetric androecium, this monosymmew being median or transversal (Tucker & Bernhardt, 2000; Horn, 2006, 2007, 2009).

Rosid Alliance (see also Endress, 2010)

Saxifragales. In Saxifragales floral monosymmetry is scarce, and two of these cases have an unusual pollination biology. In Hamamelidaceae, the bird-pollinated Rhodoleia has flat inflorescences with peripherally radiating (enlarged) petals (Bogle, 1989). In Saxifragaceae, the monosymmetric flowers of Tolmiea have a lateral entrance by a onesided floral cup and are pollinated by fungus gnats (Goldblatt et al., 2004). A number of Saxifraga species have a monosymmetric corolla and nectary (Engler, 1930). Tiarella and some Chrysosplenium species have a monosymmetric gynoecium giving rise to fruits with rain dispersal (see above). In Crassulaceae the long-tubed flowers of Tylecodon grandiflorus are curved (van Jaarsveld, 2003). In Cercidiphyllaceae, the unicarpellate female flowers are monosymmetric by reduction and the male flowers by congestion in dense inflorescences (Endress, 1986a).

Vitales. Floral monosymmetry appears to be lacking.

Rosids. The distribution of floral monosymmetry is uneven. Myrtales, Geraniales, Fabales, and, to some extent, malvids are most conspicuous for monosymmetric flowers, with a peak of elaboration in Fabales. Other orders only have monosymmetry by reduction or none at all.

Eurosids 1 (Fabids). Fabales are distinguished by large clades with elaborate monosymmetric flowers. In the other orders monosymmetry is much less conspicuous.

Zygophyllales. In Zygophyllaceae, the flowers of Bulnesia and Porlieria are slightly monosymmetric (Sheahan, 2007). Krameria (Krameriaceae) has pronounced monosymmetry in its oil flowers (Vogel, 1974; Simpson, 1982).

Fabales. Monosymmetric flowers are predominant. Those of many subclades in Fabaceae (e.g., Endress, 1994; Westerkamp, 1997; Tucker, 2002; Prenner, 2004a, c, d) and in most Polygalaceae (e.g., Westerkamp & Weber, 1997; Prenner, 2004b) are especially elaborate. The two families are unusual by the convergent evolution of pronounced keel flowers (Westerkamp, 1999). They have been relatively widely studied and are not further treated here. In the small family Surianaceae there is monosymmetry by reduction (Guilfovlia; Schneider, 2007; Stylobasium; Carlquist, 1978).

Rosales. Elaborate floral monosymmetry is lacking. However, there are several families with monosymmetry by reduction to a single stamen or carpel. This is present in Barbeyaceae (Dickison & Sweitzer, 1970), Cecropiaceae (Kubitzki, 1993), Moraceae (Rohwer, 1993a), Urticaceae (Friis, 1993), and Rosaceae (e.g., Prunus, Alchemilla, Aphanes, Margyricarpus; Murbeck, 1941). Gynoecial pseudomonomery also occurs and is sometimes difficult to distinguish from monomery (Elaeagnaceae; Bartish & Swenson, 2004; Moraceae; Ulmaceae, and Celtidaceae; Eckardt, 1937).

Fagales. The situation is similar as in Rosales, with merely monosymmetry by reduction. In Betulaceae, the perianth is affected in Betula (Abbe, 1935), and male flowers are monosymmetric by dense arrangement between bracts in Carpinus (e.g., Endress, 2008b). Male flowers are monosymmetric by reduction to a single stamen in Casuarinaceae (Flores & Moseley, 1990), and some Juglandaceae (Abbe, 1974; personal observation) and Myricaceae (Abbe, 1972).

Cucurbitales. Floral monosymmetry is rare. In Begoniaceae, some species of Begonia have a monosymmetric (or even asymmetric) inferior ovary, which acts in rainballistic seed dispersal (Tebbitt et al., 2006). In Cucurbitaceae, the oil flowers of some Momordica species are monosymmetric (or asymmetric) (Vogel, 1990). A pseudomonomerous gynoecium is present in Cyclanthera (Leins & Galle, 1971). Corynocarpaceae flowers are monosymmetric by reduction because of their monomerous or pseudomonomerous gynoecium (Philipson, 1987; Matthews & Endress, 2004).

Celastrales. Monosymmetric flowers appear to be lacking (Matthews & Endress, 2005a)

Oxalidales. Floral monosymmetry by reduction is present in unicarpellate flowers of some Connaraceae (Matthews & Endress, 2002).

Malpighiales. Floral monosymmetry does not play an important role in this large order. However, because of its sheer size (over 40 families, Wurdack & Davis, 2009), a number of families are to be listed here. Elaborate monosymmetry is mainly known from Chrysobalanaceae s.l., where it is predominantly expressed in the floral cup, androecium and gynoecium (Prance & White, 1988; Matthews & Endress, 2008), some Ochnaceae, where it occurs together with buzz pollination (Amaral, 1991), and some Violaceae, in which the anterior petal has a spur, which surrounds the nectaries formed by two stamens, and the stigma has a monosymmetric pollen collection mechanism (Melchior, 1925; Beattie, 1969). In Malpighiaceae monosymmetry is especially present in the perianth as an adaptation to oil collection by oil bees (Vogel, 1974; Wurdack & Davis, 2009; Davis & Anderson, 2010; Zhang et al., 2010) (see also above). In some Passiflora species (Passifloraceae) sigmoid curvature of the pollination organs is present, and may be an adaptation to bat pollination (Sazima & Sazima, 1978). In Lacistemataceae (Endress, 1999), some Euphorbiaceae (Radcliffe-Smith, 2001; Prenner & Rudall, 2007; Prenner et al., 2008), some Podostemaceae (Cook & Rutishauser, 2007), and some cleistogamous flowers of Violaceae (Weibel, 1941), monosymmetry by reduction is present. In Erythroxylaceae and some Linaceae only one of the three carpels is fertile, which makes the flowers monosymmetric by reduction (Matthews & Endress, 2011).

Eurosids II (Malvids). The large core orders Sapindales, Brassieales, and Malvales have some monosymmetric cases in almost every family, even if polysymmetry is dominant in each order (Endress & Matthews, 2006). This is also true for the newly here positioned orders Myrtales and Geraniales. In most cases the shape of the organs but not their number is affected. Thus malvids are more strongly characterised by monosymmetry than fabids (measured by the occurrence of monosymmetry in percentage of families). In Brassicales and Sapindales several families have oblique monosymmetry.

Myrtales. Floral monosymmetry is widespread in Melastomataceae (Krasser, 1893), and extreme in Vochysiaceae with transitions to asymmetry (Litt & Cheek, 2002; Litt & Stevenson, 2003a, b; Kawasaki, 2007). Some Combretaceae have slightly monosymmetric flowers based on unequal development of the floral base, which, in Quisqualis, results in an inner spur in the floral tube (Brandis, 1893; Engler & Diels, 1899); in Terminalia paniculata the fruit is monosymmetric by a large wing (Brandis, 1893). Monosymmetry is present in various expressions in some Lythraceae (mainly perianth; Graham, 2007) and Onagraceae (mainly sigmoid curvature of pollination organs; Raimann, 1893; Mabberley, 2000; Wagner, et al. 2007). In the genus Lagerstroemia (Lythraceae) there is polysymmetry and monosymmetry side by side; some species have heterantherous flowers with conspicuous pollinating stamens and cryptic feading stamens; in some of the latter the cryptic stamens are turned to one side (personal observations). Lopezieae (Onagraceae) are strongly monosymmetric in corolla and androecium (Eyde & Morgan, 1973; Wagner et al., 2007). In Lythraceae monosymmetry by reduction occurs in some Rotala species with low stamen number (Eichler, 1878). In Myrtaceae, Chamaelaucieae and Fenzlia have a monosymmetric ovary by a one-sided placenta (Niedenzu, 1893); it is unclear whether these gynoecia are unicarpellate or pseudomonomerous.

Geraniales. Geraniaceae exhibit different gradations between polysymmetry and monosymmetry. Geranium is polysymmetric, in a few species the pollination organs are sigmoidally curved, and in the Hawaiian Geranium arboreum also the corolla is monosymmetric (Albers & Van der Walt, 2007), Erodium is weakly monosymmetric, and Pelargonium strongly monosymmetric (Vogel, 1998). Melianthaceae also have strongly monosymmetric representatives, in addition to polysymmetric ones (Ronse de Craene et al., 2001; Linder, 2007).

Crossosomatales. In Aphloiaceae and some Glossopetalon species (Crossosomataceae), which are unicarpellate, there is monosymmetry by reduction (Matthews & Endress, 2005b).

Sapindales. In some representatives of at least five families floral monosymmetry has been found. Monosymmetry is common in Sapindaceae (Eichler, 1878; Ronse de Craene et al., 2000) but less so in the other families. In various Anacardiaceae (especially Anacardioideae), there is monosymmetry by reduction to one functional stamen or one functional carpel (Bachelier & Endress, 2007, 2009). In Meliaceae, Nymania is monosymmetric in androecium and gynoecium, with sigmoidal curvature (Harms, 1940). In Rutaceae, a group of genera (Cusparieae, Galipeae) has a monosymmetric androecium by reduction of stamens to staminodes on one side (Eichler, 1878; Engler, 1931; Kallunki & Pirani, 1998; Mabberley, 2000; Pirani & Kallunki, 2007; Groppo et al., 2008); Calodendrum and Dictamnus have sigmoidally curved pollination organs (Engler, 1931), and Cneoridium and Empleuridium are monosymmetric by reduction (Engler, 1931). In Simaroubaceae, Leitneria is monosymmetric by reduction (Abbe & Earle, 1940).

Huerteales. There are no obvious monosymmetric flowers in this small order (Worberg et al., 2009).

Brassicales. In at least 13 families floral monosymmetry has been recorded. In very few Brassieaceae monosymmetry is expressed in the perianth: the calyx in Streptanthus (Rollins, 1993), and the corolla in Iberis, Teesdalia, and Erysirnum (Appel & Al-Shehbaz, 2003; Gomez et al., 2006; Busch & Zachgo, 2007). In Bataceae the calyx is monosymmetric (Ronse De Craene, 2005). In some Capparaceae and Cleomaceae the pollination organs are sigmoidally curved (Hildebrand, 1886; Vochting, 1886; Kers, 2003). In more extreme cases the nectary is one-sided (Capparaceae; Capparis, Euadenia, Cadaba; Kers, 2003), or the androecium is unequal (Cleomaceae; Cleome, Dactylaena, Polanisia; Endress, 1992). In Cleomaceae two distinct pathways to monosymmetry were found, one starting development with disymmetry, and the other with monosymmetry (Patchell et al. 2011). Several smaller families are partly or completely characterized by floral monosymmetry (Ronse de Craene & Haston, 2006), such as Bretsehneideraceae (Ronse De Craene et al., 2002a), Emblingiaceae (Leins, 1969), Moringaceae (Ronse de Craene et al., 1998; Olson, 2003), Resedaceae (Sobick, 1983), and Tropaeolaceae (Ronse De Craene & Smets, 2001). Oblique monosymmetry was found in Bretsehneideraceae, and Moringaceae. In Gyrostemonaceae, Gyrostemon may be unicarpellate, thus there is monosymmetry by reduction (George, 2003). The same is true for some Salvadoraceae (Kubitzki, 2003).

Malvales. Almost each subfamily of Malvaceae sensu lato has some monosymmetric representatives. In Bombacoideae, the gynoecium is affected in Adansonia, the androecium in Chiranthodendron (Endress, 1994, 1999). In Grewioideae Pseudocorchorus has a monosymmetric androecium (Bayer & Kubitzki, 2003). In Helicteroideae, Helicteres and Mansonia are monosymmetric (Cheek & Dorr, 2007). In Byttnerioideae, Kleinhovia has elaborate monosymmetry (personal observation), in Melochia sigmoid curvature occurs (Machado & Sazima, 2008), and the unicarpellate Waltheria and Plagianthus are monosymmetric by reduction (Bayer & Kubitzki, 2003). In Malvoideae, species of Hibiscus and Hibiscadelphus may have curved pollination organs (and corolla) (Fryxell, 1983; Endress, 1999). In Sterculioideae, Sterculia species may have a curved staminal column (personal observation). In Coehlospermaceae, the androecium in Amoreuxia is onesided, with staminodes on one side (Poppendieck, 2003). In some Dipteroearpaceae the fruits are monosymmetric by differential further growth of the sepals (Ashton, 2003). The flowers of a number of Thymelaeaceae are monosymmetric by reduction: some Pimelea species have a single stamen, in other genera the gynoecium is pseudomonomerous (Eckardt, 1937; Herber, 2003).

Asterid Alliance (see also Endress, 2010)

Caryophyllales. Conspicuous monosymmetry is present in some bird-pollinated Caetaceae (Barthlott, 1993), and monosymmetry by sigmoid curvature in moth-pollinated Mirabilis (Nyetaginaceae, Correll & Correll, 1982) and Silene (Caryophyllaceae, Mabberley, 2000). Monosymmetry by reduction of the gynoecium to a single carpel is present in Rhabdodendraceae (Prance, 2003), in some Molluginaceae (Endress & Bittrich, 1993), in Nyctaginaceae (Rohweder & Huber, 1974; Bittrich & Kuhn, 1993), and some Phytolaccaceae (Rohwer, 1993b); monosymmetry by reduction of stamen number occurs in some Amaranthaceae (including Chenopodiaceae) (Kuhn, 1993), Caryophyllaceae (Bittrich, 1993), and Portulaeaceae (Carolin, 1993). Cases with a single (curved) ovule in a bi- or tricarpellate gynoecium are present in Basellaceae (Eckardt, 1955), Sarcobataceae (Behnke, 1997), Plumbaginaceae (De Laet et al., 1995; see also under Asymmetry), and part of Polygonaceae, Amaranthaceae (including Chenopodiaceae), and Caryophyllaceae.

Santalales. Monosymmetry by sigmoid curvature of the pollination organs is present in a number of Loranthaceae. These flowers are bird-pollinated and have an explosive opening mechanism. Such explosive flowers evolved more than once in the family (Feehan, 1985; Wilson & Calvin, 2006; Kuijt, 2007). In Eremolepidaceae, Antidaphne has monosymmetry or disymmetry by reduction (Kuijt, 1988).

Asterids. Monosymmetric flowers are prominent in both large subclades, lamiids and campanulids. In some orders they are highly elaborate.

Ericales. Although most families have predominantly polysymmetric flowers, monosymmetric taxa occur in several of them. Balsaminaceae are all pronouncedly monosymmetric, especially in the perianth (Fischer, 2004; Caris et al., 2006; Geuten et al., 2006). In Ericaceae monosymmetry with sigmoid curvature of the pollination organs occurs in Pyrola and Rhododendron (Stevens et al., 2004). In Lecythidaceae, Lecythidoideae have predominantly monosymmetric flowers, mainly with different stamen (staminode) shapes (Tsou & Mori, 2007). In Myrsinaceae some buzzpollinated taxa have a monosymmetric androecium (Ardisia speciosa, personal observation), and Coris has a bilabiate corolla (Stahl & Anderberg, 2004). In Polemoniaceae, Cobaea species and Loeseliastrum are pronouncedly monosymmetric (Grant & Grant, 1965; J. Sch6nenberger, personal communication), other genera have slighly monosymmetric flowers, either in the perianth or by curvature of the style (Grant & Grant, 1965; Wilken, 2004). In Fouquieria of Fouquieriaceae, the floral tube is slightly curved in long-tubed species (J. Sch6nenberger, personal communication).

Cornales. Flowers are largely polysymmetric, more rarely monosymmetric by reduction. In Cornaceae the gynoecium is sometimes pseudomonomerous and the flowers monosymmetric by reduction, such as in Nyssa and Mastixia (Eyde, 1963), and Alangium (Eyde, 1968). In Hydrangeaceae, Schizophragma has pronouncedly monosymmetric, onesided sterile flowers at the periphery of the inflorescences (Hufford, 2004), and Kirengeshoma has flowers with sigmoidally curved pollination organs. In Loasaceae, Petalonyx is monosymmetric (or asymmetric) by reduction of the fertile stamens to two or three and curvature of the pseudomonomerous gynoecium (Hufford, 1989; Weigend, 2004).

Euasterids I (Lamiids). Among the unplaced families in lamiids, Boraginaceae have a few genera with monosymmetric flowers. In Echium and relatives the corolla and androecium are (obliquely) monosymmetric (Gfirke, 1893), and in some Echiochilon species the corolla is strongly monosymmetric (Langstrom & Oxelman, 2003). In Lycopsis flowers have a slightly curved floral tube (Gurke, 1893). In Rochelia the gynoecium is unicarpellate and monosymmetric (Hilger, 1984), in Wigandia the two carpels are of unequal size and thus slightly monosymmetric (Hilger, 1987). Some genera are pronouncedly monosymmetric only in fruit (Asperugo, Harpagonella) (Gurke, 1893). In Metteniusaceae the gynoecium is monosymmetric by pseudomonomery (Gonzalez & Rudall, 2010).

Garryales. In Eucommiaceae the female flowers of Eucornmia are pseudomonomerous and thus monosymmetric by reduction (Eckardt, 1957). In Aucubaceae female flowers have a (pseudo)monomerous gynoecium and are monosymmetric by reduction, whereas the male flowers are polysymmetric (Philipson, 1967). Icacinaceae (which may be included in an expanded Garryales; Karehed, 2001; Stevens, 2001 onwards) have a monosymmetric unilocular and biovulate gynoecium, and the flowers may be monosymmetric by reduction (Sleumer, 1942; Fagerlind, 1945; Karehed, 2001).

Gentianales. Gentianales have largely polysymmetric flowers but most families have one or few monosymmetric taxa. In Gentianaceae the buzz-pollinated Exacum and Orphium have a curved androecium and gynoecium (Klackenberg, 2002); in Exacum dipterum also the calyx is monosymmetric (Klackenberg, 2002). In the bat-pollinated Irlbachia alata the corolla tube is sigmoidally curved (Machado et al., 1998). In the likewise bat-pollinated Fagraea racemosa the pollination organs are sigmoidally curved (Endress, 1994). In Canscora roxburghii, corolla and androecium (heteromorphic anthers) are monosymmetric (Thiv & Kadereit, 2002b). In Macrocarpaea marahuacae corolla, androecium and gynoecium are slightly monosymmetric (Struwe & Albert, 2002). In some species of Gentiana (e.g.G. lutea) the calyx is basically polysymmetric but it splits on one side ("spathaceous" calyx) (Ho & Liu, 2001). In Apocynaceae the corolla is slightly monosymmetric in Isonema and Rauvolfia vomitoria, or the flowers adjust to the final position by bending of the corolla tube (e.g., Allamanda, Beaumontia, Ceropegia, Tavaresia (Bruyns, 1985; personal observation). In Rubiaceae, although polysymmetry is by far predominant, there are a number of monosymmetric taxa. Mainly calyx and corolla are affected (survey in Robbrecht, 1988). The most conspicuous cases are flowers in which one of the five sepals is higly enlarged and acts as a flag (Mussaenda, Warszewiczia) (Weber, 1955; Classen-Bockhoff, 1996). Rarely also the androecium is affected (Posoqueria; Robbrecht, 1988). Among Loganiaceae, in Spigelia splendens the floral tube is slightly curved upwards (Erbar & Leins, 1999).

Lamiales. Lamiales are a huge clade in terms of species, genera and families with monosymmetric, often elaborate bilabiate flowers (Kampny, 1995). Monosymmetry is differentially expressed in the different families. The most common pattern is pentamerous flowers with an upper lip of two and a lower lip with three petals, and the odd stamen posterior (adaxial) (Donoghue et al., 1998). Commonly the odd stamen is more or less reduced to a staminode or even lacking. It is still present as a staminode in Gesneriaceae, many Bignoniaceae, and many Veronicaceae, but lacking in, e.g., Lamiaceae, Orobanehaceae, and many Acanthaceae (Endress, 1998, 1999). Scrophulariaceae sensu stricto are diverse in this respect. In Calceolariaceae, the flowers are tetramerous (and may almost look dimerous) (Endress, 1999; Mayr & Weber, 2006). In a few groups flowers became secondarily polysymmetric, such as Ramonda (Endress, 1998) and some Gloxinieae (Clark et al. 2011) (Gesneriaceae), and Sibthorpia (Veronicaceae) (Endress, 1998). In other groups a change from pentamery to tetramery by complete reduction of the median adaxial floral sector led to approximate polysymmetry (e.g. Veronica, Aragoa, Plantago, Veronicaceae; Donoghue et al., 1998; Reeves & Olmstead, 1998; Endress, 1999; Bello et al., 2004; or in the former Buddlejaceae, which are now in Scrophulariaceae; Olmstead et al., 2001; Oxelman et al., 2005; Tank et al., 2006). Such cases of lost monosymmetry were especially investigateded in molecular developmental studies (Citerne & Cronk, 1999; Cubas et al., 1999a; Moller et al., 1999; Citerne et al., 2000; Wang et al., 2006; Preston et al., 2011)). Pseudomonomerous gynoecia occur scattered in several groups. They add another dimension to the monosymmetry already present in corolla and androecium (e.g. Globularia, Hippuris, Litorella, Veronicaceae; Phryma, Phrymaceae; Stachytarpheta, Verbcnaceae; Eckardt, 1937). In the largely tetramerous and dimerous Oleaceae, Jasminum species with increased number of petals are polysymmetric (Torgard, 1924).

Solanales. Monosymmetry is rare in Convolvulaceae, most pronounced in Ipomoea lobata, with the corolla, androecium, and gynoecium bent, and Humbertia, with the androecium and gynoecium bent (Deroin, 1992). But it is more common in Solanaceae, with peaks in the keel flowers of Schizanthus and in the buzz-pollinated, heterantherous flowers of Solanum sect. Androceras (Whalen, 1978, 1979), which in the extreme have only one fertile stamen left (e.g., Solanum citrullifolium). Other nectariferous flowers less extremly monosymmetric than Schizanthus are, e.g., Browallia, Brunfelsia, Nierembergia, Petunia, Reyesia, Salpiglossis, Schwenkia, Solandra, and Triguera. In those mostly the corolla and androecium are affected; often only four or two of the five stamens are fertile (Robyns, 1930; Huber, 1980; Cocucci, 1991; Hunziker, 2001; Ampornpan & Armstrong, 2002).

Euasterids II (Campanulids)

Brunelliales. Monosymmetric flowers occur in Columellia, which have two stamens but otherwise pentamerous whorls (Stern et al., 1969), and in Desfontainia (both in Brunelliaceae), which have a slightly monosymmetric corolla tube (Endress, 2002).

Apiales. Monosymmetric flowers are rare. The most conspicuous monosymmetric flowers are in Apiaceae: In Orlaya, the peripheral flowers of the umbels have conspicuously enlarged petals on their peripheral side (Froebe, 1980; Classen-Bockhoff, 1992). Unequal petals are also present in a number of other genera, such as Ammi, Heracleum, and Scandix (Drude, 1897; Froebe, 1980). In some groups the two carpels are unequal, one with two main ribs and one with three main ribs (e.g., Elaeoselinum; Drude, 1897; Polemanniopsis; Burtt, 1988; Liu et al., 2004). In this case features of the pentamerous symmetry of the other floral whorls are super-imposed on the gynoecium (see also chapter "Transient monosymmetry"). In Griselinia of Griseliniacae, the female flowers are monosymmetric by their pseudomonomerous gynoecium (Kubitzki, 1963; Philipson, 1967). This is also true for Aralidium (Aralidiaceae; Philipson & Stone, 1980).

Aquifoliales. Cardiopteridaceae and Stemonuraceae have a unilocular gynoecium with two ovules (Sleumer, 1942; Fagerlind, 1945; Karehed, 2001; Tobe, 2012), and the flowers are probably monosymmetric by reduction.

Asterales. In Argophyllaceae, Corokia has a bi- or unilocular gynoecium with one ovule per locule and the unilocular flowers are monosymmetric (Philipson, 1967). A unilocular, uniovulate gynoecium is constantly present in Asteraceae (Anderberg et al., 2007) and Calyceraceae (Hellwig, 2007). However, Asteraceae also have another, more conspicuous kind of monosymmetry, which has found much more attention: the pronouncedly onesided differentiation of the corolla forming the radiating part of flat inflorescences (Harris, 1995; Leins & Erbar, 2000; Anderberg et al., 2007; Jeffrey, 2007). In Campanulaceae, Lobelioideae have elaborate monosymmetric flowers, with all four whorls of floral organs involved (Brantjes, 1983; Ayers, 1990; Leins & Erbar, 2005). This is also true for Stylidiaceae (Erbar, 1992) and Goodeniaceae (Carolin, 1959). In the three last mentioned families, in addition, a few taxa with pseudomonomerous, uniovulate gynoecia occur (Carolin, 2007a, b; Lammers, 2007).

Dipsacales. The families of Dipsacales have partially or consistently monosymmetric flowers, such as Adoxaceae, Caprifoliaceae, Diervillaceae, Dipsacaceae, Linnaeaceae, Morinaceae; only in Valerianaceae are they asymmetric) (Fukuoka, 1972; Roels & Smets, 1996; Donoghue et al., 2003; Howarth & Donoghue, 2005; Howarth et al., 2011). Monosymmetry comes about in various ways. In Caprifoliaceae the corolla is sometimes monosymmetric (especially Lonicera), also in Diervillaceae and Dipsacaceae, and in Linnaeaceae and Morinaceae, in addition, the androecium. Furthermore, in some Adoxaceae (Sinadoxa) and in Dipsacaceae the gynoecium is unilocular and uniovulate (Donoghue et al., 2003).

Basal Monocots. In basal monocots there is no elaborate monosymmetry, but monosymmetry by simplicity is more common. In Acorus (Acoraceae, Acorales), the putative sister to all other monocots, the perianth is more developed abaxially than adaxially (Buzgo & Endress, 2000). What looks like the subtending bract is in reality the outer median tepal. Among Alismatales, many Araceae (including Lemnaceae) have unicarpellate flowers (also the basal genus Gymnostachys; Buzgo, 2001), and some have unistaminate flowers (Mayo et al., 1997, 1998; Landolt, 1998). In Aponogetonaceae, flowers are often monosymmetric by the loss of the adaxial petal (Aponogeton distachyus) (van Bruggen, 1998). As an extreme, the spicate inflorescence may be condensed into a pseudanthium composed of several monosymmetric flowers as in Aponogeton ranunculiflorus (personal observation). In Cymodoceaceae flowers have one stamen or one carpel (Kuo & McComb, 1998a). In Hydrocharitaceae, similar reduction is known from several genera (Cook, 1998a), and the same applies for Juncaginaceae (Haynes et al., 1998a), Najadaceae (Haynes et al., 1998b), Positloniaceae (Kuo & McComb, 1998b), Potamogetonaceae (Haynes et al., 1998c), Zannichelliaceae (Haynes et al., 1998d), and Zosteraceae (Kuo & McComb, 1998c).

Dioscoreales. In Burmanniaceae, Afrothismia and some Thismia have a bent floral tube (Maas-van de Kamer, 1998). In Dioscoreaceae, Rajania has fruits with a wing (Correll & Correll, 1982), which are then monosymmetric.

Asparagales. Orchidaceae are a monosymmetric family with highly elaborate flowers par excellence, with all floral whorls strongly involved (e.g., Endress, 1994; Kurzweil & Kocyan, 2002; Rudall & Bateman, 2002); if the frequent floral resupination is considered, orchid flowers are strictly speaking asymmetric (see below). All other families are not or only partly monosymmetric. In Alliaceae, Gilliesia is strongly monosymmetric involving the inner perianth whorl, androecium and gynoecium, the related Gethyum only androecium and gynoecium (Rahn, 1998; Rudall et al., 2002; Fay & Hall, 2007). In several families a slight monosymmetry is present in the form of sigmoidal curvature of the floral tube and/or pollination organs; this is true for a number of Agapanthaceae (Kubitzki, 1998a), Agavaceae (Verhoek, 1998), Amaryllidaceae (Vochting, 1886; Church, 1908; Correll & Correll, 1982; Meerow & Snijman, 1998; Meerow, 2010), Anthericaceae (Conran, 1998), Asphodelaceaae (Vochting, 1886; Correll & Correll, 1982; Smith & Van Wyk, 1998), Blandfordiaceae (Clifford & Conran, 1998), Convallariaceae (Conran & Tamura, 1998), Doryanthaceae (Clifford, 1998), Hemeroeallidaceae (Vochting, 1886; Clifford et al., 1998), Hostaceae (Vochting, 1886; Kubitzki, 1998b), Iridaceae (mainly Ixioideae; Goldblatt et al., 1998), and Tecophilaeaceae (Simpson & Rudall, 1998). Other cases with stronger monosymmetry are: Sprekelia (Amaryllidaceae), in which, in addition to mere curvature, the perianth is conspicuously monosymmetric (Vochting, 1886); Haworthia and Chortolirion (Asphodelaceae) with bilabiate perianth (Smith & Van Wyk, 1998); Daubenya with three enlarged petals in basal flowers of the inflorescence, and Lachenalia (Hyacinthaceae) with a bilabiate perianth (Speta, 1998); Melasphaerula, Sparaxis, Chasmanthe and perhaps other Iridaceae-Ixioideae with bilabiate or otherwise monosymmetric perianth (Vogel, 1954; Goldblatt et al., 1998); Conanthera, Cyanella, Tecophilaea (Tecophilaeaceae) with strongly monosymmetric androecium (Simpson & Rudall, 1998).

Liliales. Most Liliales have polysymmetric or only slightly monosymmetric flowers, commonly involving sigmoidal curvature of perianth and/or pollination organs, thus similar as in Asparagales. More conspicuously monosymmetric are Alstroemeriaceae with monosymmetric nectar guides in Alstroemeria (Bayer, 1998), Colchicaceae with sharply angled style in Gloriosa (Werth, 1956; Endress, 1994, Nordenstam, 1998), Corsiaceae with a strongly monosymmetric perianth (Neinhuis & Ibisch, 1998), Liliaceae with a spur in one of the six tepals in Fritillaria sect. Theresia (Tamura, 1998a), and Melanthiaceae with the perianth developed more strongly on one side in some Chionographis (Tamura, 1998b; Tanaka, 2003).

Pandanales. Pandanales generally have polysymmetric flowers. Slight monosymmetry is present in some flowers of Pandanus (Pandanaceae), in which a unicarpellate gynoecium occurs (Endress, 1995; Stone et al., 1998). In Triuridaceae the androecium is rarely reduced to one stamen (Maas-van de Kamer & Weustenfeld, 1998).

Commelinids. In general, in commelinids floral monosymmetry is more prominent than in the other suprafamilial clades of monocots.

Arecales. Arecaceae are supposed to have largely polysymmetric flowers as floral symmetry is not considered in Uhl & Dransfield (1987) and Dransfield & Uhl (1998). However, weak monosymmetry is present in at least some palms: in Geonoma the gynoecium is pseudomonomerous (Stauffer et al., 2002; Stauffer & Endress, 2003); some other Geonomeae have a dorsiventrally flattened gynoecium and the (sterile) androecium in female flowers is monosymmetric (Stauffer & Endress, 2003). Some coryphoid palms are unicarpellate (Rudall et al., 2011).

Commelinales. In Commelinaceae many genera have monosymmetric flowers, often with heteranthery involved (e.g., Aetheolirion, Tinantia, Floscopa, Polyspatha, Aneilema, Rhopalephora, Commelina) (Faden, 1998). In Plowmanianthus the lower three stamens are strongly reduced (Hardy & Faden, 2004; Hardy et al., 2004). In Dichorisandra the stamens are sigmoidally curved (Hardy et al., 2000). In Tapheocarpa, one of the three carpels is sterile (Faden, 1998). Haemodoraceae have some monosymmetric genera in both subfamilies, such as Pyrrorhiza (with one stamen), Schiekia, Wachendorfia in Haemodoroideae, and the conspicuously bilabiate Anigozanthos in Conostylidoideae (Simpson, 1990, 1998). In Philydraceae the flowers of all genera are conspicuously monosymmetric (sometimes asymmetric) (Hamann, 1966, 1998). In Pontederiaceae all genera are monosymmetric to some degree (Cook, 1998b; Strange et al., 2004).

Poales. Bromeliaceae have largely polysymmetric flowers. In a few genera weakly monosymmetric flowers occur, such as Pitcairnia or Bilbergia (Smith & Till, 1998). In Centrolepidaceae, the unisexual flowers are monosymmetric by reduction in having a single stamen or (probably) an unilocular and uniovulate gynoecium (Hamann, 1975; Sokoloff et al., 2009). In Cyperaceae, delimitation of flowers is unclear in Hypolytreae and Chrysitricheae. If the apparent flowers are pseudanthia, the male flowers are unistaminate and thus monosymmetric by reduction. Monosymmetry by reduction is also common in Cyperoideae, in which many genera have flowers with 1-3 stamens (e.g. Scirpus, Eriophorum, Eleocharis, Cyperus) or only one stamen (Becquerelia, Diplacrum, Bisboeckelera, Calyptrocarya); in female flowers the gynoecium is unilocular and uniovulate (Goetghebeur, 1998). In many Eriocaulaceae the median sepal is smaller than the other two or even lacking, and in several Eriocauloideae the united sepals form a one-sided spathe-like structure (Stutzel, 1984). In Poaceae, floral monosymmetry by reduction is common; the petals ("lodicules") are reduced to two in most groups, some of the stamens may be reduced, and the gynoecium is unilocular and uniovulate (Dahlgren et al., 1985; Cocucci & Anton, 1988; Rudall & Bateman, 2004). In Rapateaceae, flowers of Kunhardtia have a monosymmetric corolla (Stevenson et al., 1998). In Restionaceae, floral monosymmetry by gynoecium reduction to one (uniovulate) locule has evolved several times (Linder et al., 1998; Ronse De Craene et al., 2002b). In Sparganiaceae floral monosymmetry by reduction is present by a unilocular ovary and sometimes reduction of perianth and androecium to one organ (Muller-Doblies, 1970; Kubitzki, 1998c). In Xyridaceae the calyx is monosymmetric in Abolboda and Xyris, the corolla is bilabiate in Orectanthe, and the ovary is dorsiventrally compressed in Aratitiyopea (Kral, 1998).

Zingiberales. All Zingiberales have pronouncedly monosymmetric (or even asymmetric) flowers (Kress, 1990). In Musaceae (Andersson, 1998b), Strelitziaceae (Andersson, 1998c), Heliconiaceae (Andersson, 1998a), and Lowiaceae (Larsen, 1998b) the median posterior stamen is reduced to a staminode or is missing, and the flowers are more or less conspicuously bilabiate. In Zingiberaceae (Larsen et al., 1998) and Costaceae (Larsen, 1998a), in contrast, only the median posterior stamen is fertile; in Costaceae the other five stamens are transformed into the lower lip, in Zingiberaceae only two to four of them form the lower lip, while the remaining ones may form separate petaloid organs (Endress, 1995; Specht et al., 2008).

Asymmetric Flowers

Real floral asymmetry, the lack of any symmetry plane, is much less studied than monosymmetry and is also much less common than monosymmetry, although subtle forms of asymmetry, in addition to fluctuating asymmetry, are widespread. Like monosymmetry, asymmetry is also expressed at different structural levels.

Asymmetry in flowers, if taken in a strict sense is, like monosymmetry, a mixed bag of forms that are likely not all homologous because this asymmetry comes about for different reasons. In the literature floral asymmetry is mostly addressed for cases of basically monosymmetric flowers that have lost their monosymmetry by unequal development of the two mirror symmetrical halves. The most well known cases are enantiostyly or enantiomorphy. More subtle cases of asymmetry are those in which the symmetry planes of the different organ whorls are not congruent.

Diverse Manifestations of Asymmetry in Flowers

Asymmetry as a Modification of Elaborate Monosymmetry (Enantiomorphy). Floral asymmetry is best known in enantiostylous flowers, which are basically monosymmetric but have the style deflected to the right or the left side (Jesson & Barrett, 2003; Jesson et al., 2003a, b). In such flowers with a deflected style often also other floral whorls are involved in this onesidedness (Marazzi et al., 2006; Marazzi & Endress, 2008). For those, the more inclusive term enantiomorphy is used. Enantiostyly and enantiomorphy express the presence of two mirror-image like morphs (either on the same individual or on different individuals). If only one morph exists in a species or higher group, it is non-enantiomorphic asymmetry. The latter tends to be the case in the most elaborate asymmetric flowers (e.g., Phaseoleae of Fabaceae; Endress, 1999, 2001a; Etcheverry et al., 2008). The presence of only one morph may make it easier for bees to exploit the flowers always from the same side (Endress, 1999; see also Schmucker, 1931; Woodward & Laverty, 1992; and Gegear & Laverty, 1995). In vertebrates the organisational internal asymmetry is not conspicuous externally; however, some specialized groups also have an external non-enantiomorphic asymmetry, such as snail-eating snakes with asymmetric jaws (Hoso et al., 2007). If enantiomorphic flowers occur on one individual, they are often components of a system with pendulum symmetry (Goebel, 1908; Endress, 1999; Kirchoff, 2003; Hardy et al., 2004).

A variant of curvature is torsion, such as resupination in orchids (inferior ovary), torsion in Platystigma (superior ovary), mimosoids (superior ovary), Medicago (superior ovary), Helicteres (superior ovary), Cajophora (inferior ovary), Pedicularis (corolla) (Goebel, 1920). In Haemaria (Orchidaceae) the torsion encompasses the entire flower and makes it conspicuously asymmetric (Goebel, 1920). Torsion is often a mechanism for resupination, the reorientation of otherwise monosymmetric flowers into a perpendicular position, best known from orchids. However, resupination may also be based on adjustments outside of the flower proper. Thus not all resupinated flowers are asymmetric. Besides orchids, resupination is less well studied but occurs in a number of families here and there (Papaveraceae-Fumarioideae, Melianthaceae, Leguminosae, Violaceae, Balsaminaceae, Acanthaceae, Lamiaceae, Scrophulariaceae sensu lato, Campanulaceae-Lobelioideae, Alstroemeriaceae, Zingiberaceae; Goebel, 1920).

At another level is fluctuating asymmetry, small irregularities resulting from incompletely balanced development (Moller, 2000). This is a widespread phenomenon and is not discussed here.

Asymmetry by Spiral or Irregular Phyllotaxis and by Imbricate Perianth Organ Aestivation. A kind of minor asymmetry is provided by spiral and irregular phyllotaxis. These cases are not considered in this study because the former are common and scattered through the basal angiosperms and basal eudicots, and the latter are common in flowers with an increased number of stamens throughout the angiosperms.

Likewise, all kinds of imbricate aestivation (contort, quincuncial, cochlear) of the calyx or corolla of a flower with pentamerous or otherwise odd-merous whorls lead to a kind of floral asymmetry (Reinsch, 1927). For instance, in Commelinaceae there is a cochlear aestivation in both perianth whorls (Hardy et al., 2000; Hardy et al., 2004). As imbricate aestivation of sepals and petals is the most common aestivation type in angiosperms, it is difficult to record all these taxa. The most conspicuous asymmetric pattern among imbricate aestivations is contort aestivation. Thus among the imbricate aestivations only taxa with a contort pattern were recorded in this study. Contort petal aestivation is present in a number of angiosperm groups, especially in core eudicots. Whereas in many rosids, contort petal aestivation is enantiomorphic, in almost all asterids contort aestivation is fixed to one of the two possible morphs at genus or even family level (Endress, 1999, 2001a).

Ironically, contort aestivation is strongly correlated with polysymmetric flowers (Schoute, 1935; Endress, 1994, 1999). However, the contortion superimposes on the flowers an asymmetry pattern.

Contort aestivation in many cases also results in asymmetric petals: Sida, Malva, Hibiscus, Kielmeyera, Dombeva, Linum austriacaum, Oxalis floribunda, O. ortgiesii, Plumbago capensis (Schoute, 1935), and especially conspicuous in some Apocynaceae, such as Vinca (Endress, 1999). However, others have perfectly symmetrical petals: Linum usitatissimum, L. flavum, Gypsophila paniculata, Geranium sanguineum, Plumbago larpentae (Schoute, 1935).

In many groups of eudicots with an imbricate (quincuncial) calyx, inner and outer sepals are of somewhat difference size and/or shape, which also results in a minor floral asymmetry. Such cases are not considered here.

Transient Asymmetry

Monochasial partial inflorescences in which the angles of successive branches are not 180[degrees] are asymmetric systems. This asymmetry may influence the shape of the young flowers and they may also be slightly asymmetric. Later in development this asymmetry is equalized and the anthetic flowers are monosymmetric or polysymmetric. Such transient slight early asymmetry was observed in Tradescantia (Commelinaceae) (personal observation; Hardy & Stevenson, 2000b).

Asymmetry in Special Biological Situations

Buzz Pollination and Floral Enantiostyly. Interestingly, most enantiostylous flowers are buzz-pollinated pollen flowers (Buchmann, 1983), such as Chamaecrista and Senna (Fabaceae; Gottsberger & Silberbauer-Gottsberger, 1988; Marazzi et al., 2007), Cyanella (Tecophilaeaceae; Dulberger & Ornduff, 1980), Heteranthera (Pontederiaceae), Philydrum (Philydraceae), Solanum (Solanaceae), and Paraboea (Gao et al. 2006) and Saintpaulia (Gesneriaceae). It has been argued that the lateral position of the style, away from the buzzing bee, is favorable for not obstructing this special kind of pollination (Dulberger, 1981; Barrett et al., 2000; Marazzi & Endress, 2008).

Tubular Nectar Flowers with Anthers at Different Levels. Anthers of basically polysymmetric pentamerous, tubular flowers may be presented at three or five different levels, based on either different levels of stamen insertion or different length of filaments. These levels are linked with the spiral direction of the sepals (Endress, 1999) so that the result is, strictly speaking, floral asymmetry. Such cases were reported from Ericaceae (personal observation), Polemoniaceae (Grant & Grant, 1965; Mabberley, 2000), Gentianaceae (Thiv & Kadereit, 2002a), Rubiaceae (Robbrecht, 1988), Boraginaceae (Taroda & Gibbs, 1986), Convolvulaceae (Wagner, 1989; Deroin, 1996), and Solanaceae (Robyns, 1930; Hunziker, 2001). These families are all in asterids I (lamiids), except for Ericaceae and Polemoniaceae, which are in the more isolated Ericales (basal asterids).

Flowers in Agamospermous Plants. Another source of asymmetry is irregular petal formation, associated with reduction, which is known from some taxa with agamospermous (apomictic) flowers (Potentilla aurea; Heinricher, 1907; Ranunculus auricomus; Zimmermann, 1975). This may be seen as an extreme case of fluctuating asymmetry that is not constrained if pollination of flowers is not required.

Systematic Distribution of Floral Asymmetry Across Angiosperms

Also the occurrence of floral asymmetry is relatively widespread, although it is less common than monosymmetery. Different forms of asymmetry are not equally distributed across the angiosperms (Fig. 6). In contrast to the corresponding chapter on monosymmetrie flowers in most cases only those larger taxa that contain asymmetric flowers are listed.

Basal Angiosperms. Asymmetric flowers are lacking, except for flowers with spiral or irregular floral phyllotaxis (here not considered), and flowers with contort aestivation in each perianth whorl (Cabomba, Cabombaceae) (Endress, 2008a).

Basal Eudicots. Floral asymmetry is rare and never conspicuous. It comes about by spiral or irregular phyllotaxis (here not considered), or by contort aestivation of petals (some Papaveraceae, Schoute, 1935), or with irregularity superimposed on monosymmetry (Eupteleaceae, Endress, 1986a, 2008b; Ren et al., 2007). Slight enantiomorphy occurs in otherwise monosymmetric flowers and fruits of Banksia (Proteaceae) (Renshaw & Burgin, 2008).

Saxifragales. Some Crassulaceae and Haloragaceae have contort petals (Eichler, 1878, Schoute, 1935).

Zygophyllales. In Zygophyllaeeae the petals are contort in Tribulus (Correll & Correll, 1982).

Cucurbitales. In Begoniaceae the perianth and/or the inferior, winged ovary is often asymmetric (Eichler, 1878; Fang et al., 2006), and in some species of Momordica (Cueurbitaceae) corolla and androecium are asymmetric (Vogel, 1974).

Fabales. Floral asymmetry is characteristic for some larger clades of Fabaceae, such as Senna and Chamaecrista in caesalpinioids, in which the flowers are more or less open but corolla, androecium and gynoecium (enantiostyly) can be involved in asymmetry (Tucker, 1996; Gottsberger & Silberbauer-Gottsberger, 1988; Marazzi et al., 2006, 2007; Marazzi & Endress, 2008), and Delonix, in which the androecium base with the access to the nectar is asymmetric (Troll, 1951; Endress, 1994); in Papilionoideae the flowers of Phaseoleae are largely asymmetric by sometimes extensive torsion of the keel (Troll, 1951; Brizuela et al., 1993; Endress, 1994; Etcheverry et al., 2008), or in certain Vicieae the keel is asymmetric without torsion (the asymmetry arising late in development), as in Lathyrus species (Teppner, 1988; Westerkamp, 1993; Prenner, 2003) and Ottleya (Ottley, 1944; Sokoloff, 1999; Sokoloff et al., 2007); asymmetry in the androecium is present in early development of various Papilionoideae (Prenner, 2004e). Among basal Fabaceae, Duparquetia (Prenner & Klitgaard, 2008) and Gleditsia (Tucker, 1991) are asymmetric by loss of one or more sepals. Late asymmetry of the keel is also present in Polygalaceae (some Polygala species; Brantjes, 1982; Westerkamp & Weber, 1997, Prenner, 2004b). In Quillajaceae and Suriana (Surianaceae) the petals are contort (Bello et al., 2007).

Rosales. Petals are contort in Gillenia and Raphiolepis (Rosaceae) (Eichler, 1878).

Malpighiales. A number of families have representatives with contort petal aestivation, such as Bonnetiaceae (Lleras, 1972; Maguire, 1972; Kubitzki. 1978), Clusiaceae (Eichler, 1878; Schoute, 1935; de Oliveira & Sazima, 1990; Gill et al., 1998; Stevens, 2007), Ctenolophonaceae (Narayana & Rao, 1971; Matthews & Endress, 2011), Erythroxylaceae (Eichler, 1878), Euphorbiaceae (Schoute, 1935; Rao, 1972; Endress, 1999), Euphroniaceae (Matthews & Endress, 2008), Humiriaceae (Rao & Narayana, 1965; Narayana & Rao, 1969), Hypericaceae (Eichler, 1878; Ronse De Craene & Smets, 1991), Ixonanthaceae (Steyermark & Luteyn, 1980), Linaceae (Eichler, 1878; Schoute, 1935; Narayana, 1963; Endress, 1999; Matthews & Endress, 2011), Medusagynaceae (Dickison, 1990), Ochnaceae (Eichler, 1878), Quiinaceae (Schneider et al., 2006), Rhizophoraceae (Matthews & Endress, 2011; Trigoniaceae (Schoute, 1935; Matthews & Endress, 2008), and Turneraceae (Eichler, 1878; Rao, 1949; Gonzalez, 1993).

Celastrales. Brexia (Celastraceae) has contort petals (Eichler, 1878; Matthews & Endress, 2005a).

Oxalidales. Oxalidaceae have contort petals (Narayana, 1966; Matthews & Endress, 2002).

Geraniales. The petals are contort in those genera of Geraniaceae that have otherwise polysymmetric flowers (Geranium, Monsonia) (Eichler, 1878), and also in Hypseoeharitaceae (Devi, 1991) and Ledoearpaceae (Knuth, 1931).

Myrtales. Flowers with contort petals are present in some Combretaceae (Schoute, 1935; Tiagi, 1969), Punica (Lythraceae) (Eichler, 1878), Melastomataceae (Renner, 1993), and Onagraceae (Eichler, 1878). In Vochysiaceae the spurred flowers are strongly monosymmetric and in some taxa asymmetric (enantiomorphic) (Oliveira, 1996; Litt & Stevenson, 2003a, b; Kawasaki, 2007).

Brassicales. Contort petals occur regularly or occasionally in various families, such as Akaniaceae (Ronse De Craene et al., 2002a), Capparaceae (Correll & Correll, 1982; Ronse De Craene et al., 2002a; Rankin-Rodriguez & Greuter, 2004), Caricaceae (Eichler, 1878; Schoute, 1935; Devi, 1952; Ronse De Craene & Smets, 1999), Cleomaceae (Hildebrand, 1886), Koeberliniaceae (Mehta & Moseley, 1981), Limnanthaceae (Eichler, 1878), and Moringaceae (Olson, 2003).

Malvales. Contort petals occur in Cistaceae (Eichler, 1878), Cochlospermaceae (Poppendieck, 2003), Dipterocarpaceae (Rao, 1962; Ashton, 2003), Malvaceae sensu lato (Eichler, 1878; Rao, 1952; Bayer, & Kubitzki, 2003), and Sarcolaenaceae (Bayer, 2003).

Sapindales. Taxa with contort petals are scattered in various families, e.g. Anacardiaceae (Bachelier & Endress, 2009), Burseraceae (Lain, 1932), Meliaceae (Narayana, 1958; Murty & Gupta, 1978; Correll & Correll, 1982; Castaneda-Posadas & Cevallos-Ferriz, 2007), Nitrariaceae (Eichler, 1878; Nair & Nathawat, 1958; Ronse De Craene et al., 1996), Rutaceae (Eichler, 1878; Engler, 1931; Tilak & Nene, 1978; Correll & Correll, 1982), Sapindaceae (Weckerle & Rutishauser, 2003), and Simaroubaceae (Nair & Joseph, 1957; Correll & Correll, 1982).

Caryophyllales. In several clades contort petal aestivation occurs, such as in some Lychnideae of Caryophyllaceae (Schoute, 1935), some Droseraceae (Eichler, 1878), Plumbaginaceae (Schoute, 1935), and Tamaricaceae (Eichler, 1878; Correll & Correll, 1982). In some Caryophyllaceae asymmetry by reduction also occurs, in flowers with only one stamen that is not in a symmetry plane (Mniarum, Eichler, 1878).

Cornales. Contort petals occur in Philadelphus of Hydrangeaceae (de Candolle, 1827; Eichler, 1878).

Ericales. In several families there are representatives with contort petals, such as in Ebenaceae (Schoute, 1935), Ericaceae (Eichler, 1875; Schoute, 1935), Myrsinaceae (Schoute, 1935), Polemoniaceae (Eichler, 1875; Schoute, 1935), Samolaceae (Schoute, 1935), and Sapotaceae (Eichler, 1875). The anthers are at different levels within a flower in Erica fastigiata (Ericaceae) (personal observation) and some Polemoniaceae (Grant & Grant, 1965; Mabberley, 2000; Wilken, 2004).

Boraginaceae. In several genera the petals are contort (Eichler, 1875; Schoute, 1935). Some species of Cordia have the stamens inserted at different levels in the corolla tube (Taroda & Gibbs, 1986).

Gentianales. Contort petals are common in Gentianaceae and Apocynaceae (Endress, 1999; Endress & Bruyns, 2000) and also occur in some Loganiaceae and Rubiaceae (Schoute, 1935; Robbrecht, 1988). In the otherwise monosymmetric Exacum (Gentianaceae) enantiostyly is present (Lloyd & Webb, 1992). In some Gentianaceae stamens are inserted at different levels within a flower (Canscora, Hoppea, Schinziella) or the stamen filaments are of different length (Duplipetala, Phyllocyclus) (Thiv & Kadereit, 2002a). Also in some Rubiaceae stamens are of different length within a flower (Wittmackanthus) (Robbrecht, 1988).

Solanales. Petals are contort in Convolvulaceae and some Solanaceae (Eichler, 1875; Schoute, 1935; Endress, 1999). Anthers positioned at different levels in a flower, either by different filament length or different insertion height occur in taxa of both families, such as Convolvulus, Hildebrandtia in Convolvulaceae (Wagner, 1989; Deroin, 1996) and many genera in Solanaceae (Robyns, 1930; Hunziker, 2001). In Solanaceae, species of Solanum sect. Androceras are enantiostylous (Whalen, 1979).

Lamiales. Contort petals are present in a number of Acanthaceae (Scotland et al., 1994; Schonenberger & Endress, 1998) and Oleaceae (Schoute, 1935). Saintpaulia and species of Streptocarpus (Gesneriaceae) are enantiostylous (Harrison et al., 1999; Jesson & Barrett, 2003). In a number of Pedicularis species (Orobanchaceae) the basically monosymmetric flowers have a twisted corolla (Endress, 1999). Among Acanthaceae, in Brachystephanus glaberrimus, flowers with a long and a short stamen were reported (Champluvier, 1997).

Asterales. No conspicuous floral asymmetries were found.

Dipsacales. In Valerianaceae the flowers are conspicuously asymmetric in Kentranthus because the monosymmetry planes of the corolla, androecium and gynoecium are not congruent (Eichler, 1875; Endress, 1999); in other genera this asymmetry is also present but less strong (Eichler, 1875; Goebel, 1908; Endress, 1999; Donoghue et al., 2003). A similar kind of asymmetry is also present in Viburnum (Adoxaceae), and Abelia and Linnaea (Linnaeaceae) (Eichler, 1875).

Basal Monocots. Asymmetric flowers appear to be lacking.

Asparagales. In Orchidaceae, many flowers have a twisted inferior ovary, which results in a superposed asymmetry (Ernst & Arditti, 1994); in a few genera, such as Haemaria (Goebel, 1920) and Tipularia (Stoutamire, 1978), also the upper parts of the flower are twisted. In Iridaceae, Iris and Tigridia have contort petals (Schoute, 1935). In Tecophilaeaceae, Cyanella has enantiostylous flowers (Dulberger & Ornduff, 1980; Simpson & Rudall, 1998).

Liliales. Contort petals are present in Philesia (Philesiaceae) (Conran & Clifford, 1998); both sepals and petals are contort in some Trilliaceae (Schoute, 1935; Tamura, 1998c).

Commelinales. The flowers of Cochliostema (Commelinaceae) are conspicuously asymmetric (Troll, 1961; Hardy & Stevenson, 2000a). In Haemodoraceae, some genera are enantiostylous, either based on otherwise polysymmetric or monosymmetric flowers (Simpson, 1990; Jesson & Barrett, 2002; Jesson et al., 2003b). Enantiostyly is also present in Philydrum (Philydraceae) (Simpson, 1990; Jesson et al., 2003b), and Heteranthera and Monochoria (Pontederiaceae) (Graham & Barrett, 1995; Jesson et al., 2003b).

Poales. Sepals and petals are contort in Bromeliaceae (Eichler, 1875; Smith & Till, 1998) and Rapateaceae (Pilger, 1930).

Zingiberales. Marantaceae and Cannaceae consistently have conspicuously asymmetric flowers with corolla, androecium and gynoecium involved, especially elaborate in Marantaceae (Kirchoff, 1983; Kress, 1990; Classen-Bockhoff & Heller, 2008). They may be seen as an elaboration of monosymmetric flowers as in the other six families of the order, of which a few Zingiberaceae also have asymmetric flowers (Hedychium) (Kress, 1990).
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Title Annotation:p. 345-374
Author:Endress, Peter K.
Publication:The Botanical Review
Article Type:Report
Geographic Code:1USA
Date:Dec 1, 2012
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