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Chloranthaceae were one of the first common lines during the early radiation of angiosperms, possibly reflecting adaptation to more open habitats. Phylogenetic analyses clarify the position of Cretaceous mesofossils in molecular trees of Recent taxa. Plants that produced Asteropollis pollen, with tepals adnate to a single carpel, are nested in crown group Chloranthaceae with Hedyosmum; Canrightiopsis, with three stamens and no perianth, is sister to Sarcandra and Chloranthus; and Canrightia is a stem relative that illustrates a still bisexual stage in floral reduction. Plants that produced Pennipollis pollen are related to Chloranthaceae and/or Ceratophyllum rather than monocots. Appomattoxia, which produced Tucanopollis pollen, has equivocal affinities, but Pseudoasterophyllites, with similar pollen and stems with reduced leaves, may be a link between Chloranthaceae and Ceratophyllum. These results imply that flowers became unisexual before losing the perianth, while bisexual flowers in Canrightiopsis, Sarcandra, and Chloranthus are secondarily derived from unisexual flowers.

New morphological and phylogenetic data prompt us to present an updated review of floral morphology and its evolution in the basal ANITA grade of living angiosperms, Chloranthaceae, and Ceratophyllum. Floral phyllotaxis is complex whorled in Nymphaeales and spiral in Amborella and Austrobaileyales. It is unresolved whether phyllotaxis was ancestrally whorled or spiral, but if it was whorled, the whorls were trimerous. The flowers are probably ancestrally bisexual because in most families with unisexual flowers these flowers exhibit rudiments of the opposite sex. Carpels are largely ascidiate and the closure line is short, either transverse or longitudinal. A style is usually absent or, if present, generally short and not plicate. Angiospermy (carpel sealing) is by secretion, rather than postgenital fusion, except in large-flowered Nymphaeales and in Illicium, correlated with unusual fruits. Carpels with a single, median, pendent ovule are probably plesiomorphic. Chloranthaceae and Ceratophyllaceae have an unsettled phylogenetic position, but in some phylogenetic analyses they form a clade, which may be sister to the remaining mesangiosperms (Magnoliidae, monocots, eudicots). This position is supported by their carpel characters, which are similar to those of the ANITA grade and different from those of most other mesangiosperms.

Premise of research. Discoveries of fossil flowers in Cretaceous rocks offer improved evidence for relationships with living clades, but for more secure inferences formal phylogenetic analyses are desirable. We extend previous analyses of magnoliids, monocots, and basal eudicots to Aptian, Albian, and Cenomanian fossils related to the basal "ANITA" lines and Chloranthaceae. Methodology. We performed parsimony analyses of a morphological data set of Recent angiosperms and published fossils, with the arrangement of Recent taxa constrained to backbone trees based primarily on molecular data. Pivotal results. Not only Monetianthus (as previously inferred) but also Carpestella is nested within Nymphaeaceae, while Pluricarpellatia may be a stem relative of Cabombaceae or Nymphaeaceae. Anacostia (with Similipollis pollen) is nested within Austrobaileyales. The position of Couperites (with Clavatipollenites pollen) is ambiguous: it may be on the stem lineage of Chloranthaceae (and Ceratophyllum, if this extant aquatic is related to Chloranthaceae), nested in Chloranthaceae, or more basal. Plants with Asteropollis pollen and reduced tepals are related to the chloranthaceous genus Hedyosmum. Zlatkocarpus, which also has a reduced perianth, may be either a stem relative or a crown group member of Chloranthaceae. Plants that produced loosely reticulate Pennipollis pollen are more likely related to Chloranthaceae and/or Ceratophyllum than to monocots. We confirm that Canrightia, with bisexual flowers and a reduced perianth, is a stem relative of Chloranthaceae. Despite similarities to Piperales, Appomattoxia (with Tucanopollis pollen) is more likely near the base of the ANITA grade or related to Chloranthaceae and/or Ceratophyllum. Conclusions. The Cretaceous rise of angiosperms involved the radiation not only of magnoliids, eudicots, and monocots but also of basal ANITA lines, including both aquatic Nymphaeales and woody groups. Our results reaffirm the early diversity of Chloranthaceae and clarify their floral evolution, in which a shift to unisexual flowers preceded loss of the perianth. © 2014 by The University of Chicago. All rights reserved.

Alexander Moritzi (1806-1850) is one of the most obscure figures in the early history of evolutionary thought. Best known for authoring a flora of Switzerland, Moritzi also published Réflexions sur l'espèce en histoire naturelle (1842), a remarkable book about evolution with an overtly materialist viewpoint. In this work, Moritzi argues that the (then) generally accepted line between species and varieties is artificial, that varieties can over time give rise to new species, and that deep time and turnover of species in the fossil record clearly support an evolutionary interpretation of biological diversity. Moritzi was also a gradualist and viewed relationships between taxa as best represented by a ramifying tree. Although Réflexions was the first full book to be written on the topic of evolution following Lamarck's Philosophie zoologique (1809), Moritzi's evolutionist contribution was stillborn, read by almost no one in his lifetime and ultimately absent from the many historiographies of evolutionary thought. This is unfortunate since many of the arguments Moritzi marshaled on behalf of an evolutionary explanation of life can be found in subsequent transmutationist writings by Frédéric Gérard, Robert Chambers, Henri Lecoq, Baden Powell, Charles Naudin, Herbert Spencer, Alfred Russel Wallace, and Charles Darwin-none of whom is likely to have ever known of the existence of Réflexions. Finally, Moritzi's arguments, along with those found in Darwin's private essay on evolution of the same year, provide an excellent window into the state of evolutionary thought and debate over the nature of species at the beginning of the 1840s.

With 145 species, Turnera is the largest genus of Turneraceae (Malpighiales). Despite several morphotaxonomic and cytogenetic studies, our knowledge about the phylogenetic relationships in Turnera remains mainly based on morphological data. Here, we reconstruct the most comprehensive phylogeny of Turnera with molecular data to understand the morphological evolution within this group and to assess its circumscription and infrageneric classification. We analyzed two nuclear and six plastid markers and 112 taxa, including species and infraspecific taxa, 97 from Turnera, covering the 11 series of the genus. Bayesian inference, maximum parsimony and maximum likelihood analyses show that Turnera, as traditionally circumscribed, is not monophyletic. The genus is divided into two well-supported independent clades; one of them is sister to the genus Piriqueta and is here segregated as the new genus Oxossia. According to our reconstructions, Turnera probably evolved from an ancestor without extrafloral nectaries and with solitary, homostylous flowers with yellow petals. The emergences of extrafloral nectaries and distyly, both common in extant taxa, played an important role in the diversification of the genus. An updated infrageneric classification reflecting the relationships within Turnera is now possible based on morphological synapomorphies and is here designed for further studies., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Recent advances in molecular phylogenetics and a series of important palaeobotanical discoveries have revolutionized our understanding of angiosperm diversification. Yet, the origin and early evolution of their most characteristic feature, the flower, remains poorly understood. In particular, the structure of the ancestral flower of all living angiosperms is still uncertain. Here we report model-based reconstructions for ancestral flowers at the deepest nodes in the phylogeny of angiosperms, using the largest data set of floral traits ever assembled. We reconstruct the ancestral angiosperm flower as bisexual and radially symmetric, with more than two whorls of three separate perianth organs each (undifferentiated tepals), more than two whorls of three separate stamens each, and more than five spirally arranged separate carpels. Although uncertainty remains for some of the characters, our reconstruction allows us to propose a new plausible scenario for the early diversification of flowers, leading to new testable hypotheses for future research on angiosperms.

Explosive diversification is widespread in eukaryotes, making it difficult to resolve phylogenetic relationships. Eudicots contain c. 75% of extant flowering plants, are important for human livelihood and terrestrial ecosystems, and have probably experienced explosive diversifications. The eudicot phylogenetic relationships, especially among those of the Pentapetalae, remain unresolved. Here, we present a highly supported eudicot phylogeny and diversification rate shifts using 31 newly generated transcriptomes and 88 other datasets covering 70% of eudicot orders. A highly supported eudicot phylogeny divided Pentapetalae into two groups: one with rosids, Saxifragales, Vitales and Santalales; the other containing asterids, Caryophyllales and Dilleniaceae, with uncertainty for Berberidopsidales. Molecular clock analysis estimated that crown eudicots originated c. 146 Ma, considerably earlier than earliest tricolpate pollen fossils and most other molecular clock estimates, and Pentapetalae sequentially diverged into eight major lineages within c. 15 Myr. Two identified increases of diversification rate are located in the stems leading to Pentapetalae and asterids, and lagged behind the gamma hexaploidization. The nuclear genes from newly generated transcriptomes revealed a well-resolved eudicot phylogeny, sequential separation of major core eudicot lineages and temporal mode of diversifications, providing new insights into the evolutionary trend of morphologies and contributions to the diversification of eudicots., (© 2017 The Authors. New Phytologist © 2017 New Phytologist Trust.)

Background and Aims: Apocynaceae and Orchidaceae are two angiosperm families with extreme flower synorganization. They are unrelated, the former in eudicots, the latter in monocots, but they converge in the formation of pollinia and pollinaria, which do not occur in any other angiosperm family, and for which extreme synorganization of floral organs is a precondition. In each family extensive studies on flower development and evolution have been performed; however, newer comparative studies focusing on flower synorganization and involving both families together are lacking., Scope: For this study an extensive search through the morphological literature has been conducted. Based on this and my own studies on flowers in various Apocynaceae and Orchidaceae and complex flowers in other angiosperms with scanning electron microscopy and with microtome section series, a review on convergent floral traits in flower development and architecture in the two families is presented., Key Findings: There is a tendency of protracted development of synorganized parts in Apocynaceae and Orchidaceae (development of synorganization of two or more organs begins earlier the more accentuated it is at anthesis). Synorganization (or complexity) also paves the way for novel structures. One of the most conspicuous such novel structures in Apocynaceae is the corona, which is not the product of synorganization of existing organs; however, it is probably enhanced by synorganization of other, existing, floral parts. In contrast to synorganized parts, the corona appears developmentally late., Conclusions: Synorganization of floral organs may lead to a large number of convergences in clades that are only very distantly related. The convergences that have been highlighted in this comparative study should be developmentally investigated directly in parallel in future studies., (© The Author 2015. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oup.com.)

Background and Aims: Icacinaceae sensu stricto consist of a group of early branching lineages of lamiids whose relationships are not yet resolved and whose detailed floral morphology is poorly known. The most bizarre flowers occur in Emmotum: the gynoecium has three locules on one side and none on the other. It has been interpreted as consisting of three fertile and two sterile carpels or of one fertile carpel with two longitudinal septa and two sterile carpels. This study focused primarily on the outer and inner morphology of the gynoecium to resolve its disputed structure, and ovule structure was also studied. In addition, the perianth and androecium were investigated., Methods: Flowers and floral buds of two Emmotum species, E. harleyi and E. nitens, were collected and fixed in the field, and then studied by scanning electron microscopy. Microtome section series were used to reconstruct their morphology., Key Results: The gynoecium in Emmotum was confirmed as pentamerous, consisting of three fertile and two sterile carpels. Each of the three locules behaves as the single locule in other Icacinaceae, with the placenta of the two ovules being identical, which shows that three fertile carpels are present. In addition, it was found that the ovules are bitegmic, which is almost unique in lamiids, and that the stamens have monosporangiate thecae, which also occurs in the closely related family Oncothecaceae, but is not known from any other Icacinaceae sensu lato so far., Conclusions: The flowers of Emmotum have unique characters at different evolutionary levels: the pseudotrimerous gynoecium at angiosperm level, the bitegmic ovules at lamiid level and the monosporangiate thecae at family or family group level. However, in general, the floral morphology of Emmotum fits well in Icacinaceae. More comparative research on flower structure is necessary in Icacinaceae and other early branching lineages of lamiids to better understand the initial evolution of this large lineage of asterids., (© The Author 2014. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oup.com.)

Background and Aims: Species in the holoparasitic plant family Rafflesiaceae exhibit one of the most highly modified vegetative bodies in flowering plants. Apart from the flower shoot and associated bracts, the parasite is a mycelium-like endophyte living inside their grapevine hosts. This study provides a comprehensive treatment of the endophytic vegetative body for all three genera of Rafflesiaceae (Rafflesia, Rhizanthes and Sapria), and reports on the cytology and development of the endophyte, including its structural connection to the host, shedding light on the poorly understood nature of this symbiosis., Methods: Serial sectioning and staining with non-specific dyes, periodic-Schiff's reagent and aniline blue were employed in order to characterize the structure of the endophyte across a phylogenetically diverse sampling., Key Results: A previously identified difference in the nuclear size between Rafflesiaceae endophytes and their hosts was used to investigate the morphology and development of the endophytic body. The endophytes generally comprise uniseriate filaments oriented radially within the host root. The emergence of the parasite from the host during floral development is arrested in some cases by an apparent host response, but otherwise vegetative growth does not appear to elicit suppression by the host., Conclusions: Rafflesiaceae produce greatly reduced and modified vegetative bodies even when compared with the other holoparasitic angiosperms once grouped with Rafflesiaceae, which possess some vegetative differentiation. Based on previous studies of seeds together with these findings, it is concluded that the endophyte probably develops directly from a proembryo, and not from an embryo proper. Similarly, the flowering shoot arises directly from the undifferentiated endophyte. These filaments produce a protocorm in which a shoot apex originates endogenously by formation of a secondary morphological surface. This degree of modification to the vegetative body is exceptional within angiosperms and warrants additional investigation. Furthermore, the study highlights a mechanical isolation mechanism by which the host may defend itself from the parasite., (© The Author 2014. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oup.com.)

Premise of the Study: The holoparasitic plant family Rafflesiaceae include the world's largest flowers. Despite their iconic status, relatively little is known about the morphology and development of their flowers. A recent study clarified the organization of the outer (sterile) floral organs, surprisingly revealing that their distinctive floral chambers arose via different developmental pathways in the two major genera of the family. Here, we expand that research to investigate the structure and development of the reproductive organs of Rafflesiaceae., Methods: Serial sectioning, scanning electron microscopy, and x-ray tomography of floral buds were employed to reconstruct the structure and development of all three Rafflesiaceae genera., Key Results: Unlike most angiosperms, which form their shoot apex from the primary morphological surface, the shoot apex of Rafflesiaceae instead forms secondarily via internal cell separation (schizogeny) along the distal boundary of the host-parasite interface. Similarly, the radially directed ovarial clefts of the gynoecium forms via schizogeny within solid tissue, and no carpels are initiated from the floral apex., Conclusions: The development of the shoot apex and gynoecium of Rafflesiaceae are highly unusual. Although secondary formation of the morphological surface from the shoot apex has been documented in other plant groups, secondary derivation of the inner gynoecium surface is otherwise unknown. Both features are likely synapomorphies of Rafflesiaceae. The secondary derivation of the shoot apex may protect the developing floral shoot as it emerges from within dense host tissue. The secondary formation of the ovarial clefts may generate the extensive placental area necessary to produce hundreds of thousands of ovules.

The heuristic value of drawings in research is emphasized, based on a drawing of flowers of Eupomatia bennettii.

Rafflesiaceae, which produce the world's largest flowers, have captivated the attention of biologists for nearly two centuries. Despite their fame, however, the developmental nature of the floral organs in these giants has remained a mystery. Most members of the family have a large floral chamber defined by a diaphragm. The diaphragm encloses the reproductive organs where pollination by carrion flies occurs. In lieu of a functional genetic system to investigate floral development in these highly specialized holoparasites, we used comparative studies of structure, development, and gene-expression patterns to investigate the homology of their floral organs. Our results surprisingly demonstrate that the otherwise similar floral chambers in two Rafflesiaceae subclades, Rafflesia and Sapria, are constructed very differently. In Rafflesia, the diaphragm is derived from the petal whorl. In contrast, in Sapria it is derived from elaboration of a unique ring structure located between the perianth and the stamen whorl, which, although developed to varying degrees among the genera, appears to be a synapomorphy of the Rafflesiaceae. Thus, the characteristic features that define the floral chamber in these closely related genera are not homologous. These differences refute the prevailing hypothesis that similarities between Sapria and Rafflesia are ancestral in the family. Instead, our data indicate that Rafflesia-like and Sapria-like floral chambers represent two distinct derivations of this morphology. The developmental repatterning we identified in Rafflesia, in particular, may have provided architectural reinforcement, which permitted the explosive growth in floral diameter that has arisen secondarily within this subclade.

Background and Aims: Most genera of the neotropical Galipeinae (tribe Galipeeae, Rutoideae) exhibit several forms and degrees of fusion between the floral organs, including the union of petals into an apparently sympetalous corolla, the joining of the stamens among themselves and to the corolla, and the partial to complete connation of carpels. Though these and others floral traits are currently used in the circumscription of species in Galipeinae, few studies have shown in detail in which way (postgenital or congenital) and to what extent these fusions occur. To elucidate these anatomical conditions, a structural study of the flowers of the Galipeinae species was carried out., Methods: Flowers of six species from three genera of Galipeinae were studied in their morphology, anatomy and development with stereomicroscopy, light microscopy and scanning electron microscopy (SEM)., Key Results: The floral tube is formed by synorganization of stamens with petals in all species, and exhibits three main patterns: (1) Conchocarpus heterophyllus and C. minutiflorus have a floral tube formed by marginal coherence/adherence of petals and filaments due to interwining trichomes (postgenital connection); (2) Erythrochiton brasiliensis has a tube formed by congenital fusion of petals and filaments; and (3) Galipea jasminiflora and Conchocarpus macrophyllus have a tube formed distally with the first pattern, and proximally with the second pattern. Although floral tubes seem to be homologous within Galipeinae, this is not true at the level of the family: the floral tube of Correa (from an only distantly related clade of the family) is formed by postgenital union of the petals representing a convergent structure. The gynoecium of the studied species of Galipeinae shows a great variability in the extent of fusion of carpel flanks. Even though different structures for the mature gynoecium were found in each genus, all genera show postgenitally fused carpel apices, which is related to the formation of a compitum, as described earlier for other members of Rutaceae., Conclusions: The degree and diversity of fusions of floral organs in Galipeinae is unique within the order Sapindales. A study of the amount of diversification of Galipeinae in South America and comparison with other clades of Rutaceae would be of interest.

Background and Aims: Malpighiales are one of the largest angiosperm orders and have undergone radical systematic restructuring based on molecular phylogenetic studies. The clade has been recalcitrant to molecular phylogenetic reconstruction, but has become much more resolved at the suprafamilial level. It now contains so many newly identified clades that there is an urgent need for comparative studies to understand their structure, biology and evolution. This is especially true because the order contains a disproportionally large diversity of rain forest species and includes numerous agriculturally important plants. This study is a first broad systematic step in this endeavour. It focuses on a comparative structural overview of the flowers across all recently identified suprafamilial clades of Malpighiales, and points towards areas that desperately need attention., Methods: The phylogenetic comparative analysis of floral structure for the order is based on our previously published studies on four suprafamilial clades of Malpighiales, including also four related rosid orders (Celastrales, Crossosomatales, Cucurbitales, Oxalidales). In addition, the results are compiled from a survey of over 3000 publications on macrosystematics, floral structure and embryology across all orders of the core eudicots., Key Results: Most new suprafamilial clades within Malpighiales are well supported by floral structural features. Inner morphological structures of the gynoecium (i.e. stigmatic lobes, inner shape of the locules, placentation, presence of obturators) and ovules (i.e. structure of the nucellus, thickness of the integuments, presence of vascular bundles in the integuments, presence of an endothelium in the inner integument) appear to be especially suitable for characterizing suprafamilial clades within Malpighiales., Conclusions: Although the current phylogenetic reconstruction of Malpighiales is much improved compared with earlier versions, it is incomplete, and further focused phylogenetic and morphological studies are needed. Once all major subclades of Malpighiales are elucidated, more in-depth studies on promising structural features can be conducted. In addition, once the phylogenetic tree of Malpighiales, including closely related orders, is more fully resolved, character optimization studies will be possible to reconstruct evolution of structural and biological features within the order.

Premise of the Study: Investigations of inflorescence architecture offer insight into the evolution of an astounding array of reproductive shoot systems in the angiosperms, as well as the potential to genetically manipulate these branching patterns to improve crop yield and enhance the aesthetics of horticultural species. The diversity of inflorescences in the economically important family Oleaceae was studied from a comparative developmental point of view for the first time, based on species of seven genera (Chionanthus, Fontanesia, Fraxinus, Jasminum, Ligustrum, Olea, Syringa)., Methods: Series of developmental stages of chemically fixed inflorescences were studied with epi-illumination light microscopy., Key Results: All taxa studied have inflorescences with terminal flowers. The inflorescences are mostly panicles, but in some cases thyrsoids or compound botryoids. Phyllotaxis of the flower-subtending bracts is mostly decussate, rarely tricussate (Fraxinus) or spiral (Jasminum). Accessory flowers or accessory inflorescences, almost unknown in Oleaceae as yet, were found in two genera. In Syringa, common bract-flower primordia are formed by a delay in early bract development compared to flower development. Such a delay is also expressed by the loss of bracts in the distal part of inflorescence branches in Syringa and Chionanthus., Conclusions: Significant variation in branching pattern and phyllotaxy was observed among the studied species of Oleaceae. The suppression of bracts and formation of accessory flowers were found as special features of inflorescence ontogeny. The occurrence of accessory flowers and accessory partial inflorescences is interesting from the point of view of dense and flower-rich inflorescences in ornamental species.

The angiosperm order Malpighiales includes ~16,000 species and constitutes up to 40% of the understory tree diversity in tropical rain forests. Despite remarkable progress in angiosperm systematics during the last 20 y, relationships within Malpighiales remain poorly resolved, possibly owing to its rapid rise during the mid-Cretaceous. Using phylogenomic approaches, including analyses of 82 plastid genes from 58 species, we identified 12 additional clades in Malpighiales and substantially increased resolution along the backbone. This greatly improved phylogeny revealed a dynamic history of shifts in net diversification rates across Malpighiales, with bursts of diversification noted in the Barbados cherries (Malpighiaceae), cocas (Erythroxylaceae), and passion flowers (Passifloraceae). We found that commonly used a priori approaches for partitioning concatenated data in maximum likelihood analyses, by gene or by codon position, performed poorly relative to the use of partitions identified a posteriori using a Bayesian mixture model. We also found better branch support in trees inferred from a taxon-rich, data-sparse matrix, which deeply sampled only the phylogenetically critical placeholders, than in trees inferred from a taxon-sparse matrix with little missing data. Although this matrix has more missing data, our a posteriori partitioning strategy reduced the possibility of producing multiple distinct but equally optimal topologies and increased phylogenetic decisiveness, compared with the strategy of partitioning by gene. These approaches are likely to help improve phylogenetic resolution in other poorly resolved major clades of angiosperms and to be more broadly useful in studies across the Tree of Life.

Background and Aims: Preliminary field observations in 2001 and 2002 suggested that Kingdonia uniflora (Circaeasteraceae, Ranunculales) exhibits heterodichogamy, an unusual kind of reproductive heteromorphy, hitherto unreported in Ranunculales and known from only one other genus in basal eudicots., Methods: During several subsequent years flowers were observed in the field. Flowers were fixed in FAA and studied with microtome sections series and with the scanning electron microscope., Key Results: The flowers proved to be heterodichogamous, with protandrous and protogynous morphs, which have a 1 : 1 ratio. Both morphs equally set fruit. Each year a single flower is formed at the tip of a rhizome or more rarely two flowers. The flowers are already open when they appear at the soil surface, before they are receptive and before pollen is dispersed. In both floral morphs the styles elongate early and the stigmas are positioned above the anthers before anthesis begins. In protogynous flowers the stigmas become receptive in this position; later the styles become reflexed and then the anthers dehisce. In contrast, in protandrous flowers the stamen filaments elongate during early anthesis such that the dehiscing anthers come to lie above the (still unreceptive) stigmas; after dehiscence of all anthers in a flower the styles begin to elongate and become receptive., Conclusions: This is the first record of heterodichogamy in a representative of Ranunculales, in an herbaceous eudicot, and in a plant with uniflorous ramets. The occurrence of heterodichogamy in Kingdonia in which clonal reproduction appears to be dominant might be an adaptation to avoid mating between the ramets from a common mother individual (genet).

Background and Aims Anaxagorea is the phylogenetically basalmost genus in the large tropical Annonaceae (custard apple family) of Magnoliales, but its floral structure is unknown in many respects. The aim of this study is to analyse evolutionarily interesting floral features in comparison with other genera of the Annonaceae and the sister family Eupomatiaceae. Methods Live flowers of Anaxagorea crassipetala were examined in the field with vital staining, liquid-fixed material was studied with scanning electron microscopy, and microtome section series were studied with light microscopy. In addition, herbarium material of two other Anaxagorea species was cursorily studied with the dissecting microscope. Key Results Floral phyllotaxis in Anaxagorea is regularly whorled (with complex whorls) as in all other Annonaceae with a low or medium number of floral organs studied so far (in those with numerous stamens and carpels, phyllotaxis becoming irregular in the androecium and gynoecium). The carpels are completely plicate as in almost all other Annonaceae. In these features Anaxagorea differs sharply from the sister family Eupomatiaceae, which has spiral floral phyllotaxis and ascidiate carpels. Flat stamens and the presence of inner staminodes differ from most other Annonaceae and may be plesiomorphic in Anaxagorea. However, the inner staminodes appear to be non-secretory in most Anaxagorea species, which differs from inner staminodes in other families of Magnoliales (Eupomatiaceae, Degeneriacae, Himantandraceae), which are secretory. Conclusions Floral phyllotaxis in Anaxagorea shows that there is no signature of a basal spiral pattern in Annonaceae and that complex whorls are an apomorphy not just for a part of the family but for the family in its entirety, and irregular phyllotaxis is derived. This and the presence of completely plicate carpels in Anaxagorea makes the family homogeneous and distinguishes it from the closest relatives in Magnoliales.

Background: Ovules as developmental precursors of seeds are organs of central importance in angiosperm flowers and can be traced back in evolution to the earliest seed plants. Angiosperm ovules are diverse in their position in the ovary, nucellus thickness, number and thickness of integuments, degree and direction of curvature, and histological differentiations. There is a large body of literature on this diversity, and various views on its evolution have been proposed over the course of time. Most recently evo-devo studies have been concentrated on molecular developmental genetics in ovules of model plants., Scope: The present review provides a synthetic treatment of several aspects of the sporophytic part of ovule diversity, development and evolution, based on extensive research on the vast original literature and on experience from my own comparative studies in a broad range of angiosperm clades., Conclusions: In angiosperms the presence of an outer integument appears to be instrumental for ovule curvature, as indicated from studies on ovule diversity through the major clades of angiosperms, molecular developmental genetics in model species, abnormal ovules in a broad range of angiosperms, and comparison with gymnosperms with curved ovules. Lobation of integuments is not an atavism indicating evolution from telomes, but simply a morphogenetic constraint from the necessity of closure of the micropyle. Ovule shape is partly dependent on locule architecture, which is especially indicated by the occurrence of orthotropous ovules. Some ovule features are even more conservative than earlier assumed and thus of special interest in angiosperm macrosystematics.

Angiosperms and their flowers have greatly diversified into an overwhelming array of forms in the past 135 million years. Diversification was shaped by changes in climate and the biological environment (vegetation, interaction with other organisms) and by internal structural constraints and potentials. This review focuses on the development and structural diversity of flowers and structural constraints. It traces floral diversification in the different organs and organ complexes (perianth, androecium, gynoecium) through the major clades of extant angiosperms. The continuously improved results of molecular phylogenetics provide the framework for this endeavor, which is necessary for the understanding of the biology of the angiosperms and their flowers. Diversification appears to work with innovations and modifications of form. Many structural innovations originated in several clades and in special cases could become key innovations, which likely were hot spots of diversification. Synorganization between organs was an important process to reach new structural levels, from which new diversifications originated. Complexity of synorganization reached peaks in Orchidaceae and Apocynaceae with the independent evolution of pollinaria. Such a review throughout the major clades of angiosperms also shows how superficial and fragmentary our knowledge on floral structure in many clades is. Fresh studies and a multidisciplinary approach are needed.

Background and Aims: Synorganisation of floral organs, an important means in angiosperm flower evolution, is mostly realized by congenital or post-genital organ fusion. Intimate synorganisation of many floral organs without fusion, as present in Geranium robertianum, is poorly known and needs to be studied. Obdiplostemony, the seemingly reversed position of two stamen whorls, widely distributed in core eudicots, has been the subject of much attention, but there is confusion in the literature. Obdiplostemony occurs in Geranium and whether and how it is involved in this synorganisation is explored here., Methods: Floral development and architecture were studied with light microscopy based on microtome section series and with scanning electron microscopy., Key Results: Intimate synorganisation of floral organs is effected by the formation of five separate nectar canals for the proboscis of pollinators. Each nectar canal is formed by six adjacent organs from four organ whorls. In addition, the sepals are hooked together by the formation of longitudinal ribs and grooves, and provide a firm scaffold for the canals. Obdiplostemony provides a guide rail within each canal formed by the flanks of the antepetalous stamen filaments., Conclusions: Intimate synorganisation in flowers can be realized without any fusion, and obdiplostemony may play a role in this synorganisation.

In basal angiosperms (including ANITA grade, magnoliids, Choranthaceae, Ceratophyllaceae) almost all bisexual flowers are dichogamous (with male and female functions more or less separated in time), and nearly 100 per cent of those are protogynous (with female function before male function). Movements of floral parts and differential early abscission of stamens in the male phase are variously associated with protogyny. Evolution of synchronous dichogamy based on the day/night rhythm and anthesis lasting 2 days is common. In a few clades in Magnoliales and Laurales heterodichogamy has also evolved. Beetles, flies and thrips are the major pollinators, with various degrees of specialization up to large beetles and special flies in some large-flowered Nymphaeaceae, Magnoliaceae, Annonaceae and Aristolochiaceae. Unusual structural specializations are involved in floral biological adaptations (calyptras, inner staminodes, synandria and food bodies, and secretory structures on tepals, stamens and staminodes). Numerous specializations that are common in monocots and eudicots are absent in basal angiosperms. Several families are poorly known in their floral biology.

Morphological variation in Ephedra (Gnetales) is limited and confusing from an evolutionary perspective, with parallelisms and intraspecific variation. However, recent analyses of molecular data provide a phylogenetic framework for investigations of morphological traits, albeit with few informative characters in the investigated gene regions. We document morphological, anatomical and histological variation patterns in the female reproductive unit and test the hypothesis that some Early Cretaceous fossils, which share synapomorphies with Ephedra, are members of the extant clade. Results indicate that some morphological features are evolutionarily informative although intraspecific variation is evident. Histology and anatomy of cone bracts and seed envelopes show clade-specific variation patterns. There is little evidence for an inclusion of the Cretaceous fossils in the extant clade. Rather, a hypothesized general pattern of reduction of the vasculature in the ephedran seed envelope, probably from four vascular bundles in the fossils, to ancestrally three in the living clade, and later to two, is consistent with phylogenetic and temporal analyses, which indicate that extant diversity evolved after the Cretaceous-Tertiary boundary. Notwithstanding striking similarities between living and Cretaceous Ephedra, available data indicate that the Mesozoic diversity went almost entirely extinct in the late Cretaceous causing a bottleneck effect in Ephedra, still reflected today by an extraordinarily low level of genetic and structural diversity.

* Diplostigmaty, the presence of a primary (apical) stigma and secondary (mid-stylar) stigmas along the style, is only known from the genus Sebaea (Gentianaceae). Early work indicated that the secondary stigmas provide a mechanism of autogamy, suggesting that it might ensure reproductive assurance. * Here, we test the monophyly of this unique morphological trait. Using Bayesian methods, we infer a nuclear DNA phylogeny for 96 accessions, including c. 50% of the species from the genus Sebaea. With this phylogeny, we infer the distribution of ancestral states on critical nodes using parsimony and likelihood methods. * The inferred nrDNA phylogeny shows that the genus Sebaea is divided in two statistically well-supported clades, A and B, consistent with recent estimates. The most recent ancestor (MRCA) of clade A, except the most basal species (Sebaea pusilla), is resolved as diplostigmatic. No reversal to a single stigma is observed within this clade. * We suggest that diplostigmaty is evolutionarily stable through time. We also discuss why this reproductive system is not found elsewhere than in Gentianaceae and the potential advantage of diplostigmaty as a stable mixed mating strategy.

Increasingly robust understanding of angiosperm phylogeny allows more secure reconstruction of the flower in the most recent common ancestor of extant angiosperms and its early evolution. The surprising emergence of several extant and fossil taxa with simple flowers near the base of the angiosperms-Chloranthaceae, Ceratophyllum, Hydatellaceae, and the Early Cretaceous fossil Archaefructus (the last three are water plants)-has brought a new twist to this problem. We evaluate early floral evolution in angiosperms by parsimony optimization of morphological characters on phylogenetic trees derived from morphological and molecular data. Our analyses imply that Ceratophyllum may be related to Chloranthaceae, and Archaefructus to either Hydatellaceae or Ceratophyllum. Inferred ancestral features include more than two whorls (or series) of tepals and stamens, stamens with protruding adaxial or lateral pollen sacs, several free, ascidiate carpels closed by secretion, extended stigma, extragynoecial compitum, and one or several ventral pendent ovule(s). The ancestral state in other characters is equivocal: e.g., bisexual vs. unisexual flowers, whorled vs. spiral floral phyllotaxis, presence vs. absence of tepal differentiation, anatropous vs. orthotropous ovules. Our results indicate that the simple flowers of the newly recognized basal groups are reduced rather than primitively simple.

Background and Aims: The monogeneric Kirkiaceae (Sapindales) were formerly placed as Kirkioideae in Simaroubaceae. However, recent molecular phylogenetic studies indicate that they are not in Simaroubaceae and they appear to be sister to the clade of Anacardiaceae plus Burseraceae. Such affinity was never considered or discussed since the first description of Kirkia. The present study is the first detailed analysis of the floral structure of a representative of Kirkiaceae and the first comparison with other sapindalean families, especially Anacardiaceae and Burseraceae., Methods: Floral structure of Kirkia wilmsii was studied using transversal and longitudinal microtome section series, scanning electron microscopy and light microscopy., Key Results: The flowers of Kirkia wilmsii are morphologically bisexual but functionally unisexual. They are polysymmetric, isomerous (tetramerous) and haplostemonous. The ovary is syncarpous and entirely synascidiate. The floral apex forms a hemispherical protrusion on top of the ovary. The styles are free but postgenitally united and apically form a stigmatic head with a compitum. Each carpel is uniovulate (biovulate in a few other species) and ovules are crassinucellar, bitegmic and slightly campylotropous. The micropyle is formed by both integuments and is unusually long. The unusual two radially disposed locules in each carpel in the former genus Pleiokirkia can be explained developmentally by the two offset and tightly contiguous lateral placentae., Conclusions: Paralleling the molecular results, a suite of floral features supports the position of Kirkiaceae close to the Anacardiaceae-Burseraceae clade, and not in Simaroubaceae.

Flowers exhibit tremendous variation in size (>1000-fold), ranging from less than a millimeter to nearly a meter in diameter. Numerous studies have established the importance of increased floral size in species that exhibit relatively normal-sized flowers, but few studies have examined the evolution of floral size increase in species with extremely large flowers or flower-like inflorescences (collectively termed blossoms). Our review of these record-breakers indicates that blossom gigantism has evolved multiple times, and suggests that the evolutionary forces operating in these species may differ from their ordinary-sized counterparts. Surprisingly, rather than being associated with large-bodied pollinators, gigantism appears to be most common in species with small-bodied beetle or carrion-fly pollinators. Such large blossoms may be adapted to these pollinators because they help to temporarily trap animals, better facilitate thermal regulation, and allow for the mimicry of large animal carcasses. Future phylogenetic tests of these hypotheses should be conducted to determine if the transition to such pollination systems correlates with significant changes in the mode and tempo of blossom size evolution.

The buzz-pollinated genus Senna (Leguminosae) is outstanding for including species with monosymmetric flowers and species with diverse asymmetric, enantiomorphic (enantiostylous) flowers. To recognize patterns of homology, we dissected the floral symmetry character complex and explored corolla morphology in 60 Senna species and studied floral development of four enantiomorphic species. The asymmetry morph of a flower is correlated with the direction of spiral calyx aestivation. We recognized five patterns of floral asymmetry, resulting from different combinations of six structural elements: deflection of the carpel, deflection of the median abaxial stamen, deflection or modification in size of one lateral abaxial stamen, and modification in shape and size of one or both lower petals. Prominent corolla asymmetry begins in the earl-stage bud (unequal development of lower petals). Androecium asymmetry begins either in the midstage bud (unequal development of thecae in median abaxial stamen; twisting of androecium) or at anthesis (stamen deflection). Gynoecium asymmetry begins in early bud (primordium off the median plane, ventral slit laterally oriented) or midstage to late bud (carpel deflection). In enantiostylous flowers, pronouncedly concave and robust petals of both monosymmetric and asymmetric corollas likely function to ricochet and direct pollen flow during buzz pollination. Occurrence of particular combinations of structural elements of floral symmetry in the subclades is shown.

Background and Aims: Based on molecular phylogenetic studies, the unigeneric family Eupteleaceae has a prominent phylogenetic position at or near the base of Ranunculales, which, in turn, appear at the base of eudicots. The aim of the present paper is to reveal developmental features of the flowers and to put the genus in a morphological context with other basal eudicots., Methods: Flowers in all developmental stages of Euptelea pleiosperma were collected in the wild at intervals of 7-10 d in the critical stages and studied with a scanning electron microscope., Key Results: Remnants of a perianth are lacking throughout flower development. Floral symmetry changes from monosymmetric to asymmetric to disymmetric during development. Asymmetry is expressed in that the sequence of stamen initiation is from the centre to both lateral sides on the adaxial side of the flower but starting from one lateral side and proceeding to the other on the abaxial side. Despite the pronounced floral disymmetry, a dimerous pattern of floral organs was not found. The carpel primordia arise between the already large stamens and alternate with them. Stamens and carpels each form a somewhat irregular whorl. The carpels are ascidiate from the beginning. The stigma differentiates as two crests along the ventral slit of the ovary. The few lateral ovules alternate with each other., Conclusions: Although the flowers have some unusual autapomorphies (wind pollination, lack of a perianth, pronounced disymmetry of the floral base, long connective protrusion, long temporal gap between androecium and gynoecium initiation, small space for carpel initiation), they show some plesiomorphies at the level of basal eudicots (free carpels, basifixed anthers, whorled phyllotaxis), and thus fit well in Ranunculales.

Although the relationship of angiosperms to other seed plants remains controversial, great progress has been made in identifying the earliest extant splits in flowering-plant phylogeny, with the discovery that the New Caledonian shrub Amborella trichopoda, the water lilies (Nymphaeales), and the woody Austrobaileyales constitute a basal grade of lines that diverged before the main radiation in the clade. By focusing attention on these ancient lines, this finding has re-written our understanding of angiosperm structural and reproductive biology, physiology, ecology and taxonomy. The discovery of a new basal lineage would lead to further re-evaluation of the initial angiosperm radiation, but would also be unexpected, as nearly all of the approximately 460 flowering-plant families have been surveyed in molecular studies. Here we show that Hydatellaceae, a small family of dwarf aquatics that were formerly interpreted as monocots, are instead a highly modified and previously unrecognized ancient lineage of angiosperms. Molecular phylogenetic analyses of multiple plastid genes and associated noncoding regions from the two genera of Hydatellaceae identify this overlooked family as the sister group of Nymphaeales. This surprising result is further corroborated by evidence from the nuclear gene phytochrome C (PHYC), and by numerous morphological characters. This indicates that water lilies are part of a larger lineage that evolved more extreme and diverse modifications for life in an aquatic habitat than previously recognized.

To date, molecular developmental studies have focused on vegetative rather than floral phyllotaxis because vegetative shoot apices are technically more tractable than floral apices in model plants. In contrast to evolutionary changes in the phyllotaxis of vegetative shoots, however, changes in floral phyllotaxis appear to have played a major role in angiosperm evolution. Consolidation of a whorled floral phyllotaxis in derived groups allowed synorganization of floral organs and further adaptive radiations. In basal angiosperms, floral phyllotaxis is more flexible. To study these phenomena, we need clarification of the complex relations of both spiral and whorled phyllotaxis with divergence angles, plastochrons, spiral versus simultaneous initiation of organs, parastichies, orthostichies, organ series, and whorls. Improved resolution of phylogenetic relationships and increased knowledge of the diversity of floral phyllotaxis will allow us to trace evolutionary changes in floral phyllotaxis in ever more detail. Already, such surveys have confirmed that floral phyllotaxis was unusually labile early in angiosperm evolution. Whether the original floral phyllotaxis in angiosperms was spiral or whorled is equivocal, but it appears that spiral floral phyllotaxis in Magnoliales and Laurales is derived rather than primitive.

Senna (Leguminosae) is a large, widespread genus that includes species with enantiostylous, asymmetric flowers and species with extrafloral nectaries. Clarification of phylogenetic relationships within Senna based on parsimony analyses of three chloroplast regions (rpS16, rpL16, and matK) provides new insights on the evolution of floral symmetry and extrafloral nectaries. Our results support the monophyly of only one (Psilorhegma) of the six currently recognized sections, while Chamaefistula, Peiranisia, and Senna are paraphyletic, and monotypic Astroites and Paradictyon are nested within two of the seven major clades identified by our molecular phylogeny. Two clades (I, VII) include only species with monosymmetric flowers, while the remaining clades (II-VI) contain species with asymmetric, enantiostylous flowers, in which either the gynoecium alone or, in addition, corolla and androecium variously contribute to the asymmetry. Our results further suggest that flowers were ancestrally monosymmetric with seven fertile stamens and three adaxial staminodes, switched to asymmetry later, and reverted to monosymmetry in clade VII. Fertility of all 10 stamens is a derived state, characterizing the Psilorhegma subclade. Extrafloral nectaries evolved once and constitute a synapomorphy for clades IV-VII ("EFN clade"). These nectaries may represent a key innovation in plant defense strategies that enabled Senna to undergo large-scale diversification.

Background and Aims: The morphological structure of anthetic carpels of Brasenia (Cabombaceae), a member of the phylogenetically basal ANITA grade, has not been studied before. The carpel has a long stigmatic crest on the ventral side and could give the impression of a conduplicate structure. This is in contrast to the carpel structure in other genera of the ANITA grade. Therefore, a study of carpel development and carpel structure at anthesis was carried out., Methods: Carpels of Brasenia schreberi were studied at different developmental stages up to anthesis by means of microtome section series and SEM to analyse and reconstruct the outer and inner carpel morphology., Key Results: Carpels of Brasenia are extremely ascidiate up to anthesis. The elongate stigma originates around the mouth of the young carpel, which is slightly curved toward the centre of the flower. Subsequently, the stigmatic zone below the mouth expands by massive intercalary elongation., Conclusions: In their ascidiate shape, carpels of Brasenia are similar to carpels of Cabomba, the other genus of Cabombaceae, which, in contrast, has a short stigma restricted to the tip of the carpel. Thus, the morphological structure is independent of the extent (and one-sidedness) of the stigma. The outer shape of carpels at anthesis does not allow the inference of the inner morphological surface. If an angiosperm carpel has a one-sided stigma it can be extremely conduplicate or extremely ascidiate. Therefore, caution has to be used in the interpretation of the structure of fossil carpels.

The Balanopaceae, whose flowers were poorly known, have, in the past, been variously allocated to the Fagales, Euphorbiaceae, Salicales or other hamamelids and rosids (these groups being in Fagales, Malpighiales and Saxifragales, according to the Angiosperm Phylogeny Group). This paper attempts a clarification based on flower morphology. Female flowers and cupules were studied in Balanops vieillardii, young fruits in B. australiana. The cupules are simple involucres of bracts which are spirally arranged (according to a Fibonacci pattern) on the floral axis preceding the flower. They contrast with the complicated cupules of Fagaceae which consist of a condensed cymose ramification system of axes of several orders around the flower. Flowers appear later than most of the cupular bracts, in contrast to Fagaceae. In addition to a terminal flower there may be several smaller lateral flowers in the axil of cupular bracts, each surrounded by its own small cupule. The female flowers do not have a perianth. They consist of two to three large carpels. At anthesis, the ovary is completely septate; the syncarpous part (ovary and lower style) is completely symplicate. The carpels are free for most of their length, with the free parts once, twice or three times bifurcate, in contrast to simple in Fagales. The stigmatic surface covers the ventral side of each stigmatic branch and at the margins also spreads to the dorsal side. The stigma is wet and secretion appears holocrine. The two ovules per carpel are collateral and axile in early development. However, at anthesis they appear one above the other, because in one ovule the funicle greatly elongates. As the ovary elongates only above the placenta, the ovules appear basal at anthesis. The ovules are (weakly) crassinucellar, bitegmic (not unitegmic), anatropous, and intermediate between apotropous and epitropous (not apotropous). The ovules are mature at anthesis, in contrast to Fagales. In mature ovules the upper part of the nucellus disintegrates, and a weakly differentiated endothelium is present in the inner integument. The morphological results of this study support a position of Balanopaceae in Malpighiales, and not Fagales or other orders, and are thus in accordance with recent molecular results based on chloroplast rbcL sequences data. However, within Malpighiales, as opposed to molecular results, Balanopaceae agree more with Euphorbiaceae s.l. than with Dichapetalaceae/Trigoniaceae and Chrysobalanaceae/Euphroniaceae.

With molecular analyses indicating that angiosperms are not closely related to any other extant seed plant group, information from fossils might provide the only basis for reconstructing their origin. Therefore the description of a new Early Cretaceous angiosperm, Archaefructus, placed as the sister of all extant angiosperms, has created much debate and optimism. However, we question both the interpretation and the analysis of Archaefructus, concluding that it might be a crown-group angiosperm specialized for aquatic habit rather than a more primitive relative.

Phylogenetic relationships among many lineages of angiosperms have been clarified via the analysis of large molecular data sets. However, with a data set of three genes (18S rDNA, rbcL, and atpB), relationships among lineages of core eudicots (Berberidopsidales, Caryophyllales, Gunnerales, Santalales, Saxifragales, asterids, rosids) remain essentially unresolved. We added 26S rDNA sequences to a three-gene matrix for 201 eudicots (8430 base pair aligned nucleotides per taxon). Parsimony analyses provided moderate (84%) jackknife support for Gunnerales, which comprise the two enigmatic families Gunneraceae and Myrothamnaceae, as sister to all other core eudicots. This position of Gunnerales has important implications for floral evolution. A dimerous or trimerous perianth is frequently encountered in early-diverging eudicots (e.g., Buxaceae, Proteales, Ranunculales, Trochodendraceae), whereas in core eudicots, pentamery predominates. Significantly, dimery is found in Gunneraceae and perhaps Myrothamnaceae (the merosity of the latter has also been interpreted as labile). Parsimony reconstructions of perianth merosity demonstrate lability among early-diverging eudicots and further indicate that a dimerous perianth could be the immediate precursor to the pentamerous condition characteristic of core eudicots. Thus, the developmental canalization that yielded the pentamerous condition of core eudicots occurred after the node leading to Gunnerales.

Morphology and development of the female flowers in Geonoma interrupta are described and compared with other taxa within Arecaceae. Inflorescences are pleiothyrses. Cincinni are immersed in pits and arranged according to the Fibonacci pattern along the rachillae. The gynoecium is composed of three free carpels in early stages and later becomes pseudomonomerous. Two carpels are sterile; they develop to different degrees and are commonly unequal in size. The fertile carpel contains a single, crassinucellate, anatropous ovule. Styles are formed in each carpel. The style of the fertile carpel becomes basifixed as the ovary enlarges. The stigmas remain free and plicate during development and expose unicellular papillae at anthesis. Pollen tube transmitting tracts and a compitum are present in the ventral slits of the stigmas and the postgenitally united styles during anthesis. A septal nectary is formed by incomplete union of the flanks of the carpels at the base of the gynoecium, and nectar is secreted from an epithelium. It is suggested that in Geonoma as a whole, the attraction of pollinators to female flowers is due to a combination of nectar reward and partial mimicry of male flowers.

Flowers evolved in many steps, probably starting long before flowering plants (angiophytes) originated. Certain parts of flowers are conservative and have not changed much during evolution; others are evolutionarily highly plastic. Here conservative features are discussed and an attempt is made to trace them back through their evolutionary history. Microsporangia and ovules (which develop into seeds) are preangiophyte floral elements. Angiospermy, combined with postgenital fusion, was the most prominent key innovation in angiophytes. Angiospermy and thecal organization of stamens originated earlier than all clades of extant angiosperms (the crown group of angiophytes). Differentiation of a perianth into calyx and corolla and syncarpy appeared after the first branching of the basalmost clades of extant angiosperms. Sympetaly and floral tubes as well as tenuinucellar, unitegmic ovules originated as major innovations in the clade that led to asterids. An obvious trend in flower evolution is increased synorganisation of parts, which led to new structures. Fixation of floral organ number and position was a precondition for synorganization. Concomitantly, plasticity changed from number and position of organs to shape of the new structures. Character distribution mapped onto cladograms indicates that key innovations do not appear suddenly, but start with trials and only later become deeply rooted genetically in the organization. This is implied from the common occurrence of reversals in the early history of an innovation. J. Exp. Zool. (Mol. Dev. Evol.) 291:105-115, 2001., (Copyright 2001 Wiley-Liss, Inc.)

Floral monosymmetry, which is conspicuous and prominent in many angiosperms, has attracted much attention from both developmental geneticists and pollination biologists. A combined evolutionary biological approach to studying floral monosymmetry in the Lamiales, the order that contains the model plant Antirrhinum, is just beginning to take shape. In contrast, floral left-right asymmetry has largely been neglected, although it is much in discussion in animal biology, probably because in flowers (unlike in animals) left-right asymmetry is not predominant. Nevertheless, there are patterns in the evolution of floral left-right asymmetry that are interesting enough to be addressed by developmental genetics. These are the direction of contortion in flowers with contort petal aestivation, and the direction of deflection of pollination organs in groups with enantiostylous flowers or in some groups with enclosed pollination organs, such as beans (Phaseolinae, Fabaceae) or louseworts (Pedicularis, Orobanchaceae).

Flower development and anatomy of Acorus calamus and flower anatomy of A. gramineus were studied. Findings were compared with published reports on paleoherbs. Important developmental features include an abaxially median tepal that is initiated first and is similar to a flower-subtending bract and unidirectional flower development with an inversion of organ initiation sequence in the second tepal whorl. The mature gynoecium is largely synascidiate, but early development of carpels is plicate, and the apocarpous portion persists up to anthesis. The carpels form dorsal bulges on the style, enclosing longitudinal intercarpellary slits. The dominance of the synascidiate portion and the apical position of the placenta result from a late and distinct basal elongation of the gynoecium. Stigma, pollen transmitting tract, and ovary are filled with secretion. Secretory papillae are present from the stigma to the placenta; papillae also occur on the rims of the integuments of the ovules. In the uppermost part of the inflorescence, the adaxial floral sectors are reduced in number and structure, and at the apex of the inflorescence, a peloria-like structure is formed. Developmental and morphological similarities seem to be closer between Acorus and Piperales than between Acorus and other magnoliids.

This article traces research on floral symmetry back to its beginnings. It brings together recent advances from different fields that converge in floral symmetry and new unpublished material on diversity and development of floral symmetry. During floral development, symmetry may change: monosymmetric flowers may have a polysymmetric early phase; polysymmetric flowers may have a monosymmetric or even asymmetric early phase; more than one symmetry change is also possible. In Lamiales s.l. (comprising the model plant Antirrhinum, where the cycloidea gene produces monosymmetric flowers with the adaxial side of the androecium reduced), taxa also occur in which the androecium is reduced on both sides, adaxial and abaxial. As a trend in asymmetric flowers, enantiomorphy (with two mirror-image morphs) at the level of individuals seems to occur only in groups in which the flowers are predominantly of a relatively simple construction. In contrast, one morph is fixed at the level of species or higher taxa in groups with more complicated flowers. This is indicated by the apparent lack of enantiomorphy in corolla contortion in asterids but its predominance in rosids with contort flowers, or by the apparent lack of enantiomorphy in the pollination organs of asymmetric flowers in Faboideae but its presence in asymmetric flowers in Caesalpinioideae. To study the evolution of the diverse symmetry patterns, a concerted approach from different fields including molecular developmental genetics, pollination biology, and comparative diversity research is necessary.

Lamiales s.l., a prominent clade in the Asteridae, commonly have pentamerous monosymmetric flowers with the upper (odd) stamen reduced or lacking. Of the five largest families of the Lamiales s.l. (Gesneriaceae, Scrophulariaceae, Bignoniaceae, Acanthaceae, and Verbenaceae/Lamiaceae), perhaps the most phylogenetically basal, the Gesneriaceae, is the only one with odd staminodes or stamens occurring in all genera. In addition, Gesneriaceae have relatively large odd staminodes often with a differentiation in filament and reduced anther. They also have the largest proportion of genera (approx. 8%) with more or less polysymmetric flowers with five fertile stamens. In the more advanced families of the Lamiales s.l. the pattern varies. In Bignoniaceae an odd staminode is also commonly present. The traditional Scrophulariaceae is the most diverse family with some tribes constantly having an odd staminode (e.g. Antirrhineae) and others missing it (e.g. Manuleae, Pedicularieae), reflecting its probable polyphyly. In Acanthaceae, and still more in Verbenaceae/Lamiaceae with their extremely monosymmetric flowers an odd staminode is more often missing than present. From the systematic distribution of the different floral forms it is most likely that monosymmetric flowers were already present at the base of the Lamiales s.l. but 'reversal' to polysymmetry was still easy in the basal groups with only weak expression of monosymmetry. The evolution of more pronounced monosymmetry proceeded in different lineages and loss of the odd stamen occurred in various clades. The developmentally most intriguing groups are those with loss of the perianth and reduction of floral organ numbers to two or one, Callitriche being the most extreme genus.

An unexpected variety of new fossil flowers from the Lower and mid-Cretaceous and new results on the structure, development and biology of the flowers of extant primitive angiosperms are leading to modifications of earlier concepts of early flower evolution. Most fossil flowers conform best to those of the angiosperm subclass Magnoliidae, diverse though they may be. The unusual variety in organ number and organ arrangement patterns is a characteristic not only of the fossils but also of the extant Magnoliidae. It is a feature of the still 'open' organization of the flower (without intricate synorganization of parts) at this evolutionary level, and not an expression of only distant phylogenetic relationship. On the other hand, many other predominant features of modern angiosperms are lacking in both earliest fossils and most extant Magnoliidae., (Copyright © 1987. Published by Elsevier Ltd.)