Partial Inflorescences Research Articles

Correlation between Inflorescence Architecture and Floral Asymmetry-Evidence from Aberrant Flowers in Canna L. (Cannaceae).

Floral symmetry studies often focus on the development of monosymmetric and polysymmetric flowers, whereas asymmetric flowers and their position and function within the inflorescence structure are largely neglected. Cannaceae is one of the few families that possesses truly asymmetric flowers, serving as a model to study the characters and mechanisms involved in the development of floral asymmetry and its context within the developing and mature inflorescence. In this study, inflorescence structure and floral morphology of normal asymmetric flowers and 16 aberrant flower collections from Canna indica L. and C. glauca L. were photographed, analyzed, and compared with attention to stamen petaloidy, floral symmetry, and inflorescence branching patterns anterior and posterior to the aberrant flower. In comparison with normal flowers, the aberrant flowers are arranged into abnormal partial florescences, and vary in floral symmetry, orientation, and degree of androecial petaloidy. The appendage of the fertile stamen is universally located distal from the higher order bract, indicating an underlying influence of inflorescence architecture. A synthetic model is proposed to explain the relationship between floral symmetry and inflorescence structure. Data from the observation of aberrant phenotypes strongly support the hypothesis that irregular petaloidy of the stamens is correlated with an asymmetric morphogenetic field within the inflorescence that contributes to the overall floral asymmetry in Canna flowers.

Diagnostic traits of medicinal herbal raw material of species of Thymus genus

The paper presents the peculiarities of morphological-anatomical structure of the organs, which are diagnostic traits of herbal raw materials (HRM) of the commonest species of Thymus L. genus in the moderate climatic zone. We determined that the studied species develop a subshrub life form, which grows monopodially in Th. pulegioides and Th. marschallianus, and sympodially with mono-, di- and polycyclic types of monocarpic shoots in Th. serpyllum. Inflorescence is of thyrsoid type with opposite partial inflorescences, which are constituents of simple dichasia with monochasia. Macroscopic and microscopic diagnostic features of the HRM are structure and pubescence on the stem, leaves and flowers, the anatomical structure of which was studied on 20 µm thick transversal sections using a light microscope. In Th. pulegioides, stems are tetrahedral, pubescent on ribs with unicellular and two-cellular elbow-shaped hairs. Leaves are ovoid, covered by multiangular epidermis that bears diacytic stromas, sharpened capitate unicellular and two-cellular hairs and 10–12 cellular essential oil glandules. In Th. marschallianus, the stems are poorly tetrahedral, rounded, pubescent on the entire surface. Leaves are elliptic, the cells of the epidermis are elongated, significantly tortuous, with 10–12 cellular glandules, diacytic stromas, capitate and unicellular hairs. In Th. serpyllum, the stems are cylindric, pubescent all round. The leaves are elongated-elliptic, epidermis cells are oval, tortuous, stromas are diacytic, essential oil glandules are 8–10 cellular, number of papilla-like hairs is low. The petioles and lamina base of the studied species are pubescent on the sides with short, coned and elbow-shaped, 2–3 cellular hairs, among which there are long, 4–8 cellular hairs; the hairs in Th. serpyllum are multicellular, interrupted. In mesophyll, there are schizogenous essential oil reservoirs. In Th. marschallianus and Th. pulegioídes, the calyx is campanulate, bilabiate, and pubescent. The flower corolla is purple-violet, bilabiate, has tube and outward bend, pubescent inside with coned unicellular hairs, has essential oil glandules and glandular capitate hairs, and pubescence in the fauces comprises long unicellular hairs. In Th. pulegioídes, cells of the corolla epidermis are multiangular-shaped with insignificantly wavy walls, in Th. marschallianus ‒ rectangular with significantly wavy walls. Cells of the tube epidermis are rectangular, tortuous. Surface of the corolla of Th. serpyllum is pubescent on both sides with short, cone-like and three-cellular, interrupted hairs, and also capitate hairs, and has essential oil glandules. Epidermis cells are multi-angular, tubes – rectangular and narrow. The obtained results may be used for diagnostics of HRM of morphologically similar species during their preparation.

Flower and Spikelet Construction in Rapateaceae (Poales).

The family Rapateaceae represents an early-divergent lineage of Poales with biotically pollinated showy flowers. We investigate developmental morphology and anatomy in all three subfamilies and five tribes of Rapateaceae to distinguish between contrasting hypotheses on spikelet morphology and to address questions on the presence of nectaries and gynoecium structure. We support an interpretation of the partial inflorescence (commonly termed spikelet), as a uniaxial system composed of a terminal flower and numerous empty phyllomes. A terminal flower in an inflorescence unit is an autapomorphic feature of Rapateaceae. The gynoecium consists of synascidiate, symplicate, and usually asymplicate zones, with gynoecium formation encompassing congenital and often also postgenital fusions between carpels. Species of Rapateaceae differ in the relative lengths of the gynoecial zones, the presence or absence of postgenital fusion between the carpels and placentation in the ascidiate or plicate carpel zones. In contrast with previous reports, septal nectaries are lacking in all species. The bird-pollinated tribe Schoenocephalieae is characterized by congenital syncarpy; it displays an unusual type of gynoecial (non-septal) nectary represented by a secretory epidermis at the gynoecium base.

Spicoid morphology of Mapanioideae (Cyperaceae): an evolutionary perspective

Abstract Cyperaceae subfamily Mapanioideae have reproductive units called spicoids, which are interpreted alternatively as flowers or partial inflorescence, and little is known about morphological variation of spicoids from an evolutionary perspective. Combining morphological and evolutionary analyses, we aimed to assess variation in spicoid morphology and its evolutionary significance. Spicoid morphology of 48 species of Mapanioideae was analysed, and variable characters were selected for downstream analyses. The results showed that the general spicoid morphology is consistent within genera and that character states can vary within genera, supporting molecular phylogenetic relationships. The presence of lateral keeled scales, the number of lateral stamens, inner scales, inner stamens and pistil merosity were shown to be variable. Possible ancestral spicoid morphology was reconstructed as resembling the current morphology of Hypolytrum. Based on the most frequent evolutionary changes observed, the increase in the number of parts (number of inner scales, inner stamens and pistil merosity) seems to be a recurrent process in spicoid evolution, contradicting a previous hypothesis that assumed changes in spicoid morphology towards reduction. Combining molecular and morphological analyses allows us to refine our knowledge of the evolutionary history of Mapanioideae and enhances the understanding of the evolution of Cyperaceae.

Why pendulum symmetry is absent from the cymose partial inflorescences of Cannaceae? Insights into the essential characteristic of cincinni.

In a typical cincinnus, the neighboring two flowers are generally enantiomorphic, which leads to the pendulum symmetry of the entire cyme. While in a two-flowered Cannaceae cincinnus, the flowers develop the same chirality. In this study, we observed several abnormal cincinni of Canna indica that extended longer than their normal form, which presented a second enantiomorphic flower, thus reflecting a typical pendulum symmetry. The chirality change of the second flower was strongly associated with the position of the lateral cincinnus meristem, which determines the angle size of the cincinnus zigzag shift and may serve as a key factor controlling the formation of pendulum symmetry. We propose that alternating floral chirality and the concomitant pendulum symmetry are the essential characteristics of a typical cincinnus. Accordingly, Canna flowers with the same chirality are arranged in modified cincinni.

Expression and Function Studies of CYC/TB1-Like Genes in the Asymmetric Flower Canna (Cannaceae, Zingiberales).

The asymmetric flower, lacking any plane of symmetry, is rare among angiosperms. Canna indica L. has conspicuously asymmetric flowers resulting from the presence of a half-fertile stamen, while the other androecial members develop as petaloid staminodes or abort early during development. The molecular basis of the asymmetric distribution of fertility and petaloidy in the androecial whorls remains unknown. Ontogenetic studies have shown that Canna flowers are borne on monochasial (cincinnus) partial florescences within a racemose inflorescence, with floral asymmetry likely corresponding to the inflorescence architecture. Given the hypothesized role of CYC/TB1 genes in establishing floral symmetry in response to the influence of the underlying inflorescence architecture, the spatiotemporal expression patterns of three Canna CYC/TB1 homologs (CiTBL1a, CiTBL1b-1, and CiTBL1b-2) were analyzed during inflorescence and floral development using RNA in situ hybridization and qRT-PCR. In the young inflorescence, both CiTBL1a and CiTBL1b-1 were found to be expressed in the bracts and at the base of the lateral florescence branches, whereas transcripts of CiTBL1b-2 were mainly detected in flower primordia and inflorescence primordia. During early flower development, expression of CiTBL1a and CiTBL1b-1 were both restricted to the developing sepals and petals. In later flower development, expression of CiTBL1a was reduced to a very low level while CiTBL1b-1 was detected with extremely high expression levels in the petaloid androecial structures including the petaloid staminodes, the labellum, and the petaloid appendage of the fertile stamen. In contrast, expression of CiTBL1b-2 was strongest in the fertile stamen throughout flower development, from early initiation of the stamen primordium to maturity of the ½ anther. Heterologous overexpression of CiTBL genes in Arabidopsis led to dwarf plants with smaller petals and fewer stamens, and altered the symmetry of mature flowers. These data provide evidence for the involvement of CYC/TB1 homologs in the development of the asymmetric Cannaceae flower.

Inflorescence evolution in Santalales: integrating morphological characters and molecular phylogenetics.

The sandalwood order (Santalales) includes members that present a diverse array of inflorescence types, some of which are unique among angiosperms. This diversity presents not only interpretational challenges but also opportunities to test fundamental concepts in plant morphology. Here we used modern phylogenetic approaches to address the evolution of inflorescences in the sandalwood order. Phylogenetic analyses of two nuclear and three chloroplast genes were conducted on representatives of 146 of the 163 genera in the order. A matrix was constructed that scored nine characters dealing with inflorescences. One character, "trios", that encompasses any grouping of three flowers (i.e., both dichasia and triads) was optimized on samples of the posterior distribution of trees from the Bayesian analysis using BayesTraits. Three nodes were examined: the most recent common ancestors of (A) all ingroup members, (B) Loranthaceae, and (C) Opiliaceae, Santalaceae s.l., and Viscaceae. The phylogenetic analysis resulted in many fully resolved nodes across Santalales with strong support for 18 clades previously named as families. The trios character was not supported for nodes A and C, whereas it was supported for node B where this partial inflorescence type is best described as a triad. Essentially every major inflorescence type can be found in Santalales; however, the dichasium, a type of partial inflorescence, is rarely seen and is not plesiomorphic for the order. In the family Erythropalaceae, inflorescences are mostly in small, axillary fascicles or cymes. Successive families show both cymose and racemose types and compound systems (e.g., thyrses). Inflorescences in Amphorogynaceae and Viscaceae are not dichasial and in general are difficult to compare to "standard" inflorescences.

Synchrotron X-ray imaging of a dichasium cupule of Castanopsis from Eocene Baltic amber.

The Eocene Baltic amber deposit represents the largest accumulation of fossil resin worldwide, and hundreds of thousands of entrapped arthropods have been recovered. Although Baltic amber preserves delicate plant structures in high fidelity, angiosperms of the "Baltic amber forest" remain poorly studied. We describe a pistillate partial inflorescence of Castanopsis (Fagaceae), expanding the knowledge of Fagaceae diversity from Baltic amber. The amber specimen was investigated using light microscopy and synchrotron-radiation-based X-ray micro-computed tomography (SRμCT). The partial inflorescence is a cymule, consisting of an involucre of scales that surround all four pistillate flowers, indicating a dichasium cupule. Subtending bracts are basally covered with peltate trichomes. Flowers possess an urecolate perianth of six nearly free lobes, 12 staminodia hidden by the perianth, and a tri-locular ovary that is convex-triangular in cross section. The exceptional three-dimensional preservation suggests that the fossil belongs to the extant East Asian genus Castanopsis. The amber inclusion represents the first record of Castanopsis from Baltic amber and the first pistillate inflorescence of Fagaceae from Eurasia. The partial female inflorescence reported here provides an important addition to acorns of Castanopsis described from middle Eocene strata of Europe. Furthermore, the intercontinental distribution of Castanopsis in the Eocene is confirmed. The amber fossil also broadens the picture of the Baltic amber source area, indicating oligotrophic, sandy, bog-like habitats. Finally, this study underscores the great benefit of SRμCT as a powerful tool to investigate plant inclusions from amber in a nondestructive way.

Morphological structure of inflorescences of Agastache breviflora (A. Gray) Epling, A. rugosa (Fisch. & C.A. Mey.) Kuntze, and A. rupestris (Greene) Standl. (family Lamiaceae) ex situ

The results of comparative analysis of morphological structure of inflorescences Agastache breviflora, A. rugosa, and A. rupestris introduced in the O.V. Fomin Botanical Garden are represented. It has been established that the common morphological characteristics for inflorescences of these plants are: the type of inflorescence (thyrs), its location on the shoot (terminal), type of the growth of main and lateral axes (monopodial growth of the main axis and sympodial growth of lateral axes on which are formed the partial inflorescences – cymoids), the structure of the partial inflorescences (system of dichasia). Differences are determined by the linear dimensions of inflorescences, quantity and size of flowers in partial inflorescences, orientation of the main axis and flowers in space, presence or absence of bracts and bracteoles.

Lectotypifications and a new combination in Eugenia sect. Racemosae (Myrtaceae)

Eugenia sect. Racemosae (Myrtaceae) includes about 59 species of shrubs and trees native to the Neotropics. Members of the section range from northern Mexico and the Antilles throughout South America to Uruguay and northern Argentina. Most species are trees, found in several vegetation types. The section as treated here contains species with flowers exclusively in racemes and/or rarely diplobotrys (compound inflorescence with racemose partial inflorescences), with an obvious flower-bearing axis with a pedicel to internode ratio of 2:1 or less. All typification and nomenclatural issues pertaining to the section and its component species are comprehensively examined here. Eugenia sect. Racemosae is monophyletic, as showed by previous studies. Thirty five names are lectotypified where necessary; many of these taxa were described from collections in Berlin (B) that are no longer extant. This study also provides other nomenclatural notes of species of Eugenia sect. Racemosae, such as a new synonym and a new combination. A new name for Eugenia macrophylla O. Berg is also proposed.

<I>Barjonia furlanii</I> (Apocynaceae, Asclepiadoideae), a New Species from the State of Minas Gerais, Brazil, and a Key to Genus

Abstract This paper provides descriptions and illustrations of a new species, Barjonia furlanii (Asclepiadoideae, Apocynaceae), from Serra da Canastra National Park, in the state of Minas Gerais, Brazil. This species is similar to B. laxa, especially in its frondose thyreoid inflorescence, compound corona, and linear leaves. It is, however, distinguished from B. laxa by internodes (up to 4 cm long), peduncles (4.2–5mm long) and much shorter pedicels (4–6 mm long), erect leaves, attenuated at the base, and partial inflorescences that alternate with 2–4 flowers. B. furlanii is proposed for Endangered status under IUCN guidelines (EN B2abiii), and a key to all Barjonia species is included in this study.

Comparative study of inflorescence development in Oleaceae

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). Series of developmental stages of chemically fixed inflorescences were studied with epi-illumination light microscopy. 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. 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.

Diversity and Evolution of Inflorescences in Celastrales

Inflorescences structures in context of their evolution have been conducted for 60 genera and 170 species of Celastrales (according to APG (Angiosperm Phylogeny Group) III 2009, almost 60% of genera and 15% of species from this taxon of the world flora). There are two big groups of inflorescences in Celastrales-intercalary (more often) and terminal. For many genera of Celastrales both types of inflorescences can be observed, although the frequency of their occurrence varies. There is an important difference between two types of inflorescences: character of completion of the main axis (the terminal flower present or absent). Flower opening can be in basipetal (Celastraceae, Brexia) or acropetal (Stackhousiaceae) order. Partial inflorescence types included: simple, compound or umbrellate dichasia, spike, raceme, thyrse. Simple flowers of Parnassia are large, nested on long reproductive shoots, emerging from a rosette, with amplexicaule leaf.

Papilionoid inflorescences revisited (Leguminosae-Papilionoideae)

The inflorescence structure determines the spatiotemporal arrangement of the flowers during anthesis and is therefore vital for reproductive success. The Leguminosae are among the largest angiosperm plant families and they include some important crop plants. In papilionoid legumes, the raceme is the most common type of inflorescence. However, a range of other inflorescence types have evolved via various developmental processes. A (re-)investigation of inflorescences in Swainsona formosa, Cicer arietinum, Abrus precatorius, Hardenbergia violacea and Kennedia nigricans leads to new insights into reduction mechanisms and to a new hypothesis on the evolution of the papilionoid pseudoraceme. Inflorescence morphology and ontogeny were studied using scanning electron microscopy (SEM). The inflorescence in S. formosa is an umbel with a rare type of pendulum symmetry which may be triggered by the subtending leaf. Inflorescences in C. arietinum are reduced to a single flower. An early formed adaxial bulge is the sterile apex of the inflorescence (i.e. the inflorescence is open and not terminated by a flower). In partial inflorescences of A. precatorius, the axis is reduced and its meristem is relocated towards the main inflorescence. Flower initiation follows a peculiar pendulum pattern. Partial inflorescences in H. violacea and in K. nigricans show reduction tendencies. In both taxa, initiated but early reduced bracteoles are present. Pendulum symmetry in S. formosa is probably associated with distichous phyllotaxis. In C. arietinum, strong reduction tendencies are revealed. Based on studies of A. precatorius, the papilionoid pseudoraceme is reinterpreted as a compound raceme with condensed lateral axes. From an Abrus-like inflorescence, other types can be derived via reduction of flower number and synchronization of flower development. A plea is made for uniform usage of inflorescence terminology.

Flowers and inflorescences of the seagrass Posidonia (Posidoniaceae, Alismatales)

The predominantly aquatic order Alismatales displays a highly variable flower groundplan associated with a diverse range of developmental patterns. We present the first detailed description of flower anatomy and development in Posidonia, the sole genus of the seagrass family Posidoniaceae. Existing accounts provide conflicting interpretations of floral and inflorescence structure, so this investigation is important in clarifying morphological evolution within this early-divergent monocot order. • We investigated two species of Posidonia using light microscopy and scanning electron microscopy. Our observations are interpreted in the framework of a recent molecular phylogeny. • Partial inflorescences are bracteate spikes, which are arranged into a botryoid or a panicle. The flowers are perianthless. The gynoecium is monomerous with the ventral carpel side oriented abaxially. The carpel contains a single pendent bitegmic ovule with a nucellus and long chalaza, both extending along the carpel wall. The ovule develops an integumentary outgrowth. Each flower is supplied by a vascular bundle, whereas the flower-subtending bracts are nonvascularized. • Our data support a racemose interpretation for the partial inflorescence of Posidonia and the presence of flower-subtending bracts. In common with some other Alismatales, Posidonia has simultaneous development of the flower and its subtending bract and loss of the bract vascular supply accompanied by innervation of the flower by a single vascular strand. The unusual carpel orientation could be an evolutionary reduction of a formerly tricarpellate gynoecium. The ovule of Posidonia is campylotropous and unusual within Alismatales in possessing an integumentary outgrowth.

Fruit Development and Pericarp Structure in Nypa fruticans Wurmb (Arecaceae): A Comparison with Other Palms

Nypa fruticans is a mangrove palm with current natural coastal distribution in the Old World tropics and hydrochorous fruit dispersal. Nypa comprises its own monotypic subfamily in Arecaceae, with high or moderate support of its sister position to other palms excluding Calamoideae or even sister to all other palms including Calamoideae. Apocarpous Nypa fruits develop from a spherical female partial inflorescence into large, tightly packed capitate infructescences. Important anatomical features of Nypa fruits (drupes of the Rhapis type) are the single-layered exocarp and endocarp, while the mesocarp is thick and multilayered; its differentiation into a hypodermis and three topographic layers is initiated in the gynoecium shortly after pollination. The outer mesocarp layer transforms into complex aerenchyma at late developmental stages, the massive stone constitutes the second mesocarp layer, and the innermost multilayered parenchymatous mesocarp layer is highly compressed and partly obliterated in mature fruit as a result of seed growth. The mode of pericarp development in Nypa is nearly identical in most aspects to those described for paracarpous fruits of Eugeissona (Calamoideae) and syncarpous fruits of all genera of Borasseae (Coryphoideae). Carpological features support the close relationship of Nypoideae, Calamoideae (Eugeissona), and Coryphoideae (Borasseae). Nypa fruits show little similarity to those of Coryphoideae (excepting Borasseae), Ceroxyloideae, and Arecoideae.

Reassessing Inflorescence and Floral Morphology and Development inHedyosmum(Chloranthaceae)

Hedyosmum (Chloranthaceae), one of the oldest angiosperm lineages, possesses some atypical reproductive features that remain poorly understood, including the inflorescence architecture, the stamen anatomy, and the perianth development around the allegedly inferior ovary. Our developmental survey of these characters with LM and SEM techniques showed that both staminate and carpellate partial inflorescences are indeterminate and suggests that each stamen corresponds to a single, ebracteolate flower. The tapetum is secretory and associated with orbicules; the connective apex is secretory, which—along with the reticulate pollen and the frequent visits of insects—suggests that entomophily should not be ruled out in Hedyosmum. The perianth is formed by three almost completely fused hypogynous tepals; the window is schyzogenous, unique among angiosperms, and according to the fossil record, it has been in the genus since the Early Cretaceous. Unlike any previous description, we found that the ovary is fully superior. The persistent perianth completely surrounds the fruit; in turn, they are tightly enclosed by the fused, acrescent, fleshy flower-subtending bracts, which appear to play a threefold role in protection, secretion, and dispersal.

Homologies of the flower and inflorescence in the early‐divergent grass Anomochloa (Poaceae)

The grass subfamily Anomochlooideae is phylogenetically significant as the sister group to all other grasses. Thus, comparison of their structure with that of other grasses could provide clues to the evolutionary origin of these characters. We describe the structure, embryology, and development of the flower and partial inflorescence of the monotypic Brazilian grass Anomochloa marantoidea. We compare these features with those of other early-divergent grasses such as Pharus and Streptochaeta and closely related Poales such as Ecdeiocolea. Anomochloa possesses several features that are characteristic of Poaceae, notably a scutellum, a solid style, reduced stamen number, and an ovary with a single ovule that develops into a single indehiscent fruit. Interpretation of floral patterning in Anomochloa is problematic because the ramification pattern of the florets places the bracts and axes in unusual positions relative to the primary inflorescence axis. Our study indicates that there is a single abaxial carpel in Anomochloa, probably due to a cryptic type of pseudomonomery in Anomochloa that resembles the pseudomonomery of other grasses. On the other hand, the Anomochloa flower differs from the "typical" grass flower in lacking lodicules and possessing four stamens, in contrast with the tristaminate condition that characterizes many other grasses. Using the median part of the innermost bract as a locator, we tentatively homologize the inner bract of the Anomochloa partial inflorescence with the palea of other grasses. In this interpretation, the pattern of monosymmetry due to stamen suppression differs from that of Ecdeiocolea.

Unisexual flowers as a robust synapomorphy in Cariceae (Cyperaceae)? Evidence for bisexual flowers in Schoenoxiphium

Cariceae, the largest tribe within Cyperaceae, comprises about 2000 species in five genera. Cariceae is usually considered to be distinct from other Cyperaceae by the presence of exclusively unisexual flowers and by the arrangement of the pistillate flowers in single-flowered spikelets that are enclosed by the flask-like spikelet prophyll (utricle or perigynium). The nature of several morphological features of the Cariceae inflorescence remains controversial. The staminate reproductive units, as well as earlier reported bisexual reproductive units in Schoenoxiphium have been considered to be reduced partial inflorescences, or flowers. Aims of this study are to test both interpretations, based on a floral ontogenetic investigation. Moreover, for the first time, detailed SEM micrographs are presented of the inflorescence and floral development and of bisexual flowers in Schoenoxiphium. We propose that ‘inhibition of bisexuality’ is a more robust synapomorphy in Cariceae than ‘presence of only unisexual flowers’.

Ontogeny and structure of the acervulate partial inflorescence in Hyophorbe lagenicaulis (Arecaceae; Arecoideae)

The palm tribe Chamaedoreeae displays flowers arranged in a complex partial inflorescence called an acervulus. This type of partial inflorescence has so far not been reported elsewhere in the largest palm subfamily Arecoideae, which is traditionally characterized by flowers predominantly arranged in triads of one central female and two lateral male flowers. The ontogenetic basis of the acervulus is as yet unknown and its structural diversity throughout the genera of the Chamaedoreeae poorly recorded. This study aims to provide critical information on these aspects. Developmental series and mature inflorescences were sampled from plants cultivated in international botanical gardens and wild populations. The main techniques employed included scanning electronic microscopy and serial anatomical sectioning of resin-embedded fragments of rachillae. Inflorescence ontogeny in Hyophorbe lagenicaulis demonstrates that the acervulus and the inflorescence rachilla form a condensed and cymose branching system resembling a coenosome. Syndesmy results from a combined process of rapid development and adnation, without or with reduced axis elongation. Acervulus diversity in the ten taxa of the Chamaedoreeae studied is displayed at the level of their positioning within the inflorescence, their arrangement, the number of floral buds and their sexual expression. The results show that a more general definition of the type of partial inflorescence observed within the large subfamily Arecoideae would correspond to a cyme rather than to a floral triad. In spite of their common cymose architecture, the floral triad and the acervulus present differences with respect to the number and arrangement of floral buds, the superficial pattern of development and sexual expression.