Floral vasculature and ontogeny in Canna indica

The identity of the labellum is a hot point in Zingiberales, which has long been discussed by many authors. In this study, floral vasculature and ontogeny of Canna indica (Cannaceae) was observed by LM and SEM in order to ascertain the identity of the labellum and the functional stamen of this species and provide evidence for the homologies of the floral organs in Zingiberales. The results indicate that the labellum of C. indica have incorporated two androecial members from both outer and inner whorls, rather than three, one or half member, as previously suggested by morphologists of Cannaceae flowers. The two labellum traces are here interpreted as: one from the outer androecial whorl (diverging from the carpellary dorsal bundle), while the other from the inner androecial whorl (diverging from the parietal bundle). The functional stamen also incorporates two androecial bundles, the same as the labellum: one trace from the carpellary dorsal bundle, and the other (the petaloid appendage) from the parietal bundle. In addition, the origin of the vascular system in the androecium of Zingiberales and its systematic significance are discussed.     

FLORAL DEVELOPMENT IN MANGROVE RHIZOPHORACEAE

The flowers of mangrove Rhizophoraceae (tribe Rhizophoreae) are adapted to three different pollination mechanisms. Floral development of representative species of all four genera suggests that the ancestral flower of the tribe was unspecialized, with successively initiated whorls of separate sepals, petals, antisepalous stamens, and antipetalous stamens; at its inception, the gynoecium had a united, half-inferior ovary and separate stigmatic lobes. This developmental pattern is found in Rhizophora mangle (wind-pollinated) and Ceriops decandra (insect-pollinated). In Kandelia, all floral organs distal to the sepals are initiated simultaneously, and there has apparently been an evolutionary amplification in the number of stamens to about six times the number of petals. Explosive pollen release evolved independently in C. tagal and in Bruguiera. In the former, all stamens belong to one whorl and arise simultaneously upon a very weakly differentiated androecial ring primordium. In Bruguiera, the androecial ring is pronounced, and two whorls of stamens arise upon it; the primordia of the antisepalous whorl arise first but are closer to the center of the apex than the antipetalous stamen primordia. The antisepalous stamens bend toward and are enclosed by the petals early in development. In all genera, the inferior ovary develops by zonal growth of receptacular tissue; additional intercalary growth above the placenta occurs in Bruguiera. In general, floral specialization is accompanied by an increase in the width of the floral apex compared to the size of the primordia, increasing fusion of the stylar primordia, and decreasing prominence of the superior portion of the ovary. Apparent specializations of petal appendages for water storage, including the presence of sub-terminal hydathodes (previously unreported in any angiosperm), were found in two species in which flowers remain open during the day but were absent from two species normally pollinated at night or at dawn. Distinctive tribal characteristics that may aid in phylogenetic analysis include the mode of development of the inferior ovary; the aristate, bifid, usually fringed petals that individually enclose one or more stamens; the intrastaminal floral disc; and the initially subepidermal laticiferous cell layer in the sepals and ovary.     

Floral organogenesis of Chloranthus sessilifolius, with special emphasis on the morphological nature of the androecium of Chloranthus (Chloranthaceae)

 Floral organogenesis of Chloranthus sessilifolius K. F. Wu is described. The inflorescence primordium is dome-like in the beginning and then elongates, and bract primordia initiate almost decussately. Each floral primordium, arising from the axil of a bract, soon becomes a scale-like structure, with three primordia of androecial lobes originating from its abaxial part, and the gynoecial primordium in adaxial position. As the androecial lobes become more distinct, four thecae are already in differentiation, and the gynoecial primordium appears as a shallow disc. The androecial lobes do not extend their length until the thecae approach maturity and the stigma is differentiated. The androecial lobes are united at all the stages of development, and the entire androecium falls off as a unit at the end of anthesis. Based on these results, combined with published evidence from neobotany, palaeobotany and phylogenetic studies, the morphological nature of the androecium of Chloranthus is further discussed. Our studies support the viewpoint that the androecial structure of Chloranthus may have arisen by splitting of a single stamen with 2 marginal thecae. Received May 2, 2001 Accepted December 18, 2001     

FLORAL ORGANOGENESIS IN FIVE GENERA OF THE MARANTACEAE AND IN CANNA (CANNACEAE)

The paired flowers of all species of the Marantaceae studied, except Monotagma plurispicatum, are produced through the division of an apical meristem with a tunica-corpus structure. The solitary flowers of M. plurispicatum develop from a similar meristem which does not bifurcate. The paired flowers of Canna indica are produced in the axil of a florescence bract through the formation of a bract and an axillary flower on the side of the primordium which gives rise to the largest flower of the pair. The sequence of organ initiation for both families is: calyx, corolla and inner androecial whorl, outer androecial whorl, gynoecium. The sequence of sepal formation is opposite in the two families. In the Cannaceae it leads directly into the spiral created by the formation of the other organs, while in the Marantaceae the sequence of sepal formation follows a spiral opposite to that of the other floral organs. The members of the corolla and inner androecial whorl separate from common primordia. In general these common primordia separate into a petal and an inner androecial member through the initiation of two growth centers, at the same level, in the dorsal and ventral flanks of the primordium. In Ischnosiphon elegans and Pleiostachya pruinosa the stamen is initiated at a lower position than the petal in the ventral flank of the common primordium. A similar pattern of initiation is described for the callose staminode in Marantochloa purpurea and Canna indica. This pattern is interpreted as a variation on the more generalized pattern of inner androecial formation found in the other genera.     

Structure, development and evolution of the androecium in Adansonieae (core Bombacoideae, Malvaceae s.l.)

Androecium development and vasculature were studied in nine species of the Adansonieae clade (core Bombacoideae, Malvaceae s.l.). In early androecium development either distinct pentagonal androecial ring walls or five common petal/androecium primordia are present. Ring walls give rise to five antepetalous and five alternipetalous primary androecial primordia. Common primordia divide into peripheral petal primordia and antepetalous primary androecial primordia. Antepetalous primary androecial primordia split anticlinally into ten primordia-halves, on which secondary androecial primordia are initiated in a centrifugal succession. Androecial lobes are formed by fusion of an alternipetalous primary androecial primordium and its two neighbouring antepetalous primary primordia-halves, a pattern that also occurs in other Malvatheca. Later, tertiary androecial primordia are formed by the subdivision of secondary androecial primordia (except in Adansonia and Ceiba). Each tertiary primordium differentiates into a two-locular androecial unit. At anthesis these two-locular androecial units are often present in pairs, corresponding to the two halves of the same secondary androecial primordium. Androecium development and vasculature imply that the alternipetalous androecial sectors have been reduced in Bombacoideae, a tendency that is shared with other subfamilies of Malvaceae.     

Insects and plants in the pollination ecology of the boreal zone

Pollination systems in the boreal zone range from generalist to specialist, both entomologically and botanically. The relative importance of wind pollination, insect pollination, sexual separation between and within plants, and between flowers, hermaphroditism of flowers, and various breeding systems are related to plant growth form and habitat. The diversity and specializations of anthophilous insects parallel those in other biogeographic zones, but seem less developed. We suggest that this reflects the combined effects of evolutionary youth, severity of climate, restriction of symbiont ranges within those of their hosts, and the naturally frequent perturbations by fire or insect outbreaks in the zone, requiring faunal and floral vagility and constraining specialization in mutualism. Modern perturbations by logging and pesticides seem to be well buffered because of the relative openness of the ecosystem (compared to others), although damage has been documented. Insect pollination is as much a keystone process in the boreal forest as elsewhere, despite the immediate counter-impression given by the dominance of wind-pollinated conifers. Nevertheless, there are few studies, botanical or entomologicalin situ. The boreal system offers important opportunities in general and applied research in pollination ecology and synecology generally.     

FLORAL DEVELOPMENT OF COCHLOSPERMUM TINCTORIUM AND BIXA ORELLANA WITH SPECIAL EMPHASIS ON THE ANDROECIUM

The initiation and development of the flower of Bixa orellana L. and Cochlospermum tinctorium A. Rich, were investigated using the scanning electron microscope to elucidate the nature of the androecial development and the relationships of both taxa. Initiation of floral buds starts with the inception of five sepals in a 2/5 sequence between two bracteoles. The petals are formed successively on the irregular pentagonal apex in a phyllotactic pattern different from that of the calyx. Stamen development proceeds centrifugally on a broad circular primordium or ringwall, which attains its final size at the time of initiation of individual stamens. Stamen primordia arise in successive whorls without connection to the perianth. The residue of the apex is differentiated into a gynoecial circular primordium. In Cochlospermum 3 to 4 carpels are initiated; in Bixa no individual carpels are visible. The origin of the androecial circular primordium is discussed in relation to other types of androecial development. A comparison is made with existing theories of the evolution of multistaminate androecia. Few differences were found in the floral development of Bixa and Cochlospermum, except in the gynoecium. It is proposed to retain them in a single family Bixaceae for a number of reasons. The floral development supports a thealean, dillenialean, or violalean affinity rather than a malvalean. The inception of a broad androecial circular primordium is highly different from the development in Malvales, where more space for stamens is provided by the continuous growth of a tube.     

FLORAL ANATOMY IN THE SAXIFRAGACEAE SENSU LATO. I. INTRODUCTION,PARNASSIOIDEAE AND BREXIOIDEAE

The floral morphology and anatomy of one representative of the Parnassioideae and two of the Brexioideae are described, and some of the recent literature dealing with the Saxifragaceae sensu lato is reviewed. Comparison of the floral structure in Parnassia to that typical of the Saxifragoideae, the subfamily constituting the Saxifragaceae sensu stricto and which, therefore, may be considered to show the basic saxifragaceous characteristics, reveals little similarity. Parnassia differs in pattern of both sepal and androecial vascularization, vascularization and degree of connation of the carpels, height in the gynoecium to which ventral bundles remain compound, possession of nectariferous staminodia, and the absence of epidermal appendages. Brexia and Ixerba (both of the Brexioideae) are strikingly dissimilar in floral structure and probably should be dissociated. While the position of Ixerba is problematical, it shares more floral characters with the Escallonioideae than with either Brexia or the Saxifragoideae and is better associated with that taxon. In both Parnassia and Brexia the vascular pattern suggests derivation of the androecium from a fascicled condition: the vascular supply of each filament consists of a cylinder of closely associated collateral bundles, and each staminodial set receives a single vascular complex which subsequently divides into as many vascular strands as there are staminodia in the set.     

Structure and evolution of the androecium in the Malvatheca clade (Malvaceae s.l.) and implications for Malvaceae and Malvales

Androecial development and structure as well as floral vasculature of six selected species of Bombacoideae and of several smaller lineages of the Malvatheca clade (Malvaceae s.l.) were studied. All studied taxa share a similar pattern of androecial development: initially, five antepetalous/antetepalous and five alternipetalous/alternitepalous primary androecial primordia develop on a ring wall. Two elongate secondary androecial primordia form on each antepetalous/antetepalous sector. At anthesis the androecium consists of an androecial tube crowned by five androecial lobes. Each of these lobes is the developmental product of an alternipetalous/alternitepalous primary androecial primordium and its two neighbouring antepetalous/antetepalous secondary androecial primordia. The elongate, sessile androecial units are positioned along the lateral margins of the androecial lobes and in the distal part of the androecial tube. Seen in the light of the most recent studies of floral development and phylogeny of the Malvaceae and the Malvales as a whole, our data indicate that i) elongate, sessile androecial units are ancestral in the Malvatheca clade, that ii) an obdiplostemonous floral ground plan is a synapomorphy for the Malvaceae, and that iii) diplostemony is most likely ancestral in the Malvales.     

MORPHOLOGICAL STUDIES OF THE NYMPHAEACEAE SENSU LATO. XVII. FLORAL ANATOMY OF ONDINEA PURPUREA SUBSPECIES PURPUREA (NYMPHAEACEAE)

Classification and phylogeny of the Nymphaeaceae are unresolved. This study provides floral anatomical data that will assist in elucidating generic interrelationships and systematic relationships to other taxa of angiosperms. The floral anatomy of Ondinea purpurea den Hartog subsp. purpurea has been examined utilizing light microscopy. The peduncle possesses stelar vascular bundle complexes and cortical vascular bundles. Cortical bundles terminate within the peduncle. Each bundle complex consists of 2 collateral bundles on the same radius, the inner bundle inverted; 2 protoxylary lacunae occur yet differ in structure and function. Progressing acropetally, the inner xylary lacunae become discrete mesarch strands surrounded centrifugally by a vascular cylinder formed by divisions and anastomosing of the bundle complexes. Together these become the massive receptacular vascular plexus. The plexus provides collateral traces to the floral organs. Each sepal receives 3 traces that separate from the plexus as 1–3 lateral traces. Petals are absent and no vestigial petal traces have been observed. Distally, the plexus forms several large strands of connate gynoecial and androecial traces termed the principal vascular bundles (PVBs). Ventral veins separate from the PVBs and the latter extend acropetally through the outer ovary wall. Branches of the ventrals and PVBs contribute to septal vascular reticula from which each ovule is supplied by one vascular bundle. Each stamen receives 1 trace from branches of the PVBs. The ventrals and PVBs terminate within the carpellary lobes. A comparative anatomical study is offered that supports the inclusion of Ondinea in the Nymphaeaceae sensu stricto.     

FLORAL MORPHOLOGY OF HORSFIELDIA (MYRISTICACEAE)

The pistillate flowers of Horsfieldia are morphologically similar to those of Myristica and Knema, and are composed of a single whorl of thick, fleshy tepals, and an unsealed, monocarpellate pistil bearing a single ovule. The carpel is vascularized by two ventral bundles, a pair of dorsal bundles, and several supernumerary bundles. The ovule vascularization is derived from the supernumerary bundles. Paired dorsal vascular bundles are an uncommon feature of uncertain significance. Carpels of Myristica and Knema lack any clearly defined dorsal vasculature, and the ovule vascular supply is derived from both the ventral and supernumerary bundles. The organization of the staminate flowers of Horsfieldia agrees with the myristicaceous pattern observed in Myristica and Knema. Each androecium consists of a single whorl of anthers fused or partially fused to a massive connective column. Each anther consists of a pair of bisporangiate lobes and a single vascular bundle. The androecial forms observed are interpreted as forming a series of intermediates between the monadelphous type of androecia of two South American genera, Compsoneura and Dialyanthera, and one African genus, Brochneura, and the solid, columnar androecia which are predominate in the family. Accumulating evidence supports a proposed South American or west Gondwanaland origin of the Myristicaceae.     

Variation in flowering behaviour inVulpia (Poaceae)

Some abnormalities in the androecial structure and behaviour withinVulpia sect.Vulpia are described. These features help to trace the evolutionary history of cleistogamous taxa with a single micranthous stamen from chasmogamous taxa with three macranthous stamens.     

Pollination of Ravenala madagascariensis (Strelitziaceae) by lemurs in Madagascar: evidence for an archaic coevolutionary system?

Investigations of the floral biology of the traveler's tree (Ravenala madagascariensis) and the ecology of the ruffed lemur (Varecia variegata), both endemic to the island of Madagascar, suggest a plant-pollinator relationship. Ravenala exhibits many specializations for visitation by large nonflying animals: inflorescences placed below the crown of the plant and easily accessible to arboreal animals; large flowers enclosed in tough, protective bracts that require a strong pollinator to open; stiff, rodlike styles that withstand the rough handling of the visitors; and copious, sucrose-dominant nectar that provides a renewable reward for a sizable animal for a long time period. Our observations in the field also show that Varecia variegata: consistently and almost exclusively visit the flowers of Ravenala; carry pollen on their fur between flowers on the same plant and between conspecific plants; do not destroy the flowers while obtaining the nectar; and appear to be highly dependent on nectar as a food source during specific times of the year. The basal phylogenetic position of Ravenala in the family Strelitziaceae, as indicated by molecular sequence data, and the distribution of reproductive traits in the three extant genera are consistent with the hypothesis that pollination by nonflying mammals is an archaic system, whereas bird and bat pollination are derived.     

Comparative floral structure and systematics in Rhizophoraceae,Erythroxylaceae and the potentially related Ctenolophonaceae,Linaceae, Irvingiaceae and Caryocaraceae (Malpighiales)

Within the rosid order Malpighiales, Rhizophoraceae and Erythroxylaceae (1) are strongly supported as sisters in molecular phylogenetic studies and possibly form a clade with either Ctenolophonaceae (2) or with Linaceae, Irvingiaceae and Caryocaraceae (less well supported) (3). In order to assess the validity of these relationships from a floral structural point of view, these families are comparatively studied for the first time in terms of their floral morphology, anatomy and histology. Overall floral structure reflects the molecular results quite well and Rhizophoraceae and Erythroxylaceae are well supported as closely related. Ctenolophonaceae share some unusual floral features (potential synapomorphies) with Rhizophoraceae and Erythroxylaceae. In contrast, Linaceae, Irvingiaceae and Caryocaraceae are not clearly supported as a clade, or as closely related to Rhizophoraceae and Erythroxylaceae, as their shared features are probably mainly symplesiomorphies at the level of Malpighiales or a (still undefined) larger subclade of Malpighales, rather than synapomorphies. Rhizophoraceae and Erythroxylaceae share (among other features) conduplicate petals enwrapping stamens in bud, antepetalous stamens longer than antesepalous ones, a nectariferous androecial tube with attachment of the two stamen whorls at different positions: one whorl on the rim, the other below the rim of the tube, the ovary shortly and abruptly dorsally bulged and the presence of a layer of idioblasts (laticifers?) in the sepals and ovaries. Ctenolophonaceae share with Rhizophoraceae and/or Erythroxylaceae (among other features) sepals with less than three vascular traces, a short androgynophore, an ovary septum thin and severed or completely disintegrating during development, leading to a developmentally secondarily unilocular ovary, a zigzag‐shaped micropyle and seeds with an aril. Special features occurring in families of all three groupings studied here are, for example, synsepaly, petals not retarded and thus forming protective organs in floral bud, petals postgenitally fused or hooked together in bud, androecial tube and petals fusing above floral base, androecial corona, apocarpous unifacial styles, nucellus thin and long, early disintegrating (before embryo sac is mature), and nectaries on the androecial tube. Some of these features may be synapomorphies for the entire group, if it forms a supported clade in future molecular studies, or for subgroups thereof. Others may be plesiomorphies, as they also occur in other Malpighiales or also in Celastrales or Oxalidales (COM clade). The occurrence of these features within the COM clade is also discussed. © 2011 The Linnean Society of London, Botanical Journal of the Linnean Society, 2011, 166 , 331–416.     

Phylogeny, diversity, and distribution inExostema (Rubiaceae): Implications of morphological and molecular analyses

The neotropical genusExostema comprises 25 species of trees and shrubs, ranging in distribution from Bolivia to Mexico and throughout the West Indies, with most species endemic to the Greater Antilles. Infrageneric relationships and species-level patterns of evolution were investigated in phylogenetic analyses using morphological, molecular, and combined data sets. All data sets resolved three main species groups which correspond to the three sections recognized byMcDowell (1996). However, the analyses of ITS sequence data placed the two South American species basal to the three main clades. Otherwise, the morphological and molecular data are highly compatible, and produce a more robust yet consistent phylogeny in the combined data analysis. Morphological evolution inExostema involves many specializations for xeric habitats, reflecting repeated ecological shifts from moist forest to exposed, seasonally dry environments during the diversification of the genus. Both moth and bee pollination syndromes are found inExostema, and shifts in pollination ecology appear pivotal to the differentiation of the three sections. Biogeographically,Exostema likely originated in South America and migrated via Central America to the Greater Antilles, where the morphological diversification and speciation are most extensive.     

Scanning electron microscopy of the placental interface in the viviparous lizard Sceloporus jarrovi (Squamata: Phrynosomatidae)

Placental membranes mediate maternal‐fetal exchange in all viviparous reptilian sauropsids. We used scanning electron microscopy to examine the placental interface in the mountain spiny lizard, Sceloporus jarrovi (Phrynosomatidae). From the late limb bud stage until birth, the conceptus is surrounded by placental membranes formed from the chorioallantois and yolk sac omphalopleure. The chorioallantois lies directly apposed to the uterine lining with no intervening shell membrane. Both fetal and maternal sides of the chorioallantoic placenta are lined by continuous layers of flattened epithelial cells that overlie dense capillary networks. The chorioallantoic placenta shows specializations that enhance respiratory exchange, as well as ultrastructural evidence of maternal secretion and fetal absorption. The yolk sac placenta contains enlarged fetal and maternal epithelia with specializations for histotrophic nutrient transfer. This placenta lacks intrinsic vascularity, although the vascular allantois lies against its inner face, contributing to an omphallantoic placenta. In a specialized region at the abembryonic pole, uterine and fetal tissues are separated by a compact mass of shed shell membrane, yolk droplets, and cellular debris. The omphalopleure in this region develops elongate folds that may contribute to sequestration and absorption of this material. Fetal membrane morphogenesis and composition in S. jarrovi are consistent with those of typical squamates. However, this species exhibits unusual placental specializations characteristic of highly placentotrophic lizards. J. Morphol., 2011. © 2011 Wiley‐Liss, Inc.     

Orchid pollination biology

Orchids display many unsurpassed floral specializations, as both rewarders and frauds in their interaction with animal pollinators. Accumulating evidence indicates that their floral evolution is driven by pollinator traits and that expenditure for maximized sexual reproduction is parcelled out over their lifetimes in strategies for coping with pollinator and resource limitations. Recent advances in orchid pollination biology center mainly on floral evolutionary processes, pseudocopulation and other deceptive pollination systems, and flower and fruit production in relation to costs of sexual reproduction.     

Fascicled Androecia in Dilleniidae and Some Remarks on the Garcinia Androecium

In Dilleniidae, stamen fascicles are interpreted either as phylogenetically secondary structures (derived from a single stamen primordium by dédoublement), or as the most archaic type of androecial organs in angiosperms. In context with an assumed high plasticity in the flowers at the beginning of angiosperm evolution (Endress, 1987 a, b), fascicled androecia can also be regarded as having coexisted with “magnolioid” spiral androecia since early in the evolution. On the basis of this assumption, it is easy to link the Dilleniidae, via their basal group, the Paeoniaceae, to the Magnoliidae. In Paeonia, the stamen clusters continue the spiral arrangement of the perianth members, with “limiting divergence” (Hiepko, 1964). Our investigations of Paeonia officinalis show that the fascicle primordia follow the spiral not only in their position, but also in their temporal sequence. In some Theaceae (Stewartia) and Clusiaceae (species of Garcinia) the divergence angle changes to 2/5, resulting in an epipetalous position of the stamen fascicles. The transition from fascicled (complex) androecia to simple ones (one- or two-whorled) has occurred repeatedly within the Dilleniidae. A great diversity of androecial structures, based on a fascicled androecium, can be found in the palaeotropical genus Garcinia (Clusiaceae), which comprises about 200 species. Some of these forms, including exceptional ones, are presented in this article. The diversity in the androecium in Garcinia can be interpreted phylogenetically as a secondarily increased plasticity, resulting in morphological curiosities.     

CONE AND OVULE ONTOGENY IN TAXODIUM AND GLYPTOSTROBUS (TAXODIACEAE – CONIFERALES)

Seed cones in Taxodium distichum and Glyptostrobus pensilis occupy the position of permanent shoots and are initiated in the summer preceding spring pollination. Morphological features are similar in the two genera, reflecting their close taxonomic relationship. Ovule complexes originate as two (rarely more) ovule primordia in the axil of each fertile bract but without any indication of a preceding discrete ovuliferous scale. When the nucellus, integument, and micropyle are well developed, a series of up to ll abaxial lobes forms at the base of each ovule pair. They become fused by basal growth. After pollination the common basal meristem of lobes and bract extends by intercalary growth to form the conspicuous “ovuliferous scale” of the mature cone; the lobes enlarge and exceed the ovules. Despite the topographic similarity in the cones of both genera, there are differences in vasculature such that the vascular traces to the axillary complex originate directly from the axial cylinder in Glyptostrobus but from the bract trace in Taxodium. The complex vasculature of the mature cone develops late and primarily as an expression of intercalary growth.     

Notes on the Evolution of Androecial Organisation in the Magnoliophytina (Angiosperms)

This paper aims to summarize briefly and to update our ideas about androecial architecture formulated in earlier publications. Ontogenetic evidence of stamen development, viz. the initiation, arrangement and relationship of stamens to other floral morphomes, can be translated into a semophyletic scheme reflecting the phylogeny of the androecium. The ancestral androecium is discussed in the light of recent theoriesabout angiosperm phylogeny. Two divergent androecial processes are proposed for the angiosperms starting from a spiral androecium with a moderate number of stamens. However, transitions exist between spiral polyandry, numerous stamens in whorls, and chaotic polyandry. From an androecium with several alternating whorls of paired and single stamens, outer stamen pairs are retained following the successive loss of inner stamen whorls. Single stamens instead of pairs occur at the very end of this line and represent a more advanced condition. This line is mostly present in tri- and dimerous flowers. From the same starting point diplostemony (with two alternating whorls of single stamens) originated, again giving rise to various states usually present in pentamerous or tetramerous flowers.