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Floral morphology and development reveal extreme diversification in some species of Croton (Euphorbiaceae)
Abstract
Floral diversity of Croton, the second largest genus in Euphorbiaceae, is currently under-explored. Several clades demonstrate an unusual floral morphology, e.g., lower or higher stamen number, bilateral symmetry and reduced ovary, but have never been investigated in a comparative study with typical Croton. This study examined morphology and ontogeny of flowers in nine Croton species from different clades within the genus with light and scanning microscopy, resin sectioning and micro-computed tomography. In staminate flowers, great variations of stamen number and arrangement are observed. The ancestral androecium likely consisted of two or more whorls with the outermost antepetalous stamen whorl developing centrifugally. Modification by reduction of the antepetalous whorl resulted in an outer alternipetalous stamen whorl in Croton section Moacroton, subgenus Quadrilobi. Several species in the subgenus Geiseleria show an independent reduction of stamen numbers by absence of a centrifugal development with the antepetalous whorl the first whorl to develop. Petal losses are observed in the distantly related C. setiger and C. dioicus. Chaotic stamen arrangement is found in C. celtidifolius (subgenus Adenophylli) as a result of a secondary stamen increase. In pistillate flowers, reduction of carpel numbers happened three times in the subgenus Geiseleria. C. monanthogynus has a bicarpellate ovary, while in C. setiger and C. michauxii the ovary is monocarpellate. Reduction of carpel number is linked with merism change and perianth reduction. The ovary in C. michauxii has basal placentation which is unique among all Croton. Moreover, strong bilateral sepals and nectaries are observed in species from section Julocroton. Therefore, the floral diversity of some species in the genus Croton could be explained by developmental modification of an ancestral form via reduction, rearrangement of stamen whorls, and symmetry shifts.
... Examples illustrating how the interaction between mechanical pressure and heterochrony is responsible for shifts in organ position and the establishment of a novel floral Bauplan are presented and discussed. Thaowetsuwan et al. (2024) elucidated the diversity of floral morphology in the genus Croton (Euphorbiaceae) based on the detailed observations and suggested the potential influence of mechanical forces on this diversity. Jerominek and Claßen-Bockhoff (2024) focused on the explosive movement of style in flowers of Marantaceae and investigated its mechanisms driven by internal tensile stress. ...
Trichomes are key functional traits that protect plants against abiotic stresses, herbivores and pathogens. In addition to their ecological roles, trichomes show high morphological variability in many lineages, providing useful characters for species characterization and taxon delimitation. Here, we focus on trichomes in Croton, a megadiverse genus (c. 1200 spp.) of Euphorbiaceae, in which these structures are ubiquitous and exhibit significant variation in form, location on the plant body, density and function. Owing to their functional and taxonomic relevance and the current need for further standardization of their terminology and classification, we revised previous trichome classifications used in the genus. We also provide new trichome data by expanding taxon sampling and using different methodological approaches in plant anatomy to improve understanding of the structural patterns of the trichome types present in the genus. On the basis of our results and literature review, we propose a new classification for trichomes in Croton based on well-defined parameters; this clarifies existing redundancies, is consistent with anatomical evidence and can be easily updated as new types are revealed in future surveys. This contribution will improve communication among taxonomists and promote more meaningful morphological comparability across systematics studies, including those focusing on morphological evolution and functional traits.
Acalypha (Euphorbiaceae: Acalyphoideae) is a large, monophyletic genus distributed worldwide in tropical and subtropical regions, with a few species extending into temperate areas of southern Africa, Asia, and North and South America. We reconstructed phylogenetic relationships within the genus using DNA sequences from the plastid ndhF and trnL-F regions and the nuclear ribosomal ITS region, sampling 142 species to represent the geographic, morphologic, and taxonomic diversity with the genus, resulting in a 162 (158 in Acalypha ) terminal and 3847 character combined dataset. Bayesian and maximum likelihood reconstructions based on the combined dataset yielded a tree with a generally well-supported backbone and several strongly supported clades. Our results strongly supported the monophyly of Acalypha subg. Acalypha as currently recognized but showed that A. subg. Linostachys and almost all other infrageneric taxa recognized in the most recent comprehensive classification of the genus were not monophyletic. We therefore propose a new subgeneric classification comprising A. subg. Acalypha , A. subg. Androcephala , A. subg. Hypandrae , and A. subg. Linostachys (s.s.). Our results also shed light on relationships within some species groups, including in what has been treated as a broadly defined A. amentacea , in which we recognize A. amentacea , A. palauensis comb. nov. , and A. wilkesiana as distinct species. Bayesian ancestral state estimations based on the phylogeny of Acalypha demonstrated that inflorescence position and sexuality and habit show high homoplasy, especially within A. subg. Acalypha , and that inflorescence position and habit exhibit correlated evolution.
A revised assessment of Croton L. sect. Geiseleria (A. Gray) Baill. is provided. The section as now circumscribed includes 84 species ranging across warm areas of the Americas. A nuclear ITS phylogeny of 150 accessions from 83 species and a chloroplast trnL-F phylogeny of 89 accessions from 65 species were generated to confirm correct phylogenetic placement of the species and to determine if any species previously included in the section should now be excluded. Seven new species are described, and we present a taxonomic synopsis that lists all currently accepted species along with their synonyms, distributions, and pertinent comments. We also recognize four subsections within Croton sect. Geiseleria and list their corresponding species. Based on both morphological and molecular criteria, we now recognize C. lagunillae Croizat as a distinct species, rather than as a variety of C. guildingii Griseb. (now treated as C. suavis Kunth), and we treat C. ramillatus Croizat var. magniglandulifer V. W. Steinm. as a species, C. magniglandulifer (V. W. Steinm.) B. W. van Ee. We also exclude C. tetradenius Baill. from Croton sect. Geiseleria (A. Gray) Baill. along with related species such as C. pulegiodorus Baill. and C. leptobotryus Müll. Arg., which are either members of Croton sect. Adenophylli Griseb. or else may merit new sectional status. We also exclude C. waltherioides Urb., although it is not clear to which section of Croton it belongs.
Croton is a mega-diverse genus of more than 1,200 species with great morphological diversity and highly dimorphic flowers. Staminate flowers generally possess petals and a variable number of stamens and lack of an ovary. Pistillate flowers generally lack petals or have filamentous structures instead; stamens are lacking, and the ovary is generally tricarpellate with divided styles. However, well-developed petals can be found in pistillate flowers of some African species and two New World sections, i.e., sect. Alabamenses and sect. Eluteria subsect. Eluteria. Our objectives are to compare ontogeny in dimorphic flowers of Croton which may elucidate the origin of petals, homology of the filamentous structures, nectaries, and diversity of the androecium. The development of staminate and pistillate flowers of C. alabamensis (sect. Alabamenses) and C. schiedeanus (sect. Eluteria) was studied under the scanning electron microscope (SEM) and compared with C. chilensis (sect. Adenophylli) which has filamentous structures in pistillate flowers. In staminate flowers, petals develop in alternation with sepals and later the outermost stamen whorl develops opposite to the petals. In a much later stage, nectar glands emerge in alternation with the petals. In pistillate flowers, filamentous structures and petals share the same early development in alternation with the sepals. However, growth of the filamentous structures of C. chilensis becomes arrested, while petals of C. alabamensis and C. schiedeanus develop similar to those of the staminate flowers. The main conclusions of the study are: (1) Petals and filamentous structures are homologous based on their location and shape in early development. (2) Nectaries with variable morphology develop in antesepalous position and probably represent receptacular outgrowths and are not staminodial. (3) Staminate flower development displays an unexpected diversity, including a floral cup in C. alabamensis. All investigated species have an unusual centrifugal initiation of the second stamen whorl. (4) The possible evolutionary loss and potential regain of petals among different species of Croton is discussed in relation to heterochrony. We suggest that the filamentous structures in pistillate flowers represent pedomorphic forms of petals of staminate flowers, and that the well-developed petals in C. alabamensis and C. schiedeanus are derived via a developmental reversion.
Most Euphorbiaceae are insect pollinated, but in Acalyphoideae a considerable number of species are wind pollinated and generally have inconspicuous monochlamydeous flowers with red styles and multifid or laciniate stigmata. Alchornea is within the basal alchorneoid clade, and its flowers are more conspicuous in the group. The aim of this study was to investigate the ontogeny, anatomy and vasculature of flowers of Alchornea sidifolia Müll. Arg., species from the Alchorneoids clade, a basal clade sister of the “core acalyphoid,” in order to find structural aspects related to wind pollination. Flowers and buds of A. sidifolia were embedded in Paraplast and sectioned with a rotary microtome for analysis under light microscopy. SEM was performed for additional structural and ontogenetic data. Alchornea sidifolia is dioecious and has morphologically and functionally unisexual flowers which only share a monochlamydeous, tetramerous perianth, organized in a single whorl of sepals. In the male flower, calyx begins as a ring on the floral meristem in a congenital connation, but in the female one, the calyx has a late synsepaly. Pistillodes and staminodes are absent in the male and female flowers, respectively. The filaments of the stamens are connate at the base for a short extant and free toward the apex. The gynoecium is syncarpous, and the ovary is mostly synascidiate with a short symplicate zone and two long stigmata. Our results highlight the development of features of the inflorescence and flowers of A. sidifolia which in comparison with insect-pollinated Euphorbiaceae display a set of features which are typically associated in many other unrelated angiosperm lineages with the evolution of wind pollination, such as dioecy, production of a higher number of flowers in the male inflorescences than the female inflorescences, unisexual flowers with reduced perianth and no nectaries, male flowers with a variable number of stamens and female flowers with extensive stigmatic receptive surface. We also discuss the presence of two carpels in Alchornea, when most Euphorbiaceae and many other Malpighiales have three, and its relationship with the wind pollination.
Croton macrosepalus Sodré & M.J.Silva (sp. nov.), a new species found in the states of Bahia, Goiás, and Tocantins, in Brazil, is described and illustrated. Although it is morphologically similar to Croton spica, the novel species can be differentiated from it by having large glandular stipules, entire or sometimes serrulate leaf margins, petioles with 2–8 cylindrical nectary glands at the apex, ovate pistillate sepals, and a hat-shaped caruncle. The habitat, conservation status, phenology, and morphological relationships of the species are also discussed. Morphological data and molecular analyses using ITS and trnL–trnF support the placement of C. macrosepalus and C. spica in Croton sect. Geiseleria. A geographic distribution map, images, notes about the leaf anatomy of both species, and their phylogenetic relationships are provided.
The genus Croton belongs to the Euphorbiaceae family, which comprises approximately 1300 species. Many Croton species have been used as folk medicines. This review focuses on the chemical constituents from Croton species and their relevant biological activities, covering the period from 2006 to 2018. A total of 399 new compounds, including 339 diterpenoids, were reported. Diterpenoids are characteristic components of the Croton species. These isolated compounds exhibited a broad spectrum of bioactivities, including cytotoxic, anti-inflammatory, antifungal, acetylcholinesterase inhibitory, and neurite outgrowth-promoting properties. The present review provides a significant clue for further research of the chemical constituents from the Croton species as potential medicines.
The Euphorbiaceae are notable for floral diversity and evolutionary complexity. Croton is the second largest genus in the family and exhibits particular diversity in its flowers. The aim of this study was to investigate the floral ontogeny and structure of three Croton species with distinct morphologies, with a focus on testing the hypothesis that the filaments of female flowers, which have received different interpretations in the literature and are currently described as reduced petals, are staminodes and part of a vestigial androecium. With the ontogenetic study we can understand the origin of the organs and associate these with flower evolution in the genus. Flowers in several stages of development were analysed using light microscopy and scanning electron microscopy. In the early stage of development, the sepals are the first structures to be formed, although they do not continue to grow in female Croton fuscescens Spreng. flowers. Petals are absent in female flowers, with filamentous, petaloid structures, interpreted here as staminodes, alternating with the sepals in Croton lundianus (Didr.) Müll. Arg. In Croton sphaerogynus Baill., the staminodes are located between the nectary lobes. The stamens exhibit centripetal development in the flower bud stage, and the carpels are post-genitally connate, with differences in style branching. Besides the ontogenetic interpretation for the filamentous structures, the genus shows transitional structures that we consider evolutionary reductions. Our results can explain how developmental alterations have influenced the suppression and modification of floral organs in the genus.
The presence of a gynoecium composed of carpels is a key feature of angiosperms. The carpel is often regarded as a homologue of the gymnosperm megasporophyll (that is, an ovule-bearing leaf), but higher complexity of the morphological nature of carpel cannot be ruled out. Angiosperm carpels can fuse to form a syncarpous gynoecium. A syncarpous gynoecium usually includes a well-developed compitum, an area where the pollen tube transmitting tracts of individual carpels unite to enable the transition of pollen tubes from one carpel to another. This phenomenon is a precondition to the emergence of carpel dimorphism manifested as the absence of a functional stigma or fertile ovules in part of the carpels. Pseudomonomery, which is characterized by the presence of a fertile ovule (or ovules) in one carpel only, is a specific case of carpel dimorphism. A pseudomonomerous gynoecium usually has a single plane of symmetry and is likely to share certain features of the regulation of morphogenesis with the monosymmetric perianth and androecium. A genuine monomerous gynoecium consists of a single carpel. Syncarpous gynoecia can be abruptly transformed into monomerous gynoecia in the course of evolution or undergo sterilization and gradual reduction of some carpels. Partial or nearly complete loss of carpel individuality that precludes the assignment of an ovule (or ovules) to an individual carpel is observed in a specific group of gynoecia. We termed this phenomenon mixomery, since it should be distinguished from pseudomonomery.
Croton cupreolepis (Euphorbiaceae) is newly described from montane, evergreen forests of eastern Madagascar. It is distinguished by its coppery-lepidote trichomes that form a dense covering on the floral buds and inflorescences, as well as on the dorsal side of the petals of both pistillate and staminate flowers, and also by its distinctive recurved pedicels. The young leaves are covered by the same type of coppery trichomes on the lower surface, but as the leaves expand, the coppery scales become scattered on a background of silvery scales. The species has been confused with Croton chrysodaphne, C. nobilis, or else specimens have been left determined just to genus. Its broad latitudinal distribution in Madagascar suggests that it may once have been more widespread, but has been a victim of the widespread deforestation of primary forest along the eastern slopes of the island.
Background and aims:
Obdiplostemony has long been a controversial condition as it diverges from diplostemony found among most core eudicot orders by the more external insertion of the alternisepalous stamens. In this paper we review the definition and occurrence of obdiplostemony, and analyse how the condition has impacted on floral diversification and species evolution.
Key results:
Obdiplostemony represents an amalgamation of at least five different floral developmental pathways, all of them leading to the external positioning of the alternisepalous stamen whorl within a two-whorled androecium. In secondary obdiplostemony the antesepalous stamens arise before the alternisepalous stamens. The position of alternisepalous stamens at maturity is more external due to subtle shifts of stamens linked to a weakening of the alternisepalous sector including stamen and petal (type I), alternisepalous stamens arisingde factoexternally of antesepalous stamens (type II) or alternisepalous stamens shifting outside due to the sterilization of antesepalous stamens (type III: Sapotaceae). In primary obdiplostemony the alternisepalous stamens arise before the antesepalous stamens and are more externally from initiation. The antesepalous stamen whorl is staminodial and shows a tendency for loss (type I), or the petals are missing and the alternisepalous stamens effectively occupy their space (type II). Although obdiplostemony is often related to an isomerous gynoecium, this is not essential. Phylogenetically, both secondary and primary obdiplostemony can be seen as transitional stages from diplostemony to either haplostemony or obhaplostemony. Obdiplostemony is the consequence of shifts in the balance between the two stamen whorls, affecting either the alternisepalous stamens together with the petals, or the antesepalous stamens.
Conclusions:
We advocate a broad definition of obdiplostemony, to include androecia with incomplete whorls, staminodial whorls, anisomerous gynoecia and an absence of petals. As such, the taxonomic significance of obdiplostemony is transient, although it is a clear illustration of how developmental flexibility is responsible for highly different floral morphs.
The phylogenetic relationships of Croton section Cyclostigma sensu Webster were examined using nuclear ribosomal ITS and plastid spacer trnL-F sequences from 48 of the 63 species in the group, including members of the four recognized subsections and representing the group's full morphological and geographic range. The two gene phylogenies concurred that section Cyclostigma sensu Webster is polyphyletic, with its members appearing in nine different clades throughout Croton. The clade that retains the taxonomic type of the section and about half of the species sampled is redefined here as a reduced, monophyletic section Cyclostigma, and the remaining species are placed in other sections or informal clades. The ITS and chloroplast phylogenies are discordant regarding the position of the redefined section Cyclostigma suggesting that reticulate evolution might have played a role in the evolutionary history of this group. These results highlight that the tree habit has evolved on multiple occasions in Croton, and that there are other morphological characters that can effectively distinguish different tree lineages in the genus.
•
Phylogenetic and morphological studies have helped clarify the systematics of large and complex groups such as the tribe Crotoneae (Euphorbiaceae). However, very little is known about the diversity, structure, and function of anatomical features in this tribe. Crotoneae comprises the species-rich pantropical genus Croton and six small neotropical genera. Here we characterized the anatomy of leaf secretory structures in members of this tribe and explored their function and evolutionary significance.•
Young and mature leaves of 26 species were studied using standard anatomical light microscopy techniques. Three sections of Croton and one representative of Brasiliocroton and Astraea were sampled.•
We identified five types of secretory structures: laticifers, colleters, extrafloral nectaries, idioblasts, and secretory trichomes. Laticifers were present in all species studied except Croton alabamensis, which instead presented secretory parenchyma cells. Articulated laticifers are reported in Crotoneae for the first time. Colleters of the standard type were observed in the majority of the sampled taxa. Extrafloral nectaries were present in section Cleodora and in B. mamoninha, but absent in section Lamprocroton and Astraea lobata. Idioblasts were spread throughout the palisade and/or spongy parenchyma in most of the studied species. Secretory trichomes were restricted to Lamprocroton except for C. imbricatus.•
This study revealed a high diversity of secretory structures, including novel ones, in one of the largest clades of Euphorbiaceae. Our results are promising for investigations on the anatomical and ecophysiological bases of species diversification within Euphorbiaceae.
© 2015 Botanical Society of America, Inc.
Clusia fluminensis and C. lanceolata are dioecious shrubs having resiniferous flowers with strongly distinct androecia. The aim of this study was to investigate the development and anatomy of their androecia and the ultrastructure, histochemistry and secretory process of their androecium resin glands, looking at whether the cellular aspects of resin secretion differed between these two morphologically distinct androecia. Stamens differ by being free in C. fluminensis and clustered in a synandrium in C. lanceolata. Staminode sterility is due to the undifferentiated nature of the anthers in C. lanceolata and the degeneration of meiocytes and anther indehiscence in C. fluminensis. Resin is produced by subepidermal cavities and canals with wide lumens. In the secretory stage, epithelial cells present sinuous walls, voluminous nuclei, polymorphic plastids associated with periplastidial reticulum, mitochondria, oil bodies, multivesicular bodies, endoplasmic reticulum and dictyosomes. The resin is released through rupture points on the distal surface of stamens and staminodes, associated with disrupted cavities and canals. Our results show morphological diversity associated with functional similarity. Also, a secretion pattern shared by the two species, including the initiation of the secretory process in young floral buds, compartmentalisation of the secretion in pre-anthesis buds, and the release of secretions at anthesis. Cellular aspects of resin secretion in these species are quite similar, as are the chemical identities of the main components of the floral resins of the genus. This article is protected by copyright. All rights reserved.
This article is protected by copyright. All rights reserved.
The flower morphology and its relation to the pollination biology of four species of Clusia (Guttiferae) from the Gran Sabana (Guyana highland) belonging to three sections of the genus is described. The flowers of both genders of C. schomburgkiana (sect. Polythecandra) and of C. columnaris (sect. Omphalanthera) offer resin as a reward for small bees acting as pollinators. In both species the androecial morphology is highly derived. Functionally, this can be interpreted in terms of minimization of contact of the sticky resin and the pollen. Male flowers of C. pusilla and Clusia sp. of sect. Clusiastrum offer pollen as a reward for large, middle-sized and small bees, who vibrate the anthers to collect pollen. Female flowers are rewardless and unusual in the genus by the presence of long styles. The pollinating bees are probably deceived by the combination of papillose stigmas and the yellowish staminodes directly below these. The bees pollinate the stigmas when trying to vibrate them. The evolution of the flower biology in the genus Clusia is briefly discussed.
RIINA, R., I. CORDEIRO, A. M. AMORIM & P. E. BERRY (2010). Croton thomasii Riina & P. E. Berry (Euphorbiaceae), a new species from the Atlantic Forest in the State of Bahia (Brazil) and typification of Croton sapiifolius Müll. Arg. Candollea 65: 101–107. In English, English and French abstracts. Croton thomasii Riina & P. E. Berry (Euphorbiaceae), a new tree species from the Atlantic forest of southern Bahia, Brazil, is described and illustrated. It is compared with Croton sapiifolius Müll. Arg., previously known only from the type collection, for which a detailed description and a first illustration are given. A lectotype is designated for Croton sapiifolius. Taxonomical affinities of these two species are also briefly discussed using both morphological and molecular data.
The floral development of four species of the Nyctaginaceae is described. An involucrum surrounds the dichasial inflorescences of all species studied, except for Pisonia brunoniana. In the involucrum of Bougainvillea glabra three flowers are present while in Mirabilis longiflora and Mirabilis jalapa only a single flower is maintained. The five-membered perianth tube is interpreted as a calyx with a corolloid appearance. The anthocarp, which encloses the fruit, is considered to be the transformed lower part of the perianth. A reduction in stamen number is suggested within the Nyctaginaceae resulting in the five-membered androecium of Mirabilis longiflora and Mirabilis jalapa. Nectariferous tissue is associated with the filament bases. The nectarial zones are characterized as caducous, androphyllominous nectaries of the staminal type. The monocarpellate gynoecium is characterized by a leptate carpelform. An appendage, viz. the adaxial pistillary primordium bears the single basal ovule. Therefore the ovule cannot be regarded as a terminal structure. It is argued that the origin of the ovule is carpellate and not cauline.
Computed tomography remains strongly underused in plant sciences despite its high potential in delivering detailed 3D phenotypical information because of the low X-ray absorption of most plant tissues. Existing protocols to study soft tissues display poor performance, especially when compared to those used on animals. More efficient protocols to study plant material are therefore needed. Flowers of Arabidopsis thaliana and Marcgravia caudata were immersed in a selection of contrasting agents used to treat samples for transmission electron microscopy. Grayscale values for floral tissues and background were measured as a function of time. Contrast was quantified via a contrast index. The thick buds of Marcgravia were scanned to determine which contrasting agents best penetrate thick tissues. The highest contrast increase with cytoplasm-rich tissues was obtained with phosphotungstate, whereas osmium tetroxide and bismuth tatrate displayed the highest contrast increase with vacuolated tissues. Phosphotungstate also displayed the best sample penetration. Furthermore, infiltration with phosphotungstate allowed imaging of all plants parts at a high resolution of 3 µm, which approaches the maximum resolution of our equipment: 1.5 µm. The high affinity of phosphotungstate for vasculature, cytoplasm-rich tissue, and pollen causes these tissues to absorb more X-rays than the surrounding tissues, which, in turn, makes these tissues appear brighter on the scan data. Tissues with different brightness can then be virtually dissected from each other by selecting the bracket of grayscale to be visualized. Promising directions for the future include in silico phenotyping and developmental studies of plant inner parts (e.g., ovules, vasculature, pollen, and cell nuclei) via virtual dissection as well as correlations of quantitative phenotypes with omics datasets. Therefore, this work represents a crucial improvement of previous methods, allowing new directions of research to be undertaken in areas ranging from morphology to systems biology.
A taxonomic key, synonymy, and a molecular phylogeny of the nine species and three varieties of section (Euphorbiaceae) are presented. Section is synonymized with section , and the species from outside of North America that had been placed into these two sections are shown to belong in other clades within the genus. As circumscribed here, section is a well-supported clade of annual herbs and subshrubs that ranges from central Mexico to the eastern United States. A new combination, , is made. Lectotypes are designated for var. var. , and .
Spanish
Una clave taxonómica, sinonimia, y una filogenia molecular son presentados para las nueve especies y tres variedades de Croton sección Heptallon (Euphorbiaceae). Sección Angelandra se sinonimiza con sección Heptallon, y se demuestra que las especies de fuera de Norte América que habían sido incluidas en estas dos secciones pertenecen a otros clados dentro del género. Como está circunscrita aquí, sección Heptallon es un clado con buen apoyo de hierbas anuales y subarbustos que se distribuye desde el centro de México hasta el oriente de los Estados Unidos. Se hace una nueva combinación, Croton heptalon . Se designan lectotipos para C. corymbulosus, C. elliottii, C. ellipticus, C. engelmannii var. albinoides, C. eutrigynus, C. heptalon, C. leucophyllus var. trisepalis, y Heptallon graveolens.
The present paper is part of a study dealing with various aspects of reproduction of two Costa Rican Clusia species offering resin as a floral reward. It provides data on the floral development and flower (especially stamen and staminode) anatomy of one of the species, Clusia valerioi. In the early stages, both male and female flowers develop in the same manner. The bracts are distinguished by a decussate arrangement from the five sepals and five petals, which emerge in a spiral manner. In the male flowers the apical meristem forms five meristematic mounds (common stamen primordia) that are pentagonally arranged around the apical meristem in epipetalous position. From these mounds, the primordia of the proper stamina emerge in 3–5 whorls. Direction is centrifugal. In the centre, five hemispherical bulges arise which develop into carpel primordia. These, however, cease growth, stay rudimentary and are hidden by the stamens in the mature male flower. The adult stamens consist mainly of a thick angular filament column, while the two anthers situated at the flattened top are very small. One anther is annular and surrounds a second, hemispherical one right in the centre. At the periphery, these two pollen sacs (provided with a distinct wall of customary anatomy) are surrounded by a ring-like protuberance of the filament. The resin canals are situated at the periphery of the filament. Their schizogenous development is documented in cross sections. At anthesis, the resin is released from the ring-like filament protuberance by burst of the single-layered epidermis. In the female flower, the five meristematic mounds produce two whorls of staminode primordia. The development of the staminodes does not essentially differ from that of the fertile stamens, but some staminodes lack the central pollen sac and the other tissues do not develop into pollen grains. An attempt is made to derive the peculiar stamen morphology of Clusia valerioi and similar species from conventional stamens. Three hypotheses are proposed and discussed.
New observations are presented on the ontogeny, vasculature and morphology of both staminate and pistillate flowers of Croton and Astraea. These data support earlier hypotheses that the filamentous structures in pistillate flowers represent reduced and transformed petals. Staminate flowers of both genera possess five free nectaries, which are vascularised by divergences of the sepal traces in Croton and unvascularised in Astraea. In pistillate flowers, there are five separate non-vascularised nectaries in Astraea, but in Croton there is a single nectariferous disk that is vascularised by divergences of the sepal traces. The nectaries are initiated late in floral development, but their location indicates that they could represent the outer stamen whorl transformed into secretory staminodes. Other glandular structures occur in pistillate flowers of most Croton species, resulting in flowers with two secretory organ whorls. In these cases, the inner whorl is formed by modified staminodes. Our observations support the recent segregation of Astraea species from the larger genus Croton. Despite strong similarities between the two genera, there are clear structural differences, including the presence of colleters in Astraea (absent in Croton), moniliform trichomes on petals (rather than simple trichomes in Croton), non-vascularised nectaries (vascularised in Croton) and reduced, non-secretory filamentous structures (well developed and secretory in Croton).
KeywordsCrotoneae–Euphorbiaceae–Flowers–Inaperturate crotonoids–Nectary evolution–Obdiplostemony–Petals–Staminodes
The greater Caribbean region has played an important role in the early diversification of Croton L. (Euphorbiaceae s.s.). Jamaica is also important to Croton taxonomy because several of the earliest described species were based on material from the island. The Jamaican species of Croton are found in five distinct clades indicating that there were at least five separate over-water dispersal events of the genus to the island. Croton jamaicensis (section Eluteria Griseb.), a new species endemic to limestone hills along the southern coast of Jamaica, is described and illustrated. The species is phylogenetically most closely related to C. laurinus Sw. and C. grisebachianus Müll. Arg., both also endemic to Jamaica. Several lectotypifications and novel synonymies are required to clarify the taxonomy of the species of Croton that have been described from Jamaica, some of which are widespread in the region.
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La región del Caribe ha jugado un papel importante en la diversificación inicial de Croton L. (Euphorbiaceae s.s.). Jamaica también es importante para la taxonomia de Croton dado que varias de las primeras especies descritas fueron basadas en material de la isla. Las especies Jamaicanas de Croton se encuentran en cinco clados distintos indicando que hubieron por lo menos cinco eventos de dispersión sobre agua del género a la isla. Croton jamaicensis (sección Eluteria Griseb.), una nueva especie endémica a colinas de caliza de la costa sur de Jamaica, es descrita e ilustrada. La especie es filogenéticamente mas cercanamente relacionada a C. laurinus Sw. y C. grisebachianus Müll. Arg., ambas también endémicas a Jamaica. Varias lectotipificaciones y nuevas sinonimias son requeridas para clarificar la taxonomia de las especies de Croton que se han descrito de Jamaica, algunas de las cuales están ampliamente distribuidas en la región.
A recent molecular phylogenetic study of Mallotus and related genera showed that three small Asiatic genera are clearly part of a strongly supported main Mallotus clade (Mallotus s.s. clade), rendering the genus paraphyletic. In this paper these genera, Neotrewia, Octospermum and Trewia, are merged with Mallotus. The monotypic Neotrewia and Octospermum were originally described within Mallotus and are now transferred back to it. Trewia was never included in Mallotus; it has two distinct species, for which new combinations within Mallotus are made. A full taxonomic treatment with descriptions, distribution maps and drawings is given for the species transferred, and a new generic description for Mallotus is provided. The morphology of the newly transferred species in comparison to Mallotus in general and to related species in particular is discussed.
Croton (Euphorbiaceae) is a large and diverse group of plants that is most species-rich in the tropics. We update the infrageneric classification of the New World species of Croton with new evidence from phylogenetic analyses of DNA sequence data from all three genomes. The relationships of species that were previously placed in conflicting positions by nuclear and chloroplast data, such as C. cupreatus, C. poecilanthus, and C. setiger, are further resolved by adding the nuclear EMB2765 and mitochondrial rps3 genes to the molecular sampling. Analyses of rps3 reveal an accelerated rate of evolution within Croton subg. Geiseleria, the only one of the four subgenera that contains numerous herbaceous, annual species. We provide morphological descriptions, species lists, and a key to the 31 sections and 10 subsections recognized in the New World. New taxa that we describe include C. sects. Alabamenses, Argyranthemi, Cordiifolii, Corinthii, Cupreati, Luetzelburgiorum, Nubigeni, Olivacei, Pachypodi, Prisci, and C. subsects. Cubenses, Jamaicenses, and Sellowiorum. Additional transfers are made to the ranks of subgenus, section, and subsection. A total of 712 species of Croton are currently recognized for the New World, with 702 of them assigned here to section.
This review based on a morphological and developmental perspective reveals a striking diversity in shapes and evolutionary trends in the gynoecium of core Caryophyllales that have affected the number of carpels, the formation of septa and the number of ovules. Two major developmental shifts are responsible for the diversity in gynoecial forms and are linked to the proportional development of carpellary tissue (ovary wall) and the floral apex. (1) Meristic change is caused by an expansion or reduction of the diameter of the floral apex. An expansion leads to polygyny linked with the development of more numerous small carpels; a reduction of space leads to lower carpel numbers, eventually resulting in a single carpel. (2) Different ovary shapes can be recognized at a mid-developmental stage predicting the further development of ovaries. With an equal growth of the ovary wall and floral apex, young ovaries take the shape of a salt-shaker; with more extensive development of the floral apex and delay of the ovary wall, a club-shaped ovary is formed; with faster growth of the ovary wall linked with intercalary expansion and a delayed growth of the floral apex, a (half-) inferior cup-shaped ovary develops. The different growth forms are the results of heterochronic shifts and affect the development of septa and ovule numbers. A common trend in the order implies a weakening and break-up of septa during development, leading to residual apical and basal septa and the shift to free-central and basal placentation. The club-shaped ovary is linked with an almost complete loss of septa and a reduction of the ovule number to one. The salt-shaker shape leads to ovaries with a massive placental column and several ovules. The cup-shaped ovary leads to a shift of ovules away from the floral apex. Developmental flexibility is responsible for a disconnection of carpel wall growth from ovular tissue. Subtle shifts in proportional growth lead to a high diversification of ovaries in core Caryophyllales and the establishment of predictable developmental trends. These trends clearly represent apomorphic tendencies, affecting different families of core Caryophyllales in different degrees. The ancestral gynoecium was probably pentamerous and isomerous with the other floral whorls, with ovules clearly separated from the carpellary wall and inserted on axile placentas corresponding to the central axis of the flower.
Croton section Geiseleria encompasses 82 species in the Americas, but its center of origin and diversity is in Brazil, where 53 species are distributed especially in areas of Cerrado, in the central part of the country. Taxonomic knowledge of several taxa of Croton section Geiseleria is still scarce, and they are frequently misidentified in Brazilian herbarium collections due to their very similar morphology. During a taxonomic study of this section in the Midwest region of Brazil, a new species was found in the state of Mato Grosso. Croton rhodotrichus is herein described, illustrated, and its relationships with the most morphologically similar species, C. glandulosus and C. hirtus, are discussed. Additionally, distribution, habitat, flowering, fruiting, and conservation status of the new species, as well as a key to identify all the species of Croton section Geiseleria present in the state of Mato Grosso are provided.
Despite the well‐known visual attraction function of angiosperm petals, additional roles of these floral organs (e.g. the provision of landing‐site platforms for pollinators) have rarely been examined. This is likely because most petals perform multiple functions, making it difficult to isolate the importance of landing sites in pollination success.
We investigated the landing‐site function of dull‐coloured pinnately branched petals in Mitella pauciflora flowers, which are predominantly pollinated by fungus gnats. We conducted a field experiment, in which the effects of experimental petal removal on pollinators’ approach, landing and visit duration and floral reproductive success were examined in naturally pollinated flowers.
According to direct and time‐lapse camera observations, petal removal did not influence pollinators’ approach frequency or visit duration, but did significantly decrease their landings. Fruit set and pollen dispatch both significantly decreased with petal removal, indicating that petals promote female and male reproductive success in M. pauciflora by facilitating pollinator landing.
This demonstrates that inconspicuous petals primarily have a landing‐site function rather than a visual attraction function in M. pauciflora . Discriminating between diverse petal functions is a challenging problem, and new approaches are required to elucidate the functional features of angiosperm flowers.
A lay summary is available for this article.
Floral morphology and vascular anatomy have been studied in 4 species of the subfamily Rivinoideae (Phytolaccaceae). The proposed model of the vascularization pattern of the perianth is characterized by the tendency towards different types of fusion among the main bundles. Apparently, the multistaminate androecium represents the ancestral type. Determination of number and position of the stamens resulted in the origin of the polycyclic androecium, that by the way of reduction developed into the haplostemonous one. The gynoecium is truly monomerous, but it has descended from a syncarpous one. The comparison of the vascular systems in the carpels of several species has proved once more the appendicular nature of the placenta. The concept of the “Apical Septae” (Hartl 1962) is not applicable to the Phytolaccaceae gynoecium. The obtained conclusions do not confirm the theory of secondary multiplication of floral parts in Phytolaccaceae and the hypothesis of the primitiveness of the family within the centrosperms.
Radcliffea, a new genus of Euphorbiaceae sensu stricto from western Madagascar with one species, R. smithii, is described and illustrated, and an IUCN conservation rating is suggested. Its affiliation with Crotonoideae sensu stricto is supported by crotonoid inaperturate pollen morphology and DNA sequence data.
Monoecious or dioecious trees, shrubs, or herbs, sometimes succulent or scandent; stems with or without laticifers; indumentum simple, malphighiaceous, stellate, or lepidote, sometimes glandular or 0. Leaves alternate, spiral or distichous, less commonly opposite or whorled; leaf blades simple to palmately lobed or compound, pinnately to palmately nerved, sometimes with basal, laminar, or marginal glands; stipules persistent or deciduous, sometimes reduced or 0. Inflorescences axillary or terminal, sometimes cauliflorous, thyrsoid, paniculate dichasial, glomerulate or synanthial, or flowers solitary; bracts sometimes colored, glandular, or hypertrophied. Flowers unisexual, usually actinomorphic; perianth segments distinct or connate, imbricate to valvate, rarely obsolete or suppressed; sepals (1–)3–6(−10), entire or lobed, sometimes glandular, the pistillate persistent or deciduous in fruit; petals + or 0, greenish to white, yellow, pink, or red; disk + or 0, extrastaminal or less commonly interstaminal, entire to toothed, lobed, or dissected; stamens (1–)3–50(−400), filaments distinct or connate; anthers mostly 2-locular, dehiscing longitudinally or laterally, extrorse or less commonly introrse; gynoecium syncarpous, ovary (1)2–5(−20)-locular; placentation axile (basal); ovules 1 or 2 per locule, anatropous or hemitropous, epitropous, usually inserted beneath an obturator. Fruits mostly schizocarpic, splitting from the columella, commonly explosively dehiscent, mesocarp often separating from endocarp, sometimes baccate or drupaceous. Seeds often carunculate; seed coat mostly exotegmic; endosperm copious, often fleshy, or 0, sometimes oily; embryo straight, curved or plicate.
Acalypha (Euphorbiaceae) consists of approximately 450 species. Acalypha indica L. is a common annual herb (1-3 ft high). The plant develops a peculiar terminal monocarpellary female flower at the tip of inflorescence axis, it is known as allomorphic flower. Actually, the species produces two kinds of female flower, viz. the allomorphic flower and lateral, tricarpellet regular euphorbiaceous (female) flower. Interestingly, two kinds of female flowers exhibit two opposite mode and direction of evolutionary trends. The lateral flower shows regressive evolutionary trend while the terminal flower shows the trend of progressive evolution. The allomorphic flower, from either progressive or regressive view, stands in a different state of evolution from other flowers; this is a case of heterobathmy. And, both kinds of flowers are under neutral selection pressure, and the development of unicarpellary flower in this Euphorbiacean member is a kind of neutral evolution.
Phenetic analysis of the New World Jatropha L. species generally supports the 1979 classification of the genus by Dehgan and Webster. The distinctness of subgenera Curcas (Adans.) Pax and Jatropha, the intermediate position of section Polymorphae Pax between sections of both subgenera, and the close relationship of sections Mozinna (Ortega) Pax and Loureira (Cav.) Muell. ex Pax are indicated. Phylogenetic analysis provided evidence of monophyly for subgenus Jatropha, sections Jatropha and Mozinna, and probable paraphyly in subgenus Curcas and several sections and subsections. The cladistic analyses described herein produced multiple parsimonious topologies. The circumscriptions of the subgenera and some sections and subsections are significantly clarified. The status of heretofore dubiously placed or recently described species is also elucidated. Although geographical data were not included in the phylogenetic analyses, a distinct correlation between evolutionary trends in morphological features, postulated infrageneric delimitations, and geography of the genus became evident. Evidence is presented to support the antiquity of the genus and its present distribution, which resulted from a Gondwanaland breakup and subsequent overland dispersal across the African and American continents. Adaptive gradual mosaic evolution as a series of successive speciational steps, primarily involving morphological features, in concert with migration to areas of increasing aridity and cold appears to be the norm for the genus.
A short account is given of allomorphic flowers in the genus Acalypha, with especial reference to the Series Polygynae-Pleurogynae.
Summary A new classification of the suprageneric taxa of Euphorbiaceae is presented in outline, including a key to the five subfamilies recognized. Latin descriptions are provided for seven new tribes and ten new subtribes.
Webster, G. L.: A provisional synopsis of the sections of the genus Croton (Euphorbiaceae). – Taxon 42: 793‐823. 1993. – ISSN0040‐0262.
The 19th century classification of Croton by Müller Argoviensis is highly artificial. A revised system that incorporates the sections proposed by Baillon and Grisebach is presented, with a key to the 40 sections recognized. For each section, types and synonymy are indicated, along with a description and list of representative species. Described as new are 2 sections (C. sect. Anadenocroton, sect. Corylocroton) and 3 subsections (C. subsect. Cuneati, subsect. Matourenses, subsect. Sampatik); 2 new sectional names (C. sect. Argyrocroton, sect. Luntia) represent changes in rank.
Herbs, shrubs, more or less woody climbers, or trees. Leaves alternate (opposite in Gisekia), simple, entire, usually petiolate; stipules absent (but sometimes prophylls of axillary bud resembling stipules; transformed into spines in Seguieria). Inflorescences mostly indeterminate (determinate in Seguieria and Agdestis), most frequently racemes or spikes. Flowers small, bisexual (unisexual in Monococcus and a few species of Phytolacca and Ledenbergia, plants usually dioecious), mostly actinomorphic (± weakly zygomorphic in Hilleria and Anisomeria), hypogynous (nearly epigynous in Agdestis). Perianth simple, tepals 4 or 5, free (slightly connate in Hilleria), imbricate, inconspicuous to petaloid, mostly greenish to whitish, less often yellow or reddish. Stamens (2−)4 to many, free; anthers dorsifixed, tetrasporangiate, mostly more or less linear (subglobose in Microtea), at the base incised, often also at the tip, opening by longitudinal slits. Carpels 1–17, arranged in one whorl (inserted on a gynophore in Nowickea), free or more or less united; styles usually free or absent (united in Agdestis); stigmas free. Ovules one per carpel (in Microtea and Lophiocarpus one per ovary), campylotropous, bitegmic, crassinucellate, basal or nearly so in single carpels, or axile in syncarpous ovaries. Fruit various, indéhiscent (a capsule in Barbeuia). Seed with a crustaceous or membranaceous testa (arillate in Barbeuia); perisperm copious to lacking in the mature seed; embryo curved.
This monograph presents a taxonomic revision of the neotropical species of the tribe Alchorneae (Hurus.) Hutch., which consist of the genera Alchornea Sw., Aparisthmium Endl. and Conceveiba Aubl. In Alchornea 23 species have been recognized for the neotropics. Alchornea websterii R. Secco is described as a new species and a status novum, A. glandulosa subsp. iricurana (Casar.) R. Secco is proposed. Aparisthmium is a monotypic genus with the only species A. cordatum, which presents a wide geographic distribution in South America. Conceveiba has 13 species in the neotropics. The genera Conceveibastrum and Gavarretia have been considered as synonym of Conceveiba. A discussion on the identity and systematic position of Polyandra Leal is presented. Alchornea leptogyna Diels, A. pavonioeides Posada-Ar., A. valleana Croizat and Conceveiba prealta (Leal) Punt ex J. Murillo have been considered as doubtful species. Data on the morphology of the species, including habit, indument, leaves, inflorescences, flowers, pollen, anatomy, fruits and seeds, as well as a brief chapter about the dispersal, pollination, sexual systems, seedlings, chromosomes, chemistry, ethnobotany and conservation is presented. The center of diversification of Alchornea is in Colombia, where 16 species occur. Alchornea has seven species with wide patterns of distribution, 12 species with restricted patterns of distribution and four species with very restricted distribution. Conceveiba presents three species with wide geographic distribution patterns, seven species with restricted geographic distribution patterns and three species with very restricted geographic distribution patterns. Some hypothetical evolutionary trends were found for the studied genera: in the case of Alchornea, the leaf pattern of venation, which splits up the genera in two species groups: I. With pinnate venation, including the patterns brochydrodomous or slightly eucamptodromous, and II. With venation actinodromous. In the case of Conceveiba, simple/stellate hair, 20-60 stamens free or slightly connate at the bases and without staminodes, 8-15 stamens, connate at the bases, located external to the staminodes, and inflorescences with 1-3 flowers/inflorescences with flowers in glomerules. In the case of Aparisthmium, stamens 8-10/stamens 3-5. /// Esta monografia trata da revisão taxonômica das espécies neotropicais da tribo Alchorneae (Hurusawa) Hutchinson, que compõe-se dos gêneros Alchornea Sw., Aparisthmium Endl. e Conceveiba Aubl. Em Alchornea foram reconhecidas 23 espécies. Alchornea websterii R. Secco é uma nova espécie aqui proposta para o Equador. É proposto um status novum, A. glandulosa subsp. iricurana (Casar.) R. Secco. Aparisthmium constitui-se de apenas uma espécie, A. cordatum, e apresenta ampla distribuição na América do Sul. Conceveiba apresenta 13 espécies. Os gêneros Conceveibastrum e Gavarretia foram considerados sinônimos de Conceveiba. Uma discussão sobre a identidade e posição sistemática de Polyandra Leal é apresentada. Alchornea leptogyna Diels, A. pavonioeides Posada-Ar., A. valleana Croizat e Conceveiba prealta (Leal) Punt ex J. Murillo foram consideradas espécies duvidosas. Dados sobre a morfologia das espécies, incluindo hábito, indumento, folhas, inflorescências, flores, pólen, anatomia, frutos e sementes, bem como um resumo sobre dispersão, polinização, sistemas sexuais, plântulas, cromossomos, química, etnobotânica e conservação das mesmas são apresentados. Alchornea apresenta três padrões de distribuição geográfica: espécies amplamente distribuídas (sete), espécies de distribuição restrita (12) e espécies de distribuição muito restrita (quatro endemismos). O centro de diversidade genética de Alchornea está na Colômbia, onde ocorrem 16 espécies. Conceveiba apresenta três espécies com padrões de distribuição amplos, seis espécies com distribuição restrita e duas espécies com distribuição muito restrita. Algumas tendências evolutivas, de caráter hipotético, foram assinaladas para os gêneros estudados, destacandose: no caso de Alchornea, o padrão de venação foliar, que divide o gênero em 2 grupos de espécies (I. com venação pinada, incluindo os padrões broquidódromo ou levemente eucamptódromo e II. com venação actinódroma). No caso de Conceveiba, tricomas simples/estrelados, 20-60 estames livres ou levemente concrescidos nas bases e sem estaminódios/8-15 estames, concrescidos nas bases, dispostos externamente aos estaminódios, e inflorescências com flores isoladas, aos pares ou em tríades/inflorescências com flores dispostas em glomérulos. No caso de Aparisthmium, estames 8-10/estames 3-5.
Localization of the stamens can be approached by a preliminary distinction between two characters, oligomery and polymery, occurring in two different groups of taxa, respectively the oligomerous complex and the polymerous complex. Oligomery is described by four character states standing in a close semophyletic relationship: diplostemony, obdiplostemony, haplostemony and obhaplostemony. Each character state is analysed for its distribution and systematic value. Diplostemony is the synapomorphic character state for the oligomerous line and has arisen once from a polymerous ancestor or in parallel in different lines. Obdiplostemony arises ontogenetically in three different ways. Loss of one whorl leads either to obhaplostemony, or haplostemony; both character states are believed to represent evolutionary steps of noreturn. Secondary increases and reductions of the stamens within a whorl are seen as expressions of the intrinsic variability of the character states and should not be homologized with them. Stamen numbers can be increased by the building-up of complex primordia or by secondary receptacular growth. Reductions of stamens affect one or two whorls of stamens and are caused by lack of space, interactions with the gynoecium and zygomorphy. The distribution of the different character states of oligomery is presented on Dahlgrenograms and the androecia of a number of families and their relationships are discussed. The interactions between oligomery and polymery are analysed as guidelines for a global phylogeny of the Magnoliatae.
Floral development and anatomy were investigated in the monotypic Aextoxicaceae (one species: Aextoxicon punctatum), one of two families of Berberidopsidales, to understand its putative relationship with Berberidopsidaceae and clarify floral evolution in basal core eudicots. Aextoxicon is dioecious, with unisexual flowers that display a late abortion of male and female organs in the respective genders. Flowers are pentamerous or hexamerous and are enclosed by a calyptra derived from the congenital fusion of two bracteoles. Initiation sequence and number of sepals, petals, and stamens are variable and mostly spiral, without clear transition between organ categories, and a single carpel initiates without evidence of a second carpel. The ovary contains two apical-marginal ovules. Floral anatomical data suggest that the nectaries are receptacular in nature. Aextoxicon displays floral characters found in basal angiosperms, such as a unicarpellate gynoecium, an interrupted hoodlike outer integument, little differentiation between sepals and petals, and a transition of a decussate to spiral phyllotaxis at the level of sepals. These data indicate that Aextoxicon, together with Berberidopsidaceae, might represent a basal transitional lineage of basal to core eudicots and evidence that pentamerous flowers with a biseriate perianth evolved from progenitors with a comparable floral structure.
The ecologically similar species Crotonopsis elliptica and Croton monanthogynus were described by Steyermark in his Flora of Missouri as inhabitants of acid rock outcrop communities (sandstone, granite, chert) in the Ozarks but not of limestone outcrops. However, field observations and transect data confirm that C. monanthogynus can be abundant in Ozark limestone outcrop communities, codominating with Isanthus brachiatus and Heliotropium tenellum in the 5 cm to 11 cm soil depth zone, while almost never occurring on sandstone outcrops. Greenhouse experiments revealed that C. elliptica was healthy on sandstone soil, but stunted and sickly on limestone outcrop soil, consistent with its absence from calcareous substrates. In contrast, C. monanthogynus was healthy on both soils, and had greater growth on limestone than on sandstone soil. Contrary to Steyermark's characterization, C. monanthogynus grows abundantly on limestone soils, but is rare or absent on Ozark sandstone outcrops, not because of substrate intolerance, but probably because of competing species there.
Croton sección Crotonopsis es muy distintiva dentro del género. Es uno de los pocos grupos del género que son endémicos a los Estados Unidos. Aunque las relaciones filogenéticas de sección Crotonopsis aún no se han establecido precisamente en los estudios moleculares, está claramente ubicada dentro de Croton. Hipotetizamos que puede formar un clado con algunas de las otras secciones que se encuentran en Norte América y que tienen tricomas lepidotos y estilos multífidos. Las dos especies que han sido reconocidas en la sección son muy similares morfológicamente; por lo tanto proponemos que sean incluidas como variedades de una sola especie y formalizamos sus nombres, a saber Croton michauxii var. michauxii y la nueva combinación C. michauxii var. elliptica. Se presenta una clave para distinguir las dos variedades.
A new arborescent species of Croton, C. megalocarpoides Friis & Gilbert, is described from the coastal semi–evergreen forest and bushland in S Somalia and E Kenya. Its taxonomic affinity with other African species is demonstrated, particularly the relationship to C. megalocarpus Hutch., a montane E African species with which it has long been confused, and to C. mayumbense J. Léonard, and C. mubango Mull. Arg., two species from lowland forest and forest–savannah mosaic in W Zaire and surrounding countries. The phytogeographical position of the new species is briefly discussed.
We examined the embryological development of anthers and ovules from Astraea (A. lobata and A. praetervisa) and Croton (C. floribundus, C. fuscescens, C. glandulosus, C. lundianus, C. piptocalyx, C. urucurana and C. triqueter) focusing on features with systematic significance for the group. Some of these features are common in Euphorbiaceae including: a dicotyledonous type of anther wall formation, a secretory tapetum, a mixed origin of the outer ovule integument, an epidermal origin of the inner ovule integument, the occurrence of many archesporial cells inside the ovules and a megagametophyte of the Polygonum-type. Other features, such as the presence of styloid crystals in the tapetum, an idioblast with a druse in the endothecium, simultaneous and successive microsporogenesis, and a functional micropylar megaspore, have not previously been reported in the family. These characters appear to distinguish Croton and Astraea from other Euphorbiaceae and possibly represents autapomorphies for the tribe Crotoneae.