The Schoenus Spikelet: a Rhipidium? A Floral Ontogenetic Answer

The inflorescence unit of Schoenus nigricans and S. ferrugineus consists of a zigzag axis and distichously arranged bracts, each of which may or may not subtend a bisexual flower. Each flower seems to terminate a lateral axis. These features have led to a controversy about the nature of the inflorescence unit, particularly whether it is monopodial or sympodial. It was often seen as a pseudospikelet composed of a succession of lateral axes, each subtended by the prophyll of the previous axis, as in a rhipidium. Many authors, however, consider the inflorescence units of all Cyperaceae to be indeterminate, racemose, actual spikelets. In our study, we present new SEM observations on the floral ontogeny of S. nigricans, corroborating a monopodial interpretation of the spikelet. Concaulescent growth of the flower primordium and the spikelet apex explains: (1) the presence of a peduncle under the flower, (2) the advanced development of the subtending glume compared to its own flower primordium, and (3) the position of the distal glume with regard to the distal flower primordium

Verhuellia is a segregate lineage in Piperaceae: more evidence from flower, fruit and pollen morphology, anatomy and development

Background and Aims The perianthless Piperales, i.e. Saururaceae and Piperaceae, have simple reduced flowers strikingly different from the other families of the order (e.g. Aristolochiaceae). Recent molecular phylogenies proved Verhuellia to be the first branch in Piperaceae, making it a promising object to study the detailed structure and development of the flowers. Based on recently collected material, the first detailed study since 1872 was conducted with respect to morphology, anatomy and development of the inflorescence, pollen ultrastructure and fruit anatomy. Methods Original Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Light Microscopy (LM) observations on Verhuellia lunaria were compared with Piperaceae, Saururaceae and fossils. Key results The inflorescence is an indeterminate spike with sessile flowers, each in the axil of a bract, developing in acropetal, helical succession. Flowers consist of two (occasionally three) stamens with basifixed tetrasporangiate anthers and latrorse dehiscence by a longitudinal slit. The gynoecium lacks a style but has three to four stigma branches and a single, basal orthotropous, and unitegmic ovule. The fruit is a drupe with large multicellular epidermal protuberances. The pollen is very small, inaperturate, and areolate with hemispherical microechinate exine elements. Conclusions Despite the superficial similarities with different genera of Piperaceae and Saururaceae, the segregate position of Verhuellia revealed by molecular phylogenetics is supported by morphological, developmental and anatomical data presented here. Unitegmic ovules and inaperturate pollen, which are synapomorphies for the genus Peperomia, are also present in Verhuellia

Different ways to obtain similar results: the development of the corolla and epipetaly in Rubieae (Rubioideae, Rubiaceae)

Background and aims – Rubieae is a tribe in the subfamily Rubioideae characterised by herbaceous plants with verticillate leaves and flowers with a rudimentary or absent calyx and a short, cup-shaped corolla. This is in contrast to the flowers of most other Rubiaceae, in which the tubular corolla is longer than the corolla lobes. Also, the description by Payer, a French 19th century pioneer of floral ontogenetic research, of the floral development in Asperula, Galium, and Rubia deviates from recent insights about the development of tubular corollas, which are based on investigations of flowers of tropical Rubiaceae. Tubular corollas are currently considered as resulting from the development of underlying annular intercalary meristems, whereas Payer explained the tubular corollas in the three taxa by postgenital fusion. We therefore tested both hypotheses in six Rubieae genera, including the three taxa studied by Payer.Methods – Floral ontogeny of ten species in six Rubieae genera based on scanning electron (SEM) and light microscopy (LM). Conclusions – Our results suggest that, in all species studied, the mature phenotype of the corolla as well as the epipetaly of the stamens is caused by a combination of three developmental processes (the development of a stamen-corolla tube, the development of a corolla tube sensu stricto, and postgenital fusion), and the relative moment of activation of each of these processes during floral development (plastochron variation or heterochrony)

Two new species of Phialiphora (Spermacoceae, Rubiaceae) exemplify drought adaptations in western Madagascar

Background – Phialiphora (Spermacoceae, Rubiaceae) is a herbaceous genus restricted to unconsolidated white sands in open spaces in dry forests in northwestern Madagascar. The genus is characterized by the semi-succulent nature of its leaves and stems, its basal leaf rosette, its head-like inflorescences subtended by large leaf-like involucrate bracts and its isostylous flowers. Hitherto, only two species were described.Methods – Classical methods of herbarium taxonomy are followed.Key results – Two new Phialiphora species, P. glabrata De Block and P. valida De Block, are described, which brings the number of species in the genus to four. The two new species differ from the two previously described species by their glabrous branches, bracts, ovaries, calyces and fruits. They are easily distinguished from each other by the shape and size of the calyx lobes. A detailed description, illustration, distribution map and preliminary conservation assessment is given for the new species, which are assessed as critically endangered. We also present a key for the genus and discuss in detail the typical branching system, the capsules and the dispersal mechanism of Phialiphora. The description of these two new species exemplifies the uniqueness of the dry vegetations on unconsolidated white sands in western Madagascar, which are characterized by a high level of endemicity

Floral ontogeny links Dialypetalanthus (Condamineeae) with the floral developmental morphology of other Rubiaceae

Background – Vegetative and fruit characters of the Amazonian genus Dialypetalanthus point to a position in Rubiaceae. However, its floral morphology is so deviant that the genus was often placed in a family of its own. Even relationships outside Gentianales were postulated. Current molecular phylogenetic studies firmly show that Dialypetalanthus belongs to Rubiaceae. Aims – This study aims to understand the idiosyncratic floral morphology in Dialypetalanthus and to compare it with the floral development in two other Condamineeae genera as well as in other Rubiaceae for which ontogenetic data are available. Material and methods – SEM and LM based floral ontogeny in Dialypetalanthus fuscescens, Mussaendopsis beccariana, and Pogonopus exsertus. Results and main conclusions – Flowers in Dialypetalanthus develop a stamen-corolla-calyx tube, which can be considered as a floral morphological link between the genus and the other Rubiaceae. The polyandrous androecium originates from an annular intercalary meristem at the adaxial side of the stamen-corolla-calyx tube

Development of reproductive organs in Canephora madagascariensis (Octotropideae - Rubiaceae)

Background and aims – Ontogenetic studies provide valuable morphological characters to aid interpretation of evolutionary scenarios. But they are rare within the Rubiaceae and the variation in floral ontogenetic patterns in the different Rubiaceae lineages remains underexplored. Here, we provide the first developmental study of the reproductive organs of Canephora madagascariensis, a typical representative of the poorly known tribe Octotropideae in most of its characters except for its unique, flattened, assimilating, 'phylloclade-like' inflorescences. Methods – Using SEM and LM we produced a developmental study of inflorescence, flower and fruit in C. madagascariensis . Key results – The inflorescence of Canephora madagascariensis is a condensed dichasium. Peduncle, first-order bracts and first-order branches together form a flattened, green, phylloclade-like structure, showing both leaf and stem characters. Initially, the dome-shaped floral apex becomes concave and sepals are formed. Next, from the collar of the concave apex, a tubular corolla with epipetalous stamens grows out. Meanwhile, an inferior bilocular ovary is formed. Per locule, up to seven unitegmic ovules are formed on a U-shaped placenta. Fruits are drupes with one to several fibrous seeds with folded exotesta. Conclusions – The double-organ identity of the green, flattened part of the inflorescence is due to a modification leading to leaf analogy. The peculiar inflorescence of C. madagascariensis constitutes just another state of the character 'axillary inflorescence' in Rubiaceae. It results from both reduction and congestion, two trends typical in Rubiaceae inflorescences. Floral cups form the basis of the floral structure in Canephora . The inferior ovary is due to the initial formation of a hypanthium, from which calyx lobes, stamen-corolla tube, corolla tube sensu stricto and calyx tube originate through successive outgrowth of underlaying annular intercalary meristems

Development of reproductive organs in Canephora madagascariensis (Octotropideae - Rubiaceae)

Background and aims – Ontogenetic studies provide valuable morphological characters to aid interpretation of evolutionary scenarios. But they are rare within the Rubiaceae and the variation in floral ontogenetic patterns in the different Rubiaceae lineages remains underexplored. Here, we provide the first developmental study of the reproductive organs of Canephora madagascariensis, a typical representative of the poorly known tribe Octotropideae in most of its characters except for its unique, flattened, assimilating, 'phylloclade-like' inflorescences. Methods – Using SEM and LM we produced a developmental study of inflorescence, flower and fruit in C. madagascariensis . Key results – The inflorescence of Canephora madagascariensis is a condensed dichasium. Peduncle, first-order bracts and first-order branches together form a flattened, green, phylloclade-like structure, showing both leaf and stem characters. Initially, the dome-shaped floral apex becomes concave and sepals are formed. Next, from the collar of the concave apex, a tubular corolla with epipetalous stamens grows out. Meanwhile, an inferior bilocular ovary is formed. Per locule, up to seven unitegmic ovules are formed on a U-shaped placenta. Fruits are drupes with one to several fibrous seeds with folded exotesta. Conclusions – The double-organ identity of the green, flattened part of the inflorescence is due to a modification leading to leaf analogy. The peculiar inflorescence of C. madagascariensis constitutes just another state of the character 'axillary inflorescence' in Rubiaceae. It results from both reduction and congestion, two trends typical in Rubiaceae inflorescences. Floral cups form the basis of the floral structure in Canephora . The inferior ovary is due to the initial formation of a hypanthium, from which calyx lobes, stamen-corolla tube, corolla tube sensu stricto and calyx tube originate through successive outgrowth of underlaying annular intercalary meristems