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).

Developmental sequence of the syncarpous gynoecium of Sarracenia purpurea (Sarraceniaceae) with flattened and broadened carpels and an umbrella-shaped style

The umbrella-shaped style of Sarracenia has a flattened and broadened distal half forming an umbrella canopy, and a slender cylindrical proximal half forming an umbrella stalk. The developmental sequence that gives rise to this unique structure has never been studied in detail. Data from light microscopy and scanning electron microscopy showed that the five carpels are initiated as discrete primordia, which then undergo congenital fusion and conduplicate folding and become a pentagonal syncarpous gynoecium. The distal region of the carpel then bends abaxially and undergoes significant expansion via a marginal meristem, forming the umbrella shape. Carpel closure is achieved via postgenital fusion at both transverse and longitudinal slits. Each of the five pollen tube transmitting tracts is enclosed by the adaxial surface of the carpel, and the inner epidermis of the umbrella canopy represents the expanded abaxial surface of the carpels, whereas the outer epidermis represents the expanded distal region of the fused carpellary margins. Epidermal trichomes develop first, then secretory glands and stomata appear later at the same stage on the umbrella canopy. This study provides insights into the evolution of the umbrella-shaped style utilizing both common and specialized carpel developmental programs with a novel spatial and temporal pattern.

Floral development in three species of Acacia (Leguminosae, Mimosoideae)

The complete sequence of floral development in three species of Acacia was analysed. These species were sampled from each of the three Acacia subgenera. The species were Acacia berlandieri Benth. (subg. Aculeiferum), A. pennatula (Schltdl. & Cham.) Benth. (subg. Acacia) and A. saligna (Labill.) H.L.Wendl. (subg. Phyllodineae). The aim of the study was to determine whether the different subgenera share developmental pathways during flower formation. This study showed that development in the genus Acacia is heterogeneous. Each species studied showed different inception patterns of the calyx and androecium, whereas the inception patterns of the corolla and gynoecium were similar. These differences of inception in the calyx are not necessarily constant within each subgenus. Nevertheless, each subgenus was differentiated on the basis of inception patterns of the androecium, and other features such as the presence or absence of congenital or postgenital fusion in the calyx and corolla, and the time of differentiation of calyx and corolla tubes and the style.

Floral development of Rosa setigera

The development of the flower of Rosa setigera from initiation to the onset of anthesis is described. Rosa setigera is the only known member of the genus Rosa to exhibit dioecy. Flowers of functionally staminate (male) and functionally carpellate (female) plants appear identical, a condition referred to as cryptic dioecy. Discrete sepals and petals are formed on the floral meristem. As the hypanthium forms, stamens are initiated in alternating whorls on the wall of the hypanthium and continue to develop as the hypanthium extends. Carpel primordia arise individually on the remainder of the floral meristem and show neither adnation to the hypanthial wall nor coalescence to one another as they give rise to the styles and stigmas that are exserted above the hypanthium lip. The only observable fusion in this species appears to be the postgenital fusion of the margins of the carpel primordia to form the enclosed locule. Although historically the hypanthium has been variously interpreted as either axial and (or) appendicular in nature, resulting from congenital fusion of sepals, petals, and stamens, this paper uses a more realistic, testable and functional approach to the development of the hypanthium that is in keeping with current concepts such as process morphology. Key words: Rosa setigera, dioecy, floral development, fusion, hypanthium.

Flower development in Amelanchier alnifolia (Maloideae)

The development of the flower of Amelanchier alnifolia from initiation to the onset of anthesis is described. Sepals are formed sequentially, but interprimordial zonal growth results in the initiation of the hypanthium. Petals and stamens arise in whorls around the floral meristem as the hypanthium extends. They show neither coalescence nor adnation and do not appear to contribute to the development of the hypanthium. Gynoecial primordia arise individually, give rise to the styles and stigmas, and are joined basally by zonal growth to produce the roof of the ovary. The wall of the inferior ovary is interpreted as a gynoecial hypanthium. It is difficult to determine the extent to which the gynoecial primordia contribute to the development of the ovary. They do not give rise to most of its structure but may be responsible for the initiation of the ovules. There is evidence of postgenital fusion of the septal margins as they converge in the centre of the ovary. The timing of events in floral development is recorded for the locality of the study. The observations are discussed in relation to current theories concerning the nature of the inferior ovary. Key words: Amelanchier, flower, development, inferior ovary, hypanthium.

Fleurs pléiomères et synanthie chez Hyacinthus orientalis

A score of pleiomerous flowers of Hyacinthus orientalis L. are described. About a quarter of these unquestionably result from synanthy. Morphology shows that, in some of the specimens, synanthy is caused by ontogenetic (postgenital) fusion. If a similar origin is admitted for certain nearly normal specimens encountered in this study, it would be another indication that the end product of synanthy may be an entity simulating a simple flower.