Then There Were Plenty-Ring Meristems Giving Rise to Many Stamen Whorls

Floral meristems are dynamic systems that generate floral organ primordia at their flanks and, in most species, terminate while giving rise to the gynoecium primordia. However, we find species with floral meristems that generate additional ring meristems repeatedly throughout angiosperm history. Ring meristems produce only stamen primordia, resulting in polystemous flowers (having stamen numbers more than double that of petals or sepals), and act independently of the floral meristem activity. Most of our knowledge on floral meristem regulation is derived from molecular genetic studies of Arabidopsis thaliana, a species with a fixed number of floral organs and, as such of only limited value for understanding ring meristem function, regulation, and ecological value. This review provides an overview of the main molecular players regulating floral meristem activity in A. thaliana and summarizes our knowledge of ring primordia morphology and occurrence in dicots. Our work provides a first step toward understanding the significance and molecular genetics of ring meristem regulation and evolution.

Stability Despite Reduction: Flower Structure, Patterns of Receptacle Elongation and Organ Fusion in Eriocaulon (Eriocaulaceae: Poales)

Eriocaulaceae (Poales) differ from potentially related Xyridaceae in pattern of floral organ arrangement relative to subtending bract (with median sepal adaxial). Some Eriocaulaceae possess reduced and non-trimerous perianth, but developmental data are insufficient. We conducted a SEM investigation of flower development in three species of Eriocaulon to understand whether organ number and arrangement are stable in E. redactum, a species with a highly reduced calyx and reportedly missing corolla of female flowers. Early flower development is similar in all three species. Male and female flowers are indistinguishable at early stages. Despite earlier reports, both floral types uniformly possess three congenitally united sepals and three petals in E. redactum. Petals and inner stamens develop from common primordia. We assume that scanning electron microscopy should be used in taxonomic accounts of Eriocaulon to assess organ number and arrangement. Two types of corolla reduction are found in Eriocaulaceae: suppression and complete loss of petals. Common petal–stamen primordia in Eriocaulon do not co-occur with delayed receptacle expansion as in other monocots but are associated with retarded petal growth. The ‘reverse’ flower orientation of Eriocaulon is probably due to strictly transversal lateral sepals. Gynoecium development indicates similarities of Eriocaulaceae with restiids and graminids rather than with Xyridaceae.

Quantitative developmental analysis of two phenotypes of Hibiscus rosa-sinensis in the context of homeosis

The flowers of Hibiscus rosa-sinensis L. (Malvaceae) exist in two floral morphologies: a single phenotype, and a double phenotype. This study focused on the early stages of floral development, just before the initiation of petal primordia and up until the bifurcation of the stamen primordia. The two phenotypes were compared using logistic regression and bootstrapping techniques. Four aspects of floral development were considered: (i) organogenesis of petal and stamen primordia, and stamen bifurcation; (ii) allometry of stamen primordia; (iii) morphology of stamen primordia; and (iv) size of stamen primordia. The single and double buds initiated petal primordia at the same bud radii, but double buds initiated stamen primordia and stamen bifurcation at larger bud radii than the single phenotype. Double stamen primordia were shorter, wider, and more spherical than single stamen primordia, although the sizes of the single and double stamen primordia (defined as the sum of their length and width measurements) were not different. Results suggest that the additional space on the floral meristem of the double phenotype is linked to the divergent development of stamen primordia occupying this extra space.

Comparison of synandrium structure and development in three species from the Myristicaceae

Species of Myristicaceae have diverse morphology and structure of their synandria, making them an interesting group for androecium evolution research. To clarify the morphology, structure, and origin of the synandrium, scanning electron microscopy and histology were performed on staminate flowers of Horsfieldia pandurifolia H.H.Hu, H. tetratepala C.Y.Wu & W.T.Wang, and Myristica fragrans Houtt. at different developmental stages. A whorl of stamen primordia was rapidly initiated around the margin of the floral apex in groups of two in H. pandurifolia and groups of three in M. fragrans. Each stamen primordium grew longitudinally, producing a pair of anther lobes and four microsporangia, accompanying one vascular bundle in H. pandurifolia and M. fragrans. In H. tetratepala, three stamen primordia groups were formed, and each group with several anthers was supported by one vascular bundle, indicating a secondary increase of stamen. Three types of synandrium origin were observed: in M. fragrans, the central sterile column tissues originated from the elongate receptacle; in H. pandurifolia and H. tetratepala they were derived from a combination of floral meristem and fused connectives, and a combination of receptacular tissue and stamen groups. The diverse origins of the central sterile column suggest that the synandrium develops differently and independently in different genera and species of Myristicaceae.

Acacia (wattle) and Cananga (ylang-ylang): from spiral to whorled and irregular (chaotic) phyllotactic patterns – a pictorial report

Phyllotaxis, i.e., the arrangement of leaves around the stem and leaf-like organs inside flowers is regular in most vascular plants. Thus, developmental models usually explain regular phyllotactic patterns such as Fibonacci spirals and decussate/whorled patterns that obey Hofmeister’s rule: primordia form as far away as possible from previously initiated primordia. However, flowering plants showing at first Fibonacci spirals or whorled phyllotaxes may switch to other patterns that lack an obvious order and thus may be called irregular or even chaotic. Vegetative shoot tips of various Australian wattles (<em>Acacia</em> spp., Leguminosae in eudicots) and flower buds of ylang-ylang (<em>Cananga odorata</em>) and other Annonaceae (basal angiosperms) provide examples of irregular patterning. This pictorial report provides food for thought for scientists interested in phyllotaxis patterns beyond the usual spiral and whorled patterns. Emphasis is given on irregular phyllotaxes that occur in wild-type plants, mainly correlated with geometrical parameters such as leaf and stamen primordia that are very small as compared to the size of their apical meristems. They call for additional explanatory models, combining auxin-driven development with geometrical constraints and biophysical processes.

Comparative Ontogeny of Hermaphrodite and Pistillate Florets in Helianthus annuus L. (Asteraceae)

The inflorescence of Helianthus annuus L. has two types of flowers (or florets) on a single capitulum; central hermaphrodite disc florets and peripheral pistillate ray florets. In both florets, reproductive development starts with the conversion of apical meristem into floral meristem that will produce floral organ primordia. The only difference between hermaphrodite and pistillate florets in apical meristem stage is that apical meristem of the pistillate florets is not as apparent and curvaceous as apical meristem of the hermaphrodite florets. The differentiation of apical meristem into floral meristem is in the same progress in both florets. In hermaphrodite florets, flower organs; petals, stamens and carpels develop from floral meristem. Differentiation of five petal primordia takes place in the same way in both florets. Firstly filament and then anther differentiates in a stamen. Two carpel primordia appear below the stamen primordia in hermaphrodite florets. In following stages, carpel primordia are lengthened and formed inferior ovary, style, stigma respectively. In pistillate florets, flower organs; petals and carpels develop from floral meristem. They pass directly from the periant initiation to the start of carpel formation. Stamen primordia don’t appear and the further development of carpel primordia stops in a short time, as a result, stigma and style do not exist in pistillate florets. However, an inferior ovary with no ovule forms. In the capitulum of hermaphrodite florets, the development takes place in a centripetal manner; it starts firstly on the outermost whorl, and it proceeds towards inner whorl. However, this is not the case in pistillate florets.

REVISION OF PLETHIANDRA HOOK.F.: A POLYSTAMINATE, EAST ASIAN GENUS OF MELASTOMATACEAE

Plethiandra (Melastomataceae–Melastomatoideae–Dissochaeteae) comprises seven species in Borneo and one (P. sessiliflora) in the southern part of Peninsular Malaysia and central Sumatra. Plethiandra is easily recognized by its polystaminate androecium, having 16–40 stamens with short, straight, inappendiculate anthers. The closely related Medinilla has a diplostemonous androecium with 8–12 stamens and variously appendaged anthers. Ontogenetic studies in P. hookeri show that the increase in stamen number in Plethiandra results from subdivision of the stamen primordia into three or more subprimordia.This paper provides a key, genus and species descriptions including diagnostic characters, distribution maps and ecological notes. One species, P. tomentosa, is newly described. During field observations on P. hookeri and P. cuneata pollination by bees and dispersal by birds and squirrels were observed.

Studies in the Alismataceae. X. Floral organogenesis in Luronium natans (L.) Raf.

Floral organogenesis of Luronium natans (L.) Raf. occurs at first in an alternating trimerous pattern typical of Alismataceae, with the formation of three sepals, then three bulges, corresponding to the petal-stamen primordia described in some other Alismataceae, alternating with the sepals. A petal is initiated on each bulge and a pair of stamens is initiated either on it or close to it. After this, development no longer follows a trimerous plan. Six carpels are initiated in positions alternating with the six stamens, and further carpels may then arise above and between the first six. The carpels ultimately lie in a whorled arrangement if there are only six; if more, they may appear whorled or irregularly arranged. After the initiation of the stamen pairs, floral organ primordia appear simply to be positioned between pre-existing primordia as in other phyllotactic systems. It is suggested that the number of carpel primordia formed is probably determined by the size of primordia relative to the floral apex, and the extent of continued growth of the floral apex. Luronium reinforces the concept that a form of trimery is fundamental for the Alismataceae up to the formation of three stamen pairs and adds to the possibilities for variation after this point. It is suggested for the Alismataceae in general that, according to taxon, trimerous development may be terminated at any point after the initiation of the stamen pairs, and after this the primordia are positioned individually in relation to pre-existing primordia. The switch from stamen to carpel initiation is not necessarily correlated with these phyllotactic changes.

Floral development in Melaleuca and Callistemon (Myrtaceae)

Floral development of seven species of Melaleuca and four species of Callistemon is compared. The multistaminate fascicles of Melaleuca develop from stamen primordia initiated on antepetalous pre-staminal bulges (PSBs); the resultant bundles of stamens become separated by hypanthial expansion as the flower bud enlarges. In most species of Callistemon examined the stamen primordia are initiated directly on the floral apex, and the stamens are distributed evenly around the hypanthium at anthesis. The possession of large and prominent PSBs, and thus stamen fascicles, is a feature of most species of Melaleuca and their total absence is a feature of most species of Callistemon; however, there is a continuum between these two extremes. Several taxa of both genera exhibit intermediate morphology. In C. glaucus (Bonpl.) Sweet, small but distinct PSBs develop, which influence antepetalous stamen groups that remain contiguous at anthesis. This also occurred in M. leucadendra (L.) L. This variable expression of PSBs is the result of differences in the timing of stamen initiation. Other variable features are determined by the space available for primordium initiation and the patterns of growth and expansion of the developing flower.

Comparative development of perianth and androecial primordia of the single flower and the homeotic double-flowered mutant in Hibiscus rosa-sinensis (Malvaceae)

The double-flowered variety of Hibiscus rosa-sinensis L. (Malvaceae) displays a divergent floral morphology that appears to fit the criteria for homeosis. A comprehensive definition defines homeosis as the complete or partial replacement of one part of an organism with another part. The corolla of the single flower is pentamerous. The mature flower has a staminal tube bearing 60 – 70 stamens that surrounds an exserted synstylous gynoecium with five fused stigmas. In double flowers, the outermost whorl of petals is similar in appearance to that of the single flower. The remaining floral appendages have a morphology that is intermediate between petals and stamens, to varying degrees. No two double flowers are exactly the same, even on the same plant. As with other members of the Malvaceae, floral development in both floral types is unusual: once the calyx has been initiated, a ring meristem is formed from which both petal and stamen primordia are initiated. In the single flower, petal primordia are initiated on the flank of the ring, and then stamen primordia arise in five distinct and orderly clusters. In the double flower, petal primordia are also initated on the abaxial flank, but the remainder of the ring initiates primordia that form a mixture of petals, petal – stamen intermediates, and stamens. A common ring meristem that has two different developmental pathways provides a novel opportunity to study homeosis from the perspective of comparative developmental morphology. Keywords: homeosis, Hibiscus rosa-sinensis, androecium, intermediates, ring meristem, floral development.