Sepal Initiation Research Articles

Comparative floral ontogeny in Detarieae (Leguminosae: Caesalpinioideae). 1. Radially symmetrical taxa lacking organ suppression

Flowers in detarioid legume taxa (Isoberlinia angolensis, Microberlinia brazzavillensis, M. bisulcata, Hymenostegia klainii) initiate all 21 floral organs, are radially symmetrical, and have little or no organ suppression. All share a narrow, "Omega"-shaped floral apex and massive bracteoles at initiation. All have helical sepal initiation, starting abaxially. They differ in whether the first sepal initiates medianly (Microberlinia brazzavillensis, M. bisulcata) or nonmedianly (Isoberlinia angolensis, Hymenostegia klainii), and in petal order: helical (I. angolensis) or unidirectional (M. brazzavillensis, M. bisulcata, H. klainii). Stamens initiate in unidirectional order in each whorl except in M. brazzavillensis, which has a bidirectional outer whorl. An unusual feature is the ring meristem in M. bisulcata, on which petals and stamens are initiated. Overlap in time of organ initiation between whorls occurs in I. angolensis, M. brazzavillensis, and M. bisulcata but not in H. klainii. The carpel initiates concurrently with petals in all except H. klainii, in which it initiates with the outer stamens. The carpel remains open at ovule initiation in both species of Microberlinia. These detarioid taxa represent elements of the tribe having essentially radially symmetrical flowers, with all organs initiated and persisting to anthesis, but their specialized "Omega" character-state complex is shared with specialized taxa that have zygomorphic flowers and some organs suppressed.

Comparative floral ontogeny in Detarieae (Leguminosae: Caesalpinioideae). 2. Zygomorphic taxa with petal and stamen suppression

Marked floral zygomorphy and a reduced number of petals and/or stamens are the character traits that distinguish the taxa described (species of Afzelia, Berlinia, Gilbertiodendron, Macrolobium, Neochevalierodendron, Paramacrolobium, Phyllocarpus, and Tetraberlinia). All have an "Omega"-shaped floral apex after bracteole initiation, bracteoles large when initiated, helical sepal initiation, unidirectional petal initiation (simultaneous in Afzelia, not determinable in Tetraberlinia), and unidirectional stamen initiation. Floral zygomorphy is expressed primarily by one petal being much larger than the others and by suppression of several of the stamens. Five petals are initiated in all; suppression begins in late development. Either two petals (Neochevalierodendron, Phyllocarpus) or four petals (Afzelia, Berlinia, Macrolobium, Tetraberlinia) are suppressed. All ten stamens are initiated; at midstage, suppression begins in either three stamens (Afzelia) or seven stamens (Gilbertiodendron, Macrolobium, Paramacrolobium). Other expressions of zygomorphy may include diadelphy, stamen filament connation late in development, or displacement of the carpel from a central position to the adaxial side of the hypanthium. There is no loss of organs similar to that which occurs in some other Detarieae.

Floral development and anatomy ofPentadiplandra(Pentadiplandraceae): a key genus in the identification of floral morphological trends in the core Brassicales

The development and anatomy of leaves and flowers have been investigated in the monotypic species Pentadiplandra brazzeana to discuss its position relative to the core Brassicales and to clarify floral evolutionary trends within the order. Young leaves are flanked by two large stipules that shrivel at anthesis. Young flowers develop as elliptical primordia on a flattened raceme. Sepal initiation is sequential, starting with the lateral sepals and ending with three median sepals. The petals and diplostemonous androecium arise as regular whorls. Five antesepalous carpels develop into a saccate gynoecium with axile placentation. Petals develop a cushion-like basal appendage covering the extrastaminal nectary. Several ontogenetic and anatomical data of the flower support the basal position of Pentadiplandra in the evolution of the core Brassicales and for the derivation of the order from pentamerous diplostemonous ancestors. A derivation of the tetramerous Capparaceae and Brassicaceae as well as hexamerous Tovariaceae is discussed. A number of synapomorphies are identified: sequential sepal initiation with laterals preceding median sepals, retarded petal growth, a (andro)gynophore, the extrastaminal nectary, and stipulate leaves. The peculiar androecium of the Brassicaceae is caused by the strong median compression of the flower, leading to the loss and displacement of median stamens. The derivation of a parietal placenta from an axile placenta is supported. Evidence supports a close link with either Tovariaceae or Capparaceae, concomitant with macromolecular results.Key words: core Brassicales, Capparaceae, Pentadiplandra brazzeana, Tovariaceae, molecular phylogeny, floral development, anatomy, diplostemony, placentation.

Floral development in Schotia and Cynometra (Leguminosae: Caesalpinioideae: Detarieae)

Floral development was compared among several relatively unspecialized taxa in caesalpinioid tribe Detarieae: Cynometra webberi, an undescribed species of Cynometra, and three species of Schotia. These share a relatively unspecialized caesalpinioid flower type with no loss of organs, but some organ suppression. They also share the following characters: relatively small bracteoles in early stages, a circular floral apex, helical sepal initiation, and a nonmedian abaxial position of the first sepal initiated. Floral symmetry is radial. Order of initiation is helical among sepals in all. Order of petal initiation varies: helical in Schotia afra, unidirectional in Schotia brachypetala and S. latifolia, and simultaneous in Cynometra. Stamens are initiated unidirectionally in all except in the outer stamen whorl of Schotia afra, which is helical. Overlap of time of initiation between whorls occurs in S. brachypetala, S. latifolia, and Cynometra. The carpel initiates concurrently with the petals or directly thereafter in all. A tetramerous calyx results from fusion of the two upper (adaxial) sepal primordia. The carpel initiates ovules before marginal closure in both genera. Organ suppression includes one or two petals in Schotia and sometimes one in Cynometra.

Evolution and Development of Floral Diversity of Melianthaceae, an Enigmatic Southern African Family

Melianthus has strongly modified and monosymmetric (zygomorphic) flowers, and the ontogenetic derivation and evolutionary origins of these highly modified flowers are sought in a comparative study of the floral ontogeny and anatomy of two species of Melianthus and one species of Bersama. The development of monosymmetry in the family can be observed as a stepwise process linked with displacements of organ primordia and selective losses of petals and stamens. Flowers are weakly (Bersama) to strongly (Melianthus) monosymmetric. Sepal initiation is divided into two time sequences: the three outer sepals arise simultaneously with the anterior sepals behaving as a shielding unit before the two inner sepals emerge. Similarities and differences between the two genera are shown to be related to the trend to monosymmetry and are linked to specific pollination strategies. In Melianthus, an abaxial petal primordium is initiated, but it aborts at an early stage of development; in Bersama, all petals are present, but the incipient monosymmetry is visible at the onset of initiation. In Bersama, four stamens are the rule, but flowers pentamerous in all whorls are occasionally found. The nectary develops in a pouch formed between the adaxial sepal and the androecium. There is little ontogenetic evidence for a staminodial nature of the “beak” formed between the posterior stamens; it develops as a protective extension of the filament tube. A reduction sequence in the number of ovules can be seen by the progressive size limitation of the placentas. In Bersama, only one ovule arises on a marginal placenta and becomes displaced in a central position during development. The vasculature of different species is compared. The development of vascular traces is strictly linked with the initiation and external development of the floral organs. In both Melianthus and Bersama, the nectary is partly supplied by an odd adaxial trace. Our data support a relationship of Melianthaceae with Greyiaceae and Francoaceae, concordant with macromolecular results. It may be suggested that polysymmetric flowers of Bersama are secondarily derived from monosymmetric ones.

Reversion of floral development under adverse ecological conditions in Whytockia bijieensis (Gesneriaceae)

The reversion of floral development in Whytockia bijieensis Y.Z.Wang et Z.Y.Li occurs after the normal initiation of mid-adaxial sepals, which implies that the floral organs are not determined at the initiation of the floral primordium, but progressively determined during its development. Both low humidity (less than 75%) and low night temperature (12–14˚C) contribute to the reversion of floral development. However, the low humidity or low night temperature alone do not greatly affect the normal flowering process. The combination of these two ecological factors seems to considerably disturb the normal flowering process, greatly strengthening the reversion process of flowering. High humidity and moderate temperature or at least one of the two are necessary for the normal flowering process of W. bijieensis. It is likely that low humidity and low night temperature trigger the reversal to vegetative growth during floral development. The change of a flower into a bract-like organ in the flower pairs indicates that a transition from pair-flowered to normal cymes is possible.

The floral development of Pleuropetalum darwinii, an anomalous member of the Amaranthaceae

Summary The genus Pleuropetalum differs from the Amaranthaceae in its higher stamen and carpel number, coupled with numerous ovules. The floral development of Pleuropetalum darwinii is investigated with the SEM and is compared with the Amaranthaceae and other taxa of the Caryophyllales. Inflorescences are cymose with reduced partial inflorescences. Bracteoles are closely associated with the flower only separated by a short pedicel. Sepal initiation follows a 2/5 sequence but there is an important lag between the first two and the remaining sepals. The androecium arises on a circular primordium; the first four stamens are initiated as pairs opposite the two outer sepals, and are followed by paired or single stamens opposite the inner sepals. However, the number of stamens can fluctuate, or their sequence may be variable. The gynoecium starts its growth as a ring primordium with a number of apices enclosing a flattened apical dome. In early stages septal tissue can sometimes be observed. The central dome becomes completely subdivided in different sectors on which a large number of coiled ovules arise centrifugally. The development of flower and androecium shows similarities with other octandrous Caryophyllales, especially Phytolacca. The shared development is interpreted as a primitive feature. A comparison with other Amaranthaceae on the one hand, and with the Caryophyllales on the other shows evidence that a pluristaminate androecium with outer stamen pairs is plesiomorphic in the Amaranthaceae.

Inflorescence and Flower Development in the Hedychieae (Zingiberaceae): Scaphochlamys kunstleri (Baker) Holttum

Primary inflorescence bracts are initiated in a spiral phyllotactic pattern on the flanks of the inflorescence apex. A cincinnus primordium forms in the axil of each primary bract. A prophyll is initiated in the transverse plane and later extends around the periphery of the cincinnus to produce a sheathing, but not tubular, bract. The apex of the cincinnus forms a flower. Flower development begins with the enlargement and flattening of the floral apex. Sepal initiation is sequential but differs in the first two flowers of a cincinnus. The different sequences are likely the result of developmental constraints that operate at the time of sepal initiation. After initiation, the margins of the sepals become confluent and intercalary growth produces the synsepalous calyx. The periphery of the flower, interior to the sepals, now rises to form a ring primordium composed of three common petallandroecial primordia. The common primordia enlarge, flatten, and separate to produce a petal to the exterior and an androecial member to the interior. The outer androecium forms in the regions of the ring primordium left vacant by the formation of the other floral parts. The anterior outer androecial member soon ceases growth and contributes only initially to the formation of the labellum. The gynoecium shows heterochrony in its time of initiation, which occurs much earlier than in other species of Zingiberaceae. Gynoecial initiation takes place on the margins of the central flora cup, at the time of the separation of the common primordia. Three gynoecial primordia from in antipetalous positions. Septa development differs in the basal and apical portions of the ovary. At the base, the primordia grow inward and fuse to form the central axis of the ovary. At the apex, there is little radial growth of the primordia. They cease growth soon after initiation and produce a unilocular cavity. Ovules form at the junction of these two regions and project up into the cavity. Epigynous nectaries form after all other floral organs.

A floral ontogenetic investigation of the Hydrangeaceae

As a first step towards a broader floral ontogenetic study on Cornales, the flowers of four species of Hydrangeaceae (Deutzia corymbosa, Kirengeshoma palmata, Philadelphus purpurascens and Hydrangea petiolaris) were studied. In Deutzia and Kirengeshoma five sepal primordia are generally initiated, in Philadelphus only four. Sepal initiation in Kirengeshoma shows a tendency to tetramony. Deutzia has a variable sepal initiation. Petal growth was never retarded in the studied species. Initiation of the androecium in Kirengeshoma, Philadelphus and Hydrangea starts with antesepalous primary primordia, on which secondary primordia are soon formed, leading ultimately to the formation of polystaminate androecia. In Deutzia a diplostemonous androecium is formed, starting with the initiation of the antesepalous stamen primordia. Gynoecium development is similar in all species studied: on a concave floral apex, a ring meristem is initiated; it develops into a variable number of continuous carpel primordia, while the centripetally growing common margins form the septa. Initiation of the ovule primordia starts halfway up each placenta and extends in an apical, basal and lateral direction. A number of morphological problems are discussed, such as the derivation of tetramery, and evolutionary and developmental trends in the androecium and gynoecium. Kirengeshoma is well settled in Hydrangeaceae, although its exact position within the family remains uncertain. A sistergroup relationship of Hydrangeaceae with Loasaceae is supported. However, Hydrangeaceae also share features with Saxifragaceae (e.g. similar gynoecium development).

Trends in Evolution of Floral Ontogeny in Cassia Sensu Stricto, Senna, and Chamaecrista (Leguminosae: Caesalpinioideae: Cassieae: Cassiinae); A Study in Convergence

Distinctions in floral ontogeny among three segregate genera (Cassia sensu stricto, Chamaecrista, and Senna) of Cassia L. support their separation. In all species studied, the order of floral organ initiation is: sepals, petals, antesepalous stamens plus carpel, and lastly antepetalous stamens. Sepal initiation is helical in all three genera, which however differ in whether the first sepal is initiated in median abaxial position (Senna), or abaxial and off-median (Cassia javanica), a rare character state among legumes. Order of petal initiation varies: helical in Senna vs. unidirectional in Cassia and Chamaecrista. Both stamen whorls are uniformly unidirectional. Intergeneric ontogenetic differences occur in phyllotaxy, inflorescence architecture, bracteole formation, overlap of initiation among organ whorls (calyx/corolla in Cassia; two stamen whorls in Chamaecrista), eccentric initiation on one side of a flower, anther attachment, anther pore structure, and precocious carpel initiation in Senna. The asymmetric corolla and androecium in Chamaecrista arise by precocious organ initiation on one side (left or right). The poricidal anther character can result from differing developmental pathways: lateral slits vs. sealing of lateral sutures; clasping hairs vs. sutural ridges; terminal pores (one or two) vs. none; and clamp layer formation internally that prevents lateral dehiscence. Genera differ in corolla aestivation patterns and in stigma type. Convergence is shown among the three genera, based on intergeneric dissimilarities in early floral ontogeny (floral position in the inflorescence, bracteole presence, position of the first sepal initiated, order of petal initiation, asymmetric initiation, overlap between whorls, anther morphology, and time of carpel initiation) resulting in similarities at anthesis (showy, mostly yellow salverform flowers, heteromorphic stamens, poricidal anther dehiscence, bee pollination, and chambered stigma).

Trends in evolution of floral ontogeny in Cassia sensu stricto, Senna, and Chamaecrista (Leguminosae: Caesalpinioideae: Cassieae: Cassiinae); a study in convergence

Distinctions in floral ontogeny among three segregate genera (Cassia sensu stricto, Chamaecrista, and Senna) of Cassia L. support their separation. In all species studied, the order of floral organ initiation is: sepals, petals, antesepalous stamens plus carpel, and lastly antepetalous stamens. Sepal initiation is helical in all three genera, which however differ in whether the first sepal is initiated in median abaxial position (Senna), or abaxial and off‐median (Cassia javanica), a rare character state among legumes. Order of petal initiation varies: helical in Senna vs. unidirectional in Cassia and Chamaecrista. Both stamen whorls are uniformly unidirectional. Intergeneric ontogenetic differences occur in phyllotaxy, inflorescence architecture, bracteole formation, overlap of initiation among organ whorls (calyx/corolla in Cassia; two stamen whorls in Chamaecrista), eccentric initiation on one side of a flower, anther attachment, anther pore structure, and precocious carpel initiation in Senna. The asymmetric corolla and androecium in Chamaecrista arise by precocious organ initiation on one side (left or right). The poricidal anther character can result from differing developmental pathways: lateral slits vs. sealing of lateral sutures; clasping hairs vs. sutural ridges; terminal pores (one or two) vs. none; and clamp layer formation internally that prevents lateral dehiscence. Genera differ in corolla aestivation patterns and in stigma type. Convergence is shown among the three genera, based on intergeneric dissimilarities in early floral ontogeny (floral position in the inflorescence, bracteole presence, position of the first sepal initiated, order of petal initiation, asymmetric initiation, overlap between whorls, anther morphology, and time of carpel initiation) resulting in similarities at anthesis (showy, mostly yellow salverform flowers, heteromorphic stamens, poricidal anther dehiscence, bee pollination, and chambered stigma).

Activation of Basal Cells of the Apical Meristem during Sepal Formation in Tomato.

Although great advances have been made in research on the regulation of primordium fate in the floral meristem, our understanding of the molecular events occurring during the floral transition remains incomplete. Via a careful analysis of the expression patterns of five genes encoding housekeeping functions during the floral transition in tomato (using both in situ hybridization and enzyme histochemistry), we identified a particular phase of floral development (sepal initiation) at which cells located toward the base of the meristem show a high level of cellular metabolism, whereas cells at the tip of the meristem dome show little activity. At other stages of floral development, the probes used showed genespecific patterns of expression generally consistent with our previous investigation of the vegetative apical meristem. Our data, in conjunction with other reports in the literature, enabled us to postulate that relative activation of basal cells of the meristem may be of general occurrence during the transition to flowering. Such a hypothesis could account for recent observations using periclinal chimeras on the effect of L3 genotypes on flower development and have a bearing on the expected mechanism by which the number of primordia generated by a floral meristem is regulated.

Studies in the Alismataceae. IX. Development of the flower in Ranalisma humile

The solitary flower of Ranalisma humile has three sepals, three petals, 8–12 stamens, and numerous carpels. The first six stamens appear to arise in pairs associated with the petal primordia. With respect to the perianth and the first six stamens, the flower conforms to the basic trimerous pattern detected in other Alismatalean flowers, but it differs in other aspects of development. Organogenesis is unidirectional during the period of sepal and petal initiation, since both sepal and petal initiation occur first on the same side of the flower bud. After the initiation of the first six stamens there is no evidence that subsequent organs are initiated in any semblance of a trimerous pattern. The transition from stamen to carpel initiation occurs at some point after the initiation of the first six stamens, but it does not occur at a fixed point. There are six organogenetic sites above and alternating with the first six stamens, and these may be occupied indifferently by stamen or carpel primorida. The sequence of initiation of further carpel primordia is hard to define, but ultimately carpels appear to be arranged spirally. The features of floral organogenesis are discussed in relation to the position of Ranalisma among the Alismatales. Key words: flower, organogenesis, symmetry, unidirectional development, Ranalisma, Alismataceae, Alismatales.

ORGAN INITIATION AND DEVELOPMENT OF INFLORESCENCES AND FLOWERS OF ACACIA BAILEYANA

Inflorescence and floral ontogeny are described in the mimosoid Acacia baileyana F. Muell., using scanning electron microscopy and light microscopy. The panicle includes first‐order and second‐order inflorescences. The first‐order inflorescence meristem produces first‐order bracts in acropetal order; these bracts each subtend a second‐order inflorescence meristem, commonly called a head. Each second‐order inflorescence meristem initiates an acropetally sequential series of second‐order bracts. After all bracts are formed, their subtended floral meristems are initiated synchronously. The sepals and petals of the radially symmetrical flowers are arranged in alternating pentamerous whorls. There are 30–40 stamens and a unicarpellate gynoecium. In most flowers, the sepals are initiated helically, with the first‐formed sepal varying in position. Petal primordia are initiated simultaneously, alternate to the sepals. Three to five individual stamen primordia are initiated in each of five altemipetalous sectorial clusters. Additional stamen primordia are initiated between adjacent clusters, followed by other stamens initiated basipetally as well as centripetally. The apical configuration shifts from a tunica‐corpus cellular arrangement before organogenesis to a mantle‐core arrangement at sepal initiation. All floral organs are initiated by periclinal divisions of the subsurface mantle cells. The receptacle expands radially by numerous anticlinal divisions in the mantle at the summit, concurrently with proliferation of stamen primordia. The carpel primordium develops in terminal position by conversion of the floral apex.

Location of cell cycle changes in relation to morphological changes in the shoot apex ofSilene coeli-rosa immediately before sepal initiation

Changes in morphology, the mitotic index and the proportions of cells in G1 and G2 were measured in shoot meristems ofSilene coeli-rosa immediately before floral morphogenesis in order to determine whether the known changes to the cell cycle at this time are restricted to a particular region of the apex. Twenty-eight day-old plants were given either 7 long days (LD) plus 2 short days (SD) (day 8 of the LD treatment) or 9 SD [day 8 of the SD control (SDC) treatment]. Plants were sampled on day 8 every 2 h for 12 h and the various cell cycle measurements were performed on sections of the apical meristem. In the inductive LD treatment there was a peak in the mitotic index at 13.00 h and, possibly, the start of another at 19.00 h. At 21.00 h all meristems in this treatment initiated sepals. The mitotic activity at 13.00 and 19.00 h in the LD treatment was a result of significant increases in the mitotic index in the axial, lateral and central sub-axial areas of the apex compared with the corresponding zones in the SDC treatment. At 13.00 h of day 8, 80% of cells were in G2 phase in the axial region in the LD treatment whilst 85% of cells were in G1 in the axial zone in the SDC treatment. In the other zones significantly more cells were in G2 in the LD compared with the SDC treatment as was the case at 19.00 h although not to the same extent as the axial zone at 13.00 h. Thus these data emphasize, for the first time, the mitotic activation and predominance of the G2 population of cells particularly in the axial zone of shoot meristems in the LD treatment. These data are discussed in relation to the synchronisation of cell division which could occur in the prefloral shoot meristem at this time, affecting each shoot apical zone.

Shoot Development in Citrus sinensis L. (Washington Navel Orange). I. Floral and Inflorescence Ontogeny

Floral and inflorescence development was studied in Citrus sinensis (L.) Osbeck (Washington navel orange) to establish the pattern and timing of organogenesis. Morphological characters were used to select dormant flowering and vegetative shoots. Bud break occurred on flowering shoots 3-4 wk earlier than on vegetative shoots. Initiation of sepals and petals followed the 3/8 phyllotactic pattern. These fleshy organs tightly enclose the whorl of numerous stamens and the two whorls of carpels, which form the navel. Organogenesis from sepal to carpel initiation required 2 wk, and anthesis, another 3 mo. Within the cymose inflorescence, development of the terminal flower was followed by the simultaneous initiation of lateral flowering shoots in the axils of six to seven foliar leaves. The terminal flower was always the most advanced in development, and the position 2 flower, the least. Basipetally, the other lateral floral meristems were gradually released from this apparent apical dominance, which may explai...

Synchronisation of cell division in the shoot apex of Silene in relation to flower initiation

In plants of Silene coeli-rosa, induced to flower by 7 LD, synchronisation of cell division in 20 per cent or more of the cells in the shoot apical dome was found on the 8th and 9th days after the beginning of induction, during the plastochron before sepal initiation. Synchronisation was inferred from the changes in the proportions of cells with the 2C and 4C amounts of DNA, and changes in mitotic index and labelling index. From the peaks of mitotic index a cell cycle of 10 h was measured for the synchronised cells, half that of cells in the apices of uninduced plants in short days. The faster cell cycle and synchronisation in the induced plants was associated with a shortening, of both G1 and G2, suggesting two control points, while S and M remained unchanged. These results are compared with those from other plants in which synchronisation occurs at the beginning rather than the end of evocation.

IN VITRO CULTURE OF FLORAL BUDS OF AQUILEGIA

Tepfer, S. S., R. I. Greyson, W. R. Craig, and J. L. Hindman. (U. Oregon, Eugene.) In vitro culture of floral buds of Aquilegia. Amer. Jour. Bot. 50(10): 1035–1045. Illus. 1963.—Floral buds at various stages of development, from early stages before sepal initiation to late stages with young carpel primordia present, were grown in culture on various agar media. A basic medium containing White's minerals, Nitsch's trace elements, coconut milk, sucrose, and assorted water‐soluble vitamins was developed for growth of the buds. The addition of indoleacetic acid, gibberellic acid, and kinetin to the basic medium extended the developmental limits of buds at nearly all stages and decidedly improved the continued development of carpels. On this medium buds produce all of their organ primordia, growing from early stages to about the size of flowers at anthesis, but will not develop unless the sepals are removed. This inhibiting effect of sepals is not understood at this time. Stamen development is consistently poor beyond the point of differentiation of anther and filament, even with the addition of hormones. The development of buds in culture is illustrated and compared with development in intact plants. With further improvement of the medium, it is hoped that these buds may be used for experiments testing theories of floral morphogenesis.