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Floral ontogeny in Swartzia (Leguminosae: Papilionoideae: Swartzieae): distribution and role of the ring meristem¹

Department of Biology (Ecology, Evolution, and Marine Biology), University of California, Santa Barbara, California 93106-9610 USA; and Department of Biology, Louisiana State University, Baton Rouge, Louisiana 70803 USA

Received for publication November 15, 2002. Accepted for publication April 4, 2003.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
The anomalous systematic position of Swartzieae at the base of Papilionoideae is correlated with unusual developmental features: one petal or none; a ring meristem; polystemony; heterostemony; little or no alignment of stamens as antesepalous or antepetalous; multicarpely; and absence of unidirectional order of organs except in the calyx. Symmetry is zygomorphic throughout development. Floral ontogeny of four species of Swartzia reveals five sepals are initiated successively, beginning abaxially, but intercalary growth below the separate sepals forms a tubular calyx that splits irregularly, a feature typifying the genus. A single petal is initiated adaxially or may be missing altogether (in S. sericea). The apex enlarges and forms a ring meristem concurrently with carpel initiation. Several large-stamen primordia (2–15, according to the species) initiate first on the ring, followed by 40–150 small-stamen primordia. The latter initiate in centrifugal order in S. aureosericea and S. laurifolia or in acropetal order in S. sericea and S. madagascariensis. While ring meristems are considered to be homologous among Neotropical species studied as well as in the African S. madagascariensis, they vary in extent, duration, order of initiation, and productivity. Swartzieae is unlikely to be ancestral to the rest of Papilionoideae, based on radically differing floral ontogeny in the two groups.

Key Words: Bobgunnia • development • Fabaceae • flower • initiation • Leguminosae • Papilionoideae • ring meristem • ontogeny • polystemony • Swartzia • Swartzieae

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Swartzia is a genus of woody legumes in the tribe Swartzieae, mostly Neotropical, currently placed in the legume subfamily Papilionoideae (Polhill, 1981 , 1994 ) although its members differ markedly from traditional papilionoids. The flower structure is highly unusual in that most of the approximately 135 species have only one petal or none, multistaminate dimorphic androecia, and in some taxa multicarpellate gynoecia. The flowers have pronounced zygomorphic symmetry, and the calyx is entire and undivided in bud, splitting open irregularly at anthesis.

Swartzieae sensu Polhill (1981 , 1994 ) is a tribe of uncertain affinities among legumes. It includes 11 genera and approximately 185 species, mostly inhabiting New World tropics, with a few tropical African taxa. Recent evidence asserts that Swartzieae sensu lato is not monophyletic (Herendeen, 1995 ; Ireland et al., 2000 ; Pennington et al., 2000 ) and that some taxa are more closely aligned with papilionoid tribe Sophoreae. Swartzia is retained in a core group of Swartzieae sensu stricto by these authors.

The aims of this paper are (1) to compare floral development among several species of Swartzia, (2) to examine the role and diversity of ring meristems, and (3) to elucidate the anomalous systematic position of Swartzieae at the base of Papilionoideae in the light of developmental evidence. Comparisons among species also offer an opportunity to examine the mode and consequences of organ loss (petals) and organ proliferation (up to 150 stamens per flower) and to compare the floral ontogenies with those of other legume flowers of caesalpinioids and papilionoids that show either loss or proliferation of organs. The ring meristem is an organogenetic feature restricted to Swartzieae (including Ateleia; Tucker, 1990 ) and certain Detarieae (Tucker, 2000a , b , 2002b ) among over 300 legume taxa examined (Tucker, 1987a , 1997 ). Systematic conclusions may provide evidence relevant to a proposed segregate genus (Bobgunnia, in Kirkbride and Wiersema, 1997 ) for the two African species of Swartzia, which differ in several respects from the majority of species in the genus (Ferguson and Skvarla, 1988 , 1991 ).

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Materials
Table 1 shows the species studied. Flower buds of each were dissected and studied with a dissecting microscope and prepared for scanning electron microscopy.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Developmental floral material has been obtained for four species of Swartzia (S. aureosericea Cowan, S. laurifolia Benth., S. madagascariensis Desvaux, and S. sericea Vogeli-Zuber). In addition, near-mature floral buds were available from S. simplex Sprengel and S. apetala. Fortunately, the species of Swartzia obtained include one of the two African taxa, S. madagascariensis [Bobgunnia madagascariensis (Desv.) Kirkbride and Wiersema], so that comparisons can be made between it and several of the Neotropical taxa in Swartzia sensu stricto. Among the latter, the representatives studied include species with and without a petal and also include diversity in number of large stamens per flower: two, three, four, six, or seven, in addition to approximately 40–150 small stamens in each.

Organography
Inflorescences are of two types in the genus Swartzia: cauliflorous racemes or panicles on trunks and branches in section Swartzia, as opposed to axillary racemes in section Possira (Cowan, 1968 ). The six species of Swartzia obtained for study (S. apetala, S. aureosericea, S. laurifolia, S. madagascariensis [Bobgunnia madagascariensis], S. sericea, and S. simplex) belong to section Possira, defined by Cowan (1968) as having simple racemose inflorescences that arise in leaf axils, inflorescence bracts that are obviously reduced leaves, and bracteoles usually present but not attached to the pedicels. The form of bracts is correlated with the difference in inflorescence form: nonfoliar bracts in section Swartzia and foliar ones in section Possira. All inflorescences in species studied were racemes with helically arranged bracts, each subtending a solitary flower. Further information on inflorescence development will be minimal because most of the material did not include sufficient stages needed for studying inflorescence ontogeny.

Cowan used mostly vegetative characters in his keys to species but used the following reproductive characters: number of carpels; stamens isomorphic or heteromorphic; stigma form (capitate or punctiform); relative length of style, ovary, and stipe; petal blade shape; and type and amount of indumentum on organs. Some features, such as numbers of large stamens and small stamens, vary among species and are of morphological and developmental interest, but are not used in keys.

Developmental descriptions will be given for four species of Swartzia, of which S. aureosericea (Fig. 1a, b) is considered first and has a petal, three large stamens, and many small ones. Then S. laurifolia (Fig. 2a, b) is described, which differs in having five or more large stamens; then S. sericea (Fig. 3a, b), with no petals and with two large stamens; and lastly S. madagascariensis (Fig. 4a, b), which has a petal and 9–14 large stamens, but which lacks a sharp distinction between large and small stamens. Some comments will be made on any differences in the additional two species (for which series were incomplete).

View larger version (65K): [in this window] [in a new window]   Figs. 1–4. Camera lucida drawings and floral diagrams. 1. Swartzia aureosericea. (a) Flower drawing, x1. (b) Floral diagram. 2. Swartzia laurifolia. (a) Flower drawing, x2. (b) Floral diagram. 3. Swartzia sericea. (a) Flower drawing, x6. (b) Small stamen, x8. (c) Large stamen from bud, x10. (d) Carpel. (e) Floral diagram. 4. Swartzia madagascariensis. (a) Flower drawing, x2. (b) Floral diagram. Figure abbreviations: A, large stamen; a, small stamen; Ab, abaxial side; Ad, adaxial side; B, bract; Bl, bracteole; C, carpel; F, floral apex; Gy, gynophore; P, petal; R, ring meristem; S, sepal.

Organogeny (Figs. 5–33)
The floral apical meristems (Fig. 5) are produced singly in axils of bracts. Each is wide tangentially and narrow in height. Bracteole initiation was not observed, but bracteole scars are visible early (Figs. 6–7) where bracteole primordia were removed. The first sepal primordium (Fig. 6) is median and abaxial. The order of initiation among the remaining four sepal primordia was not determined, although the larger size of the two adaxial sepal primordia (Figs. 7–8) at early stages suggests that they are initiated before the two smaller lateral sepal primordia. The sepal primordia are confluent at their adjacent margins immediately after initiation (Fig. 8), forming the beginning of the calyx cup. Order of sepals, then, appears to be bidirectional.

View larger version (173K): [in this window] [in a new window]   Figs. 5–16. Swartzia aureosericea. Floral organogenesis (scanning electron microscopy [SEM] micrographs). Abaxial side is at base in each figure. Subtending bracts have been removed in all, bracteoles removed in Figs. 6–16 , and sepals removed in Figs. 9–16 . Scale bar = 50 µm in Figs. 5 and 9 , 100 µm in all other figures. 5. Bare floral apex in axil of bract. 6. First sepal primordium is initiated on abaxial side. Bl, bracteole scar. 7. All five sepals present, polar view (three labeled). A bracteole scar is at left. 8. Calyx cup beginning by intercalary growth below sepal primordia (three labeled) around floral apex. 9. Post-sepal floral apex 109 µm in diameter, polar view. 10–11. Expanded floral apex 250 µm in diameter, with ring meristem, on which has been initiated the median adaxial petal primordium, the median abaxial large-stamen primordium, and the carpel primordium, in polar (Fig. 10 ) and oblique-lateral (Fig. 11 ) views. In Fig. 11 , all three large-stamen primordia are visible, including the two lateral ones. The two mounds at arrowheads are discontinuous parts of the ring meristem on either side of the median abaxial stamen primordium. 12–14. The ring meristem (R) is delimited from the carpel primordium at center and is now continuous abaxially with the two originally discontinuous mounds (at arrowheads); two polar views and a lateral-oblique view. In Fig. 13 , the petal primordium is visible, and in Fig. 14 , the heights of the ring meristem and organs are seen. 15. Oblique-lateral view of flower with enlarging petal and large stamens, showing the carpel primordium is highest abaxially and tapered adaxially. 16. Oblique-abaxial view showing initiation of earliest small-stamen primordia (at arrowheads) abaxially on ring meristem. The carpel primordium has an adaxial basal mound beginning (see asterisk)

The first two organs are initiated in close succession on the ring meristem, the single petal primordium adaxially, and the first large-stamen primordium abaxially (Fig. 10), both in the median sagittal plane. No vestiges of other petal primordia were ever seen. The carpel primordium is initiated concurrently with the first two organs (the single petal and the first large-stamen primordium), as well as with two abaxial mounds (arrowheads; Figs. 10–11) that are part of the ring meristem, in antepetalous sites on either side of the middle large-stamen primordium.

Two lateral large-stamen primordia are next initiated (Figs. 11–12). Positionally, the three large-stamen primordia are in antesepalous sites; no stamens form in the other two antesepalous positions, however (see Figs. 13 and 14). The large-stamen primordia are each about 108–117 µm wide when first measurable (Figs. 12 and 13).

The ring meristem continues to expand in diameter, although it is widest in the adaxial half of the flower (R; Figs. 12–15). Abaxially it becomes continuous with the two abaxial mounds and also extends between them in the space between the abaxial large-stamen primordium and the carpel primordium (Fig. 13). The floral apex is 110 µm high in Fig. 14 above the inner base of the calyx cup; this thickness is also noticeable in Figs. 18–19.

View larger version (134K): [in this window] [in a new window]   Figs. 17–24. Swartzia aureosericea. Floral organogenesis and organ development (SEM micrographs). Abaxial side is at base in Figs. 17–23 ; adaxial side is at base in Fig. 24 . Subtending bracts and bracteoles have been removed in all, and sepals removed in all figures except Fig. 24 , in which part of the calyx tube persists. Scale bar = 100 µm in Figs. 17–18 and = 200 µm in Figs. 19–24 . 17. Oblique lateral view showing adaxial mound at carpel base and enlarging organs including petal with marginal growth. 18. Oblique view showing beginning of carpel cleft and initiation of small-stamen primordia (at arrowhead) abaxially. 19–20. Lateral-oblique and near-polar views. Carpel is about 300 µm high, and the cleft and adaxial mound are visible. Rows of small-stamen primordia are being initiated starting along the inner edge of the ring meristem. Petal primordium has grown marginally in height and width. 21–23. Lateral, polar, and oblique views of one flower. The carpel is 320 µm high, with cleft ending at basal mound. Small-stamen primordia have been initiated over the entire ring meristem, in a centrifugal (basipetal) direction. Petal blade has an acute tip. 24. Adaxial-oblique view. The carpel is 600 µm high, has basal mound and cleft, with margins appressed. Two of the large-stamen primordia have each differentiated into filament and anther, the latter with an adaxial groove (at arrowheads). Small-stamen primordia are undifferentiated. The petal blade bears trichomes abaxially near the midrib. Parts of the calyx cup are visible at left and rear

Organ enlargement and development
The two bracteoles become narrowly lanceolate and acutely tipped, covered by trichomes abaxially and marginally (Fig. 27). The five sepal primordia become confluent at their adjacent margins immediately after they are all initiated (Fig. 8). The resultant calyx tube thickens considerably (Figs. 26–27), becomes densely covered by trichomes externally (Fig. 27), and encloses the rest of the flower during development. The single petal primordium, in the adaxial median position, grows marginally to become broadly orbicular (Fig. 1a), with trichomes clustering first along the midrib (Fig. 24) and later more extensively, but still absent from the margins (Fig. 32).

View larger version (185K): [in this window] [in a new window]   Figs. 25–33. Swartzia aureosericea. Late stages of floral organ development (SEM micrographs). Subtending bracts, bracteoles have been removed in all except Fig. 27 , and sepals have been removed except in Figs. 25–27 and 30 , where parts of the calyx tube persist. Scale bar = 200 µm in Fig. 33 ; 500 µm in Figs. 25 and 28 ; 1 mm in Figs. 26, 27, 30, and 32 ; and 2 mm in Figs. 29 and 31 . 25–27. Oblique lateral views (Figs. 25–26 ) and polar view (Fig. 27 ). The carpel is 1.14 mm high, and the distal portion with cleft is elongating and arching at the tip. Large-stamen anthers have median and lateral grooves, delimiting the four microsporangia, arrayed in a row adaxially. Some small-stamen primordia are indicated by arrowheads. In Fig. 26 , the innermost small-stamen primordia (closest to the carpel base) have differentiated to produce anther and filament, and their anthers have a median groove. Other small-stamen primordia produced later are shorter and not as much differentiated. In Fig. 27 , all organs are visible, including the two bracteoles and the thick tomentose calyx cup (the upper part removed). 28. Oblique-polar view. The carpel is about 1.2 mm long, arched strongly toward the adaxial side, and the cleft is sealed. Anthers of the large stamens have median and lateral grooves, delimiting four microsporangia aligned in a single plane. All small stamens have differentiated, with filament and anthers bearing median and lateral grooves (some indicated at arrowheads), delimiting the four microsporangia in a tetrad arrangement. 29. Portion of flower (side view) showing the arched gynoecium about 6.8 mm long including stipe (at arrowhead) and stigma. A large stamen is about 4.5 mm long, with the anther declinate in bud. 30. Small stamens have differentiated, each 1.5–1.8 mm high. 31. Large stamen with sparsely trichomatous filament and anther with four microsporangia on adaxial side (detail of Fig. 29 ). 32. Lateral view. The petal is 2.6 mm high, with trichomes on abaxial side of blade. The carpel is arched, narrow, and about 4 mm long. 33. Capitate stigma and style

From short, rounded primordia (Fig. 24), the 90–150 small stamens differentiate at about 300 µm in height into filament and anther with a dorsal median groove (arrowheads; Figs. 25–26). By about 420 µm in height, the small-stamen anthers have lateral grooves as well that delimit the four microsporangia (arrowheads; Fig. 28). The microsporangia are aligned so that those closest to the carpel base face away from it (Figs. 26 and 28). The microsporangia are abaxial and adaxial (Fig. 30), in contrast to the parallel array in the large stamens. The long slender filaments are glabrous.

The carpel cleft becomes evident (Fig. 18) at a height of about 230 µm. The carpel heightens (Figs. 19, 20, 22 and 23), terminating basally at a convex mound on the adaxial base of the carpel; the cleft does not continue down the mound. At a carpel height of about 640 µm, the flanges on either side of the cleft extend around to become appressed, closing the cleft except at top and bottom (Fig. 24). The basal mound elongates, and becomes the stipe (at arrowhead; Fig. 29). The distal portion of the carpel bearing the cleft elongates and becomes arcuate toward the adaxial side of the flower (Figs. 28–29), tapering gradually to a narrow style (Fig. 33). In the bud just before anthesis (Fig. 29), the gynoecium is about 6.8 mm long. The stigma (Fig. 33) is terminal, papillate, bisected by the terminus of the cleft, and no wider than the style. The ovary, stipe, and style remain glabrous throughout.

The flower of Swartzia apetala var. subcordata (not illustrated) resembles that of S. aureosericea in the number of large stamens (two to four) but lacks bracteoles and a petal.

Swartzia laurifolia Benth
Organography
This species includes small trees or shrubs of the Brazilian Amazon. The inflorescences are racemose (Fig. 34) and 10–25 cm long (Cowan, 1968 ), with individual flowers each subtended by a sparsely hairy bract but lacking bracteoles. Buds are globose. Five sepals form a seamless calyx cup that splits as four strongly reflexed segments (Fig. 2a; Cowan, 1968 ), densely strigulose abaxially, and glabrous adaxially. The single pale yellow petal is glabrous or sericeous on the midvein abaxially; the claw is 2–3 mm long, and the blade is oval, 1.1–1.5 x 0.7–1.0 cm, with an undulate margin. Four or five large stamens (Fig. 2a, b) on the abaxial side have narrowly oblong anthers 3.5–4 mm long and filaments 12–15 mm long. The 70–100 small stamens (Fig. 2a, b) are glabrous, with filaments 7–12 mm long and oblong orange-brown anthers 2 mm long. The sericeous gynoecium has a truncate stigma, a short style, and a gynophore.

View larger version (208K): [in this window] [in a new window]   Figs. 34–45. Swartzia laurifolia. Floral organogenesis (SEM micrographs). Abaxial side is at base in Figs. 35–37, 39–40, and 42–45 ; adaxial side is at the base in Fig. 38 and at left in Fig. 41 . Subtending bracts are removed in Figs. 35–45 , and sepals are removed in Figs. 39–45 . Scale bar = 50 µm in Figs. 35–45 , 100 µm in Fig. 34 . 34. Side view of inflorescence tip, with single floral apices (F) in bract axils. Most bracts are removed. 35. Bare floral apical meristem. 36. Initiation of first sepal primordium median on abaxial side. 37. Oblique side view. Two lateral sepal primordia have initiated on either side of median abaxial sepal. 38. All five sepal primordia have been initiated and have become confluent marginally to form a calyx tube around the floral apex. 39. Floral apex before organ initiation. 40–41. Adaxial-oblique and lateral views. Post-sepal floral apex 120 µm in widest diameter, surrounded by scar of calyx cup (removed). Floral apex has become more convex. 42. Single petal primordium has been initiated in median adaxial position on floral apex, which is 84 µm in widest diameter (polar view). 43–44. Polar and oblique-lateral views. Floral apex 100 µm in diameter, showing establishment of the ring meristem (R) and initiation of the carpel primordium at center. The petal primordium is visible. 45. Floral apex (adaxial-oblique view) 125 µm in diameter, with petal primordium adaxially. The carpel primordium is highest on abaxial side, showing delimitation inside the ring meristem

The floral apex after sepal initiation is high convex and rather oval in outline (Figs. 38–39). It expands greatly thereafter although remaining wide tangentially (Figs. 39–41). The single petal primordium is next initiated in median position on the adaxial side (Figs. 42 and 44), low on the flank and before other organs, on an apex about 84 µm in diameter. Soon thereafter, the carpel primordium is initiated (Figs. 43–45), followed by initiation of two or three large-stamen primordia abaxially (two in Figs. 46–47; three in Fig. 48). The large-stamen primordia are formed on a massive ring meristem (R; Figs. 46–49) surrounding the carpel primordium. Additional large-stamen primordia form laterally to the first two or three (Figs. 52–53) on the ring meristem. The carpel primordium at this stage is about 70 µm high and has an oblique slanted adaxial side and a relatively vertical abaxial side (Fig. 51).

View larger version (219K): [in this window] [in a new window]   Figs. 46–57. Swartzia laurifolia. Floral organogenesis and organ development (SEM micrographs). Abaxial side is at base in Figs. 46–47, 52–53, and 57 ; at left in Figs. 48, 50–51, and 54 ; adaxial side is at base in Figs. 49 and 55–56 . Subtending bracts and calyx have been removed in all. Scale bar = 50 µm in Figs. 46–52 , 100 µm in Figs. 53–57 . 46. Lateral-oblique view. Two large-stamen primordia are being initiated abaxially on the ring meristem. Petal primordium is visible adaxially. 47. Near-polar view of flower 160 µm in widest diameter. Additional large-stamen primordia are beginning to initiate laterally (A), and small-stamen initiation (at arrowheads) is beginning on inner periphery of ring meristem. Also visible are the petal primordium on the adaxial side and the carpel primordium at center. 48. Lateral-oblique view of a floral meristem shows the broad ring meristem, rather flat carpel primordium, and five large-stamen primordia on the abaxial side. Small-stamen initiation (at arrowheads) is beginning from abaxial to the adaxial side of the carpel. 49–50. Adaxial and lateral side views of flower 180 µm in widest diameter. The petal primordium is seen to be attached at the base of the thick ring meristem and to have started height growth. Small-stamen primordia are at arrowheads. 51–52. Lateral and polar views of a flower 185 µm in widest diameter, with four large-stamen primordia on the abaxial side. Small-stamen primordia (a) have initiated around inner periphery of ring meristem. The carpel primordium has formed a convex pedestal (see asterisk) at its adaxial base. 53. Polar view of a flower 292 µm in widest diameter. Four to five rows of small-stamen primordia have initiated in a basipetal direction on the ring meristem. Five large-stamen primordia are present, as well as carpel and petal. 54–55. Lateral and adaxial side views of a flower 300 µm in widest diameter. The carpel is 105 µm high on its adaxial side and has an adaxial cleft extending upward above the adaxial mound. Basipetal initiation of small-stamen primordia is continuing on the ring meristem. 56. Adaxial view, showing that small-stamen initiation is nearly complete on the ring meristem; the last-initiated are at the outer edge. The carpel primordium is about 150 µm high, and its sides have grown, so the cleft is deeper. Large stamens are incurved. 57. Polar view after all organs are present; five large stamens and about 110 small-stamen primordia are present

Organ enlargement and development
The sepal primordia become confluent as a calyx cup very early, immediately after the last sepal primordia are initiated (Fig. 38). The single petal primordium on the adaxial side forms a lamina by a height of about 70 µm (Fig. 51) during small-stamen initiation and grows in height comparably with the large stamens. Trichomes form sparsely on the abaxial side of the petal (Figs. 59 and 65). The petal enlarges and surrounds the large stamens (Fig. 70) and eventually encloses the incurved large stamens (not shown) in the pre-anthesis bud.

View larger version (199K): [in this window] [in a new window]   Figs. 58–70. Swartzia laurifolia. Midstage and late stages of floral organ development (SEM micrographs). Abaxial or adaxial side is labeled in some figures. Calyx has been removed in all. Scale bar = 50 µm in Figs. 61–63 ; 100 µm in Figs. 59 and 64 ; 200 µm in Figs. 58, 60, 67, and 69 ; and 400 µm in Figs. 65–66, 68, and 70 . 58. Lateral view of flower, after removal of sepals and petal. Large-stamen primordia are elongate, incurved, and about 250–400 µm high. Neither they nor the small-stamen primordia (70–90 µm high) are differentiated into anther and filament as yet. 59. Oblique-adaxial side view of flower, after removal of sepals but with petal attached. Large-stamen primordia are elongate, incurved, and have differentiated into anther and filament with a median groove. One primordium (at arrowhead) is intermediate in height between the two sizes of stamens. 60. Flower with large stamens about 625–850 µm high and small-stamen primordia about 65–80 µm high, both undergoing anther differentiation as microsporangia. 61. Differentiation of anther and filament in small-stamen primordia about 95 µm high. 62. Anther of large stamen after formation of median adaxial groove (at arrowhead). 63. Anther of large stamen after formation of lateral grooves, as well as median adaxial groove, delimiting the four microsporangia, all on the adaxial side. 64. Anther of small stamen. 65–66. Polar and lateral views of coiled large stamens with differentiating anthers, the sericeous gynoecium, and the petal in large bud. In Fig. 66 , small stamens are visible with anthers differentiated. 67. The gynoecium is arcuate, densely sericeous, and has a stigma with a cleft. 68. Adaxial view of coiled large stamens and sericeous ovary. 69. Lateral side view of sericeous gynoecium and narrow, capitate stigma. Small stamens with anthers and filaments are at left. 70. Side view of large pre-anthesis bud showing the petal, large and small stamens, and the gynoecium

The small stamens begin to heighten (Figs. 58–59), mostly synchronously, although the last-formed ones along the adaxial periphery seem to be the last to enlarge. Differentiation of anther and filament occurs by a height of about 95 µm (Figs. 60–61). The median suture is evident first, followed by the two others (Fig. 64); all are on the adaxial side of the stamen (Figs. 64, 67, and 69). The filament is attached near the base on the abaxial side of the anther (Fig. 64); filaments are variously curved in bud (Fig. 66). An occasional flower has a stamen primordium that is intermediate in height and in lateral position, next to one of the large stamens (Figs. 58–59).

The carpel primordium becomes flat at its adaxial base, forming a low pedestal on the adaxial side (Figs. 51–52) at a height of about 80 µm, that will later become the stipe or gynophore. The adaxial crease or cleft of the carpel (which will become the locule) is first visible at a height of about 105 µm (Figs. 54–55). The carpel primordium is already arched abaxially at this stage. It heightens and the margins appear appressed along the cleft (Fig. 56) at all stages seen. Trichomes begin to form from the base upward on the abaxial side of the gynoecium (Fig. 58) and nearly obscure it at a height of about 930 µm (Fig. 67). The ovary tapers distally, and the stigma is short, terminal, and two-lipped (Fig. 67).

The flower of Swartzia simplex (not illustrated) resembles that of S. laurifolia in having a single petal and 5–15 large stamens, but differs in having bracteoles. The material available did not include a complete developmental series.

Swartzia sericea Vogeli-Zuber
Organography
The inflorescence is racemose, ramigerous, or ramuligerous (Cowan, 1968 ). Bracteoles are lacking, and flower buds are globose and strigulose. The flower (Fig. 3a, e) has four (if two separate primordia become confluent) or five calyx segments, strigulose abaxially and sericeous-strigose adaxially, with a white cartilage-like inner surface. Petals are lacking. There are two large stamens (Fig. 3c) 1.1–1.3 cm long, with oblong anthers, villous at the junction with the filaments, and about 120 short stamens (Fig. 3b), 0.9–1.1 cm long, the anthers orbicular to oblate and black-dotted, with a row of glandular red trichomes on either side. The gynoecium (Fig. 3d) has a punctiform stigma, glabrous style, densely sericeous ovary, and short stipe or gynophore (Cowan, 1968 ).

Organogeny (Figs. 71–97)
The floral apex is lenticular in frontal view at first (Figs. 71–72); no bracteoles were seen. The apex becomes circular and low convex (Figs. 73–74) at a diameter of about 74 µm. Sepal organogeny was not observed; after all five are present, they immediately become marginally confluent (Fig. 75), initiating the calyx cup. The sepal primordia elongate to converge over the rest of the flower (Fig. 76).

View larger version (180K): [in this window] [in a new window]   Figs. 71–80. Swartzia sericea. Floral organogenesis (SEM micrographs). Abaxial side is at base in Figs. 71–73, 75–76, and 78–79 ; it is at right in Figs. 74 and 77 , and at left in Fig. 80 . Subtending bracts have been removed in all the figures, and parts or the entire calyx have been removed in Figs. 76–80 . Scale bar = 25 µm in Figs. 71–74 and 77–79 and 50 µm in Figs. 75–76 and 80 . 71–72. Lenticular floral apical meristems, each in axil of a bract (removed). 73–74. Circular floral apex about 74 µm in diameter in polar and lateral side views. 75. Floral apex with four sepal primordia initiated, which are marginally confluent and initiating the calyx cup. 76. Four sepals converged over rest of flower. The upper (adaxial) sepal is probably the fusion product of two sepal primordia. 77. Sagittal longitudinal section of post-sepal floral apex 55 µm in diameter. 78–79. Polar views of post-sepal floral apex, which is circular, low convex, and 70–85 µm in diameter. 80. Lateral-oblique view of floral apex at formation of an arcuate groove that will delimit a partial ring meristem at the left

View larger version (139K): [in this window] [in a new window]   Figs. 81–88. Swartzia sericea. Floral organogenesis and organ development (SEM micrographs). Abaxial side is at base in Figs. 81–87 and at right in Fig. 88 . Subtending bracts and calyx have been removed in all. Scale bar = 50 µm in Figs. 81–85 and 100 µm in Figs. 86–88 . 81. Floral apical meristem 150 µm in widest diameter in polar view, at initiation of the carpel primordium (C) 75 µm wide, and two large-stamen primordia, one on either side of the carpel. The partial ring meristem occupies the remainder of the floral apex. 82. Adaxial-oblique view, with carpel primordium, two large-stamen primordia (each about 42 µm wide; only one visible), and partial ring meristem (crack is an artifact). 83–84. Floral apex 170 µm in widest diameter (polar and lateral-oblique views), with carpel primordium about 92 µm in diameter, two large-stamen primordia (each 35–40 µm wide), and enlarged partial ring meristem at base. 85. Oblique view of flower with partial ring meristem. The carpel primordium is about 47 µm high on the adaxial side. The total height is probably twice that, but is difficult to measure. 86. Floral apex with ring meristem 280 µm in widest diameter. Large-stamen primordia are elongating and arched inward. Small-stamen initiation is beginning on the adaxial base of the ring meristem (at arrowheads). The carpel primordium is 100 µm high adaxially. 87–88. Flower (polar-adaxial and lateral views) with ring meristem 345 µm in widest diameter, on which several rows of small-stamen primordia have been initiated in acropetal order. The large-stamen primordia are about 150 µm high, but undifferentiated except for a few hairs abaxially. The carpel primordium is 170 µm high adaxially, has appressed margins along the cleft, and is densely strigulose abaxially.

Organ enlargement and development
The upper two sepal primordia become confluent laterally so that there appear to be only four sepals when they elongate and converge over the rest of the flower (Fig. 76). That only four sepals are visible in Fig. 76 and that one is median adaxial (not an expected site for a sepal) suggests that the two adaxial sepal primordia have converged as one. Subsequent sepal development is not shown, because they enlarge greatly during subsequent organogeny and must be removed to reveal stages of organ initiation. The calyx sheath becomes thick and protective in older buds (Fig. 89).

View larger version (208K): [in this window] [in a new window]   Figs. 89–97. Swartzia sericea. Midstages and late stages of floral organ development (SEM micrographs). Abaxial side is at base in Figs. 89–91, 93, and 95 , and at left in Fig. 94 . Subtending bracts and sepals have been removed in all. Scale bar = 50 µm in Fig. 92 and 200 µm in Figs. 89–91 and 93–97 . 89. Near-polar view of flower, calyx removed, showing densely strigose gynoecium with adaxial cleft visible. Large and small stamens are undifferentiated. 90–91. Polar views of androecium; carpel has been removed, as well as one of the two large stamens in Fig. 91 . Each large stamen has filament and anther (seen abaxially), with microsporangial development occurring on the adaxial side. Small-stamen primordia vary somewhat in size, with the larger ones on the periphery (at arrowheads); although undifferentiated, the peripheral ones are becoming distally enlarged. 92. Large stamen 170 µm high becoming differentiated into filament and anther, in which the median adaxial groove is present. 93. Lateral adaxial view of flower in which the small-stamen primordia are becoming distally enlarged (one at arrowhead), and the large stamens with differentiating anthers curve around the carpel. The carpel is covered with strigulose hairs, obscuring its outline. 94. Large stamen is 440 µm high; four microsporangia (arrowhead) are differentiating on its incurved adaxial side, and its filament is strigose-hairy. The small-stamen primordia are 70–120 µm high and enlarged distally. 95. Adaxial view of densely strigulose gynoecium, in which the upper part of the cleft is visible. The large stamen is about 440 µm high, the filament is thick and arcuate, and the anther has four parallel microsporangia adaxially (at arrowhead). 96–97. Oblique-lateral and adaxial views of carpel 400 µm high, densely strigulose, with appressed margins along the cleft

The short-stamen primordia remain undifferentiated until the large stamens have microsporangia. The short stamens begin to differentiate at slightly varying times; the primordia along the adaxial and abaxial periphery elongate before those at the center of the small-stamen cluster (Figs. 90–91). They first become distally enlarged at a height of 90–100 µm (Figs. 91 and 93; at arrowheads). Further stages of small-stamen development were not available.

The carpel primordium is initially broad (about 92 µm in Fig. 83) and arches inward as it grows in height, making it difficult to assess total height. It is 47 µm high on the adaxial side above the surface of the ring meristem, when the cleft is first visible adaxially (Fig. 85). The cleft deepens as the sides of the carpel grow; the two sides are appressed early by an adaxial height of about 170 µm (Fig. 87). The cleft persists the full length of the carpel through a height of 400 µm (Figs. 96–97). Trichomes begin to form abaxially on the carpel primordium by a height of 100 µm (Fig. 86) and become longer and more abundant until they obscure the outlines in older stages of the gynoecium (Figs. 93, 96, and 97). Because of the marked inward arching of the carpel in early stages, it was not possible to tell if there is an adaxial mound that presages the gynophore similar to that found in other species. No older stages of the gynoecium were available for study, so there is no information on formation of stipe, style, and stigma.

The flowers of Swartzia apetala Raddi var. subcordata Cowan, studied only in part, resemble those of S. sericea in organ number: no petal and 2–4 large stamens.

Swartzia madagascariensis Desvaux (Bobgunnia madagascariensis [Desvaux] Kirkbride and Wiersema)
Organography
This species occurs as small trees or shrubs in tropical and southern Africa. The inflorescences are racemose where terminal, and racemose or fasciculate where axillary. Racemes contain 2–14 sweet-scented flowers; the fascicles, when present, contain one or two flowers subtended by a single bract (Kirkbride and Wiersema, 1997 ) with small bracteoles at the base of the pedicel. The flower (Fig. 4a, b) has 2–4 brown hairy calyx segments and a single white petal 2.0–3.6 cm long by 1.8–2.7 cm wide. The petal is clawed, with a broadly elliptic blade that encloses the rest of the flower in bud. There are 10–16 large stamens 1.5–2.5 cm long and 40–100 small stamens 0.6–1.4 cm long (Fig. 4a, b); the size difference is slight and insignificant at anthesis. The ovary is stipitate, with a long narrow style and a capitate stigma.

Organogeny (Figs. 98–127)
The floral apical meristem, initiated singly per bract axil, is at first low convex and lenticular (Fig. 98). The first sepal primordium is initiated abaxially (Fig. 99). Although initiation of the remaining four sepal primordia was not observed, the fact that all four are equal in size shortly thereafter (Fig. 100) suggests that they are initiated either simultaneously or closely in succession. The sepal margins become confluent shortly after initiation (Fig. 101) to form a calyx tube around the floral apex. Trichomes cover the margins and abaxial surface of the sepal primordia (Figs. 101 and 105).

View larger version (203K): [in this window] [in a new window]   Figs. 98–109. Swartzia madagascariensis. Floral organogenesis (SEM micrographs). Abaxial side is at base in Figs. 98, 100–102, 104–106, and 108 and is labeled in Figs. 99, 103, and 107 . Subtending bracts were removed in all, and one or more sepals were removed in Figs. 100 and 102–108 . Scale bar = 25 µm in Figs. 98–99 and 102–103 ; 50 µm in Figs. 100–101 and 104–108 ; and 100 µm in Fig. 109 . 98. Bare floral apex in axil of bract. 99. Initiation of the first sepal, abaxially. 100. Floral meristem with all five sepals initiated. The scar of the first (abaxial) sepal primordium is at base; the four initiated subsequently are equal in size shortly after initiation. 101. The five sepals have been initiated; one is out of sight at left. They become confluent at their margins and have formed the calyx cup. 102–103. Post-sepal floral meristem nearly circular in shape and 100 µm in diameter, in nearly polar and oblique views. 104. Expanded floral meristem 155 µm in diameter, seen from lateral side. 105. Floral meristem at initiation of the petal (at arrowhead) in median adaxial position. 106. Near-polar view of flower about 160 µm wide tangentially and about 35 µm wide in the sagittal plane, with sepals removed. The petal has enlarged considerably, and two or three large-stamen primordia (at arrowheads) have been initiated on either side of the petal. 107. The initiation of large-stamen primordia (at arrowheads) has continued around the periphery of the floral meristem, with the youngest primordia close to the median on the abaxial side. The ridge is a partial ring meristem (R), tangentially broad and narrow in the sagittal plane. 108. Polar view with petal primordium removed to show the moundlike carpel primordium, the abaxial row of large stamens (at arrowheads), and a few small-stamen primordia starting to initiate laterally to the carpel and acropetally with relation to the previously initiated stamens. Most of the ring meristem ridge in the foreground has been broken away. 109. Hoodlike petal with undulate margin and trichomes beginning abaxially

Stamen initiation begins with two or three large-stamen primordia (at arrowheads; Fig. 106) produced laterally and adaxially on either side of the petal primordium. Abaxially the remainder of the floral meristem forms a thick ridge, a partial ring meristem. The carpel primordium forms next, immediately inside the single petal primordium (Fig. 108) followed by initiation of a row of additional large-stamen primordia (at arrowheads) on the abaxial periphery (Figs. 107–108). This row is continuous with the first stamens produced (Fig. 110). The 10–16 large-stamen primordia of this row (at arrowheads) form earliest at the sides and last toward the center of the row (Fig. 107). The peripheral large-stamen primordia rapidly become equal in size (A; Figs. 108–110).

View larger version (213K): [in this window] [in a new window]   Figs. 110–121. Swartzia madagascariensis. Floral organogenesis and organ development (SEM micrographs). Abaxial side is at base in Figs. 110 and 117 ; at right in Figs. 111, 114, and 119 ; at left in Figs. 118 and 120 ; adaxial side is at base in Figs. 112–113, 115–116, and 121 . Subtending bracts and sepals have been removed in all, and the petal removed in Figs. 112–121 . Scale bar = 50 µm in Figs. 110–114 ; 100 µm in Figs. 115–119 ; and 200 µm in Figs. 120–121 . 110. Flower with petal primordium covering center of flower; large-stamen primordia (A) are visible at left, and small-stamen primordia are at arrowheads. 111. The hoodlike petal covers most of the rest of the flower, except for small-stamen primordia on the abaxial side. 112–113. Near-polar views of flowers showing the lens-shaped carpel primordium, the single row of large-stamen primordia abaxially, and initiation of small-stamen primordia lateral to the carpel and on its adaxial side. 114. Lateral view of the carpel primordium (C) becoming incurved, with the large-stamen primordia also becoming incurved. 115–117. Oblique view, adaxial side, and polar views of flowers in which the large-stamen primordia are arched over toward the center of the flower. Differentiation of anther and filament is beginning (at arrowhead in Fig. 116 ). Small-stamen primordia are being initiated on the adaxial side of the carpel and laterally to it. The carpel has developed its adaxial cleft (in Figs. 115–116 ). 118–119. Lateral views of the arcuate carpel primordium (C) with a truncate tip. The large stamens are showing differentiation of anther and filament (at arrowhead). Small-stamen primordia remain much shorter and undifferentiated. 120–121. Flowers in which the large-stamen anthers are spatulate and are becoming inverted on the elongating filaments. The row of small stamens adaxially (between arrowheads) conceals other rows. The gynoecium has elongated and arched over, so that the tip touches the receptacle adaxially. The large scar is that of the petal.

The ring meristem is present in S. madagascariensis, but it is less pronounced than in other species. The early expansion in diameter and height of the floral apex at petal initiation, seen in this species, are features shared by the other Swartzia species that have evident ring meristems. The small-stamen primordia in S. madagascariensis initiate on a ridge that may be considered a partial ring meristem present on one side only, similar to that in S. sericea. Organ initiation proceeds acropetally in S. madagascariensis (compared to basipetal order in other species) until carpel initiation and then continues around the base of the carpel.

Organ enlargement and development
The sepal primordia become confluent to form a calyx cup very early (Fig. 101), which surrounds the remainder of the developing flower throughout development. It forms a seamless calyx tube, the entire calyx that typifies Swartzia. It encloses the enlarging bud throughout development and eventually splits irregularly at anthesis.

The single petal primordium enlarges relatively early to a height of about 60 µm and a width of about 110 µm (Figs. 106–107), before and during large-stamen initiation. The petal primordium enlarges by marginal growth, arches over, and covers the rest of the flower (Figs. 109 and 111) while stamen initiation is continuing. Sparse trichomes are produced abaxially (Fig. 109). The petal lamina becomes broadly circular and erect at anthesis (Fig. 4a).

The 10–16 large-stamen primordia elongate and arch inward (Figs. 112–116) and are rather uneven in size (Fig. 117). The anther and filament are differentiated by lateral expansion distally (at arrowheads; Figs. 116 and 119), beginning at a height of about 160 µm. The anther becomes tapered distally (Fig. 118) as the large-stamen primordia reach a height of 220 µm and broaden in width as microsporangia begin to form (Figs. 120–122) at a height of about 400 µm. The four microsporangia are aligned in a row on the adaxial side of each anther; only abaxial sides of the large incurved stamens are visible (Figs. 122 and 126).

View larger version (119K): [in this window] [in a new window]   Figs. 122–127. Swartzia madagascariensis. Late stages of floral organ development (SEM micrographs). Abaxial side is at left in Fig. 122 and at right in Fig. 123 . Subtending bracts, sepals, and the petal have been removed in all. Scale bar = 50 µm in Fig. 122 ; 200 µm in Fig. 125 ; and 400 µm in Figs. 123–124 and 126–127 . 122. Large-stamen primordia with anthers and filaments first differentiating. 123. Lateral side view of flower, with adaxial side at left. The large stamens are longest adjacent to the recurved gynoecium in the middle and successively shorter at the sides. 124. Adaxial side view of a flower, with large stamens removed. Numerous differentiated small stamens are present; one at left has the anther tilted to show the microsporangia aligned on the adaxial surface. 125. Single small stamen. 126–127. Two views showing the narrow, convoluted, glabrous gynoecium. Fig. 127 shows the tapered, capitate stigma

In large pre-anthesis buds of Swartzia madagascariensis, there is not a large size differential between large and small stamens (Figs. 124 and 126). At anthesis, the sizes are given by Cowan (1968) as 1.5–2.14 cm for large stamens and 0.6–1.4 cm for small stamens. This small size difference contrasts with that in other species of Swartzia.

The carpel primordium is at first wide tangentially and narrow radially (Figs. 108, 112, and 113). The tip begins to arch over toward the adaxial side (Fig. 114), after which the cleft becomes visible on the incurved side (Fig. 115). The cleft deepens as the flanges of the carpel expand (Fig. 116). Carpel elongation and adaxial curvature continue (Fig. 119) until the tip contacts the receptacular surface (Figs. 120 and 123). The carpel elongates further and becomes convoluted, twisting laterally (Figs. 124, 126, and 127). The stigma is narrow, terminal, and capitate (Fig. 127).

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Role and diversity of ring meristems
Ring meristems (a continuous, circular meristem with a less active center) have been reported in at least 29 plant families, including 23 eudicot families and six monocot families (unpublished data). But only a few legumes in two major groups have a ring meristem. It is present in most taxa investigated in papilionoid tribe Swartzieae: species of Swartzia (Tucker, 1987b , and present paper), Ateleia herbertsmithii (Tucker, 1990 ), Baphiopsis parviflora Benth. ex Baker, and Cyathostegia matthewsii (Benth.) Schery (unpublished data). A few taxa in caesalpinioid tribe Detarieae have a ring meristem: Microberlinia bisulcata A. Chev. (Tucker, 2002b ), and species of Brachystegia, Aphanocalyx, and Monopetalanthus (Tucker, 2000a ).

All species of Swartzia studied here as well as other taxa studied in Swartzieae (Ateleia herbert-smithii, Baphiopsis parviflora, Cyathostegia matthewsii) share a ring meristem as part of floral initiation, in contrast to the great majority of other legumes (about 300 species) that have been studied developmentally. Ring meristems in Swartzia species all function to greatly increase stamen number. They vary in being complete or partial, in their duration of activity, and in number of primordia initiated on them. A complete circular ring occurs in S. aureosericea and S. laurifolia vs. a partial ring in S. sericea and S. madagascariensis. The ring in the latter has more limited meristematic activity and fewer total stamens produced than the other species investigated. Ring meristems of Swartzia also vary in direction of organogenetic activity: basipetal (centrifugal) in S. aureosericea and S. laurifolia vs. acropetal in S. sericea and S. madagascariensis.

In species of Baphiopsis and Cyathostegia (also in Swartzieae sensu lato), the ring meristem functions to increase stamen number to 20–40 stamens. The stamens are initiated unidirectionally in Baphiopsis and acropetally in three irregular whorls in Cyathostegia (unpublished data). Petals are also initiated on the ring meristem in Baphiopsis but not in Cyathostegia. Ateleia is the only genus in Swartzieae that has a ring meristem that initiates only 10 stamens on the ring; they are initiated in erratic order and in atypical sites (Tucker, 1990 ). The ring meristems in these taxa, while similar in form and function to those of Swartzia species, have only a limited duration and a limited number of primordia initiated.

Unlike most taxa of Swartzieae, ring meristems in the caesalpinioid tribe Detarieae (species of Brachystegia, Aphanocalyx, Monopetalanthus, and Microberlinia bisulcata A. Chev.) do not function to increase stamen number. Five petals are initiated on the ring meristem in M. bisulcata (Tucker, 2002b ) and in the species of Brachystegia (Tucker, 2000a ), although in the latter the petals are subsequently suppressed. Stamens are initiated on the ring meristem in all the detarioid taxa mentioned, but only 10 in number. The order of stamen initiation on the ring meristem is unidirectional in M. bisulcata and Monopetalanthus, but atypical in the others: bidirectional in Brachystegia and erratic in Aphanocalyx.

Ring meristems have evolved at least twice among legumes: in papilionoid tribe Swartzieae and in caesalpinioid tribe Detarieae. They appear homologous based on similar form and function, although they differ in time of appearance (pre-petal or post-petal) and in duration and activity.

Organ loss in Swartzia compared with that in other legumes
Bracteoles are present in S. aureosericea and S. madagascariensis, but absent in S. laurifolia and S. sericea. Cowan (1968 , 1981 ) said that some individual sepals are absent in species of Swartzia, resulting in an entire, undivided calyx that splits irregularly as the flower and fruit matures. However, the irregular splitting of the calyx tube at maturity of the flower or fruit has no direct relation to the sepal number initiated or to the delimitations of the component sepals. Ontogeny shows that five sepals are initiated that become confluent early to form a solid tube around the developing flower. The minute sepal tips remain evident in young buds in at least some species. The calyx tube forms by intercalary growth below the level of individual sepal lobes. This type of calyx tube formation occurs in many other papilionoid flowers, but to a lesser degree, and the individual lobes enlarge in other papilionoids.

Calyx-tube formation via intercalary growth below the sepal primordia begins almost as soon as the primordia become visible, so that it is not always possible to be sure about the number of sepals involved. The first sepal primordium is always abaxial and largest, and its base curves around laterally, sometimes making it difficult to determine whether two lateral sepals are present in adjacent positions. Sepal initiation starts on the abaxial side, but varies thereafter between unidirectional or bidirectional according to species. Either three or four sepal primordia are usually visible in optimal preparations before individual sepals become obscured by hairs and formation of the calyx tube. A gap is often seen between the two adaxial sepals in such stages.

The entire type of calyx, considered an apomorphy for Swartzia, is shared by several other genera in the tribe (Cordyla, Baphiopsis, Mildbraediodendron; Cowan, 1968 , 1981 ; unpublished data). Cowan indicated some doubt that this character is significant, and I agree. Developmentally a tubular or "entire" calyx requires only the interpolation of intercalary growth below the separate sepal lobes after their initiation, a relatively simple developmental step similar to that of elongation of an internode. More developmentally complex characters found in some Swartzieae to date include heterostameny, multicarpely, and a ring meristem. The first two are not universally present in the tribe; the ring meristem may prove to be common to all (except for the Lecointea group; see Mansano et al., 2002 ).

The single petal in many species of Swartzia is the vexillary or standard petal, median on the adaxial side. It forms very early in development before the ring meristem, and is very low on the apical flank compared to petal initiation on a relatively flat apex in most legumes. No rudiments of other petals were seen in the present study, and Cowan (1968) reported only one species (S. dipetala Willdenow ex Vogel) with more than one petal. In species of Swartzia lacking a petal, no petal primordium is formed.

Among other taxa of Swartzieae, Ateleia herbert-smithii Pittier also has a single petal in the adaxial position (Tucker, 1990 ). Here too, no other petals are initiated. In related five-petallate taxa such as species of Exostyles, Harleyodendron, Lecointea, and Zollernia (Mansano et al., 2002 ) the adaxial petal primordium is the last to be initiated. Persistence of only the last-initiated member of an organ whorl is contrary to most examples of organ loss (Tucker, 1987a , 1988 , 1990 ) in which the last-initiated organs are usually the ones that are first to be lost. However, retaining the vexillary (adaxial) petal maintains the bilateral symmetry of the flower.

Failure of some organs to be initiated is associated with disruption of subsequent organ whorls in several legume flowers studied. The consequences of such disruptions may take the form of atypical organ positions, atypical order of organ initiation, or atypical timing of initiation such as prolonged delay (Tucker, 1988 ). Labichea lanceolata Benth., after initiating only four petals (rather than five), initiates only two stamens, and they are in positions that do not correspond to those expected in the stamen whorl (Tucker, 1998 ). Ateleia herbert-smithii Pittier shows both atypical order and positions. A single petal is initiated, after which the floral apex forms a ring meristem on which the 10 stamens are initiated in erratic order and not in the expected sites (Tucker, 1990 ). The insertion of a ring meristem as in species of Ateleia and Swartzia is a major alteration of conventional floral ontogeny among legumes, and it may produce greatly increased numbers of organs as well as changes in their positions and their order of inception. An example of atypical timing of initiation after organ loss is shown in Monopetalanthus durandii F. Hallé and Normand (Tucker, 2000a ) in which only one sepal is initiated at the usual time preceding that of a single petal, 10 stamens, and carpel. Later, after all other organs have initiated and started to enlarge, additional sepal rudiments may be initiated.

Stamen dimorphism and proliferation in Swartzia compared with that in other legumes
All but two species of Neotropical Swartzia have dimorphic androecia (Cowan, 1968 ). The two exceptions are closely related Brazilian species S. auriculata Poeppig, with about 22 identical stamens per flower, and S. arborescens (Aublet) Pittier, with 10–15 functional stamens and 15 staminodia per flower (Cowan, 1968 ). The remaining 125 Neotropical species all have dimorphic androecia with 2–20 large stamens and 20–200 small stamens per flower. Cowan (1968 , p. 12) says that two to about 25 large stamens may be present (accompanied by numerous small stamens) according to the species. He speculated that the evolutionary trend was toward loss of large stamens and that the two species known with only small stamens should be considered the most derived taxa.

Gynoecium specialization
Unfortunately no developmental material of multicarpellate species was available for this study. Multicarpely has arisen several times in Swartzia in the view of Cowan (1968) , who believed the condition to represent a specialization derived from unicarpellate species. He described two types