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(American Journal of Botany. 2002;89:875-887.)
© 2002 Botanical Society of America, Inc.


Anatomy and Morphology

Comparative floral ontogeny in Detarieae (Leguminosae: Caesalpinioideae). 1. Radially symmetrical taxa lacking organ suppression1

Shirley C. Tucker2

Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, California 93106-1467 USA; and Department of Plant Biology, Louisiana State University, Baton Rouge, Louisiana 70803 USA

Received for publication July 31, 2001. Accepted for publication December 13, 2001.


    ABSTRACT
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
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.

Key Words: Caesalpinioideae • Detarieae • Fabaceae • floral development • flower • HymenostegiaIsoberlinia • Leguminosae • Microberlinia


    INTRODUCTION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
The legume tribe Detarieae is the largest of four tribes of subfamily Caesalpinioideae and is the least studied tribe in the family. Most of the approximately 80 genera include tropical tree species that have not been adequately collected. A concerted effort is being made by several individuals and institutions to enlarge our knowledge of Detarieae by studies of systematics and floristics (Breteler, 1995 ; Breteler and Wieringa, 1999 ; Wieringa, 1999 ; Mackinder, 2000 ), molecular systematics (Bruneau et al., 2000 , 2001 ), comparative study of wood (Gasson, Trafford, and Matthews, 2000 ), vegetative anatomy (Herendeen, 2000 ), pollen (Banks and Klitgaard, 2000 ; Banks, Klitgaard, and Crane, 2001 ), reproductive biology (Lewis, Simpson, and Neff, 2000 ), and comparative floral development (Tucker, 2000a , b , c , d , 2001a , b , 2002 ).

Comparisons of floral development reveal the basis for the observed significant morphological distinctions among related taxa. These distinctions may be quantitative (e.g., heterochronic, or differences in timing of events) or qualitative (novel developmental events that are present in some groups and absent in others). While both types of distinctions occur in floral ontogeny of Detarieae, qualitative differences are of more interest because they have more evolutionary potential. The aims of this paper are (1) to compare floral ontogeny in representative taxa of the poorly known caesalpinioid legume tribe Detarieae sensu lato and (2) to use these data to seek correlations with groups currently considered to be clades (Bruneau et al., 2000 , 2001 ; Mackinder, 2000 ). The nine suprageneric "groups" within the tribe (Cowan and Polhill, 1981a , b ; Polhill, 1994 ) that were originally established by Léonard (1952 , 1957) are not supported by molecular evidence (Bruneau et al., 2000 ), but they provide a framework to assure representation of the major groups in the tribe. The Léonard groups represented here are Berlinia group (Isoberlinia, Microberlinia, Tessmannia), Hymenostegia group (Hymenostegia), and Detarium group (Phyllocarpus). Bruneau's work provides a rearranged set of relationships in Detarieae that is supported by Breteler (1995) and Breteler and Wieringa (1999) with minor differences that will be discussed later.

I have used an assemblage of floral developmental characters to group taxa in previous papers in this series. The first developmentally based group of taxa (previously treated) has a radially symmetrical flower, a circular floral apex, relatively small bracteoles directly after their initiation, all 21 floral organs, helical initiation of five sepals, and no ring meristem. Some taxa in this group have a medianly positioned first sepal (Amherstia, Brownea, and Tamarindus; Tucker, 2000c ), while other taxa in the group have the first sepal initiated nonmedianly (Crudia, Cynometra, Saraca, Schotia; Tucker 2000b , 2001a , b ). The genera Saraca and Crudia (Tucker, 2000b , 2001a ) were treated in separate papers because their floral ontogeny diverges markedly in being apetalous but differing in the causal factors resulting in apetaly.

A second developmentally based group (having the "Omega" complex) contrasts sharply with the first in having a radially elongate and tangentially narrow floral apex, relatively large bracteoles directly after their initiation, and nonhelical sepal initiation. A ring meristem functions during floral organogeny in some taxa, and organ number at anthesis may be less than the 21 organs initiated. The first sepal initiated may be either median (Aphanocalyx, Monopetalanthus; Tucker, 2000a ) or nonmedian (Brachystegia; Tucker, 2000a ).

Floral symmetry of the Omega group of Detarieae may be essentially radial or actinomorphic (as in the present paper), or zygomorphic, based on the adaxial petal being larger or solitary and on suppression of some stamens (Tucker, 2000a , 2002 ). The petals are completely missing in Brachystegia, making assessment of symmetry more difficult. Two genera (Paraberlinia, Sindora) are in the Omega group but differ significantly from any other Detarieae studied and will be described together (unpublished data).

The Detarieae treated here initiate all 21 floral organs per flower, and flowers are essentially actinomorphic. There is little or no organ suppression in the taxa studied here, including species of Hymenostegia, Isoberlinia, Microberlinia, Phyllocarpus, and Tessmannia. Taxa in the same group but showing strong suppression of petals and/or stamens after their initiation are described in the second paper of this series (Tucker, 2002 ).


    MATERIALS AND METHODS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Materials
Species to be studied and their provenance are listed in Table 1.


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Table 1. Sources for plant material in the present study. Vouchers are in Herbarium Vadense (Wageningen, the Netherlands) or in the Royal Botanic Gardens Herbarium, Kew, UK

 
Methods
Material collected by Breteler and colleagues was killed in liquid fixative, FAA (five parts formalin, five parts acetic acid, 90 parts 70% alcohol) in the field, and transferred and stored in 70% ethanol. Material from the spirit collection of the Royal Botanic Garden, Kew, UK, was preserved in Kew Mixture, which is similar but lacks acetic acid. Young buds and inflorescences of all sizes of each species were processed for scanning electron microscopy (SEM). Before dissection, they were dehydrated to 95% ethanol. Bracts and larger floral organs were removed from each piece under a dissection microscope. The resultant buds were further dehydrated through an ethanol-acetone series, critical point dried with CO2 in a Tousimus Samdri-780 drier apparatus (Tousimus Research, Rockville, Maryland, USA), mounted on aluminum stubs with carbon conductive adhesive tabs (T. Pella, Redding, California, USA), and coated with gold-palladium in an Denton Desk-1 sputter coater (Denton Vacuum, Moorestown, New Jersey, USA). The micrographs were taken with a JEOL JSM-6300V (JEOL Co., Tokyo, Japan) in the Geology Department at the University of California, Santa Barbara, California, USA.


    RESULTS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
All of the taxa described here are radially symmetrical and have five sepals, five petals, and ten stamens in two alternating whorls: Isoberlinia, Microberlinia, Hymenostegia, and Tessmannia. Three of the species are described and illustrated: Isoberlinia angolensis, which has fewest specializations; Microberlinia brazzavillensis, with some fusion in the androecium; and M. bisulcata, with a ring meristem.

Isoberlinia angolensis (Benth.) Hoyle & Brenan
Organography
The genus is in the Berlinia group of Léonard and includes 5–7 tree species, one in tropical Africa, the others in deciduous woodlands of Sudan and the Zambezi region. The medium-sized flowers of Isoberlinia angolensis (Fig. 1a, b) are in corymbose terminal panicles of racemes. The bracts are small and deciduous; bracteoles are large, covering the floral buds (Fig. 1a) and persistent after flowering. The flower (Fig. 1a, b, and e) is radially symmetrical with five subequal sepal lobes (upper two not fused) on a calyx tube, five subequal white petals (sepals and petals about the same length or petals slightly longer), ten fertile stamens with oblong pale yellow anthers and white filaments, and a stipitate ovary with elongate filiform style and a terminal stigma that is attached to the adaxial side of the hypanthium rim (Fig. 1d; Thompson, 1924 ; Hutchinson, 1964 ; Cowan and Polhill, 1981b ).



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Fig. 1. Drawings and floral diagrams of Isoberlinia angolensis. (a) Open flower. (b) Opening bud with bracteoles. (c) Large flower bud, dissected to show sepals, petals, and deep hypanthium. (d) Longitudinal section of bud to show gynoecium laterally attached to adaxial rim of hypanthium. (e) Floral diagram. Scale bar in all = 5 mm. Scale for (b) and (c) is between (b) and (c). Figure Abbreviations: A = outer or antesepalous stamen, a = inner or antepetalous stamen, Ab = abaxial side, Ad = adaxial side, B = floral bract, Bl = bracteole, C = carpel, F = floral apex, G = gynoecium, H = hypanthium, O = ovary, P = petal, R = ring meristem, S = sepal, St = stigma, Sy = style.

 
Isoberlinia angolensis, organogenesis
The inflorescence apical meristem initiates bracts in helical, acropetal succession (Figs. 2 and 3). A floral apex, narrow radially and wide tangentially, is formed in each bract axil (Fig. 4). Two bracteoles are initiated successively by the floral apex (Fig. 5). The bracteoles are large at initiation, and rapidly encircle 90% of the circumference, leaving an Omega-shaped floral apex, tapered adaxially and truncate abaxially (Fig. 6).



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Figs. 2–13. Floral organogenesis (scanning electron micrographs) of Isoberlinia angolensis. All are polar views with abaxial side at base except Figs. 2, 7, and 11 . Subtending bracts have been removed in all; bracteoles were removed in Figs. 2, 3, and 6–13 , and some or all sepals were removed in Figs. 9–13 . Scale bar = 50 µm in Figs. 5–7 ; scale bar = 100 µm in Figs. 3, 4, and 8–13 ; scale bar = 500 µm in Fig. 2 . 2–3. Polar views of inflorescence tip with helically arranged floral buds. The youngest buds are closest to the center. In Fig. 3 , the inflorescence apical meristem is at arrowhead, and the floral apex at right has one bracteole formed; the floral apex at left has two, removed. 4. Bare floral apex. 5. Floral apex after initiation of one bracteole. 6. Floral apex after initiation of two bracteoles, removed. First sepal primordium is being initiated abaxially. 7. Lateral view of floral apex with first sepal at right. The nearside bracteole has been removed. 8. Initiation of first sepal primordium nonmedianly on abaxial side, and second sepal primordium nonmedianly on adaxial side. 9. Third and fourth sepal primordia have initiated in lateral positions, and the fifth is nonmedian on the adaxial side. All five petal primordia have initiated. 10–11. Carpel initiation (polar and lateral views). 12. The five outer antesepalous stamen primordia (A) have been initiated, with the abaxial ones larger than the adaxial. Four inner antepetalous stamen primordia (a) have initiated centripetally from the petal primordia. 13. Polar view with all floral organs initiated. Sepals and two lower petals have been removed. The carpel cleft is first visible

 
The five sepals have a helical order of initiation, with the first initiating abaxially and nonmedianly (Figs. 7 and 8). The second sepal initiates adaxially and nonmedianly (Fig. 8), the third and fourth laterally in succession (Fig. 9), and the fifth adaxial and nonmedian, beside the second (Figs. 9 and 10). The petals also are initiated in helical order: the first abaxial and nonmedian, the second lateral, and the third adaxial-lateral (Figs. 8 and 9). In Fig. 9 the helical order is evident from relative sizes of petal primordia. The carpel is being initiated concurrently with the petals in Figs. 9–11.

Initiation of both stamen whorls is unidirectional (Fig. 12), beginning with the median abaxial and two laterals of the outer whorl concurrently. The last two outer-whorl stamens were just being initiated on the adaxial side in the same figure, and the first two inner-whorl stamen primordia were being initiated on the abaxial side (Fig. 12). Time of initiation overlaps in the two stamen whorls. All members of both stamen whorls are present in the midstage in Fig. 13. The carpel cleft is beginning in the same figure.

Isoberlinia angolensis, organ development
The bracteoles become thick and densely tomentose, forming a protective cover enclosing the floral bud (Figs. 1a and 17). The five sepal lobes elongate and overlap slightly, fused below into a calyx cup (Figs. 1c and 17). The two adaxial sepals remain discrete. The five petals begin to heighten and grow marginally as flat, ovate laminas (Fig. 21) at a height of 400–500 µm. Close to anthesis (Figs. 1b and 23) the petals appear subequal in height and shape. They become broad, sessile, and imbricate (Figs. 1a, 23, and 24).



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Figs. 14–25. Floral organ development (scanning electron micrographs) of Isoberlinia angolensis. Sepals removed in all, as well as other organs selectively removed to reveal inner structure. Scale bar = 100 µm in Figs. 14–16 ; scale bar = 200 µm in Figs. 18–20 ; scale bar = 300 µm in Figs. 17 and 21 ; scale bar = 1 mm in Figs. 22–25 . 14. Adaxial side view showing carpel with cleft beginning, petal and stamen primordia all initiated and nearly uniform in height. 15–16. Adaxial side view and polar view of floral bud showing petal primordia becoming laminar, outer stamen primordia larger than the inner ones, and cleft of carpel deepening. 17. Calyx of undissected floral bud, embedded in one of the bracteoles. 18–20. Lateral, adaxial, and polar views of a floral bud with petal primordia enlarged, outer stamens differentiated into filament and anther, and inner stamen primordia beginning to broaden distally. Copious hairs cover most of the carpel. 21. Side view of larger bud with petals enlarged but not yet imbricate. An antesepalous stamen at right has the anther dorsifixed. Trichomes cover the gynoecium. 22. Side view of floral bud just before anthesis, with sepals and petals removed. Both whorls of stamens are dorsifixed, with microsporangia separated by a strong abaxial median groove. 23. Adaxial side view of floral bud just before anthesis, with sepals removed. Petals are imbricately overlapped. 24–25. Polar views of floral bud just before anthesis. Fig. 24 shows the petals imbricately overlapping; they have been removed in Fig. 25 to reveal the five outer stamens and the densely trichomatous gynoecium

 
The ten stamen primordia in two whorls all differentiate as functional stamens. The outer, antesepalous stamen primordia differentiate first, by enlarging distally into the anther, each with an acute tip and a basal filament region evident relatively early (Figs. 18 and 19). The inner-whorl antepetalous stamen primordia begin to differentiate while much shorter than the outer ones (Fig. 19). The anthers are basifixed when first formed (Figs. 18 and 19), but through differential growth they become dorsifixed (Figs. 21 and 22). All stamens are equal in length at anthesis (Fig. 1a).

The carpel cleft becomes visible on the adaxial side at a height of 160–180 µm (Figs. 13–15). The cleft deepens as the margins expand (Fig. 16). By a height of 250 µm, the carpel is densely covered by trichomes (Figs. 19, 20, 22 and 23–25) that obscure further stages in development. At anthesis, the gynoecium is attached to the adaxial side of the hypanthium (Fig. 1d).

Microberlinia brazzavillensis A. Chev
Microberlinia, in the Berlinia group, includes two species of tall trees from tropical Africa with distichous branches and helical racemes or axillary panicles. The bracts are very large in bud, imbricate, covering the whole inflorescence, and soon becoming deciduous. The bracteoles are obovate, valvate, keeled, fused, and enclose the flower in bud (Fig. 26c and d). The radially symmetrical flower (Fig. 26a, b, and h) has four narrow uniform pale green sepals (Fig. 26f) that do not overlap at anthesis; the adaxial two are fused. The five subequal white petals include one that is clawed and slightly wider (Fig. 26g), up to 6.6 mm long and 3.2 mm wide, and four narrower petals, up to 2.2 mm wide but equally long or longer. The ten stamens include nine connate into a sheath split above (Fig. 26b), and the tenth is free; the anthers are yellow and uniform with white filaments. The ovary is pale green, stipitate, adnate to the margin of the tubular hypanthium (Fig. 26e). The style is slender and truncate; the pod valves have a strong wing (Hutchinson, 1964 ; Cowan and Polhill, 1981b ; Wieringa, 1999 ). Floral symmetry is essentially actinomorphic (Fig. 26h), except for the incomplete filament sheath open adaxially and the adaxial attachment of the gynoecium stipe.



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Fig. 26. Drawings and floral diagrams of Microberlinia brazzavillensis. (a) and (b) Open flowers: (b) is at right angles to (a) and has one bracteole removed. (c) and (d) Two views, one at right angles to the other, of a large floral bud. (e) Longitudinal section to show gynoecium laterally attached to adaxial rim of hypanthium. (f) Calyx of undissected bud. (g) Petal. (h) Floral diagram. Scale bar = 3 mm in (a)–(g). Scale between (c) and (d) applies to both.

 
Microbarlinia brazzavillensis, organogenesis
The inflorescence apical meristem initiates bracts acropetally and helically (Fig. 27). A floral apex, tangentially wide and sagittally narrow, is initiated in each bract axil (Fig. 28). It initiates two bracteoles, one preceding the other (Fig. 29), which enlarge rapidly so that they encircle the floral apex (Fig. 30). The latter is Omega-shaped, sagittally elongate, and tapered adaxially and abaxially.



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 Figs. 27–41. Floral organogenesis (scanning electron micrographs) of Microberlinia brazzavillensis. All are polar views with abaxial side at base except Figs. 27, 38, and 41 . Subtending bracts have been removed in all; bracteoles removed in Figs. 27 and 30–41 ; and some or all sepals removed in Figs. 38–40 . Scale bar = 50 µm in all. 27. Polar view of inflorescence apical meristem (at arrowhead) with helically arranged floral buds, of which the youngest are closest to the center. 28. Bare floral apex. 29. Initiation of first of two bracteoles. 30. Floral apex after initiation of two bracteoles (removed). 31. Initiation of first sepal primordium medianly on abaxial side. 32. Second sepal primordium is initiating on adaxial side. 33. Oblique lateral view of floral bud with fifth sepal primordium initiated adaxially (at arrow), and four of the five petal primordia initiated, as well as the carpel primordium at center. 34–35. Two views of a floral bud in which two lateral sepals have been initiated, and the fifth and last sepal primordium is visible in Fig. 35 (at arrow). Four petal primordia have initiated, the first two abaxially and the next two laterally. The carpel primordium and two lateral stamen primordia (A) of the first or antesepalous stamen whorl also have initiated. 36–37. Two views of a floral bud with the fifth and last petal primordium initiated. The carpel is a hemispherical mound. Initiation is proceeding in both stamen whorls in overlapping time: three antesepalous (A = outer) and two antepetalous (a = inner) stamen primordia have initiated, but stamen initiation has not occurred yet in the adaxial part of the young flower. 38–39. Side and polar views of a floral bud in which four stamen primordia have initiated adaxially, two to complete the antesepalous (A, outer) stamen whorl, and two of the antepetalous (a, inner) stamens; one more antepetalous stamen remains to be initiated adaxially. Sepal and petal primordia remain very short. 40–41. Polar and side views of a floral bud at midstage, with all floral organs initiated. The sepal primordia are starting to elongate and arch inward, and the carpel cleft is beginning to form

 
Sepal initiation is helical, beginning with one in the abaxial median position (Figs. 31 and 32). The second sepal is initiated adaxially (Fig. 32), the third and fourth laterally (Figs. 33–35), and the fifth is adaxial, nonmedian, and delayed (Fig. 35) so it overlaps in time with stamen initiation. Except for the first abaxial sepal, the others remain very small and do not arch over the other organs until well after all are initiated.

Petal initiation is unidirectional with the first pair abaxial and the second pair lateral (Figs. 33–35). The fifth, in median adaxial position, is difficult to discern until stamen initiation (Fig. 36). The carpel primordium initiates from the highly convex floral apex concurrently with the petals (Fig. 33).

Order of stamen initiation is bidirectional starting from the lateral positions in the outer antesepalous whorl and unidirectional from the abaxial side in the inner antepetalous whorl. The two whorls overlap with one other in time. The first stamen primordia of the outer antesepalous whorl to initiate are the two laterals (Figs. 33–36). The first two inner antepetalous stamens are also visible in these same figures. Next to initiate is the median abaxial outer stamen (Figs. 37 and 38). The remaining two antesepalous stamen primordia next initiate on the adaxial side; one is visible in Figs. 37 and 38; both are seen in Fig. 39. The lateral stamens of the inner antepetalous whorl initiate next (Fig. 39), and lastly the median adaxial of the same whorl is initiated, seen in the same figure. All five organ whorls overlap in time of initiation.

Microbarlinia brazzavillensis, organ development
The paired bracteoles are the protective structures, greatly overtopping the floral organs even in large buds (Figs. 26a and 48). The sepal primordia form a shallow, five-lobed calyx cup, with the abaxial sepal slightly larger than the rest, up to the time of midstage (Figs. 26b, f, and 41). Fusion between the two adaxial sepals is partial in Fig. 26f; the degree of fusion may vary, as Hutchinson (1964) described the flower as four-sepallate, which would indicate complete fusion of these two. The sepals remain relatively thin and narrow (Figs. 26a, f, and 49); they overlap imbricately in bud but are separate at anthesis.



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 Figs. 42–55. Floral organ development (scanning electron micrographs) of Microberlinia brazzavillensis. Most or all sepals removed in Figs. 43–47, 52, 54, and 55 . Other organs have been selectively removed to reveal inner structure. Scale bar = 50 µm in Fig. 42 ; scale bar = 100 µm in Figs. 43–47 ; scale bar = 200 µm in Figs. 48, 50, and 51 ; scale bar = 500 µm in Fig. 49 ; scale bar = 1 mm in Figs. 52–55 . 42. Lateral side view at midstage, showing sepal primordia enlarging over center, carpel with cleft, but other organs uniformly small and undifferentiated. 43. Polar view of floral bud with petal primordia elongating but remaining narrow. Outer antesepalous stamen primordia (A) have enlarged more than the inner-whorl stamen primordia (a). The carpel cleft has deepened. 44. Abaxial view of floral bud as petal primordia begin to show marginal growth, and outer (antesepalous) stamen primordia begin to enlarge distally. 45. Adaxial view showing anthers differentiating on outer, antesepalous stamen primordia (A), some of which show a median adaxial groove (one at arrowhead). The carpel cleft remains open while ovules are beginning to form within. 46–47. Adaxial and lateral side views of a carpel primordium with margins gaping open and unfused while ovules form within. Stamen anthers are differentiating microsporangia with both median groove (at arrow) and lateral grooves (at arrowheads). 48. Undissected calyx. 49. Portion of floral bud showing adaxial side of stamen anthers of both whorls. The antesepalous stamen anthers are larger and have longer filaments. The carpel has been removed; its scar is in a depression that will become the hypanthium. 50. Anther from large floral bud. 51. Gynoecium about 900 µm high with arched style. Some of the copious trichomes covering the ovary have been removed to show the form. The margins have fused by this stage. 52. Polar view of large flower bud, sepals removed, to show five uniform imbricate petals, five uniform antesepalous stamen anthers, and the coiled style. 53. Coiled style and capitate stigma. 54–55. Lateral views of large floral bud (sepals removed) showing clawed uniform petals, stamens with dorsifixed inverted anthers, style, and hypanthium. Sepal scars show that the sepals are relatively thin

 
The five petal primordia appear to be inserted in the sinuses of the calyx cup (Fig. 41). All remain relatively narrow with small laminas (Figs. 26a, b, 49, and 52) in the SEM stages studied. However, in flowers at anthesis, one petal is slightly larger than the rest (Fig. 26a and g; Hutchinson, 1964 ).

All ten stamens become functional. The outer-whorl antesepalous stamens go through differentiation before those of the inner antepetalous whorl. All stamen primordia of both whorls are equal in size at midstage (Figs. 40–42). The outer-whorl primordia become distally enlarged (Fig. 44) and broadened (Fig. 45). Anther and filament become evident (Fig. 47), and microsporangia begin to form, delimited by adaxial and lateral grooves (Figs. 49 and 50). The anthers are basifixed (Figs. 44 and 45) when first formed, but become dorsifixed later in development (Figs. 47, 49, and 54). Nine of the stamens become dorsifixed, connate, and raised up on a short filament tube (Figs. 26b and 49) split adaxially, formed by intercalary growth in the receptacle below the level of filament attachment. The median adaxial stamen remains free.

The carpel forms an adaxial cleft at a height of about 140 µm (Fig. 41). After elongating to a height of about 250 µm, ovule initiation is visible in the locule, seen between the gaping margins (Figs. 43 and 45). The carpellary margins remain open unusually long (Figs. 46 and 47) in this species. Eventually the carpellary margins become appressed and fuse. The gynoecium has a trichome-covered ovary, a revolute style in bud (Figs. 26a, 51, and 52), and a narrow stigma (Fig. 53). The carpellary stipe is attached to the adaxial side of the hypanthial rim (Fig. 26e).

Microberlinia bisulcata A. Chev
A complete organogenetic series was obtained for Microberlinia bisulcata, but only the early and midstages are illustrated because later stages do not differ appreciably from those in M. brazzavillensis. Bracts and subtended floral apices are initiated in helical acropetal order in the inflorescence (Fig. 56). Each floral apex is tangentially broad (Fig. 57) before bracteole initiation; the two bracteoles are initiated successively (Figs. 58 and 59) with broadly attached bases. Sepals are helical with the first median abaxial (Fig. 59) and the second median adaxial (Fig. 60). The last three sepals initiate in helical succession, but they remain as mere lateral shelves (at arrowheads in Figs. 61–63) through organogeny.



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Figs. 56–67. Floral organogenesis (scanning electron micrographs) of Microberlinia bisulcata. All are polar views with abaxial side at base except Figs. 56, 65, and 67 . Subtending bracts have been removed in all, and bracteoles removed in Figs. 59–67 . Scale bar = 50 µm in Figs. 57–64 ; scale bar = 100 µm in Figs. 65–67 ; scale bar = 500 µm in Fig. 56 . 56. Side view of young inflorescence with floral meristems forming in axils of bracts in acropetal succession. The inflorescence apex is at arrowhead. 57. Bare floral apex. 58. Initiation of two bracteoles in succession. 59. Initiation of first sepal primordium medianly on abaxial side. 60. Second sepal primordium is initiating on adaxial side and a third is visible at the right (at arrowhead). The floral apex is expanding to become circular. 61. Formation of a ring meristem bearing two petal primordia (at arrowheads) and an antesepalous stamen primordium (at double arrowhead) on the abaxial side. The carpel primordium has been initiated at center. Two of the last three sepal primordia have initiated as shallow shelves laterally. 62–63. Two floral buds in which most of the petals and antesepalous stamen primordia have been initiated on the abaxial and lateral sides of the ring meristem, but not on the adaxial side as yet. One or two antepetalous stamen primordia (a) of the inner stamen whorl have been initiated abaxially. The carpel primordium appears flattened on the adaxial side. 64. Additional antesepalous and antepetalous stamen primordia have been initiated to complete the two whorls. The carpel cleft is becoming visible. 65. Adaxial side view showing the adaxial carpel cleft, the calyx cup with sepal lobes and petals attached to the rim. 66. Polar view at midstage, after all organs have been initiated. The sepal lobes are beginning to overlap the center. 67. Adaxial side view showing the open carpel margins at the time ovules are initiating. Stamen anthers are starting to form thecae

 
The floral apex enlarges considerably and becomes round in polar view (Fig. 60). A ring meristem forms concurrently with the carpel primordium at the center (Fig. 61). The ring meristem is a raised circular ridge around the center of the floral meristem that remains meristematic. The petals and outer and inner stamens are initiated on a ring meristem, each whorl beginning abaxially and then proceeding unidirectionally. Two petal primordia and the first antesepalous stamen primordium appear abaxially on the ring meristem (Fig. 61). The other petals and antesepalous stamens have all been initiated by the next stages (Figs. 62 and 63), but only the abaxial members of the antepetalous stamen whorl have been initiated. The remaining antepetalous stamen primordia on the adaxial side have been initiated in Fig. 64. All organ whorls overlap at the time of initiation.

Although few older stages are shown of M. bisulcata, the carpel cleft (at a carpel height of about 190 µm) and the calyx tube are beginning in Fig. 65. The lengthening calyx lobes are shown in Figs. 65 and 66. The carpel margins are still open at an approximate height of 400 µm at the time of ovule initiation (Fig. 67); they become sealed shortly thereafter (not shown).

Hymenostegia klainii Pierre ex Pellegr
Hymenostegia, in the Hymenostegia group of Cowan and Polhill (1981a) , includes 16–20 species of trees and shrubs of tropical Africa. (This species is not illustrated, because its organogeny is very similar to that of Isoberlinia angolensis.) Hymenostegia klainii is a species of small trees about 3 m high with erect, racemose inflorescences that have pale pink helically arranged bracts. The bracteoles are large and ovate, white, petaloid, membranous, glabrous, flaring and persistent at anthesis. The two bracteoles fold imbricately in early stages but remain free and do not enclose the bud when large. The calyx in bud is pink. The small radially symmetrical flowers have a cupular or cylindric calyx tube, four imbricate sepals (the adaxial composed of two confluent sepals), five unequal free yellow petals (the outer two or three longer and broader, the abaxial two shorter and linear) that turn red over time. There are ten free stamens with white filaments and dorsifixed pale brown anthers that are inverted in bud and dehisce longitudinally. The ovary is pink with white hairs, stipitate, with filiform sigmoid style and punctiform white stigma. The carpel stipe is adnate to the side of the hypanthium (Hutchinson, 1964 ; Cowan and Polhill, 1981b ; label information from F. J. Breteler).

Developmentally, Hymenostegia klainii has an Omega-type floral apex at bracteole initiation, massive bracteoles, helical initiation of five sepals (the first nonmedian and abaxial), and unidirectional initiation in the petal and stamen whorls. The carpel initiates after the petals. The stamen primordia remain essentially similar in size in each whorl during development.

Tessmannia africana Harms
Tessmannia includes approximately 12 tree species of tropical Africa and is in the Detarium group. (This species is not illustrated.) The species are large trees with terminal or axillary racemes or panicles of flowers. Tessmannia africana has short compact terminal panicles, each branch of which has about 8–15 flowers arranged distichously. Bracts of T. africana are small and caducous at about 1 mm in length; bracteoles are free and neither aestivate nor valvate. Floral buds are glutinous, covered with short brown peglike warts, and are 7–9 mm high before anthesis. Individual flowers of T. africana are radially symmetrical and have four greenish brown sepals, slightly imbricate or valvate, verrucose or strongly warty. The five subequal petals are pale pink, narrowly oblanceolate, broadly clawed with a plicate margin on the limb. The ten stamens are declinate in bud; nine are connate in a thick hairy sheath, and one is free. The filaments are pink, the anthers red and dorsifixed, dehiscing lengthwise. The ovary is stipitate and the style is long, circinately coiled, with a small capitate stigma. The genus (Cowan and Polhill, 1981b ) is characterized by radial symmetry, five subequal petals, ten stamens with nine connate at base, and a short hypanthium.

Developmentally, T. africana is similar to the species of Isoberlinia and Microberlinia illustrated. It has an Omega-type floral apex at bracteole initiation and massive, enclosing bracteoles at an early stage. Petals are initiated unidirectionally, and the carpel initiates with the petals. No other stages of organogenesis were available for T. africana. Late stages of floral development are obscured by an abundance of hairs covering most organs.


    DISCUSSION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Omega-type assemblage of character states
Detarieae can be divided into two groups based on floral development: one group having a circular postbracteole floral apex, small bracteoles at initiation, and helical order of sepal initiation, and a second group having the "Omega"-type assemblage of character states that was reported for the Brachystegia group of Detarieae (Tucker, 2000a ). The "Omega" assemblage includes the following character states: a laterally narrow, sagittally elongate post-bracteole floral apex that is tapered adaxially and abaxially, and bracteoles that are massive just after initiation that encircle 90% of the circumference of the post-bracteole floral apex. The bracteoles become large and enclose the floral buds. The taxa studied in the present paper (Isoberlinia angolensis, Microberlinia brazzavillensis, M. bisulcata, Hymenostegia klainii, and Tessmannia africana) have the "Omega" set of character states. Other character states that the taxa studied here share include radial floral symmetry, all 21 floral organs present, no floral organs suppressed, uniform calyx, uniform corolla, and all ten stamens alike and functional. Position of the first sepal varies; it is initiated medianly in Microberlinia and nonmedianly in Hymenostegia and Isoberlinia.

The character states that they share with the "circular apex" type group (Amherstia, Cynometra, Schotia, Tamarindus, Crudia, etc.; Tucker, 2000c , 2001a , b ) include helical initiation of sepals in most taxa and complete calyx and corolla initiated. These are probably plesiomorphic character states and are shared with other caesalpinioid tribes, particularly the tribe Caesalpinieae (Kantz, 1996 ).

Comparisons among taxa studied
The developmental series shown (for Isoberlinia angolensis, Microberlinia brazzavillensis, M. bisulcata) are representative of the range of variations observed among taxa with all 21 floral organs initiated, radial symmetry, and little or no organ suppression. All share the Omega-shaped floral apex and massive bracteoles immediately after their initiation. All have helical initiation of sepals, 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 in the species studied, except for bidirectional order in the outer whorl in M. brazzavillensis. An unusual feature in M. bisulcata is the ring meristem, on which the 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 slightly later, with the outer stamens. The carpel remains open through the time of ovule initiation in both species of Microberlinia, but not in the other species studied. Stamen differentiation is essentially alike in all, with the anthers basifixed at midstage and then becoming dorsifixed in late stage of development. Hypanthium formation is pronounced, with the perianth and stamens attached to the rim, in both species of Microberlinia and in H. klainii.

Ring meristem
The floral apex forms a ring meristem (a raised meristematic circular ridge that initiates petal and stamen primordia) in Microberlinia bisulcata but not in the other taxa examined. The ring meristem is a significant developmental innovation in other legume taxa, particularly papilionoid tribe Swartzieae (unpublished data) and the Brachystegia group of Detarieae (Tucker, 2000a , d ). In Swartzieae, the ring meristem functions in greatly increasing the number of stamens per flower, but it produces only the basic ten stamens in M. bisulcata, Ateleia herbertsmithii (Tucker, 1990 ), and the Brachystegia group of Detarieae (Tucker, 2000a ). I have speculated (Tucker, 2000d ) that the ring meristem could have arisen by accelerating "overlap" between whorls and by decreasing the time interval between initiations of organ whorls. Overlap between initiation of organ whorls occurs in I. angolensis, M. brazzavillensis, M. bisulcata, and Berlinia grandiflora, but not in H. klainii, among the taxa studied here. Overlap among organ whorls is found in some other detarioid legumes (e.g., Monopetalanthus durandii, Brachystegia boehmii, B. glaucescens, Saraca declinata, S. indica; Tucker, 2000a , b ).

Specializations
At anthesis there are a few minor zygomorphic character traits in some taxa, such as adaxial attachment of the gynoecium in I. angolensis, M. brazzavillensis, and M. bisulcata, and there are differences in sinus depth between adjacent sepal lobes in several. Slight connation among stamen bases in M. brazzavillensis and M. bisulcata is another specialization. The persistent open carpel margins at ovule initiation in M. bisulcata is unusual; it is a heterochronic feature, but whether it is a specialization is open to question. Open carpels with ovules have been noted in 25 caesalpinioid species (Tucker and Kantz, 2001 ); the majority are in tribes Caesalpinieae and Detarieae.

Systematic relationships
A comprehensive phylogenetic analysis based on both molecular and morphological data and including 71 of the 84 genera of tribes Detarieae and Macrolobieae has been published by Bruneau et al. (2000) . Most of the taxa studied here belong to the Berlinia group of tribe Macrolobieae sensu Bruneau et al. (2000) . The analysis by Bruneau et al. (2000) however includes Tetraberlinia and Paraberlinia, both of which have marked organ suppression and zygomorphy and therefore will be published separately (Tucker, 2002 ; unpublished data). In a phylogenetic analysis of Berlinia and other putatively related genera, Mackinder (2000) found the genera Berlinia and Isoberlinia to be monophyletic and supported by morphology, but other clades remain cryptic and are not supported by known morphological states.

My investigations included only a few genera of the radially symmetrical members of Macrolobieae, so my comments are limited to those taxa. Floral development among these relatively unspecialized taxa, with their radial symmetry and little or no organ suppression, is essentially uniform. Shared developmental features of the Macrolobieae of Bruneau et al. (2000) that have been investigated developmentally are (1) the "Omega"-type floral apex (narrow tangentially, wide in sagittal plane) during initiation of bracteoles and the early sepals; and (2) massive bracteoles at initiation with very wide basal attachment that quickly enclose the floral apex. Many Omega-complex taxa show reduced numbers of organs at petal and sepal initiation (Tucker, 2000a ) or failure of organs to develop after initiation (Tucker, 2001a ). In the species investigated here, however, there is no consequent diminution of organ primordia of sepals or petals, as there is in other detarioid taxa having the reduced floral apex characteristic of the Omega complex.


    FOOTNOTES
 
1 The author thanks Jo Anna Bass and Andrew Douglas for their technical assistance with scanning electron microscopy and photography, Janice Beckert for drawings, and the following for collections: Franz Breteler and J. Wieringa, University of Wageningen, Netherlands; Patrick Herendeen, Colin Hughes, and the staff of the Royal Botanic Gardens, Kew, England. The research was supported by National Science Foundation grants DEB92-07671 and DEB-9420158 (DEB-9596281). The author also acknowledges funds provided for publication as a Boyd Professor Emerita from Louisiana State University, Baton Rouge, Louisiana. Back

2 Tucker{at}lifesci.ucsb.edu Back


    LITERATURE CITED
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Banks H. I. B. B. Klitgaard 2000 Palynological contribution to the systematics of detarioid legumes (Leguminosae: Caesalpinioideae). In P. S. Herendeen and A. Bruneau [eds.], Advances in legume systematics, part 9, 79–106. Royal Botanic Gardens, Kew, UK

Banks H. I. B. B. Klitgaard P. R. Crane 2001 Palynological contribution to the systematics of the Caesalpinioideae (Leguminosae). In Legumes down under, Abstracts, 7. The Fourth International Legume Conference, 2–6 July 2001. Australian National University, Canberra, Australia

Breteler F. J. 1995 The boundary between Amherstieae and Detarieae (Caesalpinioideae). In M. D. Crisp and J. J. Doyle [eds.], Advances in legume systematics, part 7, Phylogeny, 53–62. Royal Botanic Gardens, Kew, UK

Breteler F. J. J. J. Wieringa 1999 Generic delimitation in Caesalpinioideae. XVI International Botanical Congress, St. Louis, Missouri, August 1999. Abstract 16.2.1. Missouri Botanical Garden, St. Louis, Missouri, USA

Bruneau A. F. J. Breteler J. J. Wieringa G. Y. F. Gervais F. Forest 2000 Phylogenetic relationships in tribe Macrolobieae and Detarieae as inferred from chloroplast trnL intron sequences. In P. S. Herendeen and A. Bruneau [eds.], Advances in legume systematics, part 9, 121–149. Royal Botanic Gardens, Kew, UK

Bruneau A. F. Forest P. S. Herendeen B. B. Klitgaard G. P. Lewis 2001 Phylogenetic relationships in the Caesalpinioideae (Leguminosae) as inferred from chloroplast trnL intron sequences. Systematic Botany 26: 487-514[ISI]

Cowan R. S. R. M. Polhill 1981a Detarieae. In R. M. Polhill and P. H. Raven [eds.], Advances in legume systematics, part 1, 117–134. Royal Botanic Gardens, Kew, UK

Cowan R. S. R. M. Polhill 1981b Amherstieae. In R. M. Polhill and P. H. Raven [eds.], Advances in legume systematics, part 1, 135–142. Royal Botanic Gardens, Kew, UK

Gasson P. C. Trafford B. Matthews 2000 Wood anatomy of Caesalpinioideae. In Legumes down under, Abstracts, 30. The Fourth International Legume Conference, 2–6 July 2001. Australian National University, Canberra, Australia

Herendeen P. S. 2000 Structural evolution in the Caesalpinioideae (Leguminosae). In P. S. Herendeen and A. Bruneau [eds.], Advances in legume systematics, part 9, 45–64. Royal Botanic Gardens, Kew, UK

Hutchinson J. 1964 The genera of flowering plants (Angiospermae), vol. 1, Dicotyledones. Oxford University Press, Oxford, UK

Kantz K. E. 1996 Floral development in the caesalpinioid tribe Caesalpinieae (Fabaceae). Ph.D. dissertation, Louisiana State University, Baton Rouge, Louisiana, USA

Léonard J. 1952 Cynometrieae et Amherstieae. In Flore du Congo Belge et du Ruanda-Urundi, vol. 3, Spermatophytes, 279–495. Publications de l'Institut National pour l'Étude Agronomique du Congo Belge, Bruxelles, Belgium

Léonard J. 1957 Genera des Cynometreae et des Amherstieae africaines (Leguminosae-Caesalpinioideae). Memoires Academie Royale de Belgique, Classe Sciences 30: 1-314

Lewis G. P. B. B. Simpson J. L. Neff 2000 Progress in understanding the reproductive biology of the Caesalpinioideae (Leguminosae). In P. S. Herendeen and A. Bruneau [eds.], Advances in legume systematics, part 9, 65–78. Royal Botanic Gardens, Kew, UK

Mackinder B. 2000 The monophyly and taxonomic position of Berlinia Hook. f. (Leguminosae-Caesalpinioideae): a preliminary study. In P. S. Herendeen and A. Bruneau [eds.], Advances in legume systematics, part 9, 151–163. Royal Botanic Gardens, Kew, UK

Polhill R. M. 1994 Complete synopsis of legume genera. In F. A. Bisby, J. Buckingham, and J. B. Harborne [eds.], Phytochemical dictionary of the Leguminosae, vol. 1, xlix–liv. Chapman and Hall, New York, New York, USA

Thompson J. M. 1924 Studies in advancing sterility, part 1, The Amherstieae. Hartley Botanical Laboratory Publications, Liverpool University, Liverpool, UK

Tucker S. C. 1990 Loss of floral organs in Ateleia (Leguminosae: Papilionoideae: Sophoreae). American Journal of Botany 77: 750-761[CrossRef][ISI]

Tucker S. C. 2000a Evolutionary loss of sepals and/or petals in detarioid taxa Aphanocalyx, Brachystegia, and Monopetalanthus (Leguminosae: Caesalpinioideae). American Journal of Botany 87: 608-624[Abstract/Free Full Text]

Tucker S. C. 2000b Floral development and homeosis in Saraca (Leguminosae: Caesalpinioideae: Detarieae). International Journal of Plant Sciences 161: 537-549[CrossRef]

Tucker S. C. 2000c Floral development in tribe Detarieae (Leguminosae: Caesalpinioideae): Amherstia, Brownea, Tamarindus. American Journal of Botany 87: 1385-1407[Abstract/Free Full Text]

Tucker S. C. 2000d Organ loss in detarioid and other leguminous flowers, and the possibility of saltatory evolution. In P. S. Herendeen and A. Bruneau [eds.], Advances in legume systematics, part 9, 107–120. Royal Botanic Gardens, Kew, UK

Tucker S. C. 2001a The ontogenetic basis for missing petals in Crudia (Leguminosae: Caesalpinioideae: Detarieae). International Journal of Plant Sciences 162: 83-89[CrossRef][ISI]

Tucker S. C. 2001b Floral development in Schotia and Cynometra (Leguminosae: Caesalpinioideae: Detarieae). American Journal of Botany 88: 1164-1180[Abstract/Free Full Text]

Tucker S. C. 2002 Comparative floral ontogeny in Detarieae (Leguminosae: Caesalpinioideae). 2. Zygomorphic taxa with petal and stamen suppression. American Journal of Botany 89: 888-907[Abstract/Free Full Text]

Tucker S. C. K. E. Kantz 2001 Open carpels with ovules in Fabaceae. International Journal of Plant Sciences 162: 1065-1973[CrossRef]

Wieringa J. J. 1999 Monopetalanthus exit. A systematic study of Aphanocalyx, Bikinia, Icuria, Michelsonia and Tetraberlinia (Leguminosae, Caesalpinioideae). Wageningen Agricultural University Papers 99 (4): 1–320. Wageningen University, Wageningen, The Netherlands




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