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Structure and Development |
Department of Biology (Ecology, Evolution, and Marine Biology), University of California, Santa Barbara, California 93106 USA; and Department of Plant Biology, Louisiana State University, Baton Rouge, Louisiana 70803 USA
Received for publication May 23, 2000. Accepted for publication February 1, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: Caesalpinioideae • Cynometra • Detarieae • flower • legume • Leguminosae • ontogeny • Schotia.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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80 genera of caesalpinioid legumes including taxa with "repeated and spectacular modification of the flower"(Cowan, 1981
; Cowan and Polhill, 1981a, b
). It includes two former tribes: Detarieae sensu stricto and Amherstieae, which differ in the prominent bracteoles in Amherstieae, in contrast to bracteoles either absent or not prominent in Detarieae. Breteler (1995)
recently placed the large-bracteoled group including Amherstia in Detarieae, which effectively removes any consistent distinction between the two tribes. Cowan and Polhill (1981a, b)
arranged the genera of Detarieae/Amherstieae in ten informal "groups," after Léonard's (1952, 1957)
somewhat fewer "groups" present in Africa. My current series on floral development among Detarieae (Tucker 2000a, b, c, 2001
) uses these groups as a framework for developmental comparisons. As part of a continuing comparison of floral development in representative caesalpinioid legumes, this paper examines several taxa in the Cynometra group—three species of Schotia (S. afra, S. brachypetala, and S. latifolia), plus Cynometra webberi and an undescribed species of Cynometra—to elucidate the developmental bases for similarities and differences among these taxa. This group of taxa shares a relatively unspecialized caesalpinioid flower with all organs present, relatively small bracteole size after their initiation, a nonmedian position of the first sepal initiated, and a circular prepetal floral apex.
The taxa of the Cynometra group have plesiomorphic flowers, which often lack any apomorphies that would unite the group in an intellectually satisfactory way. The problems of such studies are discussed later. The useful literature on floral structure and development in Detarieae includes Baillon (1872)
, Hutchinson (1964)
, and Thompson (l924) for general floral descriptions.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Cynometra webberi E. G. Baker was collected by J. Reitsma (#131, sent to the author by F. J. Breteler) in Kenya, along a road to Bamba, 12 km west of Ganze Forest, 5 November 1979. Voucher is at the University of Wageningen, Netherlands (WAG).
Cynometra sp. was collected by F. J. Breteler, T. Nzabi & J. J. Wieringa (#12828) in Gabon: Woleu-Ntem, Cristal Mountains, on a road from Tchimbélé to Assok, 12 September 1994. This is a new, undescribed species in Cynometra sensu lato. If the latter genus is split, this species would fall in Plagiosiphon cf. Breteler (F. J. Breteler, University of Wageningen, The Netherlands, personal communication).
Material of S. brachypetala was killed in liquid fixative, FAA (5 parts formalin: 5 parts acetic acid: 90 parts 70% alcohol) in the field, and transferred and stored in 70% ethanol with a few drops of glycerine. Young buds and inflorescences of all sizes of each species were processed for scanning electron microscopy at Louisiana State University, Baton Rouge, Louisiana, USA. 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 Denton DCP apparatus (Denton Vacuum Co., Moorestown, New Jersey, USA), mounted on aluminum stubs with either Tubecote or colloidal graphite, and coated with gold-palladium in a Edwards S-150 sputter coater (Edwards High Vacuum Co. International, Wilmington, Massachusetts, USA). Micrographs of S. brachypetala were taken at Louisiana State University, Baton Rouge, Louisiana, USA with a Cambridge 260 scanning electron microscope (SEM; Cambridge Scientific Instruments, Cambridge, UK) at 25 kV.
Material of Schotia afra, S. latifolia, and Cynometra webberi was prepared at the University of California, Santa Barbara, California, USA. Buds were preserved in liquid fixative in the field, transferred to 95% alcohol, and dissected in the laboratory. The pieces were further dehydrated through an acetone-ethyl alcohol series, critical point dried with CO2 in a Tousimus Samdri-780 drier (Tousimus Research Corporation, Rockville, Maryland, USA), and mounted on aluminum stubs with carbon conductive adhesive tabs (Ted Pella Co., Redding, California, USA). They were coated with gold-palladium in a Denton Desk-1 sputter coater, and micrographs were taken at 25 kV with a Hitachi S-415A SEM (Hitachi Co., Tokyo, Japan) in the Department of Biology at the University of California, Santa Barbara.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Ross, 1977
). The showy, red, tubular, bisexual flowers are crowded into short, highly branched racemes or panicles (Baillon, 1872
; Hutchinson, 1964
), often while the tree is essentially leafless. The flowers (Fig. 1a–d) have membranous caducous bracteoles that never enclose the flowers, four imbricate sepals, five, bright-red, fleshy, imbricate petals (often subequal), ten free stamens with dorsifixed anthers declinate in bud (sometimes slightly connate basally into a filament tube), a stipitate ovary with an elongate style, small stigma, and numerous ovules. The stipe is adnate to the side of the hypanthium (Fig. 1b), and there is a long turbinate hypanthium. The flower is radially symmetrical.
Schotia brachypetala
Organogeny
The inflorescence apical meristem initiates bracts helically (Figs. 5–6), each of which subtends a single floral apex (Figs. 6–7). This tangentially broad floral apex first initiates a pair of bracteoles, one usually slightly ahead of the other (Figs. 7–8). The floral apex becomes circular and produces the first sepal primordium abaxially and slightly to one side of the median sagittal plane (Figs. 9–10). The subsequent sepal initiations are helically ordered, with the second being adaxial and nonmedian (Fig. 10), the third and fourth lateral (Fig. 11). The fifth sepal is adaxial and nonmedian (Fig. 12), beside the second.
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The carpel primordium initiates with the first of the outer stamens (Fig. 14) in a noncentral position closer to the adaxial side. The meristem surface around the carpel primordium increases in area (Fig. 14), and the first antesepalous (outer) stamen initiates in the median abaxial position (Figs. 14–16, at arrowhead). The same flowers show expanded areas of meristem laterally between adjacent petal primordia, where the next outer stamens would have been initiated. In Fig. 17, the two lateral antesepalous stamens have been initiated in addition to the median abaxial one and two antepetalous stamen primordia. The last two outer stamens are initiated adaxially (Figs. 19–20, 23). Order of initiation among outer (antesepalous) stamens is unidirectional.
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Schotia brachypetala
Organ differentiation
The two adaxial sepals become laterally confluent (Fig. 15, at asterisk) so that there appear later to be only four sepals (Fig. 1a). The petal primordia broaden and undergo marginal growth (Figs. 20, 25, 31) to form laminas, beginning with the more abaxial members. All petal and stamen primordia become inwardly tilted toward the carpel (Figs. 19–21) as hypanthium formation begins. In many flowers, one petal primordium (usually one of the abaxial petals) does not develop and remains much smaller than the rest (Figs. 1a, 22, 24, 35–36).
Differentiation of the outer, antepetalous stamen primordia begins with distal expansion for anther formation (Fig. 26), and filaments result from the narrow bases. Three of the five outer stamen primordia enlarge and differentiate before the other two (Figs. 27, 29), but eventually all members of the outer whorl are equal in size and stage (Fig. 36). Median dorsal grooves (at arrowheads, Fig. 30) form first in the anthers, followed by lateral grooves (Fig. 36, arrowheads) that delimit the microsporangia. Later, abaxial median grooves become noticeable (Fig. 36). The filaments of the outer stamens become flared basally (Figs. 31, 33, 35–36). The anthers are inverted in bud on recurved filaments (Fig. 38). The inner (antepetalous) stamens differentiate later than those of the outer whorl, and remain smaller at first (Figs. 20, 22, 26–27); their development is similar (Figs. 33, 35–36) to that of the outer antesepalous stamens. By anthesis, the filaments are connate at base (Fig. 1a).
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700 µm (Fig. 35) and is revolute by a height of 2 mm (Figs. 36–37). A stipe forms basally (Fig. 37) as the hypanthium forms to one side of the gynoecial base (Fig. 37). The capitate stigma is seen in Fig. 39.
Schotia afra (Figs. 2, 40–67)
Organography
The inflorescences are few-flowered terminal racemes of pink flowers with dark-red pedicels and sepals (Codd, 1956
; Ross, 1977
). Bracts and bracteoles are caducous. Bracteoles are small and acutely tipped and do not enclose the flowers. Floral buds are obovate (Fig. 2c) and up to 1.5 cm long. Each flower (Fig. 2a) is 3–3.5 cm long at anthesis, with radial symmetry (Fig. 2d). It has four sepals only, although one is two-lobed and wider than the others. Sepals are imbricate, with the two largest and outermost completely covering the bud. They are dark red with pale irregular spots, membranous, and glabrous (Fig. 2a, c). At anthesis (Fig. 2a) the sepals spread outward at an angle but are not reflexed. There are five petals, pink in color, membranous, and glabrous. Each is 1.5–2.2 cm long and has an ovate lamina tapering to a claw that is approximately half the total length. The ten stamens are 3 cm long and long-exserted (Fig. 2a). They are in a single whorl at anthesis (Fig. 2b). The anthers are oblong, dorsifixed, tetrasporangiate, and versatile with longitudinal dehiscence. The gynoecium has a flat-sided glabrous stipitate ovary, an elongate style, and a capitate stigma. It is adaxially attached to the cup-shaped hypanthium (Fig. 2b), which has a ten-lobed disk, the lobes alternating with the stamen filaments (Fig. 2b).
Schotia afra
Organogeny
Bracts are initiated in helical acropetal succession; each bract subtends a flower (Fig. 40). Each floral apex is at first broader tangentially than sagittally (Fig. 41). Opposite bracteoles are initiated (Fig. 42), and then the first sepal nonmedianly on the abaxial side (Fig. 43). The second sepal primordium is adaxial and nonmedian (Figs. 43–44), and the remaining three are initiated successively (Figs. 44–45) continuing the helical order of initiation. The two adaxial sepal primordia later become laterally confluent (Fig. 47) and subsequently appear as one sepal.
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The carpel primordium is initiated at the same time as the last petal primordia (Fig. 48), and soon becomes a circular, convex mound (Fig. 50). The adaxial cleft begins (Figs. 51–52) at a height of 55 µm in Fig. 53.
Stamen formation begins with initiation of a lateral antesepalous primordium (Fig. 48), an unusual site, since in most related taxa, the first stamen is initiated in median abaxial position. The floral apex at this time is still unequally pentagonal; this fact, plus the lateral position of the first stamens, suggests that stamens of this outer whorl may form in helical order. The outer (antesepalous) stamens are all initiated in Fig. 50, although the abaxial (first-initiated) ones are larger than the adaxial ones. All are equalized in size in Fig. 51, when the first two members of the second or antepetalous whorl of stamens are being initiated abaxially. The antepetalous stamens appear to be initiated unidirectionally, starting abaxially. All five are present in Figs. 51–52 and 54, where they all appear to be equal in size. The petal and outer stamen primordia remain similar in height (Fig. 53) while the carpel primordium is approximately twice their height. In an occasional flower, organ initiation on one side is delayed (Figs. 51–52).
Schotia afra
Organ development
The small paired bracteoles are attached directly below the flower and flare outward, not enclosing the obovate bud (Fig. 63). They drop off as the buds enlarge. The sepals develop broad incurved blades and overlap imbricately (Figs. 2a, 56). The two outermost sepals enclose the bud. In the open flower (Fig. 2a), the four sepals flare outward; one is wider and has a bifurcate tip.
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Differentiation of the antesepalous stamen primordia begins with distal broadening (Fig. 58) that will form the anther. The anther is at first basifixed (Fig. 58) and then becomes dorsifixed (Figs. 61–62) by differential growth. Development of a median adaxial groove (arrow in Figs. 60, 62, 64) and lateral grooves (Fig. 62, arrowheads) delimit the microsporangia. The median groove becomes the connective, broad on the adaxial side (Fig. 62) and tapered and deeply indented on the abaxial side (Figs. 64, 67). The anthers in large buds (Fig. 67) and open flowers have oblong, longitudinally dehiscent anthers and a small knob on the connective. The filaments are attached to the rim of the hypanthium and are long-exserted at maturity (Figs. 2a, b, 67). Development of the stamens is similar in the two whorls, except that the inner (antepetalous) whorl differentiates later and generally has shorter filaments than the outer whorl (Fig. 64), at least during development. At anthesis the two stamen whorls have merged into a single whorl (Figs. 2b, 67) by differential growth of the receptacle. All ten stamens are functional.
The carpel cleft remains open for an unusually long time (Figs. 58–60: 180 µm high) overlapping with the time of ovule initiation. The margins become sealed by a height of 190 µm (Fig. 61). The style becomes evident and recurves (Figs. 62–66) when the carpel is 540 µm high. The adaxial suture, the region of marginal fusion, remains visible (Fig. 66) nearly to the base. The hypanthium is formed by the upward growth of the receptacle below the organs around the gynoecium (Fig. 62). Petals and stamens are attached to the rim of the deepening depression around the gynoecium (Figs. 2b, 67). At anthesis the gynoecium consists of a flat-sided glabrous stipitate ovary, an elongate style, and a capitate stigma (Fig. 2b). The hypanthium is lined by a disk that has ten short lobes that alternate with the stamen filament bases (Fig. 2b).
Schotia latifolia (Figs. 3, 68–82)
Organography
The inflorescences are few-flowered terminal racemes. Bracts and bracteoles are caducous; the bracteoles are small and do not enclose the flowers. The flower buds are obovate and dark red. Each flower (Codd, 1956
; Ross, 1977
; Fig. 3a) is 3.2–3.5 cm long with essentially radial symmetry, except that the gynoecium is adaxially attached to the side of the hypanthium (Fig. 3b) and the style arches downward at its tip. Each flower has four dark red membranous, glabrous sepals, erect and slightly flared outward. The four sepal lobes have rounded tips, are 2 cm long, and one (on the adaxial side) is slightly wider than the others. Rarely, there is a rudiment of the fifth sepal. The lobes are attached to a short calyx cup. The five petals alternate with the sepals, are dark red in color, and differ strongly in size; three are 1.5 cm in length, with a small lanceolate lamina tapering to a claw that is approximately half the total length. The other two petals, the abaxial pair, remain as tiny filamentous rudiments. The ten stamens are in a single whorl and are long-exserted. They are attached basally to a short filament tube (Fig. 3b). The anthers are oblong, dorsifixed, tetrasporangiate, and versatile with longitudinal dehiscence. The gynoecium includes a stipitate glabrous ovary, an elongate style, and a capitate stigma, and is attached to the adaxial side of the hypanthium (Fig. 3a, b).
Schotia latifolia
Organogeny
The inflorescence apical meristem of Schotia latifolia initiates bracts and subtending flowers in helical acropetal succession. Each floral apex is at first laterally broad and sagittally narrow (Fig. 68). Paired opposite bracteoles are initiated in quick succession (Fig. 69). The first sepal primordium is initiated abaxially but to one side of the median sagittal plane (Fig. 70). Successive sepal initiations are in helical order (Figs. 71–72), either clockwise (Figs. 72–73) or counterclockwise (Fig. 71). The resulting pentagonal floral apex has unequal sides (Fig. 72). Petal order is successive and unidirectional but close in time (Figs. 72–73, 77). The median adaxial petal is last to initiate (Figs. 74–76); the petals are equalized in size by midstage (Fig. 78). Four of the five later elongate, but one of the two abaxial petals usually remains smaller than the rest (Figs. 81–82).
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The stamens are initiated in unidirectional order in each of the two whorls. The first stamens of the antesepalous whorl are initiated in lateral positions (Fig. 75), with the median abaxial following closely (Fig. 76). The two adaxial members of this whorl initiate last (Fig. 76, at arrows). Initiation of the antepetalous stamens overlaps with that of the outer antesepalous stamen whorl. Four antepetalous stamens are present in Fig. 76 (at arrowheads), and the last (adaxial antepetalous) stamen has initiated in Fig. 77. Members of each stamen whorl remain equal in size throughout development.
Schotia latifolia
Organ development
Bracts and bracteoles in the inflorescence are caducous. Sepals mature as in the other species of Schotia, with the adaxial two becoming confluent. The flower at anthesis (Fig. 3a) has a calyx cup with four dark red membranous, glabrous sepal lobes, erect and slightly flared outward. The four sepals are subequal except for the adaxial, which is slightly wider than the others. Rarely there is a rudiment of the fifth sepal.
Petal primordia grow marginally, three or four becoming broadly lanceolate and acutely tipped (Fig. 79), while one or two others remain short with a rudimentary lamina (Figs. 81–82). At anthesis the five petals differ markedly in size (Fig. 3a, b); three or four are large, each with a small lanceolate lamina tapering to a claw that is approximately half the total length. The other one or two petals, usually on the abaxial side, remain as tiny filamentous rudiments (Figs. 3b, 81–82). The petals are attached to the stamen filament tube on the rim of the hypanthium (Fig. 3b).
Stamen differentiation begins with distal enlargement of the antesepalous stamen primordia (Fig. 79). The inner antepetalous stamen primordia are delayed in development (Fig. 80) but follow the same sequence of development as the outer stamens. By anthesis, the two whorls have merged into one (Fig. 3b) by differential growth of the receptacle. All ten are functional at maturity (Fig. 3a). In the developing anthers, median grooves (at arrows) on both abaxial and adaxial sides and lateral grooves (at arrowheads) delimit the developing microsporangia (Figs. 81–82). The stamen anthers are basifixed at first (Fig. 80) but toward maturity become dorsifixed (Fig. 81) by differential growth; they are versatile with longitudinal dehiscence. The filaments are long-exserted and are attached basally to a short filament tube on the rim of the hypanthium (Fig. 3a, b).
The carpel heightens, protruding well above the stamens (Figs. 3a, b, 81). No stages were seen with margins open during ovule formation in this species. The margins become fused by a height of 260 µm (Fig. 79). The style becomes reflexed toward the abaxial side (Figs. 81–82). A hypanthium forms, within which the gynoecial stipe is at first attached centrally (Fig. 82), but later the attachment is reoriented to the adaxial side of the hypanthium (Fig. 3b) by differential growth. The gynoecium at anthesis has a stipitate glabrous ovary, elongate style, and a capitate stigma (Fig. 3b).
Cynometra webberi and Cynometra sp. (Figs. 4, 83–97)
Organography
These tree species are native to tropical Africa. The flowers are arranged in few-flowered axillary racemes (Hutchinson, 1964
) in which the lowermost, first-initiated flower expands and opens early while the remainder of the flower buds (Fig. 4b) remain small. The flowers (Fig. 4a, c) are small and radially symmetrical, have paired petalloid bracteoles; five (rarely four) thin sepals, imbricate in bud, then reflexed; five subequal, imbricate petals; ten free stamens with dorsifixed anthers in two alternating whorls, and a stipitate, centrally attached gynoecium with tomentose ovary and capitate stigma. There is a small disk, but no hypanthium. Cynometra webberi has white flowers with yellow anthers (label data). The flower structure is very simple, having homogeneity in each whorl, no fusion, and no loss or suppression of organs visible at anthesis.
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40 m high, with a diameter at breast height of 85 cm (collection label information). Its flower buds are creamy, and anthers are pale orange in bud. The inflorescences are cauliflorous and on young branches. The five sepals are pale greenish yellow, petals are creamy white, and the standard petal is folded. The three stamens have white filaments and orange-yellow anthers, and the style is white with a yellow stigma. Flowers are fragrant.
Cynometra webberi and Cynometra sp
Organogeny
Floral structure and ontogeny appear similar in these two species of Cynometra, although neither series is complete because of lack of material. Organogenetic stages of sepals were available for both species, those for petals only in Cynometra sp.; stages for stamens were available in C. webberi. The two series are combined for convenience.
The inflorescences have floral bracts subtending individual flowers, arranged helically and initiated in acropetal order (Fig. 83). The floral apex is tangentially broader than high (Fig. 84), and it initiates paired bracteoles in close succession (Fig. 85), which enlarge and eventually enclose the floral bud (Figs. 86–87).
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The stamens are initiated in unidirectional order in each whorl, starting from the abaxial side. Initiation time overlaps between the two whorls of stamens in Cynometra sp.; a late stage of stamen initiation is seen in Figs. 90–91, in which an adaxial primordium (at the arrow) is the last member initiated of the antesepalous (outer) whorl of five. In the same figures, a stamen primordium of the antepetalous (inner) whorl has initiated on the abaxial side (a), before others form adaxially in the same whorl; the expected sites of two of these are indicated at arrowheads in Fig. 90.
Cynometra webberi and Cynometra sp
Organ development
At midstage of development (Fig. 92) all 21 floral organs have been initiated. The sepals have enlarged and become imbricate, and the two adaxial sepals have become confluent as a single organ (Fig. 92). The petal and stamen primordia are uniform and equal in size in each whorl. Petal primordia have broadened due to marginal growth and have started to arch inward over the center. None of the stamen primordia of either whorl has started differentiation (Fig. 92). The petals in Fig. 93 have enlarged and developed a lamina, and begun to overlap. In older buds, one of the five petals may be smaller than the rest; in Fig. 96, it is the vexillary or adaxial petal that is smaller (at arrowhead).
The antesepalous stamen primordia broaden distally to produce incipient anthers (Figs. 91–92). The anthers are basifixed at first (Fig. 94), but later become dorsifixed (Figs. 96–97) due to differential growth of the anther. A dorsal median groove (at arrow) and lateral grooves (at arrowheads) become evident in the anthers of the three larger outer stamens (Figs. 94–96). The two slightly smaller antesepalous stamens undergo the same development but are somewhat delayed (Figs. 94, 96). With considerable enlargement, the anthers (Fig. 97) become dorsifixed and the filaments appear narrowly cylindrical compared to earlier stages.
Antepetalous stamen primordia (a) are undifferentiated pegs in Fig. 94. They subsequently undergo the same processes of development as outer stamens, but are generally smaller with shorter filaments (Fig. 97). The stamens of both whorls merge into a single whorl at anthesis by differential growth, but there is no connation.
The carpel primordium becomes indented adaxially (Figs. 91–92); this crease deepens to form the locule as the lateral walls increase in area. The margins of some carpels remain open at the time of ovule initiation (Fig. 93). Trichomes begin to form first basally and abaxially (Figs. 94–96) and then gradually cover the surface (Fig. 97).
Cynometra sp. differs from C. webberi in a few aspects of floral structure. In Cynometra sp., the paired bracteoles are linear and do not enclose the buds; there is a short hypanthium, and the gynoecium is attached to one side. In C. webberi, the bracteoles are broadly ovate and enclose the buds, no hypanthium was seen in the material available, and the gynoecium appeared centrally attached.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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20% of the circumference. The two bracteoles do not contact each other at their margins in early stages. The floral apex is circular after bracteoles are initiated, and sepals are five in number, produced successively in a 2/5 quincuncial helix beginning nonmedianly on the abaxial side. All sepals are initiated before any petals. Most of these character states (except the nonmedian first sepal position) are shared by other groups of Detarieae (Amherstia, Brownea, Crudia, and Hymenostegia groups; Tucker, 2000c, 2001
, and unpublished data).
This assemblage of developmental character states contrasts with a second assemblage (the "Omega" group) that includes taxa in the Brachystegia, Berlinia, Macrolobium and Detarium groups (Tucker, 2000a
). These share a set of alternative character states: bracteoles large and broad-based at initiation, occupying 90% of the circumference of the floral meristem. The two bracteoles are in contact at their margins on the adaxial side, and are closely adjacent abaxially, separated only by a narrow isthmus of floral meristem. The post-bracteole floral apex is lenticular and elongate in the sagittal plane. Diminished numbers of sepals and petals are initiated, the order is not helical, and there may be overlap between the sepal and petal whorls. Floral development of this assemblage of detarioid taxa will be reported in subsequent papers.
Order of organogeny
Order of initiation is helical among sepals in all. Order of petal initiation varies: unidirectional in Schotia brachypetala and S. latifolia, simultaneous in Cynometra, and bidirectional in Schotia afra. Stamens are initiated unidirectionally in all except Schotia afra, where the outer stamen whorl appears helical. Overlap of time of initiation between whorls occurs in S. brachypetala, S. latifolia, and Cynometra. The carpel initiates concurrently with the petals or directly thereafter in all. The carpel initiates ovules before marginal closure in both genera, although it was observed in only two of the three species of Schotia.
Schotia afra may differ from the other species in stamen organogeny. Stamens are initiated on one lateral side earlier than those on the other side. More sampling is needed to tell if the order of initiation is helical, bidirectional, or unidirectional as in the other taxa. This feature of asymmetrical initiation is highly unusual among legumes; it has been found in Chamaecrista (Tucker, 1996
), but in that flower, the asymmetry persists in the flower at anthesis. In S. afra, the process of organ equalization occurs during subsequent organ development, and the flowers do not reveal any asymmetry at anthesis. In S. brachypetala and S. latifolia, stamens are initiated unidirectionally in each whorl starting on the abaxial side and proceeding equally on both lateral sides.
Position of first sepal initiated
In the species examined of Cynometra and Schotia, the first sepal primordium is initiated on the abaxial side, but nonmedianly, contrary to most caesalpinioid legumes in which the first sepal is abaxial and median in the sagittal plane. Nonmedian abaxial position of the first sepal is found in many Caesalpinieae (Kantz, 1996
), in Ceratonia siliqua (Cassieae tribe; Tucker, 1992
), Cassia javanica (Cassieae; Tucker, 1996
), Petalostylis, Labichea, and Dialium (Cassieae; Tucker, 1998
), and Saraca (Tucker, 2000b
). In some Caesalpinieae, the first sepal is nonmedian abaxial at initiation, but enlarges unequally with growth so that its position gradually becomes median abaxial (Kantz, 1996
). The contrasting state (median abaxial position of the first-initiated sepal) has been reported in several other Detarieae (Tucker, 2000a, 2001
). Prevalence of the median abaxial sepal position, together with this ontogenetic shift toward the median in some taxa, invites speculation about a causal factor for median vs. nonmedian position at initiation.
Rate or timing of bracteole enlargement may be correlated with nonmedian position of the first sepal. In species of Schotia and Cynometra the two opposite but successively initiated bracteoles are the first lateral organs on their modified branch before any sepals. One bracteole is larger than the other in these taxa, but the bracteoles do not cover or enclose the flower during first sepal initiation. The first sepal appears to initiate closer to the larger bracteole and is therefore nonmedian, although relative bracteole size is difficult to determine in most micrographs, especially since the bracteoles are usually removed. A limited comparison of taxa has been made with detarioid taxa that have a median abaxial first sepal—in Aphanocalyx, Brachystegia, and Monopetalanthus (Tucker, 2000a
). The two bracteoles in these taxa enlarge early and massively and completely cover the flower during first sepal initiation. Any physiological influence from the two massive bracteoles would be equal, so that a median abaxial site could be predicted for the next organ initiated, the first sepal.
Suppression of organs
Some floral organs are suppressed after being initiated. One of the five petals is usually suppressed in species of Schotia (Tucker, 1989
) and often also in Cynometra. In Schotia brachypetala and Cynometra, three stamen primordia are larger than the others during midstages of development, but these differences disappear by anthesis.
The flowers of many other legumes, particularly in subfamily Caesalpinioideae, show loss of some organs (Tucker, 1988b
), so that each flower has fewer than the usual 21 organs. The loss can be due either to suppression of organs after they are initiated, a fairly common phenomenon as in Bauhinia species (Tucker, 1988a
), or some organs may never be initiated, as in Ateleia herbertsmithii with only one petal (Tucker, 1990
) or Ceratonia siliqua lacking petals altogether (Tucker, 1992
).
Problems of assessing significance of plesiomorphic character states
The present work is significant in the light of previously demonstrated developmental effects of suppressed vs. missing organs (Tucker, 1988a, b
), and in its support of the generalization that species in a genus share most of their floral developmental pathway (shown and discussed earlier for taxa of Sophoreae; Tucker, 1993, 1994
). Within a genus, there are often apomorphies as well. For example, Schotia species differ at anthesis primarily in corolla features: color, number of petals, and relative petal sizes. Developmental differences among the species of Schotia include basal connation among stamen filaments in two species vs. free stamens in S. brachypetala; overlap between time of initiation of whorls in two species but not in S. afra; three outer stamens precociously enlarged during development (but not at anthesis) in S. brachypetala vs. uniform enlargement per whorl in the other two species; and stamen anthers declinate in bud in S. brachypetala.
Species of Cynometra and Schotia have mostly plesiomorphic floral character states: radial symmetry, all 21 floral organs initiated, little or no organ fusion, and no suppression or dissimilarity within a whorl. Some developmental character states also may be considered plesiomorphic, e.g., circular post-bracteole floral apex and acropetal helical order of sepal initiation.
Among derived character states seen here, the unidirectional order of initiation in the stamen whorls of most taxa is considered a derived state (Tucker, 1987
et seq.). Other derived character states include overlap in time of initiation between organ whorls, the short filament tube in S. afra and S. latifolia, inverted position of anthers in bud in S. brachypetala, and the disk in Cynometra.
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FOOTNOTES |
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2 The work on Schotia brachypetala was performed at the Department of Botany, Louisiana State University, Baton Rouge, Louisiana 70803; that on the other taxa was carried out in the Biology Department (Ecology, Evolution, and Marine Biology), University of California, Santa Barbara, California 93106 USA.
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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