Comparative floral anatomy and ontogeny in Magnoliaceae

F. Xu1 and P. J. Rudall

2

1South China Botanical Garden, Academia Sinica, Guangzhou, China2Royal Botanic Gardens, Kew, Richmond, Surrey, UK

Received November 16, 2004; accepted June 9, 2005Published online: March 8, 2006� Springer-Verlag 2006

Abstract. Floral anatomy and ontogeny are de-scribed in six species of Magnoliaceae, representingthe two subfamilies Liriodendroideae (Liriodendronchinese and L. tulipifera) and Magnolioideae,including species with terminal flowers (Magnoliachampioni, M. delavayi, M. grandiflora, M. pae-netalauma) and axillary flowers (Michelia crassipes).The sequence of initiation of floral organs is fromproximal to distal. The three distinct outermostorgans are initiated in sequence, but ultimately forma single whorl; thus their ontogeny is consistent witha tepal interpretation. Tepals are initiated in whorls,and the stamens and carpels are spirally arranged,though the androecium shows some intermediacybetween a spiral and whorled arrangement. Carpelsare entirely free from each other both at primordialstages and maturity. Ventral closure of the styleranges from open in Magnolia species examined topartially closed inMichelia crassipes and completelyclosed in Liriodendron, resulting in a reduced stigmasurface. Thick-walled cells and tannins are presentin all species except Michelia crassipes. Oil cells arenormally present. Floral structure is relativelyhomogeneous in this family, although Liriodendrondiffers from other Magnoliaceae in that thecarpels are entirely closed at maturity, resultingin a relatively small stigma, in contrast to theelongate stigma of most species of Magnolia. Theflower of Magnolia does not terminate in an organor organ whorl but achieves determinacy by gradualdiminution.

Key words: Floral development, Floral morphology,Liriodendron, Magnolia, Michelia.

Introduction

Magnoliaceae are a well-defined and horticul-turally important family of about 230 speciesof trees and shrubs characterised by largeflowers with numerous tepals and fertile partsinserted separately on an elongated axis. Morethan 80% of species of Magnoliaceae aredistributed in subtropical and tropical regionsof eastern Asia; the remainder occur in Amer-ica, indicating a relictual tropical disjunction(Azuma et al. 2001). Renewed debate on thesystematics of the family has been stimulatedby several recent cladistic analyses, bothmorphological (Li and Conran 2003) andmolecular (Shi et al. 2000), but several out-standing questions remain.

Dandy (1927) proposed the first compre-hensive taxonomic treatment of Magnoliaceae,which recognised ten genera distributed in twotribes: Liriodendreae (sole genus Liriodendron)and Magnolieae, including Magnolia, Mang-lietia, Michelia, and six smaller genera. Sub-sequent authors have proposed severaldifferent infrafamilial taxonomic schemes, but

Pl. Syst. Evol. 258: 1–15 (2006)DOI 10.1007/s00606-005-0361-1

all of them divide the family into twosubfamilies, of which one, Liriodendroideae,includes the sole genus Liriodendron, and theother, Magnolioideae, includes a variablenumber of genera. Law’s (1984) Magnolioideaeincluded two tribes: Magnolieae, with terminalflowers, and Michelieae, with axillary flowers.Nooteboom (1985) and Cheng and Noote-boom (1993) reduced genera of Magnolioideaefirst to six genera (Chen and Nooteboom 1993)and later to two, and discarded all tribes andsubtribes (Nooteboom 2000). Thus, there is nodisagreement about the status of Liriodend-roideae containing only Liriodendron; thisisolated placement is also strongly supportedby analyses of nucleotide sequences in whichLiriodendron was consistently sister to all otherMagnoliaceae (Qiu et al. 1995, Ueda et al.2000, Shi et al. 2000, Kim et al. 2001). How-ever, relationships within Magnolioideaeremain equivocal; in all analyses, includingchloroplast DNA sequence data from matK(Shi et al. 2000), and ndhF (Kim et al. 2001,2004), the large genusMagnolia is paraphyleticwith respect to the other smaller genera. Li andConran (2003) recommended placement of thesmaller genera of Magnolioideae within abroadly circumscribed Magnolia, but high-lighted the need for more morphological datato improve phylogenetic resolution within thisgroup. Many species of Magnoliaceae areknown only from fossils (e.g. Frumin andFriis 1999, Kim et al. 2004), making combinedmorphological and molecular analysis highlydesirable in this group.

The large magnolia flower was once con-sidered to represent the primitive floral type(the Ranalian hypothesis), based mainly on theexistence of many fossil forms. However, recentimproved understanding of phylogenetic rela-tionships, together with new fossil discoveries,have demonstrated that small flowers withrelatively few organs predominate in early-divergent angiosperms (magnoliids). The largeflowers of Magnoliaceae are now normallyregarded as relatively specialised within thisgrade (for reviews see Crane et al. 1994,Endress 1994a). Here we examine floral

anatomy and ontogeny of a broad taxonomicrange of species of Magnoliaceae in a system-atic context. The floral morphology of Mag-noliaceae has been investigated by severalauthors, including Baillon (1866), Howard(1948), Skvortsova (1958) and Melville (1969).Influential studies of floral vasculature includethose of Canright (1960), Tucker (1961), Skip-worth and Philipson (1966), Skipworth (1970)and Ueda (1982, 1986). Earlier work on floralontogeny in Magnoliaceae includesinvestigations of the floral apex and carpel ofMichelia fuscata (Tucker 1960, 1961), carpeldevelopment in Magnolia stellata and Micheliamontana (Van Heel 1981, 1983), and floralontogeny in Liriodendron tulipifera and Mag-nolia denudata (Erbar and Leins 1994, Leinsand Erbar 1994, Leins 2000).

Materials and methods

Species examined were chosen as representatives ofthe taxa with terminal flowers (species of MagnoliaL.), those with axillary flowers (species of MicheliaT. Durand) and Liriodendron L. Specimens at arange of developmental stages were collected eitherfrom the Botanical Garden at the South ChinaInstitute of Botany, Chinese Academy of Sciences(SCBI), or the Living Collections, Royal BotanicGardens, Kew (K). Voucher specimens of samplescollected from South China Institute of Botanywere deposited in SCBI. The following species wereinvestigated: Magnolia championi Benth. (sectionGwillimia) (SCBI: FX Xu 03011), M. delavayiFranch. (section Gwillimia) (SCBI: FX Xu 03019),M. grandiflora L. (section Theorhodon) (SCBI: FXXu 03008), M. paenetalauma Dandy (SCBI: FX Xu03014), Michelia crassipes Y.W.Law (SCBI: FX Xu03016), Liriodendron chinense Sargent (SCBI: FXXu 03022) and L. tulipifera L. (K: 1939–77308).

Material was fixed in formalin acetic alcohol(FAA: 70% alcohol, formaldehyde and glacialacetic acid in a ratio of 85:10:5). For scanningelectron microscope (SEM) examination, buds weredehydrated in an ethanol series. Dehydrated mate-rial was then critical-point-dried using a BaltecCPD 030 critical point drier, mounted onto SEMstubs using double-sided adhesive tape, coated withplatinum using an Emitech K550 sputter coater,

2 F. Xu and P. J. Rudall: Floral morphology of Magnoliaceae

and examined using a Hitachi cold field emissionSEM S-4700-II at 4–5 KV. For light microscope(LM) observations, material was embedded in resinprior to sectioning. Fixed flowers and buds weredehydrated in an ethanol series to absolute ethanol,then transferred through an absolute ethanol : LRwhite resin series to absolute resin, and kept in afridge for about a week, with daily changes of resin.Specimens were then moved to gelatine capsulesand polymerized between 58–62C at 600 mbarpressure for about 21 hours. Once cooled, the resinspecimens were sectioned at 5lm thickness using aLeica microtome. Sections were stained in Tolui-dine Blue and mounted in DPX (Sigma-AldrichCo., Gillingham, UK). Photomicrographs weretaken using a Leitz Diaplan photomicroscope witha digital camera.

Results

Floral morphology and anatomy

Flowers are solitary, bisexual, and haplomor-phic, i.e. with spirally arranged organs insertedseparately onto an elongated axis. A ring ofthree bract-like structures surrounds the flow-er; these are normally interpreted as bracts, butsometimes as sepals. The perianth consists ofnormally nine free tepals which surroundnumerous free stamens and carpels respectively(Figs. 1–6).

Androecium. In all species except Lirioden-dron, the stamens have long slender non-marginal sporangia which are embedded inthe adaxial surface of the microsporophyll. InMagnolia and Michelia species examined here,the stamen apices (connective appendages) areshort, and there is no distinct filament, so thatthe stamens cannot readily be differentiatedinto filament, anther, and connective. Bycontrast, in Liriodendron the sporangia aremarginal in position and the filaments arethread-like. At anthesis, sporangia are introrsein Magnolia and Michelia but extrorse inLiriodendron. Shape of stamens in Liriodendronand several Magnolioideae was also studied byEndress (1994b).

Gynoecium. The total number of carpels ina flower varies between species. In Magnolia

championi and Magnolia paenetalauma, thenumber is around ten; but over 90 are presentin Magnolia delavayi (Fig. 1) and 40–50 inMagnolia grandiflora (Fig. 2).

In material examined here, carpels wereentirely separate from each other; no connec-tion or adnation was observed at any positionin any species examined here (Figs. 8, 9, 20, 21,30, 31, 36, 37). The carpel-bearing region ofthe reproductive apex is cylindrical in Micheliacrassipes, Magnolia championi and Magnoliapaenetalauma to sub-ovoid in Magnolia dela-vayi and Magnolia grandiflora. In Liriodendronthe carpel-bearing region of the reproductiveapex is more or less conical. This region of theflower is stipitate, formed by the sterile part of

Figs. 1–6. Flowers of Magnoliaceae. Fig. 1. Magno-lia delavayi. Fig. 2. Magnolia championi. Fig. 3.

Michelia crassipes. Fig. 4. Liriodendron tulipifera.Fig. 5. Magnolia grandiflora. Fig. 6. Magnolia pae-netalauma

F. Xu and P. J. Rudall: Floral morphology of Magnoliaceae 3

the carpels, a petiole-like stipe in Micheliacrassipes and all Magnolia species examined,but not in Liriodendron. In Magnolia champi-oni, M. paenetalauma and Liriodendron thecarpels are glabrous, but pubescent inMagnoliagrandiflora, Magnolia delavayi and Micheliacrassipes. Each carpel possesses a single stylewith three vascular traces, a median and twoventrals (Figs. 7, 17, 26, 32). Style shape andlength varies from narrow, semi-erect, andelongated in Magnolia paenetalauma to com-paratively stout and recurved in Magnoliagrandiflora. In Liriodendron the style is elon-gate, broad, flattened and wing-like, and con-tains numerous aggregations of thick-walledcells (Figs. 34, 40). The extent of the stigmaticepidermal papillae is variable between species.The stigma in Liriodendron differs from that ofspecies of subfamily Magnolioideae in that it issmall and localized, formed of epidermalpapillae (Fig. 38). Magnolia paenetalauma(Figs. 10, 11) has a small stigmatic crest ofunicellular epidermal papillae which are longerthan other epidermal cells, whereas in Magno-lia championi and Michelia crassipes the uni-cellular epidermal papillae resemble otherepidermal cells (Fig. 24). The ventral sutureof the carpels is not closed in open flowers ofthe Magnolia species examined here (Figs. 22,23), and only partially closed in Micheliacrassipes, in which the ventral suture is openat the upper part of style (Fig. 29) but firmlyfused at the lower part (Figs. 27, 28) so thatthe line of fusion completely disappears. InLiriodendron the ventral suture in the style iscompletely closed (Figs. 32, 33).

Ovules are inserted at the inner edge of thecarpel margin (see also Erbar 1983). There aretwo ovules per carpel in all species examinedhere. Crystals were not present in the integu-ments of species examined here, in contrast tothe material examined by Igersheim and En-dress (1997).

Idioblasts and sclereids. Idioblastic (soli-tary) oil cells were present in all speciesinvestigated here. They are circular and scat-tered in the carpel parenchyma from the styleto the ovary, in the tissues (Figs. 15, 18) or

subepidermally in Magnolia paenetalauma(Fig. 12). Mature oil cells are filled with alarge vacuole and a cupule, which is a commoncharacter of oil cells (Mariani et al. 1989), wasobserved in some slides (Figs. 15, 18).

Dark-staining tanniniferous cells werepresent in most species, although they aresparse or absent in Michelia crassipes. InMagnolia championi (Figs. 18, 19, 25) theyare scattered throughout the carpel from styleto ovary and also concentrated under theepidermis to form a ring of tanniniferous cells.In Magnolia paenetalauma, tanniniferous cellsare only observed aggregated in the chalazalregion (Fig. 16) or scattered sparsely in theovary. In Liriodendron, tannins are present inthe outer integument and the distal portion ofthe inner integument (Fig. 35).

Numerous aggregations of thick-walledcells or solitary idioblastic sclereids wereobserved in all species except Michelia crassi-pes, also reported for Magnoliaceae by Can-right (1960), and Igersheim and Endress (1997).These cells have lamellar thickened walls,obvious cytoplasm, large intercellular spacesand well-developed plasmodesmata (Figs. 13,14). They are distributed from the style to theovary in Magnolia championi, Magnoliapaenetalauma and Liriodendron chinense,although those of the latter possess compara-tively thinner walls (Fig. 39). In the upper partof the style ofMagnolia championi, the group ofthick-walled cells are associated with themedian veins, which is not connected inMagnolia paenetalauma. From the middle partof style, they are associated with both themedianand lateral veins in these two species. They aretotally free from the veins in Liriodendron.

In confirmation of the observations ofCanright (1960), carpel vasculature is similarin Magnolia and Michelia; the apical carpelsare supplied entirely from the central vascularcylinder of the axis, while carpels from themiddle to the base are all supplied by both thecortical and stelar systems. By contrast, inLiriodendron, all carpels are supplied byvasculature from both the cortex and centralvascular cylinder.

4 F. Xu and P. J. Rudall: Floral morphology of Magnoliaceae

Floral development

Floral apex. At initiation, the floral apex iscircular (Figs. 41, 53, 62, 71, 83) in all speciesexamined, and subsequently develops threetepals surrounding a triangular floral primor-dium (Figs. 43, 54, 64). During subsequentfloral development the shape of the floral apexvaries from flat during perianth initiation

(Figs. 44, 57, 65) to highly convex at laterfloral stages (Figs. 49, 60, 68, 74–76). The laterconvex shape of the apex is maintainedthrough appendage initiations. Tepals,stamens and carpels are initiated at slightlydifferent levels around the periphery of theapex. The members of each group of organsare initiated closely in time.

Figs. 7–16. Magnolia paenetalauma. Transverse sections of mature flower. Fig. 7. Floral apex, showing threefully developed carpels in the last tier, each with 3 vascular bundles (white arrows). Fig. 8. Carpellary region,showing carpels closely appressed, but not fused. Fig. 9. detail of Fig. 9, showing carpels closely appressed.Fig. 10. Stigmatic epidermal papillae. Fig. 11. Detail of Fig. 10. Fig. 12. Subepidermal oil cell. Fig. 13. Uppercarpels, showing aggregations of thick-walled cells in each carpel. Fig. 14.Detail of thick-walled cells in Fig. 13,free from the vascular bundles. Fig. 15. Oil cell (black arrow). Fig. 16. Tanniniferous cells in chalazal region ofovule. All bars = 50 lm

F. Xu and P. J. Rudall: Floral morphology of Magnoliaceae 5

Figs. 17–25 Magnolia championi. Transverse sections of mature flower. Fig. 17. Single carpel, showing threevascular traces (black arrows) interspersed with regions of thick-walled cells. Fig. 18. Oil cell and tanniferouscells. Fig. 19. Single carpel, showing aggregations of thick-walled cells and subepidermal tannins. Fig. 20.Carpels including ovules; carpels closely appressed but not fused to each other; note insertion to axis. Fig. 21.Detail of Fig. 20, showing carpels closely appressed. Fig. 22. Carpel below ovule, showing ventral suture.Fig. 23. Detail of Fig. 22, showing open ventral suture. Fig. 24. Stigma, showing unicellar epidermal papillae.Fig. 25. Tanniniferous cells (arrowed) in chalazal region of ovule. All bars = 50 lm

6 F. Xu and P. J. Rudall: Floral morphology of Magnoliaceae

Tepals. The three outer tepals are initiatedin sequence (Figs. 42, 56, 59, 63) but ultimatelyform a single whorl (Figs. 43, 54, 64). At thisstage, the shape of the floral primoridumchanges from circular to triangular (Figs. 43,54, 64). The second whorl of three semicirculartepal primordia are initiated at the tips of thethree angles formed by the triangular floralprimordium and alternate with the outer tepalwhorl (Figs. 44, 57, 65). One of them isinitiated slightly earlier than the other two(Figs. 65–67). Similarly, the innermost thirdwhorl of three perianth primordia differslightly from one another in time of initiationand alternate with those of the middle whorland hence are opposite those of the first whorl(Figs. 45, 46, 68, 72, 73). Thus, the tepals are

initiated in spiral acropetal succession, but aretrimerously whorled; the internodes betweenpetals seldom elongate. There is a considerabledifference in size between primordia of the firstand the second whorl during early stages(Fig. 73). Following completion of tepal initi-ation, the central floral primordium is more orless circular (Figs. 47–49, 68, 74–76).

Stamens. Stamen primordia are initiatedat the same time or slightly later than the thirdwhorl of perianth primordia. One or twostamen primordia arise opposite (in the samesector as) the first tepal primordia (Figs. 46,47, 68). Stamens are initiated acropetally,successively and rapidly around the base ofthe apex (Figs. 48, 49, 60, 76, 77, 84). Theorder of stamen initiation within each whorl is

Figs. 26–31. Michelia crassipes. Transverse sections of mature flower. Fig. 26. Single carpel, with three vasculartraces (black arrows); thick walled cells absent; the ventral suture is closed. Fig. 27. Lower part of style. Fig. 28.Detail of Fig. 27, showing closed ventral suture. Fig. 29. Upper part of style, showing open ventral suture.Fig. 30. Carpellary region, showing free carpels. Fig. 31. Detail of Fig. 30. All bars = 50 lm

F. Xu and P. J. Rudall: Floral morphology of Magnoliaceae 7

Figs. 32–40. Liriodendron chinense. Transverse sections of mature flower. Fig. 32. Single carpel, with threevascular traces (black arrows); thick-walled cells absent. Fig. 33. Detail of Fig. 32, showing closed ventralsuture. Fig. 34. Middle part of mature flower, showing carpel arrangement. Fig. 35. Ovule, showing tanninspresent in outer integument and distal portion of inner integument. Fig. 36. Detail of Fig. 37. Fig. 37.

Carpellary region, showing free carpels. Fig. 38. Stigma, showing localized epidermal papillae. Fig. 39.Detail ofFig. 40, showing thick-walled cells. Fig. 40. Winged styles, each containing a group of thick-walled cells, freefrom vascular bundles. All bars = 50 lm, except in 35 = 200 lm

8 F. Xu and P. J. Rudall: Floral morphology of Magnoliaceae

Figs. 41–52. Magnolia paenetalauma. Floral development (SEM). Figs. 41–43. Differentiation of three outertepals surrounding the triangular floral apex. Fig. 44. Differentiation of second tepal whorl, one tepal slightlyearlier than the other two. Fig. 45. Initiation of three outer and three middle tepals, and first tepal of innerwhorl. Figs. 46–48. Differentiation of third tepal whorl and stamens. Fig. 49. Acropetal initiation of stamens.At this stage the floral apex reaches its greatest height and diameter. Fig. 50.Differentiation of carpels, showingcarpel primordia larger than those of stamens. Fig. 51. Differentiation of carpels, showing the carpel primordiainitiated alternately and in series of four to five. Fig. 52.Older stage. Abbreviations: c= carpel; f=floral apex;s= stamen; t1= tepal of first whorl; t2= tepal of second whorl; t3= tepal of third whorl. All bars = 100 lm

F. Xu and P. J. Rudall: Floral morphology of Magnoliaceae 9

not determined. During stamen development,the floral apex displays its greatest height anddiameter. In Liriodendron tulipifera and Mag-nolia delavayi the outermost stamens arebroader and petaloid at older stages (Figs. 61,88).

Carpels. When all stamen primordia havebeen initiated and begun to broaden, theremaining floral apex becomes slightly flatter.Some rounded bulges are initiated in series offour to five, which are larger than the stamenprimordia (Figs. 50, 51, 55, 69, 78, 80, 81, 85,

86). Carpel primordia are free and are initiatedin acropetal succession (Figs. 50, 51, 58, 69, 70,79, 87). During carpel initiation, the floral apexgradually diminishes in height and diameter.At the middle or late stage of ontogeny, themargins of each carpel are incurved, forming adeep ventral groove which extends to the tip(Figs. 51, 55, 58, 79–82, 87, 88). There is nodifferentiation of stigma and style at this stage.In older buds of all species examined here,stamens and carpels are arranged irregularlyon the floral axis (Figs. 52, 61, 70, 82, 88).

Figs. 53–61. Magnolia delavayi. Floral development (SEM). Figs. 53, 56, 59. Differentiation of outer tepals.Fig. 54. Three outer tepals initiated surrounding triangular floral primordium. Fig. 55. Differentiation ofcarpels, showing carpel primordia initiated in series of four to five. Fig. 57. Initiation of three middle tepals.Fig. 58. Differentiation of carpels, showing deep ventral groove extending to the tip of each carpel. Fig. 60.Initiation of stamens. Fig. 61. Older stage of flower bud, showing arrangement of stamens and carpels, andoutermost petaloid stamens. Abbreviations: c = carpel; f = floral apex; s = stamen; ps = petaloid stamens;t1 = tepal of first whorl; t2 = tepal of second whorl; t3 = tepal of third whorl. All bars = 100 lm

10 F. Xu and P. J. Rudall: Floral morphology of Magnoliaceae

Discussion

Our observations correspond with those ofother investigations, such as Tucker’s (1961)observations on Michelia fuscata, that apicalgrowth continues during floral development inMagnoliaceae, but the floral apex graduallydiminishes in diameter and height duringcarpel initiation. Thus, the flower of Magnoli-aceae is not a ‘‘true’’ determinate structure,since it does not terminate in an organ ororgan whorl, as in typical eudicot flowers.

Rather, the floral meristem achieves determi-nacy by gradual diminution (Tucker 1960,1979), as with the indeterminate apex ofracemose inflorescences.

Floral ontogeny inMagnoliaceae is remark-ably homogeneous throughout the family, withtepals arranged in a more or less whorledpattern surrounding more or less irregularlyarranged fertile organs. Erbar and Leins (1994)observed an intermediate organisation inMagnolia denudata and Liriodendron tulipifera,

Figs. 62–70. Magnolia cha (SEM). Fig. 62. Floral apex. Fig. 63. Differentiation of first tepal of outer whorl.Fig. 64. Subsequent differentiation of outer tepal whorl surrounding the triangular floral primordium. Fig. 65–67. Differentiation of middle tepal whorl, one tepal slightly earlier than the other two. Fig. 68. Initiation ofstamens (arrow). Fig. 69. Differentiation of carpels, showing the carpel primordia initiated in series of four tofive. Fig. 70. Older stage, showing irregular arrangement of stamens and carpels. Abbreviations: c = carpel; f= floral apex; s= stamen; t1= tepal of first whorl; t2= tepal of second whorl; t3= tepal of third whorl. Allbars = 100 lm

F. Xu and P. J. Rudall: Floral morphology of Magnoliaceae 11

Figs. 71–82. Magnolia grandiflora. Floral development (SEM). Fig. 71. Floral apex. Figs. 72–73. Initiation ofthree tepal whorls. Fig. 74–76. Initiation of third tepal whorl and stamens. At this stage the floral apex reachesits greatest height and diameter. Fig. 77. Acropetal initiation of stamens. Fig. 78. Initiation of carpels, showingcarpel primordia larger than stamen primordia. Figs. 79–81. Differentiation of carpels, showing carpelprimordia initiated alternately, and in series of four to five. A deep ventral groove extends to the tip of eachcarpel. The floral apex gradually diminishes in height and diameter. Fig. 82. Older stage of flower bud, showingirregular arrangement of stamens and carpels. Abbreviations: c = carpel; f = floral apex; s = stamen; t1 =tepal of first whorl; t2 = tepal of second whorl; t3 = tepal of third whorl. All bars = 100 lm

12 F. Xu and P. J. Rudall: Floral morphology of Magnoliaceae

and suggested that a whorled condition isderived from a spiral one in basal angiosperms(Erbar 1983, 1988; Erbar and Leins 1982, 1983,1994). In some Magnolioideae not examinedhere, such as Pachylarnax, Dugandiodendron,and Woonyoungia (Li and Conran 2003) carpelnumber is reduced to less than ten. Van Heel(1983) described early carpel formation inMichelia montana, which is unusual in possess-ing only two to four stalked carpels arranged inpairs.

One outstanding question of floral mor-phology in Magnoliaceae is whether the out-ermost organs represent bracts, as indicated bytheir mature structure, or tepals, as Ueda(1986) proposed. The three distinct outermostorgans are initiated in sequence, but ultimatelyform a single whorl; thus their ontogeny isconsistent with a tepal interpretation.

Both species of Liriodendron examined herediffer from other Magnoliaceae in that the

carpels are entirely closed at maturity, resultingin a relatively small stigma, in contrast to theelongate stigma of most species of Magnolia.No carpel fusion was observed here in species ofMagnolioideae, either in primordial or maturestructures. In some other early-diverging an-giosperms, including the ANITA grade andsomemagnoliids (Endress and Igersheim 2000),carpel closure is entirely by secretion ratherthan by postgenital fusion. However, this char-acter may be variable in Magnoliaceae, andrequires further investigation. Nooteboom(1985) reported carpel fusion in some of thesmaller genera of Magnolioideae, such as Tal-auma, Aromadendron and Tsoongiodendron, inwhich the fruit is a syncarp. Li and Conran(2003) reported that in all Magnoliaceae thecarpels are connate to varying degrees beforedehiscence; this conflicts with our data, butindicates that some late fusion or concrescencemay occur. In Michelia crassipes, the ventral

Figs. 83–88. Liriodendron tulipifera. Floral development (SEM). Fig. 83. Floral apex. Fig. 84. Initiation ofstamens. Figs. 85, 86. Initiation of carpels, showing carpel primordia initiated in series of four to five. Fig. 87.Carpel differentiation, showing a deep ventral groove extending to the tip of each carpel; the floral apexgradually diminishes in height and diameter. Fig. 88. Older stage of flower bud, showing arrangement ofstamens and carpels, and outermost petaloid stamens. Abbreviations: c= carpel; f=floral apex; ps=petaloidstamen; s = stamen. All bars = 100 lm

F. Xu and P. J. Rudall: Floral morphology of Magnoliaceae 13

carpel suture is closed in the lower part of thestyle, so that the stigmatic region is relativelyshort. Michelia crassipes also differs from theother species examined in the absence of thick-walled cells and tannins. Wider sampling isnecessary to determine the significance of thesecharacters. However, we concur with Noote-boom (1985) that concrescence of the carpelsalone is not a reliable character for delimitationof genera in Magnoliaceae.

We thank Chrissie Prychid (Royal Botanic Gar-dens, Kew) for help in the laboratory. The projectwas supported by the National Sciences Founda-tion of China (grant number 30000011, 30370108)and the National Sciences Foundation of Guang-dong province, China (grant number 000991). Weare grateful to Peter Endress and an anonymousreviwer for their comments on the manuscript.

References

Azuma H., Garcia-Franco J. G., Rico-Gray V.,Thien L. B. (2001) Molecular phylogeny of theMagnoliaceae: The biogeography of tropical andtemperate disjunctions. Amer. J. Bot. 88: 2275–2285.

Baillon H. E. (1866) Sur la famille des Magno-liacees. Adansonia 1: 133–192.

Canright J. E. (1952) The comparative morphologyand relationships of the Magnoliaceae I. Trendsof specialization in the stamens. Amer. J. Bot.39: 484–497.

Canright J. E. (1960) The comparative morphologyand relationships of the Magnoliaceae III. Car-pels. Amer. J. Bot. 47: 145–155.

Chen B. L., Nooteboom H. P. (1993) Notes onMagnoliaceae III: the Magnoliaceae of China.Ann. Missouri Bot. Gard. 80: 999–1104.

Crane P. R., Friis E. M., Pedersen K. R. (1994)Paleobotanical evidence on the early radiation ofmagnoliid angiosperms. Pl. Syst. Evol. s8: 51–72.

Dandy J. E. (1927) The genera of Magnoliaceae.Kew Bull. 1927: 275–264.

Endress P. K. (1994a) Floral structure and evolu-tion of primitive angiosperms: recent advances.Pl. Syst. Evol. 192: 79–97.

Endress P. K. (1994b) Shapes, sizes and evolution-ary trends in stamens of Magnoliidae. Bot.Jahrb. Syst. 115: 429–460.

Endress P. K., Igersheim A. (2000) Gynoeciumstructure and evolution in basal angiosperms.Int. J. Plant. Sci. 161 (6 Suppl.): S211–S223.

Erbar C. (1983) Zum Karpellbau einiger Magnoli-iden. Bot. Jahrb. Syst. 104: 3–31.

Erbar C. (1988) Early developmental patterns inflowers and their value for systematics. In:Leins P., Tucker S. C., Endress P. K. (eds.)Aspects of floral development J. Cramer, Berlin,pp. 7–23.

Erbar C., Leins P. (1982) Zur Spirale in Magnolien-bluten. Beitr. Biol. Pflanzen 56: 225–241.

Erbar C., Leins P. (1983) Zur Sequenz vonBlutenorganen bei einigen Magnoliiden. Bot.Jahrb. Syst. 103: 433–449.

Erbar C., Leins P. (1994) Flowers in Magnoliidaeand the origin of flowers in other subclasses ofthe angiosperms. I. The relationships betweenflowers of Magnoliidae and Alismatidae. Pl.Syst. Evol. Suppl. 8: 193–208.

Frumin S., Friis E. M. (1999) Magnoliid reproduc-tive organs from the Cenomanian-Turonian ofnorth-western Kazakhstan: Magnoliaceae andIlliciaceae. Pl. Syst. Evol. 216: 265–288.

Howard R. A. (1948) The morphology and sys-tematics of the West Indian Magnoliaceae. Bull.Torr. Bot. Club 75: 335–357.

Igersheim A., Endress P.K. (1997) Gynoeciumdiversity and systematics of the Magnolialesand winteroids. Bot. J. Linn. Soc. 124: 213–271.

Kim S., Soltis D. E., Soltis P. S., Suh Y. (2004)DNA sequences from Miocene fossils: an ndhFsequence of Magnolia latahensis (Magnoliaceae)and an rbcL sequence of Persea pseudocarolin-ensis (Lauraceae). Amer. J. Bot. 91: 615–620.

Kim S., Park C. W., Kim Y. D., Suh Y. (2001)Phylogenetic relationships in family Magnolia-ceae inferred from ndhF sequences. Amer. J. Bot.88: 717–728.

Law Y. W. (1984) A preliminary study on thetaxonomy of the family Magnoliaceae. ActaPhytotax. Sin. 22: 80–109.

Leins P. (2000) Blute und Frucht. Schweizerbart:Stuttgart.

Leins P., Erbar C. (1994) Flowers in Magnoliidaeand the origin of flowers in other subclasses ofthe angiosperms. II. The relationships betweenflowers of Magnoliidae, Dilleniidae, and Caryo-phyllidae. Pl. Syst. Evol. Suppl 8: 208–218.

Li J., Conran J. G. (2003) Phylogenetic relation-ships in Magnoliaceae subfam. Magnolioideae: a

14 F. Xu and P. J. Rudall: Floral morphology of Magnoliaceae

morphological cladistic analysis. Pl. Syst. Evol.242: 33–47.

Mariani P., Cappeletti E. M., Campoccia D.,Baldan B. (1989) Oil cell ultrastructure anddevelopment in Liriodendron tulipifera L. Bot.Gaz. 150: 391–396.

Melville R. (1969) Studies in floral structure andevolution. I. Magnoliales. Kew Bull. 23: 133–180.

Nooteboom H. P. (1985) Notes on Magnoliaceae,with a revision of Pachylarnax and Elmerrilliaand the Malesian species of Manglietia andMichelia. Blumea 31: 65–121.

Nooteboom H. P. (2000) Different looks at theclassification of the Magnoliaceae. In: Liu Y. H,Fan H. M., Chen Z. Y., Wu Q. G., Zeng Q. W.(eds.) Proceedings of the international sympo-sium of the family Magnoliaceae 26–37. SciencePress, Beijing, China.

Qiu Y. L., Chase M. W., Parks C. R. (1995) Achloroplast DNA phylogenetic study of theeastern Asia–eastern north America disjunctsection Rytidospermum of Magnolia (Magnolia-ceae). Amer. J. Bot. 82: 1582–1588.

Shi S., Jin H., Zhong Y., He X., Huang Y., Tan F.,Boufford D. E. (2000) Phylogenetic relationshipsof the Magnoliaceae inferred from cpDNA matKsequences. Theor. Appl. Genet. 101: 925–930.

Skipworth J. P. (1970) Development of floralvasculature in the Magnoliaceae. Phytomorphol-ogy 20: 228–236.

Skipworth J. P., Philipson W. R. (1966) Thecortical vascular system and the interpretationof the Magnolia flower. Phytomorphology 16:463–469.

Skvortsova N. T. (1958) On the flower anatomy ofMagnolia grandiflora L. Bot. Zhurn. 43: 401–408. [in Russian]

Tucker S. (1960) Ontogeny of the floral apex ofMichelia fuscata. Amer. J. Bot. 47: 266–277.

Tucker S. (1961) Phyllotaxis and vascular organi-zation of the carpels in Michelia fuscata. Amer.J. Bot. 48: 60–71.

Tucker S. (1979) Ontogeny of the inflorescence ofSaururus cernuus (Saururaceae). Amer. J. Bot.66: 227–236.

Ueda K. (1982) On the fundamental vascularpattern in the flowers of the Magnoliales. ActaPhytotax. Geobot. 33: 383–391. [in Japanese]

Ueda K. (1986) Vascular systems in the Magnoli-aceae. Bot. Mag. Tokyo 99: 333–349.

Ueda K., Yamashita J., Tamura M. N. (2000)Molecular phylogeny of the Magnoliaceae. In:Liu Y. H., Fan H. M., Chen Z. Y., Wu Q. G.,Zeng Q. W. (eds.) Proceedings of theinternational symposium of the family Magnoli-aceae 205-209. Science Press, Beijing, China.

van Heel W. A. (1981) A SEM investigation on thedevelopment of free carpels. Blumea 27: 499–522.

van Heel W. A. (1983) The ascidiform earlydevelopment of free carpels, a SEM investiga-tion. Blumea 28: 231–270.

Addresses of the authors: Fengxia Xu (e-mail:xfx@scib.ac.cr), South China Botanical Garden,Academia Sinica, Guangzhou, 510650, China.Paula J. Rudall (e-mail: p.rudall@rbgkew.org.uk),Royal Botanic Gardens, Kew Richmond, Surrey,TW9 3AB, UK.

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