Aspects of Floral Morphology and Development in the Seagrass Syringodium filiforme (Cymodoceaceae) Author(s): P. B. Tomlinson and U. Posluszny Source: Botanical Gazette, Vol. 139, No. 3 (Sep., 1978), pp. 333-345 Published by: The University of Chicago Press Stable URL: http://www.jstor.org/stable/2474059 . Accessed: 30/08/2011 15:57

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http://www.jstor.org BOT.GAZ. 139(35: 333-345. 1978. (B 1978by The Universityof Chicago.0006-8071/78/3903-0005$01.16

ASPECTS OF FLORAL MORPHOLOGY AND DEVELOPMENT IN THE SEAGRASS SYRINGODIUM FILIFORME (CYMODOCEACEAE)

P. B. TOMLINSONAND U. POSLUSZNY1 HarvardForest, Harvard Universit, Petersham,Massachusetts 01366

Inflorescencesin Syringodigmfiliforme are initially racemoseand subsequentlyextensively cymose, with terminalunits referredto as "flowers."The speciesis dioecious.Each unit is normallyenclosed by a pair of bractswhich may subtendrenewal shoots. The male flowerconsists of four pairs of microsporangiawhich ariseby equal subdivisionof two lobes of the floralapex. Each pair of microsporangiais associatedwith a late-developingprotuberance, which collectivelyfuse to form a "perianthridge," soon obscuredby the enlargingmicrosporangia. The female floweris naked and consists of two carpelswhich arise separately on the floralapex. The carpelis initially peltatebut maturesas a closedstructure with a single pendulous ovule and two styles. The homologyof these structureswith the parts of a normalflower is discussedand comparedwith severalrelated genera for which similarinformation is available.

Introduction and a dissociationfrom the fresh-watergroup Zanni- Detailed studies of floral developmentin sea- chelliaceae,in contrastto the opinionof J. E. DANDY grassesare lacking because of the difficultyof obtain- (in HUTCHINSON1959), who includesboth groupsof ing material.Nevertheless, this informationis use- generain a morewidelsr conceived Zannichelliaceae. ful in providingcomparative data for s-stematic Informationon floral morphologyin Cymodo- purposesand is helpfulin understandingbiological ceaceae,summarized by DEN HARTOG(1970), has processes associated with underwaterpollination beenaugmented b- the studiesof ISAACS(1969) and (e.g., PETTITTand JERMY1975; DUCKERand KNOX KAY(1971) on Thalassodendronand Cymodoceaand 1976; YAMASHITA1976). DUCKERand KNOX (1976) on A mphibolis. Plants We undertookthis study largely to supply the are apparentlydioecious; the reproductivestruc- necessarybackground information for this biological tures are associatedwith reducedleaves (bracts); approachand also to circumscribemore clearly the the female"flower" consists of a pairof carpelseach family Cymodoceaceae(sexsu TAYLOR1909), since with a pair of filamentoussty les; and the male the same kind of informationhas been made avail- "flower"has a single axis, sometimesappendaged able for the Zannichelliaceae(sensu DUMORTIER) distallyand bearingfour pairs of pollensacs. (POSLUSZNYand TOMLIN-SON1977). Since these We extendthese studiesto Syringodiumand pro- aquaticmonocotyledons are representedat an ele- vide detailedinformation about floraldevelopment mentarvlevel of organization,presumably b) reduc- and some aspects of anatomy,adding information tion, it is appropriateto continueto establishpre- which helps to establishthe fundamentalconstruc- cise morphologicalinformation by which their dis- tion of the cymodoceaceousflower. cretenessand affinitiescan be assessed. Materialand methods TAXONOMYOE THE CYMODOCEACEAE.The five of Syringodium filiforme Kutz., generaof seagrasses Amphibolis (two species),Cy- Inflorescences collectedduring several recent modocea(five species), Halodule (perhapsseven spe- fixed in FAA, were Wading Beach, cies), Syringodium (two species),and Thalassoden- summersat MathesonHammock Florida.The most generally dron (two species) form a naturalentity in terms Dade County,Miami, of habit, floral morphology,pollen structure,and useful specimensin terms of developmentalstages aspectsof anatomy;they have long been recognized werecollected in Februar 1976.In addition,mate- of Mexico, as a distinctgroup, either as Cymodoceeae(ASCHER- rial from the Enconana River, Gulf F. BITTAKER proveduseful for SONand GRAEBNER1907) or Cymodoceoideae(DEN suppliedby H. has HARTOG1970) within a larger Potamogetonaceae, comparisonwith the Miamipopulation. of early floral stages were prepared, or as a separatefamily Cymodoceaceae(TAYLOR Dissections 1909), supportedby several subsequentauthors stained, and photographedby the techniquesof morphologicalstudy was (AIRY-SHAW1966; TAKHTAJAN1966; TOMLINSON SATTLER(1968). Direct by study of serialsections of paraffin- and POSLUSZNY1976). Accumulatingmorphological complemented information,of whichthis study formsa part, con- embeddedmaterial, stained in either safraninand tinues to supportthe categorizationat higherrank fast greenor safraninand Delafield'shematoxylin, accordingto standardtechniques. 1 Present address: Department of Botany and Genetics Universityof Guelph,Guelph, Ontario, Canada N1G 2W1. Grossmorphology Manuscript receivedFebruary 1978; revised manuscript received Syringodium filiJorme ("manatee grass") is a April 1978. commonconstituent of shallow-watermarine com- 333 334 BOTANICALGAZETTE munities in appropriate stable substrates in the emptxr(fig. 6) and a linear sympodium results. Least Caribbean.Plants show shoot dimorphismsince they commonly, the lateral buds abort (fig. 8). All axes extend by means of a horizontalmonopodial rhizome, are terminated br the third kind in which neither bearing scale leaves separated by quite long inter- bract subtends a lateral branch so that the system nodes (fig. 1). The scale leaves each subtend erect cannot extend further (figs. 10, 11). shoots with foliage leaves and usuallSrquite short Bracts (fig. 4) are well-developed structures. The internodes; the erect shoots are normally ultimatel) basal sheath includes a conspicuous median and determinate by flowering (DEN HARTOG1970; four to six lateral veins which unite at the top of the TOMLINSON1974). Proliferative branching of the sheath anc form the pair of lateral veins of the blade. rhizome is facultative, since it usually is stimulated Prophylls (fig. 3) are always scarious, bladeless, and by damage to rhizomes.The species is dioecious, and with a median and one or two obscure lateral veins; apparent clones of one sex are often quite extensive, they- neither develop a blade nor subtend a lateral suggesting that proliferative branching maintains bud. The prophyll remains within the enclosing and extends individual genets. In South Florida, bract, enveloping the base of its parent lateral axis. flowering apparently begins as earl- as Januarx=,so Squamules (intravaginal scales) are common at all that ripe fruits may be collected as earl as June. nodes, including the prophxllar nodes, and usually However, the flowering season is clearly extended, form a lateral pair. In earlv stages squamules may although there are no detailed phenological data. be proportionatelylarge in relation to the primordia The extended flowering season and elaborately of other organs (e.g., fig. 24). cymose construction of the inflorescence make it FLORALMORPHOLOGY.-For descriptive purposes relatively easy to assemble material showing a wide we have applied the term "flower" to each biologi- range of developmental stages. cally discrete unit enclosed bs a bract pair within INFLORESCENCEMORPHOLOGY. Branching of the the branch complex we have called the "inflores- inflorescence is essentiallyrcymose, at least in later cence." The descriptive convention is not intended stages. The inflorescence is superficiall- paniculate to conve- any interpretative information, which is (fig. 2) and branched in one plane. The size of the discllssed later. The female unit can be recognizedby inflorescence depends on the age and vigor of the the extended stigmata (fig. 10); in the male unit the plant; larger ones may include 30-SO flowers of anther remains enclosed until anthesis (fig. 14). differentages (fig. 2). For the first two or three nodes The female flower consists of a pair of carpels on the inflorescencebranches racemosels (fig. 5), with the common axis but without any additionalperianth a single bract subtending a branch that bears a basal or ridge, as is clear in the unit with bracts removed prophxrll(fig. 3), both axes extending. Soon branches (fig. 12). Each carpel includes a single bitegmic ovule become cymose, with pairs of bracts enclosing a pendulous from the apex of the locule (fig. 13). The single flower. The internode between the bracts of carpel continues as a short style which further one pair is suppressed; that between bracts of suc- extends into two rather stout stigmata. At maturity cessive pairs is usually quite extended (fig. 9). The the stigmata protrude between the enveloping lowest bracts on an inflorescenceconsist of a basal bracts, but there is no extension of the floral axis sheath and a distinct blade (fig. 4) that is often (figs. 10, 11). The pair of carpels originate in a plane several centimeters long. Distally bracts at increas- at right angles to the plane of distichw-of the sub- ingly higherlevels have progressivelSrreduced blades; tending bract (figs. 6, 34), but stlbsequentlSrthev are the hlade in the ultimate bracts is usuallxrvestigial. often displaced into an oblique position. Both Three possible kinds of arrangementoccur in asso- carpels can develop into fruits, but there is frequent ciation with each bract pair, with the axis above the abortion of one or both of them. bracts terminatingin a flower in each kind (figs. 6-8). The male flower terminates a unit of the sympo- In the most complex condition both bracts subtend dium and consists of four pairs of microsporangia a prophyllate branch, both branches growing out as inserted directl) on the axis and enclosed bv the renewal shoots so that the inflorescenceis elaborated bract pair (fig. 16). A shallow ridge, obscured by the (figs. 7, 14). In the most common condition onl+ the basal extension of the microsporangia,encircles the lowest bract subtends a branch; the upper bract is axis at the level of insertion of the microsporangia

FIGS. 1-8.-Syringodiumfiliforme, habit and inflorescenceconstruction. Fig. 1, Olderpart of rhizomewith erect leafy shoots one with a terminalinflorescence; X 3/8. Fig. 2, Mature female inflorescencefrom a vigorousplant; X 1/2 (numbersindicate examplesof the types of branchingin figs. 5-8). Fig. 3, Prophyllfrom branchbase laid flat to show venation; X 4. Fig. 4, Bract with short blade, basal sheath laid flat to show venation; X 5. Figs. 5-8, Diagramsof differentpossible kinds of units within a single inflorescence(regardless of whether male or female); L= bract, Ls= prophyll,S= squamule,outline= main axis or flowerparts, solid black = branchaxis; fig. 5, lover node with monopodialbranching from the axil of a single bract;fig. 6, node with pseudomonopodialbranching below a terminalfemale flower; one renewalshoot in the axil of the lowest bract;fig. 7, lower node with sympodialbranching (two renewalshoots) belowa terminalmale flower;fig. 8, terminalunit without renewalshoots, belowa male flower;no axillarybranching. 3

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(fig. 30). The surfaceof the antheris providedwith floral axis (figs. 22, 23). For a limited period the numerouslarge tannin cells, exceptfor the narrow ridge is relatively conspicuous (fig. 23), but with the longitudinalline of dehiscence. final expansion of anther lobes it is essentially ob- The maleflower is initiallyenclosed by its pair of scured. This expansion of the anther lobes, I;oth bracts,but at the time of dehiscencethe floralaxes basally and apically, continues so that the floral apex elongateabout 10-fold.This pushesthe bractsaside becomes overtopped and obscured while the length- (althoughthe flowerdoes not usuallyextend beyond ening floral axis is also enveloped (figs. 22-28). them) sufficientlyfor the pollen sacs to be released During this period of enlargement the renewal (fig. 15). As in all Cymodoceaceae,the pollen is shoot (V) in the axil of the lowest bract becomes filamentous. differentiated, and the prophyll (LS) and bracts (L) of the next unit of the sympodium are produced Development (figs. 20, 24, 27, 28). In both sexesthe floralapex originates as a lateral Further growth involves the enlargement of all axis in the axil of a bract of the previousflower parts except squamules up to the stage when the complex;most oftenthis is the lowestbract (figs. 17- anther is fully mature but still enclosed within the 24, 31-37). The lateral axis initiates prophylland leaf bracts. The girdling ridge (B) does not take bractsbefore there is any evidenceof floraldifferen- part in this enlargement (fig. 30). tiationin the apexitself. FEMALEFLOwER. In early stages the floral apex MALEFLOWER. The developmentalsequence is of the future female flower closely resembles the observedby freeingthe complexof outerenveloping male, with the same arrangementof a prophyll (Ls) bracts (figs. 17-30), a randomselection revealing and two bracts together with associated squamules units at differingages. As an indicationof the rela- (figs. 31, 32). As in the male flower, early differenti- tive stages of developmentof successiveunits, the ation is indicated by the broadening of the floral lateralapex (fig. 28) is at an earlierstage than the apex (G), which becomes bilobed in a plane at right floralprimordium (fig. 17). angles to the plane of distichy of the bracts. Each The first indication of floral differentiationis lobe is a carpel initial. Carpel development is indi- expansionof the shootapex and developmentof two cated by the elaboration of each lobe as a peltate lobes at right anglesto the plane of distichyof the structure (figs. 32, 33). The single ovule of each bracts (fig. 17). At this stage the floweris partially carpel is developed on the inner (adaxial) side (i.e., envelopedby its prophyll(Ls)n and the two bracts fig. 34,0). This early stage of the open carpel is are well developed(fig. 18,L and L arrow).Subse- rapidly supersededby intercalarygrowth so that the quently the two lobes themselvesbecome bilobed carpel becomes flask shaped, enclosing the ovule (fig. 19),so that the floralapex is nowrepresented by which is carried to the apex of the locule (fig. 13). four equal lobes which are the primordiaof the Stigmata are initiated at the apex of each carpel microsporangialpairs (fig. 20). At this stage the (figs. 35-37); they elongate markedly later, with complexis partiallyenveloped by the prophyll(fig. intercalary growth producing the style. 20nEs)nand the lowest bract has to be removedto Relative stages of development of the next lateral reveal its subtendedbud (V) and the anther (A). branch of the ss mpodium (V) in relation to the The subtendedlateral meristem(V) has the pos- parental unit show that the axes of two successive sibility of forminga continuingrenewal shoot. The orders maintain the same developmentalproportions secondbract occupies a positionon the sameside of (figs. 33-37). At the time the stigmata (Si) on one the axis as the prophyll. The situation is made unit are initiated, the primordia of the carpels of somewhatcomplex by the seriesof squamuleswithin the next higher order are evident (fig. 36,G) and eachleaf axil which are revealed when the subtending another developmental cycle can be reconstructed leaf is removed(e.g., fig. 21,S). (cf. fig. 31). At the time the floralapex takeson its definitive Where two renewal shoots develop, one from the four-lobedshape, a subtendingappendage is initiated axil of each bract, the upper unit is always later in below each lobe (fig. l9,B). Throughextensions of development than the lower unit so that there is an the distal portionsof these appendages,a shallow acropetal sequence of development maintained with- ridgeof tissueis developedencircling the baseof the in one complex.

FIGS. 9-16.-Syringodiumfiliforme.Details of inflorescencecomplexes and flowers.Figs. 9-13, female;figs. 14-16, male. Fig. 9, Portionof a femaleinflorescence showing several kinds of brancharrangement, mostly correspondingto types in figs. 5-8; X 3/2. Fig. 10, Terminalunit, bract pair envelopinga terminalfemale flower;X 4. Fig. 11, Same as fig. 10 with Lower(outer) bract re- moved to show abortedaxillary renewal shoot; X 4. Fig. 12, Femaleflower, the bicarpellatestructure enclosed by the bractpairs- X 6. Fig. 13, Longitudinalsection of bicarpellateflower with centralvascular system- X 4. Fig. 14, Male flowerwith two renewai shoots, one from each axil of the bract pair; X 3 (cf. fig. 7). Fig. 15, Male flowerat anthesis,the upperbract pushedaside by extensionof the floralaxis; X 3. Fig. 16, Male flowerwith bractsremoved to show superficialtannin cells; X 4 (cf. fig. 30). 338 BOTANICALGAZETTE

Floral vasculature becomes the ovular trace, extending and bending The vascular system of the floral axes corresponds into the funiculus) and a ventral trace (which to that in the vegetative axes of the rhizome and extends as far as the base of the style, which it does leafy erect shoots, both in its general histology not enter, ending blindly at about the level of inser- and in the distribution of traces to appendages tion of the ovule on the opposite side) (fig. 13). (SAUVAGEAU189 1; MONOYER192 7 ; TOMLINSON, Consequently, in a slightly oblique section (fig. 46), unpublished observation). There is a central axial both traces are visible in the left-hand carpel, but strand, which includes a central xylem lacuna sur- only the ventral trace is visible in the right-hand rounded by two to four phloem strands together carpel cut at a higher level. with four to six cortical strands (figs. 38, 39). Median traces of leaves, bracts, prophxTlls,and Discussion branch axes diverge from the central strand (e.g., A universal feature of the Cymodoceaceae is the figs. 40, 41, arrows), whereas lateral traces of leaves terminal position of the reproductive unit. Syringo- and bracts are derived from the cortical sxstem. dium offers immediate contrast to other Cymodocea- There is a direct connection between the cortical ceae because of its expanded, sympodially con- vascular s) stem of a parent axis and its branch axis. structed inflorescence (OSTEN-FELD1916), whereas In the male flower (fig. 42) the central strand other genera seem superficially distinct in having continues to the floral apex, where it expands, flowers either on lateral shoots of the erect leaf developing an extensive tracheary mass, but ends system (Amphibolis, Thalassoden(lron)or immedi- blindly. The cortical system ends blindly at the level ately terminal on the leafy erect shoots themselves of the inconspictlousridge of tissue near the insertion (Cymo(locea,Halo(lule). In the latter two genera, of the microsporangia.This ridge, however, is not shoot dimorphism is neither consistent nor regular vasculated. (TOMLINSON1974). KAY (1971) emphasized that C. In the female flower (figs. 43-46) the cortical serrulata has a terminal flower (his description is system ends blindly at the base of the common restricted to the female flower) but that sympodial carpellary axis. The central strand at this level en- branchingcan occur by the development of a renewal larges and divides equally to produce one trace to shoot from the axil of a subfloral foliage leaf. His each carpel (fig. 44); at a higher level each carpel description corresponds to the account by DEN trace divides (fig. 45) to give a dorsal trace (which HARTOG(1970) of the genus Cymo(loceaas a whole,

FIGS. 17-24. Syringodilfm751iforme, early stages in the development of the male floner; all X 120. Fig. 17, Young male primor- dium enclosed in a sheathing prophyll (Ls) from above; the first bract removed (rL), the apex of the second bract just protruding on the same side as the prophyll (Ls); the primordium (A) showing by lateral expansion the first indications of microsporangial development. Fig. 18, Lateral (adaxial) view of a similar stage to that of fig. 1 with prophyll removed, both bracts (L) present- A = floral apex. Fig. 19, Oblique view of male flower showing the two lobes that will each develop into two pairs of microsporangia, the perianth ridge (B) is just evident below the lobed apex (A). Figs. 20-24, Lateral view of progressively older male flowers- fig. 20, the initial lobes seen in fig. 18 have divided to form the primordia of the four pairs of microsporangia (A); Ls is the proFhyll partly enclosing the shoot and opposite the apex of the renenal shoot (V) developing in the a.xil of the first bract; figs. 21-23, stages in the development of the perianth ridge (B); squamules (S) are evident in the axils of bracts which have been removed some in dorsal position (e.g., S in fig. 23); fig. 24, male floner primordia of two orders, L.^is a possible third bract on the abaxial side; V is the apex of the renewal shoot in the axil of bract which has been removed but whose squamules (S) remain; L' is the secondleaf producedby V.

FIGS.25-30. Syringodium;liforme, lateral view of male flowers at later stages in development; all X 120 (cf. fig. 17). In these structures, the perianth ridge (B) is more or less obscured by surrounding bracts (L), squamules (S), and microsporangia, except in figs. 29 and 30 where enveloping organs have been removed; Ls is prophyll of axillary renewal shoot (V). Fig. 25, Flower from abaxial (i.e., prophyll-opposed) side; the renewal shoot in the axil of the first bract, which has been removed, has initiated its own prophyll (Ls) and first bract (L). Fig. 26, Flower enveloped b sheath of second bract; renenal shoot in axil of first bract, which has been removed, overtopped by its own first bract. Fig. 27, Flower with first bract removed to show precociously enlarged squa- mules (S) and renewal shoot at about same age as in fig. 17. Fig. 28, Flower with bisporangiate condition of each anther sac clearly established; renewal shoot (V) with all appendages initiated. Fig. 29, Male flower nith all appendages removed to show perianth ridge (B) contiguous with microsporangia. Fig. 30, Perianth ridge (B) displaced and obscured by expansion of axis and micro- sporangia; surface tannin cells clearly differentiated.

FIGS. 31-37. Syringodium;liforme, early stages in the development of the bicarpellate gynoecium of the female flower; all X 120 (cf. fig. 17). Fig. 31, Young renewal shoot, future female flower indicated by laterally expanded apex (G); prophyll (Ls) and second bract (on same side of axis as prophyll) still attached; first bract removed (rL). Fig. 32, Female flower obliquely from above, differential upgrowth of carpel primordia (arrons) initiating their peltate form. Fig. 33, Later stage, with apex of renewal shoot (V) in axil of first bract. Fig. 34, Ovule primordia (O) initiated on the inner margin of each peltate carpel; renen al shoot with its prophyll. Fig. 35, Late stage of carpel development, enclosure of the ovule complete; stigma lobes (Si) evident, renewal shoot (V) with all its appendages initiated; squamules (S) disproportionately large in relation to other primordia. Figs. 36-37, Late stages in flower development, the common style of stigmata (Si) still unelongated; the primordium of the next floner (G) is well developed, in fig. 36 already with a further renewal shoot (V) evident. > F : .

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341 342 TOMLINSON& POSLUSZNY-FLORALMORPHOLOGY OF SYRINGODIUAI 343 followingthe detaileddescriptions of suchauthors as and becomes a rim of tissue separate from the micro- BORNET(1864) and OSTENFELD(1916). KAY(1971) sporangia, can be used as evidence that the struc- also recorded?in T. ciliatum from Kenya, male ture is a true flower. However, the sequence of floweringbranches with a secondmale flowerin the initiation (staminal lobes preceding the subtending axil of the third(penultimate) bract of the flowering appendages) is in contrast to the acropetal sequence unit in about 10So of examplesstudied. There is of development in most flowers. Furthermore,there thus an incipienttendency in other generafor the is no comparable structure in the female flower. branchingcondition strongly expressed in Syringo- DEN HARTOG(1970) made no detailed commen- dium. tary about the morphologt, of the female flower, Most authorshave adopteda neutralterminology which resembles the male in major features. The in describingthe reproductiveparts in the Cymodo- male floweleof Thalasso(Sendron,however, has one ceaceae and referredto "flowers"in a biological fewer bract than the female flower. His family sense for the individualunits associatedwith spe- description of the unit as including two separate cializedbracts (e.g., OSTENFELD1916; ISAACS 1969; "ovaries" (where we have used the term carpels) DEN HARTOG1970; KAY 1971). In an interpretative might suggest that he saw it as two separate flowers, sense the male flowerby descriptionor implication but this may not have been his intention in view of has been consideredto consistof two dorsallycon- his statement about the male flower. nate anthers (DEN HARTOG1970), which seems An alternative explanationis offeredby MARKGRAF appropriatein Halodule because thev are clearly (1936) (see also ASCHERSONand GRAEBNER1907), insertedat differentlevels and becausein all genera who considered each anther (set of four micro- they consist of four bisporangiatepollen sacs (i.e., sporangia) as a separate male flower so that the with eight separate microsporangia,not four as unit is seen as a partial inflorescence, its stalk a suggested in the illustrationof Syringod1um by peduncle and not a pedicel. Similarly, the two MARKGRAF[19362, his TafelVIII, 6d). In all genera carpels of the female unit would be regarded as except Syringodium the anthers are also apicallv belonging to two morphologically separate flowers. appendaged;in Amphibolis these appendagesbe- This interpretation is also implied in the description come quite elaborate.Syringodium thereforeshows by DUCKERand KNOX(1976) of the unit in Amphi- least evidenceof a pairof discreteanthers at matur- bolis as "paired female flowers." The multiple (two ity, althoughthe initialbilobed floral apex couldbe to three) sts les of all genera except Halodulecertainly interpretedas demonstratingtheir existence.This support this view. evidence, however, is quite tenuous. Vasculature In Syringodium we have shown that the female againdoes not supportthe interpretationof a dual unit has no associated perianth, and its extreme structuresince the stalkhas a singlevascular supply, simplicity both in development and vascular ana- somewhatelaborated distally, but scarcelyforming tomy provides no clues to its tspological organiza- two discretestrands. tion. Amphibolisis somewhat exceptional because of DEN HARTOG(1970) pointed out that, if two anthersare involved,the commonaxis is not a fila- the four unequal lobes which crown the carpels ment but a pedicel,implying that he views the unit below the styles. These might be interpreted as as a flowerwith two stamens.All previousauthors tepals (which wotlld produce an inferior ovary), emphasizedthe absence of any specializedfloral but since they develop late (S. C. DUCKERand envelope(MARKGRAF 1936). Our present demonstra- R. B. Wnox, personal communication), they seem a tion of a vestigial"perianth ridge," which is initiated specialized feature related to the pronounced vivip- as separateprimordia subtending each of the four ary and method of anchorage of seedlings in this developingstaminal lobes (pairsof microsporangia) genus.

FIGS. 38-46.-Syringodiumfiliforme, male and femalefloral units in transversesection (TS), all fromserial sections of paraffin- embeddedmaterial. Scale in figs. 38, 42, 44-46 = 0.5 mm; in fig. 39 = 100 ,um,in figs.40, 41, 43 = 1 mm. Figs. 38-42, Male in- florescence;figs. 43-46, female inflorescence.Fig. 38, TS axis belowfloral unit with centraland corticalvascular strands. Fig. 39 Detail of centralvascular strand with xylem lacuna (X) and two phloemstrands (P), remainsof secondarywalls of tracheary elementsin the xylem lacuna.Fig. 40, TS same floralunit ca. 1.5 mm above 38 to show bract pair L1and L,1,the latter empty the formersubtending a branchrenewal shoot with prophyll (Ls); the leaf trace of the first bract of the renewalshoot evident (arrow,lower right). Fig. 41, TS same floralunit 320 ,umabove 40 to show basal lobes of anthersacs (A)- renewalshoot, lower right, shows the primordiaof two furtherbranch orders in the axil of L1, the leaf trace of the second bract of the first branch evident (arrow).Fig. 42, TS floralunit 600 Hm above41 to showfour pairsof microsporangiaof terminal"flower." At this level parts of the perianthridge are evident (B); the corticalvascular system ends blindlyabout 30 ,umab.ove; region of dehiscenceof microsporangialpairs shown by arrows;note numerousepidermal tannin sacs. Fig. 43, TS femaleunit with pair of bractsL1 and L,1,each subtendinga renewalshoot with prophyllLs correspondingto fig. 6, the uppermostunit, top right, shows one further orderof branching.Fig. 44, TS female"flower" terminating the centralaxis in fig. 43 at a level 960 ,umhigher; the centralvascular strand bifurcatingto supply each separatecarpel; cortical vascular system absent blades of bracts L1'and Lll' evident above enclosedby prophyllLst. Fig. 45, TS 450 ,umabove 44, carpellarytrace dividing to give a dorsaland ventralstarnd in each carpel Fig. 46, TS 450 ,umabove 45, slightly oblique,the level of sectionpassing through the bitegmicovule of the right-handcarpel. TABLE 1 COMPARISONSOF FLORALMORPHOLOGY AND DEVELOPMENT

CYMODOCEACEAE ZANNICHELLIACEAE

SyringodiumJiliforme Lepilaenabilociaris Altheniafiliformis Vleisia vschersoniana (Dioecious) (Monoecious) (Monoecious) (Monoecious) Developmentof fertilebranches First three nodes of inflorescence Repeatedly branched sympodia with Male and female flowers aggregated in Axial sympodialclusters. Male flower racemosely branched, soon becomes pairs of subopposite reduced foliage complex lateral sympodia. Male usually terminatingfirst-order branch cymose with pairs of bracts leaves. Sympodia terminate with flower usually terminating main axis Femaleflower terminating higher- enclosing a single male or female either male or female flowers of each flower complex. Female orderbranches and short branches flower flower terminal on higher branch close to the rhizome orders in each flower cluster Stamens...... Possibly two One One One Male flower...... Four pairs of microsporangia. Apex Two to six pairs of microsporangia. Single pair of microsporangia. Three Four pairs of naked microsporangia. initially bilobed, then tetralobed, Three scalelike bracts at the base of scalelike bracts at the base of the Connectiveterminating in a blunt each lobe subtended by a late- the microsporangia initiate and microsporangia initiate and develop appendageand with two vestigial forming appendage; becomes develop concurrently with the concurrently with the microsporangia outgrowthsopposite each other on "perianth ridge", encircles axis and microsporangia they subtend; they subtend the centralportion, initiated after obscured at maturity by anther; connective extended into a pointed the microsporangiaand obscured apex of connective unappendaged terminal appendage at maturity Femaleflower.... Two naked carpels, each initiated as Three carpels, each stlbtended by a Three carpels, each subtended by a Singlecarpel basally enveloped by a peltate primordium. Single ovule membranous scale which is initiated membranous bract which is initiated tubularmembrane which is initiated formed on adaxial portion of concurrently with the carpel concurrently with the carpel late in carpeldevelopment (at ovule carpel wall. Bilobed stigma primordium it subtends. Each primordium it subtends. Each inception).Carpel initiated as peltate carpel initiated as peltate carpel initiated as peltate primordium.Single ovule formedon primordium. Single ovule formed on primordium. Single ovule formed on side wall of carpel.Funnel-shaped adaxial portion of carpel wall. adaxial portion of carpel wall. stigma Feathery-branched stigma Peltate stigma Pollen...... Filamentous Spherical Spherical Spherical TOMLINSON& POSLUSZNY-FLORAL MORPHOLOGY OF SYRINGODIUM 345

Parallelsmight be sought betweenthe Cymodo- A particularemphasis on developmentalmorphology ceaceae and Zannichelliaceaesince the latter are and comparative histology, especially in the direc- most directly comparableto Syringodiumin their tion of fruit anatomy, would be welcome, although essentiallysympodial inflorescence. We have shown, the chief limitation has always been the availability however (POSLUSZNYand TOMLINSON1977), that of material. the reproductiveorgans in Zannichelliaceaeare more variableand complexthan previouslysuspected and Acknowledgments that evidencefor typologicalanalysis is very limited. We are indebted to PRISCILLAFAWCETT, botanical Syrirgodiumis compared with certain taxa in illustrator, Fairchild Tropical Garden, who drew Zannichelliaceaefor which detailed informationis figures 1-16, to ANN FAULKNER,who made the known(table 1). A commonfeature of theseaquatic histological preparations, and to REGULAZIMMER- monocotvledonsseems to be the structureof the MANNfor photographic assistance. The work was carpeland its developmentalpattern. carried out while U. POSLUSZNYheld a postdoctoral Certainly,more morphological information can be fellowship provided by the Maria Moors Cabot added to the present level of understanding,but Foundation for Botanical Research of Harvard biologicallyoriented work will be most profitable. University.

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