Organography, Branching, and the Problem of Leaf

Organography, Branching, and the Problem of Leaf Versus Bud Differentiation in the Vining Epiphytic Fern Genus Microgramma Author(s): Ann M. Hirsch and Donald R. Kaplan Source: American Journal of Botany, Vol. 61, No. 3 (Mar., 1974), pp. 217-229 Published by: Botanical Society of America Stable URL: http://www.jstor.org/stable/2441600 . Accessed: 09/08/2013 14:06 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp

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Amer. J. Bot. 61(3):

217-229. 1974.

ORGANOGRAPHY, BRANCHING, AND THE PROBLEM OF LEAF VERSUS BUD DIFFERENTIATION IN THE VINING EPIPHYTIC FERN GENUS MICROGRAMMA' ANN M. HIRSCH AND DONALD R. KAPLAN2 Departmentof Botany, Universityof California,Berkeley94720, and the FairchildTropicalGarden,Miami,Florida A B S T R A C T Investigation of the development and organography of the shoot systemsof Microgramma vacciniifoliaand M. squamulosawas undertakenfor the purposeof determining:(1) the viningcharacterof these featuresof shoot growththat are responsiblefor the distinctive epiphytic ferns;and (2) themode of originof branchesand theircontrastwithleaf initiation. Shoots of both species are dorsiventral and plagiotropic(i.e., parallel to the substrate)in symmetry, shootdorsiventralhabit. Since the shootapical meristem is radial in transectional event,and its inceptionis ity in Microgrammais a postgenitalor secondarydevelopmental relatedto the initiationof lateralappendages.Leaves and buds arise in a distichousphyllotaxis and occupyoppositeand alternating positionson the dorsal surfacesand flanksof the rhizome. Endogenousrootsare initiatedin two rows fromthe ventralsurfaceof the stem, in thevicinityof therhizomemeristem; however,theydo not emergefromthe rhizomeuntil some distancebehindthe tip and do not elongateuntilthe regionof substratecontact. We elongaconcludethattheviningnatureof thisfernrhizomeis a resultof precociousinternodal tion and the concomitant delay of leaf and bud expansionin the regionof stemelongation. In addition,observationof branchoriginconfirmsprevioussuggestionsthat branchingin and not a dichotomousderivativeas prolateraland extra-axillary Microgrammais strictly posed by some workers.Leaf and bud primordiadiffernot only in the natureof theirrespectivevascularsuppliesbut also in theiractual courseof initiation.In the case of the leaf, is precociouslyemergentand exhibitsa lenticularapical cell at its summit the primordium when it is only one plastochronremovedfromthe flanksof the apical meristem.By condivideless activelyand remainin a sunkenpositionfor trast,initialsof the bud primordium at least 5-6 plastochrons; only when the bud apex becomesexpandedand emergentdoes a Because of the tetrahedral apical cell becomerecognizableat the tip of thebud promeristem. and phyllodistinctive patternof branchand leaf origin,as well as the lack of adventitious genous originof branchprimordia,we suggestthat the shoot of Microgrammais a useful of theproblemof leaf and bud determination in theferns. testorganismforthere-examination MICROGRAMMA VACCINIIFOLIA (Langsd.& Fisch) the arrestof root growthuntil contact with the Copeland and M. squamulosa (Kaulf.) de la Sota substrateis made. Morphologistshave long been interestedin the are simple-leavedfernswithcreepingor climbing rhizomes covered with tapering,asymmetricallynatureof branchingin the ferns,and particularly peltatescales. The two species are mainlySouth Bower (1923), Goebel (1928), and Troll (1937) American in distributionwhere they are found have proposed various hypothesesto explain the growingas epiphyteson moss-coveredrocks or varietyof branchingpatternsobserved. However, tree trunks(de la Sota, 1960). Their epiphytic as Guttenberg(1966) has pointed out, there habit has resultedin a numberof morphological actuallyhave been veryfewhistologicalinvestigaadaptationssuch as the viningnatureof the rhi- tions of branchinitiationin the ferns;most prezome, the coriaceous textureof the fronds,and vious studies having been concerned only with examinationof the gross morphologyof branch 1 Receivedforpublication27 June1973. originand development.These studieshave led 2 Research collaboratorFairchild Tropical Garden, to the conclusionsof Bower (1923) and Goebel Miami, Florida. The authorswish to thankMr. Stefan of the (1928) thatbranchingin the fernsis characterJ. Kirchanskifor assistancein the formulation dorsiventrality index and Ms. CharlotteMentges for isticallydichotomousor some derivativeof it. We also wishto express Guttenbergfurtheremphasized,however,that in of the illustrations. preparation our gratitudeto Dr. RudolfSchmidand Mr. StefanJ. of thegenusNephrolKirchanskifor theircriticalreadingof the manuscript thehistologicalinvestigation epis by Sperlich (1908), the data presenteddo and theirhelpfulsuggestions.

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not agree withdichotomousbranchingor deriva- (Sass, 1958), embedded in Paraplast, and sections thereof,but ratherin this fernthe branch tionedseriallyat 10 Mm.The sectionswerestained primordiaarise lateralto the shoot apex. In his with Heidenhain's iron-alum hematoxylinand withsafraninand fastgreen. Clearreviewof branchingin thepterido- counterstained comprehensive phytes, Troll (1937) described branching in ings of rhizomesectionswere made accordingto Microgrammavacciniifolia,and althoughhe did the techniqueof Foster (1942) and a modificanot examinethe originof branchprimordiafrom tion of Boke's (1970) procedure. Camera lucida drawingswere made of transthe apex, he likewiseconcludedbranchingof the rhizomeof Microgrammawas strictly lateral and versesectionsat 50glmintervalswitha Wild-M20 microscopein orderto calculate the dorsiventralnot a dichotomousderivationas Goebel (1928) had previouslysuggested. Therefore,investiga- ity index (see below). The consistencyof the tions of branchingin ferns,to be trulyrigorous, methodwas checked using four rhizomesof M. mustincludeobservationson the histologicalfea- vacciniifoliaand threeof M. squamulosa. Linear turesof branchorigin. In thispaper,histogenetic regressionlines were calculatedusing an Olivetti studiesof bud originin M. vacciniifoliaand M. UnderwoodProgramma101 computer.Measuresquamulosa are presentedwhichdemonstrate con- mentof the angle of divergenceof leaves of M. clusivelythatrhizomebranchingin thesefernsis squamulosa and M. vacciniifoliawas determined forleaves 1 and 2 and leaves 4 and 5. Both speindeed lateraland not dichotomousin nature. Since a numberof morphogeneticstudies on cies were found to have a similarangle of leaf pteridophytes have dealt withthe problemof leaf divergence. versusbud determination (Cutter,1956; Steeves, 1961; Haight and Kuehnert,1971), the present OBSERVATIONS-Generalorganographyof the studywas also undertakento investigate the actual shoot-The shoot systemof M. vacciniifoliaand differences in initiationand developmentof leaf M. squamulosa is a plagiotropicrhizome with and bud primordia.Microgrammais ideallysuited leaves and buds arrangedin a distinctdistichous for investigationof this problembecause leaves phyllotaxis(Fig. 1, 2). Of the two species, M. and branchesappear in a highlyregularorderand squamulosa (Fig. 1) is the larger,havinga rhican easily be followed back to the shoot apex zome diameterof approximately6 mm in the whereprimordiaare being initiated.From histo- matureaxis whereasthatof M. vacciniifolia(Fig. logicalexaminationsof therhizomeapex, we have 2) is 2 mm. The large coriaceous leaves of M. foundthatdistinctdifferences in bud and leaf ini- squamulosa are lanceolate to oblong with attiationdo exist and that these differencesfore- tenuatetips and average 11 cm in lengthand 3 shadow the final morphogeneticexpressionof a cm in widthforthe sterileleaves (Fig. 1 ). The primordium as eithera leaf or a branch. Observa- simple frondsof M. vacciniifoliaare ovate and tions on the anatomyand symmetry of the shoot measureapproximately 3 cm longand 1.2 cm wide axis are also includedin thepresentaccountsince (Fig. 2). In bothspecies,theleaves are displaced theyare intimately correlatedwiththe viningna- towardsthe dorsal surfaceof the rhizomein two tureand uniquemode ofbranchingoftheseplants. ranks with an angle of divergenceof 124.83 + 7.15?. The lateral buds are positionedapproxiANDMETHODS-Materialof Micro- matelyoppositethe leaves but are situatedon the MATERIALS grammavacciniifoliawas suppliedby the Univer- flanksof the rhizomeinsteadof the more dorsal sity of California,Berkeley,Botanical Garden. location of leaf insertion(Fig. 3). Hence, they Plantsof M. squamulosa were originallyobtained are also two-ranked and alternating.The alternatfromTalnadge'sFern Gardens,Chula Vista, Cali- ing positionsof leaf and bud are consistentalong fornia. Microgrammavacciniifoliaand M. squa- the axis and can easily be seen in serial transecmulosa exhibitdimorphismof leaves; the fertile tions and longisectionsof the shoot (Fig. 4, 8, leaf is narrowerand longerthan the sterileone. 31 ). Therefore,the determinationof whether Under greenhouseconditions,M. squamulosa oc- an early primordiumis a leaf or a bud can be casionally produced fertileleaves, whereas M. predictedby tracingback fromolder appendages vacciniifoliawas completelysterile. on the axis. In actuality,microscopicmeasureSegmentsof the rhizomewere fixedin Craf V ment of internodaldimensionsin the terminal

KEY

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B, bud; L, leaf;AC, apical cell; BA, bud apex.

Fig. 1-5. Morphologyand anatomyof rhizomesof Microgramma. 1. Rhizomemorphology of M. squamulosa. of M. vacciniifolia.x 0.5. 3. Decapitatedrhizomeof M. vacciniifolia, x 0.25. 2. Rhizomemorphology showing expandedlateral buds oppositethe leaves. x 0.42. 4. Nonmedianlongisection of the rhizomeapex of M. vacciniifolia,showingalternating positionsof buds. A leaf primordiumis oppositethat of a bud. X 62. 5. Median longisection of the rhizomeapex of M. vacciniifolia, showingthe largevacuolateapical cell at the summit.x 245.


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regionof the shoot of M. vacciniifoliashow that theterminalportionof the rhizometo plagiotropy leaf and branch are initiatedalmost exactlyop- is a concomitantchangein the transectional symposite one another. However,withincreasingin- metryof theshootaxis fromradialto dorsiventral. in the older internodes ternodal elongationtheir positions of insertion This changein symmetry become more and more separated (Fig. 15); al- can best be seen fromthe studyof serial transecthoughthe degreeof separationis not enoughto tionsof M. vacciniifoliaand M. squamulosa (Fig. can also distorttheiroppositeappearanceat the grosslevel 6-9). Increasingshoot dorsiventrality be expressedquantitatively of observation(Fig. 3). by the measurement index (D. I.), shown The vine-characterof the shoot of Micro- termedthe dorsiventrality in Fig. 11, 12. grammais due to precocious intemodalelonga- graphically The dorsiventrality index was formulatedas tion. Figure 15 showsthatin M. vacciniifoliathe most rapid rate of elongation occurs between follows. Since there is a change in transection nodes 4 and 7. In M. vacciniifoliafullyexpanded froma circleto an ellipse,a methodwas devised intemodesare approximately1 cm in lengthand to expressthis change in symmetry utilizingthe in M. squamulosa theyrangebetween2 and 2.5 geometryof transversesections fromthe tip of cm. Also contributing to the creepingnatureof theshootto themorebasal regionsof the rhizome the rhizomeis the delay of crozieruncoilingand at 50/ m intervals.Camera lucida drawingswere bud developmentfor some distance behind the made of the transversesectionsand a circle was apex (Fig. 2). In M. vacciniifoliathe firstun- constructedwitha compass on the drawingsuch rolled leaf is usually found 5-8 cm behind the that one edge of the circle was confluentwith apex and in both species,the lateralbuds remain eitherthe dorsal or ventralsurfaceof the rhizome suppressed and exist as "restingbuds" (Troll, (Fig. 11). The distance between the opposite 1937) (Fig. 2). Troll indicatedthatin M. vac- rhizomesurfaceand the opposite portionof the ciniifoliathereis a "correlativerelationship"be- circlegivesa difference (d') whichwillbe greater tweenthe lateralbuds and the apex of the main for rhizomesections approachingdorsiventrality axis: when the terminalportionof the shoot is than forthose havingradial symmetry.To cominjuredor removed,the lateralsare releasedfrom pensate for the increase in size as one proceeds theirrestingstate and continueto grow out. A fromthe apical regionto the more basal interrhizomeof M. vacciniifoliawhich had been de- nodes, the d' at each 501xmwas divided by the capitatedand the lateral buds released is shown radius (r) of the constructedcircle. Several atin Fig. 3. If the terminalmeristemis leftintact, temptswere made to obtain the best possible the lateral buds closer to the apex will usually circle to fit eitherthe dorsal or ventralsurface. expand to about 1 or 2 mm in length,producing Regressionlines were calculatedfromthe dorsione or two leaf primordiauntiltheyare displaced ventrality indicesand are includedin Fig. 11 and fartherfromthe apex (Fig. 34). 12 to give the trendsof increasingdorsiventrality The rhizome grows plagiotropicallyover the of the rhizome. IncreasingD. I. of the dorsal substrateexcept for the firstseveral millimeters and ventralsurfacesgive a measure of the inwhichare slightlyelevatedabove the surface. As creased difference betweenthe respectivesurface earlierauthors (Weber, 1936; Troll and Wetter, and a theoreticalcircle of the same dimensions. 1952) observed,the roots are arrangedin two This is especially obvious for M. squamulosa ranks,alternatingin positionon the ventralsur- whichshows a D. I. of zero 10-50gm behindthe face of therhizome. The rootsare endogenousin apex (Fig. 12). Fluctuationsin the positionof originand a rootprimordium in its incipientform pointsforM. vacciniifolia(Fig. 11), particularly can be recognizedby its four-sidedapical cell at the beginningof the line, are due to regions (Fig. 23). The rootsare protostelic, have a large where primordiaare being initiated. Bilaterality root cap, and are usually hidden among the at- is expressedwhen the d' of the dorsal surfaceis tenuate,asymmetrically peltatescales whichcover equal to thed' of theventralsurface,but whered' the rhizomesurface. Root growthis usually ar- is not equal to zero. At approximatelylOO1m restednear the tip of the rhizome,but older por- behind the apex of M. squamulosa (Fig. 12), tionsof theshootwhichalreadycontactedthe soil the rhizomebecomes more bilateraland increashave rootselongatingand branchingintothe sub- ing separationof the dorsal pointsfromthe venstratum. tral points gives a measurementof the progresRelated to the change fromslightelevationof sive increasein dorsiventrality of the shoot. The

Fig. 6-10. M. squamulosa.-Fig. 6-9. Progressivechangein transectional symmetry along the lengthof the rhizomeof M. squamulosa. X 31.5. 6. lOO,umfromthe apex. 7. 140,umfromthe apex. 8. 96O0,mfromthe apex. Note thatthe leaf primordium is on the dorsal surfaceand thatthelateralbud is positionedon theflanksof the rhizome. 9. 3.12 mm fromthe apex. 10. Transectionof an expandedlateralbud nearthe pointof insertion to the main axis, showingthe lJ-shaped meristele.X 40.



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change fromradial to bilateralsymmetry is conditionedby the appearance of the firstleaf and branch primordia;greater cell division on the adaxial surfaceimpartsa more roundedappearance in contrastto the flattenedventralsurface. Therefore,the dorsiventrality of the stem of Microgrammais derivedsecondarily(i.e., postgenitally) ratherthan congenitallyand is intimately associatedwiththeinitiation oflateralappendages.

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Structureof the shoot apex-Rhizomes of Microgrammahave an apical meristemmeasuring 100-150,tm in height and 240-275/mmin diam above the insertionof the firstleaf primordium (P1). The apical dome of Microgramma shows a histologicalzonation similarto that describedfor Aspleniumruta murariaand Adiantum cuneatum by Hagemann (1964). At the summitof the apex is a zone of faintlystaining tabularor prismaticcells,the largestbeinga welldefinedtetrahedralapical cell (Fig. 5, 16). The apical cell has threecuttingfaces (Fig. 17) and thisapical cell shape is correlatedwiththe radial of the tip of the rhizome. Below and symmetry surroundingthis "initialzone" of prismaticcells are small, darklystainingcells whichHagemann findscomparableto the zone of primarymorphogenesisor leaf initiationin seed plants (Fig. 5). A rib meristemof anticlinally orientatedcell files occursdirectlyunderthe apical initialsin thecenteroftherhizome(Fig. 5, 16). The diameterof theapical domeincreasesfrom about 150 umin the youngsporophyteto a maximum of about 250 /m in the matureplant. Cell divisionsin the pithresultin the increaseof rhizome diameterfromabout 0.2 mmin the juvenile plantto 2 mm in the matureaxis of M. vacciniifolia, thus giving the shoot its obconic shape. This progressiveincrease in stem diameter is thereforedue to primarythickeninggrowth(see Wetterand Wetter,1954, for a detaileddescriptionof primarythickening in the leptosporangiate ferns).

Shoot anatomy and vascular organizationShoot anatomyis similarin the two species (Fig. 9). The epidermisis a single layer of smallD crogromms squOmu0oso 0V20 Y 0-ventrXl diameteredcellsfromwhichthepeltatescales arise 0.15 t/ -dorsal by oblique divisions. The groundtissue consists of large,uniformly isodiametricparenchymacells. Vetrl 9 12. M. The vascularpatternis a dictyostele Y0.507XO454 ro. consistingof l12 Distance behind the apex inj,um five meristeles,each withthe xylemarrangedin a diarch patternwith protoxylemoccupyingthe Fig. 11, 12. Graphical representation of the dorsi- poles. The phloem occurs as two arcs flanking ventralityindex (D.I. ), showing an increase in dorsi- the xylem. Each meristeleis surroundedby two ventral symmetry in transection of the rhizome of layersof endodermis(van Tieghemand Douliot, Microgramma. See text for elaboration. l l. M. vac1888). Meristeles anastomose throughoutthe = 0.0005X + 0.5126; r 0.9747. ciniifolia. Dorsal: Y lengthof the rhizome. Two of the strandsare Ventral: Y =_0.00087X + 0.4504; r = 0.9719. 12. M. squamulosa. Dorsal: Y = O.OO1X + 0.0579; r = 0.9746. located ventrallyand innervatethe developing Ventral: Y =_0.001I2X + 0. 1197; r = 0.966 1 roots (Fig. 9). The lateral bundles ramifyinto //

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eitherleaf or bud primordiaand anastomoseperiodicallywiththelarge,dorsalbundle (Fig. 9). The anastomosesolfthevascularbundles,which can be easily followedin clearingsof matureregionsof the rhizome,occurin relationto a leaf or branch (Fig. 13, 14). The species studiedvary slightlyin the extentof fusionof the lateral and dorsal meristelesbeforetheyentera leaf primordium and in the angle of divergenceof a trace fromthe dorsal bundle into a lateral bud. For

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dorsal meristele(large solid line, Fig. 14) joins fittedbyeye. withthe lateral bundle (dashed line) whereupon they immediatelyseparate and two traces enter the develoqping leaf (Fig. 14). By contrast,in M. (Fig. 13). However,the bud trace in M. squathelateralbundleand thebranchof mulosa branchesfromthe dorsal meristeleeither vacciniifolia the dorsal meristelefuse and remainunitedfor a above thebud site,resultingin an acute angle,or long distancewhereuponat the base of the pri- at the same level at a 900 angle (Fig. 14). In mordium,the bundle separates and two traces contrastto the leaf, branches from the lateral, entertheleaf (Fig. 13). Withreferenceto lateral dorsaland ventral(small solid line) bundlesenter differencein the bud primordium(Fig. 13, 14). buds, there also is an interspecific the angle of divergenceof a branchfromthe dorVascular differentiation in the rhizomeis first sal bundle and its subsequent fusion with the evidentin M. squamulosa 70-100 xmbelow the lateralmeristele.In M. vacciniifolia the pointof apex and in M. vacciniifolia60-120gm fromthe divergenceis below the pointof bud insertionand apex. Procambium is recognizable as discreet the trace divergeswith an angle less -than90 ? bundles of denselycytoplasmiccells differing in


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Fig. 16-29. Camera lucida drawingsof the rhizomeapex and stagesof leaf,bud, and root development in M. vacciniifolia. 16. Longisectionof the apex. 17. Transectionof the apex, showingtriangularconfiguration of the apical cell. 18-22. Median longisections of stagesof initiationof leaves 1-4. 22. Leaf of anotherrhizome. 23. Median longisection of a root primordium, of buds 1-5, showingendogenousorigin. 24-29. Stagesin development 7. 29. Shaded cells are tannin-filled.


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theirstainingpropertiesfromthe contiguous,increasingly vacuolategroundmeristem.Subsequent of procambiuminto phloic and differentiation xylaryprovascularelements(Esau, 1965) is first evidentin the dorsal meristeleand later in the othervascular bundles of the rhizome.

225

on the meristemflank (Fig. 24, 30). In subsequentstagesof development(indicatedby plastochronor bud numberin Fig. 24-29), the apical initialcells undergorepeatedanticlinaldivisionto forma smallgroupof lateralprismaticcells which constitute the futureinitialsof thebud apex (Fig. 25). At the same timethatthe initialsare formLeaf initiationand earlydevelopment-The in- ing lateralderivatives, periclinaldivisionof flankternodeswithinthe first5 mm of the shoot are ing cells causes the surrounding tissueof the rhimuch shorterthan the average 1 cm forM. vac- zome axis to become elevated,givingthe initial ciniifoliaand 2.5 cm for M. squamulosa (Fig. zone a sunken appearance (Fig. 25, 26). In 15). There are fouror fiveleaf-budpairs in the older bud positions,increased periclinaldivision elevated portionof the rhizomeof M. vaccinii- in the lateralderivativescauses the bud meristem folia, the firstleaf-budpair arising 100-150 tm to become even more deeply sunkenin the rhibehindthe apex (Fig. 15). The leaf primordium zome flank (Fig. 27, 31). By bud stage 5 or 6, originatesfromthe flankmeristemof the apical the central initial cells also become more endome 100-250pm below the apex. In Asplenium largedlaterally(Fig. 27, 28), and it is possibleto rutamurariaand Adiantumcuneatum,Hagemann recognizea centraltetrahedralapical cell (Fig. (1964) described a darklystainingsmall-celled 28). meristemcomplex at the point where a leaf priThe bud apex, however,does not show a sigmordiumis expected. Accordingto Hagemann nificantrib meristematic activityin the firstfive (1964), at a slightlylaterstage an apical cell can plastochronsof development(Fig. 25-28, 30-32). be distinguished fromthe outerlayerof thismeri- In fact, the centralprocambialstrandof a bud stemcomplexand is characterizedby itslargesize becomes differentiated just beneaththe apical iniand vacuolation. By contrast,in M. vacciniifolia tial cells and its close proximity to the centraliniand M. squamulosa P1 appears as a slightpro- tial regionis indicativeof the lack of rib merituberanceon the dorsal side of the rhizomeand stematicactivity(Fig. 28, 31 ); it is onlywhenthe is alreadymarkedat thispointby the appearance bud meristembecomes emergentthat anticlinal of a large lenticularapical cell (Fig. 18). Since divisionsare evidentin the central pith region theflanksof the apical dome of Microgrammaare (Fig. 29, 33), and these,togetherwithactiveperidenselycytoplasmic(Fig. 4, 5), it is difficult to clinal divisionof flankmeristemderivatives,are delimita meristemcomplex of cells, and hence actuallyresponsiblefor the early extensionof a earlierstagesin theformation of P1 werenotseen. branchaxis (Fig. 29, 33). Coincidentwithapical Anticlinaland periclinaldivisionsof the deriva- emergenceis an enlargement of all the prismatic tivesof theapical cell resultin the elevationof the initialcells and the clear, by virtueof its central primordium above the rhizomesurface(Fig. 19). positionand characteristic shape, definitionof a Cells internalto the apical cell undergofurther tetrahedralapical cell at the summitof the apical periclinaldivisionand thus broaden the base of dome (cf. Fig. 29 with28, Fig. 33 withFig. 32). the leaf primordium(Fig. 20, 21). Differentia- Afterits emergence,the bud begins to initiate tion of the procambiuminto the leaf primordium bud, leaf,and rootprimordia;but thesestructures is acropetal. Two vascular traces are evidentin will not extenduntilthe branchis displaced farthe leaf primordium and these formthe vascular therfromthetip of the main axis (Fig. 34). Like supply of the base of the petiole. The lateral the leaf, the directionof differentiation of bud prismaticderivativesof the apical cell formthe vasculatureis acropetal (Fig. 26-28). However, marginalmeristemof the leaf primordium(Fig. in contrastto theleaf,thevascularsupplyis more 22) and divisionof thesecells formsthewing-like complicatedin origin since it is derived from protrusions fromthe leaf axis whichwill become branches of both dorsal and lateral bundles as the halves of the simple lamina. The leaf pri- well as one of the ventralstrands(Fig. 13, 14). mordiumis erectfrompositionsP1 to P4 (Fig. 18- As the bud enlarges,the single vascular bundle 22), and crozier formationbegins at approxi- whichtraversesthe axis becomes branched. It is matelyP5. The youngleaves remainas croziers at first U-shaped, then the U-shaped meristele until5-8 cm behindthe apex at whichtimethey splitsinto threebundles,and finallyfivevascular beginto unroll. bundlesare evidentin transectionnear the tip of thebranch(Fig. 10). The branchshowsthesame Initiationand developmentoflateralbuds-Bud obconic form as the main axis, having a small primordiaarisefromtheflanksof theshootapical diameterat the pointof its insertionand progresmeristemof the parentaxis 100-250/,mfromthe sivelyincreasingin girthas its departsfromthe apex. In M. vacciniifoliaa bud meristemis first rhizome. evidentas two to severaltabular (prismatic)surface cells that are somewhatmore vacuolate and DIscussIoN-Although Goebel (1928) recogelongatedanticlinallythan theirneighboring cells nized that branchingin Microgramma (Poly-


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