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Vascular Construction and Development in the Aerial Stem of Prionium (Juncaceae) Author(s): Martin H. Zimmermann and P. B. Tomlinson Source: American Journal of , Vol. 55, No. 9 (Oct., 1968), pp. 1100-1109 Published by: Botanical Society of America Stable URL: http://www.jstor.org/stable/2440478 . Accessed: 19/08/2011 13:58

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http://www.jstor.org Amer.J. Bot. 55(9): 1100-1109.1968. VASCULAR CONSTRUCTION AND DEVELIOPMENT IN THE AERIAL STEM OF PRIONIUM (JUNCACEAE)1

MARTIN H. ZIMMERMANN AND P. B. TOMLINSON HarvardUniversity, Cabot Foundation,Petersham, Massachusetts and FairchildTropical Garden, Miami, Florida

A B S T R A C T The aerial stemof Prioniumhas been studiedby motion-pictureanalysis which permits the reliabletracing of one amonghundreds of vascularstrands throughout long series of transverse sections.By plottingthe path of many bundles in the maturestem, a quantitative,3-dimensional analysisof their distribution has beenmade, and by repeatingthis in theapical regionan under- standingof vasculardevelopment has been achieved.In the maturestem axial continuityis maintainedby a verticalbundle which branches from each tracejust beforethis enters the leaf base. Lateral continuityresults from bridges which link leaf traceswith nearbyvertical bundles.Development of the provascularsystem involves a meristematiccap into whichthe blindends of verticalbundles can be followed.Leaf tracesare prodlucedcontinuously in associa- tionwith developing leaf primordia for a periodof over30 plastochrones;they connect with the verticalbundles in the meristematiccap and so establishthe essentialvascular configuration whichis laterreorientated through about 900 by overallgrowth of the crown. The last bundlesto differentiatefrom the leaf do so outsidethe meristematic cap and thusfail to makecontact with the axial system;they appear in the matureaxis as blind-endingcortical bundles. Prionium is only distantlyrelated to palms and its vascularhistology is quite different.Nevertheless, the courseof vascularbundles and the originof thispattern in the stemresembles that of a palm. It is suggestedthat we are examiningthe fundamental pattern of vascular development in large .

RECENT reexaminationof the vascular system in relevatnceto the situation as it exists in nature the stems of largermonocotyledons, particularly (Tomlinson and Zimmermann,1966). Neverthe- the palms, shows that earlier anatomists had less, since the time of von Mohl, the "palm- incompletelyobserved the fundamentalprinciple type" of vascular arrangementhas been regarded underlyingconstruction of these axes and could as the one basic for the stem in most groups of thereforearrive at no true understandingof monocotyledons.Continuing reexaminationof developmentalprocesses (Zimmermann and Tom- monocotyledonousanatomy, far from demolishing linson,1965, 1967). Using the small palm Rhapis this concept,is likelyto strengthenit by putting excelsa as a model it has been shown that the it on a firm factual foundation which was classical researches of Hugo von Mohl (1824, originallylacking. The only literatureto which 1849) and de Mirbel (1843-44) on the palm stem modernwork has any directrelevance is that of were correctbut incomplete.They did not reveal about 100 years ago because in subsequentyears the axial continuityof vascular strandsalthough anatomistshave virtuallyneglected a topicwhich, this is essentialto an appreciationof the pathway like many others in the classical botanical forlong-distance transport. It has also been shown tradition,has been regardedas "fullyworked out" that any understandingof this pathway which and thereforedismissed by later generationsof modern textbooks may claim to illustrate has investigators. arisen by a processof "paper evolution" with no Investigationof the palm Rhapis revealed a well-definedorganization of the maturevascular 1 Receivedfor publication 30 April1968. system,as well as a clearlyrecognizable pattern Researchof one of theauthors (P.B.T.) on theanatomy of development. The question naturally arose of monocotyledonsis supportedby N.S.F. Grant GB- 5762-X. whetherthis vascular patternis fundamentalfor the palms and perhaps even forthe monocotyle- The writersexpress their appreciation to Dr. D. F. dons as a whole.The answerto this questionwill Cutlerof the Royal BotanicGardens, Kew, England,and Dr. R. G. Streyof the Natal ,Durban, South requirea great deal of work.A numberof Africa,for the materialon which the investigationis in various familieshave been observed but any based, and to Miss AnneBellenger and GeorgeBailey for vascular pattern fundamentallydifferent from preparationof sections.They thankProfessor Ralph H. Wetmorefor his criticalreview of the manuscriptand for that of Rhapis has not been found.The present manyvaluable discussions. articleprovides further evidence in this direction

1100 October,1968] ZIMMERMANN AND TOMLINSON STEM OF PRIONIUMt 1101 by demonstratingthat the vascular pattern, subsequently only every third section was whichseems characteristicof the palm stem,also kept to give a total of 200 slides. occurs in a having no obvious systematic Sectionswere bleached briefly in Clorox,washed affinitywith the palms. The presentstudy of the well in runningwater and stainedin safraninand "-rush," Prionium serr-atum(Juncaceae), Delafield's haematoxylin. They were then amply confirmsour prediction.This species was mountedone per slide to permitvascular analysis selected because it was a suitably large mono- by meansof our opticalshuttle (Zimmermann and cotyledonand yet proved amenable to investiga- Tomlinson, 1966). For a study of histological tion by the methods we have employed. The details thinner sections were cut from small anatomy of Prionium has been described in blocks which had been embedded in celloidin. considerable detail by Buchenau (1893) but These sections were stained either in safranin without attention to the arrangement and and Delafield's haematoxylinor in the tannic developmentof the vascular bundles. acid-ferricchloride-resorcin blue combinationof Cheadle, Gifford,and Esau (1953). MATERIAL AND METHODS-Material-Prionium apices wveredehydrated and embedded serratum(L.f.) Drege is a gregariousplant of wet in Paraplast by normal procedures.Continuous places in the Cape and elsewherein South Africa series of sections 10,u thick, in transverseand whereit is knownas "Palmiet." Livingspecimens longitudinalplanes, were cut on a rotarymicro- have not been seen but Hooker (1868) illustrated tome, stained in safraninand Delafield's haema- cultivated specimens and fromhis figuressub- toxylinand mountedin the usual way. Individual sequent drawingsare largelyderived (e.g., transversesections were so large that only two Buchenau, 1893, 1906; Vierhapper,1930). It is could be mounted on each 50 X 75-mm micro- describedby Buchenau as a plant withperennial scope slide. This proved beneficialin the sub- erect arisingfrom creeping, scale-bearing sequent plotting of the developing vascular . The aerial stems considered in this systemsince the optical shuttlecould be used in study are erect shoots up to 2 m tall with a the way describedbelow. terminalcluster of lanceolate up to 1 m long,each shoot eventuallyproducing a terminal, Serial analysis-The vascular system of the paniculate .Stems are clothed with adult axis was unravelled with the aid of the the fibrousremains of persistentleaf bases, the cinematographicmethods, including the optical axis proper being of the order of 5 cm in diam. shuttle, described in detail elsewhere(Zimmer- Internodesare close set and only 2-3 mm long. mann and Tomlinson,1965, 1966). This involves Branching of the erect axis is infrequent,but the use of a cine camera to photograph,frame by accordingto Buchenau(1893) it may sometimesbe frame,successive sectionsobserved throughthe sympodial and associated with flowering,as in microscope.The optical shuttleis a device which Yucca and many other arborescentmonocotyle- uses two microscopesto superimposethe images dons. Otherwise branching is irregular and fromtwo successivesections in such a way that determinedby no obvious morphologicalrule. they are preciselyregistered in the filmplane of Axillary , protected by a conspicuous the camera and then photographedin turn. A prophyll,are distributedwithout obvious order furtherimprovement of the method described and are stronglysuppressed. in the earlier article is a foot-operateddevice A portion of the aerial stem togetherwith a whichswitches each microscopeon or offin turn, numberof shoot apices fixedin FAA were pro- leaving both hands freeto adjust the microscope vided by Dr. R. G. Streyof the Natal Herbarium, stage. Optical alignmentof subsequent sections Durban, South Africa.The axis whichwe studied was then achieved by reductionof the apparent was mature but only slightly hardened and motionbetween the twoimages viewed alternately by operating the foot thereforeeasy to cut, it a switch, rather than by making subject suitable of seen at the time. forour purpose.This is in superimposition images same contrastto the experi- In addition a number of of cine sequences were ence Buchenau (1893) who evidentlyhad much photographedthrough a Wild M-5 stereomicro- older and more lignifiedstems from which he scope at a magnificationlower than that which could get useful sections only by means of a could be obtainedwith the compoundmicroscope, circularsaw. using the drawingmethod to align the series of sectionsfor the camera. Sectioning-Foranalysis of the maturevascular The vascular systemin the developingcrown systema seriesof sections 60 ,uthick were cut from was plotted using the paraffin-embeddedseries the unembedded material using a Reichert oftransverse sections. The methodwas outlinedin slidingmicrotome. Central and peripheralregions detail in a paper describingthe analysis of the were analyzed separatelybecause single sections developing shoot system of Rhapis excelsa of the whole stem would not fit on a standard (Zimmermannand Tomlinson, 1967). This in- 25 X 75-mmmicroslide. In each regionthe first volved carefullymapping the course of the leaf 100 sectionswere retained as a continuousseries; traces on an outlineplan of the axis in quantita- 1102 AMERICAN JOURNAL OF BOTANY [Vol. 55

tive detail as shownin Fig. 14. A major tracefrom tissues which are described subsequently as each successivelyyounger leaf as well as a number corticalbundles (Fig. 1). There is no specialized of minor and cortical traces were plotted in a layer which delimits the wide central cylinder basal directionto the bottom of the series of fromthe .Towards the centerof the stem sections. Vertical bundles were plotted in the the groundtissue is again somewhatspongy and reversedirection as far as possible towards the withoutspecial differentiation, except for frequent shoot apex proper.Plotting involves followinga tannin cells (Fig. 2). The central cylinder is particular bundle continuouslythroughout the recognized by its crowded peripheralvascular series of sections and measuringits position at bundles and a fairlyabrupt transitionto more intervalsof about 0.2 mm. Measurementswere compact ground parenchyma.Vascular bundles taken fromthe selected bundle to the centerof becomewider and somewhatmore diffuse towards the stem which was determinedby eye in each the stemcenter. Outgoing leaf traces together with section. The fact that the stem center is not the derivative bundles described below are con- preciselydefinable inevitably led to some irregu- spicuous in the peripheryof the centralcylinder. laritiesin the originalplots; theirrepresentation Individual bundles are amphivasal. Each in- in Fig. 14 omits these irregularitiesand draws cludes a centralphloem strand surroundedby a the course of bundles as smooth curves. This completecylinder of angular vessels, the vessels more nearly represents the actual course of in turn surroundedby short fairlynarrow and bundles in the stem. slightlythick-walled fibers. The outer limit of The successof this plottingmethod depends on the bundle is representedby thin-walledcells the certaintywith which an individualvascular which are sharply delimited from the ground strand can be followedfrom section to section parenchyma(Fig. 5-9). These sheathinglayers withoutan inadvertentjump beingmade fromone become much thickerwalled in older stems,as is bundle to another. The optical shuttle method evidentfrom Buchenau's description.Conducting makesthis possible. The filmwhich is made during elements of metaxylem and metaphloem are the plottingprocess is not essentialbut providesa short (rarely more than 150,u long), with co- usefulrecord for later review.The optical shuttle incidentend walls (Fig. 4). The phloemincludes can be used even with two sectionson one slide conspicuoussieve tubes up to 25 ,i wide,with more if sections are lined up in the order 1A-2A, or less transverse,simple sieve plates, together 1B-2A, 1B-2B, 2B-3A, etc., wherebythe num- with narrow parenchyma and densely bers indicateslides, A and B the two sectionson stained companion cells. Vessel elementsup to each slide, and italics the sequence of pho- 70,u wide slightlyoblique, scalariformlyor re- tography.The resultingcine filmdemonstrates ticulately perforatedend walls and scalariform the generalconstruction of the vascular systemin pittingon the lateral walls. The trachearyele- the developing crown, but the informationof ments are incompletely sheathed by con- most value is the measurementsincorporated spicuouslypitted parenchymacells, the vessels in Fig. 14. otherwiseoften abutting directlyon sheathing fibers.Narrow protoxylemelements with spiral OBSERVATIONs-Mature stemn-Theanatomy of wall thickeningsare distributedon the innerside the stem of Parioniumhas been described in of leaf tracesat certainlevels in the mannerindi- detail by Buchenau (1893). The followingnotes cated below. The specialized phloem of bridge our own observations. brieflysummarize his and bundlesin the leaf trace complexis also The surface layers include a narrow zone of described suberized cells about eight cell layers beneath below. the thin-walledepidermis. The cortexis 5-8 mm The peripherallayers of the centralcylinder are wide and consists of loose, lobed but otherwise complicatedby the insertionof tracesto dormant undifferentiatedparenchyma within which are initialswhich do not, however,disrupt the large conspicuous collateral leaf traces together overall picture.No axillarybuds were presentin with a fewnarrow bundles with reduced vascular the piece of stem which was analyzed.

Fig. 1-9. Prionium serraturm,vascular anatomy of mature stem.-Fig. 1. Transverse section throughperipheral region of mature stem, X 10. The upper half illustrates the cortex with few leaf traces and cortical bundles. The lower half with the numerousvascular bundles illustratesthe peripheralportion of the central cylinder.Outgoing leaf traces, cut at an angle, appear oval in shape.-Fig. 2. Transverse section throughthe central region of the mature stem at the same low magnification,X 10. Vascular bundles are less crowded, ground parenchyma lacunose.-Fig. 3. Transverse section throughperipheral region of central cylinder at higher magnification,X 37. A leaf trace is shown entering the cortex (upper-third of illustration). Bridges and vertical bundle, which are branches fromthis leaf trace, appear below and can be recognized by their densely stained phloem (arrows). Compare this transversesection with the plotted diagram, Fig. 13.-Fig. 4. Longitudinal section through an individual vascular bundle, showing coincident end walls of sieve-tube elements and vessel elements, X 104.-Fig. 5. Vertical bundle withoutprotoxylem, X 104.-Fig. 6. Vertical bundle with protoxylem, X 104.-Fig. 7. Vertical bundle or bridge, immediately above the point of branching from parent leaf October,1968] ZIMMERMANN AND TOMLINSON-STEM OF PRIONIUM 1103

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shownin Fig 8, iI.but a itehge,a onffso,X14 trac, X104.Not th peclia phoem.Fig 8.Bride (eft above tr 1104 AMERICAN JOURNAL OF BOTANY [Vol.55

Course of the vascular bundles-The vascular creases to reach a maximumat the level where systemof Prionium correspondsexactly to that the sharp outward curve of the bundle begins. describedfor the palm Rhapis excelsa (Zimmer- There is littlechange in the numberof sheathing mann and Tomlinson,1965) except forquantita- fibersthroughout the bundle, as in Rhapis. tive differences.Each leaf is supplied by a series Bridges-The above systemdescribes longitu- of vascular bundles (leaf traces). Major leaf dinal continuityin the vascular system.Lateral traces divergefrom the center of the stem into continuity,as in Rhapis, involvesbridge bundles the leaf. Bundles which diverge at a shallower which branch from the outgoing leaf trace, angle fromprogressively more peripheralregions always in an upward direction,and link with may be arbitrarilydesignated intermediate and adjacent vertical bundles in the stem periphery minor bundles. The relative dispositionof each (Fig. 3, 13). Bridges are amphivasal,often quite type of bundle going to a singleleaf is shown in long and somewhatnarrower than theirparental the plotted diagram used to make Fig. 13. To- leaf trace. They occasionallysplit so that they wards the peripheryof the central cylindereach make distal contactwith two verticalbundles. A outgoingleaf trace gives offone (sometimestwo) peculiarityof derivative bundles (both bridges continuingvertical bundles which proceed up the and the lowest part of the continuingvertical stem to repeat the processof branchingin asso- bundle) which distinguishesthem readily in ciation with some higherleaf (Fig. 3, 13). The single sectionsis theirmodified phloem (Fig. 7). series analyzed have all been too short to The familiarphloem of the normalbundles measure leaf-contactdistances directly,but by- is replaced by a small-celled tissue without extrapolation (adding the distal part of one clearlycircumscribed sieve tubes. This tissue has bundle to the proximalpart of another)it can be not been examined in detail, but it appears to estimated that this distance is about 30 inter- consist of horizontalbands of narrow-,nucleate nodes fora major bundle. The courseof vertical cells alternatingirregularly with bands of some- bundles in the stem is not uniformlyaxial but whatwider cells with callose depositson indistinct shows the so-called "double-curve"principle of pit-like areas but not aggregated into definite von Mohl. Traced in an upwarddirection bundles sieve plates. Similarphloem has been encountered can be followed gradually towards the stem in the bridge bundles of those membersof the centerbefore they bend out sharplyat the next Pandanaceae whose vascular anatomv we have leaf contact. Major bundles extend inwards all investigated(results unpublished). These modi- the way to the stem centerduring this upward fiedphloem cells also recall those in the glomeruli course and describe the longest path between of the node in members of the Dioscoreaceae leaf contacts. Intermediateand minor bundles (Braun, 1957). In Prioniumthe modifiedphloem are progressivelyrestricted to the stemperiphery, of the bridgecontinues into the vertical bundle accounting for the vascular congestionat the above the point of union for some distance so periphery of the central cylinder.Their leaf- that a recent fusion is easily recognized.It is contact distances are correspondinglyshorter. equally persistentin vertical bundles (Fig. 3, Apart fromthe large leaf traces which cross 7-9). the cortex in their outward course there are M\lajorleaf traces give off up to six bridges, narrowbundles restrictedto the cortex.Traced intermediate and minor traces progressively downwardsfrom the leaf insertionthese cortical fewer.The outgoingleaf trace loses almost all bundlesend blindlybelow, never extending more of its metaxylemto the derivative bundle and than two or threeinternodes. Although insignifi- passes into the leaf base as a collateral bundle, cant in termsof vasculature,these bundles are which constructionis retained throughoutthe very meaningfulwhen developmentof the vascu- leaf (Buchenau, 1893). Unlike Rh.apis there is lar systemis considered. no elaborationof the leaf-tracecomplex in associa- The anatomyof each vascular bundle changes tion with lateral (Tomlinson and in its courseup the stemin exactlythe same way Zimmermann, 1968) since Prionium has a as in Rhapis. On divergingfrom the leaf tracethe terminalinflorescence. bundle is as shown in Fig. 5 and includes no protoxylem.2At the higherlevel, protoxylemis 2 Protoxylemis distinguishedstructurally from meta- xylemby its narrowelements with annular or helicalwall developedon the innerface of the bundle(Fig. 6), thickeningsand by its situationon the innerside of the and the number of elements progressivelyin- bundle.

Fig. 10-12. Prionium serratum,vascular anatomy of the meristematiccrown.-Fig. 10. Transverse section through the meristematiccrown of the aerial stem at the level of the apical ,X 11.-Fig. 11. Transverse section through meristematiccrown 0.5 mm below leaf insertions22-24, X 98. Four leaf traces are shown. The procambial strands are cortical traces. Below the leaf insertionthey turn out radially towards the stem periphery.They do not enter the central cylinderbut end blindly below in the cortex (see also Fig. 14).-Fig. 12. Median longitudinal section through the meri- stematic crown. The overall shape, given by the leaf insertions,is a shallow 'bowl. The shape of the meristematiccentral cylinderis t,hatof a very shallow cone. It can be recognizedby the whitishline of tissue lacking tannin-filledcells. October,1968] ZIMMERMANN AND TOMLINSON-STEM OF PRIONIUM 1105 ___~~ 1 ~~~~~~~~~~~0

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Topographyof the crown-The general con- formationprovided in Fig. 14 and 16 permitsa figurationof the developingcrown is shown in reasonable assessmentof the process of vascular longitudinal section in Fig. 12. The overall differentiationwhich is summarizedin Fig. 17. outline is a shallow bowl, and that of the meri- Procambial strands initiated by the leaf base stematiccentral cylinder is a shallow cone. It is link with a verticalbundle differentiatingon the crowned by the shoot apex proper. Developing lower(inner) face of the cap. The firststrands are vascular strands are representedby series of major traces and the linkage is in the centerof procambial initials which are progressivelyless the stem. Linkage is effectedwith a bundlewhich easily recognizedas one approaches the apex. has grown blindly in associationwith the meri- Leaves are arrangedin a one-thirdspiral phyllo- stematic cap for the maximum length of time. taxis, each leaf having a short, closed tubular This makes a maximum leaf-contactdistance. base (Fig. 10). Since thereis no regularinternal Subsequently differentiatedbundles (on the helix and because the phyllotaxisis simpler,the steeply risingpart of the curve in Fig. 16) link plotteddiagram (Fig. 14) representsmore realistic- progressivelywithin the meristematiccap with ally a radial sectionof the apex than did the cor- progressivelymore peripheral and shortervertical responding diagram which resulted from our bundles. These are the intermediateand minor analysisof Rhapis (Zimmermannand Tomlinson, bundlesof whichone of each is shownin Fig. 14 1967; Fig. 2). In Rhapis the leaves have a two- continuousinto leaf 16. At a level when the pri- fifthsspiral and individual bundles describe a mordiumis about the 16th youngest,differentia- helix in the directionof the phyllotacticspiral. tion of the vascular bundleswithin the leaf slows In Prionium major bundles merely describe a down (Fig. 16). Differentiationof these last sharp 360? turnin the stem center(Fig. 15). formedbundles occurs outside the cap and after Figure 14 representsthe major dorsal leaf trace all possible leaf trace connections have been fromeach leaf along one orthostichy. completed. Consequently these bundles fail to Comparingthe arrangementof maturevascular unite with the axial systemand they are seen as bundleswith that in the crown(Fig. 13 with 14) bundles which end blindly in the cortex after it is evident that primarygrowth reorientates traversingtwo or three internodes.Three such the vascular systemthrough about 900 so that corticalbundles belongingto P23, P24, and P25 leaf traces whichrun more or less parallel to the are shown in Fig. 14. Figure 11 shows some of axis in the crown become obliquely radial, them as procambialstrands in a transversesec- whereasvertical bundles change from an obliquely tion 1.1 mm below the apical meristem,and ca. radial directionto one more or less parallel to 100l below leaf insertions23-25. They are not the axis. numerousenough (about 50 per leaf) to make In the crown vertical bundles, of which an contactwith each otheras occursin Rhapis,where inner (IVB) and an outer (OVB) are shown in there are about 1,000 per leaf. There is thus a Fig. 14, can be traced upwards until theiriden- complete developmental explanation for the tity is lost in a histologicallywell-defined region dispositionof leaf traces betweencortex and cen- here called the "meristematiccap." That bundles tral cylinder.Both bundles are developmentally end blindlyin this regionis a matterof observa- homologous,but the last to differentiatedo so tion whichcan be furtherverified bv calculation. outsidethe meristematiccap and fail to establish On this basis many traces which end blindlyin axial continuity. the cap will make connectiononly with a leaf In serial sections the peripheryof the meri- primordiumwhich will not be visible before stematiccap cannot be determinedvery precisely some 10 othershave appeared. because of the way in which procambialstrands Leaf traces (with the exception of cortical graduallydifferentiate out of it. The information bundles) are always continuousbelow with ver- in Fig. 14 and 16, however,provides two methods tical bundles and this connection must take of estimatingits extent. From Fig. 14 its outer place withinthe meristematiccap as is discussed limit must be about the upper end of OVB and below. Therefore leaf traces which have below the base of leaf 21, since OVB was one of established continuityextend throughthe cap. the outermostvertical bundles which could be The numberof tracesin successivelyolder leaves plotted. Furthermorethe extent of the cap is followsapproximately a sigmoidcurve (Fig. 16). given by the distance fromthe apical meristem There is of course some variationin the number properto the firstleaf base havingthe fullcomple- of bundles in individual leaves and some guess- mentof vascular bundles.Allowing generously for work is involved in counting the number of the subjective errorresulting from the need to recognizableprocambial strands. For each mature distinguishbetween vascular and corticalbundles, leaf thereare about 95 traces continuousinto the it is seen in Fig. 16 thatthe edgeof the cap is where central cylinder and 45 cortical traces. Leaf- a leaf has producedbetween 80 and 100 bundles. trace differentiationcontinues for about 30 This lies on a steep part of the curve so the limit plastochrons. is fairly sharply defined as somewhere below leaves 14 to 16,even allowingfor error in counting. DIscussIoN--Vascular differentiation-Thein- Returningto Fig. 14 in which an outer vertical October,1968] ZIMMERMANN AND TOMLINSON-STEM OF PRIONIUM 1107

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S O 4 6 8 0t MILLIMETERS FROM STEM AXIS Fig. 13-16.-Fig.13. Plotted diagram of leaf-trace departure in themature stem.-Fig. 14. Plotted diagram of pro- vasculartraces in the meristematic crown. Note that the axial scale is stretchedabout two times for the sake of clarity. Onemajor leaf trace is shownfor all leafprimordia in oneorthostichy. In addition, the following traces are shown: one intermediate(ILT) and oneminor leaf trace (MLT) fromP16, one cortical trace each from P23, P24, and P25. An inner(IVB) andan outervertical bundle (OVB) are shown ending blindly in the cap. The arrow points to the location of theapical meristem-Fig. 15. Diagramshowing the 360? turn of a majorleaf trace in thestem center.-Fig. 16. Number of tracesin leaves and leaf primordiaat thepoint of theirinsertion. The extentof themeristematic cap is givenby the positionof the youngest leaf which has the complet;eset ofvascular traces, but no corticaltraces. 1108 AMERICAN JOURNAL OF BOTANY [Vol.55 bundle (OVB) is shown to fuse with the cap 1. MAJORLEAF TRACE directly below leaf 21, it is evident that this LINKS IN CAP 2. MINORLEAF TRACE would have to establish leaf contact,within the LINKSIN CAP 3.CORTICALTRACE 4.VERTICAL l INCEPTIONOUT- BUNDLES cap, with a bundle continuouswith leaf 15 to (4 SIDE CAP BRANCH make the last minor bundle. Beyond the meri- - ~~~~~~~FROM stematic cap in leaf positions later than about -- ~~~~~~LEAF P15, vascular bundles continue to differentiate - - ..z - - T~RACES in the leaf (forabout 15 more plastochrons),but they can make no vascular connectionwith the central cylinder and end blindly in the area distinguishedas cortexin the mature stem. Because corticaltraces are directeddownwards we inferthat their predecessors,the leaf traces whichenter the centralcylinder, are "sent down" from the leaf primordiuminto the cap, rather than in the reverse direction. However, the "direction" of bundle differentiationbetween leaf primordiumand cap remainsunknown; the Fig. 17. Diagrammatic representationof vascular differentiationin the crown of the monocotyledonous stem. presentpaper is not specificallyconcerned with The meristematiccap is indicatedby dashedlines. Three this question. positionsof leaf trace inception are shown by arrowswhich, however,are notintended to implythe unknown direction Renewal of vertical bundles-Although the ofdifferentiation. 1. Major leaftrace links with a potential longestpiece of crownwhich could be accommo- verticalbundle in the cap nearstem center. 2. Minorleaf dated on the microtomewas sectioned,the series tracelinks later with a potentialvertical bundle near the was still not sufficientlycomplete to show a leaf cap periphery.3. Leaf tracesappearing still later, when contact between two major leaf traces. The the leaf is situatedoutside the cap, fail to make contact trace to the oldest leaves in Fig. 14 do not show withthe vascular system of the centralcylinder and thus the youngest vertical bundles becomecortical traces, ending blindly below in thecortex. continuing along Cap renewalis fromleaf tracesat the pointwhere they the radius illustrated. However, the sections begin to branch(4). were slightlyoblique and on anotherradius they were just evident in the lowest slides. The readeris also referredto Fig. 2 and 3 in Zimmer- series permits this. For these reasons previous mann and Tomlinson(1967) where several such investigators have described development in verticalbundle branchesare shown.It is evident terms of cytohistological zonation and cell that as vertical bundles,initially ending blindly seriation.In many species the meristematiccap in the cap, become connectedto newly differen- can be recognizedin singlelongitudinal or trans- tiating leaf traces they must be replaced. This verse sections.As a result,it acquired the name must occur on the outer (upper) side of the cap "primarythickening meristem." As shownin this and newly differentiatedbundles must be con- and in the previouspaper on the vasculardevelop- tinuous with leaf traces in the downwarddirec- mentof Rhapis, the meristematiccap has nothing tion (Fig. 17). The originof new verticalbundles to do with primarythickening. Primary thicken- is also closelyassociated with the originof bridges ing involvesboth elongationand expansionof the whichresemble vertical bundles in theiranatomy stem, and most important,a reorientationof and the directionin whichthey divergefrom the all tissue throughabout 900. It is suggestedthat leaf trace. Bridges and vertical bundles are the term "primary thickeningmeristem" be developmentalhomologues. One of these deriva- discarded,because the tissue whichreally merits tive bundles makes contact with the margin of the name is diffuselylocated below the tissue the meristematiccap. This contact is maintained for which the term was originallycoined. The as the bundle continuesto differentiateuntil it alternate possibilityof maintainingthat name linkswith a leaf trace. This is the verticalbundle. forwhat has been called in thisstudy meristematic Other bundles make contact only with existing cap would be incorrectand thereforemisleading. verticalbundles and so become bridges. In conclusionthe foregoingremarks might be strengthenedby saying that the vascular topog- "Primary thickeninqmeristem' Previous in- raphyof furthermonocotyledonous crowns which vestigationsof the apical developmentof large fully confirms previous findings has been monocotyledons(Helm, 1936; Ball, 1941; Eck- analyzed. We are progressingtoward an under- hardt, 1941) have apparently relied on serial standing of the monocotyledonous vascular longitudinaland transversesections which had systemand its development. been studied singly.No attempt had been made to reconstructthe vascular systemin the crown LITERATURE CITED three-dimensionally.Only careful plotting of BALL, E. 1941. The developmentof the shoot apex individual strands throughout entire section and of the primarythickening meristem in Phoenix October,1968] ZIMMERMANN AND TOMLINSON-STEM OF PRIONIUM 1109

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