pole 1991 modified terminology for angiosperm leaf architecture

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A modif ied terminology forangiosperm leaf architectureMike Pole

ab

aDepart ment of Geology, University of Otago, P.O. Box 56,

Dunedin, New Zealandb

Department of Plant Science, University of Tasmania, GPOBox 252C, Hobart , 7001, Aust ral ia

Available online: 05 Jan 2012

To cite this article: Mike Pole (1991): A modified t erminology for angiosperm leaf architecture,Journal of the Royal Societ y of New Zealand, 21:4, 297-312

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Journal of the Royal Society ofNew Zealand.Volume 21. Number 4, December 1991, pp 297-312

Amodified terminology for angiosperm leaf architectureMike Pole*

Amodifiedterminologyfor describingleaf architectureis presented,which incorporateslamina development and gives greater emphasis to the patterns formed by groupsofvenationelements than to their size. Leaf architectureis describedusinga hierarchyofvenation"elements", which are grouped into "patterns", which are located in "zones"and "segments" of the leaf lamina.Keywords: macropaleobotany. leaf venation, leaf ontogeny, constructional morphologyINTRODUCTION

In the study of angiosperm leaf impressions within paleobotany it soon becomes apparentthat there is a need to develop a comprehensive terminology to describe leaf "architecture".This term was defined by Hickey (1979: 25) as"... the position and form of the elements constituting the outward expression of leafstructure. These include venation pattern, marginal configuration, leaf shape, and glandposition.Architecture is theaspectofmorphologywhichappliesto thespatialconfigurationand coordinationof those elementsmaking uppart of a plant without regard to histology,function, origin, or homology."

Leaf fossils are usually found as impressions only, with no anatomical detail, and theoutline of the leaf and the pattern of lines left by the venation on fossilisation is all one has towork with. An understanding of venation, its formation and the taxonomic distribution ofdifferent venation types and patterns, is then very important (a taxonomic survey of venationpatterns based on the modifications presented here is beyond the scope ofthis paper, but is inprogress and will be presented later). The addition of anatomical information, such ascuticular structure and stomatal distribution, adds a new range of characters and requiresfurther terminology.The terminology of leaf architecture must develop to a point where it can allow plantsknown only from leaf impressions to be integrated into the accepted Linnean-style taxonomicedifice. I propose a modified leaf architecture terminology which I suggest is an improvementon all existing systems.DEFICIENCIES OF EXISTING LEAF ARCHITECTURE TERMINOLOGY

Several different terminologies have been devised for describing leaf architecture, ofwhich the most comprehensive is that presented by Hickey (1973,1979) and Dilcher (1974).Melville (1976) objected to this terminology because it is restricted to dicotyledons and alsobecause its basis is in Greek rather than Latin. He presented a new terminology as asubstitute. Melville's criticisms were answered by Hickey (1979) and his terminology is notconsidered further here.The terminology of Hickey (1973, 1979) and Dilcher (1974) has as its cornerstone therecognition of vein orders, i.e.* Departmentof Geology, Universityof Otago,P.O. Box 56, Dunedin,New Zealand.Present address: Departmentof Plant Science,Universityof Tasmania,GPOBox252C,Hobart7001,Australia.

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298 Journal of the Royal Society ofNew Zealand, Volume 21,1991"... the recognition of vein orders is essential in describing leaf architecture ... "

and"... The fundamental rule for the determination of the order of a vein is its relative size atits point of origin" (Hickey, 1979: 32).

The patterns formed by the venation have secondary importance in Hickey's scheme, i.e..".. Subsidiary to the rule of size at the point of origin in the recognition of vein order is aconsideration of the behaviour of a vein in relation to veins of other orders and to marginalfeatures of the leaf blade" (Hickey 1979: 32).

I agree with Hickey on the importance of establishing vein orders, but believe that his useof size at the point of origin to determine the orders introduces a serious problem into histerminology. Hickey himself (1979) noted that important ambiguities may arise when applyingthese rules, for instance in distinguishing a "lateral primary vein" from a "basal secondaryvein". I f such ambiguity arises, it may not be possible to ordinate veins to at least the sixthorder, as Hickey does. In an ideal terminology one needs to know that something referred toas a "secondary" vein in one leaf is homologous to a "secondary" in another. In order toreveal phylogenetic relationships, a terminology must incorporate knowledge of thesehomologies.Thickness of veins at the point of origin is surely a variable thing, depending on the extentof growth. Troll (1935) and Arber (1950) both refer to the effects on venation of differinggrowth rates over the lamina, even to the point where lateral veins may dominate over themidrib (see below). It is the behaviour of the veins which expresses their identity, not theirthickness.Spicer (1986) criticised Hickey's terminology, including the intergradation of pinnate andpalmate organisation and the difficulty of distinguishing between primary and secondaryveins. His solution places emphasis (p. 379) " ... upon the pattern of vein courses andbranching hierarchy which provides unambiguous reference points 00'" I agree with hiscomments, but disagree with the details of his solution. Spicer proposed the term "pectinalveins" for lateral veins which

'''00differ from the main secondaries in that they produce abmedial veins almost throughouttheir length and a greater number of abmedial branches than any other laterals" (Spicer1986: 382).In his scheme only one pair of pectinals is recognised in anyone leaf, and this could beused as a point of reference. 1 do not accept that Spicer's "pectinal" veins are significantlydifferent from other lateral veins, and believe that they result only from a greater amount of

progressive, or ramifying, growth (see below). Often there is a complete gradation apically inthe number of abmedial branches on a lateral, but sometimes there is not. To single out thelaterals with the most branches on them is similar to singling out those which are thickest asprimaries. Both phenomena result from a localised increase in development (the summary ofArber's comments below is relevant to this discussion) furthermore, Spicer's terminologydoes not deal with abmedial branches on his "superior secondaries".Differential development of the lamina results in some end-member conditions whichHickey and Dilcher have included along with venation patterns in their terminology. In leafarchitecture, "venation pattern" refers to the interrelationships between veins or betweenveins and the margin. The amount of lamina development between veins, or the thickness ofthe veins themselves, is a separate phenomenon and should not be incorporated in aclassification of venation pattern.Careful consideration should be made before slotting features into a terminology. Forinstance, study of many leaves, both fossil and extant, has revealed that the "loops" ofvenation, usually referred to as "brochidodrornous" (e.g. by Hickey, 1979), are in fact almostalways composite structures, formed of several elements. Brochidodromous in the sense of aloop formed by the joining of just two elements may be rare.

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Pole - Modified leafarchitecture terminology 299WAYS OF LOOKING AT VENATION

The underlying principle of my approach is to look at leaf venation as a coherentconfiguration or set of patterns. Emphasis should be placed on describing the patterns formedby groups of elements rather than the behaviour of individual elements. I take the stance thatfeatures of venation, or other aspects of leaf architecture, can be labelled and have taxonomicor phylogenetic importance. They are comparable with the various diagnostic features inbones used in osteology or vertebrate zoology.ONTOGENY OF VENATION

Looking at leaves from a developmental point of view may provide a conceptual tool forordering the patterns of leaf venation and allow the construction of a comprehensiveterminology. Once the patterns are understood, homologies may be postulated.Leaf development in dicotyledons (Esau, 1965; Pray, 1955, 1963; Slade, 1957, 1959)begins with the progressive formation of the midrib during apical growth, controlled by theapical meristem. Laterals develop progressively away from the midrib during marginal

growth, controlled by the marginal meristems. Lateral veins continue to be formed until theend of apical growth. In plants with well-developed areolation, higher-order veins developboth simultaneously and progressively during intercalary growth, controlled by the intercalarymeristem. Vein endings within these areoles appear to differentiate progressively. In plantswith imperfect or poor areolation, all vein orders appear to differentiate progressively. Fromthe point of view of the terminology developed here, the division ofvenation into "progressive"and "simultaneous" may be better expressed as "ramifying" and "non-ramifying". The termramifying applies to any vein which can be related to a hierarchical system of branchingwithin the lamina, while non-ramifying applies to those veins which cross-link ramifyingveins or other non-ramifying veins.In several stimulating essays on plant form, Arber (1950) discussed leaf architecture. Sheadvanced the "partial-shoot theory of the leaf' which holds that the shoot is the basic unit "forthe fundamental interpretation of the plant body" and that the leaf itselfis a partial shoot. Thiscontrasts with the concept of root, stem, and leaf as separate units. She states

"... Thewholeplantmaybe said to consistof a seriesof shootgenerations, togetherwithaseries of root generations; every individual lateral branch is a repetition, modified invaryingdegrees,of theoriginalprimaryshoot,whileevery lateralroot similarlyrepeatstheprimary root" (Arber, 1950:77).And with reference to the leaf

"... thepartial-shoot hasan inherent urgetowards thedevelopment ofwhole-shoot characters"(Arber 1950: 78).With particular relevance to the problem of "lateral primaries", "palmate venation", and"acrodromous venation" sensu Hickey (1979), Arber writes of the

.".. tendencyforthemembersof eachgenerationtocompetewith,andattemptto dominate,those of the previousgeneration"(Arber, 1950: 93).This results in the phenomenon of "daughter-shoot dominating parent-shoot" and"ascendancy oflaterals in shoot and leaf'. Leaf examples given include Bauhinia yunnanensisFranch. and Passiflora capsularis L. var. acutiloba , bi-lobed leaves in which the terminalpinna or lobe are reduced to points. The leaf of Cercis siliquastrum L. apparently simple, issurmised to "consist of two lateral pinnae in a state of union, the median pinna being absent".

According to Arber,"... Palmatevenationmaybe envisagedas a type inwhichthemainveinis shortenedby thesuppression of itsearlier 'internodes'; the lateralsare thusmore importantrelatively to themidrib than in pinnate leaves, in which the midrib is elongated and dominates its ownbranches" (Arber, 1950: 102).3



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300 Journal of the Royal Society ofNew Zealand, Volume 21, 1991

... -. -.

Increasing dcvclopmcnt of the looping zone

Apicallaminadcvclopmcnt

-.

-+

Midlaminadevelopment

-.Basallaminadevelopment

I Development of externals-.

Development of externals and counter-externals

Fig. 1 - Schematic leaves showing the effects of differential development of various portions of thelamina.

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Pole - Modified leafarchitecture terminology 301Examples are given of Campanula rotundiflora L. and Gaya lyallii E.G. Baker, whichshow a range of palmate to pinnate venation even within a single plant. According toMeyerhoff (1952: 31), Troll (1935) also believed that palmate venation resulted from morerapid development of the basal one or two pairs of secondary veins. Arber (1950) also

pointed out, using TWa tomentosa Moench as an example, the effects of repetition of shootgenerations and symmetry on leaf venation. Because the generations of veins remainpermanently attached to one another and within the constraints of a two dimensional lamina,the production of successive generations of laterals (external laterals in the present terminology)takes place along the side of the parent vein remote from the midrib, the abmedial side. In thecase of the basal laterals, this is the side on which growth can develop freely. Cordate basesare the result of extended growth of vein generations rotating about the pivot of the midribbase."Cinnamomum" venation, i.e., two strong basal acrodromous veins extending for abouttwo-thirds of the length of the lamina with a few closely-spaced laterals near the apex, maybe an example of Arber's "ascendancy of laterals". Alternatively, it may be that the bulk ofleaf expansion during intercalary growth is centered in the basal region of the leaf. Thisphenomenon was discussed by Comer (1958) with respect to a brochidodromous-acrodromousgradation in Ficus. Total reduction of the apical laterals would lead to acrodromous veins,running all the way to the apex, as in various Myrtaceae and Melastomataceae. Eliminationof externals and of most growth outside the lateral veins by cessation of marginal growthimmediately after initiation of the lateral veins (Carr et al., 1986) would lead to an"intramarginal" vein of the Eucalyptus type.Carr et al. (1986) show that two fundamentally different venation patterns may co-existwithin the genus Eucalyptus, and sometimes even within a single species. Most juvenileleaves ofEucalyptus are brochidodromous, their lateral veins linking to form a "paramarginalvein", while adult leaves are acrodromous, their "intramarginal veins" ontogenetically distinctand forming before the finer veins along the midrib. They describe ways in which the twoforms could be distinguished in fully developed leaves. The brochidodromous type isdistinguished by

"... the diminishing thickness of the lateral veins in approaching the leaf margin, as well asthe gaps often found in the pararnarginal vein, where it has failed to be completed by thethickening of interconnecting minor veins" (Carr et al., 1986: 57).The acrodromous type is distinguished by the intramarginal veins being

.".. of uniform thickness and as thick or thicker than the primary lateral veins."They discuss the problem of what ordination to give the intramarginal veins, and conclude

that since they were" ... independent of the midrib at the base of the lamina ... (they are therefore) ... undoubtedlyprimary" (Carr et al., 1986: 59).Despite this they recommend the use of the non-ordinated term "intramarginal" to describethem. Non-ordinated terminology is attractive, but not very useful if used to circumventproblems of homology. In the presently proposed terminology, a single midrib is recognisedand the two intramarginal veins are described as first order laterals. I prefer not to use theterm "intramarginal vein" as I feel it leads to semantic problems when there is a significantamount of venation external to this vein. Instead, such veins will be termed "longitudinal"(see below).The terms "outer secondaries" as used by Romero and Dibbern (1985), and "secondary ortertiary branches" sensu Hickey (1979), refer to craspedodromous venation, mostly when thelaterals terminate in a tooth. However, there seems to be a link between a toothed margin andcraspedodromous venation, and between an entire margin and brochidodromous venation(van Steenis, 1953).Inthe leaves of several plants, normally dentate and with craspedodromous

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302 Journal of the Royal Society ofNew Zealand, Volume 21,1991

a -+ ....Development of a single pair of longitudinal veins by expansion of thebasal interlateral segment

b -+ -+

Development of several longitudinal veins by expansion of basal interlateraland basal loop segments

Fig. 2 - Possible development of leaves with single (a), or multiple (b), longitudinal veins from anoriginal camptodromous condition.secondaries and externals (eg Nothofagus ), occasionally there is no tooth for these veins toenter. The result is that the external vein loops back to its "parent" lateral and/or the lateralvein loops up to the next more apical lateral. This suggests that "outer secondary loops"(Hickey, 1979) and the loops in "festooned brochidodrornous" leaves are homologous withexternal veins. This condition is referred to here as "external veins looped".The classic work of D'Arcy Thompson (1952) on growth and form has particular relevanceto leaf architecture. He proposed that

"... The rate of growth deserves to be studied as a necessary preliminary to the theoreticalstudy of form, and the organic form itself is found, mathematically speaking, to be afunction of time ... We might call the form of an organism an event in space-time, and notmerely a configuration in space" (D' Arcy Thompson, 1952: 283).

Figs 1 and 2 illustrate some of the changes in basic leaf architecture resulting fromdifferent directions and amounts of laminar and marginal growth, either ontogenetically orphylogenetically, and the effect these may have on the venation and form of a leaf. I stressthat these sequences are hypothetical. The terms used are explained below.A REVISED VENAnON TERMINOLOGY

Unless redefined below, terminology follows Hickey (1973, 1979).A simple set of working assumptions may be stated to distinguish growth phases in a leaf,and to incorporate these distinctions in a terminology:1. Progressive growth is indicated by decreasing thickness along a vein and/or by freeends.2. Simultaneous development is indicated by veins of equal width along their lengthwhich terminated against a lower order vein at each end.

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Second Orde rExternal LoopThird Orde rExternal LoopLateralInternal LoopBasiscoplcInternal LoopFirst OrderExternal Loop

a

Pole - Modified leaf architecture terminology 303

Firs t Order LateralSecond Order LateralTh ird Order Latera lCounter-external

\ c ( Joining Vein;;:s:: Firs t Order Externale:f Interangular Vein

' Second Order Exte rna l

Basic Venation Elements

Looping Zone ItIShaded!

Lateral Lone(Un.hadtdl

Interloop Segment

Iu terlateral Segment

Joining Vein Segment

'-Y =r - Interangular Segment- - - ": Interexternal Segment

b Lamina Zones and SegmentsFig. 3 - Schemat ic leaves to illustrate terminology of (a) basic venation clements and (b) laminar zonesand segments.

3. Thic ker veins are ont ogenet ically old er than thinn er ones in the immediateneighbourhood."Ramified" tertiary veins sensu Hickey (1979) are considered to be a result of progressivegrowth, and "percurrent" tertiaries to be produced by simultaneous growth. There will nodoubt be subjectivity in their recognition. Veins often show some taper, but. for reasons oftheir general location and behaviour, one might nevertheless suspect that they developedsimultaneously. For instance, external veins or second or third order laterals in some leaves

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304 Journal of the Royal Society ofNew Zealand, Volume 21,1991may appear to intergrade with non-ramifying veins. Nevertheless, relative vein size shouldprovide a simple way of estimating the pattern of growth over a leaf lamina. It should beemphasised that this procedure is an estimation with the purpose of providing a conceptualtool for describing leaf venation. The actual details of how venation develops must be studiedon growing plants.HIERARCHY IN LEAF ARCHITECTURE

For ease of comprehension, this revised terminology of leaf venation is presented in ahierarchical fashion.A. Elements

The veins themselves are termed "elements" and, depending on their behaviour and size,can be divided into ramifying or non-ramifying; first, second, third order elements; etc. Nonramifying elements are most conveniently discussed as being portions of patterns (Figs 3 and7).B. Patterns

The way in which venation elements interact with each other provides "patterns". Dependingon the order of the elements concerned, and their location, these patterns can be divided intofirst, second, third order patterns, etc (Figs 6 and 7).C. Zones and Segments

These both refer to areas of the lamina which are defined by the course of elements andfilled in with patterns. "Zone" is the most general category, and it may be divided into"segments" (Fig. 3).BASIC VENATION ELEMENTS (see Fig. 3)Midrib

Only one primary vein exists per leaf, for which the term "midrib" is used. The midrib isthe central vein of a leaf, acting more or less as an axis of symmetry, and it is the main pointof reference for the following vein terminology. The midrib need not be the strongest vein inthe leaf.Laterals

Any ramifying veins developed along the midrib, and when applicable, the largest pair atthe midrib base, are referred to as "laterals". The largest of these ("secondaries" sensuHickey, 1979) are here termed "first order laterals".Higher order laterals, i.e. second or third order laterals (here termed "extra-laterals"), referto Hickey's "intersecondary veins". Higher order laterals can usually be recognised by theirbehaviour as well as by their relative thickness. Often they do not extend across the fulllength of the lateral segment, or else join the lateral loops with little effect on them, whereasfirst order laterals join at a sinus, or kink. At other times they will be recognised because theyinterrupt the normal style of joining veins in a percurrent pattern. Collectively, higher orderlateral veins are termed "extra-laterals".Sometimes a vein which fits in with a pattern of "interangular veins" (see below) willprogressively thicken as it nears the midrib. It may be difficult to distinguish depending onhow pronounced is the thickening. The two categories may well intergrade.

Depending on the growth pattern along the midrib the spacing between first order lateralsmay decrease apically, increase apically, decrease both apically and basally from midlamina, or be relatively constant.The pattern of higher-order laterals, which form subsequent to the first order laterals,depends on how the lateral segment expands (Fig. 6c). New higher-order laterals may be

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lateral Loop formed bythe junction of FirstOrder Lateral \YUhSecond Order External

1.=first Order External Loop2. =Second Order External Loop3. = Third Order External Loop

f. O. E.=First Order.ExternalS.O. E.. SKOnd Orde r r m . T. O.E.. Third Ordf'r E f ' m

Fig.4 - Annotated venation of the extantMuehlenbeckia australis.


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306 Journal of the Royal Society ofNew Zealand, Volume 21,1991

Hypho drornous I'arallclodromous Craspcdoclroruous M.ixcdFig. 5 - Schematic leaves illustrating four kinds of first order venation patterns.

LoopsLoops refer to closed circuits of ramifying venation (Figs. 3a, 4). As mentioned previously,they are often composite structures formed by the linkage of several elements. The thickestvein in the loop is the "base" from which the "origins", the next thickest veins, depart. Theopposite end of the loop from the base is the "arch".

a) Lateral loopsThese are loops formed by first order lateral veins (the origins) growing to join an adjacentfirst order lateral or an external branch of it. The midrib forms the base, and the arch mayinclude first order lateral, first order external, second order external, and possibly higher

order elements.b) External loops (Figs. 3 and 4)

1. First-order external loops. These loops have first order externals as their origins andare based on first order laterals. The arch is frequently a second order external.2. Second-order external loops. These have second order externals as their origins andare based on a first order external. The arch is frequently a third order external.3. Third-order external loops. These have third order externals as their origins and arebased on a second order external.c) 1nternalloops

These loops are found within the interlateral segments (cf, Fig. 3b) defined by the lateralloops.1. Lateral internal loops. These are similar to lateral loops but are formed of higher orderlateral veins, generally second order laterals.2. Basiscopic internal loops. These are formed by the linking up of external veins,exactly the same as external loops, but within the lateral loop. Included are second orderlateral veins which loop up to an external.Loops may not show any particular distortion or may be elongated widthwise (perpendicularto the midrib), or be elongated lengthwise (parallel to the midrib).FIRST ORDER VENATION PATTERNS

The "traditional" patterns of ramifying venation are somewhat arbitrary with little taxonomicsignificance, and intergradation is common as the distinctions may be due merely to varyingamounts of meristematic growth. The recognition of a single primary vein, or midrib, in thepresent terminology requires the redefinition of some of the basic types of venation, as givenin Hickey (1979) where one or more primaries are recognised. These "basic" venation10



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a

l

ExternodromousEucamptodromous(l Inkrd by rnl. rgrd joiningveins)

l

EucamptodromousIUnkrd by j o i n l n ~ . rin51

5

E

Brochidodromous

b

Joining veins fromdevel. of N.E.quadrant.

Cascade venationfrom devel, ofS.W. quadrant.Development ofall quadrants.

Development ofS.E. and S.W.quadrants.

cl

.....

.....

Development ofsymmetricallyplacedextra-lateralsresulting in the"symmetrical"pattern

Development ofbasally initiatedextra-lateralsresulting in the"axial" pattern

Fig. 6 - (a) types of the camptodromous first order venation pattern, (b) development of four differenttypes of second order venation as a result of differential laminar growth (based in stippled areas) froma basic "quartered" interlateral segment (modified from Comer, (958), (c) symmetrical growth withinan interlateral segmen t resulting in the "symmetrical" pattern of extra-laterals (top) and growth in theaxils of first order laterals result ing in the "ax ial" pattern of extra-laterals (bottom).

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308 Journal of the Royal Society ofNew Zealand, Volume 21, J991patterns, formed by the midrib and the first order laterals, are the first order patterns of a leaf.Seven first order venation patterns are recognised.a) Hyphodromous venation

Used in the sense of Hickey (1979) where all veins but the midrib are absent, rudimentary,or concealed within a coriaceous or fleshy mesophyll (Fig. 5).b) Parallelodromous venation

Several veins originating beside each other at the leaf base and running parallel to the apexwhere they converge, with no obvious decrease in thickness along their length, and no strongconnecting or joining veins (Fig. 5). [It should be noted that Hickey's illustration (1973, fig.45) of parallelodromous venation is not consistent with the definition given in his text. In hisillustration the veins do not originate beside each other at the leaf base.]c) Craspedodromous venation

All first order lateral veins terminate at the margin (Fig. 5). Similar to Hickey (1979).d) Mixed venation

Some first order lateral veins terminate at the margin and some otherwise (Fig. 5).e) Camptodromous venation

First-order lateral veins not terminating at the margin. Three basic conditions exist here(Fig. 6a), but in practice there may be occasions where it is difficult or impossible todistinguish between them.1. Brochidodromous venation

This term is reserved for that condition where adjacent first order laterals directly join oneanother.2. Externodromous venation

This is a new term for the condition where a first order lateral joins only to an externalbranch of an adjacent first order lateral (see also Fig. 4).3. Eucamptodromous venation

This is as used by Di1cher(1974) and Hickey (1979) where first order laterals are upturnedand gradually diminish apically with no direct lateral-lateral contact. They are linked toadjacent laterals by joining veins, and although the laterals diminish to the thickness ofjoining veins, there is no break in the pattern of the joining veins. Sometimes a particularjoining vein may be thicker than the rest, forming a "pseudo-lateral loop". It is possible thatsuch veins are externals ontogenetically.Hickey's "cladodromous" and "reticulodromous" are rejected. Hickey (1973: 24) appliedthese terms respectively to laterals "freely ramified toward the margin", and laterals "losingtheir identity toward the leaf margin by repeated branching into a vein reticulum". I believethat closer observation would indicate an externodromous pattern.SECOND ORDER VENATION PATTERNS

Second order venation pattern refers to the most basic pattern formed in the regionbetween first order lateral veins, externals, or loops (Figs 6 and 7). The word "pattern" asused here refers to a group of veins showing some coherence in their order. Groups ofthinner/higher order veins may be found within an area defined byone pattern, but not be partof that pattern. They could be "noise" or separate patterns of a higher order.Astute observations on the development of the lateral segment venation of Ficus havebeen published by Corner (1958). He noted that simplified secondary segment venation12



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Pole - Modified leafarchitecture terminology 309

-- RamifyingLateral Vein

Lattice Percurrcnt

Joining Vein

Transverse

Second Order Patterns Third Order PatternsFig. 7 - Diagrams to illustrate common second and third order venation patterns.could be found in the unexpanded segments towards the apex of a leaf. In effect, anontogenetic sequence is preserved from the apex to the widest part of the lamina. Using thisas a guideline, Corner showed how several apparently different venation patterns in Ficuswere related by the initial pattern of subdivision of the segment. The divergent venationpatterns could be produced by differential expansion of different parts of the segment (Fig.6b). The principle corresponds to that of the similarity of juvenile forms of related taxaknown in the field of zoology.Percurrent venationThis refers to non-ramifying venation which runs directly between two veins. It may besimple or it may branch, but the term percurrent does not apply to relatively continuousnetworks of veins which run between two veins and are also interconnected with veins ofsimilar size. In a leaf showing percurrent venation, two kinds of percurrent veins (Fig. 3) maybe defined. These were named by Meyerhoff (1952), who grouped them as "tertiaries":1. Joining veins

Non-ramifying veins which join two adjacent lateral veins (abbreviated to j.v.).2. lnterangular veins

Non-ramifying veins which join lateral veins with the midrib (abbreviated to i.a.v.). Therelative development of these veins within a lateral segment depends on the amounts ofgrowth along the midrib between two laterals, and across the lamina between the midrib andleaf margin or lateral loop.Cascade venation

Corner (1958) recognised a special case where interangularveins are not directly percurrent,but are staggered so that the end of a vein originating at the midrib terminates along a veinoriginating on a lateral, and vice-versa. Growth producing this pattern is initiated in the anglebetween midrib and laterals. It can also be recognised in loops.Ramifying venationThe segments between the veins forming the first order patterns are occupied by aramifying vein network. A ramifying set is anchored to the first order pattern at some pointand then generally branches in a transverse, admedial, or exmedial direction (equivalent toHickey's (1979) ramified veins).

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310 Journal of the Royal Society ofNew Zealand, Volume 21,1991T HI RD O R DE R VENATION PATTERNS

Third order refers to repeating patterns of venation which fill the segments defined byveins of the second order pattern (Fig. 7), for instance a pattern which is consistently foundbetween two percurrent veins.A large number o f third order patterns may be recognised, but a comprehensive treatmento f them and their interelationships will require comprehensive study o f recent plants and isbeyond the scope o f this paper. The three most obvious are as follows:

Lattice venationFound as segment "fill", often between percurrent veins, but also found between firstorder laterals. It is formed by a zigzag vein connecting two rows o f offset veins projectingfrom the two veins defining the segment. In this sense it is similar to the construction of acascade vein, but there is no marked trend in the size of its constituent veins in any particulardirection, and it is not oriented towards the axil between two major veins.

Percurrent venationSimilar to a percurrent second order pattern, except that the veins run between adjacentjoining veins.Transverse venation

The veins have a somewhat ramified appearance but are aligned parallel to enclosingjoining veins.F I M B R I A L VEIN

Hickey (1973, 1979) and Dilcher (1974) proposed "fimbrial vein" for "higher vein ordersfused into a vein running just inside o f the margin". I retain this term but suggest that it mayrepresent an independent entity rather than fusion of higher vein orders.BASIC L AM IN A Z ON ES AND SEGMENTS (see Fig. 3b)1. Lateral zone

The area of lamina between the lateral loops and the midrib. In a craspedodromous leaf,where lateral loops are absent, the whole lamina forms the lateral zone.a) Interlateral segments

The area o f lamina bounded by two lateral veins, the midrib, and either the margin or thelateral loop.b) Joining vein segments

The area of lamina bounded by two joining veins and two lateral veins.c) Interangular (vein] segments

The area o f lamina bounded by two interangular veins, the midrib, and a lateral vein.d) Interexternal {vein] segments

The area o f lamina bounded by at least two external veins and the margin.2. Looping zone

This refers to the area o f lamina between the lateral loops and the margin. It contains the"external loops". It may be nearly absent on some leaves, or so developed in others that itmakes up almost half the width of the lamina.

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Pole - Modified leafarchitecture terminology 311Inter/oop segmentsThe area of lamina bounded on all sides by a loop.

LAMINA DEVELOPMENT1. Basic developmentThis proceeds as a smooth gradation of development (expansion) over the lamina.2. Actinodromous developmentThis is a condition of craspedodromous venation where the basal laterals are stronglydeveloped often with concurrent development on them of external veins and counter-externalveins, sometimes forming lobes in the lamina, and/or a cordate base (Fig. 1).

3. Acrodromous developmentThis refers to disproportional development of the basal, or suprabasal, interlateral segments.

It is normally a condition of camptodromous venation (Fig. 2).SUMMARYI consider that the currently-accepted leaf architecture terminology of Hickey (1973,1979) and Dilcher (1974) needs modification, for several reasons, including: (1) ambiguousranking of low order veins; (2) confusion of vein relationships with lamina development; (3)non-resolution of important elements of architecture, such as "loops"; and (4) low levelimportance of patterns.The terminology proposed here: (1) utilises the difference between veins resulting from"ramifying" growth and "non-ramifying" growth; (2) recognises only one primary vein, themidrib, per lamina; (3) recognises the difference between vein relationships and the effects of

differential lamina development (expansion); (4) recognises that venation "loops" of earlierterminologies are often complex structures of several venation elements, and introduces anew term, "externodromous", for this condition; (5) recognises the equivalence of "external"veins ("outer secondaries" and "pectinal" veins of earlier terminologies) with the elements of"loops"; (6) introduces a number of terms referring to areas of the lamina, for instance the"looping zone", which are helpful in describing the difference between architectural patterns;(7) recognises three distinct orders of venation patterns in an attempt to produce a terminologywhich expresses more closely the ontogenetic development of the leaf; and (8) emphasisesthe importance of patterns in recognising venation elements and growth directions in thelamina.ACKNOWLEDGEMENTSI would like to thank my supervisor, J.D. Campbell, and also R.S. Hill, J.D. Lovis andM.M. Dettmann, and two anonymous referees, who reviewed the manuscript at variousstages. This work was financed by a UGC scholarship and University ofOtago BridgingFinance.REFERENCESArber, A., 1950. The natural philosophy of plant form. Cambridge University Press. Cambridge.Carr, DJ., Carr, S.G.M. and Lenz, J.R., 1986. Leaf venation in Eucalyptus and other genera ofMyrtaceae: implications for systems of classification of venation. Australian Journal ofBotany 34:53-62.Comer, E.J.H., 1958. Transference of function. Botanical Journal of the Linnean Society 56: 33-40.Dilcher, D.L., 1974. Approaches to the identification of angiosperm leaf remains. The BotanicalReview 40: 1-85.Esau, K., 1965. Plant Anatomy, second edition. John Wiley and Sons Inc., New York.Hickey, L.J., 1973. Classification of the architecture of dicotyledonous leaves. American Journal ofBotany 60: 17-33.

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312 Journal of the Royal Society ofNew Zealand, Volume 21,19911979. A revised classification of the architecture of dicotyledonous leaves. In: Metcalfe, e.R.and L. Chalk (eds.) Anatomy ofthe Dicotyledons. Vol. I. second edition, pp. 25-39. Clarendon Press,Oxford.Hickey, L.l. and Wolfe, l.A., 1975. The bases of Angiosperm phylogeny: vegetative morphology.Annals of the Missouri Botanical Garden 62: 538-589.

Melville, R., 1976. The terminology of leaf architecture. Taxon 25: 549-561.Meyerhoff, A.A., 1952. A study of leaf venation in the Betulaceae, with its application to paleobotany.Unpublished PhD thesis, Stanford University.Pray, T.R., 1955. Foliar venation of angiosperms, II. Histogenesis of the venation of Liriodendron.American Journal ofBotany 42: 18-27.--- 1963. Origin of vein endings in angiosperm leaves. Phytomorphology 13: 60-81.Romero, E.l. and Dibbern, M.e., 1985. A review of the species described asFagus and Nothofagus byDusen. PalaeontographicaAbt B 197: 123-138.Slade, B.F., 1957. Leaf development in relation to venation as shown inCercis siliquastrum L., Prunusserrulata Lind!., and Acer pseudoplatanus L. New Phytologist 56: 28 -300.--- 1959. The mode of origin of vein-endings in the leaf of Liriodendron tulipifera L. NewPhytologist 58: 299-305.Spicer, R.A., 1986. Pectinal veins: a new concept in terminology for the description of dicotyledonousleaf venation patterns. Botanical Journal of the Linnean Society 93: 379-388.Steenis, C.G.G.J. van, 1953. Results of the Archbold expeditions. Papuan Nothofagus . Journal of theArnold Arboretum 34: 301-373.Thompson, D'Arcy, 1952.On growth and form, second edition. Cambridge University Press. Cambridge.Troll, W., 1935. Vergleichende Morphologie der hoheren Pflanzen. Gebriider Borntaeger, Berlin.

Received 31 August 1990; accepted 10 May 1991



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pole 1991 modified terminology for angiosperm leaf architecture

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