Botanical Journal of the Linnean Sociely (1986), 93: 379-388. With 5 figures
Pectinal veins: a new concept in
terminology for the description of
dicotyledonous leaf venation patterns
ROBERT A. SPICER
Departmen/ of LiJe Sciences, University of London, Goldsmith's College,
Rachel McMillan Building, Creek Road, London SE8 3BU
Received September 1985, accepted f o r publication December 1985
SPICER, R. A., 1986. Pectiual veins: a new concept in terminology for the description of
dicotyledonous leaf venation patterns. Mid-Cretaceous dicotyledonous leaves frequently
exhibit continuous variation in venation patterns. This poses considerable problems when concise
description and taxonomic partitioning of leaf material is attempted. Often it is extremely difficult
to differentiate consistently between pinnate and palmate organization. A new terminology is
proposed that dispenses with the need to differentiate between pinnate and palmate organization or
between primary and secondary vein orders which commonly intergrade. Emphasis is placed upon
the pattern of vein courses and branching hierarchy which provides unambiguous reference points
for architectural descriptions and analyses.
- fossil leaves - leaf architecture.
ADDITIONAL KEY WORDS:-Cretaceous
CONTENTS
Introduction . .
The new terminology
Examples.
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References.
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382
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387
INTRODUCTION
Palaeobotanists have long been interested in the taxonomic potential of leaf
remains and have sought to describe them in concise and unambiguous
language. Ettinghausen (1861) was the first to develop a comprehensive
terminology for leaf architecture which was utilized in part by a number of
workers, including Lesquereux ( 1892), Berry ( 1916) and Hollick ( 1930, 1936).
Others, sometimes with palaeobotanical interests, have subsequently proposed a
variety of schemes describing leaf characters in taxonomic or environmental
research: Dilcher (1974), Dolph (1975, 1976a, b), Ferguson (1971), Hickey
( 1973, 1979), Hill (1980), Lam (1925), Lee (1948), Madler (1975), Melville
( 1976), Mouton (1966, 1970).
Most of the proposed schemes of leaf character analysis were intended as aids
to written descriptions (e.g. Hickey, 1973, 1979), but a few authors have either
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379
01986 T h e Linnean Society of London
380
R. A. SPICER
modified the descriptive terminologies for numerical analyses (Dolph, 1976a, b)
or have devised original schemes for use with computer techniques (Hill, 1980).
Perhaps one of the more widely accepted descriptive terminologies is that of
Hickey (1973) which was subsequently republished with the addition of
cuticular characters by Dilcher (1974). Hickey (1979) has since expanded his
original version, but has not substantially altered the terminology. Hickey
defined leaf architecture as “the aspect of morphology which applies to the
spatial configuration and co-ordination of those elements making up part of
a plant without regard to histology, origin or homology” (Hickey, 1973:
17). As Hickey points out, the relationships between veins are not based on
function or ontogeny but purely on geometry.
Classification seeks to divide populations into groups, and segregation of
individuals into groups or classes is easy where discontinuities in trends naturally
occur. Unfortunately, natural discontinuities in the range of variation seen in
leaf architecture do not always occur in populations of fossils. This gradation of
form is particularly marked in Cretaceous leaves.
My own work on Alaskan Cenomanian dicotyledonous leaf forms (Spicer, in
press) and reports of others (e.g. Doyle & Hickey, 1976; Dilcher, 1979) suggest
that the rapid mid-Cretaceous angiosperm radiation involved mosaic evolution
of both vegetative and reproductive organs. Consequently, Cretaceous
angiosperms exhibit character combinations that today are found straddling the
boundaries normally recognized for extant families and even orders.
Furthermore, Upper Cretaceous angiosperm leaves display a high degree of
morphological plasticity. These two factors have contributed greatly to the
confused state of Cretaceous angiosperm systematics, but an additional problem
has been the inconsistent descriptions and often inadequate illustrations of the
leaf forms. The continuous nature of the architectural variation exhibited by
these leaves, and the constraints imposed by restrictive terminologies, have
frequently forced similar leaves to be assigned to markedly different taxa. Subtle
relationships, which are necessary for detailed evolutionary studies, may
therefore have been masked.
Hickey (1973, 1979) was one of the few people to carry out extensive surveys
of both extant and fossil leaves in the course of developing his terminology.
Nevertheless his scheme is somewhat limited when it is necessary to describe
Cretaceous forms in a way that does not obscure complex intergradation of
venation patterns. In his classification of the architecture of dicotyledonous
leaves, a major distinction is made between pinnately veined leaves and those
with actinodromous (or other palmate) venation. Pinnate venation is described
as consisting of a single primary vein (midvein) serving as the origin of the
higher-order venation (Hickey, 1973: 23), while actinodromous leaves have
“three or more primary veins diverging radially from a single point” (Hickey,
1973: 24). Superficially it would seem that clear distinctions can be drawn
between the two forms. However, as with most venation classification, the
recognition of vein orders (in this case primary veins) is essential for correct
analysis. In Hickey’s scheme “veins of the primary order are the thickest veins of
the leaf and occur singly or as a medial vein accompanied by others (lateral
primaries) of roughly equal thickness” (Hickey, 1973: 25). The recognition of
branches as primary veins is based on the criterion that the branch be roughly
equal in thickness to its primary source when both are measured just above the
LEAF ARCHITECTURE
38 1
point of branching. The secondaries are those branches which are of “markedly
smaller size than their primary source” (Hickey, 1973: 25).
The terms “roughly” and “markedly smaller” reflect the fact that the relative
vein thicknesses are seen to be very variable. For example, in Cretaceous fossil
leaves, which possess characters found in extant members of the
Hamamelididae, there is no natural grouping of veins based on thickness.
Therefore, it may be impossible to make consistent, and thereby meaningful,
decisions regarding vein order in many leaves, even at such a fundamental level
as recognizing primary or secondary veins. The problem is compounded in fossil
specimens where relative vein thicknesses may be distorted by preservation.
In the course of examining a large number of hamamelid and hamamelid-like
leaf forms, particularly Cretaceous specimens, it became apparent that
considerable variation exists in the relative thickness of veins branching from the
midvein. In many leaves one or more pairs of laterals near or at the base of the
lamina are clearly as thick as the midvein and the leaf is unmistakably
palmately veined (Fig. IA). If all the laterals depart from the same point the
venation is said to be actinodromous (Hickey, 1973). In some leaves these
laterals form the median veins of lobes, while in others lobing is absent. In all
cases these ‘lateral primaries’ branch a number of times on their abmedial side
(the side furthest from the midvein) to give off laterals which sometimes branch
and terminate in marginal teeth (craspedodromous), or form loops by joining
Figure I . Diagrams of stylized leaf venation patterns. Right side brochidodromous, left side
craspedodrornous, for each diagram. A. Strong pectinal veins-actinodromous venation. B. Weak
pectinal veins actinodrornous venation. C. Palinactinodromous venation. D. Palmate venation
with several radiating pairs of pectinal veins. E. Acrodromous venation-inwardly curving pectinal
veins.
382
R. A. SPICER
the superadjacent vein (brochidodromous). Where the abmedials are
craspedodromous they usually branch in a similar manner to that of identifiable
secondary veins which arise as the more apical laterals from the midvein.
However, in many cases the abmedial branches (or outer secondaries) of the
'lateral primaries' are brochidodromous in contrast to the craspedodromous
secondary veins. In these situations it could not be said that the so-called 'outer
secondaries' branch in a similar way to the main secondaries. Irrespective of
vein branching, the 'outer secondaries' are never thicker than the vein from
which they originate and, except near the leaf margin, are thicker than thirdorder veins.
In many hamamelid and hamamelid-like forms the lateral veins which give
rise to these abmedial branches are markedly thinner than the midvein, just
above the point of departure, and form part of a range of vein thicknesses
associated with the main secondaries which lie between them and the leaf apex.
Therefore, they should be considered secondaries rather than lateral primaries.
However, in all other respects they resemble lateral primaries in that they
branch abmedially more frequently than superadjacent true secondaries; have
either craspedodromous or brochidodromous (sometimes both) abmedial
branches; and terminate in the apices of lobes where lobing is present.
In some leaves, particularly the more recent hamamelid forms, the
prominence of these abmedially branching laterals may be markedly reduced to
the extent that they are considerably less developed than the more apical true
secondaries (Fig. IB). These weakly developed veins could hardly be called
primaries, but nevertheless they support numerous abmedial 'outer secondaries'.
There is no clear break in the continuum of venation to enable a distinction to
be made between palmate and pinnate venation. Therefore it is impossible,
using Hickey's classification, to describe in a reliable and consistent manner the
range of architecture found in hamamelid forms. A small variation in relative
vein thickness could easily result in the need to describe a leaf in fundamentally
different terms and the venation of one leaf might be described as pinnate
whereas an almost identical form would have to be considered palmate.
THE NEW TERMINOLOGY
If the concept of vein order, and in particular of vein thickness, is removed
from the description of the lateral veins which subtend the numerous abmedial
branches, the problem outlined above can be overcome. These lateral veins
differ from the main secondaries in that they produce abmedial veins almost
throughout their length and a greater number of abmedial branches than any
other laterals. The abmedial veins arising from the laterals never join a more
basal secondary directly, although may join each other by arching. Because of
these conspicuous features they serve as an important reference point in the
description of leaf venation. Such veins, which I shall designate PECTINAL VEINS
from the Latin pecten-a comb-may be of any order, although they will
normally be either primary or secondary. They differ from all other veins in
sub tending abmedially a distinct series of more or less parallel branches as thick
as, or thinner than, themselves, in a similar manner to teeth of a comb. Pectinal
veins may be basal or suprabasal in origin and bear admedial branches in
addition to the abmedials. A leaf may still be described as essentially pinnately
LEAF ARCHITECTURE
383
or palmately veined if the venation pattern lies towards one end or other of the
range of variation. Leaves with intermediate venation need only be described in
terms of the overall observable characters and whether the pectinal veins are
well or poorly developed in terms of the area of lamina that they or their
branches serve.
Once recognized, pectinal veins may be used as a point of reference. The
secondary veins originating from the midvein above (nearer the apex) the
origins of the pectinal veins can be termed, for the sake of brevity in written
descriptions, ‘superior’ secondaries. Any secondaries originating basal to the
departure point of the pectinals can be referred to as ‘inferior’ secondary veins.
Often the behaviour and strength of the two sets of secondaries are markedly
different.
Because pectinal veins support more abmedial branches than any other
subsidiary of the midvein there can only be one pair of pectinals per leaf.
However, in some palmately veined leaves several ranks of pectinal veins may be
present. Leaves with palinactinodromous venation are described by Hickey
(1973: 24) as possessing “primaries having one or more subsidiary points of
radiation above the lowest point”. Again, the criterion for the recognition of
primary veins is one of vein thickness and, as with actinodromous venation,
serious problems arise. The palinactinodromous condition may be thought of as
a variation of the actinodromous in which one of the pairs of the pectinal
abmedial veins (usually the most basal) is (in terms of area of lamina supplied
by it or its subsidiaries) strengthened, accompanied by an increase in the
number of abmedial veins it supports (Fig. IC). This vein then also behaves as a
pectinal vein and must be distinguished from its ‘parent’ pectinal. This is done
by referring to the parent pectinal as an a-pectinal and the daughter pectinal as
a P-pectinal. Should additional pectinal veins be present, for example when an
abmedial of the P-pectinal is strengthened and branched abmedially, they may
be referred to by the appropriate Greek letter. U p to 24 ranks of pectinal veins
( a number which is unlikely to be exceeded) may be referred to in this way. As
in the actinodromous venation described above, such pectinals may be strongly
or weakly developed.
More than a single pair of pectinal veins may depart from the midvein in a
palmately veined leaf. Such a condition arises when the P- (or other) pectinal
departs not from the a-pectinal but from the junction of the a-pectinal with the
midvein (Fig. 1D). The pectinal vein with the smallest angle between its
admedial side and the midvein is then designated the a-pectinal, and subsequent
lettering proceeds basally. The point of origin of the pectinals, whether it be on
a higher-rank pectinal or the midvein, should be specified (see examples) as
subtle changes may have evolutionary significance.
Inward curving of strong pectinal veins (Fig. 1E) geometrically produces the
acrodromous (sensu Hickey, 1973, 1979) condition. The vein orders of
acrodromously veined leaves are often difficult to differentiate, particularly with
respect to those veins running between the midvein and the inwardly curving
pectinals. The veins departing from the midvein at, or apical to, the fusion of
the pectinals with the midvein (or branches from it) are easily recognized as
superior secondaries. However, those enclosed by the pectinals often behave as
tertiary veins. This is particularly visible in leaves of the extant Cercidiphyllum
juponicum S. & Z. (Fig. 2). In fact, some of these enclosed veins have all the
R.A. SPICER
384
Superior secondary vein
Enclosed superior
a-pectinol obrnediol
/
'p-pectinol
abmediol
Figure 2. Line drawing of a leaf of extant Cercidiphyllum japonzcum S. & Z. illustrating enclosed
superior secondary veins and brochidodromous pectinal abmedial veins.
characteristics of intersecondary veins which, although most commonly seen in
leaves of members of the Magnoliidae, are present in hamamelids, including the
so-called 'platanoid' forms. However, these veins may be thought of as modified
superior secondary veins and distinguished from the others by being referred to
as 'enclosed superior secondary veins'. Where enclosure is not complete this can
be described. Often the distinction between the enclosed and normal superior
secondaries is gradational.
The brochidodromous pectinal abmedials seen in C. japonicum are a common
feature of acrodromously veined leaves in general. Similarly, the basal abmedial
frequently supports brochidodromous abmedials and geometrically is the
equivalent of the b-pectinal vein of actinodromous and palinactinodromous
leaves (Fig. 2).
Examples
Three leaf forms from the Upper Cretaceous of Alaska are presented which
illustrate the use of the new terminology.
The first leaf form (Fig. 3) is similar in many respects to Pterospermites whitei
Ward. It is characterized by having somewhat weak but readily identifiable apectinal veins, serving about half the lamina area.
Description: Leaf: simple; asymmetrical; wide elliptical; apex missing, but
probably obtuse; base auriculate or truncate; margin irregularly dentate, teeth
with acute apices, concave sides and wide, shallow rounded sinuses; venation
imperfect marginal suprabasal acrodromous; midvein more or less straight,
moderate; pectinal veins weak, angle of divergence from the midvein almost 90"
on one side and 65" on the other, both curving towards the apex but more so on
the side of greater angle of divergence, and meeting the margin less than
halfway between base and apex; pectinal abmedials depart at varying angles
but mostly approximately 75", simple or forked; /I-pectinal veins weakly
developed but more strongly developed on the side of the lamina supporting the
LEAF ARCHITECTURE
385
Figure 3. Line drawing of a Cretaceous (Coniacian) leaf from the North Slope of Alaska illustrating
weak but readily identifiable pectinal veins.
greater a-pectinal angle, departing 61-pectinal at 70" and 50", irregularly
apically curved, craspedodromous, P-abmedials departing at 50", basally to 70"
near margin, craspedodromous; y-pectinal only developed on side of lamina
with greatest a-pectinal angle, departing P-pectinal at 60" slightly curved,
craspedodromous; superior secondary veins diverging from midvein at varying
angles from 55" to 70" on one side and approximately 50" on the other, slightly
curved towards the apex, often branched abmedially near the margin,
branching becoming less near the apex; inferior secondary veins weak, recurved;
tertiary veins percurrent, straight to convex, often branched with a tendency
towards random reticulation in places, the angle formed with both ad- and
abmedial sides of the secondaries usually acute or right-angles, but sometimes
obtuse on the admedial side; fourth-order veins orthogonal percurrent, simple or
forked.
The second example (Fig. 4)is of a form akin to Crednaria longfolia Hollick. Here
the pectinal veins are less well-developed and the leaf has a more pinnately
veined appearance.
Figure 4. Line drawing of a Cretaceous (Cenornanian) leaf from west central Alaska illustrating
weak pectinal veins and an overall pinnate organization.
386
R. A. SPICER
Description: Leaf: simple; symmetrical (?) ; elliptic or narrow obovate; apex
missing; base missing; margin serrate-dentate; teeth with angular obtuse apices,
concave sides, shallow rounded sinuses; midvein moderate, straight to weakly
zig-zag; pectinal veins weaker than midvein, suprabasal departing midvein at
40-50°, initially curved then straight or slightly recurved, craspedodromous,
dichotomously forking near margin; abmedial veins departing at 70-40"
becoming more acute apically, moderate, curved, sometimes recurved near
margin, apical abmedials craspedodromous, basal abmedials branching,
looping, semicraspedodromous; superior secondaries moderate, slightly curved
or straight, departing midvein at 30-40°, more basal veins once or twice near
margin; inferior secondaries weak, straight, departing midvein at approximately
45", branching near margin to form festooned brochidodromous loops, arching
of loops angular and abrupt; tertiary veins moderate, percurrent, simple or
forked; usually convex, joining ad- and abmedial sides of secondaries at acute or
right-angles; fourth order veins indistinct.
The third example (Fig. 5) is of a leaf referred to Menispermites seplentrionalis
Hollick. Leaves of this form have a series of major veins radiating from a single
point near the base of the lamina. Geometrically this condition can be achieved
in two ways. The first is by condensation of the basal part of the midvein with
the result that superior secondary veins and pectinal veins depart from the same
point. The second way is by having a, /3 and subsequent pectinal veins radiating
from the same point. Examination of Cretaceous leaves reveals no evidence that
condensation of the midvein has occurred; no intermediate forms with reduced
spacing between secondaries have been observed. However, the points of
departure of subsidiary pectinal veins from parent pectinals, while more or less
consistent within a given leaf form, are variable between forms. This strongly
suggests that the radiating major veins of Menispermites and similar leaves are
comparable to the pectinal vein system of other forms. The following description
is based on several specimens.
E;-
pectinat
Figure 5. Line drawing of a Cretaceous (Cenomanian) leaf from west central Alaska referred to
Menispermites septmtrionalis Hollick. This leaf possesses four pairs of pectinal veins radiating from a
single point.
LEAF ARCHITECTURE
387
Description: Leaf: simple; symmetrical; orbiculate; apex missing; base lobate;
margin dentate, teeth with rounded glandular apices, glands projecting beyond
the apex, sides convex or concave, sinuses wide, shallow and rounded; margin
entire at base; venation basal actinodromous, nine radiating veins; midvein,
weak to moderate, straight or slightly curved, occasionally sinuous, alternately
branched along the apical half; a-pectinal veins strong, well-developed forming
an angle of 30-35" with the midvein at the base, straight or slightly recurved,
becoming curved above, curvature often abrupt at points of departure of
abmedial veins; a- exmedial veins moderate, curved, brochidodromous; fipectinal veins strong, well-developed, basally departing a-pectinal at angle of
30-35" curving by abrupt changes of course at points of departure of abmedial
veins, curvature increasing near the margin to join most basal a-abmedial at an
acute or obtuse angle; 8-abmedials moderate, slightly curved, curvature
increasing abruptly at departure of veins which form medial veins of teeth,
looping to join superadjacent P-abmedial at acute or obtuse angles, medial veins
to one or two teeth depart from each loop; y-pectinal veins moderate to strong,
at the base forming an angle of 30-35" to the fi-pectinals, slightly curved except
near margin where curvature abruptly increases at departure of tooth medial
veins to join basal b-abmedial at an acute or obtuse angle; y-abmedials weak,
curved, looping near margin to join superadjacent abmedial, one or two veins
depart the loops abmedially to form medial veins of teeth; h-pectinals weak,
forming angle of 30-35" to y-pectinal, curved, joining basal y-abmedial, giving
off brochidodromous weak abmedials; tertiary venation moderate to weak,
orthogonal to random reticulate, often joining to form weak composite
intersecondary veins; fourth- and fifth-order veins orthogonal, areolation
moderately well-developed, polygonal with a tendency to be quadrangular,
veinlets usually branched once; ultimate marginal venation looped to
incomplete.
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