PART OF A SPECIAL ISSUE ON INFLORESCENCES
Pair-ﬂowered cymes in the Lamiales: structure, distribution and origin
Department of Structural and Functional Botany, Faculty Center of Biodiversity, University of Vienna, Rennweg 14, A-1030 Vienna,
* For correspondence. E-mail email@example.com
Received: 6 December 2012 Revision requested: 22 January 2013 Accepted: 22 April 2013 Published electronically: 24 July 2013
†Background and Aims In the Lamiales, indeterminate thyrses (made up of axillary cymes) represent a signiﬁcant
inﬂorescence type. However, it has been largely overlookedthat there occur two types of cymes: (1) ordinary cymes,
and (2) ‘pair-ﬂoweredcymes’ (PFCs), with a ﬂower pair (terminal and frontﬂower) topping each cyme unit. PFCs are
unique to the Lamiales and their distribution, origin and phylogeny are not well understood.
†Methods The Lamiales are screened as to the occurrence of PFCs, ordinary cymes and single ﬂowers (constituting
†Key Results PFCs are shown to exhibit a considerable morphological and developmental diversity and are docu-
mented to occur in four neighbouring taxa of Lamiales: Calceolariaceae, Sanango, Gesneriaceae and
Plantaginaceae. They are omnipresent in the Calceolariaceae and almost so in the Gesneriaceae. In the
Plantaginaceae, PFCs are restricted to the small sister tribes Russelieae and Cheloneae (while the large remainder
has single ﬂowers in the leaf/bract axils; ordinary cymes do not occur). Regarding the origin of PFCs, the inﬂores-
cences of the genus Peltanthera (unplaced as to family; sister to Calceolariaceae, Sanango and Gesneriaceae in
most molecular phylogenies) support the idea that PFCs have originated from paniculate systems, with the front-
ﬂowers representing remnant ﬂowers.
†Conclusions From the exclusive occurrence of PFCs in the Lamiales and the proximity of the respective taxa in
molecular phylogenies it may be expected that PFCs have originated once, representing a synapomorphy for this
group of taxa and fading out within the Plantaginaceae. However, molecular evidence is ambiguous. Depending
on the position of Peltanthera (depending in turn on the kind and number of genes and taxa analysed) a single, a
double (the most probable scenario) or a triple origin appears conceivable.
Key words: Inﬂorescence, thyrse, raceme, cyme, pair-ﬂowered cyme, Lamiales, Calceolariaceae, Gesneriaceae,
Peltanthera, Plantaginaceae, Sanango, Scrophulariaceae, Stilbaceae.
Some taxa of Lamiales exhibit a special and otherwise unknown
type of axillary inﬂorescences: cymes with ﬂower pairs termin-
ating the cyme units instead of single ﬂowers (‘pair-ﬂowered
cymes’, PFCs). Although already noted by Bravais and Bravais
(1837) and Wydler (1851a,b), this type of cymes and the ‘super-
numerary’ ﬂowers within these cymes have been, and largely still
are, either disregarded (e.g. in virtually all descriptions of
Penstemon and other genera of Plantaginaceae) or thought to
be of rare and/or accidental occurrence and/or have been mor-
About 40 years ago, the author directed attention to this special
type of cymes and realized its wide distribution in the Gesne-
riaceae and occurrence in a few genera of Scrophulariaceae
(sensu lato,s.l.)(Weber, 1972,1973). Some special aspects,
such as particular variations, ontogeny and origin, have been
treated in later publications (Weber, 1975,1978b,1982,1995).
However, as these papers are partly written in German and/or
access to some journals is difﬁcult, little notice has been taken
of them apart from a small circle of gesneriad specialists. In
the meantime, the systematical peripherals have changed consid-
erably: based on the results of molecular analyses, the deﬁnition
of several families now included in the order Lamiales, particu-
larly Scrophulariaceae, has changed dramatically (for a review
see Tank et al., 2006). In addition, new taxa having PFCs have
been detected by me, especially by recent investigation of
The ﬁrst part of this paper provides an overview of the princi-
pal structure, diversity, ontogeny and possible interpretations of
the PFC, with a focus on Gesneriaceae. In the subsequent parts
the families of Lamiales (in their current circumscription) are
surveyed as to the occurrence of PFCs.
With regard to the phylogenetical origin of PFCs, only theor-
etical speculations have been offered so far. Weber (1973,1982)
hypothesized that the front-ﬂowers within the PFCs are remnant
ﬂowers and the PFCs originated from paniculate systems. In the
present paper, the inﬂorescences of Pelthanthera are described
and shown to represent a possibly ancestral type. The genus
Peltanthera has not satisfactorily been assigned to a family
thus far, but has been shown to be related in some way to
Calceolariaceae and Gesneriaceae in recent molecular analyses.
Based on the morphological, taxonomic and phytogeographi-
cal patterns established, the relationships and phytogeographical
origin of the relevant taxa, with a focus on Plantaginaceae, are
The paper is written in a mixed style. In some parts it is a
review, but in other parts presents new results. The relevant lit-
erature is discussed ad hoc. Documentation of some previously
published data and the new ﬁndings (Plantaginaceae) is
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Annals of Botany 112: 1577– 1595, 2013
doi:10.1093/aob/mct156, available online at www.aob.oxfordjournals.org
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essentially provided in the Supplementary Data (Tables S1– 4
and Figs S1– 29), which thus forms an integral part of the paper.
MATERIAL AND METHODS
Analyses of inﬂorescences from various families of Lamiales
have been carried out, based on the following. (1) Living material
( plants in ﬂower, in bud and/or in fruiting stage) cultivated at the
Botanical Garden of the University of Vienna (HBV) and the
Royal Botanic Garden Edinburgh, living plants of Russelia equi-
setiformis (cultivated) and R. sarmentosa (encountered in the
ﬁeld) studied in Costa Rica; fresh ﬂowering material of
Peltanthera ﬂoribunda collected in Costa Rica (San Pablo de
Pamichal) and studied from pickled material. (2) Herbarium spe-
cimens from the herbaria W and WU (a list of representative spe-
cimens examined is given in Table S3). (3) Early illustrations in
distinguished horticultural magazines such as Curtis’s Botanical
Magazine (1787 onwards),Edwards’s Botanical Register
(1815– 1847) or van Houtte’s Flore de Serre (van Houtte
et al., 1845– 1888). (4) Photographs on the internet and kindly
provided at high resolution by the relevant authors.
Throughout the text and in the supplementary ﬁguresthe plant
names are given without authorities. A list with complete names
is given in Table S4.
INTRODUCTORY NOTES ON THE
INFLORESCENCES OF LAMIALES
In the Lamiales, as presently circumscribed (e.g. Olmstead et al.,
1993;Bremer et al., 2002;Kadereit, 2004;APG III, 2009), there
is considerable variation in inﬂorescence architecture. In all fam-
ilies of the ‘core’ Lamiales (sensu Olmstead, 2002; e.g.
Lamiaceae, Acanthaceae, Bignoniaceae, Orobanchaceae) the
inﬂorescences (¼the whole terminal ﬂower-bearing parts of
the seasonal shoot system lack a terminal ﬂower: they are ‘inde-
terminate’ (‘open’, ‘indeﬁnite’). In the synﬂorescence concept of
Troll (1964) they belong to the ‘polytelic’ type and represent
‘ﬂorescences’. The components of these inﬂorescences
(‘partial ﬂorescences’ sensu Troll) are either cymes or single
ﬂowers. In the former case, the inﬂorescence can be referred to
as an indeterminate thyrse (thyrsus), and in the latter as an inde-
terminate raceme (botryum). The cymes and single ﬂowers, re-
spectively, emerge from the axils of subtending leaves and
bracts. Depending on the nature of the subtending leaves
(foliage leaves, bracts or transitional forms) the so-deﬁned in-
ﬂorescence can be denoted as frondose, bracteose or frondo-
bracteose (Troll, 1964;Weberling, 1992), and is very different
in its outward appearance. As can be seen (rarely) in the
Lamiaceae and (more frequently) in the Acanthaceae and
Bignoniaceae, thyrses and racemes co-occur in several families
and even genera (e.g. Salvia). Thyrses and racemes – and thus
cymes and single ﬂowers, respectively – can be considered as
closely related inﬂorescence forms, and changes from the
former to the latter type, that is by reduction of the cyme to a
single ﬂower (¼primary cyme unit), have probably occurred
often in the evolution of Lamiales. As long as prophylls are
present, the converse way is also a possibility.
In the ‘basal’ Lamiales inﬂorescence morphology is more
varied. Here, we ﬁnd both ‘determinate’ (‘closed’, ‘deﬁnite’,
Troll: ‘monotelic’) inﬂorescences, with a terminal ﬂower
topping the main inﬂorescence axis (e.g. Oleaceae, Tetrachon-
draceae), and ‘indeterminate’ inﬂorescences (Calceolariaceae,
Gesneriaceae, Plantaginaceae, Scrophulariaceae, Stilbaceae)
and the spectrum of forms is wider (a more detailed reference
is made in the main body of the text). The presence of determinate
inﬂorescences in the families preceding those with indeterminate
inﬂorescences in all available molecular phylogenies indicates
that indeterminate inﬂorescences have originated from determin-
ate ones in this lineage.
Some families of the ‘basal’ Lamiales, with indeterminate and
mainly thyrsic inﬂorescences, exhibit a peculiarity that is not
known from other families in the angiosperms: all units of the
cymes seem to end in a ﬂower pair (‘PFC’, Weber, 1973).
Before turning to this special type of cyme in more detail, it
seems appropriate to say a few words about the ‘ordinary’
cyme, its structural elements and its principal branching patterns.
A cyme is a branching system in which equally structured units
(modules), each ending in a terminal ﬂower, are concatenated in a
sympodial manner. Branching, that is the production of consecu-
tive units, is from the axils of the prophylls (bracteoles ¼bracts
within the cyme). Owing to the essentially decussate leaf ar-
rangement in the Lamiales, the two prophylls (a,b) belong to
a single node, stand opposite and no internode (mesopodium)
is developed between them. The internode below the ﬂower is
the pedicel (epipodium), and the internode below the prophylls
is the hypopodium. In the ﬁrst ( primary) cyme unit the hypopo-
dium is commonly referred to as ‘peduncle’.
If in each cyme unit both prophylls produce consecutive units
from their axils, the resulting cyme is a ‘dichasium’ (with the
‘simple dichasium’ or ‘triad’ consisting of just three cyme
units/ﬂowers, and the ‘compound dichasium’ consisting of
several to many units). If only one of the prophylls is ‘fertile’,
the resulting cyme is a ‘monochasium’. Principally, there are
four types of monochasia, but for the Lamiales only the ‘scorpi-
oid’ cyme (cincinnus) is of major relevance. This is a monocha-
sium in which the consecutive units develop on alternating sides
of each sequential axis. Very often the dichasial and the mono-
chasial patterns are combined, in that the ﬁrst branching is dia-
chsial, and the following ones are monochasial (‘double cyme’
or ‘double cincinnus’).
The ‘pair-ﬂowered’ cyme must not be confused with the
so-called ‘geminiﬂorous’ cyme, which is simply a special type
of ordinary cyme. Although the literal meaning of ‘pair-
ﬂowered’ and ‘geminiﬂorous’ is the same, the underlying struc-
tures are completely different. In some cases the term ‘gemini-
ﬂorous’ simply means that the inﬂorescence (e.g. a raceme) is
numerically reduced to two ﬂowers (e.g. Astragalus gemini-
ﬂorus,Cassia geminiﬂora – Fabaceae). In other (here more rele-
vant) cases, the term refers to species in which cymes bear ﬂower
pairs (e.g. Dianthus geminiﬂorus – Caryophyllaceae,
Dipteracanthus geminiﬂorus,Jussieua geminiﬂora –
Acanthaceae, Gomphocarpus geminiﬂorus – Apocynaceae).
Here the cymose branching pattern is dichasial, but in one of
the two lateral leaf/bract axils the axillary shoot is represented
by a single ﬂower only. Together with the terminal ﬂower of
the cyme unit, this ﬂower forms a pair. In the other leaf/bract
axil the cymose branching is continued (Fig. 1B) (for examples
see Weberling, 1958;Troll, 1969).
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Pair-ﬂowered cyme (Fig. 1C). In the PFC each ﬂower is accom-
panied by a second ﬂower. This ﬂower does not emerge from
the axil of one of the prophylls, but is an extra (additional or
‘supernumerary’) ﬂower. Each cyme unit, therefore, seems to
end in a ﬂower pair. The following characterization is based
largely on the examination of PFCs in the Gesneriaceae, in
which family this inﬂorescence type is not only a characteristic
feature, but also reaches its culmination in the range of morpho-
logical and ontogenetic diversity.
STRUCTURE, DIVERSITY, ONTOGENY AND
INTERPRETATIONS OF PFCs
Position, basic structure and ﬂowering sequence
Position. Like the conventional cymes of the ‘core’ Lamiales the
PFCs are always of axillary origin and are never placed in termin-
al position. The thyrse they build is thus indeterminate. In the
Calceolariaceae and most Gesneriaceae the cymes often
emerge from the axils of foliage leaves, while bracteose inﬂores-
cences are rare. In the Plantaginaceae, they more commonly
emerge from the axils of leafy or small bracts, often forming a
gradual reduction series within the inﬂorescence.
Basic structure. In the PFC, each terminal ﬂower (T) is associated
with a second ﬂower in frontal (abaxial-median ¼median-
phylloscopic) position. This second ﬂower is called the ‘front-
ﬂower’ (F). It appears to be inserted at the level of the prophylls
(a,b) and consecutive cyme units, respectively, but usually lacks
a subtending bracteole. However, in some cases (observed both
in Gesneriaceae and in Penstemon,Weber, 1973; and frequently
found in Keckiella, see below) the front-ﬂower is subtended bya
distinct bracteole (g-bracteole) (Fig. S1). As already noted by
Bravais and Bravais (1837) and Wydler (1851a,b), the
g-bracteole is probably best perceived as a remnant of a
bracteole-pair (g,d) positioned above the prophyll bracteoles
(a,b). Owing to the decussate leaf arrangement, with alternation
of the bracteole pairs, the orientation of gand dis median. While
the d-bracteole is always suppressed, the g-bracetole and its ax-
illary product, the front-ﬂower, ‘survived’ in the PFC. In some
cases the front-ﬂower bears bracteoles itself and these even
may produce axillary branches (e.g. Penstemon serrulatus,
Sequence of ﬂower opening. The ﬁrst ﬂower to open in the PFC
units is the terminal ﬂower (T
). Then the front ﬂower (F
follows. Finally, the terminal ﬂowers of the subsequent cyme
) open, followed by their front-ﬂowers (F
). The se-
quence of ﬂower opening is thus descending (T
Structural diversity. A considerable diversity of PFC forms is
accomplished by simple variation of the length of the internodia
within the cyme ( peduncle, hypopodia of subsequent cyme
units, pedicels). This variation gives rise to long-pedunculate/
epedunculate cymes and a lax or congested ﬂower arrangement
(Fig. S2). A signiﬁcant feature is the presence/absence, size,
form and coloration of the bracteoles. By displacement of the
bracteoles (e.g. Agalmyla tuberculata,Drymonia coccinea and
other species of the genus) a branching pattern is reached as is
found in Solanaceae-Solaneae, with two perpendicular brac-
teoles of unequal size at each node (Weber, 1982; Fig. S3). By
the enlargement and fusion of the ﬁrst bracteole pair aconspicu-
ous cupule embracing the ﬂowers may be formed (e.g. Cyrtandra
Diversity in branching symmetry. As each cyme unit bears two
bracteoles (a,b), and each of them is principally capable of pro-
ducing a consecutive axillary cyme unit, dichasial branching is
the basic pattern of PFC branching. Compound dichasia, such
as found in Sinningia bulbosa (Weber, 1973; Fig. S1) are rare.
Much more common is that the ﬁrst cyme unit branches dicha-
sially, and the following ones monochasially (pair-ﬂowered
‘double cyme’; common in Gesneriaceae: Fig. S2, or
Penstemon:Fig.2). In certain alliances of Gesneriaceae
(Epithemateae, Cremosperma,Tylopsacas, etc.) also the ﬁrst
branching is monochasial, giving rise to pair-ﬂowered ‘unilat-
eral’ cymes. This type of cyme is often associated with the pres-
ence of many and small ﬂowers, sometimes also with a reduction
of the bracetoles (Epithemateae), and a pseudo-monopodial de-
FIG. 1. Principal cyme types. (A) Cyme of the ordinary type, with a single ﬂowerterminating each cyme unit. (B) Geminiﬂorous cyme: special type of ordinary cyme,
with one of the two dichasial branches reduced to asingle ﬂower. (C) Pair-ﬂowered cyme: eachterminal ﬂower of the cyme units accompanied by the ‘front-ﬂower’ in
frontal (abaxial-median) position.
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Diversity in cyme elaboration (ﬂower number per PFC). Flower
number per PFC may vary considerably, dependent on the spe-
cies, the position within the thyrse (usually there is a decrease
from the base to the top) and/or the vigour of the individual
plant. Conventionally, such a series is referred to as a reduction
series (many-ﬂowered several-ﬂowered few-ﬂowered
two-ﬂowered one-ﬂowered with bracetoles one-ﬂowered
without bracteoles, Fig. 3). Phylogenetically, however, the series
can be read also in the opposite direction. So it may be better to
refer to this series as the ‘standard series’. In this series the front
ﬂowers hold a strong position, in that they are more resistant to
reduction than the lateral cyme units emerging from the axils
of the braceteoles. Notable steps in the series are, therefore,
four- and two-ﬂowered cymes. The four-ﬂowered cyme consists
on both sides (T
; Fig. 3D). The
biﬂorous cyme produces T
only (Fig. 3E). Examples can be
found in many taxa (e.g. Chirita micromusa, Fig. S8). The
extreme case is the presence of a single ﬂower (Fig. 3F, G). If it
has bracteoles (Fig. 3F), a return to cymose branching seems
easy. Complete phylogenetic loss of the bracteoles (Fig. 3G)
may mean deﬁnite ﬁxation of the single-ﬂowered state.
Irregular reduction and loss of front-ﬂowers.Occ asional loss of the
front-ﬂowers has been documented in several species of
Gesnericeae (Weber, 1978b). The most illustrative example is
perhaps Chrysothemis friedrichsthaliana (Fig. S4). In this
species, the most complete form is a PFC with six ﬂowers, that
is consisting of three cyme units each with a ﬂower pair
). Reduction of ﬂowers does not occur
along the standard series, but is irregular (e.g. T
Reduction of the front-ﬂowers may be to a rudiment or may be
complete (‘phylogenetic loss’).
FIG. 3 . Diagrams of PFCs illustrating the ‘standard series’. (A) Many-ﬂowered
cyme. (B) Several-ﬂowered cyme. (C) Six-ﬂowered cyme. (D) Four-ﬂowered
cyme. (E) Two-ﬂowered cyme. (F) Single ﬂower with bracteoles. (G) Single
ﬂower without bracteoles. T
, terminal ﬂowers of cyme units; F
ﬂowers, a,b, transverse bracteoles.
FIG.2.Penstemon digitalis. (A) Pair-ﬂowered cyme in fruiting stage.
(B) Corresponding diagram. T
, terminal ﬂowers (fruits) of cyme units
, front-ﬂowers (fruits); only right side of cyme is labelled.
Photograph: A. Weber.
Weber —Pair-ﬂowered cymes in the Lamiales1580
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Taxa such as Chrysothemis show quite plainly how PFCs can
progress into a cyme of the ordinary type. Such cases may give
some support to the idea that the ordinary cymes of ‘higher’
Lamiales may have originated from PFCs.
Initiation of the front-ﬂowers. The ontogeny of PFCs was studied
by Weber (1995) based on cleared whole mount preparations
(according to the technique described by Ritterbusch, 1974)
and scanning electron microscopy (Figs S5, 6). Signiﬁcant is
that the ontogenetic sequence of the side ﬂowers of the cyme
units (front-ﬂower, lateral ﬂowers) is converse to the sequence
of ﬂower opening. The terminal ﬂower is followed by the trans-
verse ﬂowers, and afterwards (!) the primordium of the front-
ﬂower is initiated. If a g-bracteole is present, the primordium
of this bracteole emerges later than the transverse bracteoles
(a,b). The primordium of the front-ﬂower, however, develops
more quickly, gets ahead and opens earlier than the lateral
ﬂowers, subsequent to the terminal ﬂower.
The acropetal initiation sequence of the bracteoles (a,bg)
and their axillary structures demonstrates that the front-ﬂower
and its (usually suppressed) bracteole do not arise from the
same node as the lateral bracteoles/branches, but from a distinct
node situated above the prophyll node. If a g-bracteole is present,
the primordium of the front-ﬂoweroriginates clearly in the axil of
the this bracteole (Figs S5, 6).
Initiation of the consecutive cyme units. In general, the consecutive
cyme units arise successively, with some time interval between
emergence of the primordia. However, especially in small- and
many-ﬂowered unilateral cymes development may be acceler-
ated: the primordia of two or several units are already formed
in a stage when the mother unit is still in a primordial stage
itself. In this case, a kind of ‘false apical meristem’ or ‘pseudo-
shoot apex’ is present, from which the common primordia of
the cyme units appear to be detached successively to the left
and the right ( pseudomonopodial development, e.g. Monophy-
llaea,Epithema,Weber, 1976a,b,1982,1988; Fig. S7).
Interpretations and possible phylogenetic origin
Regarding its phylogenetic origin, at least four interpretations
of the PFC appear conceivable (Fig. 4). They can be divided into
two groups. (A) The PFC originated from the ordinary cymes.
This interpretation implies three possibilities (Aa, Ab, Ac). (B)
The PFC originated from an originally more complex, panicle-
like branching system.
Hypothesis Aa: the front-ﬂowers are accessory ﬂowers. At ﬁrst
sight, this interpretation may appear as the most plausible one
and indeed has been applied to the PFCs of Gesneriaceae (in
the few cases where noted) and Calceolaria (Goebel, 1931;
Troll, 1964;Hartl, 1965;Molau, 1978;Weberling and Troll,
FIG. 4 . Conceivable interpretations and hypothetical origin of the PFC. (A) The PFC originatedfrom the ordinary cyme. Hypothesis Aa: the front-ﬂowers are acces-
sory ﬂowers congenitally fused with the axes of the cyme units; both the terminal ﬂowers and the front-ﬂowers originate from the same leaf axil. Hypothesis Ab: the
ﬂower pairs originated through splitting of the primordia of the terminal ﬂowers. Hypothesis Ac: the front-ﬂowers are de novo acquisitions. (B) The PFC originated
from a complex, paniculate branching system. Ba: pair-ﬂowered cyme (PFC), the front-ﬂowers are remnant ﬂowers. Bb: reduction of the front-ﬂowers gave rise to
ordinary cymes. Note: ordinary cymes may also have originated directly from paniculate branching systems (right arrow).
Weber —Pair-ﬂowered cymes in the Lamiales 1581
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Note that the term ‘accessory’ has several meanings and can be
misleading. Its literal meaning is ‘additional’ or ‘adventitious’
and this evokes the interpretation that an accessory structure is
a phylogenetically secondarily acquired structure (in that sense
it is also relevant for hypothesis Ac). This is, of course, a possi-
bility, but runs the risk that it blocks alternative interpretations.
In traditional German morphology the term ‘accessory’ means
the formation of one or several extra shoots (vegetative shoots/
partial inﬂorescences/ﬂowers) following the ‘legitimate’ ﬁrstax-
illary shoot in a leaf/bract axil. Position is usually serial (more
rarely biserial or ‘collateral’, see, for example, Troll, 1964:
‘Beispross’, ‘Beiknospe’, ‘Beiblu
¨te’). For devel-
opmental aspects see Sandt (1925): the axillary meristem is not
fully used up for the production of an axillary shoot or ﬂower.
There remains a residual meristem portion, which – after some
time of recovery – grows up to form another shoot/ﬂower (in
the dicotyledons usually in serial position). The same may
happen several times so that a leaf or bract axil may bear
several (serial) axillary shoots/ﬂowers. Although emerging in a
temporally and spatially consecutive order, they are equivalent
as to their origin from the same leaf axil. A review on the occur-
rence of accessory partial inﬂorescences/ﬂowers in the
Scrophulariaceae, with instructive photos and diagrams, has
been given by Weberling and Troll (1998: 368 –372). True acces-
sory cymes and ﬂowers occur also in the Calceolariaceae and the
Gesneriaceae, for example in the repetitive series of biﬂorous ax-
illary cymes in many species of the genus Microchirita (¼trad-
itional Chirita sect. Microchirita,Wang et al., 2011;Weber
et al., 2011; Fig. S8). In other words, accessory shoots/ﬂowers
and front-ﬂowers may well occur side by side.
If the front-ﬂowers represent accessory ﬂowers, they should
emerge from the same leaf/bract axil as the cyme unit. This is
clearly not the case: they emerge at the base of the pedicel of
the terminal ﬂower, at or slightlyabove the prophyll node. To re-
trieve the interpretation as an accessory shoot, an auxiliary hy-
pothesis would have to be introduced: that the peduncle of the
front-ﬂower (and the hypopodia in all following cyme units) is
(are) congenitally fused with the peduncle (hypopodium) of
the terminal ﬂower (Fig. 4Aa). Curiously, this hypothesis has
never been explicitly expressed in words. If it is true one
should expect that the peduncles of the PFCs exhibit a ‘double’
or ‘composite’ nature. However, no factual evidence (e.g. exter-
nally visible concrescence or internal presence of a double or
two-parted vascular ring) is available. The peduncles are
simple internodes, in no way different from the peduncles of or-
The strongest counter-argument is that the front-ﬂowers are in
some species subtended by a distinct bracetole (g-bracteole,
Weber, 1973; Fig. S1) and that the ontogeny shows that the front-
ﬂowers originate like normal axillary ﬂowers. This makes it at
once clear that the front-ﬂower is a regular side shoot of the
cyme axis, although its subtending bracteole is commonly sup-
Hypothesis Ab: the ﬂower pairs result from primordium splitting.
This hypothesis can be disproved immediately as the ontogeny
does not show any kind of primordial splitting: the primordium
of the front-ﬂower develops much later than the primordium of
the terminal ﬂower, emerging separately as an axillary structure
of the g-bracteole (if present) (Figs S5, S6).
Hypothesis Ac: the front-ﬂowers are de novo acquisitions. This
means that the front-ﬂowers in the PFCs have originated
through a novel evolutionary step. On the one hand, this hypoth-
esis says clearly that the PFC has arisen from the ordinary cyme
by a kind of phylogenetic ‘addition’ of the front-ﬂowers, but does
not give an explanation for where these ﬂowers come from. It is
thus difﬁcult to prove or to disprove this hypothesis from a mor-
phological point of view, but support may come from the system-
Hypothesis B: the PFC originated from a paniculate branching
system. This interpretation was proposed by Weber (1973,
1982), but no systematic evidence could be provided at that
time. It rests on the evidence that the g-bracteole belongs to a dis-
tinct node above the prophyll node. As, moreover, the front-
ﬂower sometimes bears bracteoles itself and may even branch
(Weber, 1973; Fig. S16), it seems safe to assume that the front-
ﬂower is the remnant of a branch.
Conclusions. From the four conceivable hypotheses addressed,
two (Aa and Ab) can be excluded, as there is no factual evidence.
Hypothesis Ac is difﬁcult to prove or disprove, but may become
relevant if other interpretations fail. Hypothesis B will be dis-
cussed in more detail in the context of Peltanthera.
OCCURRENCE OF PFCs IN THE
GESNERIACEAE AND PLANTAGINACEAE
The occurrence of PFCs in the Gesneriaceae and some
‘Scrophulariaceae’ (Calceolaria,Tetranema,Penstemon) has
been documented by Weber (1972,1973). However, at that
time neither the frequency of occurrence nor the taxonomic
bearing was clear. By the wide circumscription of the
Scrophulariaceae, the distribution of PFCs in that family
seemed totally erratic. By the recent split and reorganization of
the Scrophulariaceae (Olmstead and Reeves, 1995;Olmstead
et al., 2001;Olmstead, 2002;Oxelman et al., 2005;Albach
et al., 2005; APG III, 2009; for details and history see Tank
et al., 2006) the situation has changed dramatically. In the fol-
lowing, the distribution of PFCs is reviewed acrossthe new taxo-
nomic entities. Calceolariaceae, Sanango and Gesneriaceae are
only brieﬂy treated, as little new and original information can
be provided. The Plantaginaceae are treated in more detail and
evidence is presented that all taxa said to have ‘cymose’ or ‘pani-
culate’ inﬂorescences have PFCs. Reports are, however,negative
in Scrophulariaceae sensu stricto (s.s.) and other segregates from
the ‘original’ Scrophulariaceae, as well as in all other families of
The three families with PFCs belong to the ‘basal’ Lamiales in
the sense of Olmstead (2002) (for the families included in this
group and their rough position see Fig. 7). They are in some
way closely related, but there is no general consensus about the
details. The various topologies published in the literature will
be discussed later, in the context of whether PFCs have originated
once or several times.
Presently, the family includes two genera, Calceolaria (ap-
proximately 250 spp., Mexico to Patagonia) and Jovellana
Weber —Pair-ﬂowered cymes in the Lamiales1582
on 11 September 2017
(4 spp., Chile, New Zealand). The third genus traditionally
recognized, Porodittia (¼Stemotria; 1 sp., Chile), has been
recently included in Calceolaria, but is addressed here as well.
Calceolaria (Fig. 5A; Fig. S9). The curious inﬂorescences, with
two ﬂowers between the dichasial cyme branches, have
puzzled botanists for a long time, and different interpretations
have been proposed (Molau, 1978;Andersson and Molau,
1980;Weberling and Troll, 1998). Clear recognition that the
Calceolaria inﬂorescences represent PFCs was given by
Weber (1973). More recently, Ehrhart (2000,2005) has shown
that this inﬂorescence type is omnipresent in the genus. The
only exceptions are the species in which the cymes are usually
reduced to single ﬂowers (e.g. C. uniﬂora,C. darwinii,
C. fothergillii,C. tenella). In C. biﬂora and similar species the
PFCs are often (but not consistently) reduced to the primary
ﬂower pair (T
). Between many-ﬂowered cymes and single
ﬂowers many transitions are represented and variation is
clearly along the standard series. No species has been detected
so far, in which occasional or constitutive loss of front-ﬂowers
Jovellana (Fig. 5B; Fig. S10). According to personal observations,
all species of the genus (recently reduced to four; Nylinder et al.,
2012) have several-ﬂowered PFCs.
Porodittia ( =Stemotria) (Fig. S11). The only species of the genus,
P.triandra, has been recently included in Calceolaria on mo-
lecular grounds (Andersson, 2006;Cosacovet al., 2009). As per-
fectly shown in the original illustration of Cavanilles (1799,as
Jovellana triandra, see Mayr and Weber, 2006: Fig. 11), the
species has PFCs.
FIG. 5. Examples of PFCs in the families Calceolariaceae, Gesneriaceae and Plantaginaceae. (A) Calceolaria integrifolia (Calceolariaceae). (B) Jovellanasinclairii
(Calceolariaceae). (C) Streptocarpus dunnii (Gesneriaceae). (D) Sinningia macrostachya (Gesneriaceae). (E) Tetranema gamboanum (Plantaginaceae-Russelieae).
(F) Penstemon arkansanus (Plantaginaceae-Cheloneae). (G) Nothochelone nemorosa (Plantaginaceae-Cheloneae). (H) Pennellianthus frutescens
(Plantaginaceae-Cheloneae). (I) Keckiella breviﬂora (Plantaginaceae-Cheloneae). For more details and labelling of the ﬂowers see Figs S9– 29.
Weber —Pair-ﬂowered cymes in the Lamiales 1583
on 11 September 2017
Sanango racemosum (Fig. S12). Sanango comprises a single
species, S. racemosum, distributed in the sub-Andean region of
Peru and south-east Ecuador. The genus/species has had a
varied taxonomic history, having been assigned to
Scrophulariaceae, Loganiaceae and Buddlejaceae. Wiehler
was the ﬁrst to suggest that Sanango would belong to
Gesneriaceae, in particular to subfam. Gesnerioideae tribe
Gesnerieae. The molecular data of Smith et al. (1997) seemed
in perfect accordance, but proved erroneous (J. F. Smith, Boise
State University, ID, USA, pers. comm.). Later molecular
work (Oxelman et al., 1999;Perret et al., 2013) revealed that
Sanango is sister to Gesneriaceae.
At the request of H. Wiehler, I examined the axillary inﬂores-
cences. The results, including a drawing of an inﬂorescence and a
diagram, were published by Wiehler (1994). The inﬂorescences
clearly represent cymes of the pair-ﬂowered type, usually com-
prising around six ﬂowers (F
In the Gesneriaceae (c.150 genera, .3200 spp.), PFCs are
almost omnipresent. Exceptions do not occur in the more basal
lineages, so that it can be safely stated that PFCs represent the
basic type of axillary partial inﬂorescences in the family.
Flower number varies extensively, from numerous (e.g. 40–50
in PFCs of unifoliate Streptocarpus) through several, few, two
to one (with and without bracteoles) (see Fig. 5C, D and Fig.
S12). If reduction of the cymes to solitary ﬂowers is combined
with reduction of the subtending leaves to bracts, the ﬂower ag-
gregate forms a distinct phenetic entity: a terminal raceme.
Such racemes are constitutive for just a few genera: Gloxinia (in-
cluding Anodiscus and Koellikeria,Roalson et al., 2005a,b),
Gloxiniopsis,Smithiantha and Diastema (all belonging to the
tribe of Gloxinieae). By shortening of the pedicels, the ﬂowers
may become sessile and the raceme transgresses into a spike
(e.g. Sinningia allagophylla,Chautems and Weber, 1999). In
the Old World Gesneriaceae, only a single genus has bracteose
racemes: Rhynchoglossum (tribe Epithemateae) (Weber,
1978a). Taxa with cymes of the ordinary type are extremely
rare in the Gesneriaceae. To the best of my knowledge, only
two species (both belonging to genera of the morphologically
advanced tribe Epithemateae) can be quoted: Loxonia hirsuta
and Stauranthera coerulea (Weber, 1977a,b). Loxonia is a
genus of three species, two of which have PFCs cymes, and
one has ordinary cymes. Stauranthera comprises ﬁve species
(my unpubl. data), and only the one discussed has ordinary
cymes. No intermediate forms (occasional loss of front-ﬂowers)
between pair-ﬂowered and ordinary cymes have been found in
these genera. In these cases there is little doubt that cymes of
the conventional type have evolved from PFCs. The two
genera can serve as model cases for the evolution of ordinary
cymes from PFCs.
In their new circumscription (Albach et al., 2005;Oxelman
et al., 2005), the Plantaginaceae include 12 tribes with around
90 genera and roughly 2000 species. Ten of the 12 tribes can
be excluded immediately from further consideration, as these
only comprise genera with single axillary ﬂowers and ‘racemose’
The two remaining tribes are Russelieae (Russelia,
Tetranema) and Cheloneae (?Brookea,Chelone,Chionophila,
TABLE 1. Tribes of Plantaginaceae that include genera with pair-ﬂowerd cymes (no cymes of the ordinary type are present in the
Russelieae Russelia 50 (?) +– – Central and tropical South
Tetranema 5+– – Costa Rica to Mexico
Uroskinnera 4– – +Guatemala to Mexico
Cheloneae Chelone 4– +– Eastern half of North America,
Chionophila 2– +– Central North America
(Montana, Idaho, Wyoming,
Collinsia 20 – – +North America, many spp. in
Keckiella 7++ – South-western North America
(California, Oregon, Nevada,
Nothochelone 1+– – Western North America (British
Columbia to northern California)
Pennellianthus 1+– – North-east Eurasia (incl.
Kamchatka peninsula), Japan
Penstemon 270 ++ – Guatemala to Alaska
Tonella 2– – +Vancouver Island to central
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on 11 September 2017
Tonella,?Urokinnera)(seeTable1). In the molecular phylogenies
of Albach et al. (2005) and Oxelman et al. (2005) Russelieae and
Cheloneae represent sister clades. Inthe phylogeny of Wolfe et al.
(2006), relating particularly to Penstemon and its allies,
Russelieae, Uroskinnera (molecular data incomplete) and
Cheloneae form successive clades.
The tribe Russelieae was for a long time considered monoge-
neric (Pennell, 1935;Thieret, 1954,1967), but, based on molecu-
lar data, has been recently expanded to include the genus
Tetranema (Wolfe et al., 2002). Russelia and Tetranema have, al-
though sharing a similar distribution, morphologically little in
common. No morphological characters (synapomorphies) have
come to light so far that would show quite plainly their close re-
Russelia (Fig. S14). In this genus some 50 species have been
recognized (Carlson, 1957), ranging from north-east Mexico to
Colombia. The species, collectively known as ﬁrecracker
plants or coralblows, are very similar and often used as ornamen-
tals. The inﬂorescences emerging from the axils of small, oppos-
ite leaves or bracts are known to be cymosely branched (Carlson,
1957; without mention of the pair-ﬂowered condition), with
loose or dense clustering of the ﬂowers. As to early illustrations,
Russelia coccinea (¼R. multiﬂora;?¼R. sarmentosa)pro-
duces many-ﬂowered cymes which together form a dense, elon-
gated, terminal head. R. sarmentosa has also dense axillary
ﬂower clusters, but these are well separated by long internodia
and the subtending phyllomes are leafy [there is, however,
some discussion regarding whether R. coccinea and
R. sarmetosa (kept distinct by Carlson, 1957) are conspeciﬁc].
The fact that the ﬂowers of ‘Russelia juncea’(¼R. equiseti-
formis) are arranged as in Penstemon (PFCs) was already
brieﬂy mentioned by Wydler (1851a,b) and is conﬁrmed here
for that species and R. sarmentosa.InR. sarmentosa (the type
species of Russelia), the ﬂowers are arranged in +dense clus-
ters. Branching is 2-3-times dichasial at the base and then
monochasial. In contrast, in R. equisetiformis ﬂower number
is low, with the PFCs often being reduced to the primary
ﬂower pair or a single ﬂower. ‘Russelia alata’(Chamisso and
Schlechtendal, 1828) has consistently single ﬂowers in the leaf
axils. More recently, this species has been accommodated in a
genus of its own, Cubitanthus, and placed in Gesneriaceae
(Barringer, 1984). This placement was doubted by Weber
(2004). New molecular data suggest that the genus/species
belongs neither to Plantaginaceae nor to Gesneriaceae, but to
Linderniaceae (Perret et al., 2013). This taxon is therefore not
considered further here.
Tetranema (Fig. 5E; Fig. S15). The genus, represented by ﬁve
species in Central America, has been revised twice in the last
two decades (Me
´ndez-Larios and Villasen
Christenhusz, 2010), but in neither was the presence of PFCs
mentioned. The habit of the plants is rosette-like (rosulate),
with long-scapose axillary inﬂorescences emerging from the
leaf axils and bearing a dense and many-ﬂowered ﬂower
cluster at the top. The inﬂorescences of T. roseum have been ana-
lysed by Weber (1972;asT. mexicanum), with the result that they
correspond to the PFCs of Gesneriaceae. The other species
apparently conform to this pattern. An illustration of the recently
described T. gamboanum (Grayum and Hammel, 1996: ﬁg. 1)
shows nicely the pair-ﬂowered structure of the cyme (Fig. S15).
Conclusions regarding Russelieae. In view of the substantial mor-
phological differences of Tetranema and Russelia, the presence
of PFCs in the two genera is certainly a welcome common char-
acter and it is reasonable to assume that the pair-ﬂowered condi-
tion represents a synapomorphy (see Wolfe et al., 2002).
However, this interpretation is ambiguous (see below).
In tribe Cheloneae presently 8 –10 genera are accepted (Wolfe
et al., 2002,2006): except the doubtful Brookea, the alliance is
essentially of North American distribution, with one outlier
(Pennellianthus frutescens) in the far north-east of Eurasia and
Japan. A few species of Penstemon reach Central America
(Mexico) in the south. The position of Uroskinnera (Mexico,
Guatemala) is still somewhat problematic. Wolfe et al. (2006)
pleaded for exclusion from Cheloneae, but as it forms the sister
genus of Cheloneae, it is at least closely allied. In the following
the genera are (formally) grouped as follows: Penstemon,
Nothochelone,Pennellianthus,Keckiella (all with PFCs),
axillary ﬂowers with or without bracteoles) and Brookea (prob-
ably misplaced in Cheloneae and Plantaginaceae, respectively).
Penstemon (Fig. 5F; Figs S16, 17). This is by far the largest genus
of tribe Cheloneae, comprising .270 species (Lodewick and
Lodewick, 1999;Wolfe et al., 2006), from which approx. 75
have been included in the present anaylsis (Table S3). In the taxo-
nomic literature, the Penstemon inﬂorescence is almost exclu-
sively referred to as a panicle (e.g. Keck, 1932–1940;Pennell,
1935;Keck and Cronquist, 1957;Straw, 1966) and neither the
thyrsic nature nor the presence of ﬂower pairs has been noted.
Only in two species descriptions (out of approx. 90) in horticul-
tural magazines is the inﬂorescence designated as ‘thyrsoid’
(Lindley, 1842;van Houtte et al., 1845– 1883). Only Lindley
(1842: t.3884) made explicit reference to the ﬂower pairs in the
axillary inﬂorescences: ‘Gradually, towards the upper part of
the stem, the leaves become smaller, and, in proportion,
broader, till at length they constitute ovate bracteas; from the
axils of these leaves the bracteated peduncles arise, each gener-
ally with two branches, and two rather large, handsome
ﬂowers, which are subsecund.’ More frequently, the pair-
ﬂowered condition can be seen in illustrations (e.g. Penstemon
palmeri, Fig. S16). For simple descriptive purposes the reference
as ‘panicle’ may be sufﬁcient, but it is not acceptable for morpho-
phylogenetic analyses. The Penstemon inﬂorescence must be
classiﬁed as an indeterminate thyrse, which is consistently
made up of PFCs. In principle, this was already recognized by
early morphologists such as Bravais and Bravais (1837) and
Wydler (1851a,b), but was largely overlooked and/or ignored
in taxonomy. Wettstein (1891: ﬁg. 19) even presented an inﬂor-
escence diagram of P.digitalis in which he completely dismissed
the pair-ﬂowered condition and showed a cyme of the ordinary
The important point is that in Penstemon PFCs do not only
occur here and there, but seem to be a characteristic feature of
the whole genus. In the present analysis no species was detected
Weber —Pair-ﬂowered cymes in the Lamiales 1585
on 11 September 2017
in which the cymes were of the ordinary type. Due to the poor
sampling their occasional occurrence cannot be excluded, but
there is little doubt that PFCs are the typical and original form
in the genus.
As in panicles, the degree of elaboration of the partial inﬂores-
cences decreases from the base to the top, so that the outward
form of the Penstemon inﬂorescence is usually a pyramid or
slender cone. According to the average ﬂower number, the
Penstemon cymes fall roughly into four groups (see Fig. S17):
(1) cymes (including the uppermost ones) many- and usually
dense-ﬂowered (e.g. P. cyananthus,P. procerus); (2) cymes
several-ﬂowered, becoming few- to one-ﬂowered towards the
top of the inﬂorescence (bulk of species); (3) cymes two-
(rarely three- or four-)ﬂowered (the two ﬂowers representing
, the occasional third and fourth ﬂower are the lateral
; e.g. P. barbatus,P. centranthifolius,P. gentianoides,
P. richardsonii,P. wrightii); (4) cymes consistently reduced to
single ﬂowers (e.g. P. angustifolius,P. azureus,P. fruticosus,
P. humilis,P. menziesii). No species with ebracteolate single
ﬂowers has been found.
Nothochelone (Fig. 5G; Fig. S18). This genus is monotypic with
N. nemorosa. Common names such as woodland beardtongue
or woodland penstemon indicate that the species is very similar
to Penstemon, in which genus it has been originally described
(Straw, 1966,1967). The axillary inﬂorescences are made up
of two to several ﬂowers. In several-ﬂowered cymes the presence
of front-ﬂowers can be well observed and the cymes have the
typical appearance of a Penstemon PFC.
Pennellianthus (Fig. 5H; Fig. S19). This is another segregate of
Penstemon, comprising a single species, P.frutescens
(Crosswhite and Kawano, 1970), with distribution in north-east
Eurasia including Japan. The inﬂorescences are rather dense-
ﬂowered heads, held tightly above the foliage, with the ﬂowers
much like in Penstemon with insect-pollinated ﬂowers. The
partial inﬂorescences emerging from the bract axils are one- to
four-ﬂowered, with a front-ﬂower placed beneath the terminal
ﬂower of the cyme.
Keckiella (Fig. 5I; Figs S20 –24). This is a genus of seven woody,
sometimes squarrosely branched shrubs, with main distribution
in California. The genus has been separated from Penstemon
by Straw (1966,1967). The species can be roughly referred to
two groups, one with yellow, cream or whitish, short-tubed and
strongly bilabiate ﬂowers (apparently bee-pollinated: K. anti-
rrhinoides,K. breviﬂora,K. lemmonii,K. rothrockii) and
another with brilliant red, long-tubed and galeate ﬂowers (obvi-
ously hummingbird-pollinated: K. cordifolia,K. corymbosa,
K. ternata). In the ﬁrst group the inﬂorescences are rather lax,
in the second group sometimes dense- and multi-ﬂowered (but
variation is considerable, even between the individual plants of
a given species). Freeman et al. (2003) showed that the relation-
ships do not conform to these anthecological groups.
With the exception of K. rothrockii, the inﬂorescence of all
species represents a pyramidal or elongate thyrse. The cymes
represent PFCs, with ﬂower number decreasing towards the
apex. They are special in that the front-ﬂowers are usually
(always?) subtended by a distinct bracteole (g-bracteole) and
bear bracteoles themselves. In other words, the front-ﬂowers
seem to be ‘replaced’ by (poorly developed) cyme branches.
In Keckiella antirrhinoides,K. breviﬂora and K. lemmonii the
inﬂorescence organization is very similar (Figs S20, 21). The ter-
minal inﬂorescence is an indeterminate, rather lax thyrse with
pedunculate cymes. The cymes are few-, often four- and rarely
up to six-ﬂowered. In general, only one or two ﬂowers seem to
open in a cyme, while the others remain in bud stage (at least
for a long time). In the upper part of the inﬂorescence only
single, bracteolate ﬂowers are produced (characteristic of
K. antirrhinoides). Analysis of four-ﬂowered cymes shows that
below the terminal ﬂower (T
) there is a front-ﬂower in
median-abaxial position (F
) and two lateral ﬂowers (T
ging from the axils of the bracteoles (a,b). Remarkably, the
front-ﬂower has (always?, occasionally?) a subtending bract
and frequently bears bracteoles itself. This is a condition some-
times found in Penstemon (Weber, 1973; Fig. S16) which pro-
vides evidence that the front-ﬂower is a reductional form of a
branch and the PFC a reduced form of an originally more
complex branching system.
K. rothrockii (Fig. S22) has similar ﬂowers as the previous
species, but the inﬂorescence is different: the ﬂowers emerge
singly from the bract axils, they are sessile and bear two brac-
teoles below the calyx. The inﬂorescence looks like a simple (oc-
casionally basally branched) spike. Freeman et al. (2003)
concluded from molecular data that K. rothrockii would be the
‘most basal species’ of the genus Keckiella, while it appears as
a derived form with respect to inﬂorescence morphology.
The terminal inﬂorescences of K. cordifolia,K. corymbosa
and K. ternata (Figs S23, S24) are lax or dense pyramidal
thyrses in downcurved position (K. cordifolia) or many-ﬂowered
‘clusters’ in suberect (K. corymbosa) or erect position
(K. ternata). For K. cordifolia and K. ternata it can be demon-
strated clearly that front-ﬂowers are present and the cymes thus
represent PFCs. Analysis of the dense and many-ﬂowered
cymes of K. corymbosa proved difﬁcult, but the presence of
front-ﬂowers could be conﬁrmed by examination of soaked
samples of herbarium specimens.
Chelone (Fig. S25). This genus, comprising four species
(Nelson and Elisens, 1999) is uniform with respect to the inﬂor-
escences and ﬂowers. The inﬂorescences are dense, cone-like
terminal aggregates. From the axils of the opposite bracts
there emerge single subsessile, bracteolate ﬂowers, which
form together four tight longitudinal rows. No ﬂower pairs
and front-ﬂowers, respectively, are present.
Chionophila (Fig. S26). This genus comprises two species,
C. jamesii and C. tweedyi. The ﬁrst has inﬂorescences (and
ﬂowers) very similar to Chelone, but, due to sectorial anisoclady,
only two of the four rows of bracts bear axillary ﬂowers, so that
the spike is secund and bears two adjacent rows of ﬂowers
only. According to the original description of Bentham (1846:
325) the ﬂowers bear two braceteoles (‘Pedunculi brevissime
bibracteati’). C. tweedyi has elongate, lax inﬂorescences with
shortly stalked ﬂowers. Again only two rows of ﬂowers are
present, in that in every bract pair only one bract bears an axillary
ﬂower. No ﬂower pairs and front-ﬂowers, respectively, are
Collinsia (Fig. S27). This is a genus of about 20 annual species
with main distribution in California. The inﬂorescences
roughly fall into two groups thathave been referred to as ‘pedicel-
Weber —Pair-ﬂowered cymes in the Lamiales1586
on 11 September 2017
ﬂowered’ and ‘sessile-ﬂowered’ (Gray, 1880,1886;Newsom,
1929). Recently, Baldwin et al. (2011) showed that this grouping
has little taxonomic relevance. In the ‘pedicel-ﬂowered’ group
the ﬂowers are not only long-stalked, but emerge singly in the
axils of opposite bracts (e.g. C. parviﬂora,C. violacea), while
in the ‘sessile-ﬂowered’ group the pedicels are short, and the
large ﬂowers form conspicuous circles around the stem (e.g.
C. heterophylla,C. tinctoria). As the ﬂowers open almost syn-
chronously, the latter type results in a pagoda-like appearance
of the inﬂorescences (therefore the common name ‘Chinese
houses’). C. verna occupies a transitional position, in that the
circles are formed by long-stalked ﬂowers. The reason for the
ring-like arrangement of the ﬂowers is that the decussate leaf ar-
rangement in the vegetative region switches to the formation of
6– 8-merous pseudowhorls. The bracts are inserted at almost
the same levels and the pseudowhorls are separated by elongated
internodes (compare with Kwiatkowska, 2012). The stalks of the
axillary ﬂowers, regardless of being long or short, lack brac-
teoles, so that the formation of cymes is impossible. Also, no
front-ﬂowers have been observed.
Tonella (Fig. S28). This is the putative sister genus of Collinsia
(Baldwin et al., 2011), comprising two delicate, annual species
(T. ﬂoribunda,T. tenella). The inﬂorescence organization is
very similar to Collinsia.InT. tenella, a single ﬂower emerges
per leaf axil, comparable to C. parviﬂora.InT. ﬂoribunda
circles of ﬂowers are formed, each ﬂower being stalked. These
inﬂorescences correspond to C. verna. Again, no PFCs are
Uroskinnera (Fig. S29). This is a little known genus of four
species native to southern Mexico and Guatemala (Schultes,
1941;Daniel and Breedlove, 1992). As can be concluded from
the drawings and photos shown in Fig. S27, the inﬂorescences
are short or long, many- and dense-ﬂowered terminal racemes,
with the single, short and apparently ebracteolate ﬂowers emer-
ging from small, opposite (U. almedae) or alternately arranged
bracts (U. hirtiﬂora). There is no indication of the presence of
front-ﬂowers or PFCs, respectively.
Brookea. This is a genus of four species from Borneo, represent-
ing shrubs or small trees. It was originally placed in Scrophular-
iaceae (Bentham, 1876b), but transferred to Gesneriaceae by
Hallier (1903).Burtt (1965) and Thieret (1967) considered a
position in the Scrophulariceae-Cheloneae. Fischer (2004)
placed the genus in Scrophulariaceae-‘Stilbaceae’, a position
which, however, does not ﬁt with the mainly South African dis-
tribution of Stilbaceae. The inﬂorescences are said to be racem-
ose and this has been observed in an unidentiﬁed species on
Borneo by me. With regard to both morphology and phytogeog-
raphy (molecular data are lacking), Brookea is completely alien
to the Cheloneae and does not have PFCs.
Conclusions regarding Cheloneae. Of the eight (to ten) genera
presently accepted in the tribe Cheloneae, four have PFCs
four (to six) have single ﬂowers building up terminal racemes
or spikes (Chelone,Chionophila,Collinsia,Tonella; the inclu-
sion of Uroskinnera and Brookea in Cheloneae is doubtful, see
Wolfe et al., 2006). Nothochelone and Pennellianthus have
been originally described in Penstemon and segregated by
Straw (1966,1967) and Crosswhite and Kawano (1970). In the
molecular phylogenies of Cheloneae (Wolfe et al., 2002,2006)
the three genera appear at different places: Pennellianthis is
sister to the rest of Cheloneae, Nothochelone appears within a
clade comprising also Chionophila and Chelone, and
Penstemon forms the crown group. Keckiella occupies a sister
position to the clade made up of Penstemon +the clade compris-
ing Chionophila,Nothochelone and Chelone. This topology
gives support to the idea that PFCs represent the original condi-
tion in the Cheloneae. They apparently have given rise to single
ﬂowers in several alliances, but have been retained in
Notochelone and Penstemon as an ancestral condition. The op-
posite interpretation, namely that the pair-ﬂowered condition
evolved several times independently, appears less probable.
Collinsia and Tonella, genera included in the Cheloneae only
recently for molecular reasons (Wolfe et al., 2002), appear mor-
phologically somewhat out of place in the Cheloneae. Their
species never produce cymes and the single ﬂowers are always
ebracteolate. Moreover, the increase and whorled arrangement
of bracts in some species has no parallel in other members of
Cheloneae. Their divergent morphology, however, may be
seen in context with their progression to an annual habit.
Although the outer appearance of the inﬂorescences is very
different, Chelone and Chionophila ﬁt much better to
Penstemon,Nothochelone and Pennellianthus. The ﬂower
form is similar (except to bird-pollinated species of
Penstemon) and the single ﬂowers can be explained as reduced
forms of cymes.
THE INFLORESCENCES OF THE REMAINING
‘Core’ Lamiales (sensu Olmstead, 2002). In this large series of
families PFCs are not found (Table 2). Cymose branching of
the axillary inﬂorescences (and thus thyrsic structure of the inﬂor-
escence) is most common in the Lamiaceae, Acanthaceae and (to
a lesser degree) Bignoniaceae. In the families Byblidaceae,
Lentibulariaceae, Linderniacae, Phrymaceae-Mazoideae (some-
times considered as a family of its own: Mazaceae) and
Orobanchaceae cymes are not found. The only exceptions of
taxa having cymes in the ‘upper’ clades are the Phrymaceae
(with Leucocarpus and Hemichaena having cymes, while the
remaining genera, e.g. Mimulus, have single axillary ﬂowers)
and the small and monogeneric family Paulowniaceae. The
overall impression is that racemic inﬂorescences have evolved
many times in parallel from thyrsic ones and that the switch
from the one type to the other (by reduction of the cyme to its
primary cyme unit) is an ‘easy’ evolutionary process. It would
be worthwhile to performan ancestralstate reconstruction analysis
to conﬁrm/reject this view and to get more detailed informationon
the evolutionary patterns.
‘Basal’ Lamiales. Although comprising fewer families, the
‘basal’ Lamiales are more varied in their inﬂorescence organiza-
tion. The inﬂorescences are either determinate or indeterminate,
and the form spectrum includespanicles, thyrses (both withordin-
ary and PFCs) and racemes. There is essential agreement in all
available molecular phylogenies that the families preceding the
Calceolariaceae, Gesneriaceae and Plantaginaceae form a grade
of clades comprising (from bottom to top) Plocospermataceae,
Weber —Pair-ﬂowered cymes in the Lamiales 1587
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Oleaceae +Carlemanniaceae, and Tetrachondraceae. Unfortu-
nately, the inﬂorescences of most are still insufﬁciently known.
The monotypic Plocospermataceae are said to have ‘inﬂores-
cences subtended by two leaves, axillary, in 1 –7 congested
racemes or dichasia, often reduced to only 1– 2 ﬂowers’ (Struwe
and Jensen, 2004: 330). It is not clear what this means. The
Carlemanniaceae (two genera, ﬁve species) are said tohave ‘axil-
lary or terminal cymes’ (Thiv, 2004). Also this reference is
ambiguous. The same holds true for the brief descriptions
of Solereder (1897:Plocosperma, under Loganiaceae) and
Schumann (1897:Carlemannia, under Rubiaceae). The
Tetrachondraceae (two genera, three species) have been analysed
by E. M. Sehr and A. Weber (unpubl. data). Here, terminal ﬂowers
are present, each preceded by two pairs of small bracts. Branching
is cymose. Theinﬂorescences of Oleaceae are variable and still in-
completely known. Terminal ﬂowers are said to be always present
(Knoblauch, 1895), so that the inﬂorescences can be classiﬁed as
determinate (‘monotelic’, Troll, 1964: 177). Branching of the
inﬂorescences is said to be racemose, paniculate or cymose, but
statements such as ‘a simple cyme ...is rare, but more commonly
the cyme is thyrsoid and more or less paniculate’ (Green, 2004:
297) are of little help toget a clear picture of the inﬂorescence or-
ganization of Oleaceae. The only inﬂorescences of the family that
have experienced a proper analysis are those of Forsythia and
Abeliophyllum (Troll, 1969: 529 ff.). Their partial inﬂorescences
are reduced to single ﬂowers and these constitute rich-ﬂowered
Detailed studies in the ‘balsalmost’ families are badly needed
to elucidate the morphological and phylogenetic basement of the
Lamiales. Regardless, it can be said that racemic inﬂorescences
do not play a major role. The picture changes in the families fol-
lowing the Calcelariaceae–Sanango – Gesneriaceae clade(s).
The ﬁrst family in the succession is the Plantaginaceae, in
which, apart from the two genera of Russelieae and four
genera of Cheloneae, all (around 90) genera have racemic inﬂor-
escences. The two successive families, Scrophulariaceae and
Stilbaceae, are the ﬁrst ones having indeterminate inﬂorescences
with ordinary cymes. Both include also racemic inﬂorescences.
In the Scrophulariaceae thyrses/cymes of the ordinary type are
represented in the majority of tribes. In the Aptosimeae,
Buddlejeae, Freylinieae, Scrophularieae and Teedieae all or
the majority of genera have cymes (see Table S1). In the
Stilbaceae only one of the three tribes includes genera with
cymes (Bowkerieae, with two genera having cymes and one
having single ﬂowers in the leaf axils; see Table S2), while the
rest (ten genera) have racemic inﬂorescences.
TABLE 2. The families of Lamiales (+unplaced genera) and their basic types of inﬂorescences
Name of family or genus
(+)/indeterminate (– )
Raceme (s.l.), ﬂowers
singly in leaf/bract axils
Plocospermataceae 1 1 –? ?
Carlemanniaceae 2 5 +??
Oleaceae 25 600 +++
Tetrachondraceae 2 3 ++
Calceolariaceae 2 270 – ++ +
Sanango 1–++ +
Gesneriaceae .150 3300 – ++ (+)(+)
Plantaginaceae (incl. Callitrichaceae,
90 2000 – +++
Scrophulariaceae (incl. Myoporaceae,
60 1700 – ++ +
Stilbaceae (incl. Retziaceae) 10 40 – ++ +
Linderniaceae 13 195 – ++ +
Byblidaceae 1 6 – ++ +
Martyniaceae 5 15 ++ +
Pedaliaceae 15 60– 85 – (+)++
Schlegeliaceae 4 25–30 – ++ (+)
Bignoniaceae .100 850 – ++ +
Acanthaceae (incl. Avicenniaceae) 250 2500 – ++ +
Lentibulariaceae 3 (-5) 350 – ++ +
Thomandersiaceae 1 6 – ++ +
Verbenaceae 35 1200 – +++
Lamiaceae (incl. Symphoremataceae) 235 7000 – ++ + (e.g. Holmskioldia,
Phrymaceae 11 160 – +(Leucocarpus,
Paulowniaceae 1 7– ++ +
Rehmannia-Triaenophora – clade (Xia
et al., 2009)
29 – ++ +
Orobanchaceae (incl. Cyclocheilaceae) 90 .2000 – ++ +
Arrangement is roughly according to presumed phylogeny (based on Olmstead et al., 2001;Scha
¨ferhoff et al., 2010; and N. Refulio and R. G. Olmstead, pers.
comm.). Data regarding the number of genera and species are according to the family treatments in Kadereit (2004) and current online information. Rough
assessment of the frequency of the occurrence of cyme types and single ﬂowers is according to the author’s experience; +++ all genera, ++ predominance, +
rare, ( +)/(– ) very rare.
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What is important here is that none of the families mentioned
so far seems to have inﬂorescencesthat could be regarded as par-
ticularly close to PFCs. However, there is one taxon which is,
according to recent molecular data, in some way related to the
families with PFCs: the (as to family unplaced) genus
Peltanthera. Its phylogenetic position and its inﬂorescence
structure will be addressed in the following section.
THE GENUS PELTANTHERA AND THE ORIGIN
OF THE PFC
Peltanthera and its position within the Lamiales
The genus Peltanthera was previously placed in Loganiaceae
and Buddlejaceae (e.g. Bentham, 1876a;Leeuwenberg and
Leenhouts, 1980;Norman, 2000). Its only species, P. ﬂoribunda,
is represented by small trees occurring in Central and northern
South America and having small, actinomorphic ﬂowers.
Oxelman et al. (1999) were the ﬁrst to recognize that
Peltanthera belongs to the Lamiales and suggested a position
sister to Sanango +Gesneriaceae (represented by a single
species of Streptocarpus and Nematanthus). Since then
Peltanthera has been included in several molecular analyses.
Its position varies as to the particular studies. This will be
addressed in more detail in the section ‘Phylogenetic scenarios’.
In any topology Peltanthera diverges earlier than Gesneriaceae
and Sanango, sometimes ‘earlier’ and sometimes ‘later’ than
Calceolariaceae. The genus is possibly the last remnant of a
once larger group which evolved from oleaceous-like ancestors
(with tree-like, woody habit!) and which gave rise to the remain-
ing Lamiales with generally zygomorphic ﬂowers. If the axillary
partial inﬂorescences are cymes of the ordinary type, it would be
almost inevitable to assume that the PFCs have originated from
ordinary cymes by elaboration, that is by de novo acquisition
of the front-ﬂowers (hypothesis Ac above). If they represent elab-
orate branching systems (more elaborate than PFCs, panicles in
the widest sense) it would seem likely that the PFCs originated
by reduction, with the front-ﬂowers being remnant ﬂowers (hy-
pothesis B above).
The inﬂorescences of Peltanthera
The most detailed reference to the inﬂorescences was by
Norman (2000, under Buddlejaceae), who described the inﬂores-
cences as many- and small-ﬂowered panicles and referredto their
position as terminal and axillary. The somewhat close relation-
ship of Peltanthera to Gesneriaceae (and Calceolariaceae)
gives some general support to the idea thatthe PFCs of these fam-
ilies have originated from panicle-like branching systems as
represented in Peltanthera. The two opposite basal branches of
the panicle are strong (Fig. 6C) and it is easy to imagine that
they would ‘survive’ in case of reduction of the panicle to afew-
The author’s investigations in Costa Rica (unpubl. data)
revealed that Peltanthera ﬂoribunda is a small tree (Fig. 6A)
with the branches showing monopodial (but seasonally inter-
rupted) growth. The most interesting point is that at the end of
the panicle branches usually (but not always)four-ﬂowered aggre-
gates are present that consist of a terminal ﬂower, a second ﬂower
in frontal (median-abaxial) position, and two lateral ﬂowersemer-
ging froma node below the frontal ﬂower (Fig.6D, E). The termin-
al and the frontal ﬂower show some precocious development,
opening before the lateral ﬂowers and most other ﬂowers of the
panicle branch. Remarkably, neither the lateral ﬂowers nor the
frontal ﬂower are subtended by bracteoles, while the more basal
branches of the partial inﬂorescence are.
FIG.6. Peltantheraﬂoribunda. (A) Tree habit. (B) Branchletof tree with inﬂorescences. (C) Inﬂorescence; note positionin axilof foliage leaf and paniculate structure,
with two vigorous side branches at base. (D) Detail of (C); note four-ﬂowered aggregates on top of panicle branches. (E, F) Corresponding diagrams. Photographs:
Weber —Pair-ﬂowered cymes in the Lamiales 1589
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It may be that the terminal ﬂower paircorresponds to the ﬂower
pair at the end of the cyme units of Calceolariaceae, Ges-
neriaceae and Plantaginaceae. The PFC now can be (formally)
derived from the Peltanthera inﬂorescence by assuming the fol-
lowing steps: (1) genetic ﬁxation of the frontal ﬂower, (2) succes-
sive reduction of the nodes between the basal ¼prophyll node
and the node of the frontal ﬂower (that is including the node
of the two lateral ﬂowers in the four-ﬂowered agrregates of
Peltanthera), (3) increasing morphological homogenization of
the branching units and (4) progression toa sympodial branching
pattern (successive branching from the axils of the prophylls,
perhaps in compensation for the loss of nodes and their ﬂowers).
The truly paniculate structure of the inﬂorescences of
Peltanthera favours an origin of PFCs through reduction. None-
theless, it must be clear that the inﬂorescence of Peltanthera is
not immediately ancestral to the PFC of Calceolariaceae and
Gesneriaceae (and perhaps Plantaginaceae). At the moment no
taxa are known which could serve to bridge the morphological
gap between the two types.
PHYLOGENETIC SCENARIOS OF PFC ORIGIN
At ﬁrst sight, the phylogenetic interpretation of the distribution
of PFCs in the Lamiales seems easy. As they are found in three
families that form two or three successive clades in molecular
phylogenies, it seems likely that PFCs have originated once,
that is in the ancestors of Calceolariaceae and Gesneriaceae,
and have been inherited to the Plantaginaceae (Fig. 7). In the
Plantaginaceae they have survived in the clade Russelieae +
Cheloneae, but became extinct in the large remainder of the
family in that the PFC was reduced to a single ﬂower and the
thyrse progressed to a raceme. Extinction was apparently
forever: PFCs do not turn up in any of the subsequent families
of ‘basal’ Lamiales and in the ‘core’ Lamiales. If this scenario
is true, the ordinary cymes found in these families may have
evolved from PFCs by reduction of the front-ﬂowers in the
cymes, as was noted above for two species in the Gesneriaceae.
Remarkably, the situation is not as simple as that. Molecular
phylogenies are partly, but not unambiguously, in agreement.
While the ‘basement’ of the Lamiales seems to be rather stable
in the molecular phylogenies, the topologies of Calceolaria-
ceae, Gesneriaceae and Plantaginaceae vary. The statement by
Oxelman et al. (2005: 414) that: ‘There is emerging support
that the bulk of “core” Lamiales does not include Calceolaria-
ceae, Gesneriaceae, Sanango, and Peltanthera. The relation-
ships among these are, however, contradictory’ is still valid.
Moreover, so far no major attempts have been made to correlate
the molecular phylogenies with the distribution and evolution of
morphological characters. Apart from inﬂorescence morph-
ology, the present alliance is of particular interest as to ﬂoral sym-
metry. The Plocospermataceae, Carlemanniaceae, Oleaceae and
Tetrachondraceae have radially symmetric and (except
Plocospermataceae) usually tetramerous ﬂowers. Bilateral sym-
metry or slight zygomorphy is reached in the Carlemanniaceae
and Oleaceae by reduction of the four stamens to two. The
Calceolariaceae and the Gesneriaceae are the ﬁrst diverging fam-
ilies in which the ﬂowers are strongly zygomorphic. There seems
to be a strong correlationbetween pronounced ﬂoral zygomorphy
and the lack ( phylogenetic loss) of a terminal ﬂower in the inﬂor-
escences. In the famil ies under consideration and their ancestors,
respectively, the switch from determinate to indeterminate
inﬂorescences and radial to zygomorphic ﬂowers seems to go
hand in hand, but may have not been reached in one step. This
is the possible reason that phylogenetic reconstruction is rather
difﬁcult. Apparently there is a deep gap (resulting from almost
complete extinction of linking lineages) between the ‘basalmost’
(Plocospermataceae to Oleaceae) and the ‘higher’ families of the
‘basal’ Lamiales. Peltanthera could be the last survivor of an al-
liance that once gave rise to the evolution of families with PFCs
and zygomorphic ﬂowers.
There are are two critical points to be discussed in detail:
(1) the position of Peltanthera, and (2) the relationships within
Position of Peltanthera
Conﬂicting phylogenies have been published as to the position
of Peltanthera, which also concern the phylogenetic relationship
between Calceolariaceae and Gesneriaceae. As compared with
Peltanthera, the position of Sanango is much more stable. This
genus appears always in closer vicinity of Gesneriaceae than
Peltanthera and holds a sister position to that family in all mo-
lecular studies where Sanango has been included. From the dif-
ferent positions of Pelthanthera, a single, a double and a triple
origin of the PFCs can be inferred.
Scenario 1: PFCs originated once (Fig. 8A). From all available
molecular phylogenies only Qiu et al. (2010) support this scen-
ario to some extent. This phylogeny is interesting as it is the only
one based on (four) mitochondrial genes. Support for the
Lamiales, Oleaceae (Syringa) and Peltanthera is moderate
[bootstrap support (BS) values between 75 and 80] and high
for the sister clades branching off from the node following
(Russelieae + Cheloneae)
FIG. 7. Phylogeny of the ‘basal’ Lamiales (sensu Olmstead, 2002) under the as-
sumption that PFCs have originated once, have survived in the sister clades
Russelieae and Cheloneae of Plantaginaceae, and became extinct in the
common ancestor of Scrophulariaceae, Stilbaceae and ‘core’ Lamiales (C).
Note, however, that the position of Peltanthera is uncertain (dotted line).
Weber —Pair-ﬂowered cymes in the Lamiales1590
on 11 September 2017
Peltanthera: one clade comprises Calceolariaceae +
Gesneriaceae (BS 96) (Sanango was not included in the study),
and the other clade (BS 92) comprises a grade of the ‘higher’
Lamiales (nine families, including Plantaginaceae). Support
for the particular families within the grade is low (BS between
20 and 40).
On the one hand, the position of Peltanthera supports well the
idea that PFCs have originated once in the ‘basal’ Lamiales. On
the other hand, the tree does not show where they became extinct.
The Plantaginaceae (and Scrophulariaceae) turn up at the top of
the ‘core’ Lamiales. However, in view of the generally low
support values in the grade this position should not be weighed
Scenario 2: PFCs originated twice (Fig. 8B). This is the scenario
suggested by Olmstead (2002),Soltis et al. (2011), and
N. Refulio and R. G. Olmstead (unpubl. data). In these phy-
logenetic trees Peltanthera, Calceolariacae, Sanango and
Gesneriaceae are on a clade which is sister to a grade of
Plantaginaceae, Scrophulariaceae, Stilbaceae and the ‘core’
Lamiales. According to this topology, the assumption of a
double origin of the PFC is almost inevitable: PFCs have devel-
oped in the ancestors of Calceolariaceae and Gesneriaceae, from
which group Peltanthera is perhaps the last survivor, and inde-
pendently in the Plantaginaceae (more precisely: in the
common ancestors of Russelieae and Cheloneae). The question
remains: from which type of inﬂorescences did the PFCs of
Russelieae and Cheloneae evolve?
Scenario 3: PFCs originated triply (Fig. 8C). A triple origin of the
PFC (and zygomorphic ﬂowers) is suggested by the phylogenies
of Wang et al. (2004),Oxelman et al. (2005) and Perret et al.
(2013). The content of the clade with Gesneriaceae is the same
as in the trees of scenario 2, but the succession of the taxa is dif-
ferent: Calceolariaceae branch off earlier than Peltanthera,
Sanango and the Gesneriaceae.
Another remarkable difference is that the clade sister to the
Gesneriaceae contains a (topologically) basal clade of
Gratiolaceae +Plantaginaceae, while in the trees of scenario 2
the Gratiola-alliance is an ingroup of Plantaginaceae.
Gratiolaceae (like Linderniaceae) were at ﬁrst recognized as a
distinct family by Rahmanzadeh et al. (2005) but (unlike
Linderniaceae) this family has not been generally accepted
(APG III, 2009).
Again the question remains open: from which inﬂorescence
type did the PFCs of Calceolariaceae and Plantaginaceae evolve?
Relationships and distribution of PFCs within the Plantaginaceae
The family Plantaginaceae has been essentially shaped by
Olmstead et al. (2001),Oxelman et al. (2005) and Albach
et al. (2005). These studies and in more detail those of Wolfe
et al. (2002,2006) show (1) that Russelia and Tetranema are
sister genera, constituting the tribe Russelieae, and (2) that the
Russelieae and Cheloneae are sister groups, forming (together
with Uroskinnera) a clade.
There is slight uncertainty about the position of Collinsia:in
some topologies it branches off earlier than the remainder of
Cheloneae (Oxelman et al., 2005), while in the targeted studies
on Cheloneae (Wolfe et al., 2002,2006)Collinsia appears as
an ingroup of Cheloneae, branching off at the node following
Racemes in the Cheloneae. Within Cheloneae, the genera that
have racemic inﬂorescences do not form a coherent group, but
appear scattered over the tribe. If, as some studies suggest,
Russelieae and Cheloneae are sister groups, and given the scat-
tered distribution of taxa with racemic inﬂorescences within
Cheloneae, then the most parsimonious hypothesis is that the an-
cestor of both tribes had PFCs and that racemic inﬂorescences
evolved at least twice independently in Cheloneae. From its pos-
ition sister to Cheloneae, the racemic inﬂorescence of
Uroskinnera apparently evolved from PFCs too.
FIG. 8 . Phylogentic scenarios regarding the origin of PFCs in the ‘basal’
Lamiales as emanating from different molecular phylogenies. (A) Single origin
(Qiu et al., 2010: extract of ﬁg. 2/6 relating to the Lamiales); weakly supported
internal topology of R ( ¼remainder of Lamiales, including Plantaginaceae)
not shown. (B) Double origin (Olmstead, 2002; N. Refulio and R. G.
Olmstead, pers. comm.). (C) Triple origin (Wang et al., 2004;Perret et al.,
2013); Plocospermataceae and Carlemanniaceae +Oleaceae not shown. Black
bar: origin of PFCs in the common ancestor. For simplicity, the (groups of)
generic names are replaced by family names.
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on 11 September 2017
Presence of PFCs: a synapomorphy or a symplesiomorphy? There
is reasonable consensus about the internal structure of the family,
at least with respect to the nodes relevant here (Fig. 9). Similar
topologies, but usually with fewer genera, are seen in
Olmstead et al. (2001: ﬁgs 1, 2), Scha
¨ferhoff et al. (2010: ﬁgs
2, 3) and in the ten-gene analysis of N. Refulio and R. G.
Olmstead (unpubl. data; R. G. Olmstead, pers. comm.): the
clade of Russelieae and Cheloneae does not branch off from
the ﬁrst node, but from a second one.
Interpretation 1: PFCs are synapomorphic of the Russelieae-
Cheloneae clade (Fig. 9A). In view of the fact that in the
Plantaginaceae the bulk of the genera have racemic inﬂores-
cences it appears reasonable to interpret the occurrence of the
discordant inﬂorescence type as a synapomorphy. Indeed,
Wolfe et al. (2002,2006) explicitly argue that the ‘cymose’
inﬂorescences of Russelieae-Cheloneae would represent a syna-
pomophy of the two clades. In view of the fact that (ordinary)
cymes are common in the Lamiales (especially the ‘core’
Lamiales), this interpretation is indeed most plausible, at least
as long as the Plantaginaceae are seen in isolation.
Interpretation 2. PFCs are plesiomorphic in the Russelieae-
Cheloneae clade (Fig. 9B). This interpretation assumes that the
PFCs have been inherited from (gesnerioid-like?) ancestors.
This would mean that PFCs were lost in two successive steps:
(1) in the Angelonieae +Gratioleae clade, and (2) in the clade
sister to Russelieae +Cheloneae, comprising the whole remain-
der of Plantaginaceae (R).
Other topologies do exist, but are of little relevance. In
the rps16 tree of Albach et al. (2005) the backbone branching
is similar, but Gratioleae appear as the sister group of
Russelieae +Cheloneae and the remainder includes the
Angelonieae. The two above interpretations apply in the same
way as discussed. In the phylogeny of Perret et al. (2013; includ-
ing ﬁve genera of Plantaginaceae) Tetranema (the only member
of Russelieae-Cheloneae included in the analysis) appears as the
topologically lowermost clade in Plantaginaceae. At ﬁrst sight,
this appears to be in agreement with the idea that the Russelieae-
Cheloneae are the most primitive alliance in the Plantaginaceae.
However, the family is topologically preceded by the Linder-
niaceae and Gratiolaceae, both with racemic inﬂorescences.
Conclusions. The many scenarios emanating from the molecular-
phylogenetic studies clearly illustrate the difﬁculties of interpret-
ing the phylogenetic origin and inheritance of PFCs in the given
alliance. It is clear that the scanarios are not equivalent (being
based on different numbers and kinds of genes and taxa) and
are not all equally well supported. It is impossible to discuss
and to evaluate the studies mentioned above in detail. The
most authoritative studies suggest a combination of scenario 2
(double origin of PFCs) and interpretation 1 (PFCs are synapo-
morhic to Russelieae and Cheloneae), meaning that PFCs origi-
nated twice: in the common ancestors of Calceolariaceae and
Gesneriaceae and, within Plantaginaceae, in the common ances-
tors of Russelieae and Cheloneae.
If PFCs represent characters such as an inferior ovary, indehis-
cent fruit or racemic inﬂorescence, in other words characters that
are known to have originated many times in angiosperm evolu-
tion, there would be no reason for considering seriously alterna-
tive interpretations. PFCs, however, are special in several
respects: (1) they are not known from families outside the
Lamiales, (2) within the Lamiales they are only known from
three major families (+Sanango) that turn up in close proximity
in all recent molecular phylogenies, (3) they are almost omni-
present in the Calceolariaceae and Gesneriaceae, representing
here surely the original condition of inﬂorescence organization,
and (4) appear to ‘fade out’ in the clade following these families
(Plantaginaceae). Therefore, the combination of scenario 1
(single origin of PFCs) and interpretation 2 (PFCs are plesio-
morphic in the Russelieae and Cheloneae) should not be dis-
missed completely, although molecular support is not optimal.
The main problem is the position of Peltanthera. From a mor-
phological point of view (as to both the presence of PFCs and
zygomorphic ﬂowers) the most logical position is ‘ahead’ of
all taxa mentioned, as shown in the ‘combined tree’ of Fig. 7.
The only molecular support for this position so far is the study
of Qiu et al. (2010). Less problematic is the occurrence of
PFCs in the Plantaginaceae. The idea that PFCs were inherited
from the common ancestors of Calceolariaceae and
(Sanango+) Gesneriaceae and conserved in the Russelieae
and Cheloneae, but were lost (by reduction to single ﬂowers)
in two successive steps in the remaining Plantaginaceae, is prob-
ably an reasonable option, although it is not the most parsimoni-
ous one. If it is true that Peltanthera is a relict of a once larger
group (see above), the Russelieae (and successively the
Cheloneae) may have evolved from a different ( perhaps northern
Meso-American) section of this group, which possibly had
similar inﬂorescences as Peltanthera. A double origin of PFCs
then would be no longer a great surprise.
PFCs are shown to represent a morphological character that has
been largely overlooked or ignored by plant systematists so far.
Their occurrence in four, in some way coherent taxa seems
easy to explain: they have originated in the ancestors of
Calceolariaceae, Sanango and Gesneriaceae and inherited to
the Russelieae-Cheloneae alliance in the Plantaginaceae. In the
remainder of that family and in the ancestors of the subsequent
FIG. 9 . Contrasting interpretations regarding the occurrence of PFCs in the
Plantaginaceae. (A) Interpretation 1: PFCs have originated de novo in the ances-
tors of Russelieae and Cheloneae. (B) PFCs have been inherited from an ancestor
with PFCs; loss (reduction of PFCs to single ﬂowers) was in two lineages and suc-
cessive steps: Angelonieae +Gratioleae and the remainder of Plantaginaceae
(R). Topology according to Olmstead et al. (2001) and Oxelman et al. (2005),
genera subsumed under tribal names.
Weber —Pair-ﬂowered cymes in the Lamiales1592
on 11 September 2017
families of Lamiales they became extinct, by reduction either to
ordinary cymes or to single ﬂowers, giving rise to conventional
thyrses and racemes, respectively. The available molecular phy-
logenies are, however, ambiguous and alternative interpreta-
tions, suggesting a double (or even triple) origin of PFCs, have
to be taken into consideration. Virtually nothing is known
about the ecological signiﬁcance of PFCs. Generally speaking,
this applies also to the ordinary cymes and racemes in the
Lamiales. Presently, we have at best a rough picture of the occur-
rence of various ‘inﬂorescence types’ in the Lamiales (and even
that is very fragmentary in the basalmostfamilies), but our phylo-
genetic and ecological understanding is very poor. Questions
such as ‘why did thyrsic inﬂorescences proceed to racemic
ones (and vice versa)?’ and ‘what is the correlation of inﬂores-
cence structure with the habitat, with the ecophysiology, with
the ﬂower structure, with the pollinators etc.?’ have been little
addressed so far. One of the few studies in which inﬂorescence
structure, plant habit and habitat are seen in a context is that of
Chautems and Weber (1999), relating to the species of
Today, molecular studies provide an increasingly reliable
basis for recognizing and understanding morphological
changes and progressions. Traditional inﬂorescence morph-
ology, especially the ‘typology’ of Wilhelm Troll and collabora-
tors (Troll, 1964,1969;Troll and Weberling, 1989;Weberling
and Troll, 1998;Weberling, 1992), is a rather static approach,
attempting to establish a solid classiﬁcation throughout the
angiosperms and to develop an adequate terminology. With
regard to phylogenetic inferences, readers are largely left to
their own devices. The Lamiales, in particular the large series
of families with indeterminate inﬂorescences, could serve as a
model system to address various phylogenetic and ecological
questions. A prerequisite is an increasing solidity of phylogenet-
ic reconstructions, that is analysis of more taxa and more genes.
Understandably, in available multigene analyses such as the
17-gene analysis of Soltis et al. (2011) many families are repre-
sented by a single species only. A forthcoming paper by Refulio
and R. G. Olmstead, dealing speciﬁcally with the Lamiales, and
based on ten genes (R. G. Olmstead, pers. comm.), will provide a
better basis. However, for understanding variations and progres-
sions in inﬂorescence morphology and the driving forces behind
them, molecular phylogenies at the generic and speciﬁc level
will be needed. The application of modern methods (such as an-
cestral statereconstruction methods) and conceptual frameworks
(such as the ontogenetic approach of Classen-Bockhoff and
˜u, 2013) must and will play a signiﬁcant role in
future concepts and understanding of inﬂorescence morphology.
Supplementary data are available online at www.aob.oxford-
journals.org/ and consist of the following. Table S1: inﬂores-
cences of Scrophulariaceae (s.s.). Table S2: inﬂorescences of
Stilbaceae. Table S3: representative herbarium specimens of
Plantaginaceae with PFCs (W, WU). Table S4: full scientiﬁc
names of all genera and species referred to in the paper. Figs
S1– 4: structure and variation of PFCs. (S1) Presence of
g-bracteoles and axillary position of front-ﬂowers. (S2)
Variation of PFCs as to length of internodia and branching sym-
metry. (S3) Displacement of bracteoles. (S4) Irregular reduction
and loss of front-ﬂowers. Figs S5– 7: ontogeny of PFCs. (S5)
Ontogeny of PFC, documentation by cleared whole mounts.
(S6) Ontogeny of PFCs, SEM documentation. (S7) Homaxonic
(scorpioid) cymes showing pseudo-monopodial development.
Fig. S8: front-ﬂowers and accessory cymes in Gesneriaceae.
Figs S9– 29: presence of PFCs in Calceolariaceae, Sanango,
Gesneriaceae and Plantaginaceae (where appropriate, showing
variation along the standard series). Figs S9– 11: Calceolari-
aceae. (S9) Calceolaria, (S10) Jovellana, (S11) ‘Porodittia’
(¼‘Stemotria’). Figs S12, 13: Sanango (S12) and Gesneriaceae
(S13). Figs S14, 15: Plantaginaceae-Russelieae: (S14) Russelia,
(S15) Tetranema. Figs S16– 19: Plantaginaceae-Cheloneae,
genera with PFCs (1): (S16, S17) Penstemon, (S18) Nothoche-
lone, (S19) Pennellianthus. Figs S20– 24: Plantaginaceae-
Cheloneae, genera with PFCs (2): (S20) Keckiella antirrhinoides,
(S21) K. breviﬂora and K. lemmonii, (S22) K. rothrockii,
(S23) K. cordifolia and K. corymbosa, (S24) K. ternata.
Figs S25 – 27: Plantaginaceae-Cheloneae, genera with single
axillary ﬂowers (1): (S25) Chelone, (S26) Chionophila. Figs
S27 –29. Plantaginaceae-Cheloneae, genera with single axillary
ﬂowers (2): (S27) Collinsia, (S28) Tonella, (S29) Uroskinnera.
Prof. R. Classen-Bockhoff (University of Mainz) is thanked for
the invitation to contribute a paper to the special issue on inﬂor-
escences of Annals of Botany. She as well as Prof. R. G. Olmstead
(University of Washington) and an anonymous reviewer are
thanked for critical reading of the manuscript and for suggesting
substantial improvements. Moreover, I am indebted to many pro-
fessional and amateur botanists for granting permission to use
their photos posted in the internet. For their names, details and
electronic links to websites see Figs S9– 29. Susanne Pamperl
and Susanne Sontag (University of Vienna) are thanked for tech-
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