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PART OF A SPECIAL ISSUE ON INFLORESCENCES
Pair-flowered cymes in the Lamiales: structure, distribution and origin
Anton Weber*
Department of Structural and Functional Botany, Faculty Center of Biodiversity, University of Vienna, Rennweg 14, A-1030 Vienna,
Austria
* For correspondence. E-mail anton.weber@univie.ac.at
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
racemic inflorescences).
†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.
INTRODUCTION
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-
phologically misinterpreted.
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
Plantaginaceae.
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
presented.
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
#The Author 2013. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved.
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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).
Weber —Pair-flowered cymes in the Lamiales1578
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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,
Fig. S16).
Sequence of flower opening. The first flower to open in the PFC
units is the terminal flower (T
1
). Then the front flower (F
1
)
follows. Finally, the terminal flowers of the subsequent cyme
units (T
2
) open, followed by their front-flowers (F
2
). The se-
quence of flower opening is thus descending (T
1
F
1
T
2
).
Morphological diversity
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
cupulata,C. burbidgei).
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-
velopment.
TTTF
ABC
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.
Weber —Pair-flowered cymes in the Lamiales 1579
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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
of T
1
,F
1
and T
2
on both sides (T
2
—T
1
F
1
—T
2
; Fig. 3D). The
biflorous cyme produces T
1
F
1
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
(F
2
T
2
—T
1
F
1
—T
2
F
2
). Reduction of flowers does not occur
along the standard series, but is irregular (e.g. T
2
—T
1
—T
2
F
2
).
Reduction of the front-flowers may be to a rudiment or may be
complete (‘phylogenetic loss’).
T1
a
ab
b
b
b
ba
a
a
a
b
T2
A
B
C
D
E
F
G
F2
F1T3
F3T5
F5
F2
T2
T1
T1
F1
T1T2
F1
T1
F1
T1
T1
T2F2
F1T2
T4F4
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
1
–T
n
, terminal flowers of cyme units; F
1
–F
n
, front-
flowers, a,b, transverse bracteoles.
T1
A
B
ab
T3T4
T5
T2F2
F1
F3
F4
F5
FIG.2.Penstemon digitalis. (A) Pair-flowered cyme in fruiting stage.
(B) Corresponding diagram. T
1
–T
5
, terminal flowers (fruits) of cyme units
1–5; F
1
–F
5
, 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.
Ontogeny
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,
1998;Endress, 2010).
A
Aa Ab
Ac
B
Ba
?
Bb
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’, ‘Vorderblu
¨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-
dinary cymes.
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-
pressed.
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-
atic side.
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
CALCEOLARIACEAE, SANANGO,
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 Lamiales.
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.
Calceolariaceae
Presently, the family includes two genera, Calceolaria (ap-
proximately 250 spp., Mexico to Patagonia) and Jovellana
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(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
1
F
1
). 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
occurs.
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.
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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
2
T
2
—T
1
F
1
—T
2
F
2
).
Gesneriaceae
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.
Plantaginaceae
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’
inflorescences, respectively.
The two remaining tribes are Russelieae (Russelia,
Tetranema) and Cheloneae (?Brookea,Chelone,Chionophila,
Collinsia,Keckiella,Nothochelone,Pennellianthus,Penstemon,
TABLE 1. Tribes of Plantaginaceae that include genera with pair-flowerd cymes (no cymes of the ordinary type are present in the
family)
Tribe Genus
Species
number
PFCs present/
absent (+/–)
Single, bracteolate
flowers present/absent
(+/–)
Single, ebracteolate
flowers present/absent
(+/–) Distribution
Russelieae Russelia 50 (?) +– – Central and tropical South
America
Tetranema 5+– – Costa Rica to Mexico
Clade between
Russelieae and
Cheloneae
Uroskinnera 4– – +Guatemala to Mexico
Cheloneae Chelone 4– +– Eastern half of North America,
incl. Florida
Chionophila 2– +– Central North America
(Montana, Idaho, Wyoming,
Colorado)
Collinsia 20 – – +North America, many spp. in
California
Keckiella 7++ – South-western North America
(California, Oregon, Nevada,
Arizona)
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
California
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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.
Plantaginaceae-Russelieae
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-
lationship.
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
˜or, 1995;
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).
Plantaginaceae-Cheloneae
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),
Chelone,Chionophila,Collinsia,Tonella,Uroskinnera (single
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
type.
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
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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
T
1
F
1
, the occasional third and fourth flower are the lateral
flowers T
2
; 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
1
) there is a front-flower in
median-abaxial position (F
1
) and two lateral flowers (T
2
) emer-
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
present.
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-
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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
present.
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
(Keckiella,Nothochelone,Pennellianthus,Penstemon) and
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
LAMIALES
‘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,
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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
spikes.
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
No. of
genera
No. of
species
Inflorescence determinate
(+)/indeterminate (– )
Thyrse
with PFCs
Thyrse with
ordinary cymes
Raceme (s.l.), flowers
singly in leaf/bract axils
Plocospermataceae 1 1 –? ?
Carlemanniaceae 2 5 +??
Oleaceae 25 600 +++
Tetrachondraceae 2 3 ++
Peltanthera 1+
Calceolariaceae 2 270 – ++ +
Sanango 1–++ +
Gesneriaceae .150 3300 – ++ (+)(+)
Plantaginaceae (incl. Callitrichaceae,
Hippuridaceae, Globulariaceae)
90 2000 – +++
Scrophulariaceae (incl. Myoporaceae,
Buddlejaceae, Selaginaceae)
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,
Scutellaria,Tinnea,
Teucrium p.p.,)
Phrymaceae 11 160 – +(Leucocarpus,
Hemichaena)
++
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-
flowered system.
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.
T
AB
C
EF
D
T
3’ 3’
3’
2
T
3
33 2
2
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:
A. Weber.
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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).
Conclusions
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
Plantaginaceae.
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
‘Core’ Lamiales
Stilbaceae
Scrophulariaceae
PFC Plantaginaceae
(Russelieae + Cheloneae)
Gesneriaceae
Sanango
Calceolariaceae
Peltanthera
Tetrachondraceae
Oleaceae
Carlemanniaceae
Plocospermataceae
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).
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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
too strongly.
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
Pennellianthus.
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.
PFC
A
B
C
PFC
PFC
PFC
PFC
R
R
Plantaginaceae
Gesneriaceae
Calceolariaceae
Calceolariaceae
Tetrachondraceae
Tetrachondraceae
R
Gratiolaceae
Plantaginaceae
Gesneriaceae
Peltanthera
Peltanthera
Sanango
Sanango
Gesneriaceae
Calceolariaceae
Oleaceae
Peltanthera
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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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.
CONCLUDING REMARKS
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
Angelonieae
Gratioleae
R
Russelieae
Cheloneae
Angelonieae
Gratioleae
R
Russelieae
Cheloneae
PFC
PFC
A
B
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.
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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
Sinningia (Gesneriaceae).
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
Bull-Heren
˜u, 2013) must and will play a significant role in
future concepts and understanding of inflorescence morphology.
SUPPLEMENTARY DATA
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.
ACKNOWLEDGMENTS
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-
nical assistance in the preparation of figures, computer work and
sourcing of literature.
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