Pollen Transfer within Flowers: How Pollen Is Secondarily Presented

Full text

PRIMERS IN THE PLANT SCIENCES
POLLEN TRANSFER WITHIN FLOWERS: HOW POLLEN IS SECONDARILY PRESENTED
Juliana Hanna Leite El Ottra,
1,
* João Felipe Ginefra Toni,†Pakkapol Thaowetsuwan,‡Patricia Dos Santos,§
Julius Jeiter,
2,
∥Louis Ronse De Craene,# Kester Bull-Hereñu,** and Regine Claßen-Bockhoff††
*Departamento de Botânica, Instituto de Biociências, Universidade de São Paulo, São Paulo, Brazil, and Universidade Virtual do Estado de São
Paulo, São Paulo, Brazil; †Faculty of Biosciences, University of Jena, Jena, Germany; ‡Department of Biology, Faculty of Science, Silpakorn
University, Sanam Chandra Palace Campus, Nakhon Pathom, Thailand; §cE3c Centre for Ecology, Evolution and Environmental Changes,
CHANGE Global Change and Sustainability Institute, Faculdade de Ciências, Universidade de Lisboa, Lisboa, Portugal, and Department of
Environmental Sciences, Botany, University of Basel, Basel, Switzerland; ∥Nees-Institute for Biodiversity of Plants, University of Bonn, Bonn,
Germany; #Royal Botanic Garden Edinburgh, Edinburgh, United Kingdom; **Fundación Flores, Ministro Carvajal 30, Santiago, Chile;
and ††Faculty of Biology, Institute of Organismic and Molecular Evolution, Johannes-Gutenberg University Mainz, Mainz, Germany
Guest Editor: Jennifer Ison
In several flowering plants, pollen is not delivered directly from the anthers to the pollinators but is transferred to
and presented by another floral structure, a phenomenon known as secondary pollen presentation (2PP). The new
pollen-presenting structure, that is, the pollen presenter, is usually the style, but other floral organs may also be in-
volved in this within-flower pollen transfer. This transfer usually happens during late bud development when pollen
is already released and thus is associated with extreme protandry. The evolutionary significance of 2PP is reflected
by its occurrence in at least 29 families of angiosperms, suggesting many instances of independent evolution. The
functional role of 2PP includes pollen saving and pollination accuracy, by reducing pollen loss and increasing the
precision of pollen transfer to the stigma. Many types of 2PP have been described, distinguished either by the struc-
ture of the pollen-presenting organs or by the different processes involved in pollen transfer and presentation to
pollinators. Herein three main mechanisms of 2PP are distinguished: (1) The deposition mechanism is the most com-
mon type of 2PP among angiosperms. Pollen is transferred to the presenter, often with the assistance of sticky se-
cretions or hair tracts. (2) In the brush mechanism, pollen is wiped off of the anthers by attached hairs as the style
elongates or moves relative to the anthers. (3) In the pump mechanism, the style acts as a piston, pushing the pollen
out of a cylinder formed by laterally contiguous floral organs. In this article, we present a short introduction into the
diversity and functional significance of 2PP, providing examples as case studies that can be easily observed in the lab
or field courses. Potential avenues for future research on 2PP are also discussed.
Keywords: secondary pollen presentation, pollen transfer, explosive mechanism, floral development, functional
morphology.
Online enhancement: video.
Introduction
In flowering plants, anthers produce pollen grains in their pol-
len sacs (thecae, which produce microspores that endogenously
generate the male gametophyte) and present them to floral visitors.
A successful pollination of the flower depends on the successful
transfer of pollen from the anthers tothestigmathroughpollenvec-
tors, which may be animals or else abiotic factors such as wind
and water. However, in several flowering plants, pollen is not
delivered directly from the anthers but is transferred and pre-
sented on another floral structure. This phenomenon is known
as secondary pollen presentation (2PP) and has been confirmed
in the present study to occur in 29 angiosperm families (table 1).
Pollen transfer takes place during late floral development, after
pollen has been released in the advanced bud stage but before
the stigma is receptive (i.e., protandry); more rarely, it can occur
at flower opening. Pollen is predominantly deposited onto the
style, from which it is presented during anthesis (fig. 1; table 1).
However, perianth organs (sepals, petals, or tepals) and even
parts of the stamens may also be involved in the within-flower
pollen transfer (table 1).
Mechanisms, Diversity, and Classification of 2PP
In flowers with 2PP, pollen follows a longer path from anther
dehiscence to pollen transfer, relative to flowers with primary
1
Author for correspondence; email: juliana.elottra@yahoo.com.
2
Current address: Faculty of Biology, TUD Dresden University of Tech-
nology, 01062 Dresden, Germany.
Manuscript received April 2023; revised manuscript received September 2023;
electronically published December 14, 2023.
International Journal of Plant Sciences, volume 185, number 1, January 2024. q2023 The University of Chicago. All rights reserved. Published by
The University of Chicago Press. https://doi.org/10.1086/727514
15

Table 1
Distribution of Plant Taxa with Secondary Pollen Presentation (2PP) across Main Angiosperm Clades and Summary of Important Related Features and References
Phylogenetic position: clade, order,
family (total no. genera)
a
No. genera with
2PP per family
Secondary pollen
presenter(s)
Mechanism of pollen
transfer to the
presenter
b
Main reference(s)
c
Magnoliids:
Magnoliales:
Myristicaceae (20) 1 (Myristica) Tepals, anther
appendages
Deposition Yeo 1993
Monocots:
Alismatales:
Hydrocharitaceae (18) 1 (Blyxa) Petals Deposition Yeo 1993
Zingiberales:
Zingiberales (58) 1 (Zingiber) Staminodes Deposition Fan et al. 2015
Heliconiaceae (1) 1 (Heliconia) Style Deposition Nascimento et al. 2018
Cannaceae (1) 1 (Canna) Style Deposition Stiles and Freeman 1993; Yeo 1993; Ladd 1994; Glinos and Cocucci 2011; Maruyama
et al. 2015
Maranthaceae (31) All Style Deposition Kunze 1984; Claßen-Bockhoff 1991; Claßen-Bockhoff and Heller 2008a, 2008b;
Pischtschan and Claßen-Bockhoff 2008; Ley and Claßen-Bockhoff 2009, 2012;
Pischtschan et al. 2010; Jerominek et al. 2015, 2018
Eudicots (basal grades):
Ranunculales:
Papaveraceae (44) 21 Style, petals Pump, deposition Dahl 1989; Yeo 1993; Ladd 1994; Yang et al. 2019
Proteales:
Proteaceae (80) 60 Style Deposition Yeo 1993; Ladd 1994; Vaughton 1996; Ladd et al. 1996, 1998; Devoto et al. 2006;
Gross and Caddy 2006; Gardner 2008; Welsford et al. 2016; Ladd and Bowen 2020
Rosids—Fabids clade:
Fabales:
Fabaceae (776) 46 Style, petals,
stamens
Pump, brush,
deposition
Arroyo 1981; Lavin and Delgado 1990; Yeo 1993; Westerkamp 1997; Westerkamp
and Weber 1999; Etcheverry et al. 2008, 2012a, 2012b; Galloni et al. 2007; Willmer
et al. 2009; Delgado-Salinas et al. 2011; Alemán et al. 2014; Stanley et al. 2016;
Fleming and Etcheverry 2017; Etcheverry and Vogel 2018
Polygalaceae (29) 3 Style Pump, deposition Yeo 1993; Ladd 1994; Westerkamp 1997; Westerkamp and Weber 1999; Castro et al.
2008a, 2008b; De Kock et al. 2018; Uluer et al. 2022
Malpighiales:
Rhizophoraceae (16) 2 Petals Deposition Yeo 1993; Aluri 2022
Cucurbitales:
Cucurbitaceae (98) 1 (Sechium) Petals Deposition Jadeja 2015
Rosids—malvids clade:
Myrtales:
Myrtaceae (131) 5 Style, staminodes Deposition Slater and Beardsell 1991; Yeo 1993; Beardsell et al. 1993; Ellis and Sedgley 1993;
Ladd 1994; O’Brien 1996; Ladd et al. 1999
Vochysiaceae (7) 1 (Vochysia) Style Deposition Yeo 1993; Gimenes 2007; Rodríguez and Sanoja 2008; Santos et al. 1997
16

Sapindales:
Rutaceae (161) 2 Staminodes Deposition El Ottra et al. 2016
Meliaceae (58) 3 Style Deposition Yeo 1993; Ladd 1994
Malvales:
Malvaceae (243) 4 Petals, staminode Deposition Yeo 1993; Brodie et al. 1998; Prenner 2002; Kubitzki and Bayer 2003; Won 2009
Cytinaceae (2) 1 (Cytinus) Perianth Deposition Yeo 1993
Superasterids:
Santalales:
Santalaceae (44) 1 (Santalum) Petals Deposition Howell et al. 1993
Asterids:
Cornales:
Loasaceae (21) 2 Filaments, ovary Deposition Yeo 1993
Ericales:
Ericaceae (126) 2 Petals, deposition Deposition Yeo 1993; Radcliffe et al. 2010
Asterids—lamiids clade:
Gentianales:
Apocynaceae (400) 15 Style Deposition Yeo 1993; Darrault and Schlindwein 2005; Araújo et al. 2011, 2014; Koptur et al.
2020; Ramos et al. 2022
Loganiaceae (13) 2 Style Deposition Endress et al. 1998; Erbar and Leins 1999; Shotts 2021
Rubiaceae (614) 38 Style Deposition Nilsson et al. 1990; Yeo 1993; Imbert and Richards 1993; Igersheim 1993; Ladd 1994;
Puff et al. 1996; Block and Igersheim 2001; Lin et al. 2012; Tilney et al. 2014; Xu
et al. 2022; Romero et al. 2022
Lamiales:
Orobanchaceae (104) 1 (Brandisia) Anther hairs Deposition Ren et al. 2018
Asterids—campanulids clade:
Asterales:
Campanulaceae (84) Nearly all Style, stamens Deposition, brush,
pump
Erbar 1989; Leins and Erbar 1990, 2005, 2006, 2010; Yeo 1993; Ladd 1994; Erbar
and Leins 1995; Koch and Helversen 2006; Vranken et al. 2014; Crowl et al. 2016;
Goodwillie et al. 2018; Eisen et al. 2017
Calyceraceae (8) All Style Deposition Erbar 1993; Ladd 1994; Leins and Erbar 2010
Goodeniaceae (12) 12 Style, corolla Pump, brush Erbar and Leins 1988; Leins and Erbar 1989; Yeo 1993; Ladd 1994; Xu et al. 2022
Asteraceae (1620) Nearly all Style, stamens,
petals
Deposition, brush,
pump
Small 1917; Pesacreta et al. 1991; Yeo 1993; Leins and Erbar 1990, 2006, 2010; Torres
and Galetto 2007; Shabir et al. 2013; Vogel 2015; Erbar and Leins 2015a, 2015b;
Erbar 2016; Katinas et al. 2016; Valentin-Silva et al. 2016; Rao 2017; Amorim et al.
2021; Erbar and Leins 2021; Cuffia et al. 2022
a
Phylogenetic positioning follows APG IV (2016), and total number of genera follows Stevens (2001–).
b
brush pbrush mechanism; pump ppump mechanism (or noodle-squeezer); deposition pdeposition mechanism.
c
See further references in the reviews of Yeo (1993), Howell et al. (1993), and Ladd (1994).
17

pollen presentation. This path can be separated into three differ-
ent processes: (1) pollen deposition (i.e., transfer) onto a pollen
presenter, (2) pollen presentation to pollinators, and (3) pollen
transfer to pollinators. Pollen deposition on the presenter can
be assisted by gravity, by vibration, or by mechanical anther
opening; in some flowers, pollen lies in a bowl-shaped structure
(e.g., Polygala, Polygalaceae [Westerkamp 1997]; Thalia,Ma-
rantaceae [Claßen-Bockhoff and Heller 2008a]), while in others,
pollen adheres to sticky substances or hairs (trichomes) of the
presenter (e.g., Darwinia, Myrtaceae [Beardsell et al. 1993]; many
Rubiaceae and Apocynaceae [Yeo 1993; Igersheim 1993]). In
the pollen presentation process, the whole pollen mass can be
freely accessible at once, or it can be apportioned by special
mechanisms (as in pump or brush mechanisms; fig. 1B–1E). Al-
ternately, pollen can be hidden (not presented at all) and trans-
ferred only when the pollinator releases a specific mechanism,
often by means of physical force (Córdoba and Cocucci 2011;
see the discussions of Rosids—Fabales and Monocots—Zingi-
berales in “Case studies”). Finally, in the pollen transfer process,
pollen can be transferred to the pollinator with the aid of sticky
substances usually secreted by the anthers (i.e., pollenkitt), as
they do in primary pollen presentation (Pacini and Franchi 2020;
e.g., Vangeria,Cephalanthus, Rubiaceae [Tilney et al. 2014; Ro-
mero et al. 2022]; Asteraceae [Pacini 1996]).
Many examples of 2PP have been described and classified
(Yeo 1993; Howell 1993; Ladd 1994; Leins and Erbar 2006;
Erbar and Leins 2021), either according to the structure of the
pollen-presenting organs (hereinafter called pollen presenters;
sensu Howell 1993; Ladd 1994) or according to the different
processes involved in pollen transfer and presentation. Struc-
tures and processes occur in different combinations and vary
among plant families, rendering a general classification difficult.
Fig. 1 Schematic representation of the three main mechanisms of secondary pollen presentation (2PP). A, Deposition mechanism. B,C, Brush
mechanism. B, Pollen (yellow) wiping out of the anthers (dark brown) by stylar hairs; note that the stylar hairs (black lines) are at the same level as
the pollen sacs (areas within the anthers bounded by hatched lines; arrows indicate direction of pollen shedding [i.e., pollen transfer to the pollen
presenter]). C, Pollen presentation to pollinators after style elongation; hatched arrows indicate the direction of elongation movement of the style
(light brown) relative to the anthers. D,E, Pump mechanism. D, Pollen deposition on top of the style; note that the style tip is below the level of the
pollen sacs. E, Pollen presentation to pollinators. Modified from Westerkamp (1989).
18 INTERNATIONAL JOURNAL OF PLANT SCIENCES

The most used concept of 2PP follows Yeo (1993; following
Westerkamp 1989) who distinguishes among the pollen heap,
the pump mechanism (or “noodle-squeezer”), and the pseudo-
stamen. The pollen heap represents any exposed deposit of pol-
len in a flower (a pure deposition mechanism; fig. 1A), which is
readily presented to pollinators. The pump mechanism occurs when
a pollen presenter acts as a piston that pushes pollen up and
presents it at the tip of a cylindrical structure (usually the anthers
or petals; fig. 1D,1E). The pseudostamen refers to a pollen pre-
senter that mimics a stamen (fig. 2A,2B).
The classification presented in this study modifies the types in-
troduced by Yeo (1993), as it is focused on the different processes
involved in 2PP mechanisms. We distinguish pure deposition
mechanisms from pump and brush mechanisms (see “Glossary”),
as the latter two mechanisms involve additional processes after
pollen transfer to the presenter, as detailed below.
Deposition mechanism. The deposition mechanism is the
most common type of 2PP in angiosperm families (fig. 1A; ta-
ble 1). In this mechanism, anthers and pollen-presenting organs
(e.g., styles, petals, or sterile stamens, i.e., staminodes) are closely
attached. Pollen is transferred to the pollen presenter in the late
bud stage and presented during anthesis, usually after anther
wilting. Examples of this phenomenon can be seen in the distal
part of the style in Grevillea (fig. 2A–2C; Proteaceae; Ladd 2020),
in the tepals and stamen appendages of Myristica (Myristicaeae;
Yeo 1993), or in petals and staminodes of Dombeya (Malvaceae;
Yeo 1993; Prenner 2002; table 1). There are many variations that
fall within the category of deposition mechanisms. Pollen deposi-
tion sites may occur in structurally different parts of the presenter.
For instance, the deposition process may be aided by hair tracts in a
specific position on the pollen presenter, typically the style, desig-
nated the stylar brush or stylar hairs (Yeo 1993; Howell 1993;
Ladd 1994). The stylar brush—nottobeconfusedwiththebrush
mechanism, discussed below—is found in most representatives of
Campanuloideae (fig. 2D,2E; e.g., Campanula,Jasione, Cam-
panulaceae; Erbar 1989; Yeo 1993; Vranken et al. 2014; Leins
and Erbar 2006).
Brush mechanism. In the brush mechanism, the two pro-
cesses of pollen release and style elongation, or upward move-
ment, occur simultaneously. Because anthers are at the same
level as the stylar hairs, pollen release and deposition go along with
the process of wiping off the pollen from the anthers by the
brush (i.e., the stylar hairs) of the pollen presenter (Erbar and
Leins 2021). The best-known examples are found in Asteraceae,
in which pollen is wiped off by stylar hairs during style elonga-
tion. Only part of the pollen mass is presented at a given time
(which is known as pollen partitioning) due to the gradual elon-
gation of the style (figs. 1B,1C,3E,3
F).
A stylar brush can be present in both deposition and brush
mechanisms. In the first, stylar hairs have the function to hold
pollen grains and release them in doses, while, in the second,
the brush moves along the anthers, or another tubular structure,
and wipes off the pollen, such as in Asteraceae (see the dis-
cussions of Asterids—Asterales and Rosids—Fabales in “Case
studies”). Therefore, detailed observation of the function of
the stylar brush is needed to avoid confusion between the brush
and deposition mechanisms.
Pump mechanism. In the pump mechanism, the style acts
as a piston, pushing the pollen out of a cylinder formed by lat-
erally contiguous organs, such as perianth parts or anthers
(figs. 1D,1E,3B,3C,3H,3I). For that to occur, the pollen must
first be shed within the cylinder in bud stage, or—more rarely—
at flower opening, and then any force that pushes the style up-
ward can lead to the release of pollen at the tip of the cylinder.
In this mechanism, pollen partitioning also takes place. Examples
include some Asteraceae, in which the style pumps the pollen out
of the anther tube by elongation, or some Fabaceae, in which the
petals form a keel, whose mechanical deformation produces an
upward movement of the style that squeezes the pollen out at
the keel’s tip (see the discussions of Asterids—Asterales and
Rosids—Fabales in “Case studies”).
Functional and Evolutionary Aspects of 2PP
Plants exhibiting 2PP are predominantly pollinated by ani-
mals (i.e., zoophilous). Bees are common pollinators among
Fig. 2 Illustration of the deposition mechanism. A–C,Grevillea
banksia R. Br. (Proteaceae). A, Flower at successive developmental
stages: i pflower bud; ii ppollen-presenting (“male”) phase; iii p
receptive (“female”) phase. Note that in i, the anthers are hidden within
the tip of the perianth lobes, and the distal part of the style (arrow) is hid-
den within this “hood”(arrowhead) formed by the perianth and an-
thers; in a later stage, ii, the style elongates exposing the pollen-presenting
area (arrow) outside this “hood”; iii, at the receptive phase the stigmatic
region protrudes. B, Detail of the tip of the style, at the pollen-presenting
phase (corresponds to A, ii); note pollen deposited on the tip of the pol-
len presenter (immature style; arrowhead). C, Same region at the recep-
tive phase (corresponds to A, iii); note the outgrowth of the stigmatic
region (arrow) and the absence of pollen on the presenter. D,E,Cam-
panula sp. (Campanulaceae). D, Pollen-presenting phase; note pollen
deposited on the many stylar hairs (arrowhead) and anthers wilted at
the bottom of the flower tube (arrow). E, Nonvisited flower at the recep-
tive phase (arrow, curled style lobes). Photos from J. H. L. El Ottra.
EL OTTRA ET AL.—POLLEN TRANSFER WITHIN FLOWERS 19

insects, but also flies, moths, hawk moths, crickets, and butter-
flies can be involved. Among vertebrates, birds, but also rodents,
marsupials, and lizards, may function as pollinators (Yeo 1993;
Sazima et al. 2005; Raju and Rao 2006; Vogel 2015; Ladd and
Bowel 2020; table 1). Some plant families may have still more
exclusive groups of pollinators, and reports on abiotic wind-
pollinated (anemophilous) groups presenting 2PP, such as a few
Asteraceae (e.g., Artemisia; Ladd 1994) and Proteaceae (Leuca-
dendron; Ladd 1994; Ladd and Bowen 2020), are rare. For the
last groupsthere is evidence that the mechanism isinherited from
biotic-pollinated relatives (Welsford et al. 2016), reaffirming the
close relation between zoophily and 2PP.
2PP has multiple roles in the reproductive biology of plant
species. One role is pollen saving, thanks to the increase of pre-
cision in the pollen transfer process (Yeo 1993; Howell 1993).
The pollen grain transports the male gametophyte and, at the
same time, serves for pollinator attraction and feeding. Balanc-
ing these two necessary roles via 2PP can thus be essential for the
plant’s reproductive success, especially in the case of blossoms
that suffer enormous pollen loss from pollen-collecting bees.
The different modes of 2PP show different degrees of pollen
saving. The deposition mechanism transfers all pollen grains
onto a freely accessible site within the flower. Once presented,
the pollen is not particularly saved, or precisely transferred, as
in other mechanisms. In the pump and brush mechanisms, pol-
len is retained within keels and floral tubes (Papaveraceae, many
families of Asterales, Fabaceae, Polygalaceae; table 1) pending
its gradual presentation in small doses (see the discussions of
Asterids—Asterales and Rosids—Fabales in “Case studies”),
an arrangement that can be advantageous for the plant because
it favors the transfer of male gametophytes to a variety of differ-
ent female mates (Harder and Wilson 1994; Castellanos et al.
2006). In flowers that hide pollen after its transfer to the pollen
presenter (e.g., Marantaceae), pollen loss is minimized and trans-
fer to pollinators is usually highly precise. Depending on flower
construction, the process of pollen transfer to pollinators can be
either reversible or irreversible (i.e., explosive [see “Glossary”];
see the discussions of Rosids—Fabales and Monocots—Zingi-
berales in “Case studies”). In the latter case, only a single pollina-
tor visit is required, which might be advantageous if pollinators
are scarce.
Considering that most species with 2PP use part of the style
as the pollen-presenting organ, and thereby place the area of
pollen presentation close to the receptive stigma, 2PP likely
optimizes the reproductive functions of a flower both by reduc-
ing pollen loss and by increasing the precision of pollen transfer
to the stigma (i.e., pollination accuracy; Ambruster et al. 2014).
Both functions increase the paternal and maternal chances of
the gametes and, thus, the male and female fitness of the plant.
However, the close proximity of stigma and pollen grains may
facilitate self-pollination, which, by decreasing the genetic vari-
ability of the progeny, can have negative effects for the fitness
of plants. Different flower mechanisms help minimize the poten-
tial for self-pollination in flowers with 2PP, such as (1) protan-
dry (i.e., dichogamy, or the temporal separation of pollen and
stigma maturation; Lloyd and Webb 1986), (2) herkogamy
(i.e., spatial separations of the male and female functions; Webb
and Lloyd 1986), and (3) self-incompatibility (i.e., the inability
of self-pollen to germinate or fertilize ovules within the same
genotype).
Case Studies
The evolutionary significance of 2PP is reflected by its occur-
rence in 29 families from 17 angiosperm orders (following the
classification of APG IV 2016), with many instances of indepen-
dent evolution (Yeo 1993). The phenomenon occurs scattered
in all three large clades (i.e., groups of lineages that share a
common and exclusive ancestor) of angiosperms—magnoliids,
monocots, and eudicots—but it is not found in the three early-
diverging orders of angiosperms (i.e., Amborellales, Nymphaeales,
and Austrobaileyales; fig. 4; table 1). In the case studies that fol-
low, three taxa are presented in detail to illustrate the necessary
arrangement among floral organs, including synoragnization of
the floral structures involved in 2PP. Synorganization means
that independent organs develop in such a closely related way
that they act as a functional unit in the mature flower (Endress
2016; see also “Glossary”).
Asterids—Asterales: Asteraceae and Allied Families
Among asterids, secondary pollen presentation is widespread
in the order Asterales, particularly in the families Asteraceae,
Campanulaceae (including the former Brunoniaceae and Lobelia-
ceae), Goodeniaceae, and Calyceraceae (Leins and Erbar 2006).
Types of 2PP in Asteraceae. Asteraceae stand out as one
of the largest angiosperms families, with 1620 genera and
Glossary
Brush mechanism. A type of 2PP in which pollen is wiped off of the anthers by hairs on the pollen presenter as it elongates and moves along the open
anthers.
Deposition mechanism. The most common type of secondary pollen presentation (2PP), in which the anthers and pollen-presenting organs are spatially
proximate. Pollen is deposited on the pollen presenter in the late bud stage, usually held in place by sticky substances or hairs, and presented at anthesis.
Explosive pollen transfer. A specificfloral mechanism of pollen transfer to pollinators, wherein a mechanical tension is created during floral development
between the anthers and other floral organs. This tension is released when the flower is visited, releasing pollen very rapidly, either in a cloud or else applied
directly onto floral visitors.
Pump mechanism. A type of 2PP in which the pollen presenter acts as a piston, pushing pollen up and presenting it at the tip of a cylindrical structure,
usually the anthers or petals.
Secondary pollen presenter. Any floral organ that receives self-pollen from stamens (anthers) after pollen release, taking over the function of presenting
pollen to the pollinators at flower opening.
Synorganization. Floral organs that are united, either by lateral contiguity or by tissue fusion. As a result, synorganization of floral organs may lead to a
higher complexity in flower structure (Endress 2016).
20 INTERNATIONAL JOURNAL OF PLANT SCIENCES

Fig. 3 Secondary pollen presentation in Asterales. A–F, Asteraceae. A–C,Emilia sonchifolia (L.) DC. A, Flower head, with many small flowers
surrounded by green involucral bracts. B,C, Pump mechanism. B, Detail of two flowers at different developmental stages: the pollen-presenting
phase with the anther cylinder indicated (arrowhead; left) and the receptive stage with spread stylar arms (arrow; right). C, Detail of B; note
the strings of pollen clumps being liberated between the anther appendages (arrowhead). D–F,Centratherum punctatum Cass. D, View onto a
flowering head, with flowers at the pollen-presenting phase (arrowhead) and the receptive phase (arrow). E,F, Brush mechanism. E, Initial phase
of style elongation (arrowhead) wiping off pollen from the inner side of the greenish anther cylinder. F, Later stage, with the style fully elongated
and loaded with pollen (arrowhead); note several sweeping trichomes (arrows). G–J, Campanulaceae, Hippobroma longiflora (L.) G. Don.
G, Long, tubular flower. H,I, Pump mechanism. Detail of flowers at the pollen-presenting phase. H, Anther cylinder (arrowhead) filled with pollen
(asterisk). I, Transection through the anther cylinder, with the closed style branches (arrows) below the pollen mass (asterisk) acting as the piston.
J, Detail of a flower at the receptive phase, with the opened style branches. Photos from J. H. L. El Ottra.

Fig. 4 Phylogenetic tree of angiosperms representing the occurrence of secondary pollen presentation (2PP). Different types of 2PP are plotted
in the column at the right side of the tree. Tree topology follows APG IV (2016), and colors indicate different angiosperm groups, as in Cole at al.
(2019).

25,040 species (Stevens 2001–). Among Asteraceae, 2PP is nearly
universal, excepting some wind-pollinated species (Thiele 1988;
Jeffrey 2009). The family is characterized by heads (capitula)
with small and at least partially tubular flowers arranged on a
common receptacle (fig. 3A,3D). The flowers have a sympetal-
ous corolla, five stamens, and a dimerous, inferior gynoecium.
Anthers are introrse (dehiscing toward the flower center) and
postgenitally fused to form a cylinder. Flowers are protandrous,
starting with the pollen-presenting (“male”) phase. In the young
flower, the two stylar arms (stylodia) are parallel to each other,
but they elongate through the cylinder during anthesis and
spread when they become receptive (fig. 3B). The surface features
of the stylar arms, hairy or smooth, and the relative timing of
pollen release determine which mechanism of 2PP will occur:
a brush mechanism, a pump mechanism, or a combination of
the two (Erbar and Leins 2021).
The brush mechanism follows the general scheme, with a style
elongating when the flower opens and wiping off pollen with its
stylar hairs. Pollen is finally presented above the anther cylinder
but below the still-closed and immature stigmas (fig. 3E,3F). In
the receptive phase, the two style branches spread and expose
the stigmatic region (fig. 3D).
In the pump mechanism, pollen is released into the anther cyl-
inder before the style begins to elongate. When the style does
elongate, the accumulated pollen deposited on top of it is pre-
sented to the pollinators. It is common that apical appendages of
anthers (i.e., projections, usually triangular in shape, at the tip of
each of the five anthers) push outward with the pump mecha-
nism, forming slits that portion the pollen at its tip (fig. 3B,
3C). The receptive phase follows, as in the brush mechanism
(fig. 3B).
Apart from these two main types, the most common and
widely known in the family, other subtypes of 2PP combine ele-
ments of the deposition, brush, and pump mechanisms. For
instance, a mixed mechanism of pump and brush occurs when
part of the pollen is deposited on top of the style and presented
via the pistonlike movement of the elongating style, as in the
pump mechanism, while the remaining pollen is swept up from
the anthers by the stylar brush as it glides along, as in the brush
mechanism (Yeo 1993; Erbar and Leins 2015a, 2015b, 2021;
Erbar 2016).
In some representatives of Asteraceae, especially those within
the Carduoideiae that use the pump mechanism (e.g., Centau-
rea,Carduus,Cirsum), 2PP may additionally involve an element
of response to floral visitors. Once the anther cylinder is touched,
it may contract, moving downward or toward contact, liberat-
ing a dose of pollen at its tip in this motion. This mechanism,
called irritating stamens, is reversible, because the cylinder re-
turns to its initial form after some time (Small 1917; Thiele
1988; Pesacreta et al. 1991; Yeo 1993; Leins and Erbar 2006;
López-Vinyallonga et al. 2009).
2PP in other families of Asterales. Other families in the or-
der Asterales developed 2PP potentially as the result of simi-
lar floral features they shared, including (1) tubular flowers,
(2) united anthers (fig. 3H; either fused or only laterally contiguous)
releasing pollen inside the tube, (3) protandry, and (4) gradual
growth of the style through the anther cylinder (fig. 3I–J). Al-
though this general pattern of development can be traced among
most Asterales with 2PP, each of its taxa presents specific partic-
ularities related to this phenomenon (Erbar and Leins 1988,
1995; Erbar 1989, 1993; Leins and Erbar 1990, 1997, 2003a,
2003b, 2005, 2010; Yeo 1993; Ladd 1994).
Asteraceae, Calyceraceae, and Goodeniaceae are closely re-
lated families and most likely inherited their similar forms of
2PP from a common ancestor within Asterales (Stevens 2001–).
In contrast, the similarities noted within Campanulaceae arose
independently, as this is a distantly related family from other
Asterales with 2PP (Leins and Erbar 2006). Still, the pump
mechanism of some Campanulaceae—subfamily Lobelioideae
(fig. 3H–3J)—is strikingly similar to that of Asteraceae but with
some differences in floral morphology. Flowers in these Cam-
panulaceae are usually larger, not in heads, and characterized
by a staminal tube that curves toward the lower side of the flower
(fig. 3G,3H). In this family, there are active mechanisms of pol-
len transfer triggered by pollinators (e.g., Lobelia; Brantjes 1983;
Erbar 1989; Leins and Erbar 1990, 2006, 2010;Yeo 1993; Koch
and von Helversen 2006; Crowl et al. 2016).
Rosids—Fabales
Among rosids, 2PP is most widespread in the order Fabales,
notably in the family Fabaceae (Leguminosae). 2PP also evolved
independently within Fabales in the family Polygalaceae, as in
other rosid orders (table 1).
Fabaceae is one of the largest angiosperm families, with
503 genera (Stevens 2001–); among these, 46 genera exhibit 2PP
(Arroyo 1981; Polhill 1981; Lavin and Delgado 1990). All spe-
cies with 2PP belong to the subfamily Papilionoideae and are
characterized by keel flowers.
Keel blossoms in Fabaceae. Most taxa of the papilionoids
have flowers organized as “keel blossoms”(also known as “keel
flowers”or “flag blossoms”; Westerkamp 1997). A keel flower
is monosymmetric and consists of five free petals arranged in
three functional parts: the large, upright, and usually showy
adaxial-median petal called the “flag”(“banner,”“vexillum”);
two lateral petals forming “wings”; and two abaxial petals (usu-
ally united) forming the keel, which encloses partially, or com-
pletely, the reproductive organs (fig. 5A–5C; Polhill 1981;
Endress 1994; Westerkamp 1997). Usually, 10 anthers (belong-
ing to two whorls) surround the style of the single carpel. Either
they are all fused to a tube (mainly in pollen blossoms, i.e., flow-
ers without nectar) or else the median adaxial stamen stands
alone, allowing access to nectar accumulated at the base of the
staminal tube (Westerkamp 1997; fig. 5B,5E,5I).
Pollinators, which are commonly bees in Fabaceae (Arroyo
1981; Yeo 1993), use the keel-wing formation as a landing plat-
form (fig. 5K). To get access to floral rewards, they need to open
the keel. Holding onto the wings, they insert their mouthparts in
the small opening between the base of the flag and the keel-wing
complex. This manipulation of the keel flower by the bee can
eventually activate or “trip”the mechanism and release the re-
productive organs from within the keel. In the simplest flower
construction, the weight of the bee is heavy enough to lower
the keel downward sufficiently to free the reproductive organs
from the keel. This enables the stigma to receive foreign pollen
from the pollinator and then the anthers to deposit self-pollen
onto the pollinator. This form of pollen transfer, called a valvu-
lar mechanism, is associated with primary pollen presentation
and is usually reversible (i.e., the floral display returns to its ini-
tial condition being tripped; fig. 5A,5C; Yeo 1993; Westerkamp
EL OTTRA ET AL.—POLLEN TRANSFER WITHIN FLOWERS 23

Fig. 5 Primary and secondary pollen presentation (2PP) in Fabales. A–L, Fabaceae. A–C,Tipuana tipu (Benth.) Kuntze with primary pollen
presentation. Note the arrangement of floral organs in a keel flower divided in two sectors: an upper (adaxial) sector, with large, upright, and
showy median petal (the “flag”) and a middle to lower (abaxial) sector used as landing platform by pollinators, composed of four petals: two lateral
“wing”petals and two inner, smaller petals that are usually postgenitally united to variable degrees, forming a “boatlike”structure (i.e., a “keel”or
“carina”). A, Flower not tripped. B, Dissected flower; k pseparated keel petals; w pwing petals; f pflag; ft pfurrow in the filament tube.
C, Flower being artificially tripped (primary pollen presentation, valvular). D–M, Taxa with 2PP. D,E, Deposition mechanism in Lathyrus sylvestris
L. D, Flower longitudinally opened, with part of the petals removed, exposing the pollination organs at the time pollen is being deposited on the

1997). However, in some papilionoids, the process of pollen
transfer is irreversible. In these cases, the keel and the concealed
reproductive organs press in opposite directions, creating a
springlike tension (Yeo 1993; Ladd 1994). Contact from a pol-
linator can release this tension all at once, transfering a cloud
of pollen to the pollinator (Yeo 1993) in a so-called explosive
mechanism that offers only a single chance for pollen exchange
(López et al. 1999; Suzuki 2003; Galloni et al. 2007; Alemán
et al. 2014). Explosive pollen transfer to pollinators occurs both
in primary (e.g., Desmodium grahamii A. Gray; Miguel-Peñaloza
et al. 2019) and in 2PP (e.g., Spartium junceum L.; fig. 5G;
Galloni et al. 2007) mechanisms.
2PP in Fabaceae. Early studies in floral biology had already
recognized different pollen presentation mechanisms in papilio-
noids (Knuth 1908). At least three forms of 2PP occur (Polhill
1981); they developed based on a keel-blossom construction (Ar-
royo 1981). Present in protandrous species, these three forms
most likely evolved from the valvular mode of pollen presenta-
tion and often demand a specific manipulation from pollinators.
In the deposition mechanism, pollen is usually transferred to
hairs at the tip of the style (e.g., Crotalaria; Le Roux and Van
Wyk 2012). As in the valvular mechanism, the keel moves
downward when a pollinator lands on the flower, but the stiff
reproductive organs do not move with the keel and are thus
freed from it (Westerkamp 1997). First, the stigma touches the
pollinator (usually at its ventral side) and receives foreign pol-
len; then, self-pollen is loaded from the anthers onto the same
body part of the pollinator. In contrast to the valvular mecha-
nism, the stylar brush holds the pollen grains, and only some
are transferred during a single visit (fig. 5D–5F). This mecha-
nism is reversible, which allows in pollen to be issued in small
doses to pollinators on each visit (Arroyo 1981; Yeo 1993;
Westerkamp 1993, 1997; López et al. 1999; Galloni et al. 2007;
Etcheverry et al. 2012a). Additionally, it is common that the stylar
brush is positioned only at the side that touches the pollinators
(fig. 5F; e.g., Colutea,Coursetia,Robinia [Lavin and Delgado
1990]; e.g., Pisum [Yeo 1993]).
In some papilionoids, the deposition mechanism is combined
with an explosive pollen transfer to the pollinators (fig. 5J; video 1,
available online). It is commonly reported in representatives
of tribes Desmodieae (e.g., some species of Desmodium), Indi-
gofereae (Indigofera), and Genisteae (e.g., Cytisus,Spartium;
fig. 5J) and in some Mucuna (Phaseolae; Arroyo 1981; Yeo
1993; Agostini et al. 2006; Fleming and Etcheverry 2017). Al-
though this form of pollen transfer is usually irreversible, there
may be some exceptions, such as that described in Desmodium
setigerum E. Mey. (Willmer et al. 2009; Stanley et al. 2016).
Although a brush mechanism is commonly described in some
papilionoids, it is not clear whether this mechanism actually
occurs in the group. Some of the relevant studies lack sufficient
detail on the process of pollen transfer to the presenter—a stylar
brush (Arroyo 1981)—but describe the action of stylar hairs as
sweeping pollen from inside the keel (Leppik 1966; Lavin and
Delgado 1990; Etcheverry et al. 2008). Other studies do not
mention a “brush mechanism”but instead describe pollen de-
position on a stylar brush, on a stylar brush and thus a depo-
sition mechanism in our definition (Knuth 1908; Yeo 1993).
Conflicting descriptions regarding this issue are found in the
literature for the same taxa, such as Lathyrus,Pisum,Vicia,
Phaseolus, and Robinia (Lavin and Delgado 1990 vs. Knuth
1908; Yeo 1993). Alternately, other descriptions do not allow
a clear determination of the mechanism (e.g., Vicia sepium
L.; Knuth 1908). Therefore, the process of pollen transfer to
the stylar brush should be critically reevaluated in papilionoids.
In our view, this mechanism could be interpreted as a within-
flower pollen transfer based on deposition, followed by pollen
presentation in doses based on the successive release of pollen
grains from the stylar brush each time the keel is depressed.
A pump mechanism occurs in papilionoids, in which pollen
accumulates at the tip of a very narrow keel. In this flower con-
struction, the keel acts as the cylinder while the style acts as the
piston, pumping pollen out of the keel once a flower is tripped
(fig. 5G–5I; artificially tripped in fig. 5L). In some taxa, the dis-
tal part of the stamens is swollen, contributing further to the
piston function (e.g., Lotus,Lembotropis,Lupinus,Crotalaria
[Arroyo 1981; Yeo 1993]; in Crotalaria spectabilis Röth, this
function seems to be performed by the smaller anthers [fig. 5I]).
Similar to the deposition mechanism, the pump mechanism is
reversible and presents pollen in doses (Faegri and van der Pijl
1979; Haynes and Mesler 1984; Harder and Thomson 1989;
Harder 1990; Harder and Wilson 1994; Proctor et al. 1996).
The different types of 2PP are considered to have evolved
multiple times within papilionoids, although more studies are
needed to corroborate this view (Fleming and Etcheverry 2017).
Also, explosive mechanisms are often viewed (incorrectly, from
our perspective) as a process of 2PP itself (e.g., Arroyo 1981;
Yeo 1993; Westerkamp 1993, 1997). In our view, what is “ex-
plosive”in such mechanisms is the means of pollen transfer to
the pollinator, not the within-flower transfer of pollen to the
pollen presenter, which is better classified as a simple deposition
in cases where 2PP occurs.
Keel flowers and 2PP in Polygalaceae (Fabales). Although
the functioning of the floral mechanism in Polygalaceae is strik-
ingly similar to the mechanism in Fabaceae, this functional sim-
ilarity is the result of convergent evolution (table 1; Westerkamp
and Weber 1999; Uluer et al. 2022). The flower morphology
differs considerably between flowers of the two families. In Po-
lygalaceae, the attractive flags are usually formed by two inner
sepals and two smaller upper petals—as opposed to Fabaceae’s
single median petal—while the keel is formed by one large petal
folded medially—versus two united petals in Fabaceae (fig. 5M;
Yeo 1993; Westerkamp 1997; Westerkamp and Weber 1999).
stylar tip. E, Detail of the staminal tube (arrow) and of the 2PP by the style (arrowhead). F,Colutea arborescens L. Detail of the style with an
adaxial brush. G–I, Pump mechanism in Crotalaria spectabilis Röth. G, Flower not tripped. H, Detail of a tripped flower with one wing petal
removed to reveal a mass of pollen being pumped out of the opening of the keel (arrowhead). I, Flower longitudinally dissected, exposing the pol-
lination organs, pollen deposit at the tip of the keel (white arrowhead), and nectar chamber at the base of the flower (white arrow); note the smaller
anther at the base of the piston (blue arrowhead). J,Spartium junceum L., artificially tripped flower at the moment of explosive pollen release
(video 1). K,L,Astragalus sp. K, Bee visiting a flower. L, Flower being artificially tripped, showing how to stimulate the pump mechanism.
M, Polygonaceae. Keel flower of Polygala myrtifolia (Polygalaceae); note the brushlike outgrowth at the tip of the keel (arrow). Photos A–C,G–L,
Mfrom J. H. L. El Ottra; D–Ffrom Claßen-Bockhoff (2023).
EL OTTRA ET AL.—POLLEN TRANSFER WITHIN FLOWERS 25

Fig. 6 Secondary pollen presentation (2PP) in Zingiberales. A–D,Canna indica L., Cannaceae. A,Inflorescence. B, Flower with one half-fertile
stamen (hf), petaloid staminodes (ps), one modified into a labellum (la), and another in the flat style (st). C, In the bud, the fertil theca (th) is closely
attached to the style (st). D, In the adult flower, self-pollen is secondarily presented on the style. Note the gluey regions of the style (arrowheads),
involved in pollen transfer to pollinators. E–M, Marantaceae. E, Flower diagram. The outer whorl of the androecium forms one or two outer
staminodes (ost), the inner whorl the fleshy stamindode (fs), the hooded staminode (hs), and the monothecate stamen (sm). Additional organs: style
(st), petal (pe), and sepal (se). F–M,Calathea lutea (Aubl.) Schult. F, Flower from the front view, revealing the empty theca (th), that the style is
enveloped by the hooded staminode (hs), and that the trigger (tr) impedes access to nectar. G, Style in the unreleased stage; tension is shown by its

Some Polygalaceae species have a keel with a knob- or brushlike
outgrowth at its tip, thought to function as a lever, prying open
the keel when a bee lands on it (De Kock 2018).
Monocots—Zingiberales
Among the monocots, there are at least seven families with
2PP, three of them belonging to the order Zingiberales (table 1).
Whereas in the Heliconiaceae, pollen is deposited on the stylar
hairs during anthesis (Nascimento et al. 2018), in the recently
diverging lineages Cannaceae and Marantaceae, pollen has al-
ready been transferred to the style in the bud stage. Flowers in
these sister families follow the general trimerous and pentacyclic
(i.e., two perianth whorls, two androecial whorls, and one gy-
noecial whorls) organization of monocot flowers, but with some
important peculiarities: flowers are asymmetrical, the perianth is
inconspicuous and functionally replaced by petaloid staminodes
(i.e., sterile stamens that resemble petals), and the androecium
forms only a single half-fertile (monothecate) stamen while all
other androecial organs are petaloid staminodes (fig. 6A–6E).
The theca matures already in the bud, illustrating an extreme
case of protandry.
In Canna indica (Cannaceae), there are four to five flat sta-
minodes, one of them forming the labellum (in this case, a pet-
aloid staminode positioned in the lower median region of the
flower) of the flower (fig. 6A,6B). During the bud stage, the ma-
ture theca is positioned close to the style and is held in position
by the staminodes. Ongoing growth processes build a mechan-
ical pressure by which the pollen is squeezed out of the theca and
deposited on the flat side of the style (fig. 6C,6D). The style has
two gluey regions, one at the stigma at its tip and a second one
positioned laterally (fig. 6D). Pollen exchange requires two visits
of pollinating hummingbirds (Glinos and Cocucci 2011). In a
young flower, the bird comes into contact with the secondarily
presented pollen grains when it leaves the flower. It first touches
the gluey tissue and then gets pollen deposited on its beak. In an
older flower, the bird transfers pollen to the stigma when it ap-
proaches the flower, as the style has bent downward and now
presents the stigma in the flower entrance.
In Marantaceae, the androecial elements are functionally
modified. One or two outer staminodes serve as petaloid structures
(fig. 6E,ost).Thefleshy staminode forms the roof of the flower
and is named after the stiff ridge guiding the growth of the floral
organs in bud stage (fig. 6E,6I,6J, fs). The hooded staminode is
characterized by the namesake cap at its tip. It envelops the style
and hides the pollen plate at the back of its head where the pollen
is secondarily presented (fig. 6E,6F,6G). The fertile theca of the
anther is already faded in the open flower (fig. 6E,6F,th).
Secondary pollen transfer occurs a few hours before flow-
ering. It is linked to an early maturation of the theca (fig. 6I)
and its position above the style. All staminodes and perianth
elements closely surround the theca-style complex guiding their
further development (fig. 6J,6K). When the style elongates, it
presses against the theca and forces it to open, thus releasing pol-
len grains onto the pollen plate at the back of the style head
(fig. 6M; Claßen-Bockhoff and Heller 2008a). After pollen de-
position, all structures elongate to their final length. The style
elongates more than the jacketing hooded staminode, and this
difference produces a tension often visible in the arched shape
adopted by the staminode (fig. 6G; Pischtschan and Claßen-
Bockhoff 2008a). The pollen lies hidden in the hood and is spa-
tially separated from the stigmatic cavity by a gluey gland. This
arrangement is an example of herkogamy and avoids self-
pollination (Ley and Claßen-Bockhoff 2012).
The style tension is released when a pollinator (most com-
monly, a bee or bird; Kennedy 2000; Clausager and Borchsenius
2003; Ley and Claßen-Bockhoff 2009) touches the trigger ap-
pendage of the hooded staminode, which restricts access to nec-
tar (fig. 6F, tr). Pollen transfer occurs when the style springs for-
ward in a split second (“explosive mechanism”; Kunze 1984;
Claßen-Bockhoff 1991) and scrapes the foreign pollen off of
the pollinator into its stigmatic cavity (i.e., a cavity where pollen
will later start to germinate). Facilitated by the style movement,
self-pollen is pasted on the same site of the pollinator’s mouth-
parts by a glue originating from the gland on the stylar head
(fig. 6J,6L, gl). This unique pollen transfer process demands
an incredibly precise synorganization (i.e., organs forming a sin-
gle functional unit) between floral structures and pollinator body
proportions, as well as synchronization between style movement
and pollinator behavior. The explosive style movement isa purely
mechanical process caused by the overstretched upper epidermis
(Jerominek et al. 2015, 2018). It is irreversible and results in the
curled shape of the released style (fig. 6H).
The evolution of 2PP within Zingiberales went along with an
increase in complexity of the processes and structures involved.
Cannaceae and Marantaceae evolved an extreme form of pro-
tandry in which 2PP is already underway in the bud stage.
Whereas Canna flowers need two pollinator visits for pollen
transfer, flowers in Marantaceae get only a single chance to ex-
change pollen. Precision is paramount and supported by this
unique form of 2PP and pollen transfer.
How to Study Secondary Pollen Presentation
To properly study 2PP, it is necessary to study the flower con-
struction, to determinate the relative timing of anther dehiscence
and stigma receptivity, and to observe the pollinators’behavior.
If flowers are not protandrous, 2PP can be excluded. Addition-
ally, it is necessary to dissect late buds and young flowers to rec-
ognize the moment of anther opening and to see whether pollen
is presented by the anther (primary pollen presentation) or de-
posited on another floral structure (2PP). For this purpose, it
may be helpful to isolate the buds with nylon mesh bags to
overstretched shape, and the self-pollen (op) is hidden by the hood of the hooded staminode. H, Style in the released, curled stage; foreign pollen
(fp) has been transferred to the stigmatic orifice. I–M, Bud stages. I, The theca develops first and overtops the young style (so pstigmatic orifice);
both structures are closely packed by the surrounding staminodes (bud artificially opened; fs pfleshy staminode). J, The style elongates and forms a
glue at its tip (gl); note trigger appendage at the hooded staminode (tr). K, Several hours before anthesis, the elongating style presses against the
mature theca. L, Pollen is squeezed out of the theca and deposited at the pollen plate (pp) at the opposite side of the stigmatic orifice. M, Self-pollen
is deposited onto the pollen plate and hidden by the hood of the hooded staminode. Photos A,Dfrom Claßen-Bockhoff (2023), F–Hfrom Claßen-
Bockhoff and Heller (2008a).
EL OTTRA ET AL.—POLLEN TRANSFER WITHIN FLOWERS 27

forestall any disturbances that animal visitation might cause in
the within-flower pollen transfer. If a pollen presenter is puta-
tively identified, pollination studies should be conducted, to an-
alyze flower-pollinator interaction. If pollen transfer from the
pollen presenter to the stigma is verified, 2PP can be confirmed.
As the process of pollen transfer often occurs inside syn-
organized floral structures (e.g., inside an anther tube, or keel)
or in temporally concealed floral surfaces (i.e., inside the floral
bud itself), different methods of visualization can be employed
to aid the understanding of 2PP. Furthermore, surfaces of floral
organs, trichomes, and glands are best identified using micro-
scopic techniques. Methods such as scanning electron microscopy,
serial section for light microscopy, and microcomputed tomog-
raphy (mCT) allow the study of the floral surfaces involved
throughout flower development, as well as the contiguity and
position among different floral organs at various developmental
stages of 2PP (e.g., Igersheim 1993; Leins and Erbar 2003a, 2005;
Pischtschan and Claßen-Bockhoff 2008). If a modern approach,
such as mCT, is employed, the organs can be observed in situ
(i.e., without any preparation of the different bud stages; e.g.,
Jeiter et al. 2020; Vasile et al. 2022), offering a new perspective
on the study of 2PP. Microscopic and developmental studies
might not be indispensable to the study of 2PP, but they add im-
portant information on the (micro-) morphological basis of the
underlying mechanisms and provide insight into processes oth-
erwise hidden from the unaided eye of the observer.
Concluding Remarks
Although 2PP has been known since early studies of floral bi-
ology from the nineteenth century, much remains to be investi-
gated. Some plant groups need further detailed studies that
would complete the necessary steps (discussed above) to confirm
2PP. This would be the case of Arisarum (Araceae; Galil and
Meiri 1992; Howell 1993), Xyris (Xyridaceae; Yeo 1993; Rem-
izowa et al. 2012); Meliosma,Ophiocaryon (Sabiaceae et al.
2007; Thaowetsuwan et al. 2017), and species of Salvia (Kriebel
et al. 2023). In other groups, 2PP must be reevaluated (e.g., Zan-
tedeschia, Araceae [Pacini and Hesse 2004], some urticalean
rosids [Pedersoli et al. 2019], and Leptomeria and Jordania,
Cervantesiaceae [Luna et al. 2022]) in light of a clear conceptual
framework like that presented in this study. In addition, the
common occurrence of 2PP in pollination systems that involve
explosive pollen transfer to pollinators, such as in Fabaceae,
Papaveraceae, Rhizophoraceae, and others, is striking and
deserves further study. Finally, the efficiency of 2PP in terms
of pollen saving and pollen transfer to pollinators is rarely ad-
dressed in quantitative ecological studies (e.g., Eisen et al.
2017; Fleming and Etcheverry 2017). For such scientific analysis,
all steps of pollen transfer from anther opening to stigma need to
be well documented, quantified, and statistically analyzed. Be-
cause the study of 2PP is the study of functional floral morphol-
ogy, a full understanding requires that structural, developmental,
and ecological studies be combined.
Acknowledgments
J. H. L. El Ottra acknowledges Professor José R. Pirani (IB-
USP) for assistance with the identification of plants exhibiting
secondary pollen presentation and Roberto Baptista P. Almeida
(IB-USP) for helping to film the phenomenon. J. H. L. El Ottra
is also grateful to Professor Claudia Erbar (COS) for providing
bibliography on Asteraceae and to two anonymous reviewers
for their suggestions on an earlier version of the manuscript.
Literature Cited
Agostini K, M Sazima, I Sazima 2006 Bird pollination of explosive
flowers while foraging for nectar and caterpillars. Biotropica
38:674–678.
Alemán M, T Figueroa-Fleming, Á Etcheverry, S Sühring, P Ortega-
Baes 2014 The explosive pollination mechanism in Papilionoideae
(Leguminosae): an analysis with three Desmodium species. Plant
Syst Evol 300:177–186.
Aluri JSR 2022 A review of the reproductive ecology of mangrove
plant species. Pages 33–70 in SC Das, T Pullaiah, EC Ashton, eds.
Mangroves: biodiversity, livelihoods and conservation. Springer,
Singapore.
Ambruster WS, SA Corbet, AJM Vey, S-J Liu, S-Q Huang 2014 In the
right place at the right time: Parnassia resolves the herkogamy dilemma
by accurate repositioning of stamens and stigmas. Ann Bot 113:97–103.
Amorim MD, D da Silva Costa, DRP Krahl, E Fischer, AR Rech 2021
Gongylolepis martiana, an Asteraceae pollinated by bats in the Ama-
zon. Plant Biol 23:728–734.
APG (Angiosperm Phylogeny Group) IV 2016 An update of the An-
giosperm Phylogeny Group classification for the orders and families
of flowering plants. Bot J Linn Soc 181:1–20.
Araújo L, ZGM Quirino, I Machado 2011 Fenologia reprodutiva, bio-
logia floral e polinização de Allamanda blanchetii, uma Apocynaceae
endêmica da Caatinga. Rev Bras Bot 34:211–222.
——— 2014 High specialisation in the pollination system of Man-
devilla tenuifolia (JC Mikan) Woodson (Apocynaceae) drives the ef-
fectiveness of butterflies as pollinators. Plant Biol 16:947–955.
Arroyo MTK 1981 Breeding systems and pollination biology in Legu-
minosae. Pages 723–769 in RM Polhill, PH Raven, eds. Advances in
legume systematics. Royal Botanic Gardens, Kew.
Beardsell DV, SP O’Brien, EG Williams, RB Knox, DM Calder 1993
Reproductive biology of Australian Myrtaceae. Aust J Bot 41:511–
526.
Block P, A Igersheim 2001 Stigma of the African genera Rutidea and
Nichallea (Rubiaceae-Ixoroideae-Pavetteae): highly modified recep-
tive surfaces. Int J Plant Sci 162:567–578.
Brantjes NBM 1983 Regulated pollen issue in Isotoma, Campanu-
laceae, and evolution of secondary pollen presentation. Acta Bot
Neerl 32:213–222.
Brodie S, M Cheek, M Staniforth 1998 Plate 334. Trochetiopsis ebenus.
Curtis’s Bot Mag 15:27–36.
Castellanos MC, P Wilson, SJ Keller, AD Wolfe, JD Thomson 2006
Anther evolution: pollen presentation strategies when pollinators
differ. Am Nat 167:288–296.
Castro S, P Silveira, L Navarro 2008aHow does secondary pollen
presentation affect the fitness of Polygala vayredae (Polygalaceae)?
Am J Bot 95:706–712.
——— 2008bHow flower biology and breeding system affect the re-
productive success of the narrow endemic Polygala vayredae Costa
(Polygalaceae). Bot J Linn Soc 157:67–81.
Claßen-Bockhoff R 1991 Untersuchungen zur Konstruktion des Best-
äubungsapparates von Thalia geniculata (Marantaceen). Bot Acta
104:183–193.
28 INTERNATIONAL JOURNAL OF PLANT SCIENCES

——— 2023 Die Pflanze: Morphologie, Entwicklung und Evolution
von Vielfalt. Springer, Heidelberg. (In German.)
Claßen-Bockhoff R, A Heller 2008aFloral synorganization and sec-
ondary pollen presentation in four Marantaceae from Costa Rica.
Int J Plant Sci 169:745–760.
——— 2008bStyle release experiments in four species of Maran-
taceae from the Golfo Dulce area, Costa Rica. Stapfia 88:557–
571.
Clausager K, F Borchsenius 2003 The Marantaceae of Sabah, north-
ern Borneo. Kew Bull 58:647–678.
Cole TCH, HH Hilger, PF Stevens 2019 Angiosperm phylogeny poster
(APP): flowering plant systematics. https://doi.org/10.7287/peerj
.preprints.2320.
Córdoba SA, AA Cocucci 2011 Flower power: its association with
bee power and floral functional morphology in papilionate legumes.
Ann Bot 108:919–931.
Crowl AA, NW Miles, CJ Visger, K Hansen, T Ayers, R Haberle, N
Cellinese 2016 A global perspective on Campanulaceae: biogeo-
graphic, genomic, and floral evolution. Am J Bot 103:233–245.
Cuffia C, MC Cerino, PA Tomas, E de Luján Exner 2022 Winter flow-
ers for bees: reproductive biology of Trixis praestans (Asteraceae).
Plant Syst Evol 308:27.
Dahl ÅE 1989 Taxonomic and morphological studies in Hypecoum
sect. Hypecoum (Papaveraceae). Plant Syst Evol 163:227–280.
Darrault RO, C Schlindwein 2005 Limited fruit production in Han-
cornia speciosa (Apocynaceae) and pollination by nocturnal and diur-
nal insects. Biotropica 37:381–388.
De Kock C, C Minnaar, K Lunau, P Wester, C Verhoeven, MJ Schulze,
MR Randle, C Robson, RH Bolus, B Anderson 2018 The functional
role of the keel crest in Polygala myrtifolia (Polygalaceae) and its ef-
fects on pollinator visitation success. S Afr J Bot 118:105–111.
Delgado-Salinas A, M Thulin, R Pasquet, N Weeden, M Lavin 2011
Vigna (Leguminosae) sensu lato: the names and identities of the
American segregate genera. Am J Bot 98:1694–1715.
Devoto M, NH Montaldo, D Medan 2006 Mixed hummingbird: long-
proboscid-fly pollination in “ornithophilous”Embothrium cocci-
neum (Proteaceae) along a rainfall gradient in Patagonia, Argentina.
Austral Ecol 31:512–519.
Eisen K, AL Case, CM Caruso 2017 Variation in pollen-dispensing
schedules of Lobelia siphilitica. Int J Plant Sci 178:79–84.
Ellis MF, M Sedgley 1993 Gynodioecy and male sterility in Euca-
lyptus leucoxylon F. Muell. (Myrtaceae). Int J Plant Sci 154:314–
324.
El Ottra JHL, JR Pirani, ER Pansarin 2016 Floral biology and polli-
nation of two sympatric species of Galipeinae (Galipeeae, Rutaceae)
endemic to the Brazilian Atlantic Forest. Flora 221:107–116.
Endress PK 1994 Diversity and evolutionary biology of tropical flow-
ers. Cambridge University Press, Cambridge.
——— 2016 Development and evolution of extreme synorganization
in angiosperm flowers and diversity: a comparison of Apocynaceae
and Orchidaceae. Ann Bot 117:749–767.
Endress M, A Igersheim, P Garnock-Jones 1998 Sexual dimorphism
and secondary pollen presentation in Geniostoma ligustrifolium (Ge-
niostomaceae). Am J Bot 85:1–183.
Erbar C 1989 On the early floral development and the mechanisms of
secondary pollen presentation in Campanula,Jasione and Lobelia.
Bot Jahrb Syst 111:29–55.
——— 1993 Studies on the floral development and pollen presenta-
tion in Acicarpha tribuloides with a discussion of the systematic
of the family Calyceraceae. Bot Jahrb Syst 115:325–350.
——— 2016 Bi- to multi-seriate stylar hairs in Eremothamneae, Ol-
denburgieae, Stifftieae, and Wunderlichieae (Asteraceae). Syst Bot
40:1144–1158.
Erbar C, P Leins 1988 Studien zur Blütenentwicklung und Pollen-
präsentation bei Brunonia australis Smith (Brunoniaceae). Bot Jahrb
Syst 110:263–282.
——— 1995 Portioned pollen release and the syndromes of secondary
pollen presentation in the Campanulales-Asterales-complex. Flora
190:323–338.
——— 1999 Secondary pollen presentation and a curious rupture of
the style in Spigelia (Spigeliaceae, Gentianales). Plant Biol 1:389–402.
——— 2015aCuticular patterns on stylar hairs in Asteraceae: a new
micromorphological feature. Int J Plant Sci 176:269–284.
——— 2015bDiversity of styles and mechanisms of secondary pollen
presentation in basal Asteraceae: new insights in phylogeny and func-
tion. Flora 217:109–130.
——— 2021 Style diversity in Asteraceae: morphology, anatomy, phy-
logeny, and function. Schweizerbart Science, Stuttgart.
Etcheverry AV, MM Alemán, T Figueroa-Fleming 2008 Flower mor-
phology, pollination biology and mating system of the complex flower
of Vigna caracalla (Fabaceae: Papilionoideae). Ann Bot 102:305–316.
Etcheverry AV, MM Alemán, T Figueroa-Fleming, D López-Spahr, CA
Gómez, CN Yáñez, DM Figueroa-Castro, P Ortega-Baes 2012aPollen-
ovule ratio and its relationship with other floral traits in Papilionoideae
(Leguminosae): an evaluation with Argentine species. Plant Biol 14:171–
178.
Etcheverry AV, DM Figueroa-Castro, T Figueroa-Fleming, MM Ale-
mán, VD Juárez, D López-Spahr, CN Yáñez, CA Gómez 2012bGen-
eralised pollination system of Erythrina dominguezii (Fabaceae:
Papilionoideae) involving hummingbirds, passerines, and bees. Aust
J Bot 60:484–494.
Etcheverry AV, S Vogel 2018 Interactions between the asymmetrical
flower of Cochliasanthus caracalla (Fabaceae: Papilionoideae) with
its visitors. Flora 239:141–150.
Faegri K, L van der Pijl 1979 Principles of pollination ecology. Per-
gamon, Oxford.
Fan Y-L, WJ Kress, Q-J Li 2015 A new secondary pollen presentation
mechanism from a wild ginger (Zingiber densissimum) and its func-
tional roles in pollination process. PLoS ONE 10:e0143812.
Fleming FT, AV Etcheverry 2017 Comparing the efficiency of pollina-
tion mechanisms in Papilionoideae. Arthropod Plant Interact 11:273–283.
Galil J, L Meiri 1992 Effect of Wetting on the Germinability of the Pol-
len of Arisarum vulgare Targ.-Tozz. Flora 186:145–152.
Galloni M, L Podda, D Vivarelli, G Cristofolini 2007 Pollen presen-
tation, pollen-ovule ratios, and other reproductive traits in Mediter-
ranean Legumes (Fam. Fabaceae-Subfam. Faboideae). Plant Syst
Evol 266:147–164.
Gardner RO 2008 Gender dimorphism in Toronia toru (Proteaceae).
N Z Nat Sci 33:43–46.
Gimenes M 2007 Polinização de Vochysia lucida C. Presl (Vochysiaceae)
em uma área de restinga na Bahia. Rev Bras Entomol 51:465–470.
Glinos E, AA Cocucci 2011 Pollination biology of Canna indica (Can-
naceae) with particular reference to the functional morphology of the
style. Plant Syst Evol 291:49–58.
Goodwillie C, HR Patel, DM Dvorak 2018 Does stigma curvature
promote delayed selfing? an experimental investigation in Triodanis
perfoliata (Campanulaceae). Plant Biol 20:199–204.
Gross CL, HAR Caddy 2006 Are differences in breeding mechanisms
and fertility among populations contributing to rarity in Grevillea
rhizomatosa (Proteaceae)? Am J Bot 93:1791–1799.
Harder LD 1990 Pollen removal by bumble bees and its implications
for pollen dispersal. Ecology 71:1110–1125.
Harder LD, JD Thomson 1989 Evolutionary options for maximizing
pollen dispersal of animal-pollinated plants. Am Nat 133:323–344.
Harder LD, WG Wilson 1994 Floral evolution and male reproduc-
tive success: optimal dispensing schedules for pollen dispersal by
animal-pollinated plants. Evol Ecol 8:542–559.
Haynes J, M Mesler 1984 Pollen foraging bybumblebees: foraging pat-
terns and efficiency on Lupinus polyphyllus. Oecologia 61:249–253.
Howell GJ, AT Slater, RB Knox 1993 Secondary pollen presentation
in angiosperms and its biological significance. Aust J Bot 41:417–
438.
EL OTTRA ET AL.—POLLEN TRANSFER WITHIN FLOWERS 29

Igersheim A 1993 Gynoecium development in Rubiaceae-Vanguerieae,
with particular reference to the “stylar head”-complex and secondary
pollen presentation. Plant Syst Evol 187:175–190.
Imbert FM, JH Richards 1993 Protandry, incompatibility, and sec-
ondary pollen presentation in Cephalanthus occidentals (Rubiaceae).
Am J Bot 80:395–404.
Jadeja S 2015 First record on secondary pollen presentation in the Cu-
curbitaceae family. Plant Ecol Evol 148:297–299.
Jeffrey C 2009 Evolution of Compositae flower. Pages 131–138 in V
Funk, A Susanna, TF Stuessy, RJ Bayer, eds. Systematics, evolution,
and biogeography of Compositae. International Association for Plant
Taxonomy, Vienna.
Jeiter J, S Langecker, M Weigend 2020 Towards an integrative under-
standing of stamen–corolla tube modifications and floral architecture
in Boraginaceae s.s. (Boraginales). Bot J Linn Soc 193:100–124.
Jerominek M, R Claßen-Bockhoff 2015 Electrical signals in prayer
plants (Marantaceae)? insights into the trigger mechanism of the ex-
plosive style movement. PLoS ONE 10:e0126411.
Jerominek M, M Will, R Claßen-Bockhoff 2018 Insights into the in-
side: a quantitative histological study of the explosively moving
style in Marantaceae. Front Plant Sci 9:1695.
Katinas L, MP Hernández, AM Arambarri, VA Funk 2016 The ori-
gin of the bifurcating style in Asteraceae (Compositae). Ann Bot
117:1009–1021.
Kennedy H 2000 Diversification in pollination mechanisms in the Ma-
rantaceae. Pages 335–344 in LW Karen, DA Morrison. Monocots:
systematics and evolution. Collingwood, Sydney.
Knuth P 1908 Handbook of flower pollination. Clarendon, Oxford.
Koch CU, O von Helversen 2006 Secondary pollen presentation in
Cuban bat pollinated Lobelia: precise deposition of pollen and me-
chanical species isolation. Bot Jahrb Syst Pflanzengesch Pflanzengeogr
127:105–114.
Koptur S, B Barrios, I Valdes, M Nusrat 2020 A fishing expedition to
discover the pollinators of several subtropical Apocynaceae. Appl
Plant Sci 8:e11326.
Kriebel R, BT Drew, R Claßen-Bockhoff, KJ Sytsma 2023 Evolution
of anther connective teeth in sages (Salvia, Lamiaceae) under bee
and hummingbird pollination. Flora 298:152199.
Kubitzki K, C Bayer 2003 The families and genera of vascular plants.
Flowering plants: dicotyledons: Capparales, Malvales and on-
betalain Caryophyllales. Vol 5. Springer, Heildelberg.
Kunze H 1984 Vergleichende Studien an Cannaceen-und Maranta-
ceenblüten. Flora 175:301–318.
Ladd PG 1994 Pollen presenters in the flowering plants: form and
function. Bot J Linn Soc 115:165–195.
Ladd PG, AJ Alkema, GJ Thomson 1996 Pollen presenter morphology
and anatomy in Banksia and Dryandra. Aust J Bot 44:447–471.
Ladd PG, BJ Bowen 2020 Pollen release in the Proteaceae. Plant Syst
Evol 306:81.
Ladd PG, I Nanni, GJ Thomson 1998 Unique stigmatic structure in
three genera of Proteaceae. Aust J Bot 46:479–488.
Ladd PG, JAN Parnell, G Thomson 1999 Anther diversity and func-
tion in Verticordia DC. (Myrtaceae). Plant Syst Evol 219:79–97.
Lavin M, SA Delgado 1990 Pollen brush of Papilionoideae (Legu-
minosae): morphological variation and systematic utility. Am J
Bot 77:1294–1312.
Leins P, C Erbar 1989 On the development and secondary pollen pre-
sentation in Selliera radicans Cav. (Goodeniaceae) [Zur Blütenen-
twicklung und sekundären Pollenpräsentation bei Selliera radicans
Cav. (Goodeniaceae)]. Flora 182:43–56.
——— 1990 On the mechanisms of secondary pollen presentation in
the Campanulales-Asterales complex. Bot Acta 103:87–92.
——— 1997 Floral developmental studies: some old and new ques-
tions. Int J Plant Sci 158(suppl):S3–S12.
——— 2003aFloral developmental features and molecular data in
plant systematics. Pages 419–432 in T Stuessy, V Mayer, E Hörandl,
eds. Deep morphology: toward a renaissance of morphology in plant
systematics. Guther, Liechtenstein.
——— 2003bThe pollen box in Cyphiaceae (Campanulales). Int J
Plant Sci 164(suppl):S321–S328.
——— 2005 Floral morphological studies in the south African
Cyphia stenopetala Diels (Cyphiaceae). Int J Plant Sci 166:207–217.
——— 2006 Secondary pollen presentation syndromes of the Aster-
ales: a phylogenetic perspective. Bot Jahrb Syst Pflanzengesch Pflan-
zengeogr 127:83–104.
——— 2010 Flower and fruit: morphology, ontogeny, phylogeny,
function and ecology. Schweizerbart, Stuttgart.
Leppik EE 1966 Floral evolution and pollination in the Leguminosae.
Ann Bot Fenn 299–308.
Le Roux MM, BE Van Wyk 2012 The systematic value of flower
structure in Crotalaria and related genera of the tribe Crotalarieae
(Fabaceae). Flora 207:414–426.
Ley AC, R Claßen-Bockhoff 2009 Pollination syndromes in African
Marantaceae. Ann Bot 104:41–56.
——— 2012 Floral synorganization and its influence on mechanical
isolation and autogamy in Marantaceae. Bot J Linn Soc 168:300–322.
Lin H, X Fan, X Zhou, J Gao 2012 Self-interference is reduced in a
secondary pollen presentation species, Duperrea pavettifolia (Ru-
biaceae). Flora 207:895–902.
Lloyd DG, CJ Webb 1986 The avoidance of interference between the
presentation of pollen and stigmas in angiosperms. I. Dichogamy.
N Z J Bot 24:135–162.
López J, T Rodríguez-Riaño, A Ortega-Olivencia, JA Devesa, T Ruiz
1999 Pollination mechanisms and pollen-ovule ratios in some Genisteae
(Fabaceae) from southwestern Europe. Plant Syst Evol 216:23–47.
López-Vinyallonga S, I Mehregan, N Garcia-Jacas, O Tscherneva, A Su-
sanna, JW Kadereit 2009 Phylogeny and evolution of the Arctium-
Cousinia complex (Compositae, Cardueae-Carduinae). Taxon 58:153–
171.
Luna ML, DG Gutiérrez, MA Grossi, JPR Giacosa, MD Arana 2022
Possible functions of the petal hairs of Jodina Hook. & Arn. ex
Meisn. (Cervantesiaceae) during pollination. Flora 291:152071.
Maruyama PK, J Vizentin-Bugoni, B Dalsgaard, M Sazima 2015 Pol-
lination and breeding system of Canna paniculata (Cannaceae) in a
montane Atlantic rainforest: asymmetric dependence on a hermit
hummingbird. Acta Bot Bras 29:157–160.
Miguel-Peñaloza A, A Delgado-Salinas, K Jiménez-Durán 2019 Polli-
nation biology and breeding system of Desmodium grahamii
(Fabaceae, Papilionoideae): functional aspects of flowers and bees.
Plant Syst Evol 305:743–754.
Nascimento TO, PC Silva, V Loges, S Mariotto, W Krause, CA Silva
2018 Secondary pollen presentation and floral traits of Heliconia
psittacorum. Ornam Hortic 24:451–458.
Nilsson LA, E Rabakonandrianina, B Pettersson, J Ranaivo 1990
“Ixoroid”secondary pollen presentation and pollination by small
moths in the Malagasy treelet Ixora platythyrsa (Rubiaceae). Plant
Syst Evol 170:161–175.
O’Brien SP 1996 Timetable of stigmatic receptivity and development
and pollen tube growth in Chamelaucium uncinatum (Myrtaceae).
Aust J Bot 44:649–659.
Pacini E 1996 Tapetum types in the Compositae: form and function.
Pages 21–28 in DHJ Hind, H Beentije, GV Pope, eds. Proceedings
of the International Compositae Conference. Vol 1. Royal Botanic
Gardens, Kew.
Pacini E, GG Franchi 2020 Pollen biodiversity: why are pollen grains
different despite having the same function? a review. Bot J Linn Soc
193:141–164.
Pacini E, M Hesse 2004 Cytophysiology of pollen presentation and
dispersal. Flora 199:273–285.
Pedersoli GD, FM Leme, VG Leite, SP Teixeira 2019 Anatomy solves
the puzzle of explosive pollen release in wind-pollinated urticalean
rosids. Am J Bot 106:489–506.
30 INTERNATIONAL JOURNAL OF PLANT SCIENCES

Pesacreta TC, VI Sullivan, KH Hasenstein, JM Durand 1991 Thig-
monasticity of thistle staminal filaments. Protoplasma 163:174–180.
Pischtschan E, R Claßen-Bockhoff 2008 Setting-up tension in the
style of Marantaceae. Plant Biol 10:441–450.
Pischtschan E, AC Ley, R Claßen-Bockhoff 2010 Ontogenetic and
phylogenetic diversification of the hooded staminode in Maran-
taceae. Taxon 59:1111–1125.
Polhill RM 1981 Papilionoideae. Pages 191–204 in RM Polhill, PH Raven,
eds. Advances in legume systematics. Pt 1. Royal Botanic Gardens, Kew.
Prenner G 2002 Secondary pollen presentation on petals of Dom-
beya cacuminum Hochr. (Sterculiaceae). Stapfia 80:323–326.
Proctor M, P Yeo, A Lack 1996 The natural history of pollination.
Harper Collins New Naturalist, London.
Puff C, E Robbrecht, R Buchner, P Block 1996 A survey of secondary
pollen presentation in the Rubiaceae. Opera Bot Belg 7:369–402.
Radcliffe CA, JM Affolter, HY Wetzstein 2010 Floral morphology
and development in Georgia Plume, Elliottia racemosa (Ericaceae),
a rare coastal plain endemic. J Am Soc Hortic Sci 135:487–493.
Raju AJS, SP Rao 2006 Explosive pollen release and pollination as a
function of nectar-feeding activity of certain bees in the biodiesel
plant, Pongamia pinnata (L.) Pierre (Fabaceae). Curr Sci 90:960–967.
Ramos SMB, MAS Carvalho, CCF Possobom, EFA Almeida, S
Nietsche 2022 Biology and structure of flowers in Adenium obesum
(Forssk.) Roem. & Schult. (Apocynaceae) accessions with notes on the
significance of these features for floriculture. Braz J Bot 45:689–702.
Rao MM 2017 Secondary pollen presentation, psychophily and ane-
mochory in some Asteraceae species. J Palynol 53:25–82.
Remizowa MV, AN Kuznetsov, SP Kuznetsova, PJ Rudall, MS Nuraliev,
DD Sokoloff 2012 Flower development and vasculature in Xyris
grandis (Xyridaceae,Poales): a case study for examining petal diversity
in monocot flowers with a double perianth. Bot J Linn Soc 170:93–111.
Ren Y-Q, G-Y Deng, X-P Li, T Marczewski, Y-P Ma 2018 Second-
ary pollen presentation on anther hairs enhances reproductive fit-
ness in Brandisia hancei, a protogynous perennial with autonomous
selfing. Plant Ecol Divers 11:373–381.
Rodríguez L, E Sanoja 2008 Fenología, biología floral y de polini-
zación de especies de la familia Vochysiaceae en la Guayana Ve-
nezolana. Acta Bot Venez 31:331–336.
RomeroMF,RMSalas,AMGonzalez 2022 Floralanatomy,embryology,
seed, and fruit development in Cephalanthus (Naucleeae-Rubiaceae), with
emphasis on C. glabratus. Protoplasma 259:237–261.
Santos ML, A de Paula Afonso, PE Oliveira 1997 Biologia floral de
Vochysia cinnamomea Pohl (Vochysiaceae) em cerrados do Triân-
gulo Mineiro, MG. Braz J Bot 20:127–132.
Sazima I, C Sazima, M Sazima 2005 Little dragons prefer flowers to
maidens: a lizard that laps nectar and pollinates trees. Biota Neotrop
5:185–192.
Shabir PA, IA Nawchoo, AA Wani 2013 Floral phenology, secondary
pollen presentation and pollination mechanism in Inula racemosa
(Angiosperms: Asteraceae). J Threat Taxa 5:4498–4503.
Shotts G 2021 Floral biology of Alabama’sSpigelia species (Family
Loganiaceae). MS thesis. Auburn University, Auburn, AL.
Slater AT, DV Beardsell 1991 Secondary pollen presentation in the
Chamelaucium alliance of the Myrtaceae: a compact substigmatic
ring in Chamelaucium. Aust J Bot 39:229–239.
Small J 1917 The origin and development of the Compositae. New
Phytol 16:253–276.
Stanley DA, M Otieno, K Steijven, ES Berlin, T Piironen, P Willmer, C
Nuttman 2016 Pollination ecology of Desmodium setigerum (Fa-
baceae) in Uganda: do big bees do it better? J Pollinat Ecol 19:43–49
Stevens PF 2001–Angiosperm phylogeny website. http://www.mobot
.org/MOBOT/research/APweb/.
Stiles FG, CE Freeman 1993 Patterns in floral nectar characteristics of
some bird-visited plant species from Costa Rica. Biotropica:191–205.
Suzuki N 2003 Significance of flower exploding pollination on the re-
production of the Scotch broom, Cytisus scoparius (Leguminosae).
Ecol Res 18:523–532.
Thaowetsuwan P, EN Honorio Coronado, LP Ronse De Craene 2017
Floral morphology and anatomy of Ophiocaryon, a paedomorphic
genus of Sabiaceae. Ann Bot 120:819–832.
Thiele E 1988 Bau und Funktion des Antheren Griffel Komplexes der
Compositen. Diss Bot 117.
Tilney PM, AE van Wyk, CF van der Merwe 2014 The epidermal
cell structure of the secondary pollen presenter in Vangueria in-
fausta (Rubiaceae: Vanguerieae) suggests a functional association
with protruding onci in pollen grains. PLoS ONE 9:e96405.
Torres C, L Galetto 2007 Style morphological diversity of some
Asteraceae species from Argentina: systematic and functional impli-
cations. J Plant Res 120:359–364.
Uluer DA, F Forest,S Armbruster, JA Hawkins 2022 Reconstructing an
historical pollination syndrome: keel flowers. BMC Ecol Evol 22:45.
Valentin-Silva A, MAS Godinho, KC Cruz, SM Lelis, MF Vieira
2016 Three psychophilous Asteraceae species with distinct repro-
ductive mechanisms in southeastern Brazil. N Z J Bot 54:498–510.
Vasile M-A, M Weigend, A Ziegler, J Jeiter 2022 Flower and fruit
development of three species of Hydrophylleae sheds new light on
flower evolution in Hydrophyllaceae. Int J Plant Sci 183:331–347.
Vaughton G 1996 Pollination disruption by European honeybees in
the Australian bird-pollinated shrub Grevillea barklyana (Protea-
ceae). Plant Syst Evol 200:89–100.
Vogel S 2015 Vertebrate pollination in Compositae: floral syndromes
and field observations. Stapfia 103:5–26.
Vranken S, R Brys, M Hoffmann, H Jacquemyn 2014 Secondary pol-
len presentation and the temporal dynamics of stylar hair retraction
and style elongation in Campanula trachelium (Campanulaceae).
Plant Biol 16:669–676.
Wanntorp L, LP Ronse De Craene 2007 Flower development of Me-
liosma (Sabiaceae): evidence for multiple origins of pentamery in
the eudicots. Am J Bot 94:1828–1836.
Webb CJ, DG Lloyd 1986 The avoidance of interference between the
presentation of pollen and stigmas in angiosperms. II. Herkogamy.
N Z J Bot 24:163–178.
Welsford MR, N Hobbhahn, JJ Midgley, SD Johnson 2016 Floral trait
evolution associated with shifts between insect and wind pollination in
the dioecious genus Leucadendron (Proteaceae). Evolution 70:126–
139.
Westerkamp C 1989 Von Pollenhaufen, Nudelspritzen und Pseudo-
Staubblättern: Blütenstaub aus zweiter Hand. Palmengarten 53:146–149.
——— 1993 Co-operation between the asymmetric flower of Lathyrus
latifolius (Fabaceae-Vicieae) and its visitors. Phyton 33:121–137.
——— 1997 Keel blossoms: bee flowers with adaptations against
bees. Flora 192:125–132.
Westerkamp C, A Weber 1999 Keel flowers of the Polygalaceae and
Fabaceae: a functional comparison. Bot J Linn Soc 129:207–221.
Willmer P, DA Stanley, K Steijven, IM Matthews, CV Nuttman
2009 Bidirectional flower color and shape changes allow a second
opportunity for pollination. Curr Biol 19:919–923.
Won H 2009 Phylogenetic position of Corchoropsis Siebold & Zucc.
(Malvaceae s.l.) inferred from plastid DNA sequences. J Plant Biol
52:411–416.
Xu Y, Z Luo, J Wang, N Pei, D Zhang 2022 Secondary pollen presen-
tation: more than to increase pollen transfer precision. J Syst Evol
60:1027–1036.
Yang S, G Chu, X Shi, S Wang 2019 Elaborated pollen packaging and
dispensing mechanism induced by petal architecture from a Pa-
paveraceae species. PeerJ 7:e7066.
Yeo P 1993 Secondary pollen presentation: form, function and evolu-
tion. Springer, Vienna.
EL OTTRA ET AL.—POLLEN TRANSFER WITHIN FLOWERS 31