Uniting pattern and process in plant-animal mutualistic networks: a review

BACKGROUND

Ecologists and evolutionary biologists are becoming increasingly interested in networks as a framework to study plant-animal mutualisms within their ecological context. Although such focus on networks has brought about important insights into the structure of these interactions, relatively little is still known about the mechanisms behind these patterns.

SCOPE

The aim in this paper is to offer an overview of the mechanisms influencing the structure of plant-animal mutualistic networks. A brief summary is presented of the salient network patterns, the potential mechanisms are discussed and the studies that have evaluated them are reviewed. This review shows that researchers of plant-animal mutualisms have made substantial progress in the understanding of the processes behind the patterns observed in mutualistic networks. At the same time, we are still far from a thorough, integrative mechanistic understanding. We close with specific suggestions for directions of future research, which include developing methods to evaluate the relative importance of mechanisms influencing network patterns and focusing research efforts on selected representative study systems throughout the world.

The ant-pollination system of Cytinus hypocistis (Cytinaceae), a Mediterranean root holoparasite

BACKGROUND AND AIMS

The genus Cytinus is composed of rootless, stemless and leafless parasites whose flowers are only visible during the reproductive period when they arise from the host tissues. Most of the taxa occur in Madagascar and South Africa, where mammal pollination has been suggested for one species. There is only one species in the Mediterranean region, and its pollination system has been unknown. Here, a long-term field observation study is combined with experimental pollination treatments in order to assess the pollination biology and reproductive system in the Mediterranean species Cytinus hypocistis.

METHODS

Field studies were carried out in six populations in southern Spain over 4 years. Temporal and spatial patterns of variation in the composition and behaviour of floral visitors were characterized. Pollen loads and pollen viability were observed, and exclusion and controlled-pollination treatments were also conducted.

KEY RESULTS

Cytinus hypocistis is a self-compatible monoecious species that relies on insects for seed production. Ants were the main visitors, accounting for 97.4 % of total floral visits, and exclusion experiments showed that they act as true pollinators. They consistently touched reproductive organs, carried large pollen loads and transported viable pollen, although the different ant species observed in the flowers differed in their pollination effectiveness. The abundance of flying visitors was surprisingly low, and only the fly Oplisa aterrima contributed to fruit production and cross-pollination.

CONCLUSIONS

Mutualistic services by ant are essential for the pollination of Cytinus hypocistis. Although this parasite does not exhibit typical features of the 'ant-pollination syndrome', many other characteristics indicate that it is evolving to a more specialized ant-pollination system. The striking interspecific differences in the pollination systems of Mediterranean Cytinus (ant-pollinated) and some South African Cytinus (mammal-pollinated) make this genus an excellent model to investigate the divergent evolution of pollination systems in broadly disjunct areas.

Tropical deforestation alters hummingbird movement patterns

Reduced pollination success, as a function of habitat loss and fragmentation, appears to be a global phenomenon. Disruption of pollinator movement is one hypothesis put forward to explain this pattern in pollen limitation. However, the small size of pollinators makes them very difficult to track; thus, knowledge of their movements is largely speculative. Using tiny radio transmitters (0.25 g), we translocated a generalist tropical 'trap-lining' hummingbird, the green hermit (Phaethornis guy), across agricultural and forested landscapes to test the hypothesis that movement is influenced by patterns of deforestation. Although, we found no difference in homing times between landscape types, return paths were on average 459+/-144 m (+/-s.e.) more direct in forested than agricultural landscapes. In addition, movement paths in agricultural landscapes contained 36+/-4 per cent more forest than the most direct route. Our findings suggest that this species can circumvent agricultural matrix to move among forest patches. Nevertheless, it is clear that movement of even a highly mobile species is strongly influenced by landscape disturbance. Maintaining landscape connectivity with forest corridors may be important for enhancing movement, and thus in facilitating pollen transfer.

Limited mate availability decreases reproductive success of fragmented populations of Linnaea borealis, a rare, clonal self-incompatible plant

BACKGROUND AND AIMS

Small populations of rare plant species are increasingly reported to have high levels of reproductive failure. The objective of this study was to understand the principal constraints on sexual reproduction in small fragmented populations of a rare clonal self-incompatible plant.

METHODS

The pollinator spectrum, diversity of flower colour, natural pollination and fruit-set levels of L. borealis were examined in Scotland. Artificially crossed seed production was compared within and between different flower colour types and patches.

KEY RESULTS

Linnaea borealis was pollinated by a diverse spectrum of insect species and the principal pollinators were muscid, syrphid and empid flies which mostly moved only small distances (<0.25 m) between flowers when foraging. Natural pollination levels were high, indicating high pollinator effectiveness, but fruit set was very low in most patches. Flower colour diversity was low in most patches and only those with a diversity of flower colour types had high fruiting success. Pollination experiments showed L. borealis to be highly self-incompatible and artificial crosses within and between patches and flower colour types confirmed that low fruit success was the result of a lack of compatible mates and limited pollen movement between them. Evidence of isolation from pollen exchange was apparent at as little as 6 m and severe at 30 m and beyond.

CONCLUSIONS

Limited mate availability and isolation from pollen exchange compromise the reproductive success of fragmented populations of L. borealis in Scotland. A diversity of compatible mates situated within close proximity (<6 m) is the key requirement to ensure high natural fruiting success. This study emphasizes that an understanding of the breeding system, pollinator spectrum and potential for interconnectivity via pollinator movement are fundamental to identify isolation distances and to establish when conservation intervention is necessary for rare species.

Sources of floral scent variation: can environment define floral scent phenotype?

Studies of floral scent generally assume that genetic adaptation due to pollinator-mediated natural selection explains a significant amount of phenotypic variance, ignoring the potential for phenotypic plasticity in this trait. In this paper, we assess this latter possibility, looking first at previous studies of floral scent variation in relation to abiotic environmental factors. We then present data from our own research that suggests among-population floral scent variation is determined, in part, by environmental conditions and thus displays phenotypic plasticity. Such an outcome has strong ramifications for the study of floral scent variation; we conclude by presenting some fundamental questions that should lead to greater insight into our understanding of the evolution of this trait, which is important to plant-animal interactions.

Effect of pollination on floral longevity and costs of delaying fertilization in the out-crossing Polygala vayredae Costa (Polygalaceae)

BACKGROUND AND AIMS

The effect of pollination on flower life span has been widely studied, but so far little attention has been paid to the reproductive consequences of delayed pollination in plants with long floral life spans. In the present study, Polygala vayredae was used to answer the following questions. (1) How does male and female success affect the floral longevity of individual flowers? (2) How does delaying fertilization affect the female fitness of this species?

METHODS

Floral longevity was studied after experimental pollinations involving male and/or female accomplishment, bagging and open pollination. The reproductive costs of a delay in the moment of fertilization were evaluated through fruit set, seed-ovule ratio and seed weight, after pollination of flowers that had been bagged for 2-18 d.

KEY RESULTS

Senescence of the flowers of P. vayredae was activated by pollen reception on the stigmatic papillae, while pollen removal had no effect on floral longevity. Nonetheless, a minimum longevity of 8 d was detected, even after successful pollination and pollen dissemination. This period may be involved with the enhancement of male accrual rates, as the female accomplishment is generally achieved after the first visit. Floral life span of open-pollinated flowers was variable and negatively correlated with pollinator visitation rates. Delayed pollination had a major impact on the reproductive success of the plant, with fruit set, seed-ovule ratio and seed weight being significantly diminished with the increase of flower age at the moment of fertilization.

CONCLUSIONS

A strong relationship between pollination and floral longevity was observed. Flowers revealed the ability to extend or reduce their longevity, within some limits, in response to the abundance of efficient pollinators (i.e. reproductive fulfilment rates). Furthermore, with scarce or unpredictable pollinators, a long floral life span could maintain the opportunity for fertilization but would also have reproductive costs on production of offspring. Reduced female fitness late in the flower's life could shift the cost-benefit balance towards a shorter life span, partially counteracting the selection for longer floral life span potentially mediated by scarce pollination services.

Comparative anatomy and morphology of nectar-producing Melastomataceae

BACKGROUND AND AIMS

Most neotropical Melastomataceae have bee-pollinated flowers with poricidal anthers. However, nectar rewards are known to be produced in about 80 species in eight genera from four different tribes. These nectar-producing species are pollinated by both vertebrates and invertebrates.

METHODS

The floral morphology and anatomy of 14 species was studied in six genera of nectar-producing Melastomataceae (Blakea, Brachyotum, Charianthus, Huilaea, Meriania and Miconia). Anatomical methods included scanning electron microscopy, and serial sections of paraffin-embedded flowers.

KEY RESULTS

All vertebrate-pollinated melastome flowers have petals that do not open completely at anthesis, thus forming a pseudo-tubular corolla, while closely related species that are bee pollinated have rotate or reflexed corollas. In most species, nectar secretion is related to stomatal or epidermal nectaries and not filament slits as previously reported. Moreover, the nectar is probably supplied by large vascular bundles near the release area. Blakea and Huilaea have nectary stomata located upon the dorsal anther connective appendages. Brachyotum also has nectary stomata on the anther connectives, but these are distributed lengthwise along most of the connective. Meriania may release nectar through the anther connective, but has additional nectary stomata on the inner walls of the hypanthium. Miconia has nectary stomata on the ovary apex. Charianthus nectaries were not found, but there is circumstantial evidence that nectar release occurs through the epidermis at the apex of the ovary and the lower portions of the inner wall of the hypanthium.

CONCLUSIONS

Nectar release in Melastomataceae is apparently related to nectary stomata and not filament slits. The presence of nectary stomata on stamens and on ovary apices in different lineages suggests that the acquisition of nectaries is a derived condition. Nectary location also supports a derived condition, because location is strongly consistent within each genus, but differs between genera.

Pollination biology of Jacaranda oxyphylla with an emphasis on staminode function

BACKGROUND AND AIMS

Bignoniaceae is a Neotropical family with >100 genera, only two of which, Jacaranda and Digomphia, have a developed staminode. Jacaranda oxyphylla, whose flowers possess a conspicuous glandular staminode, is a zoophilous cerrado species. Here, the composition of the secretion of the glandular trichome and the influence of the staminode on the pollination biology and reproductive success of J. oxyphylla were studied.

METHODS

The floral morphology, pollen viability, stigma receptivity, nectar volume and nectar concentration were studied. Compatibility system experiments were performed and floral visitors were observed and identified. Experiments comparing the effect of staminode presence and absence on pollen removal and pollen deposition efficiency were conducted in open-pollinated flowers. Histochemistry, thin-layer chromatography (TLC) and gas chromatography coupled to flame ionization detection (GC-FID) analyses were performed to determine the main chemical components of the staminode's glandular trichome secretion.

KEY RESULTS

Flower anthesis lasted 2 d and, despite the low frequency of flower visitation, pollination seemed to be effected mainly by medium-sized Eulaema nigrita and Bombus morio bees, by the small bee Exomalopsis fulvofasciata and occasionally by hummingbirds. Small bees belonging to the genera Ceratina, Augochlora and Trigona were frequent visitors, collecting pollen. Jacaranda oxyphylla is predominantly allogamous. Staminode removal resulted in fewer pollen grains deposited on stigmas but did not affect total pollen removal. The secretion of capitate glandular trichome occurs continually; the main chemical compounds detected histochemically were phenolic and terpenoid (essential oils and resins). Monoterpene cineole, pentacyclic triterpenes and steroids were identified by TLC and GC-FID.

CONCLUSIONS

The staminode of J. oxyphyllla is multifunctional and its importance for female reproductive success was attributed mainly to the secretion produced by capitate glandular trichomes. This secretion is involved in complex chemical interactions with pollinating bees, including the solitary bees Euglossini. These bees are common pollinators of various species of Jacaranda.

Pollination systems of Colchicum (Colchicaceae) in southern Africa: evidence for rodent pollination

BACKGROUND AND AIMS

Plants adapted for pollination by rodents tend to exhibit a distinct floral syndrome that includes dull coloured and geoflorous inflorescences and nocturnal anthesis and nectar production. On the basis of their floral traits, it was predicted that two African Colchicum species (C. scabromarginatum and C. coloratum) are rodent-pollinated.

METHODS

Field studies were carried out in the semi-arid Succulent Karoo region of South Africa. Live trapping of rodents was conducted and pollen loads on the rodents were quantified. The daily periodicity of nectar production was determined. Selective exclusion and controlled pollination experiments were also conducted.

KEY RESULTS

Live-trapped rodents were found to carry large amounts of Colchicum pollen on the fur of their snouts, and in their faeces. Birds were occasional pollinators of flowers of C. coloratum. During the evening, nectar volume and concentration increased for both species. When vertebrates were excluded from C. scabromarginatum and C. coloratum plants, there was a significant decrease in seed set compared with open control plants. By contrast, vertebrate exclusion did not significantly affect seed production of a congener, C. hantamense, which has floral traits associated with insect pollination. Breeding system experiments revealed that both C. scabromarginatum and C. coloratum require pollinators for seed production. Colchicum scabromarginatum is strictly self-incompatible, whereas C. coloratum is partially self-compatible.

CONCLUSIONS

Pollination by rodents occurs in two African Colchicum species. C. scabromarginatum appears to depend exclusively on rodents for seed production, while birds and autonomous selfing may contribute to seed production in C. coloratum. These are the first records of rodent pollination in the Colchicaceae.

ANTI-HERBIVORE PROTECTION BY MUTUALISTIC SPIDERS AND THE ROLE OF PLANT GLANDULAR TRICHOMES

Although specific associations between spiders and particular types of plants have been reported for several taxonomic groups, their consequences for spiders and plants are still poorly understood. The most common South American lynx spiders, Peucetia flava and P. rubrolineata, live strictly associated with various plant species that have glandular trichomes. To understand more about these spider-plant relationships, we investigated the influence of the spiders on the fitness of a neotropical glandular shrub (Trichogoniopsis adenantha) and on the arthropod community structure on the plant. We also tested whether glandular hairs provided any benefit to the spiders. Spiders reduced the abundance of several species and guilds of herbivores on the leaves and inflorescences. Consequently, damage to the leaves, capitula, ovaries, corollas, and stigmas caused by leaf-mining and chewing insects, as well as endophagous insects, were strongly reduced in the presence of Peucetia spp. Although the spiders fed on flower visitors, their negative influence on ovary fertilization was only marginally nonsignificant (P = 0.065). Spiders on plants of Trichogoniopsis adenantha that fed on common fruit flies that had died before adhering to the glandular trichomes did not lose body mass. However, those living on plants without stalked glandular trichomes (Melissa officinalis) did not feed on dead flies and lost 13-20% of their biomass. These results indicate that Peucetia spiders are effective plant bodyguards and that when there is limited live prey they may feed on insect carcasses adhered to glandular trichomes. Since several spider species of the genus Peucetia live strictly associated with glandular trichome-bearing plants in neotropical, Neartic, Paleartic, and Afrotropical regions, this type of facultative mutualism involving Peucetia and glandular plants may be common worldwide.

Cockroaches as pollinators of Clusia aff. sellowiana (Clusiaceae) on inselbergs in French Guiana

BACKGROUND AND AIMS

A report is made on a new species of Clusia related to C. sellowiana that dominates the vegetation of the Nouragues inselberg in French Guiana. The focus is on the pollination biology and on the remarkable relationship of this plant species to Amazonina platystylata, its cockroach pollinator. This appears to be only the second record of pollination by cockroaches.

METHODS

Pollination ecology was investigated by combining morphological studies, field observations and additional experiments. Floral scent was analysed by gas chromatography-mass spectrometry. The role of acetoin, the major component of the scent of this species of Clusia, in attracting pollinators was examined in field attraction experiments. The ability of cockroaches to perceive acetoin was investigated by electroantennography (EAG).

KEY RESULTS

The Clusia species studied produces seeds only sexually. Its nocturnal flowers are visited by crickets, ants, moths and cockroaches. A species of cockroach, Amazonina platystylata, is the principal pollinator. The reward for the visit is a liquid secretion produced by tissues at the floral apex and at the base of the ovary. Although the cockroaches have no structures specialized for pollen collection, their body surface is rough enough to retain pollen grains. The cockroaches show significant EAG reactions to floral volatiles and acetoin, suggesting that the floral scent is a factor involved in attracting the cockroaches to the flowers.

CONCLUSIONS

The results suggest that the plant-cockroach interaction may be quite specialized and the plant has probably evolved a specific strategy to attract and reward its cockroach pollinators. Acetoin is a substance involved in the chemical communication of several other cockroach species and it seems plausible that the plant exploits the sensitivity of cockroaches to this compound to attract them to the flowers as part of the pollination syndrome of this species.

Flower morphology, pollination biology and mating system of the complex flower of Vigna caracalla (Fabaceae: Papilionoideae)

BACKGROUND AND AIMS

Vigna caracalla has the most complex flower among asymmetrical Papilionoideae. The objective of this study was to understand the relationships among floral characteristics, specialization, mating system and the role of floral visitors under different ecological contexts.

METHODS

Five populations were studied in north-western Argentina, from 700 to 1570 m a.s.l. Anthesis, colour and odour patterns, stigmatic receptivity, visitors and pollination mechanism were examined and mating-system experiments were performed.

KEY RESULTS

The petals are highly modified and the keel shows 3.75-5.25 revolutions. The sense of asymmetry was always left-handed. Hand-crosses showed that V. caracalla is self-compatible, but depends on pollinators to set seeds. Hand-crossed fruits were more successful than hand-selfed ones, with the exception of the site at the highest elevation. Bombus morio (queens and workers), Centris bicolor, Eufriesea mariana and Xylocopa eximia trigger the pollination mechanism (a 'brush type'). The greatest level of self-compatibility and autonomous self-pollination were found at the highest elevation, together with the lowest reproductive success and number of pollinators (B. morio workers only).

CONCLUSIONS

Self-fertilization may have evolved in the peripheral population at the highest site of V. caracalla because of the benefits of reproductive assurance under reduced pollinator diversity.

Context- and scale-dependent effects of floral CO2 on nectar foraging by Manduca sexta

Typically, animal pollinators are attracted to flowers by sensory stimuli in the form of pigments, volatiles, and cuticular substances (hairs, waxes) derived from plant secondary metabolism. Few studies have addressed the extent to which primary plant metabolites, such as respiratory carbon dioxide (CO(2)), may function as pollinator attractants. Night-blooming flowers of Datura wrightii show transient emissions of up to 200 ppm above-ambient CO(2) at anthesis, when nectar rewards are richest. Their main hawkmoth pollinator, Manduca sexta, can perceive minute variation (0.5 ppm) in CO(2) concentration through labial pit organs whose receptor neurons project afferents to the antennal lobe. We explored the behavioral responses of M. sexta to artificial flowers with different combinations of CO(2), visual, and olfactory stimuli using a laminar flow wind tunnel. Responses in no-choice assays were scale-dependent; CO(2) functioned as an olfactory distance-attractant redundant to floral scent, as each stimulus elicited upwind tracking flights. However, CO(2) played no role in probing behavior at the flower. Male moths showed significant bias in first-approach and probing choice of scented flowers with above-ambient CO(2) over those with ambient CO(2), whereas females showed similar bias only in the presence of host plant (tomato) leaf volatiles. Nevertheless, all males and females probed both flowers regardless of their first choice. While floral CO(2) unequivocally affects male appetitive responses, the context-dependence of female responses suggests that they may use floral CO(2) as a distance indicator of host plant quality during mixed feeding-oviposition bouts on Datura and Nicotiana plants.

Behavioral consequences of innate preferences and olfactory learning in hawkmoth-flower interactions

Spatiotemporal variability in floral resources can have ecological and evolutionary consequences for both plants and the pollinators on which they depend. Seldom, however, can patterns of flower abundance and visitation in the field be linked with the behavioral mechanisms that allow floral visitors to persist when a preferred resource is scarce. To explore these mechanisms better, we examined factors controlling floral preference in the hawkmoth Manduca sexta in the semiarid grassland of Arizona. Here, hawkmoths forage primarily on flowers of the bat-adapted agave, Agave palmeri, but shift to the moth-adapted flowers of their larval host plant, Datura wrightii, when these become abundant. Both plants emit similar concentrations of floral odor, but scent composition, nectar, and flower reflectance are distinct between the two species, and A. palmeri flowers provide six times as much chemical energy as flowers of D. wrightii. Behavioral experiments with both naïve and experienced moths revealed that hawkmoths learn to feed from agave flowers through olfactory conditioning but readily switch to D. wrightii flowers, for which they are the primary pollinator, based on an innate odor preference. Behavioral flexibility and the olfactory contrast between flowers permit the hawkmoths to persist within a dynamic environment, while at the same time to function as the major pollinator of one plant species.

Elaiophore structure and oil secretion in flowers of Oncidium trulliferum Lindl. and Ornithophora radicans (Rchb.f.) Garay & Pabst (Oncidiinae: Orchidaceae)

BACKGROUND AND AIMS

Many orchid flowers have glands called elaiophores and these reward pollinating insects with oil. In contrast to other reward-producing structures such as nectaries, the anatomy of the elaiophore and the process of oil secretion have not been extensively studied. In this paper, elaiophore structure is described for two members of Oncidiinae, Oncidium trulliferum Lindl. and Ornithophora radicans (Rchb.f.) Garay & Pabst.

METHODS

Elaiophores of both species were examined using light microscopy, scanning electron microscopy and transmission electron microscopy.

KEY RESULTS AND CONCLUSIONS

In flowers of Oncidium trulliferum and Ornithophora radicans, oil is secreted by morphologically distinct elaiophores associated with the labellar callus. However, in O. trulliferum, elaiophores also occur on the lateral lobes of the labellum. In both these species, the epithelial elaiophores are composed of a single layer of palisade-like epidermal cells and a distinct subepithelial layer. Secretory elaiophore cells may contain numerous, starchless plastids, mitochondria and smooth endoplasmic reticulum profiles. In O. trulliferum, the cytoplasm contains myelin-like figures but these are absent from O. radicans. In the former species, cavities occur in the cell wall and these presumably facilitate the passage of oil onto the elaiophore surface. In O. radicans, the accumulation of oil between the outer tangential wall and the cuticle causes the latter to become distended. Since it is probable that the full discharge of oil from the elaiophores of O. radicans occurs only when the cuticle is ruptured by a visiting insect, this may contribute towards pollinator specificity. The structure of the elaiophore in these species resembles both that found in previously investigated species of Oncidiinae and that of certain members of the Malpighiaceae.

Pollination in Nicotiana alata stimulates synthesis and transfer to the stigmatic surface of NaStEP, a vacuolar Kunitz proteinase inhibitor homologue

After landing on a wet stigma, pollen grains hydrate and germination generally occurs. However, there is no certainty of the pollen tube growth through the style to reach the ovary. The pistil is a gatekeeper that evolved in many species to recognize and reject the self-pollen, avoiding endogamy and encouraging cross-pollination. However, recognition is a complex process, and specific factors are needed. Here the isolation and characterization of a stigma-specific protein from N. alata, NaStEP (N. alata Stigma Expressed Protein), that is homologous to Kunitz-type proteinase inhibitors, are reported. Activity gel assays showed that NaStEP is not a functional serine proteinase inhibitor. Immunohistochemical and protein blot analyses revealed that NaStEP is detectable in stigmas of self-incompatible (SI) species N. alata, N. forgetiana, and N. bonariensis, but not in self-compatible (SC) species N. tabacum, N. plumbaginifolia, N. benthamiana, N. longiflora, and N. glauca. NaStEP contains the vacuolar targeting sequence NPIVL, and immunocytochemistry experiments showed vacuolar localization in unpollinated stigmas. After self-pollination or pollination with pollen from the SC species N. tabacum or N. plumbaginifolia, NaStEP was also found in the stigmatic exudate. The synthesis and presence in the stigmatic exudate of this protein was strongly induced in N. alata following incompatible pollination with N. tabacum pollen. The transfer of NaStEP to the stigmatic exudate was accompanied by perforation of the stigmatic cell wall, which appeared to release the vacuolar contents to the apoplastic space. The increase in NaStEP synthesis after pollination and its presence in the stigmatic exudates suggest that this protein may play a role in the early pollen-stigma interactions that regulate pollen tube growth in Nicotiana.

Darwin's bee-trap: The kinetics of Catasetum, a new world orchid

The orchid genera Catasetum employs a hair-trigger activated, pollen release mechanism, which forcibly attaches pollen sacs onto foraging insects in the New World tropics. This remarkable adaptation was studied extensively by Charles Darwin and he termed this rapid response "sensitiveness." Using high speed video cameras with a frame speed of 1000 fps, this rapid release was filmed and from the subsequent footage, velocity, speed, acceleration, force and kinetic energy were computed.

Is floral longevity influenced by reproductive costs and pollination success in Cohniella ascendens (Orchidaceae)?

BACKGROUND AND AIMS

Although studies have shown that pollen addition and/or removal decreases floral longevity, less attention has been paid to the relationship between reproductive costs and floral longevity. In addition, the influence of reproductive costs on floral longevity responses to pollen addition and/or removal has not yet been evaluated. Here, the orchid Cohniella ascendens is used to answer the following questions. (a) Does experimental removal of flower buds in C. ascendens increase flower longevity? (b) Does pollen addition and/or removal decrease floral longevity, and does this response depend on plant reproductive resource status?

METHODS

To study the effect of reproductive costs on floral longevity 21 plants were selected from which we removed 50 % of the developing flower buds on a marked inflorescence. Another 21 plants were not manipulated (controls). One month later, one of four flowers on each marked inflorescence received one of the following pollen manipulation treatments: control, pollinia removal, pollination without pollinia removal or pollination with pollinia removal. The response variable measured was the number of days each flower remained open (i.e. longevity).

KEY RESULTS

The results showed significant flower bud removal and pollen manipulation effects on floral longevity; the interaction between these two factors was not significant. Flowers on inflorescences with previously removed flower buds remained open significantly longer than flowers on control inflorescences. On the other hand, pollinated flowers closed much faster than control and removed-pollinia flowers, the latter not closing significantly faster than control flowers, although this result was marginal.

CONCLUSIONS

The results emphasize the strong relationship between floral longevity and pollination in orchids, as well as the influence of reproductive costs on the former.

Integrated signaling in flower senescence: an overview

Flower senescence is the terminal phase of developmental processes that lead to the death of flower, which include, flower wilting, shedding of flower parts and fading of blossoms. Since it is a rapid process as compared to the senescence of other parts of the plant it therefore provides excellent model system for the study of senescence. During flower senescence, developmental and environmental stimuli enhance the upregulation of catabolic processes causing breakdown and remobilization of cellular constituents. Ethylene is well known to play regulatory role in ethylene-sensitive flowers while in ethylene-insensitive flowers abscisic acid (ABA) is thought to be primary regulator. Subsequent to perception of flower senescence signal, death of petals is accompanied by the loss of membrane permeability, increase in oxidative and decreased level of protective enzymes. The last stages of senescence involve the loss of of nucleic acids (DNA and RNA), proteins and organelles, which is achieved by activation of several nucleases, proteases and wall modifiers. Environmental stimuli such as pollination, drought and other stresses also affect senescence by hormonal imbalance. In this article we have covered the following: perception mechanism and specificity of flower senescence, flower senescence-associated events, like degradation of cell membranes, proteins and nucleic acids, environmental/external factors affecting senescence, like pollination and abiotic stress, hormonal and non-hormonal regulation of flower/petal senescence and finally the senescence associated genes (SAGs) have also been described.

Variability in floral scent in rewarding and deceptive orchids: the signature of pollinator-imposed selection?

BACKGROUND AND AIMS

A comparative investigation was made of floral scent variation in the closely related, food-rewarding Anacamptis coriophora and the food-deceptive Anacamptis morio in order to identify patterns of variability of odour compounds in the two species and their role in pollinator attraction/avoidance learning.

METHODS

Scent was collected from plants in natural populations and samples were analysed via quantitative gas chromatography and mass spectrometry. Combined gas chromatography and electroantennographic detection was used to identify compounds that are detected by the pollinators. Experimental reduction of scent variability was performed in the field with plots of A. morio plants supplemented with a uniform amount of anisaldehyde.

KEY RESULTS

Both orchid species emitted complex odour bouquets. In A. coriophora the two main benzenoid compounds, hydroquinone dimethyl ether (1,4-dimethoxybenzene) and anisaldehyde (methoxybenzaldehyde), triggered electrophysiological responses in olfactory neurons of honey-bee and bumble-bee workers. The scent of A. morio, however, was too weak to elicit any electrophysiological responses. The overall variation in scent was significantly lower in the rewarding A. coriophora than in the deceptive A. morio, suggesting pollinator avoidance-learning selecting for high variation in the deceptive species. A. morio flowers supplemented with non-variable scent in plot experiments, however, did not show significantly reduced pollination success.

CONCLUSIONS

Whereas in the rewarding A. coriophora stabilizing selection imposed by floral constancy of the pollinators may reduce scent variability, in the deceptive A. morio the emitted scent seems to be too weak to be detected by pollinators and thus its high variability may result from relaxed selection on this floral trait.

New insights into the functional morphology of the lever mechanism of Salvia pratensis (Lamiaceae)

BACKGROUND AND AIMS

The functional morphology of Salvia pratensis flowers was re-investigated, after new insights revealed that pollen dispensing is one of the main functions of the staminal lever. In particular, no detailed information was available regarding the process of pollen transfer and the forces arising between the pollen-bearing thecae and the pollinating bee's body. The assumption was made that these forces play a significant role in pollen dispensing.

METHODS

The functional morphology of S. pratensis flowers and the interaction between flowers and bees (Apis mellifera) were studied by reconstructing stress and strains by using qualitative and semi-quantitative theoretical analysis. Flowers were manipulated to study the spatial arrangement of the filament and lever, and of the head and proboscis of the visiting bee inside the tube. Photographs and films of bee visits on flowers were used to analyse the interaction of pollinator and staminal lever.

KEY RESULTS

The spoon-shaped lower lever of S. pratensis has a small hole through which a bee introduces its proboscis into the corolla tube. Although mentioned for the first time by Kerner von Marilaun in 1891, presented here is the first drawing and the first photograph showing this interaction in detail. The analysis of the interaction of flower visitor and the lever mechanism revealed that the position of bees on different flowers is spatially very similar. Flower morphology constrains postures of legitimately nectar-probing bees within narrow bounds. A theoretical discussion on structural elements and force progression in the flower allows the principles of lightweight architecture in flower morphology to be recognized.

CONCLUSIONS

The staminal lever of S. pratensis is a pollen-dispensing device. It seems to influence the amount of pollen deposited on pollinators by determining the forces arising between the pollinator and the pollen. The relevant forces occur either during the first, dynamic phase or during the second, almost static phase of a flower visit.

Post-zygotic reproductive isolation between sympatric taxa in the Chamaecrista desvauxii complex (Leguminosae-Caesalpinioideae)

BACKGROUND AND AIMS

Differences in the mating systems and the mechanisms of reproductive isolation between Chamaecrista desvauxii var. graminea and C. desvauxii var. latistipula were examined in the Chapada Diamantina, Brazil. These taxa occur sympatrically, and their populations demonstrate marked morphological differences. The objective of the present work was to determine if reproductive isolation mechanisms exist between these two populations of C. desvauxii, and to determine the influence of these putative mechanisms on their genetic differentiation.

METHODS

Field observations were made of floral biology, phenology and floral visitation, and experiments on intra- and interpopulation pollination and germination rates of the resultant seeds were performed. A genetic examination of the populations was undertaken using four allozyme loci.

KEY RESULTS

The varieties examined demonstrated overlapping of flowering periods during the months of June to September. The main pollinator for both varieties was the bee Bombus brevivillus. Both varieties are self-compatible, and a large number of fruits are formed in cross-pollinations with high seed germination rates. Inter-taxa pollinations result in high levels of fruit production, but no seeds are formed. Two of the four loci examined were diagnostic for the varieties, and exclusive high-frequency alleles were encountered at the other loci, leading to a high genetic distance between the two populations (0.495).

CONCLUSIONS

Pre-zygotic barriers were not found between the two varieties, and these remain isolated due to post-zygotic events. The two varieties demonstrate marked differences in their morphology, floral biology, phenology and genetic make-up, all of which indicate that they should be treated as two distinct species. A complete revision involving the other varieties of the C. desvauxii complex will be necessary in order to define these two taxa formally.

Labellar micromorphology of Bifrenariinae Dressler (Orchidaceae)

BACKGROUND AND AIMS

The two closely related subtribes Bifrenariinae Dressler and Maxillariinae Benth. are easily distinguished on morphological grounds. Recently, however, molecular techniques have supported the inclusion of Bifrenariinae within a more broadly defined Maxillariinae. The present paper describes the diverse labellar micromorphology found amongst representatives of Bifrenariinae (Bifrenaria Lindl., Rudolfiella Hoehne, Teuscheria Garay and Xylobium Lindl.) and compares it with that found in Maxillaria Pabst & Dungs and Mormolyca Fenzl (Maxillariinae).

METHODS

The labella of 35 specimens representing 22 species of Bifrenariinae were examined by means of light microscopy and scanning electron microscopy and their micromorphology compared with that of Maxillaria sensu stricto and Mormolyca spp. The labellar epidermis of representatives of Bifrenaria, Xylobium and Mormolyca was tested for protein, starch and lipids in order to ascertain whether this tissue is involved in the rewarding of pollinators.

KEY RESULTS AND CONCLUSIONS

The labella of Bifrenaria spp. and Mormolyca spp. are densely pubescent but those of Xylobium, Teuscheria and Rudolfiella are generally papillose. However, whereas the trichomes of Bifrenaria and Mormolyca are unicellular, those found in the other three genera are multicellular. Hitherto, no unicellular trichomes have been described for Maxillaria, although the labella of a number of species secrete a viscid substance or bear moniliform, pseudopollen-producing hairs. Moniliform hairs and secretory material also occur in certain species of Xylobium and Teuscheria and these genera, together with Maxillaria, are thought to be pollinated by stingless bees (Meliponini). Differences in the labellar micromorphology of Bifrenaria and Mormolyca are perhaps related to Euglossine- and/ or bumble bee-mediated pollination and pseudocopulation, respectively. Although Xylobium and Teuscheria share a number of labellar features with Maxillaria sensu stricto, this does not necessarily reflect taxonomic relationships but may be indicative of convergence in response to similar pollinator pressures.

Elaborate petals in Australian Spermacoce (Rubiaceae) species: morphology, ontogeny and function

BACKGROUND AND AIMS

Australian Spermacoce species display various types of elaborate petals. Their precise morphology, ontogenetic origin, and function are hitherto unknown. The aim of the present paper is to unravel the development and nature of the diverse types of elaborate petals in Spermacoce through a floral ontogenetic study.

METHODS

The floral ontogeny of six species characterized by different types of corolla appendages was studied by scanning electron microscopy and light microscopy. In order to elucidate the possible functions of the elaborate petals, field observations were conducted as well.

KEY RESULTS

Scanning electronmicrographs show that full-grown petals of Spermacoce lignosa, S. phaeosperma and S. redacta bear appendages on their ventral side. Despite their different appearance at anthesis, the appendages develop very similarly in all three species. They are initiated at the same developmental stage and are first visible as two arcs of primordia converging from the upper margins of the petal towards its midvein and downwards. In S. brevidens, S. caudata and S. erectiloba, the full-grown petals have two long, concave protuberances, which develop from the tissue at both sides of the petal's mid-vein. In these three species, initiation of appendages on the ventral side of the petals is also observed, but they are hardly visible on the mature petals. The two types of elaborate petals tightly enclose the anthers, both in bud and during most of the flowering period.

CONCLUSIONS

Among Australian Spermacoce species, two types of elaborate petals can be distinguished. The former hypothesis that the two types of elaborate petals are essentially homologous is here rejected. Field investigations point out that the elaborate petals might play a role in the pollination biology of the species.

Sex in advertising: dioecy alters the net benefits of attractiveness in Sagittaria latifolia (Alismataceae)

The flowers and inflorescences of animal-pollinated dioecious plants are generally small and inconspicuous in comparison with outcrossing cosexual species. The net benefits of an attractive floral display may be different for dioecious compared to cosexual populations because dioecious species experience a more severe reduction in pollen delivery when pollinators forage longer on fewer individuals. Here, we develop a model that predicts the decrease in pollen delivery in dioecious relative to cosexual populations from female-female, female-male and male-male visit sequences as the number of individuals visited varies. To evaluate the predictions of our model we conducted a common garden experiment with dioecious and monoecious (cosexual) arrays of the insect-pollinated herb Sagittaria latifolia. We find that, although increasing the advertisements of floral rewards (i.e. increasing floral display) attracts more pollinators to individuals, the probability that these pollinators subsequently deliver pollen to neighbouring plants depends on sexual system. Because the number of individual plants visited per foraging trip did not increase significantly with floral display, the relative pollination success of dioecious versus monoecious populations decreases with increased floral display. We propose that this could explain why dioecy is strongly correlated with reduced floral display among angiosperm species.

Loasoideae (Cornales)

BACKGROUND AND AIMS

Loasaceae subfam. Loasoideae are mostly distributed in South America (sea level to over 4500 m) with a wide range of animals documented as pollinators. The aim was to investigate correlations between nectar parameters, flower morphology, pollination syndrome and phylogeny.

METHODS

Nectar was collected from 29 species from seven genera in the subfamily. Concentration and volumes were measured and the amount of sugar calculated. Correlations of nectar data were plotted on a ternary graph and nectar characteristics compared with flower visitors, floral morphology and phylogenetic data.

KEY RESULTS

Sugar concentrations are generally higher than reported for most plant families in the literature. The species investigated can be roughly grouped as follows. Group I: plants with approx. 1.5(-3.5) microL nectar with (40-)60-80% sugar and 0.19-2 mg sugar flower-1; with small, white, star-shaped corollas, pollinated by short-tongued bees. Groups II, III and IV: plants with mostly orange, balloon-, saucer-, bowl- or bell-shaped corollas. Group II: plants with approx. 9-14 microL nectar with 40-60% sugar and 4-10 mg sugar flower-1; mostly visited by long-tongued bees and/or hummingbirds. Group III: plants with 40-100 microL nectar with 30-40% sugar and 14-36 mg sugar flower-1, mostly visited by hummingbirds. Group IV: geoflorous plants with 80-90 microL with 10-15% sugar and 8.5-12 mg sugar flower-1, presumably visited by small mammals. Groups II and III include species visited by bees and/or hummingbirds.

CONCLUSIONS

Pollinator switches from short-tongued bees via long-tongued bees to hummingbirds appear to have taken place repeatedly in the genera Nasa, Loasa and Caiophora. Changes in nectar amount and concentration appear to evolve rapidly with little phylogenetic constraint.

Diurnal and nocturnal pollination of Marginatocereus marginatus (Pachycereeae: Cactaceae) in Central Mexico

BACKGROUND AND AIMS

Chiropterophillous and ornithophillous characteristics can form part of a single reproductive strategy in plants that have flowers with diurnal and nocturnal anthesis. This broader pollination strategy can ensure seed set when pollinators are scarce or unpredictable. This appears to be true of hummingbirds, which presumably pollinate Marginatocereus marginatus, a columnar cactus with red nocturnal and diurnal flowers growing as part of dense bat-pollinated columnar cacti forests in arid regions of central Mexico. The aim of this study was to study the floral biology of M. marginatus, and evaluate the effectiveness of nocturnal vs. diurnal pollinators and the contribution of each pollinator group to overall plant fitness.

METHODS

Individual flower buds were marked and followed to evaluate flower phenology and anthesis time. Flowers and nectar production were measured. An exclusion experiment was conducted to measure the relative contribution of nocturnal and diurnal pollinators to seed set.

KEY RESULTS

Marginatocereus marginatus has red hermaphroditic flowers with nocturnal and diurnal anthesis. The plant cannot produce seeds by selfing and was pollinated during the day by hummingbirds and during the night by bats, demonstrating that both pollinator groups were important for plant reproduction. Strong pollen limitation was found in the absence of one of the pollinator guilds.

CONCLUSIONS

Marginatocereus marginatus has an open pollination system in which both diurnal and nocturnal pollinators are needed to set seeds. This represents a fail-safe pollination system that can ensure both pollination, in a situation of low abundance of one of the pollinator groups (hummingbirds), and high competition for nocturnal pollinators with other columnar cacti that bloom synchronously with M. marginatus in the Tehuacan Valley, Mexico.

Nectar concentration and composition of 26 species from the temperate forest of South America

BACKGROUND AND AIMS

Floral nectar concentration and chemical composition of 26 plant species native to the temperate forest of southern South America are reported and the relationships with the flower type are evaluated.

METHODS

Nectar concentration was measured with a hand refractometer and sugar composition was analysed by gas-liquid chromatography. Plant species were classified into flower type categories based not only on floral features but also on data from the literature and field observations on their pollinators.

KEY RESULTS

Most data on nectar are new reports at the generic and/or specific level. Plant species in which more than one population was studied showed significant among-population variation in nectar sugar concentration and composition. Results showed a weak relationship between nectar traits and flower type. Many species had nectar containing 50 % or more sucrose (17 of 26 species), independent of the main pollinator.

CONCLUSIONS

Considering that (a) nectar characteristics did not show a clear association with different flower types or with plant taxonomic membership, and (b) different populations of the same species showed large variability in sugar composition, the results suggest that other factors (e.g. historical and environmental) could be involved in determining the sugar composition of the highly endemic plant species from this region.

Imaging and quantifying carbohydrate transport to the developing ovaries of maize

BACKGROUND AND AIMS

Shade or inadequate water can inhibit photosynthesis and limit the development of maize (Zea mays) ovaries around the time of pollination, potentially reducing the number of kernels at harvest. This study investigated whether the decreased photosynthesis diminished only the sugar supply or also altered the transport path to the ovaries.

METHODS

Photosynthesis and water potentials (Psiw) were measured in the leaves while dry matter delivery was monitored in the ovaries. Ovary glucose, starch and acid invertase activities were measured in situ. Stems were fed xylem-mobile safranin or phloem-mobile carboxyfluorescein (CF), and the dye transport to the ovaries was determined.

KEY RESULTS

Under normal conditions, the ovaries gained in dry mass, and starch accumulated in the pedicel and ovary wall. Glucose accumulated in the pedicel, apparently in the apoplast where insoluble (cell-wall-bound) acid invertase acted on the arriving sucrose. A glucose gradient developed from pedicel to nucellus. Safranin moved in the xylem and did not reach the ovary, but CF moved in the phloem and arrived at the ovary. CF also spread into the pedicel but unlike glucose it did not enter the nucellus. Low Psiw or shade decreased leaf photosynthesis, ovary dry mass accumulation, invertase activities, pedicel glucose, starch accumulation and CF delivery. Removal of these treatments reversed the effects.

CONCLUSIONS

The success of CF in tracing the general path and rate of carbohydrate transport gave visual evidence that phloem transport to the ovary decreased at low Psiw or in the shade but otherwise remained functional. The decreases indicated that losses in carbohydrate delivery are central features of failed ovary development under these conditions. The selectivity of transport into the nucellus resembled the situation later when embryo and endosperm are present and selective uptake occurs from the apoplast.

Pollination in Brazilian Syngonanthus (Eriocaulaceae) species: evidence for entomophily instead of anemophily

BACKGROUND AND AIMS

The reproductive biology of Syngonanthus mucugensis and S. curralensis (Eriocaulaceae) was studied in areas of 'campo rupestre' vegetation in the Chapada Diamantina, north-eastern Brazil. These species are herbaceous and the individuals have a grouped distribution. Their leaves are united in a rosette, and their inflorescence is monoecious, of the capitulum type. The staminate and pistillate rings mature in a centripetal manner on the capitulum.

METHODS

A field study was conducted, including observations concerning the morphology and biology of the flowers, fruit development, insect visits and anemophily, in both S. mucugensis and S. curralensis. Experimental pollinations were also carried out to study the mating systems of S. mucugensis.

KEY RESULTS

Both species flower from June to August. The staminate cycle lasts approx. 7 d, and the pistillate cycle from 3 to 4 d, with no temporal overlap between them on the same capitulum. The pollen viability of S. mucugensis was 88.6%, and 92.5% for S. curralensis. The inflorescences of both species demonstrated ultraviolet absorbance, and a sweet odour was detected during both the staminate and pistillate phases. No nectar production was ever noted, although nectaries are present. Both species were visited by numerous groups of insects, with the Diptera being the principal pollinators, especially the species of Syrphidae and Bombyliidae. There were secondary pollinators among species of Coleoptera and Hymenoptera. There was no evidence of wind pollination. Syngonanthus mucugensis is a self-compatible species, and forms fruits by agamospermy at low frequencies.

CONCLUSIONS

This is apparently the first report for pollination biology and mating systems of Eriocaulaceae. Conversely to that stated by some authors, entomophily, mainly effected by species of Diptera but also by species of Coleoptera and Hymenoptera, is probably the only pollination system in these species. In spite of the monoecious inflorescences without overlap of the staminate and pistillate phases, geitonogamy may occur in S. mucugensis, as the species is self-compatible and different capitula in the same plant at different phases is common.

Chiropterophily in Sinningieae (Gesneriaceae): Sinningia brasiliensis and Paliavana prasinata are bat-pollinated, but P. sericiflora is not. Not yet?

BACKGROUND AND AIMS

Based on the bell shape and greenish colour of the flowers, bat-pollination was suggested for some Sinningieae species (Gesneriaceae). Actually, there are no reports on pollination biology and visitors for these species. This paper reports on pollination biology of Sinningia brasiliensis, Paliavana prasinata and P. sericiflora in south-eastern Brazil.

METHODS

Flowers were observed in situ to determine phases of anthesis, colour patterns and scent intensity. Corolla measures were taken from fresh flowers. Nectar production and concentration were measured in flowers bagged at the pre-anthesis phases. Direct visual observations of visitors were made during the day and night, and photographs were taken to analyse their visiting behaviour.

KEY RESULTS

Some floral features of the three species fit the bat-pollination syndrome: large, robust and gullet-shaped corollas, colour patterns and large amount of nectar. However, other floral features-such as onset of anthesis, nectar attributes and odour intensity-differ among the species. Nectar volume and total sugar production increased significantly at midnight in S. brasiliensis and P. prasinata, but in P. sericiflora there were no significant differences in the total nectar and sugar production during anthesis. Scent intensity is much higher in S. brasiliensis and P. prasinata than P. sericiflora. Flowers of S. brasiliensis and P. prasinata, whose features fit the chiropterophilous syndrome, are pollinated by glossophagine bat species. Paliavana sericiflora, on the other hand, presents floral features intermediate between bat and hummingbird syndromes, but is visited and pollinated only by hummingbirds.

CONCLUSIONS

These data strengthen the statement that the bat syndrome in Sinningieae originated independently in Sinningia brasiliensis and in Paliavana species. Paliavana sericiflora may be another example of a plant 'in transition' from the hummingbird to the bat pollination, but a reversion in the direction of bat to hummingbird might not be excluded. It is also possible that this is a case of speciation on both bat and hummingbird pollination. Studies on Paliavana sister species may provide insights about origins and the evolutionary directions of the pollination systems of these species.

Effects of pollen shortage and self-pollination on seed production of an endangered tree, Magnolia stellata

BACKGROUND AND AIMS

Pollen limitation is a significant determinant of seed production, and can result from both insufficient pollen quantity (pollen shortage) and quality (mainly relating to self-pollination). For animal-pollinated tree species with large floral displays, pollen limitation may be determined by a balance between increased pollen quantity due to increased attractiveness for pollinators, countered by increased self-pollination due to increased geitonogamy. The contributions of pollen shortage and self-pollination on seed production were quantitatively examined in the natural pollination of an insect-pollinated, dichogamous, endangered tree, Magnolia stellata, which has a large, showy floral display.

METHODS

Manual self- and cross-pollinations were conducted to determine the effects of selfing on seed production. The outcrossing rate was measured using microsatellite analyses of open-pollinated seeds, and the embryo mortality rate caused by self-pollination was indirectly estimated. The frequency of ovule mortality due to pollen shortage was also inferred using the embryo mortality and ovule survival rates from natural pollination.

KEY RESULTS

The average fruit set, seed set per fruit, and ovule survival rate per tree from hand cross-pollination were 1.37, 3.15, and 3.34 times higher than those from hand self-pollination, respectively, indicating that self-pollination causes inbreeding depression for fruit and seed set. The multilocus-outcrossing rate (t(m)) was intermediate, 0.632, and the primary selfing rate was 0.657. This indicates that frequent geitonogamous selfing occurs. The ovule mortality rate due to pollen shortage and the embryo mortality rate due to self-pollination were estimated to be 80.8 % and 45.9 %, respectively.

CONCLUSIONS

It is concluded that seed production of M. stellata is strongly limited by both pollen shortage and self-pollination. Inefficient beetle-pollination and the automimicry system via asynchronous flowering might be responsible for the high level of pollen shortage and frequent geitonogamy. This is despite a large, showy floral display and the dichogamous system of the species.

Comparative account of nectary structure in Hexisea imbricata (Lindl.) Rchb.f. (Orchidaceae)

BACKGROUND AND AIMS

Despite the number of orchid species that are thought to be pollinated by hummingbirds, our knowledge of the nectaries of these orchids is based solely on a single species, Maxillaria coccinea (Jacq.) L.O. Williams ex Hodge. Nevertheless, it is predicted that such nectaries are likely to be very diverse and the purpose of this paper is to compare the nectary and the process of nectar secretion in Hexisea imbricata (Lindl.) Rchb.f. with that of Maxillaria coccinea so as to begin to characterize the nectaries of presumed ornithophilous Neotropical orchids.

METHODS

Light microscopy, transmission electronmicroscopy and histochemistry were used to examine the histology and chemical composition of nectary tissue and the process of nectar secretion in H. imbricata.

KEY RESULTS AND CONCLUSIONS

The nectary of H. imbricata has a vascular supply, is bound by a single-layered epidermis with few stomata and comprises two or three layers of subepidermal secretory cells beneath which lie several layers of palisade-like parenchymatous cells, some of which contain raphides or mucilage. The secretory cells are collenchymatous and their walls have numerous pits with associated plasmodesmata. They contain the full complement of organelles characteristic of secretory cells as well as intravacuolar protein bodies but some of the secretory epidermal cells, following secretion, collapse and their anticlinal walls seem to fold. Nectar secretion is thought to be granulocrine and, following starch depletion, lipid droplets collect within the plastids. The nectar accumulates beneath the cuticle which subsequently forms swellings. Finally, nectar collects in the saccate nectary spur formed by the fusion of the margins of the labellum and the base of the column-foot. Thus, although the nectary of H. imbricata and M. coccinea have many features in common, they nevertheless display a number of important differences.

Pseudocopulatory pollination in lepanthes (orchidaceae: pleurothallidinae) by fungus gnats

BACKGROUND AND AIMS

Lepanthes is one of the largest angiosperm genera (>800 species). Their non-rewarding, tiny and colourful flowers are structurally complex. Their pollination mechanism has hitherto remained unknown, but has been subject of ample speculation; the function of the minuscule labellum appendix is especially puzzling. Here, the pollination of L. glicensteinii by sexually deceived male fungus gnats is described and illustrated.

METHODS

Visitors to flowers of L. glicensteinii were photographed and their behaviour documented; some were captured for identification. Occasional visits to flowers of L. helleri, L. stenorhyncha and L. turialvae were also observed. Structural features of flowers and pollinators were studied with SEM.

KEY RESULTS

Sexually aroused males of the fungus gnat Bradysia floribunda (Diptera: Sciaridae) were the only visitors and pollinators of L. glicensteinii. The initial long-distance attractant seems to be olfactory. Upon finding a flower, the fly curls his abdomen under the labellum and grabs the appendix with his genitalic claspers, then dismounts the flower and turns around to face away from it. The pollinarium attaches to his abdomen during this pivoting manoeuvre. Pollinia are deposited on the stigma during a subsequent flower visit. The flies appear to ejaculate during pseudocopulation. The visitors of L. helleri, L. stenorhyncha and L. turialvae are different species of fungus gnats that display a similar behaviour.

CONCLUSIONS

Lepanthes glicensteinii has genitalic pseudocopulatory pollination, the first case reported outside of the Australian orchid genus Cryptostylis. Since most species of Lepanthes have the same unusual flower structure, it is predicted that pollination by sexual deception is prevalent in the genus. Several morphological and phenological traits in Lepanthes seem well suited for exploiting male fungus gnats as pollinators. Correspondingly, some demographic trends common in Lepanthes are consistent with patterns of male sciarid behaviour.

Perfect syncarpy in apple (Malus x domestica 'Summerland McIntosh') and its implications for pollination, seed distribution and fruit production (Rosaceae: Maloideae)

BACKGROUND AND AIMS

The gynoecium of the domestic apple, Malus x domestica, has been assumed to be imperfectly syncarpic, whereby pollination of each stigmatic surface can result in fertilization within only one of the five carpels. Despite its implied effect on fruit quantity and quality, the resulting influence of flower form on seed set and distribution within the apple fruit has seldom been investigated. Instead, poor fruit quality is usually attributed to problems with pollination, such as low bee numbers and/or ineffective pollinators within apple agro-ecosystems. The objective of this study was to determine the true nature of gynoecial structure and its influence on fruit production in the apple cultivar 'Summerland McIntosh'.

METHODS

A stigma-excision method was used to determine the effects of uneven pollination among the five stigmas on fruit quantity (as measured by fruit set), and quality (seed number and distribution). In addition, flowers were examined microscopically to determine pollen tube pathways.

KEY RESULTS

Fruit set, seed number, seed distribution, and the microscopic examination of flower gynoecial structure reported in this study indicated that the gynoecium of the cultivar Summerland McIntosh is perfectly syncarpic and not imperfectly syncarpic as previously thought.

CONCLUSIONS

Pollination levels among the five stigmas need not be uniform to obtain full seed development within Summerland McIntosh fruit; even if one stigmatic surface is adequately pollinated, a full complement of seeds is likely. The importance of perfect syncarpy in recognizing true causes of poor fruit quality in apple is discussed.

Heteromorphic incompatibility and efficiency of pollination in two distylous Pentanisia species (Rubiaceae)

BACKGROUND AND AIMS

Distyly has been hypothesized to promote cross-pollination by reducing intrafloral and geitonogamous self-pollination, and enhancing intermorph pollination. Distylous plants typically display both reciprocal herkogamy and a heteromorphic incompatibility system, which allows mating only between morphs. Distyly and its pollination consequences were examined in two Pentanisia species with long-tubed flowers which are pollinated almost exclusively by butterflies.

METHODS

Anther and stigma heights were measured to quantify reciprocal herkogamy. The type of incompatibility system was determined by observing pollen tubes and seed production following controlled hand pollination. Pollen loads on pollinators and stigmas were also examined to assess the efficiency of intermorph pollen flow.

KEY RESULTS

Pentanisia prunelloides and P. angustifolia exhibit reciprocal herkogamy and a host of ancillary dimorphisms, including pollen colour, exine sculpturing, stigmatic papilla shape and floral-tube pubescence. Controlled hand-pollinations revealed the presence of a strong heteromorphic incompatibility system in both species. The site of incompatibility differed between the morphs; intramorph pollen tubes were blocked in the style of the short-styled morph and on the stigmatic surface of the long-styled morph. Butterflies carried pollen from the short- and long-styled morphs primarily on their head and proboscis, respectively. Natural pollination resulted in a higher proportion of pollen transfer from long- to short-styled plants than vice versa. Nevertheless, fruit set did not differ between morphs.

CONCLUSIONS

Both Pentanisia species are fully distylous. Reciprocal herkogamy results in pollen from the two morphs being carried on different locations on pollinators' bodies, which in turn promotes intermorph pollination. Intramorph pollination does not result in fertilization, because of an effective heteromorphic incompatibility system.

Flower size variation in Rosmarinus officinalis: individuals, populations and habitats

BACKGROUND AND AIMS

Flowers are relatively invariant organs within species, but quantitative variation often exists among conspecifics. These variations represent the raw material that natural selection can magnify, eventually resulting in morphological divergence and diversification. This paper investigates floral variability in Rosmarinus officinalis, a Mediterranean shrub.

METHODS

Nine populations were selected in three major southern Spanish habitats (coast, lowland and mountains) along an elevation gradient. Flower samples from randomly chosen plants were collected from each population, and a total of 641 flowers from 237 shrubs were weighed while still fresh to the nearest 0.1 mg. Leaves from the same plants were also measured. Variations among habitats, sites and plants were explored with general linear model ANOVA. Leaf-flower covariation was also investigated.

KEY RESULTS

Most (58%) mass in flowers was accounted for by the corolla, whose linear dimensions correlated directly with flower mass. Averaged over plants, the mass of a flower varied between 12 mg and 38 mg. Habitat, site (within habitat) and shrub identity had significant effects on mass variance. Flowers from the coast were the smallest (17 mg) and those from the mountains the largest (25 mg on average). A pattern of continuously increasing flower size with elevation emerged which was largely uncoupled from the geographical pattern of leaf size variation.

CONCLUSIONS

As regards flower size, a great potential to local differentiation exists in Rosmarinus. Observed divergences accord with a regime of large-bodied pollinator selection in the mountains, but also with resource-cost hypotheses on floral evolution that postulate that reduced corollas are advantageous under prevailingly stressful conditions.

Resource partitioning among flower visitors and evolution of nectar concealment in multi-species communities

The plant community within which flower evolution takes place has largely been ignored. We develop two models for the evolution of nectar concealment when flowers are visited by legitimate pollinators and flower parasites. When there is a single plant species, no level of nectar concealment is evolutionarily stable: any population can be invaded by mutants exhibiting a higher level of nectar concealment. However, the presence of a second flower species with exposed nectar and not subject to evolution breaks the runaway process. In the presence of open flowers, depending on the fitness function there may be an evolutionarily stable level of nectar concealment, or more complex evolutionary dynamics, with nectar concealment fluctuating within a bounded range. Concealment of nectar from flower parasites can evolve even if it implies decreasing the accessibility of nectar to legitimate pollinators.

Breeding system in a population of Trigonella balansae (Leguminosae)

BACKGROUND AND AIMS

Although some taxonomic studies in the genus Trigonella have been conducted, there has been no concerted effort to study the breeding system. This paper examines the floral structure and pollination system in a population of T. balansae, an annual pasture legume.

METHODS

Floral morphology, hand and vector pollination, stigma receptivity, pollen tube growth, using scanning electron and fluorescence microscopy, were conducted.

KEY RESULTS

Measurements of floral structure from before to after anthesis indicates an inability for T. balansae to self-pollinate and a requirement for an external vector to effectively transfer pollen from the anthers onto the stigmas of this species. Seed set can be obtained by hand or honeybee manipulation of T. balansae flowers.

CONCLUSIONS

Trigonella balansae is a self-compatible species, but which requires vectors such as honeybees to bring about pollination.

Sugar-responsive gene expression, invertase activity, and senescence in aborting maize ovaries at low water potentials

BACKGROUND AND AIMS

Ovary abortion can occur in maize (Zea mays) if water deficits lower the water potential (psiw) sufficiently to inhibit photosynthesis around the time of pollination. The abortion decreases kernel number. The present work explored the activity of ovary acid invertases and their genes, together with other genes for sucrose-processing enzymes, when this kind of abortion occurred. Cytological evidence suggested that senescence may have been initiated after 2 or 3 d of low psiw, and the expression of some likely senescence genes was also determined.

METHODS

Ovary abortion was assessed at kernel maturity. Acid invertase activities were localized in vivo and in situ. Time courses for mRNA abundance were measured with real time PCR. Sucrose was fed to the stems to vary the sugar flux.

KEY RESULTS

Many kernels developed in controls but most aborted when psiw became low. Ovary invertase was active in controls but severely inhibited at low psiw for cell wall-bound forms in vivo and soluble forms in situ. All ovary genes for sucrose processing enzymes were rapidly down-regulated at low psiw except for a gene for invertase inhibitor peptide that appeared to be constitutively expressed. Some ovary genes for senescence were subsequently up-regulated (RIP2 and PLD1). In some genes, these regulatory changes were reversed by feeding sucrose to the stems. Abortion was partially prevented by feeding sucrose.

CONCLUSIONS

A general response to low psiw in maize ovaries was an early down-regulation of genes for sucrose processing enzymes followed by up-regulation of some genes involved in senescence. Because some of these genes were sucrose responsive, the partial prevention of abortion with sucrose feeding may have been caused in part by the differential sugar-responsiveness of these genes. The late up-regulation of senescence genes may have caused the irreversibility of abortion.

Pollination and reproductive biology of twelve species of neotropical Malpighiaceae: stigma morphology and its implications for the breeding system

BACKGROUND AND AIMS

This study on reproductive biology examines the stigmatic morphology of 12 Brazilian Malpighiaceae species with regard to their pollination and breeding system.

METHODS

The species were studied in natural populations of a semi-deciduous forest fragment. Style tips were processed for observation by SEM and pollen-tube growth was analyzed under fluorescence microscopy. The breeding system was investigated by isolating flowers within waterproof bags. Floral visitors were recorded through notes and photographs.

KEY RESULTS

Flowers are yellow, pink or white, protogynous, herkogamous and sometimes lack oil glands. While Banisteriopsis pubipetala has functional female flowers (with indehiscent anthers), 11 species present hermaphrodite flowers. Stigmas of these species may be terminal, with a slightly concave surface, or internal, consisting of a circular cavity with a large orifice, and are covered with a thin, impermeable cuticle that prevents pollen from adhering, hydrating, or germinating. Malpighiaceae have a special type of 'wet' stigma, where a secretion accumulates under the cuticle and is released by mechanical means-mainly rupture by pollinators. Even though six species show a certain degree of self-compatibility, four of them present a form of late-acting self-incompatibility, and the individual of B. pubipetala is agamospermous. Species of Centris, Epicharis and Monoeca bees pollinate these flowers, mainly collecting oil. Some Epicharis and Monoeca species collected pollen by vibration. Paratetrapedia and Tetrapedia bees are pollen and oil thieves.

CONCLUSIONS

The Malpiguiaceae species studied are pollinator-dependent, as spontaneous self-pollination is limited by herkogamy, protogyny and the stigmatic cuticle. Both the oil- and pollen-collecting behaviours of the pollinators favour the rupture of the stigmatic cuticle and the deposition of pollen on or inside the stigmas. As fruit-set rates in natural conditions are low, population fragmentation may have limited the sexual reproduction of these species.

Glucose localization in maize ovaries when kernel number decreases at low water potential and sucrose is fed to the stems

BACKGROUND AND AIMS

Around the time of anthesis, young ovary development in maize (Zea mays) is vulnerable to 2 or 3 d of water deficits that inhibit photosynthesis. Abortion can result, and fewer kernels are produced. A breakdown of stored ovary starch is associated with the abortion and was investigated in the present study by localizing the breakdown product glucose in the ovaries.

METHODS

Ovary glucose was localized with fluorescent Resorufin. Insoluble invertase was localized in vivo and soluble invertase in situ. Sucrose was infused into the stems to vary the sugar flux to the ovaries.

KEY RESULTS

At high water potential (high Psi(w)), photosynthesis was rapid in the parent. The upper pedicel of the ovaries had a high activity of insoluble acid invertase and a large amount of glucose and starch. Because the invertase was wall-bound, sucrose hydrolysis appeared to occur in the pedicel apoplast. Soluble invertase was undetected inside the pedicel cells but was present in the nucellus cells where low concentrations of glucose occurred. This created a glucose gradient between pedicel and nucellus that favoured glucose uptake by the developing ovary. At low Psi(w), photosynthesis was inhibited, pedicel glucose and starch were depleted, the glucose gradient became negligible, and abortion occurred. When sucrose was fed, glucose, starch and the glucose gradient were maintained somewhat and were normally distributed in the ovaries. Abortion was diminished.

CONCLUSIONS

The apoplast hydrolysis of sucrose unloaded from phloem is similar to that described by others during later development when embryo and endosperm are present and separated from the parent by an apoplast. The disappearance of the glucose gradient at low Psi(w) may have inhibited glucose movement into the ovary. The low glucose in the ovaries may have a role in the abortion response.

Sexual mimicry in Mormolyca ringens (Lindl.) Schltr. (Orchidaceae: Maxillariinae)

BACKGROUND AND AIMS

Pollination through sexual mimicry, also known as pseudocopulation, has been suggested to occur in some genera of the Neotropical orchid subtribe Maxillariinae. However, it has been demonstrated so far only for Trigonidium obtusum. This study reports and illustrates pollination through sexual mimicry in Mormolyca ringens.

METHODS

A total of 70 h were dedicated to the observation of flowers and pollinator behaviour, which was photographically recorded. Flower features involved in pollinator attraction were studied using a stereomicroscope and by SEM analyses. Preliminary observations on the plant breeding system were made by manually self-pollinating flowers. The chemical composition of the fragrance volatiles was determined by GC/MS analysis.

KEY RESULTS

The flower features of M. ringens parallel those of other pseudocopulatory flowers. The labellum shape and indument are reminiscent of an insect. Sexually excited drones of Nannotrigona testaceicornis and Scaptotrigona sp. (both in the Apidae: Meliponini) attempt copulation with the labellum and pollinate the flower in the process. In both bee species, the pollinarium is attached to the scutellum. Pollinator behaviour may promote some degree of self-pollination, but preliminary observations indicate that M. ringens flowers are self-incompatible. Flowers are produced all the year round, which ties in with the production of bee males several times a year. The phylogenetic relationships of M. ringens are discussed and a number of morphological and phenological features supporting them are reported.

CONCLUSIONS

It is expected that further research could bring to light whether other Maxillariinae species are also pollinated through sexual mimicry. When a definitive and robust phylogeny of this subtribe is available, it should be possible to determine how many times pseudocopulation evolved and its possible evolutionary history.

Critical stages in the recruitment process of Rhamnus alaternus L

BACKGROUND AND AIMS

Rhamnus alaternus is a Mediterranean shrub commonly used in reforestation programs. Although several aspects of its reproductive biology have been studied, little is known about the importance of the different recruitment stages in the overall regeneration process of this species, which limits its proper use in Mediterranean forests and shrubland management. The aim of the present work was to quantify the importance of the different recruitment stages in the regeneration process of R. alaternus.

METHODS

Two populations of Rhamnus alaternus on the island of Mallorca that differ in climatic conditions, type of habitat and sex ratio were studied. The importance of seed production, seed dispersal and predation, seedling emergence and seedling survival for the regeneration of this species were quantified.

KEY RESULTS

In both populations, fruit set and fruit removal by animals were not critical stages, since almost half of the flowers became mature fruits and 90% of those were dispersed. Most seeds were deposited under female conspecifics (86 and 47%, at Lloret and Esporles, respectively), and very few were found in open inter-spaces (1 and 5%). Post-dispersal seed predation (mostly by ants and rodents) was very high in both populations. Seedling emergence took place during autumn and early winter and it ranged from 31 to 68% depending upon year and microhabitat. The majority of emerged seedlings died during the first year, mainly due to desiccation; such mortality was influenced by rainfall and differed among microhabitats (varying from 67 to 100%). The general spatial distribution of seed rain was concordant with the seedling emergence and survival pattern in both populations.

CONCLUSIONS

The recruitment of Rhamnus alaternus appeared to be mainly limited by seed and seedling survival, regardless of the type of habitat in which the species is found.

Does the 'old bag' make a good 'wind bag'?: Comparison of four fabrics commonly used as exclusion bags in studies of pollination and reproductive biology

BACKGROUND AND AIMS

Fabrics used in pollination bags may exclude pollen carried by biotic vectors, but have varying degrees of permeability to wind-borne pollen. The permeability of bags to wind-borne pollen may have important consequences in studies of pollination and reproductive biology. The permeability of four fabrics commonly used in the construction of pollination bags was examined.

METHODS

Deposition of wind-borne pollen on horizontally and vertically oriented microscope slides was assessed on slides enclosed in pollination bags, as well as on control slides.

KEY RESULTS

It was found that the permeability of fabrics to wind-borne pollen, as measured by deposition on both horizontally and vertically oriented slides, decreased with pore size. However, deposition on horizontal slides was always greater than on vertical slides for a given fabric; this could manifest itself as differential success of pollination of flowers in bags-dependent on flower orientation.

CONCLUSIONS

Obviously, bags with mesh size smaller than most pollen grains are impermeable to pollen. However, material for such bags is very expensive. In addition, it was also observed that bags with even moderately small pore size, such as pores (approx. 200 microm) in twisted fibre cotton muslin, offered highly significant barriers to passage of wind-borne pollen. Such bags are sufficiently effective in most large-sample-size reproductive biology studies.

Lip anatomy and its implications for the pollination mechanisms of Bulbophyllum species (Orchidaceae)

BACKGROUND AND AIMS

The lip structure of six Brazilian and one Asiatic species of Bulbophyllum with wind-assisted fly pollination (B. involutum, B. ipanemense and B. weddellii) and non-wind-assisted fly pollination (B. epiphytum, B. glutinosum, B. regnellii and B. rothschildianum) was studied to investigate the presence of secretory tissues related to these pollination mechanisms.

METHODS

The lip study was carried out through scanning electron microscopy (lip surface) and light microscopy (anatomical features).

KEY RESULTS

In most of the species studied, the osmophores (odour glands) were located in the lobes and in the upper surface of the lip callus. Differences in the lip structure were observed between the two groups (the presence of a nectary and the extent of osmophore surface), depending on the mechanism of pollination. Nectaries were found in the cavity callus in B. ipanemense, B. involutum and B. weddellii, even though their pollinators were presumably attracted by the instinct to oviposit.

CONCLUSIONS

These findings corroborate the hypothesis that, because pollination in these species is dependent on an unpredictable external factor (wind), nectar is necessary to keep the insect in the flower for a long period. Despite the occurrence of a liquid-like nectar in the flowers of B. epiphytum, B. glutinosum, B. regnelli and B. rothschildianum, no anatomical evidence for nectaries was found in the lips of these species, although a similar structure may occur in another region of the flowers. This observation agrees with the fact that pollination by lip movement in the latter species requires only gravity, with no additional mechanism being needed to keep the flies in the flower.

The pollination ecology of an assemblage of grassland asclepiads in South Africa

The KwaZulu-Natal region of South Africa hosts a large diversity of asclepiads (Apocynaceae: Asclepiadoideae), many of which are endemic to the area. The asclepiads are of particular interest because of their characteristically highly evolved floral morphology. During 3 months of fieldwork (November 2000 to January 2001) the flower visitors and pollinators to an assemblage of nine asclepiads at an upland grassland site were studied. These observations were augmented by laboratory studies of flower morphology (including scanning electron microscopy) and flower colour (using a spectrometer). Two of the specialized pollination systems that were documented are new to the asclepiads: fruit chafer pollination and pompilid wasp pollination. The latter is almost unique in the angiosperms. Taxa possessing these specific pollination systems cluster together in multidimensional phenotype space, suggesting that there has been convergent evolution in response to similar selection to attract identical pollinators. Pollination niche breadth varied from the very specialized species, with only one pollinator, to the more generalized, with up to ten pollinators. Pollinator sharing by the specialized taxa does not appear to have resulted in niche differentiation in terms of the temporal or spatial dimensions, or with regards to placement of pollinaria. Nestedness analysis of the data set showed that there was predictability and structure to the pattern of plant-pollinator interactions, with generalist insects visiting specialized plants and vice versa. The research has shown that there is still much to be learned about plant-pollinator interactions in areas of high plant diversity such as South Africa.

Floral longevity and nectar secretion of Platanthera chlorantha (Custer) Rchb. (Orchidaceae)

Flowering and nectar secretion were studied in Platanthera chlorantha in two years. Nectar was secreted and accumulated in this orchid's spur, originating from part of the labellum. The nectary spur was, on average, 32 mm long. It produced 6.86 micro l nectar in 1999 and 7.84 micro l in 2000. The number of flowers per inflorescence and the volume of nectar secreted per flower were not correlated. Nectar secretion and flower longevity differed depending on pollination and flower position in the inflorescence. Among pairs of pollinated and unpollinated flowers there was no difference in the volume of nectar produced; however, the life span of pollinated flowers was shorter than that of unpollinated ones. Within an inflorescence, the lowest-positioned flowers had the largest nectar production and the longest life compared with flowers positioned higher up.

Opportunistic out-crossing in Nicotiana attenuata (Solanaceae), a predominantly self-fertilizing native tobacco

BACKGROUND

Although Nicotiana attenuata is entirely self-compatible, chemical and other floral traits suggest selection for the maintenance of advertisement for moth pollinators.

RESULTS

Experimental exclusions of pollinators from plants with emasculated flowers in natural populations in southern Utah during an outbreak of the hawkmoth Hyles lineata revealed that 24% of the seed set could be attributed to insect pollination, and eliminated wind pollination and apomixis as contributing to seed set. Hence these moths can mediate gene flow when self-pollen is unavailable. To quantify gene flow when self-pollen is available, plants were transformed with two marker genes: hygromycin-B resistance and beta-glucuronidase. The utility of these genetic markers to measure gene flow between plants was examined by mixing pollen from plants homozygous for both genes with self-pollen in different ratios and hand-pollinating emasculated flowers of plants growing in a natural population. The proportion of transformed seeds was positively correlated with the amount of transformed pollen applied to stigmas. In glasshouse experiments with the hawkmoth Manduca sexta and experimental arrays of transformed and wild-type plants, pollination mediated by moths accounted for 2.5% of the seed set.

CONCLUSIONS

Even though moth pollination is rare and highly variable for this largely selfing plant, N. attenuata opportunistically employs a mixed-mating system.

Lipoidal labellar secretions in Maxillaria ruiz & pav. (Orchidaceae)

The labella of Maxillaria acuminata Lindl., M. cerifera Barb. Rodr. and M. notylioglossa Rchb.f., all members of the M. acuminata alliance, produce a viscid wax-like secretion. Histochemical analysis revealed that the chemical composition of the secretion is similar in all three species, consisting largely of lipid and protein. Light microscopy and low-vacuum scanning electron microscopy were used to investigate the secretory process. In a fourth taxon, M. cf. notylioglossa, transmission electron microscopy showed that lipid bodies are associated with smooth endoplasmic reticulum or occur as plastoglobuli within plastids. Lipid bodies vary in appearance and this may reflect differences in chemical composition. They become associated with the plasmalemma and eventually accumulate between the latter and the cell wall. The wall contains no pits or ectodesmata, and it is speculated that lipid passes through the wall as small lipid moieties before eventually reassembling to form lipid globules on the external surface of the cuticle. These globules are able to coalesce forming extensive viscid areas on the labellum. The possible significance of this process to pollination is discussed.