FLORAL PARAMETERS,BREEDING SYSTEM AND POLLINATOR TYPE IN TRICHOSTEMA (LABIATAE)

Diversity in flower size and degree of exsertion of anthers and stigma from the corolla in the California species of Trichostema appear to be correlated with breeding system and pollinator type (bee vs. bird). Autogamous (self-pollinating) species unlike xenogamous (cross-pollinating) species lack spatially separate anthers and stigma and nototribic pollination. The outcrossing species have significantly larger flowers, significantly larger nectar volumes and significantly higher pollen-ovule ratios than do selfing species. Because autogamous species are less reliant on flower visitors to facilitate pollination, there may be relaxed selection for large nectar producing flowers. Pollen-ovule ratios are correlated with breeding system and reflect the efficiency of pollen transport. Data on floral parameters suggest xenogamous species expend more energy per floral unit and are less efficient seed producers than closely related autogamous species.     

Patterns and development of floral asymmetry in Senna (Leguminosae, Cassiinae)

The buzz-pollinated genus Senna (Leguminosae) is outstanding for including species with monosymmetric flowers and species with diverse asymmetric, enantiomorphic (enantiostylous) flowers. To recognize patterns of homology, we dissected the floral symmetry character complex and explored corolla morphology in 60 Senna species and studied floral development of four enantiomorphic species. The asymmetry morph of a flower is correlated with the direction of spiral calyx aestivation. We recognized five patterns of floral asymmetry, resulting from different combinations of six structural elements: deflection of the carpel, deflection of the median abaxial stamen, deflection or modification in size of one lateral abaxial stamen, and modification in shape and size of one or both lower petals. Prominent corolla asymmetry begins in the earl-stage bud (unequal development of lower petals). Androecium asymmetry begins either in the midstage bud (unequal development of thecae in median abaxial stamen; twisting of androecium) or at anthesis (stamen deflection). Gynoecium asymmetry begins in early bud (primordium off the median plane, ventral slit laterally oriented) or midstage to late bud (carpel deflection). In enantiostylous flowers, pronouncedly concave and robust petals of both monosymmetric and asymmetric corollas likely function to ricochet and direct pollen flow during buzz pollination. Occurrence of particular combinations of structural elements of floral symmetry in the subclades is shown.     

Floral ecology of Oreocharis pumila (Gesneriaceae): a novel case of sigmoid corolla

Attraction of pollinators and successful pollen transfer represent the primary targets of selection during flower evolution, leading to repeated evolutionary shifts between pollinators and consequently to the diversification of floral forms. However, most studies in floral evolution focus on the characteristics of flowers with straight corolla tube. Here, we report on an unusual form of sigmoid corolla combined with protandry and herkogamy in a Chinese species of Gesneriaceae, Oreocharis pumila (formerly Opithandra pumila). Contrary to species with sigmoid corollas studied previously, the base of the corolla tube of this species is inclined at an oblique angle downwards before the tube bends forward, and the stigma and anthers are included in the upper part of the corolla tube. The plants were found to be self‐compatible but incapable of autonomous selfing. Successful pollination was found to depend fully on the presence of insect pollinators (Nomia sp.) and pollen grains are the greatest reward for the visitors. Different from the other sigmoid flowers, the sigmoid corolla of O. pumila was not found to favor insect pollinators with long flexible proboscises. A mechanical fit between floral morphology and pollinator was found, in which only small insect visitors with specialized visiting behavior are legitimate pollinators. The protandry combined with herkogamy in the sigmoid corolla tube strongly ensures pollination efficiencies. Oreocharis pumila is the only species with sigmoid corolla in the genus Oreocharis. We hypothesize that such a corolla has arisen through selection due to inadequate pollination in early spring in the mountainous habitat that O. pumila occupies.     

The pollination biology of Burmeistera (Campanulaceae): specialization and syndromes

The floral traits of plants with specialized pollination systems both facilitate the primary pollinator and restrict other potential pollinators. To explore interactions between pollinators and floral traits of the genus Burmeistera, I filmed floral visitors and measured pollen deposition for 10 species in six cloud forest sites throughout northern Ecuador. Nine species were primarily bat-pollinated (84-100% of pollen transfer); another (B. rubrosepala) was exclusively hummingbird-pollinated. According to a principal components analysis of 11 floral measurements, flowers of B. rubrosepala were morphologically distinct. Floral traits of all species closely matched traditional ornithophilous and chiropterophilous pollination syndromes; flowers of B. rubrosepala were bright red, lacked odor, opened in the afternoon, and had narrow corolla apertures and flexible pedicels, which positioned them below the foliage. Flowers of the bat-pollinated species were dull-colored, emitted odor, opened in the evening, and had wide apertures and rigid pedicels, which positioned them beyond the foliage. Aperture width appeared most critical to restricting pollination; hummingbirds visited wide flowers without contacting the reproductive parts, and bats did not visit the narrow flowers of B. rubrosepala. Aperture width may impose an adaptive trade-off that favors the high degree of specialization in the genus. Other floral measurements were highly variable amongst bat-pollinated species, including stigma exsertion, calyx lobe morphology, and pedicel length. Because multiple species of Burmeistera often coexist, such morphological diversity may reduce pollen competition by encouraging pollinator fidelity and/or spatially partitioning pollinator's bodies.     

Sexual differences in reproductive characters and pollinator attractiveness in gynodioecious Glechoma longituba (Lamiaceae)

We quantified the differences in floral characters and attractiveness to flower visitors under natural conditions between the sexual types in the gynodioecious plant Glechoma longituba. We also manipulated flowers by altering corolla size or nectar volume, or by removing anthers, to examine the effect of these primary and secondary attractants (i.e. rewards and advertisements) on attractiveness. A change in corolla size and shape reduced visiting frequency and pollen load. Removal of anthers did not affect visiting rates, but significantly reduced pollination rates and stigmatic pollen load. A decrease in the nectar volume of a flower was associated with a reduction in handling time and pollen loads on stigmas. These results show that corolla size is an important advertisement to pollinators (particularly at greater distance), which associate hermaphrodite flowers with a larger corolla and a larger volume of nectar than female flowers. We found that artificial changes in population structure affected the behavior of pollinators as well as the pollination rates of flowers. We suggest that the pattern of distribution of hermaphrodite and female clones in a population may serve to avoid pollen limitation in a female clone or patch. This effect may ensure female reproductive success and allow for the maintenance of female individuals in natural populations of this gynodioecious plant.     

山莨菪(茄科)的传粉生物学

茄科的多数种类具有自交不亲和的特点, 主要通过异花传粉结实; 但是, 一些物种或者物种内的部分种群或者个体却高度自交亲合, 转变为自交的繁育系统。该科植物山莨菪(Anisodus tanguticus)主要分布在青藏高原, 开花较早, 比其他晚开花的植物种类更加缺少有效的异花传粉昆虫。我们选择了位于不同海拔高度的2个种群进行比较研究, 主要目的是检验该物种的繁育系统是否在极端环境下由于传粉者的缺乏而发生了部分改变。研究发现,山莨菪的花不完全雌性先熟, 柱头和花药间的平均距离随着花开放时间的延长而不断缩小, 但两者在多数花的单花花期结束时并没有发生接触。因此, 山莨菪花主要表现为适应异花传粉的雌雄异位特征。然而, 少数花 (4.9％)的柱头和花药发生接触, 为“自动自交”的传粉解除了空间隔离。2个种群的多数个体存在自交不亲和机制, 应具有异花传粉的繁育系统; 但是部分个体具有明显的自交亲和能力, 为自交提供了生理基础。高海拔种群的传粉昆虫主要是厕蝇(Fannia sp.), 它们在不同植株间的活动能够保证异花传粉结实; 同时该种群的部分个体存在“自动自交”。低海拔种群的主要访花昆虫是蚂蚁, 它们在花内的活动导致花粉在同一朵花内传递, 而引起“协助自交”; 而异花传粉昆虫厕蝇的访花频率则较高海拔种群低。两个种群的结实均由于异花传粉者不足而受到传粉限制。因此两种不同类型的自交机制为该早期开花植物异花访花昆虫的不足提供了一定程度上的繁殖补偿。     

Pollination Ecology and Endemism of Pedicularis pulchella Pennell (Scrophulariaceae)

Abstract Pedicularis pulchella Pennell, endemic to sparsely vegetated areas of alpine tundra scree in southwestern Montana and northwestern Wyoming, was found to be roat-hemiparasitic and obligately dependent for its pollination upon bumblebees of three non-tundra species that forage primarily in an inverted position for pollen scraped from anthers concealed in the galea, This sternotribic pollination syndrome is considered related to the short (5–6mm) proboscides of pollinators on flowers with deep (12.4mm), nectariferous corolla tubes and favoring pollen as an insect attractant. Photographic and spectropholomelric analysis of the red-purple floral color included prominent blue corolla and ultraviolet calyx reflections corresponding to dominant colors of bumblebee visual spectra. Nectar analysis by thin-layer chromatography and refractomctry indicated, respectively, the presence of fructose and sucrose, and a sucrose-equivalent concentration of dissolved solids exceeding 50%; both coincide with the general pattern of the genus. Analysis of pollen from corbicular loads of P. pulchella pollinators indicated relatively low pollen-constancy of insects and substantial foraging association with Vaccinium in the montane-subalpine zone. By contrast, pollinators on P. cystopteridifolia and Phyllodoce glanduliflora in a diverse tundra plant community on rich alpine turf revealed broad pollinator spectra (7–8 species), high pollen-foraging constancy on Pedicularis, and high nectar-foraging frequency on Phyllodoce : The pollination ecology and endemism of Pedicularis pulchtella are interpreted in terms of resource sharing, spatial competition of plant species, and selective stress of the physical environment.     

大王马先蒿的传粉综合征状及其生物地理学意义

对云南西北部中甸和昆明黑龙潭的大王马先蒿（ＰｅｄｉｃｕｌａｒｉｓｒｅｘＣｌａｒｋｅＣ．Ｂ）的不同居群进行了传粉生物学观察。大王马先蒿花为黄色,花冠短管、无喙、具二齿、无蜜腺,是一种原始的花冠类型。该种由红腹熊蜂（ＢｏｍｂｕｓｆｒｉｓｅａｎｕｓＳｋｏｒｉｋｏｖ）和明亮熊蜂（ＢｏｍｂｕｓｌｕｃｏｒｕｍＬ．）的工蜂在直立的位置上,通过震动花粉背触式传粉;同时也观察到红腹熊蜂的工蜂在倒悬的位置上,通过震动花粉腹触式传粉。熊蜂的工蜂在大王马先蒿上的这两种传粉综合征状的发现,可能是仅存在于马先蒿原始类型中的一种特殊现象。     

Shortened anther-stigma distance reduces compatible pollination in two distylous Primula species

Aims Distyly has been regarded as an adaptation to improve compatible pollination between two floral morphs with reciprocal herkogamy. The hypothesis that the different positions of anthers and stigmas within flowers as well as their reciprocal position between morphs, reduce the probability of self pollination raised by Darwin has been rarely tested. In this study, we measured stigmatic pollen loads in response to reduced reciprocal herkogamy in two Primula species.Methods To see whether reciprocal herkogamy can increase compatible and/or reduce incompatible pollen deposition, thus promoting compatible pollination, we shortened the distance between anthers and stigmas within the flowers by changing the position of the corolla tube, to which the anthers were fused, i.e. reduced herkogamy in natural populations of Primula secundiflora and P. poissonii and quantified stigmatic pollen loads in the field over 2 years.Important findings In both species, stigmatic pollen loads were significantly higher in the long-styled (L-morph) than in the short-styled morph (S-morph) in both control and manipulated flowers, but percentage of compatible pollen in S-morph were higher. Flowers manipulated to halve the anther–stigma distance showed a similar pattern for 2 years: total pollen grain counts on stigmas did not differ significantly but compatible pollen grains in L- and S-morphs were significantly decreased in both species. The percentage of compatible pollen loads was decreased by 68.7% in P. secundiflora and 65.3% in P. poissonii in L-morphs, while it decreased by 30.6% and 2.9% in S-morphs, respectively. Our manipulation of the relative position of anthers and stigmas in the two distylous species indicated that a lower degree of herkogamy reduced compatible but incompatible pollen transfer was likely to increase. The higher proportion of compatible pollen in the S-morph than in the L-morph in the two Primula species could be attributed to the accessibility of two-level sexual organs, floral orientations and pollinator behaviors. This is a first attempt to manipulate intraflower herkogamy for understanding adaptation of heterostyly, shedding insights into how the reciprocal herkogamy promotes compatible pollination.     

马先蒿属花冠形态的多样性与传粉式样的关系

马先蒿属(Pediculais)是有花植物中花冠形态多样化最为集中的属。该属主要的传粉者是熊蜂属(Bormbus)昆虫;在北美,熊蜂和蜂鸟是马先蒿植物一些种类有效的传粉者;也发现壁蜂(Osmia)为其传粉。不同的传粉机制要求某一特定的取食式样储藏和释放花粉。本文讨论了花冠类型的进化趋势与传粉式样和花粉形态的关系。传粉者的选择压力是决定花冠多样化的重要因素之一;花冠类型与传粉者和传粉行为紧密相关。马先蒿植物和传粉者的相互依赖与其花冠类型、功能和物候互相适应,但花冠类型与花粉形态两者之间似乎没有明显的一一对应关系。通过北美、日本和喜马拉雅不同地理分布马先蒿种类的比较研究表明,具有相同花冠类型的种类有着相同的传粉方式,花冠形态与传粉式样存在紧密的协同进化关系。     

Population-level variation in the expression of heterostyly in three species of Rubiaceae: does reciprocal placement of anthers and stigmas characterize heterostyly?

Heterostyly (i.e., reciprocal placement of anthers and stigmas between two or three floral morphs) is hypothesized to enhance outcrossing and reduce selfing. However, few studies have documented reciprocity among individual plants; instead, mean anther and stigma heights for floral morphs are usually reported, masking interindividual variation. We measured eight floral dimensions for individuals in five populations of three heterostylous Rubiaceae. The three methods used to quantify reciprocity yielded different conclusions regarding the degree to which populations conformed to expectations for heterostylous plants. Only Psychotria poeppigiana had stigma and, to a lesser degree, anther heights in discrete classes. Variation among plants of Bouvardia ternifolia and Psychotria chiapensis yielded a continuum of anther and stigma heights across populations. Comparison of distances between stigma and anthers indicated that only flowers of B. ternifolia had, as expected, a constant value for this distance. Finally, regression relationships between anther and stigma heights and corolla length showed that only in one population each of B. ternifolia and P. poeppigiana, and in P. chiapensis, was distance between anthers and stigmas the same across the range of corolla sizes for both floral morphs. Variation among these species in expression of heterostyly was not clearly linked to phylogenetic relationship or pollinator syndromes. Two approach herkogamous (AH) species were studied for comparison. Flowers of Psychotria brachiata were consistently AH, but flowers of P. pittieri were highly variable. Determining fitness consequences of population-level variation in sexual systems requires studies linking floral morphology to pollinator behavior and pollen transfer.     

The pollination ecology of Pedicularis rex subsp. lipkyana and P. rex subsp. rex (Orobanchaceae) from Sichuan, southwestern China

Pedicularis is one of few genera for which pollination ecology has been studied extensively. Although over half of the species of Pedicularis are found in the mountains of southwestern China, pollination ecology has been studied there on a few species only. The present paper reports pollination ecology of Pedicularis rex subsp. lipskyana and P. rex subsp. rex from Sichuan, southwestern China. The two subspecies are nectariferous, with sugar content 28% in P. rex lipskyana and 22% in P. rex, respectively. The flowers of the two subspecies are adapted to pollination by bumblebee workers. Bumblebees are the primary and effective pollinators although honeybees were also observed pollinating P. rex subsp. lipskyana at one studied population. Usually bumblebees entered the corolla tube from the right side in an upright or a nearly half-inverted position to imbibe nectar located at the base of the corolla tube. The stigma usually contacts the side region of the thorax and abdomen or occasionally directly contacted pollen loads, leading to pollination of flowers. Much less commonly, bumblebees foraged on flowers of P. rex subsp. rex in an inverted position for pollen, which pollinated flowers sternotribically. We suggest that pollination mechanism is closely associated with floral morphology. Some important differences were found from a previous study on P. rex in floral morphology and the primary pollination mechanism.     

臭椿花器官分化的初步研究

利用扫描电镜(SEM)和光镜(LM)对臭椿花序及花器官的分化和发育进行了初步研究,表明:1)臭椿花器官分化于当年的4月初,为圆锥花序;2)分化顺序为花萼原基、花冠原基、雄蕊原基和雌蕊原基。5个萼片原基的发生不同步,并且呈螺旋状发生;5个花瓣原基几乎同步发生且其生长要比雄蕊原基缓慢;雄蕊10枚,两轮排列,每轮5个原基的分化基本是同步的;雌蕊5,其分化速度较快;3)在两性花植株中,5个心皮顶端粘合形成柱头和花柱,而在雄株中,5个心皮退化,只有雄蕊原基分化出花药和花丝。本研究着重观察了臭椿中雄花及两性花发育的过程中两性花向单性花的转变。结果表明,臭椿两性花及单性花的形成在花器官的各原基上是一致的(尽管时间上有差异),雌雄蕊原基同时出现在每一个花器官分化过程中,但是,可育性结构部分的形成取决于其原基是否分化成所应有的结构:雄蕊原基分化形成花药与花丝,雌蕊原基分化形成花柱、柱头和子房。臭椿单性花的形成是由于两性花中雌蕊原基的退化所造成,其机理有待于进一步研究。     

The Pollination Syndrome of Pedicularis rex (Scrophulariaceae) and Its Biogeographic Significance

The pollination biology of different populations of Pedicularis rex Clarke C B. was studied in Zhongdian of Northwest Yunnan and in Kunming of central Yunnan, respectively. Seed production depends exclusively upon pollination by bmnblebees ( Bombus Latr. ). The flowers are yellow, with short-tubed, erostrate and nectarless corolla. Two species of the bumblebee workers, Bombus friseanus Skorikov and B. lucorum L. foraged in an upright (nototribic) position for pollen. Meanwhile, Bombus friseanus workers also foraged in an inverted (stemotribic) position on P. rex, vibrating pollen from the anthers within the galea. This pollination syndrome was previously reported in the North American species, e.g.P, labradorica which has the same corolla-type with P. rex. A comparison of the corolla types and pollination syndrome on the species of Pediadaris in East Asia and North America indicates that such a pollination syndrome may be only found in the primitive corolla type of Pedicularis. In spite of the separation in the geological history, P. rex and P.labradorica share the same corolla type and pollination syndrome. Further study is needed to elucidate the biogeography of the Pedicularis.     

Generalized and complementary pollination system in the Andean cactus Echinopsis schickendantzii

The considerable floral diversity present in the cactus family has often been associated with the specificity of its pollinators. However, many cactus pollination systems are generalized as their flowers are pollinated by a wide spectrum of animals. For example, cactus species with white flowers, nocturnal anthesis and extended floral cycles would present generalized pollination systems in which both nocturnal and diurnal visitors could be effective pollinators. In this article, we tested this hypothesis by studying the pollination biology of Echinopsis schickendantzii, an Andean cactus with sphingophilous flowers. In addition, we evaluated whether the cactus’s pollination system is complementary or redundant regarding the relative contributions of nocturnal and diurnal pollinators. Specifically, we studied the floral cycle, the reproductive system and the pollination effectiveness of floral visitors. The flowers of E. schickendantzii are self-incompatible; they opened at crepuscule and have an extended floral cycle. Moths were frequent visitors at night, whereas bees were frequent visitors during the day; both were effective pollinators of the cactus. Our results indicated that the flowers of this species present phenotypic, functional and ecological generalization, and their fruit set is determined by the contributions of both pollinator functional groups, i.e., they have complementary pollination systems. These results support the hypothesis that cacti in the extra-tropical deserts of South America have generalized pollination systems.     

Spectral polymorphisms in angiosperm flowers determined by differential ultraviolet reflectance

The ultraviolet reflectance pattern of flowers from 54 species representing 22 families, many native or introduced to Japan, were surveyed. Their differential ultraviolet patterns, i.e. spectral polymorphisms, were recorded by paired photographs of fresh flowers: one taken in full sunlight and the other through a visible spectrum absorbing filter. The different types of polymorphisms observed varied from total absorption by the whole flower to limited absorption by selected floral parts, such as corolla bases (nectar guides) or anthers. Some visible spectrum polymorphisms, e.g. inAnemone andHepatica, were not associated with different ultraviolet patterns. In three species ofIris, a distinct ultraviolet absorbing landing spot and nectar, tunnel guide as well as reflecting anthers are reported. The downward exerted anthers ofSolanum nigrum which shed pollen through bee wing vibration were ultraviolet absorbent. The implications of these ultraviolet patterns for biochemical (flavonoid), taxonomic, pollination and eco-evolutionary research are discussed. Attention is drawn to the seasonal distribution of ultraviolet radiation in the northern hemisphere and its selective role on northern floras.     

PATTERNS OF FLORAL DEVELOPMENT IN AGALINIS AND ALLIES (SCROPHULARIACEAE). II. FLORAL DEVELOPMENT OF AGALINIS DENSIFLORA

Initiation of floral primordia begins in Agalinis densiflora with production of two lateral adaxial calyx lobe primordia followed by a midadaxial primordium, and then primordia of two abaxial calyx lobes. Initiation of three abaxial corolla lobe primordia is succeeded by that of two stamen pairs and then by primordia of two adaxial corolla lobes. The primordium of the abaxial carpel appears before the adaxial one. Except for the calyx, initiation of primordia proceeds unidirectionally from the abaxial to the adaxial side of the floral apex. Zygomorphy in the calyx, corolla, and androecium is evident during initiation of primordia and is accentuated during organogenesis. The calyx undergoes comparatively rapid organogenesis, but the inner three floral series undergo a protracted period of organogenesis. The perianth series reach maturation prior to meiosis in the anthers. Maturation of the androecium and gynoecium are postmeiotic events.     

The filament curtain: a structure important to systematics and pollination biology in the Acanthaceae

The filament curtain is a complex structure found in Acanthaceae inside the corolla. It usually divides the corolla into two compartments, enclosing the main nectar bulk behind two adjacent filament bands. Four independent floral characters are involved in the structure of the filament curtain: a fusion of the filaments, decurrent filaments along the corolla wall, a slanting border between the synstapetal and apostapetal corolla regions (i.e. stamen corolla tube and corolla tube s.s .), and geniculate lower, lateral corolla lobe traces at this border. The distribution of a filament curtain within Acanthaceae suggests an enlarged tribe Ruellieae s.l ., including Louteridieae and Trichanthereae, excluding subtribe Barleriinae, and raises a question mark as to the systematic position of the genera Calacanthus, Glossochilus, Lankesteria and Mhitfieldia . The evolutionary origin of the filament curtain is probably connected with its functions in pollination biology, which are proposed to be those of restricted nectar access, prevention of nectar evaporation, lever arm function facilitating dorsal pollen deposition, and stabilizing of posticous position of anthers and style. Four types of filament curtains are distinguished: phaulopsoid, corolla fold, reduced and strobilanthoid. Variation in the filament curtain structure appears to be related to different pollination syndromes of the flowers. Mapping of this variation on to an existing phylogeny shows an evolution from a phaulopsoid type diverging into the other three types, possibly a development responding to shifts of pollinators.     

A preliminary study on pollination biology of Omphalogramma souliei Franch. (Primulaceae), a species endemic to China

The pollination biology of Omphalogramma souliei Franch., a species endemic to southwestern China, was investigated. Floral phenology, flower visitors, pollen/ovule ratio, attractants and rewards to the visitors were observed, measured, and recorded. Bagging experiments to exclude pollinators were carried out in the wild for two years. Our results revealed five important aspects of the reproductive biology of Omphalogramma souliei. 1) The pollen-ovule ratio was 1748±233. The breeding system was self-compatible, with facultative xenogamy. 2) The pollination syndrome is entomophily, and this species could not be pollinated by wind if the pollinators were unavailable. 3) Six insect species were observed visiting the flowers of Omphalogramma souliei in the wild, of them, three species of hymenoptera, Lasioglossum sp., Heriades parvula Cockerell and Micrapis florae Fabricius, are the principal floral visitors and effective pollinators. 4) The visual attractants to the visitors are floral color and shape, the large yellow anthers, and the rewards for visitors are pollen and nectar. 5) Cleistogamy may also occur, since the anthers of some flowers dehisced before opening of the corolla. The results of floral biology and pollination characters suggest that xenogamy predominated and autogamy played an assistant role in the evolution of reproduction and breeding system of O. souliei.     

Pollen flow in the distylous<Emphasis Type="Italic"> Palicourea fendleri</Emphasis> (Rubiaceae): an experimental test of the Disassortative Pollen Flow Hypothesis

The Disassortative Pollen Flow Hypothesis proposed by Darwin postulates that the relative position of anthers and stigmas in distylous flowers enhances pollen flow between flowers of different morphs (legitimate pollination), in comparison to flow between flowers of the same morph (illegitimate pollination). In order to test this hypothesis, we measured pollen transport, mediated by a trained Copper-rumped Hummingbird (Amazilia tobaci), between flowers of the distylous Palicourea fendleri under laboratory conditions. In individual tests, we offered to the hummingbird a pollen donor flower and two emasculated recipient flowers in a controlled sequence. After each foraging bout, we counted the number of pollen grains transported from the donor flower to the stigmas of both recipient flowers. In agreement with Darwin's hypothesis, we found that hummingbirds transport pollen of "pin" flowers in significantly higher numbers to legitimate "thrum" stigmas, even if previously visiting a "pin" flower. However, "thrum" pollen was deposited in greater numbers on illegitimate "thrum" stigmas. We interpret this asymmetry largely as the consequence of floral morphology; pollen flow was greater between anthers and stigmas that exhibit greater spatial matching. In P. fendleri, the position of floral organs along the corolla tube does not always precisely correspond. In our experimental system, the probability that the pollinator extracts a pollen grain from the anther and the probability of self-pollination were both dependent on the type of floral morph. We discuss the relevance of the latter findings in relation to other studies of pollen flow in heterostylous species.