Phenotypic models of selection are used to predict the ESS (evolutionarily stable strategy) allocation to pollen, seeds and pollination mechanisms (corolla, etc.) as a function of the frequency of self-fertilization in plants with different modes of self-fertilization. The results show that: (1) As autogamous self-fertilization increases, the ESS allocation to seeds increases and the allocations to pollen and pollination mechanisms simultaneously decrease. (2) Whether or not there is any allocat
ion to pollination mechanisms with 100% selfing depends on whether self-pollination requires the activity of pollinators or occurs autonomously. (3) The best measure of the importance of self-fertilization is the proportion of adults derived from selfing. (4) If any autogamy occurs, the allocations to pollen and seeds are not equivalent to those to male and female functions. (5) Self-fertilization is not a special case of local mate competition. (6) Geitonogamous self-fertilization requires the same structures as outcrossing and has little effect on allocations. The models can explain the allocation patterns and high fruit set commonly associated with self-fertilization and they suggest further types of data that should be collected.
Pollen loads of honeybees, bumblebees and hoverflies were analysed and pollination efficiency estimated, while they were visiting the flowers of Prunus spinosa L. (Blackthorn); Crataegus monogyna Jacq. (Hawthorn); Rosa canina L. (Dog Rose) and Rubus fruticosus L. (Bramble). The relative efficiency of the different insect groups varied, depending on the plant species being visited. In every instance the high constancy of the visitors, their rapid foraging rates and the amounts of pollen they carr
y, would ensure that they are adequate pollen-transfer agents.
The perennial herb Delphinium nelsonii Greene is pollinated by hummingbirds and bumble-bees in Colorado, USA. Birds and bees were induced to forage in an aviary and the resulting carryover of pollen and fluorescent dye powder from an initial male-phase donor flower to emasculated female-phase recipients was compared. Bumble-bee visitation caused dye transfer whose properites closely resembled those of pollen transfer on average, while hummingbird visitation caused more variable deposition of dye
and pollen on recipients and longer distances of pollen than of dye movement on average. The last result implies that pollen dispersal and genetic neighbourhood area are more extensive than estimated previously from dye dispersal in D. nelsonii populations. Pollen transfer in this species also is comparable to that in two sympatric herbs, Ipomopsis aggregata (Pursh) V. Grant and Erythronium grandiflorum Pursh.
Rates of ingestion of sucrose varied with sucrose concentration and butterfly sex, age and size for Speyeria mormonia Edwards (Lepidoptera: Nymphalidae). Peak rates of ingestion occurred between sucrose concentrations of 30% and 40%, as predicted by previous theory. Males fed at a faster rate than did females under most experimental conditions. Rates were also high for medium to large individuals as opposed to small individuals. Ingestion rates decreased with age for males but not for females. T
hese results are compared with data available for other species and considered in the context of information on nectar concentrations and composition in the field and activity budgets for S. mormonia. Finally, the possible impact on plant-pollinator interactions is evaluated.
Bumble-bees (Bombus spp) vibrated ('buzzed') flowers of Actinidia, Symphytum, Borago and Polygonatum when collecting pollen. In Actinidia anthers, fresh pollen becomes available repeatedly as surface pollen dries, becoming mobile and powdery. When individual anthers were vibrated at known frequencies, pollen surged out at a critical frequency and at its harmonics. In Symphytum, Polygonatum and Borago, the resonating chamber of the vibrational pollen release mechanism may be defined by the intror
se, connivent anthers. In a 24-h study of Symphytum, vibration was most frequent, and bee visits peaked, at times of high temperature and vapour pressure deficit. Our investigation indicates that the operation of the mechanism requires the pollen to be dry and powdery and we suggest that some features of the shooting star flower form, notably the reflexed corolla lobes and the exposed, often dark staminal cone, can be interpreted as adaptations related to the floral microclimat
1. The question of whether flowers reabsorb nectar was investigated in Grevillea robusta (Proteaceae), which bears large inflorescences with copious dilute nectar exposed to the atmosphere. 2. Each flower secreted nectar for 2 days and secretion rates were strongly dependent on temperature. 3. Reabsorption of nectar was shown indirectly, as has been done for other species, by comparing the accumulated nectar (standing crop) with the much greater cumulative rate of secretion measured during repea
ted sampling. 4. Experimental manipulation of nectar volumes and concentrations permitted the direct demonstration of nectar reabsorption under field conditions, shown by a decrease in sugar content of overfilled flowers. This occurred from both a high volume and a high concentration. 5. The results indicate active regulation of nectar sugar content by the flowers, with a balance between secretion, evaporation and reabsorption.
1. Shrubs of Caryocar brasiliense are intensively visited by many ant species in the cerrado vegetation of south-east Brazil. Ants collect nectar from the extrafloral nectaries (EFNs) located on the buds at the apex of the plant. 2. The proportion of C. brasiliense shrubs occupied by ants greatly surpassed that of neighbouring plants without EFNs and this resulted in many more termites being attacked and removed on this plant than on plants lacking EFNs. Ant attacks on Caryocar were more frequen
t near the EFNs. 3. Ant-exclusion experiments with Caryocar shrubs revealed that ants significantly reduce the infestation levels by the butterfly Eunica bechina, the sucking bud Edessa rufomarginata, the bud-destroying fly Prodiplosis floricola and by a stem-galling wasp. Ants had no effect on the infestation by a leaf-galling wasp. 4. Control (ants present) and treatment (ants excluded) plants produced equal numbers of buds but flower and initial fruit production was significantly greater for antvisited plants. Higher abortion rates of initial fruits by control plants resulted in similar final fruit and seed sets for plants in either experimental category. 5. The extremely low fertility of cerrado soils is suggested as a possible factor constraining the ability of Caryocar shrubs to provide the resources necessary for growth and development of their heavy fleshy fruits. Higher production of initial fruits by antvisited shrubs would be neutralized by competition among developing fruits and subsequent abortion. 6. Possible ant-derived benefits to C. brasiliense include: protection of vegetative tissues during plant growth, larger floral display and increased attractiveness to bat pollinators, increased male contribution to fitness through pollen donation and selective abortion of genetically inferior progeny as the maternal investment is adjusted to match available resources.
1. The functional significance of inflorescence morphology of Daucus carota L. (Apiaceae) was tested using an inflorescence manipulation experiment. Specifically, we sought to explain the role (if any) in pollination of the dark central floret of this species. 2. In central England, D. carota was found to be pollinated by a taxonomically wide range of insects that varied in their inflorescence visitation rates and in their pollen loads. There was also variation in abundance of some taxa between
the 2 years of this study. 3. By removing the dark central floret and adding false florets we sought to affect pollinator visitation rates and seed set in a manner consistent with the ‘fly catcher effect’ hypothesis of Eisikowitch (1980). Taxa responded to these manipulations but not in ways that shed light on the adaptive role of the dark central floret. Seed set was likewise unaffected in any consistent way. 4. The role of the dark central floret remains an enigma. We speculate, following Darwin (1888), that the dark central floret may now be functionless and possibly represents a trait that has persisted long after its original function has been lost.
1. Patterns of pollinator responses to variation in floral display size have significance for pollen flow among plants. Here we test a theoretical model for explaining such patterns by simultaneously assessing bumble bee behaviour and nectar availability in two native stands of Cirsium purpuratum with different spatial densities.
2. A bumble bee (Bombus diversus) foraging on a plant remembered and avoided only one or two flower heads that it had probed before, so that the flower‐head revisitatio
n rate increased as it stayed longer on a plant. Moreover, the revisitation rate increased less rapidly on larger displays.
3. The number of heads probed per plant increased less than proportionally with display size, and this increase was smaller at higher plant density. This pattern is consistent with our expectation that a bee leaves a plant when the cost of flower‐head revisitation exceeds that of interplant movement. However, bees left plants slightly earlier than predicted.
4. As predicted, the visitation rate of bees per plant showed a decelerated increase with floral display size, and this increase was greater at higher plant density.
5. As a result of these complementary changes in the number of heads probed per plant and visitation rate per plant across plant densities, nectar rewards per head were equalized among displays (an ideal free distribution was achieved).
Inflorescence architecture and floral morphology vary extensively within the Poaceae, but the functional significance of this variation remains largely unknown. As grasses are wind-pollinated, their inflorescence diversity probably reflects alternate solutions to manipulating airstreams to enhance pollen export and import. We tested this hypothesis with two field experiments that contrasted pollen removal and receipt by compact and diffuse inflorescences.
In the ‘aggregation’ experiment, we tied
together panicle branches of two species with diffuse inflorescences, creating more compact inflorescences. Aggregation reduced pollen removal from both species, probably by increasing boundary-layer thickness. The effects of inflorescence aggregation differed between the two species in a manner that is consistent with pollen-size differences, which could affect the ability of pollen grains to pass through the thickened boundary layer around stigmas.
The ‘staking’ experiment constrained inflorescence motion and revealed that culm characteristics contribute to the interaction between grass inflorescences and airstreams. In particular, inflorescence oscillation principally serves pollen removal for species with compact inflorescences, but is of primary importance in pollen receipt for species with diffuse architectures.
These results suggest that inflorescence architecture interacts with wind in a complex manner to facilitate pollination and supports the hypothesis that the extensive diversity of inflorescence architecture within the Poaceae represents evolutionary solutions to the physical constraints of wind pollination.
Resolving the controversy over the prevalence of generalization in plant–pollinator interactions requires field studies characterizing the pollination effectiveness of all a plant's floral visitors. Herein, the pollination effectiveness of all visitors to two species of barrel cactus (Ferocactus) was quantified. Flowers of both species were pollinated almost exclusively by cactus-specialist bees: 99% (F. cylindraceus (Engelm.) Orcutt) and 94% (F. wislizeni (Engelm.) Britt. and Rose) of all seeds
produced in this study resulted from cactus bee visits. For F. cylindraceus, the cactus-specialist Diadasia rinconis was the most abundant visitor. For F. wislizeni, three cactus-specialists (including D. rinconis) plus generalists in the family Halictidae (which did not act as pollinators) each accounted for a quarter of all visits. Diadasia rinconis visits to F. wislizeni flowers were more effective (per-visit) than visits by the other two cactus-specialists. Pollen-collecting and nectar-collecting visits were equally effective. Nectar-collecting visits were the most abundant. Apart from the non-pollinating halictids, floral visitors surprisingly did not include commonly co-occurring generalist bees. These data suggest that, just as apparently specialized flowers may be visited by a diverse assemblage of generalists, so apparently generalized flowers may be visited predominantly by specialists, and that these specialists may perform virtually all of the pollination.
1. Under natural selection for sexual success, the reproductive organs of plants should evolve to become highly effective pollen receptors. Among wind-pollinated plants, larger reproductive structures appear counter-adapted to accumulate pollen by impaction on their windward surfaces, because airborne particles are less able to penetrate the thicker boundary layer of larger targets. Therefore, it has been proposed that wind-pollinated plants with pollen receptors on relatively large structures,
like some grasses (family Poaceae), are architecturally adapted to create downstream vortices in which airborne pollen recirculates before accumulating on leeward surfaces. From this basis, the striking diversity among the grasses in the architecture of their flowering stems has been attributed in part to the existence of these contrasting mechanisms for effecting pollen receipt, namely impact collection and recirculatory collection.
2. We investigated the relative importance of impact and recirculatory collection in grasses by analysing a model system in silico using Computational Fluid Dynamics and by conducting in vivo experiments, both in a wind tunnel and outdoors, using two grass species with compact inflorescences, Alopecurus pratensis and Anthoxanthum odoratum.
3. Irrespective of the experimental approach, we found that although pollen recirculated in the leeward eddies of inflorescences, over 95% of the accumulated pollen was collected by windward surfaces.
4. In A. pratensis, the collection efficiency (proportion of oncoming pollen collected) was between 5% and 20%, depending on wind speed in the range 0·5–1·9 m s−1 and these levels conform to those predicted by a mechanistic model of impact collection.
5. Our results demonstrate that grass species with larger inflorescences are, like those with smaller inflorescences, primarily impact collectors of airborne pollen, which suggests that dissimilar reproductive morphology among species cannot be attributed to differentiation in the mode of pollen capture and, instead, requires reference to other factors, such as the need to produce, protect and disperse seeds of different sizes in different environments.