Article

The Aerodynamics of Plumed Seeds

Authors:
To read the full-text of this research, you can request a copy directly from the authors.

Abstract

The aerodynamics of plumed seeds are examined using 4 species of Compositae and Asclepias syriaca (Asclepiadaceae). The hairs comprising the plume of the seeds are modelled as a single long cylinder experiencing an ambient wind velocity equivalent to the measured terminal velocity in still air. The relevant measure of area is the total project area of the plume rather than the plan area of the imaginary "disk' in which the hairs reside. The hairs of Asclepias begin to bend significantly at a wind speed of 0.5-0.9 m s-1, and have an angle of attack of almost 90° at wind speeds of 4.0-5.1 m s-1. Consequently, there is a strongly curvilinear increase in terminal velocity as mass is artificially added to specimens. Asymmetric samaras and plumed seeds are about equally "efficient' at reducing descent velocity when the ratio of mass to projected area is high. At lower ratios of mass to projected area, however, the plumed design is more "efficient' at reducing descent velocity. -Authors

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... At lower wing loadings, plumes were more "efficient" in reducing terminal velocity. This is because the drag coefficient rises sharply as the Reynolds number declines [6]. For larger loads, different designs (i.e., small wings) have evolved due to the constraints involved in building bigger structures. ...
... For larger loads, different designs (i.e., small wings) have evolved due to the constraints involved in building bigger structures. In order for larger seeds to evolve, a species must face the following mechanical problem [4]: if the physical dimensions of the plume and the mass of the seed are scaled isometrically, plume loading increases with the bending of the hairs [6]. ...
... This species is exceptional in terms of performance. In fact, the plant produces plumed seeds with mass and size comparable to winged seeds [6]. ...
Article
Full-text available
Tragopogon pratensis is a small herbaceous plant that uses wind as the dispersal vector for its seeds. The seeds are attached to parachutes that increase the aerodynamic drag force and increase the total distance travelled. Our hypothesis is that evolution has carefully tuned the air permeability of the seeds to operate in the most convenient fluid dynamic regime. To achieve final permeability, the primary and secondary fibres of the pappus have evolved with complex weaving; this maximises the drag force (i.e., the drag coefficient), and the pappus operates in an "optimal" state. We used computational fluid dynamics (CFD) simulations to compute the seed drag coefficient and compare it with data obtained from drop experiments. The permeability of the parachute was estimated from microscope images. Our simulations reveal three flow regimes in which the parachute can operate according to its permeability. These flow regimes impact the stability of the parachute and its drag coefficient. From the permeability measurements and drop experiments, we show how the seeds operate very close to the optimal case. The porosity of the textile appears to be an appropriate solution to achieve a lightweight structure that allows a low terminal velocity, a stable flight and a very efficient parachute for the velocity at which it operates.
... However, the filaments of Tragopogon dubious like those of Taraxacum officinale radiate at different angles and the projected disk area of the pappus will be less then the calculated area. In the paper written by Greene and Johnson [10] the projected disk area was given by the following equation: ...
... We tried this procedure, unfortunately the fragility of the stalk gave troubles during the wrapping while the brightness of the thin foil was confusing the motion detection algorithm. Greene and Johnson [10] modified the mass of four species of Compositae and Asclepias syriaca by adding small pieces of solder (with glue) to the lower end of the achene. In order to have a reliable procedure, which did not damage the sample and worked with the motion detection algorithm we tried different materials. ...
... Many researches have been conducted to examine the efficacy of this flight mechanism and numerous models have been put forward in order to explain how the pappus interacts with the surrounding air while it descends. It is commonly thought that the drag-enhancement is the sum of the drags experienced by the individual lamellae which compose the pappus as shown in [10]. These models have a filament's diameter as characteristic length scale and it leads to a low Reynolds number Re. ...
Thesis
Full-text available
Anyone who has ever blown a dandelion clock, will know that its fruit (the feathery, seed-bearing structure) fly off with the slightest breeze. Each fruit is made of a group of stringy hollow tubes called lamellae, which together form the parachute, the pappus. Contrary to a popular belief, the physics of a conventional parachute cannot be applied, because of the parachute's high porosity. This structure gives outstanding flight capability to the fruit while, at the same time, saving weight. In steady descent, the flight of the fruit is characterized by two dimensionless parameters: the Reynolds number and the Darcy number. We measured the Reynolds number for the dandelion fruit by performing terminal velocity measurements for a variety of fruit weights. We tested previous theories for the flight of the dandelion by artificially weighting the dandelion, and determining the variation in the terminal velocity as a function of the parachute loading. Our experiments reveal that the parachute behaves more like a porous disk than individual independent lamellae. We performed direct numerical simulations of a porous disk of a certain Darcy number (as measured by us), and compared our results to flow visualizations realized in a specially designed vertical wind tunnel.
... Herein we use the drag in cross-wind direction for flow sensing while the foot of the stem of the pappus is fixated on a flexible membrane at the wall. It is not known if Dandelion use the pappus for sensing but all studies argue that the structure has evolved as a passive means to maximize dispersion with wind [9,10]. Therefore the use as the head of a flow sensor herein is new, while the objective to improve the sensor response is directly correlated with the aerodynamics related with maximization of dispersion. ...
... For the Dandelion pappus used in our study the medium length of the hairs is 7mm and the total radial diameter D of the structure is about D = 14mm, see Fig 1. As discussed in [9] the typical Reynolds-number of the flow around the hairs in nature is of order of Re d % 1. So, drag forces are dominated by viscous friction rather than inertia. ...
Article
Full-text available
This work describes the development and use of pappus-like structures as sensitive sensors to detect minute air-flow motions. We made such sensors from pappi taken from nature-grown seed, whose filiform hairs’ length-scale is suitable for the study of large-scale turbulent convection flows. The stem with the pappus on top is fixated on an elastic membrane on the wall and tilts under wind-load proportional to the velocity magnitude in direction of the wind, similar as the biological sensory hairs found in spiders, however herein the sensory hair has multiple filiform protrusions at the tip. As the sensor response is proportional to the drag on the tip and a low mass ensures a larger bandwidth, lightweight pappus structures similar as those found in nature with documented large drag are useful to improve the response of artificial sensors. The pappus of a Dandelion represents such a structure which has evolved to maximize wind-driven dispersion, therefore it is used herein as the head of our sensor. Because of its multiple hairs arranged radially around the stem it generates uniform drag for all wind directions. While still being permeable to the flow, the hundreds of individual hairs on the tip of the sensor head maximize the drag and minimize influence of pressure gradients or shear-induced lift forces on the sensor response as they occur in non-permeable protrusions. In addition, the flow disturbance by the sensor itself is limited. The optical recording of the head-motion allows continuously remote-distance monitoring of the flow fluctuations in direction and magnitude. Application is shown for the measurement of a reference flow under isothermal conditions to detect the early occurrence of instabilities.
... In order for seeds to spread over long distances, their structure is generally a disc structure composed of flexible filaments. During the process of seed flight, due to the action of hydrodynamic pressure, the flexible filament will deform to form an aerodynamic shape, thereby reducing the flow resistance [1][2][3][4][5][6][7][8]. ...
... Dandelion have evolved mechanisms to use wind for seed dispersal over a wide area [1][2][3][4][5] including creating lightweight diaspores with plumose or comose structures that act as drag-enhancing parachutes [6][7][8]. A bundle of flexible filaments bend in a wind in order to absorb deformation energy and reduce the drag force of the wind on the dandelion as shown in Fig.1. ...
Preprint
The common dandelion uses a bundle of drag-enhancing bristles (the pappus) that enables seed dispersal over formidable distances; however, the scaling laws of pneumatic/aerodynamic drag underpinning pappus-mediated flight remains unresolved. In this paper, we will study the pneumatic/aerodynamic shape of dandelion and the scaling law of resistance, and find that the drag resistance coefficient is proportional to the -2/3 power of the dandelion pappus Reynolds number. As a by-product, the terminal velocity analytical expression of the dandelion seed is also obtained.
... We examine contemporary differences in population-level dispersal trait values to test for alterations in the trajectory of dispersal evolution. Wind-dispersed plant species, like G. triflorum, have evolved multiple mechanisms that promote movement by wind, including structures such as plumes, wings, or samaras that increase time aloft and in turn increase dispersal distances (Greene and Johnson 1990;Lentink et al. 2009;Varshney et al. 2012). Population-specific variation in dispersal traits would indicate that selection likely contributes to regional differences, influencing the evolutionary trajectory of dispersal traits across the range of G. triflorum. ...
... When considering the ability for diaspores to move, terminal velocity is critical (Wilson 2000). This composite trait takes into account mass, morphology, and physical structures, such as hairs, to create differences in the length of time a diaspore can stay aloft in a column of air (Greene and Johnson 1990). Terminal velocity plays an important role in a species' response to habitat fragmentation (Schleicher et al. 2011), and indirectly gene flow and the maintenance of connectivity across populations. ...
Preprint
Full-text available
Understanding the evolution of dispersal under changing global environments is essential to predicting a species ability to track shifting ecological niches. Two important, but anthropogenically altered, sources of selection on dispersal are climate and habitat continuity. Despite the likelihood these global drivers of selection act simultaneously on plant populations, their combined effects on dispersal are rarely examined. To understand the interactive effect of climate and habitat continuity on dispersal potential, we study Geum triflorum - a perennial grassland species that spans a wide range of environments, including both continuous prairie and isolated alvar habitats. We explore how the local climate of the growing season and habitat continuity (continuous vs isolated) interact to alter dispersal potential. We find a consistent interactive effect of local climate and habitat continuity on dispersal potential. Across continuous prairie populations, an increased number of growing degree days favors traits that increase dispersal potential. However, for isolated alvar populations, dispersal potential tends to decrease as the number of growing degree days increase. Our findings suggest that under continued warming, populations in continuous habitats will benefit from increased gene flow, while isolated populations will become increasingly segregated, with reduced potential to track shifting fitness optima.
... Plants cover a large fraction of the Earth's land mass despite most species having limited to no mobility. To transport their propagules, many plants have evolved mechanisms to disperse their seeds using the wind [1][2][3][4] . A dandelion seed, for example, has a bristly filament structure that decreases its terminal velocity and helps orient the seed as it wafts to the ground 5 . ...
... Plants have evolved various mechanisms to use wind for seed dispersal over a wide area [1][2][3][4] including creating lightweight diaspores with plumose or comose structures that act as drag-enhancing parachutes 6,7 . Asteraceae plants, such as the common dandelion, produce plumed seeds containing a pappus, which is a bundle of bristly filaments 1 . ...
Article
Full-text available
Plants cover a large fraction of the Earth’s land mass despite most species having limited to no mobility. To transport their propagules, many plants have evolved mechanisms to disperse their seeds using the wind1–4. A dandelion seed, for example, has a bristly filament structure that decreases its terminal velocity and helps orient the seed as it wafts to the ground5. Inspired by this, we demonstrate wind dispersal of battery-free wireless sensing devices. Our millimetre-scale devices weigh 30 milligrams and are designed on a flexible substrate using programmable, off-the-shelf parts to enable scalability and flexibility for various sensing and computing applications. The system is powered using lightweight solar cells and an energy harvesting circuit that is robust to low and variable light conditions, and has a backscatter communication link that enables data transmission. To achieve the wide-area dispersal and upright landing that is necessary for solar power harvesting, we developed dandelion-inspired, thin-film porous structures that achieve a terminal velocity of 0.87 ± 0.02 metres per second and aerodynamic stability with a probability of upright landing of over 95%. Our results in outdoor environments demonstrate that these devices can travel 50–100 metres in gentle to moderate breeze. Finally, in natural systems, variance in individual seed morphology causes some seeds to fall closer and others to travel farther. We adopt a similar approach and show how we can modulate the porosity and diameter of the structures to achieve dispersal variation across devices. A dandelion-inspired wireless solar-powered sensing device weighing 30 milligrams that transmits data through radio backscatter achieves dispersal over a wide area by travelling on the breeze, and successfully lands upright.
... Traditional mathematical models of the pappus of a dandelion seed rely on the assumption that each of the filaments of the seed can be treated as a translating cylinder, with the total drag on the pappus being the sum of the contributions from each filament 2,14 . However, our numerical modelling and experimental measurements revealed that the flow through the seed entails strong interactions between neighbouring filaments 19,20 , causing the pappus to behave as a permeable membrane. ...
... The porosity (p) of the flattened sample was obtained by calculating the ratio of empty area to the total plan area of the pappus. The porosity ε of the original sample was then calculated to be ε = 1 − 2L(1 − p)/D = 0.916 (0.907-0.923) (mean (95% confidence interval); n = 10 seeds) 2 . Error analysis using different magnifications. ...
Article
Full-text available
Wind-dispersed plants have evolved ingenious ways to lift their seeds1,2. The common dandelion uses a bundle of drag-enhancing bristles (the pappus) that helps to keep their seeds aloft. This passive flight mechanism is highly effective, enabling seed dispersal over formidable distances3,4; however, the physics underpinning pappus-mediated flight remains unresolved. Here we visualized the flow around dandelion seeds, uncovering an extraordinary type of vortex. This vortex is a ring of recirculating fluid, which is detached owing to the flow passing through the pappus. We hypothesized that the circular disk-like geometry and the porosity of the pappus are the key design features that enable the formation of the separated vortex ring. The porosity gradient was surveyed using microfabricated disks, and a disk with a similar porosity was found to be able to recapitulate the flow behaviour of the pappus. The porosity of the dandelion pappus appears to be tuned precisely to stabilize the vortex, while maximizing aerodynamic loading and minimizing material requirements. The discovery of the separated vortex ring provides evidence of the existence of a new class of fluid behaviour around fluid-immersed bodies that may underlie locomotion, weight reduction and particle retention in biological and manmade structures.
... In our study, the settling rate did not vary with seed mass for V. nigrum seeds that dispersed short distances, whereas for seeds that dispersed long and middle distances, settling rates increased with increasing seed mass and then reached a plateau. Because both small and high-mass seeds fell near the release point, the plume area of the coma may be more variable in short-dispersing V. nigrum seeds (Greene and Johnson 1990). Although we did not correlate coma diameter of V. nigrum seeds to the distance traveled, coma diameter in this species was much more variable (4.0 to 5.8 cm; mean = 5.2 cm) in the 30 samples we measured compared with coma diameter for V. rossicum (4.3 to 4.7 cm; mean = 4.5 cm). ...
... The response of V. rossicum was also largely what was expected, except for the increased distance traveled at high wind speeds and high settling rate. It may be that these propagules have smaller coma diameters, which we did not measure, resulting in a higher plume-loading ratio and hence settling rate (Greene and Johnson 1990). Further experiments could examine the relationship of coma size or plume area and distance traveled, coupled with the other variables included here. ...
Article
Black swallowwort [ Vincetoxicum nigrum (L.) Moench] and pale swallowwort [ Vincetoxicum rossicum (Kleopow) Barb.] are perennial vines of European origin that invade natural areas and perennial cropping systems in the northeastern United States and southeastern Canada. Both species reproduce via wind-dispersed seeds in the form of achenes with comas, but little is known about the extent of dispersal of these seeds. We studied the relationship of seed release height (0.75 m, 2 m), wind speed at the time of release, seed mass, and settling rate on distance traveled. Vincetoxicum nigrum and V. rossicum seeds traveled up to 72.1 and 79.6 m, respectively. Seeds of both species released from 2 m traveled greater distances than seeds released from 0.75 m, which fell within 20 m of the release point. Release height was the most important factor influencing long-distance dispersal events. Wind speed also strongly interacted with release height for long-distance dispersal of V. nigrum . Vincetoxicum nigrum seed mass was greater and settling rates faster than for V. rossicum . Increasing seed mass generally increased settling rate, which in turn decreased distance traveled, except in V. rossicum , for which longer distance–dispersing seeds had a faster settling rate. Our findings suggest that management efforts focus on reducing the presence of these two vines, especially if there is potential for them to climb up taller vegetation such as trees. Seeds released from these greater heights are more likely to travel far from source populations and initiate new populations. Preventing seed production in small, nonclimbing patches will also help suppress the expansion of these two vines, as seeds can still disperse up to 20 m away from parent plants.
... From physical principles, one may expect a positive correlation between the distances of primary and secondary dispersal by wind. This is because both terminal velocity and lift-off velocity are proportional to the square root of a seed's wing loading (the ratio between seed mass and the seed's planform area exposed to airflow; Greene and Johnson 1990;Johnson and Fryer 1992;Norberg 1973). For a perfectly spherical seed, terminal and lift-off velocity should thus be directly proportional. ...
Article
Full-text available
The seeds of most plant species are dispersed by multiple mechanisms. Whether functional traits mediate positive correlations or negative correlations (trade-offs) between different dispersal mechanisms has important consequences for ecological and evolutionary dynamics. We investigate how traits affect wind-driven seed dispersal through the air and across the ground (primary and secondary wind dispersal, respectively). We hypothesized that primary and secondary wind dispersal were positively correlated because they should both decrease with the wing loading of seeds. We test this hypothesis with wind-tunnel experiments using different seed morphologies of Zygophyllum xanthoxylon (heterocarpous) and Calligonum species. We measured primary dispersal distance at varying wind speeds and release heights, and quantified secondary dispersal potential as the threshold wind speed for seed movement on four ground surfaces. Contrary to our expectation, we found a context-dependent trade-off between primary and secondary wind dispersal. The smoother the ground surface, the stronger this trade-off becomes. The trade-off results from a positive relationship between wing loading and the ratio of vertical to horizontal seed projection (v/h-ratio): an increasing v/ h-ratio not only promotes secondary dispersal on smooth surfaces by increasing wind interception of seeds, but also decreases primary dispersal distance by increasing wing loading and terminal velocity of seeds. The trade-off contradicts the widespread assumption of a positive correlation between primary and secondary dispersal. A simple classification into poorly and well-dispersed seeds is thus not possible. The trade-off may affect dynamics of succession and the expansion of pioneer vegetation, while potentially slowing down evolutionary responses to selection on seed dispersal.
... Samara (i.e., winged seed) is present in 25 orders, 45 families, and 140 genera of angiosperms (der Weduwen and Ruxton, 2019). It contributes to long-distance dispersal of seeds by wind (Augspurger and Franson, 1987;Greene and Johnson, 1990;Nathan et al., 2002). Dipterocarpaceae has 16 genera and approximately 500 species widely distributed in Asian tropical forests. ...
Article
Full-text available
Propagule dispersal is a crucial life history stage, which affects population recruitment and regeneration as well as community structure and functions. The windborne process of samara dispersal is affected not only by samara traits and other plant traits, but also by environmental factors. Therefore, studying samara traits related to its dispersal and intraspecific variation in relation to other plant traits and environmental factors could help to understand population distribution and dynamics. Hopea hainanensis, a Dipterocarpaceae tree species dominant in lowland rainforests in Hainan (China) but endangered due to anthropogenic disturbances, is dispersed mainly by wind because of its sepal-winged samara. Here, we measured dispersal-related intraspecific samara traits of H. hainanensis, and analyzed their variation and correlation in relation to plant height, DBH (diameter at breast height), and elevation plant location. Great variations in the samara traits existed, and the variations were larger within than among individuals, which indicated a “bet-hedging” strategy of this species. Plant height, DBH, and elevation explained slight variation in the samara traits. Samara dispersal potential is mainly affected by the samara mass and morphological traits. Samara settling velocity was significantly positively correlated with fruit mass, seed mass, length and width, as well as samara wing loading, and negatively correlated with wing mass ratio, wing area, and wing aspect ratio. Substantial proportions of intraspecific variation in samara dispersal are explained by the samara mass and morphological traits. Natural regeneration with human-aided dispersal is necessary for recovering the H. hainanensis population. This finding contributes to the generalization of trait-based plant ecology, modeling of seed dispersal in tropical forests, and conservation and recovery of rare and endangered species such as H. hainanensis.
... Simple physical modifications, such as choice of background colour (here we 267 used white to contrast with the dark brown samaras) may also be necessary for some species, while 268 species that require a greater vertical distance to reach terminal velocity would require a longer 269 tube. A suggested rule of thumb for the vertical distance required to reach terminal velocity is the 270 square of the terminal velocity (Greene & Johnson 1990;Askew et al. 1997). The device could 271 potentially also be extended to measure the terminal velocity of very small seeds (or possibly 272 spores) by addition of lenses to magnify the falling propagules, akin to the telescope that Gómez- 273 Noguez et al. (2017) observed in this trait is not unique to P. radiata. ...
Article
Seed or samara terminal velocity is a key trait affecting the dispersal potential of wind‐dispersed plants. However, this trait is often represented in dispersal models by a single mean value per species. This is despite considerable variation in dispersal traits within species and individuals that may have implications for both phenotypic selection and rates of spread. Methodological constraints may have acted as a barrier for robust assessments of intraspecific variation in seed terminal velocity. To quantify intraspecific variation in wind dispersal traits, we develop a low‐cost, time‐efficient method to measure the terminal velocity of a large number of samaras. We made three separate terminal velocity measurements for each of 750 Pinus radiata samaras, allowing partitioning of variation among individual cones, trees, and source populations. We use the mechanistic WALD model to assess the potential influence of observed variation in samara terminal velocity on predicted dispersal kernels under a variety of realistic conditions. We demonstrate a two‐fold range in samara terminal velocity within P. radiata, with the highest variation occurring within individual cones, and the lowest among cones within individual trees. We identify a potential influence of source population on terminal velocity. Our modelling results demonstrate that this within‐species variation is sufficient to affect the shape of the predicted dispersal kernels, particularly the kernel tails and therefore the likelihood of long‐distance dispersal events. The effect of samara terminal velocity on dispersal is especially pronounced under environmental conditions that enhance seed dispersal. Our findings illustrate the scale of within‐species variation in a key dispersal trait, and the likely effect of this variation on dispersal distance. We suggest that the high level of variation observed within individual cones of P. radiata is likely to reduce the potential for phenotypic selection, and may either be indicative of a ‘bet‐hedging’ strategy, or simply the result of the constraints of cone morphology on samara development. To obtain accurate dispersal models for wind‐dispersed species we highlight the necessity of capturing this variation for inclusion in future modelling approaches, and describe a device that can achieve such measurements easily and at low cost. This article is protected by copyright. All rights reserved.
... The geometry and kinematics of rotary seeds have been described previously (Azuma and Yasuda, 1989;Greene and Johnson, 1990; Minami and Azuma, 2003; Yasuda and Azuma, 1997). ...
... It was found for example that the settling velocities of the pappus differed between Asteraceae species due to their different pappus anatomy and seed weight (Andersen 1992). The plume loading (diaspore weight/ plume area) was shown to strongly influence the fall speed also for other taxa (Matlack 1987;Greene and Johnson 1990;Minami and Azuma 2003). For a better understanding of dispersal mechanisms of wind dispersed plant species, it is essential not only to know distribution details of the diaspores while they are in the air, but also to know the mechanisms by which they are fixed within the soil. ...
Article
Full-text available
Aims We analysed the surface anisotropy of Taraxacum officinale cypselae, without their pappus, microscopically and biomechanically. Methods A scanning electron microscope was used for visual analyses. We then measured the angle, at which cypselae start to slide from tilted substrates of different roughness and compared cypselae sliding along their structure with those sliding against their structure. Finally, forces during pushing cypselae into the soil and pulling them out were measured with a force transducer moved by a micromanipulator. Results The cypselae had a ribbed surface pattern with apically pointing spines, plenty of scale-like structures and small protuberances. In general, the angle at which cypselae started to slide-off from an inclined surface increased with increasing roughness of the inclined surface, but this did not hold for cypselae sliding along their structure and for the glass surface. The angles at which the cypselae started to slide were on all but the glass surface significantly higher for cypselae sliding against the structure compared to cypselae sliding along the structure. Pull-out forces were higher when pulling cypselae out of the soil against their structure compared to pulling along their structure. Conclusions Anisotropic structures hinder cypselae from being drawn from soil by wind.
... Once stable autorotation is established, the lift force of a descending samara balances its gravitational force, leading to the so-called "terminal" descent velocity (indicated as vt in the inset of Fig. 1) that remains constant until landing [11][12][13][14][15][16] . Further studies have identified that, during stable autorotation, the lift generated by the samara is sustained by a leading-edge vortex [17][18][19][20] . ...
Preprint
Full-text available
A single-winged maple seed (samara) is dispersed laterally by a crosswind in contrast to simply descending straight down (zero dispersion) in quiescent air. This article presents the general kinematic response of a particular type of samaras ( Acer buergerianum) in stable autorotation to the disturbance of a concentrated crosswind (simulated via slot jet) with the crosswind strength varied distinctively from weak to strong. A relatively weak crosswind slower than the tip velocity of the stably autorotating samara causes only damped undulations of its descent trajectory. In contrast, we demonstrate that the samara exhibits a bi-modal response when disturbed by a relatively strong crosswind (velocity greater than samara tip velocity). The strong crosswind enables the samara either to float laterally with the crosswind or drop-out through the crosswind with the switching of its rotational direction. Regardless of crosswind strength, stable autorotation is re-established after the samara leaves the crosswind zone, albeit accompanied by large-scale undulations in its descent trajectory. More importantly, before landing, the samara regains its original terminal descent velocity achieved in quiescent air.
... Terminal velocity is defined as a steady and maximum speed when air resistance is equal to the pull of gravity during a free fall of seeds in motionless air (Green, 1980). It is considered as the most useful indirect parameter of primary dispersal capacity in diaspore wind dispersal models, because diaspores with a low terminal velocity may fly longer in the air and thus disperse a longer distance than diaspores with a high terminal velocity (Okubo and Levin, 1989;Greene and Johnson, 1990;Caplat et al., 2012). However, it is not known if the terminal velocity can replace wing loading in characterizing lift-off velocity of diaspores. ...
Article
Full-text available
Lift-off velocity may be the most useful surrogate to measure secondary dispersal capacity of diaspores. However, the most important diaspore attribute determining diaspore lift-off velocity is unclear. Furthermore, it is not known whether terminal velocity used to characterize the primary dispersal capacity of diaspores also can be used to predict their secondary wind dispersal capacity. Here, we investigate how diaspore attributes are related to lift-off velocity. Thirty-six species with diaspores differing in mass, shape index, projected area, wing loading and terminal velocity were used in a wind tunnel to determine the relationship between diaspore attributes and lift-off velocity. We found that diaspore attributes largely explained the variation in lift-off velocity, and wing loading, not terminal velocity, was the best parameter for predicting lift-off velocity of diaspores during secondary wind dispersal. The relative importance of diaspore attributes in determining lift-off velocity was modified by both upwind and downwind slope directions and type of diaspore appendage. These findings allow us to predict diaspore dispersal behaviors using readily available diaspore functional attributes, and they indicate that wing loading is the best proxy for estimating the capacity for secondary dispersal by wind.
... The correct estimation of drag force, however, is the key to study the orientation and distribution of cylindrical objects moving in a viscous fluid. Examples cover, but are not limited to, the dynamics of flexible fibres in polymeric resins (Schulte et al. 2005), the aerodynamics of plumed seeds (Greene & Johnson 1990) and the 376 A. Khalili and B. Liu settling of cylindrical micro-organisms such as diatoms and filamentous green algae in aquatic systems. The latter are considered as important components of phytoplankton. ...
Article
Full-text available
Finite container sizes in experiments and computer simulations impose artificial boundaries which do not exist when they are meant to mimic ambient fluid of infinite extent. We show here that this is the case with flows past an infinite cylinder placed in an infinite ambient fluid (Stokes’ paradox). Using a highly efficient and stable numerical method that is capable of handling computational domains several orders of magnitude larger than in previous studies, we provide a criterion for the minimum necessary extent around an object in order to provide accurate velocity and pressure fields, which are prerequisites for correct calculation of secondary quantities such as drag coefficient. The careful and extensive simulations performed suggest an improved relation for the drag coefficient as a function of Reynolds number, and identify the most suitable experimental data available in the literature.
... However, some plants and animals also use filament-like structures for their aerial journey. Plumed seeds, such as those of the dandelion and thistle, use a pappus as a parachute for their passive dispersal (Greene and Johnson 1990;Minami and Azuma 2003;Casseau et al. 2015;Cummins et al. 2018). Why do all these micro-organisms, seeds, and cells utilize hairy structures for their locomotion or aerial dispersal? ...
Article
Synopsis Many flying insects utilize a membranous structure for flight, which is known as a “wing.” However, some spiders use silk fibers for their aerial dispersal. It is well known that spiders can disperse over hundreds of kilometers and rise several kilometers above the ground in this way. However, little is known about the ballooning mechanisms of spiders, owing to the lack of quantitative data. Recently, Cho et al. discovered previously unknown information on the types and physical properties of spiders’ ballooning silks. According to the data, a crab spider weighing 20 mg spins 50–60 ballooning silks simultaneously, which are about 200 nm thick and 3.22 m long for their flight. Based on these physical dimensions of ballooning silks, the significance of these filament-like structures is explained by a theoretical analysis reviewing the fluid-dynamics of an anisotropic particle (like a filament or a high-slender body). (1) The filament-like structure is materially efficient geometry to produce (or harvest, in the case of passive flight) fluid-dynamic force in a low Reynolds number flow regime. (2) Multiple nanoscale fibers are the result of the physical characteristics of a thin fiber, the drag of which is proportional to its length but not to its diameter. Because of this nonlinear characteristic of a fiber, spinning multiple thin ballooning fibers is, for spiders, a better way to produce drag forces than spinning a single thick spider silk, because spiders can maximize their drag on the ballooning fibers using the same amount of silk dope. (3) The mean thickness of fibers, 200 nm, is constrained by the mechanical strength of the ballooning fibers and the rarefaction effect of air molecules on a nanoscale fiber, because the slip condition on a fiber could predominate if the thickness of the fiber becomes thinner than 100 nm.
... There are two kinds of seeds that dispersed by wind, pappose seeds (parachute World Journal of Engineering and Technology type) and winged seeds. Pappose seeds utilized rag force acting on the pappi [1], while winged seeds mainly use lift force on their wings [2].The dispersal of winged seeds can be divided into two categories according to the locations of their center of gravity. When located in front of the seed's aerodynamic center, the seed exhibits a glider type translation during fall, without any rotation [3]. ...
... amatically. I do however think the suggestion a helpful one, to show how narrative provides momentum, so I include it here. 12 Greene, David F. and Mauricio Quesada. 2011. The differential effect of updrafts, downdrafts and horizontal winds on the seed abscission of Tragopogon dubius. Functional Ecology 25 (3):468-472. 13 Greene, D., and Edward A. Johnson. 1990 assumptions about the drag laws operating on the individual seed, varying the wind angles in their simulations, and likewise finding that updrafts mattered most for distance. 14 Fourth, plant morphologists interested in biomechanics had investigated the structure and material composition of the individual seed, and its behaviour in the ...
Chapter
The new gene-editing tool, CRISPR-Cas9, been described as “revolutionary” This paper takes up the question of what sense, if any, might this be true and why it matters. I draw from the history and philosophy of technology to develop two types of technological revolutions (Hughes, Technological momentum in history: Hydrogenation in Germany 1898–1933. Oxford University Press, New York, 1969; Wimsatt, Re-engineering philosophy for limited beings. Harvard University Press, Cambridge, MA, 2007; Constant, The origins of turbojet revolution. The Johns Hopkins University Press, Baltimore, 1980; Scaife, Sci Am 252(4), 1985). One type of revolution involves a technology that enables users to change a generatively entrenched structure (Wimsatt, Re-engineering philosophy for limited beings. Harvard University Press, Cambridge, MA, 2007). The other type involves a technology that works within a generatively entrenched structure, but as a result of incremental improvement becomes the “new normal” technology for a community (Scaife, Sci Am 252(4), 1985). In what follows, I argue that if CRISPR-Cas9 is revolutionary at all – and I do not take a stand on the issue – it is in becoming the “new normal” molecular technology across biology labs. By contrast, a technology that has the potential of being revolutionary in Wimsatt’s sense is the orthogonal tRNA technique developed by Peter Schultz’ synthetic biology lab. Whether or not CRSIPR-Cas9 or the orthogonal tRNA technologies are revolutionary, I propose to treat these two types of putative revolutions as distinct types of technological innovation. I argue further that observing distinctions between types of technological innovation can be useful for tracking the epistemic and normative consequences that technology raises.
... Sundberg (2010), however, reported settling velocities in Sphagnum spores that were about 48 % slower than expected for spherical particles of the same diameter. In fact, although the impact of seed morphology on density and drag coefficients has been extensively documented (Greene and Johnson, 1990;Matlack, 1987;McGinley and Brigham, 1989;Cody and Overton, 1996), and although 'numerous hypotheses pertain to the selective aerodynamic advantage of complex morphology such as hollow sacs to reduce density or numerous minute spines to effectively increase the volume' in pollen (Niklas, 1985), the impact of the shape and ultrastructure of spores on their settling velocity has seldom been taken into account (Roper et al., 2008;Hussein et al., 2013). ...
Article
Full-text available
Background and aims: The settling velocity of diaspores is a key parameter for the measurement of dispersal ability in wind-dispersed plants and one of the most relevant parameters in explicit dispersal models, but remains largely undocumented in bryophytes. The settling velocities of moss spores were measured and it was determined whether settling velocities can be derived from spore diameter using Stokes' Law or if specific traits of spore ornamentation cause departures from theoretical expectations. Methods: A fall tower design combined with a high-speed camera was used to document spore settling velocities in nine moss species selected to cover the range of spore diameters within the group. Linear mixed effect models were employed to determine whether settling velocity can be predicted from spore diameter, taking specific variation in shape and surface roughness into account. Key results: Average settling velocity of moss spores ranged from 0·49 to 8·52 cm s(-1) There was a significant positive relationship between spore settling velocity and size, but the inclusion of variables of shape and texture of spores in the best-fit models provides evidence for their role in shaping spore settling velocities. Conclusions: Settling velocities in mosses can significantly depart from expectations derived from Stokes' Law. We suggest that variation in spore shape and ornamentation affects the balance between density and drag, and results in different dispersal capacities, which may be correlated with different life-history traits or ecological requirements. Further studies on spore ultrastructure would be necessary to determine the role of complex spore ornamentation patterns in the drag-to-mass ratio and ultimately identify what is the still poorly understood function of the striking and highly variable ornamentation patterns of the perine layer on moss spores.
... Dandelions (Taraxacum officinale): The seed dispersal technique exercised by the dandelions has paved new insights for futuristic aerospace vehicle designs. The simplistic aerodynamic design of dandelion seeds as seen in Fig. 3e (Greene and Johnson 1990) quadruples its efficiency in maintaining altitude, when compared to an artificial parachute (Qiu et al. 2020;Roos and Willmarth 1971). The efficient flight of dandelion is attributed to the bundle of pappus/pappi (thin filament tufts/ bristles surrounding the seed) and the separated vortex ring formed due to the porosity of pappi, as noticed in Fig. 3f. ...
Article
Full-text available
Bio-inspired designs have been serving as a great inspiration for Aerodynamists for the past several decades. The biological features of each species have evolved through generations that play a substantial role in their ultimate survival. Manmade technologies have significantly benefited from these evolutionary attributes by mimicking such optimal features through artificial means of engineering. Biologically mimicking a particular feature of a natural species for scientific applications has introduced an interdisciplinary field of study that is popularly termed as “Biomimetics”. Biomimetic solutions have gained greater scope because of their unique potential to address the existing design challenges. The present review article approaches the state-of-the-art biomimetic techniques in a qualitative as well as quantitative perspective by mainly focusing on the potential aerodynamic applications. Through a quantitative approach, emerging researchers can have a firm basis from a pool of existing techniques. Drag reduction, lift enhancement, and flow separation control are the major design optimization challenges existing in the field of aerodynamics. The Biomimetic techniques discussed herein are evaluated on such qualities through consecutively organized sections which are the key instruments for non-flapping applications. A critical review on the well-liked biomimetic techniques for flow control is addressed with their potential applications in relevance to the aerospace industry. Few patented techniques from allied fields like hydrodynamics are also discussed because of their conceptual effectiveness in aerodynamic applications. Furthermore, the flow control effectiveness of Humpback Whale (HW) inspired biomimetic leading edge tubercles is numerically investigated with straight and sweptback wing configurations. The results clearly indicate the potentials of biomimetic techniques to transform the future designs of aerodynamic surfaces.
... Wind seed dispersal models consider seed terminal velocity as the most useful indirect parameter of dispersal ability because seeds with a low terminal velocity may fly longer in the air thus have a long dispersal distance (Caplat et al. 2012;Green 1980;Greene and Johnson 1990;Okubo and Levin 1989). Wing loading, reflecting air resistance during the flight, is a frequently used parameter in studying seed dispersal by wind (Andersen 1993;Green 1980;Lentink et al. 2009), determined by diaspore mass, shape and appendage. ...
Article
Full-text available
Aims How seed dispersal distance is related to various factors is a major challenge for seed ecologists. However, there are different answers as to which factor is most important in determining wind dispersal distance. This study is to quantitatively describe the relationship between various factors and primary wind dispersal distance of winged diaspores. Methods The dispersal distances of five morphologies of winged diaspores in Zygophyllum xanthoxylum (Zygophyllaceae) were measured under controlled conditions in a wind tunnel. The explanatory power of environmental factor (i.e. wind speed), plant trait (i.e. release height) and diaspore attributes (i.e. wing loading (the ratio of diaspore mass to projected area), settlement-velocity, shape index (the variance of diaspore length, width and thickness)) to the variation in dispersal distance was assessed by releasing diaspores at varying wind speeds and release heights. Important Findings Wind speed and seed release height were the strongest explanatory factors to dispersal distance, contributing 41.1 and 24.8% (P < 0.01) to total variation in dispersal distance, respectively. Wind speed accounted more for relatively light disc-shaped seeds than for relatively heavy spherical seeds. Wing loading, shape index and settlement-velocity explained 9.0% (P < 0.01), 1.4% (P < 0.01) and 0.9% (not significant) of the variation in dispersal distance, respectively. From disc-shaped to four-winged diaspores, relative contributions of wing loading and shape index decreased but contribution of settlement-velocity increased. The relative contributions of various factors to wind seed dispersal distance may change with the change in seed morphology.
... In parachute seeds, key parameters for their dispersal performance include the horizontal dispersal distance, which is inversely proportional to their terminal velocity during vertical falling (v f ) (Greene and Johnson, 1990), and changes with the seed mass (m) and the pappus projected area (A). Measurements of these parameters were reported for a Taraxacum officinale seed, with a m 0.62 mg has a v f 0.27 m/s, and for a Tragopogon pratensis seed with m 11.2 mg has a v f 0.57 m/s (Casseau et al., 2015). ...
Article
Full-text available
Plants have evolved different mechanisms to disperse from parent plants and improve germination to sustain their survival. The study of seed dispersal mechanisms, with the related structural and functional characteristics, is an active research topic for ecology, plant diversity, climate change, as well as for its relevance for material science and engineering. The natural mechanisms of seed dispersal show a rich source of robust, highly adaptive, mass and energy efficient mechanisms for optimized passive flying, landing, crawling and drilling. The secret of seeds mobility is embodied in the structural features and anatomical characteristics of their tissues, which are designed to be selectively responsive to changes in the environmental conditions, and which make seeds one of the most fascinating examples of morphological computation in Nature. Particularly clever for their spatial mobility performance, are those seeds that use their morphology and structural characteristics to be carried by the wind and dispersed over great distances (i.e. “winged” and “parachute” seeds), and seeds able to move and penetrate in soil with a self-burial mechanism driven by their hygromorphic properties and morphological features. By looking at their motion mechanisms, new design principles can be extracted and used as inspiration for smart artificial systems endowed with embodied intelligence. This mini-review systematically collects, for the first time together, the morphological, structural, biomechanical and aerodynamic information from selected plant seeds relevant to take inspiration for engineering design of soft robots, and discusses potential future developments in the field across material science, plant biology, robotics and embodied intelligence.
... Second, variation in seed orientation also affects seed lift-off velocity by changing both seed height and seed area i.e. exposed to airflow ( Fig. 1) (Schurr et al. 2005). Variation in seed area changes 'effective' wing loading, the ratio of seed mass to seed area (Norberg 1973), which strongly affects seed lift-off velocity (Greene and Johnson 1990;Johnson and Fryer 1992). Variation in seed height changes the wind interception parameter, defined as a dimensionless ratio between the wind velocity averaged over the seed height ( V ) and the wind velocity at a reference height ( − − → U ref ) (Schurr et al. 2005), determining the wind speed that a seed experiences (Fig. 1). ...
Article
Full-text available
Aims Understanding how diaspore (hereafter “seed”) morphology and orientation affect secondary seed dispersal by wind is important to link seed dispersal and post-dispersal processes, such as seed lodging, predation and germination. This study aims to describe the effects of seed morphology and orientation on secondary seed dispersal by wind via mechanistic modelling. Methods We extend the mechanistic model of Schurr et al. (2005) in order to describe how secondary seed dispersal by wind is affected by wind conditions, ground surface, seed morphology and orientation. The model simulates the initial landing orientations, dispersal distances and stopping orientations of individual seeds. To parameterise the model, we measured orientation-specific vertical seed projection and seed lift-off velocity (the wind speed at which a seed starts moving on the ground) of the asymmetric seeds of heterocarpous Zygophyllum xanthoxylon, and determined orientation-specific model parameters that depend on properties of seeds and/or the environment. To validate the model, we conducted wind-channel experiments in which we released seeds of Z. xanthoxylon onto a sand-coated tar paper, and recorded the initial landing orientations, dispersal distances and stopping orientations of the seeds. Important findings The extended model could precisely predict secondary dispersal distance, and explain up to 99% of variation in the observed proportions of seeds which stopped in various orientations. The model predicts that secondary dispersal distance increases with wind speed and decreases with aerodynamic roughness length, and that there might be a positive correlation between dispersal distance and germination success.
... Approximately 35–40 d after the onset of flowering, all heads in a branch have matured (Jones 1978c). The relationship between various achene characteristics (those that facilitate wind dispersal) and aerodynamics was summarized previously (Greene and Johnson 1990). ...
Article
Full-text available
Symphyotrichum ericoides, the heath aster, is a herbaceous perennial, arising from woody, corm-like rootstocks or herbaceous rhizomes. This facultative upland species occurs in open, dry ground in plains, railway sidings, prairies, ranges, glades, grassy hillsides, dunes, sand bars, river banks, shore salt flats, and thickets. A weed in Canada, and sometimes declared so in the United States and other times not, S. ericoides has an extensive North American distribution, occurring from Nova Scotia and Maine in the east, westward to southem British Columbia, and southward to northern Mexico. No infraspecific taxa are recognized in this treatment of the species. Symphyotrichum novae-angliae, the New England aster, is a herbaceous perennial, arising from thick, short-branched, woody root-stocks. This facultative wetland species occurs in open places such as along roadside ditches and fence rows, on dry ground in plains, prairies and glades, in clearings, meadows and abandoned fields, and along stream banks and swamps, or in moist open or sometimes wooded places. A weed in both Canada and the United States, S. novae-angliae occurs commonly throughout the northem two-thirds of the eastern deciduous forest region of North America and onto the Great Plains. In the east, Nova Scotia populations are likely escaped cultivars, but in the west, isolated natural populations occur from the Black Hills region of South Dakota south to New Mexico. No infraspecific taxa are recognized in this treatment of the species. In those habitats where the two species co-occur, the morphologically intermediate F1 hybrid S. amethystinum is often found.
... The period of each stage depends on the initial attitude of the samara when it breaks free from the tree [1]. Once stable autorotation is reached, the lift force of the samara is in equilibrium with its gravitational force, leading to the so-called "terminal" descent velocity [5][6][7][8][9][10]. This unique aerodynamic behavior of the samaras during dispersal, thus has inspired studies in biomimetics [11][12][13][14][15][16][17]. ...
Article
Full-text available
During descent, a single-winged maple seed (samara) can naturally reach a delicate equilibrium state, stable autorotation, before landing. This article reveals the intrinsic equilibrium of a particular type of samaras in terms of measurable aerodynamic and geometric parameters. To this end, we conducted a series of in situ measurements for the rate of vertical descent (exclusive of crosswind) of an autorotating samara in a natural range of samara sizes and masses. We then extended the range of size and mass by introducing artificial samaras, with discrete mass elements purposely designed to approximate the asymmetrical and nonuniform distribution ofmass found with natural samaras. Based on the widened range, a fundamental nondimensional correlation of dynamic pressure and disc loading was generalized, where all stable autorotation descent profiles collapse to a single descent characteristic curve, irrespective of the size and mass of the natural and artificial samara’s specimens. Results reveal that for stably autorotating (both natural and artificial) samaras, their terminal descent velocity (expressed as dynamic pressure) and disc loading attained equilibrium at a value that is inversely proportional to the coefficient of lift.
Article
Plants exhibit wide variation in reproductive traits, and these traits often covary to form distinct strategies. Uniparental reproduction via self-fertilization is often associated with high reproductive effort and enhanced seed dispersal, the combination of which is viewed as an adaptive strategy in ephemeral or disturbed habitats. We investigated the hypothesis that uniparental reproduction via asexual apomixis is associated with a similar suite of traits and more frequent occurrence in disturbed habitats among populations of Antennaria parlinii, a dioecious species that exhibits wide variation in sexuality at the population level. By comparing 18 sexual and 21 asexual populations in Ohio, USA, we found that, as predicted, sexual populations were more likely to occur in wooded habitats than asexual populations, which were found more frequently in disturbed roadside ditches and fallow fields. As expected, plants in asexual populations produced, on average, twice as many seeds per inflorescence as plants in sexual populations. Asexual plants also exhibited much higher reproductive effort than sexual plants when compared in a common greenhouse environment, indicating a genetic basis for the difference in reproductive output observed in natural populations. Seeds of A. parlinii are equipped with plume-like pappus that enhances wind dispersal. Settling velocity in still air was lowest for diaspores with long, highly barbed pappus bristles and small seeds. Compared to sexual populations, asexual populations produced diapsores with longer, more highly barbed pappus and smaller seeds. Taken together, our results suggest that divergent selective pressures in different habitats have produced two distinct reproductive strategies in A. parlinii.
Article
What determines the number and size of the seeds produced by a plant? How often should it reproduce them? How often should a plant produce them? Why and how are seeds dispersed, and what are the implications for the diversity and composition of vegetation? These are just some of the questions tackled in this wide-ranging review of the role of seeds in the ecology of plants. The authors bring together information on the ecological aspects of seed biology, starting with a consideration of reproductive strategies in seed plants and progressing through the life cycle, covering seed maturation, dispersal, storage in the soil, dormancy, germination, seedling establishment, and regeneration in the field. The text encompasses a wide range of concepts of general relevance to plant ecology, reflecting the central role that the study of seed ecology has played in elucidating many fundamental aspects of plant community function.
Article
Stimuli-responsive materials, especially those inspired by natural organism, have been extensively studied because of their versatile applications. Here, this study demonstrates that the unique hydroactuated reversible configuration alternation of the dandelion pappus, featured by the responsive spatial closing and opening the umbrella-like pappus depending on the environmental humidity, is a determining factor that allows the pappus to regulate its transport behavior. It is proposed that the anisotropic hydroactuated swelling of tracheids, which are typically arranged with an oblique crossing orientation, helps to generate the directional stress on the pappus. By this strategy, the pappus control its transport behavior in response to the environmental moisture, which allows long-distance and effective seeds dispersal selectively to local sites for optimal reproduction. Inspired by this, this study develops a hydroresponsive fiber array by integrating the hydrogel, whose configuration can be altered reversibly between the opening and closing state depending on the environmental humidity. This study believes the research can offer inspirations for designing and developing adaptive devices by using fibers array, as smart aerocraft.
Chapter
Some 200 papers dealing with seed dispersal have been published during the period covered by the present review, April 1987 to April 1991. This represents no increase over the 4 years dealt with previously (Progr. Bot. 49, 1987). Most work on seed dispersal is published in Ecology, Oikos, Biotropica, Oecologia, Journal of Tropical Ecology, American Journal of Botany, Canadian Journal of Botany, Journal of Ecology, Revue dTxologie (Terre et Vie), and Journal of Biogeography. The comparative stagnation of the field may partly be due to a decrease in funding, at least in the United States, for the natural history- type of work associated with seed dispersal studies.
Article
Full-text available
Manifera talaris, a voltzian conifer from the late early to middle Permian (ca. 270 Ma) of Texas, is the earliest known conifer to produce winged seeds indicative of autorotating flight. In contrast to autorotating seeds and fruits of extant plants, the ones of M. talaris are exceptional in that they have variable morphology. They bore two wings that produced a range of wing configurations, from seeds with two equal-sized wings to single-winged specimens, via various stages of underdevelopment of one of the wings. To examine the effects of various seed morphologies on aerodynamics and dispersal potential, we studied the flight performance of paper models of three morphotypes: symmetric double-winged, asymmetric double-winged, and single-winged. Using a high-speed camera we identified the mode of descent (plummeting, gliding, autorotation) and quantified descent speed, autorotation frequency, and other flight characteristics. To validate such modeling as an inferential tool, we compared descent of extant analogues (kauri; Agathis australis) with descent of similarly constructed seed models. All three seed morphotypes exhibited autorotating flight behavior. However, double-winged seeds, especially symmetric ones, failed to initiate slow autorotative descent more frequently than singlewinged seeds. Even when autorotating, symmetric double-winged seeds descend faster than asymmetric double-winged ones, and descent is roughly twice as fast compared to single-winged seeds. Moreover, the relative advantage that (effectively) single-winged seeds have in slowing descent during autorotation becomes larger as seed weight increases. Hence, the range in seed wing configurations in M. talaris produced a wide variation in potential dispersal capacity. Overall, our results indicate that the evolutionarily novel autorotating winged seeds must have improved conifer seed dispersal, in a time when animal vectors for dispersion were virtually absent. Because of the range in wing configuration, the early evolution of autorotative flight in conifers was a functionally imperfect one, which provides us insight into the evolutionary developmental biology of autorotative seeds in conifers.
Article
Is there necessarily a trade-off between seed size (mass) and dispersal capacity for wind-dispersed diaspores? Within three families (Pinaceae, Aceraceae, and Leguminosae) with asymmetric samaras, shape is maintained (isometry) despite size change. Consequently, within these three families, equilibrium descent velocity is proportional to samara mass raised to the 1/6 power and, necessarily, larger samaras are more poorly dispersed.
Article
1 Vulpia ciliata is a short winter annual grass that occurs primarily amongst open vegetation on sandy soils and shingle in southern and eastern England. In such habitats it is often found in disturbed areas alongside tracks and paths. Dispersal occurs as a result of wind or mechanical disturbance fracturing the diaspores from the spikelets on the infructescence. The aim of this study was to examine the dispersal and fates of the seeds of V. ciliata and compare them with those of the closely related V. fasciculata. 2 In a population relatively undisturbed by people the mean dispersal distance of diaspores from infructescences of V. ciliata sprayed with paint was 6 cm; the distribution of the dispersal distances was skewed to the right and leptokurtic. Diaspores were dispersed in all directions but the mean angle of dispersal was in the direction of the prevailing south westerly winds. The distance of dispersal from individual infructescences was positively correlated with the height of the infructescence. 3 A person walking through a population of plants resulted in the diaspores being dispersed three times further (19 cm) than where dispersal was primarily by wind. The diaspores were also moved disproportionately in the direction of the disturbance. 4 A transient seed bank occurs over the summer months but all of the seeds that are viable germinate in the year that they are produced; there is no bank of persistent seed. Mortality was high during the emergence of seedlings before the radicles had extended more than 2 mm. Establishment was higher if the seeds were buried and it is argued that the morphology of diaspores is as critical in promoting the establishment of seedlings as in promoting dispersal. 5 The dispersal and fates the seeds of V. ciliata are compared with those of the closely related V. fasciculata which produces fewer but heavier seeds. The mean distance of dispersal for the two species in the absence of active disturbance was very similar, although for a given height of infructescence the diaspores of V. ciliata were dispersed less far then those of V. fasciculata. Both species possess only a transient seed bank which can be related to the open but seasonally predictable habitats in which they are found. The proportion of seeds and seedlings dying was however much greater in V. ciliata. This greater mortality can be related to the higher seed production of this species in a persistent and relatively stable population. The establishment of the seedlings of both species is increased if the seeds are buried vertically.
Article
This paper presents a reduced order model for describing the steady-state and dynamic behavior of a single-winged samara seed pod undergoing autorotative descent. Blade Element Theory is employed to capture the span-wise behavior of a samara. By neglecting lateral motion of the center of mass and prescribing a roll angle, a simplified and compact model is developed while retaining reasonable accuracy in comparison to experimental data published in the literature. Steady-state results confirm the validity of assuming a small roll angle, which helps reduce model complexity. Simulation of the dynamics yields reasonable transient behavior and convergence to equivalent steady-state values. Accuracy of the reduced order model suggests the plausibility of designing and controlling simple autorotative mechanisms.
Article
Full-text available
Orchids are globally distributed, a feature often attributed to their tiny dustlike seeds. They were ancestrally terrestrial but in the Eocene expanded into tree canopies, with some lineages later returning to the ground, providing an evolutionarily replicated system. Because seeds are released closer to the ground in terrestrial species than in epiphytic ones, seed traits in terrestrials may have been under selective pressure to increase seed dispersal efficiency. In this study, we test the expectations that seed airspace-a trait known to increase seed flotation time in the air-is (i) larger in terrestrial lineages and (ii) has increased following secondary returns to a terrestrial habit. We quantified and scored 20 seed traits in 121 species and carried out phylogenetically informed analyses. Results strongly support both expectations, suggesting that aerodynamic traits even in dust seeds are under selection to increase dispersal ability, following shifts in average release heights correlated with changes in habit.
Chapter
In most plants there is a continuum of gas-filled intercellular space which can serve for the transport of atmospheric or photosynthetic oxygen from shoot to root. Despite this, the lack of oxygen in flooded soils will often restrict root growth, but the development of aerenchyma can substantially improve transport leading to deeper rooting, a greater degree of rhizosphere oxygenation and improved nutrient acquisition. In roots, gas-phase transport is chiefly by diffusion and within the cortex; in aerial parts and rhizomes, in addition to diffusion there may be a large convective component operating in both pith and cortex. Aerenchyma is formed either by wall separation and cell collapse (lysigeny) (Figure 3.1a) or by cell separation only (schizogeny) (Figure 3.1b); lysigeny may be stimulated by ethene, low oxygen and nutrient deficiency.
Article
I made measurements of morphology and settling velocity on seeds of 19 species of wind-dispersed Asteraceae. From the morphological measurements I calculated Reynolds numbers and approximate plume loadings for the species. Diaspore settling velocity increases linearly with the square root of plume loading. This relationship varies among species and among subfamilies, but not among life history types. Reynolds number is highly variable among subfamilies, less so within subfamilies. Diaspores with beaked achenes have significantly lower settling velocities than diaspores with unbeaked achenes, even though beaked and unbeaked achenes do not differ in plume loading or in Reynolds number. Reynolds numbers of all diaspores examined are well above the range in which Stokes' Law applies. I recommend that the use of formulae based on Stokes' Law be curtailed in studies of the relationship between plume loading and settling velocity. The results suggest that many seed characters may have evolved due to selection on dispersal ability. This is in spite of phyletic constraints on morphology reflected in the relative uniformity of Reynolds numbers within subfamilies.
Article
Dispersal is an important life history component. Seed settling velocity may be a useful surrogate for the measurement of dispersal ability in wind-dispersed plants, particularly those whose seeds have plumose dispersal structures. I measured settling velocities on seeds of eight species of Asteraceae, including annuals, biennials, and perennials, and including both native and introduced species. The species are Aster exilis, Picris echioides, Chrysopsis villosa, Heterotheca grandiflora, Conyza bonariensis, Sonchus oleraceous, Senecio vulgaris, and Taraxacum officinale. From these data I estimated components of total variation in seed settling velocities due to differences among species, among plants within species, and among inflorescences and seeds within plants. Significant amounts of variability were found at all levels. Contrasts among mean settling velocities showed that the five introduced species have lower settling velocities than the three native species; this result continues to be true when annuals are considered separately from biennials and perennials. Also, over all eight species, annuals have lower settling velocities than biennials and perennials. Variability among species apparently reflects different dispersal “strategies” employed by the species; these different strategies may be correlated with other life-history traits and with ecological characteristics. Variability within species also may have ecological consequences in that such variability may represent an example of risk-spreading.
Article
Full-text available
Shape optimization of tumbling wings - Volume 889 - Lionel Vincent, Yucen Liu, Eva Kanso
Article
Research on unstable airflow in nature has long been an important subject in work on aerodynamics, and most principles of flight have been found in nature. The design of an aircraft requires considering not only the lift required to balance gravity but also the stability of the state of balance during the cruise. Dandelion seeds happen to have a similar flow stability during flight. These seeds have a pore-like disk structure such that when air flows through them, a separated vortex ring is generated above them. Dandelions stabilize the airflow by changing the tip structure of the fluff. This study uses computational fluid dynamics to replace the fluff structure of dandelion seeds with a rigid porous disk structure and numerically simulates the resulting flow model to examine the flight of dandelion seeds. The results show that when the porosity of the disk structure exceeds a threshold, an axisymmetric and stable separated vortex ring is generated. A comparison of the simulation with observations from a wind tunnel experiment showed that the two yielded similar results, which confirms the principle of the model of flight of dandelion seeds. The authors then explore the condition for the separation of the vortex ring and conclude that the change in porosity affects the characteristics of the vortex ring.
Chapter
This chapter introduces philosophers of engineering to a new research agenda currently permeating the history and philosophy of science, one concerned with the functions of narrative in science. The functions of narrative that I am here interested in contribute to two particular kinds of epistemic positioning. First, that of the individual researcher’s epistemic position in relation to a field of inquiry. Second, the positioning of a community of researchers gathered around and looking at newly acquired evidence, assessing its significance. In the first, the kind of inference and hypothesis making that narrative affords stimulates and orders inquiry. In the second, narrative supplies a means of reasoning from the particulars of a case to something deeper or broader. The case analysed concerns an interdisciplinary project between engineers, applied mathematicians, and biologists dedicated to understanding how dandelion seeds fly. My analysis draws on the concepts of ‘tellability’ from literary study and ‘synoptic judgment’ from the philosophy of history. Tellability is used to explore question generation in science and engineering, in particular the making of more or less ‘askable’ questions. Synoptic judgement is used to interrogate my own case, key elements of which resemble synoptic judgement without assimilating to it.
Article
Dandelion seeds can stably diffuse owing to the dominant drag rather than the lift-based mechanism of the streamlined leaves of the plant, where this is known to favor their long-distance dispersal with the steady vortex attached. However, the generation mechanism of the vortex and the aerodynamic force exerted on the seeds through multiple filaments remain unknown. Clarifying these subjects may help realize the optimal performance of porous structures under different flight conditions. This study conducts numerical simulations to illuminate the influence of gaps and the Reynolds number (Re) on the wake structures and consequent drag force of dandelion seeds. We fabricate the seeds into circular disks composed of evenly distributed square cylinders placed in a vertical flow field with Re of 100 and 400, with the porosity of the pappus (ε) ranging from 0.887 to 0.964. We explain the geometric properties of the attached, steady vortex rings and clarify their generation mechanism, i.e., the base bleed and convection effects competed with vorticity generation, based on which the gaps are confirmed to delay chaotic vortices from occurring compared with the solid case. The weakened leeward pressure is critical for the increase in the drag coefficient to reach the peak level. The enhanced drag coefficient is several times higher than that in the solid case, endowing the seeds with a high loading capacity, and the porosity corresponding to its peak is beneficial for the structural design. These conclusions provide positive insights into the design of ventilated aircrafts with optimal long-distance dispersal performance.
Article
Inspired by the reproductive success of plant species that employ bristled seeds for wind-borne dispersal, this study investigates the gust response of milkweed seeds, selected for their near-spherical shape. Gust-response experiments are performed to determine whether these porous bodies offer unique aerodynamic properties. Optical motion-tracking and particle image velocimetry (PIV) are used to characterize the dynamics of milkweed seed samples as they freely respond to a flow perturbation produced in an unsteady, gust wind tunnel. The observed seed acceleration ratio was found to agree with that of similar-sized soap bubbles as well as theoretical predictions, suggesting that aerodynamic performance does not degrade with porosity. Observations of high-velocity and high-vorticity fluid deflected around the body, obtained via time-resolved PIV measurements, suggest that there is minimal flow through the porous sphere. Therefore, despite the seed’s porosity, the formation of a region of fluid shear, accompanied by vorticity roll-up around the body and in its wake, is not suppressed, as would normally be expected for porous bodies. Thus, the seeds achieve instantaneous drag exceeding that of a solid sphere (e.g. bubble) over the first eight convective times of the perturbation. Therefore, while the steady-state drag produced by porous bodies is typically lower than that of a solid counterpart, an enhanced drag response is generated during the initial flow acceleration period.
Article
A stable and axisymmetric “separated vortex ring” formed above the pappus can greatly improve the aerodynamic efficiency of dandelion seeds and reduce the material requirement for flight. Since the dandelion seed is not always in a “regular” state (i.e., the pappus plane parallel to the horizontal plane) when flying, this paper uses a simplified rigid pappus model to simulate and analyze the influence of flight attitude on the flow patterns and the resulting changes in aerodynamic characteristics under different porosities. The numerical results are obtained by solving the three-dimensional incompressible steady RANS equations with the Spalart–Allmaras turbulence model in ANSYS Fluent software package. The results show that when there is an angle of attitude, the separated vortex ring will break with one of the vortices, losing its original recirculating structure, which leads to a considerable loss of the drag performance. In addition, the high-pressure region below the pappus plane shifting to one side will produce a horizontal force and a recovery moment against the rolling direction. The horizontal force component generated by a large-porosity pappus disk is much larger than that of a solid disk, which shows the good flight efficiency of dandelion seeds. At the same time, the inclined pappus will produce a recovery moment to keep the seed from rolling away from the horizontal plane. These conclusions show that the pappus structure has the potential to adjust the flight attitude and aerodynamic characteristics, which provides a preliminary understanding for further study on the flight dynamics of dandelion seeds.
Article
The aerodynamic performance and wake structure of dandelion seed pappus have been numerically studied based on a simplified quasi-dandelion pappus (QDP) model with its filaments represented by rectangular cylinders. The filament width is chosen as the major geometric parameter for investigation. A rigorous measuring strategy is developed for the identification of the recirculation region width in the wake of the QDP model. Three regimes are distinguished as the filament width increases, i.e., a dandelion-like regime, a transition regime, and a disk-like regime. In the dandelion-like regime, the recirculation region widths are relatively large and monotonously decrease with the increase in Reynolds number. In the transition regime, the recirculation region widths are moderate and first decrease sharply at low Reynolds number and subsequently maintain an approximately invariant value. In the disk-like regime, the recirculation region widths are relatively small. The Reynolds number based on the recirculation region width is defined, and its correlation to the drag coefficient in a different regime is also discussed. In addition, as the QDP model turns from the dandelion-like regime to the disk-like regime, the pressure distribution in the wake turns from the recirculation region type to the flow stagnation type. The current study may provide a reference for the design of more efficient dandelion-like aircraft.
Book
This chapter introduces philosophers of engineering to a new research agenda currently permeating the history and philosophy of science, one concerned with the functions of narrative in science. The functions of narrative that I am here interested in contribute to two particular kinds of epistemic positioning. First, that of the individual researcher’s epistemic position in relation to a field of inquiry. Second, the positioning of a community of researchers gathered around and looking at newly acquired evidence, assessing its significance. In the first, the kind of inference and hypothesis making that narrative affords stimulates and orders inquiry. In the second, narrative supplies a means of reasoning from the particulars of a case to something deeper or broader. The case analysed concerns an interdisciplinary project between engineers, applied mathematicians, and biologists dedicated to understanding how dandelion seeds fly. My analysis draws on the concepts of ‘tellability’ from literary study and ‘synoptic judgment’ from the philosophy of history. Tellability is used to explore question generation in science and engineering, in particular the making of more or less ‘askable’ questions. Synoptic judgement is used to interrogate my own case, key elements of which resemble synoptic judgement without assimilating to it.
Article
Due to autorotation, samaras can fly efficiently and stably to be dispersed over a great distance under various weather conditions. Here, we provide a quantitative analysis of the dynamic stability of free-falling maple samara (Acer grosseri Pax) and verify whether they are dynamically stable as observed. Morphological and kinematic parameters were obtained based on the existing experimental data of the maple seed. Then the linearized equations of motion were derived, and the stability derivatives were calculated by a computational fluid dynamics method. The techniques of eigenvalue and eigenvector analysis were also used to examine the stability characteristics. It is found that there are five natural modes of motion of the maple seed: one stable oscillatory mode, one fast subsidence mode, one slow subsidence mode, and two neutral stable modes. The two neutral modes are manifested as the seed moving horizontally at a low speed under disturbance. Results show that the maple seed has dynamic stability in sustaining the steady autorotation and descent, exhibiting a minor horizontal motion when disturbed. These findings can be applied to biomimetic aircraft.
Article
The study of a flexible body immersed in a flowing medium is one of the best way to find its aerodynamic shape. This Letter revisited the problem that was first studied by Alben, Shelley and Zhang (Nature 420, 479-481, 2002). To determine the aerodynamic shape of the fibre, a simpler approach is proposed. A universal drag scaling law is obtained and the universality of the Alben-Shelley-Zhang scaling law is confirmed by using dimensional analysis. A complete Maple code is provided for finding aerodynamic shape of the fibre in the flowing medium.
Thesis
When in a flow, an object deviates it and from this deviation are generated vortices and flow reaction forces, such as drag and lift. If yet the object is free to move, its movement can couple with the surrounding flow, falling into the domain of fluid-structure interactions. In this PhD thesis, the coupling between a pendular system and an air flow is studied both experimentally and theoretically. Placed in a wind tunnel, a disk pendulum presents a bistability for a range of flow velocity, while a rectangular one does not. By varying the aspect ratio of such rectangle and visualizing the wake behind a fixed disk, we propose an origin on whether or not the bistability emerges, linking it to stall phenomenon. The influence of ambient turbulence on this phenomenon is then investigated together with the link between angular fluctuations and flow variations, both upstream and downstream. Going back to the bistability itself, spontaneous transitions between stable states are observed and a model inspired from the transition to turbulence suggest certain mechanisms in the wake triggering such transitions, in particular rare aerodynamical events. Modifying geometrical parameters of the pendulum enables the adjustment of the range of velocity for which the bistability occurs, and with it, we could observe jumps between both transitions at the same flow rate. Finally, when the pendulum is balanced, its movement is only driven by the flow and while a quasi-static modelling is not sufficient to describe the real dynamics of the pendulum, we introduce two ways of accounting for the dynamical retroaction of the flow in the equation of movement, one empirical and the other based on vortex-induced vibration theory.
Article
The prevailing model of wind-borne seed dispersal assumes that variation in seed size and weight controls patterns of seed distribution by limiting exposure to horizontal airstreams. To test the applicability of this model in a natural dispersal situation, seeds of Betula lenta were collected downwind of a population along transects which trapped seeds (a) after primary dispersal through the air, and (b) after primary aerial dispersal plus secondary dispersal across a snow cover. In order to examine mechanisms of seed flight, weights of 296 seeds were experimentally modified and duration and distance of seed flight measured in laboratory trials. Rate of descent of modified seeds depended strongly on individual seed weight and wing loading, and lateral movement in a horizontal airstream was determined by rate of descent, as predicted. In the field, however, distance traveled by aerial movement was only weakly linked to seed size and weight, and the relationship did not hold during secondary dispersal near the ground. Thus, seed size and weight are not the principal factors controlling seed movement under all conditions. It may be necessary to invoke models based on airstream turbulence.
ResearchGate has not been able to resolve any references for this publication.