Wide Wings Give Seeds a Soaring Advantage

Biological Strategy

Javan cucumber

Leon Wang

Move in/Through Gases

Living systems must move through gases (which are less dense than liquids and solids) such as those in the earth’s atmosphere. The greatest challenge of moving in gases is that because the living system is heavier than the gas, it must overcome the force of gravity. Moving efficiently in this light medium presents unique challenges and opportunities for living systems. As a result, they have evolved countless solutions to optimize drag and increase lift so that they can stay aloft and take advantage of variable currents. Additionally, they must overcome gravity when moving from a liquid or solid into the air. The fairyfly, the smallest known insect, is a tiny wasp that must move through the air. To the wasp, air feels like a heavy liquid and to move through it, it uses special feathery oars rather than wings.

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Flowering Plants

Clade Angiosperms (“receptacle seed”): Dandelions, oaks, grasses, cacti, apples

With 416 families containing some 300,000 known species, angiosperms are the most diverse group of plants, and they can be found around the globe in a wide variety of habitats. They are characterized by seeds that grow enclosed in ovaries, which are enclosed in flowers. The floral organs then develop into fruits of myriad kinds and dimensions, from simple seed casings on maples to elaborate fleshy growths like papayas. The oldest flower known from fossils, Montsechia vidalii, appeared during the Jurassic Period 130 million years ago. They are the primary food source for herbivorous animals, which in turn makes them the indirect food source for carnivores as well.

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A thin plate and centered configuration help Javan cucumber seeds float long distances on the wind.

The Javan cucumber (Alsomitra macrocarpa) is a vine that climbs the trees of tropical forests toward the canopy and sunlight. At great heights it grows pods that contain hundreds of winged seeds called samara. As the wind blows against the opening of the pods, the samara are peeled away and released. Unlike many seeds that make a gliding flight using auto-rotation, the seed of the Javan cucumber vine exhibits a stable gliding flight with its paper-thin wings. The seed’s design is efficient enough to achieve a low descent angle of only 12 degrees and therefore it is able to achieve a slower rate of descent (0.41 meter per second) compared to that of rotating winged seeds (1 meter per second). This aerodynamic advantage allows the seed to be easily carried by the wind.

The construction of the seed and wing gives it this advantage. The seed itself is thin, about 1 millimeter in thickness, and positioned almost exactly at the structure’s center of gravity to give it balance. The wings are even thinner, about a few micrometer to some 10 micrometer. Because the wings are so thin, as the samara is angled up or down, the center of pressure from the wind will shift to reduce that angle. This effect stabilizes the seed and also prevents it from diving. When viewed from above, the wings are angled behind the center of the seed to give it more stability and are slightly tapered toward the tip to make it lighter with less drag. When viewed from the front, the wings are angled upward which helps it fly in a straighter path and prevents spiral instability. The wings also have a sharp leading edge and an aspect ratio (AR=3~4) that results in an appropriate lift-to-drag ratio (L/D= 3~4) to support their gliding flight.

The form of the samara allows it to travel long distances in the wind. It is possible for the seeds to glide up to hundreds of meters, ensuring that they spread far from each other as well as the parent pod. This wide dispersal prevents the seeds from competing for resources once they fall to the ground and begin growing.

For a visual explanation of the Javan cucmber gliding, watch this video.

This summary was contributed by Leon Wang.

Last Updated April 28, 2017

References

“The steady gliding flight of samples of Alsomitra macrocarpa samara was filmed and analysed. The lift-to-drag ratio or the gliding ratio was about 3~4 and the rate of descent was 0.3~0.7 m/sec, which was smaller than those of other rotary seeds. The flight was so stable that samples were seen to take their optimal trimmed angle of attack with a value between the maximum gliding ratio and the minimum rate of descent.” (Azuma and Okuno 1987: 236)

“A pale species from Java, Alsomitra marcocarpa, has a sail wing and performs a stable gliding flight without any tail surface… The seed itself is very thin, about 1 mm in thickness, and is located nearly at the center of gravity, which is a slightly forward position of the wing center. The wing is also very thin (from a few µm to some 10 µm) and has a swept and tapered planform, twisted (washout) angle, reflected trailing edge, and adequately arranged position of the center of gravity… By thin wing theory, the reflected aerofoil has a positive moment at positive angle of attack, and shifts the center of pressure backward as the angle of attack increases, and thus has a tendency to stabilize the pitching motion of the seed. The pitching stability is further strengthened by the sweep angle of the wing if the center of gravity is located in the front of the mean aerodynamic center.” (Azuma and Okuno 1987: 264)

“The geometrical characteristics of the wing of Alsomitra macrocarpa, such as the slightly swept and twisted wing, the reflected trailing edge of the airfoil, the lightly loaded wing and adequately arranged CG [center of gravity] position, are well fitted to assure the good performance and stability in gliding flight of the winged seed. The thin wing with a sharp leading edge and adequate aspect ration (AR = 3 ~ 4) produce the appropriate lift-to-drag ratio (L/D [approx = to] 3 ~ 4) for the flight in small Reynolds number (Re [approx = to] 4 X 103). The low wing loading (mg/S [approx = to] 0.5 N/m2) also guarantees a smaller rate of descent (w = 0.3 ~ 0.7 m/sec) than those of the rotary seeds. The flight is performed at a life coefficient of CL = 0.34, which not only gives the maximum gliding ratio but also guarantees approximately the minimum rate of descent. The above small life coefficient is adopted because of the large drag coefficient at high angles of attack…It is also made clear that the dispersal of the seeds is assisted by the wind surrounding the husk, and the resonant pendulum motion of the hung husk.” (Azuma and Okuno 1987:274)

Journal article