Structures Facilitate Pollen Dispersal

a pine cone in development on a pine tree with a forest background

Biological Strategy

Pines

AskNature Team

Image: Andy Morffew / Flickr / CC BY - Creative Commons Attribution alone

Disperse Seeds

Plants disperse their seeds using a variety of mechanisms, including wind, rain, and attachment to animals. Seed dispersal can be important if plants growing too close together will compete for resources, such as nutrients and sunlight. Since plants are literally rooted in place, seed dispersal is also an important mechanism for expanding their range or reaching new environments.

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Pollinate

One of the ways that plants can produce offspring is by making seeds that contain the genetic information to produce a new plant. Seeds can only be produced when pollen grains reach the hidden ovaries within a flower. Flowers have developed wildly diverse forms and activities, such as the production of sweet nectar to attract organisms such as bees and butterflies, that move from flower to flower, transferring pollen. Flowers will also adapt to make use of environmental influences such as wind and water as pollinators.

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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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Conifers and Others (Pines, Spruce, Gingko)

Clade Gymnosperms (“naked seeds“): Conifers, cycads, Gingko, gnetophytes

There are approximately 1,000 known species of Gymnosperms. These include well known needle-bearing plants like pines, firs, and spruce, as well as some with broad leaves like the Gingko and the gnetophytes. Instead of flowers, gymnosperms produce unenclosed (“naked”)seeds exposed on the surface of cone scales. Economically, conifers make up 45 percent of the world’s lumber production. The first gymnosperms evolved over 390 million years ago in the Paleozoic Era and dominated landscapes for a quarter of a billion years before the evolution and diversification of flowering plants.

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Air-filled sacs in the pollen grains of pine trees allow pollen to travel farther through the air

One mechanism by which conifers promote cross-pollination is by increasing the distance their pollen grains travel. Each pollen grain is attached to two or three air-filled sacs, or sacci, which develop from the outer layer of the pollen wall. These air sacs increase surface area but do not substantially increase the overall pollen mass. The increased surface area of the pollen grain by the addition of the sacci increases the amount of air resistance on the grains, so they fall down to the ground more slowly. This allows the grains to float in the air for longer and be dispersed farther.

The amount of time the pollen grain remains airborne is also correlated with the thickness of the sacci wall and the pattern on the surface of the sacci known as “ornamentation”. Similar to the dimples of a golf ball, ornamentation may provide lift and overcome inertia forces to slow momentum. The slowing down of momentum allows the pollen grain to remain airborne longer and to travel longer distances by the wind.

This strategy was contributed by David Zaitz.

Last Updated July 20, 2017

References

“The morphology of pollen grains may also affect the aerodynamics of wind pollination…Sacci increase the surface area of the grains, while ideally adding a minimum of mass. This, in turn, increases the amount of drag on pollen grains. Increased drag reduces the settling speed of pollen, causing dispersal distances to increase” (Schewendemann et al. 2007:1371).

Journal article