Wild About Seeds

Seeds can be large as coconuts or as tiny as dust. They may have wings or hooks or they may be perfectly round and smooth. Some seeds are covered with hairs while others have a single long tail. Seeds are flat or round or jagged or smooth. The variations are endless. But why are there so many differences?

A seed pod bursts open and multiple seeds emerge, each one has a fluffy parachute that will help it travel.
Butterfly weed seeds by VershaKGupta, CC BY-SA 4.0, via Wikimedia Commons

The shape and size of seeds are related to two factors: dispersion and germination. Seeds, of course, do not exist as static entities. Seeds have a purpose—to grow and propagate their species. 

Before seeds can grow, they need to find their way to a suitable habitat. Plants have five main ways of dispersing their seeds: wind, water, animals, gravity, and as ballistic projectiles. 

Consider the familiar dandelion. Its seeds are perfectly designed for wind dispersal, floating on the air until they find a patch of disturbed soil. Dandelions share a plant family with sunflowers, and yet the two species have vastly different seed dispersal strategies. Where dandelion seeds are distributed primarily by wind, sunflowers rely on animals to carry their seeds. 

A small black and white bird hangs upside down from a sunflower head.
A mountain chickadee helps disperse sunflower seeds by JerryFriedman, CC BY-SA 4.0, via Wikimedia Commons

In grassland ecosystems, where wind dominates the landscape, seeds have evolved a variety of forms to take advantage of this dispersal opportunity. Plants like cottonwood trees and milkweed have light fluffy seeds that are easily blown to new locations. Other plants, like maple trees, have wings that help them move through the air. 

Seeds not only need to adapt in ways that help them disperse across the landscape, they also need to germinate in their new location. For some plants, this means evolving very specific strategies that allow seeds to germinate under the right conditions. For example, consider needle-and-thread grass (Hesperostipa comata). The seeds of this grass have a long tail, called an awn. In dry conditions, the awn shrinks and coils up into a tight spiral. When humidity or moisture increases, the spiral unwinds, driving the seed into the soil, the perfect adaptation in a landscape where moisture to germinate is scarce.

A person holds long seeds between their thumb and forefinger. The seeds have a long, wiry tail.
Needle-and-thread grass seeds by Matt Lavin, CC BY-SA 2.0, via Wikimedia Commons

Seed germination strategies can be fickle. For trees like cottonwoods, who rely on river floods to provide the silty sandbars that are optimal for germination, human interference and flood control has caused decreased germination. Likewise, for the lodgepole pine, which requires fire to open its hard cones, releasing seeds into a scorched landscape where its seedlings have an advantage, neither fire suppression nor the increased heat of climate change enhanced fires give the same ecological advantage.

White fluffy seeds cover the ground almost like snow.
Cottonwoods release thousands of seeds but most won’t germinate. Photo by EnLorax G. Edward Johnson, CC BY 3.0, via Wikimedia Commons

Humans have influenced seed dispersal, and germination for millennia—choosing seeds to plant and helping them grow. Regardless of their original dispersal strategies, we choose seeds not only for their ability to produce food, but for aesthetic reasons. Whether it’s the deliberate act of planting a garden or the accidental transport of seeds on our clothing, we are an important seed dispersal mechanism.

Whether it’s the wind blowing a milkweed seed or a person planting a garden, there’s something miraculous about the promise of a seed. There’s nothing more hopeful than seeing something as small as a grain of sand grow into a full plant in a matter of months. As Audrey Hepburn said, “to plant a garden is to believe in tomorrow.” A statement made even more true by the lengths seeds go to survive and germinate.