Traveling Seeds
By Megan Wilde
“Pop! Pop! Pop! Pop! Pop! Wheee!”
That’s Patty Manning, mimicking the way hummingbird bush and desert petunia scatter their seeds. Manning runs Sul Ross State University’s native-plant greenhouses in Alpine, Texas. So she appreciates the diverse strategies desert plants have evolved to spread around their offspring.
Hummingbird bush and desert petunia have little club-shaped fruits—fruits being the plant part that usually encloses seeds. After these fruits dry out, they split.
Manning: And there’s a mechanism inside that actually flings the seed. You can hear them pop. It usually happens when the fruit is exposed to some amount of moisture. It pops it open and sends the little seed inside flying. They’re sort of disc-shaped.
So these ballistic discusses are hurled every which way. How do other plants in the Chihuahuan Desert region disperse their seeds?
Animals, obviously, are more mobile than plants. So plants have evolved ways to entice, or trick, animals into transporting their seeds.
Manning: So a lot of seeds are enclosed in a fruit that is juicy, or sweet, or attractive to varying species of animals to eat.
Consider, Manning says, the holly-like agarita bush. Their juicy red berries are irresistible to many birds, who sometimes retreat to a nearby tree to digest their meal. But the agarita seeds aren’t digested, and pass out with the bird’s droppings. There, the seeds can sprout and grow in relative comfort, nestled in fertilizer, sheltered by a tree.
Mesquite beans are a favorite snack of grazing animals. The seeds inside the beans pass through the animal unscathed, exiting in their nutrient-rich manure. That’s not the only advantage of being eaten.
Manning: Mesquite beans just laying on the ground before being eaten, it’s very hard to get the seed out of the bean. It needs the action of being eaten, or being in the soil where it’s exposed to some microbial action, to break through the fruit to get the seed out. So it’s very well dispersed by animals that like to eat it.
Some fruits hitch rides on feet or fur. Devil’s claw fruits look like okra when green. But when dry, the pods split, forming two curled, spine-tipped claws.
Manning: They’re perfectly shaped so they can catch onto the hoof of an animal and be dragged along. And eventually knock loose in another location. And when the fruit continues to dry, the seeds eventually come out.
But beware, some hitchhikers are more sinister.
Manning: Goatheads are from the Mediterranean originally. And they evolved to be dispersed by flip flops and bicycle tires.
All kidding aside, Manning says goatheads are one of many invasive species that exploit us to transport seeds. Russian thistle, or tumbleweed, is another. Its tiny seeds get caked in mud on tires and shoes. But tumbleweeds mostly rely on wind to drive them around. And they’re actually following the footsteps of some native grasses, which scatter seeds as they tumble along in the desert breeze.
Other wind-dispersed seeds are packaged in fruits with papery propellers, feathery wings or tufted parachutes. Bigtooth maples, desert willows, and milkweeds produce seeds well-suited to blowing in the wind. As well as ocotillo.
Manning: Ocotillo. They have a dried capsule that splits open at the top. And the seeds are very light and papery. And they fly!
Many plants use multiple tactics. Seeds of three-awn grasses are shaped like slender torpedoes, with wiry tails at one end. These tails can catch wind or get tangled in animal fur.
When exposed to moisture, these seeds start twisting and turning, drilling their pointy tip into whatever surface they’ve landed on. By waiting for moisture, the seeds have a better chance of drilling into moist, soft soil. This can be an advantage in the desert, where the ground can be quite hard for a little seed trying to get established.
Have a question or comment about this episode? Contact Nature Notes Coordinator Megan Wilde at mwilde [at] cdri [dot] org. Or discuss this episode on Nature Notes’ Facebook page. This episode originally aired on November 11, 2010.
References & Resources for Educators
- Seed Dispersal from The Seed Site
- Fruit: Edible, Inedible, Incredible by Wolfgang Stuppy and Rob Kesseler (Firefly Books, 2008)
- Seeds: Time Capsules of Life by Rob Kesseler and Wolfgang Stuppy (Firefly Books, 2009)
- Sul Ross State University native plant propagation program
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Nature Notes is sponsored by the Meadows Foundation and the Dixon Water Foundation and is produced in cooperation with Marfa Public Radio.
