Avocado Seeds

Introduction

People usually encounter avocados only after they've been mashed up into pasty green guacamole. This Science Update considers the fruit in its natural state--investigating how the great big avocado seed manages to "get around."

Audio File

Avocado Seeds

Transcript

How avocados get around. I'm Bob Hirshon and this is Science Update.

Dandelion seeds spread in the wind. Coconuts float on the waves. And berry seeds get eaten by birds and, well, dropped someplace else. But Science Update listener Tim Appenzeller of Washington, DC wanted to know how the enormous avocado seeds get around.

Appenzeller:
What animal could possibly eat the avocado and spread that seed?

We asked Gary Bender, a farm advisor with the University of California Cooperative Extension. He says a ripe avocado makes a tasty treat for wild animals.

Bender:
And, they'll pick the whole fruit up and, either eat it right there, or carry it off under a bush. Or actually, I've seen them bury the avocado. And it's a perfect way to start a seedling tree.

He says avocados, like all other fruit, evolved specifically to spread seeds.

Bender:
Of course, they have to do that, or else you'd have all these little seedlings coming up right under the mother tree. And that is really why fruit, we think, a good-tasting fruit evolved. It's there to attract an animal, and a human, to pick it up and to move it. And that's exactly what happens with avocados.

If you've got a science question, don't worry if it's a little fruity, just call us at 1-800-WHY-ISIT. If we use your question on the show you'll get a free Science Update mug. For the American Association for the Advancement of Science, I'm Bob Hirshon.

Making Sense of the Research

Plants have a variety of methods for spreading their seeds. By dispersing over large areas, away from the parent plant, seeds are able to avoid competition for resources (light, water, minerals, and space) which would adversely affect their ability to survive. Many seeds, such as those of the dandelion, are dispersed by wind; others, such as the pond lily and coconut, float on water. Some plants spread their seeds by exploding or catapulting them away. Seeds of edible fruits and berries are widely distributed by birds and animals that eat but do not digest them. The seeds pass through an animal's digestive system and are dropped (excreted) far away from where they were eaten. Still other types of seeds may be buried by animals and forgotten, thus allowing for eventual germination. So-called "hitchhiker" seeds have hooks, barbs, or sticky surfaces that allow them to catch a free ride on an animal's fur or human's clothing.

The mechanisms for seed dispersal are diverse, and quite ingenious, really. But what about the large avocado seed? This story investigates how the avocado seed is spread, providing an interesting introduction to the larger topic of seed dispersal.

Now try to answer the following questions:

1. How do avocado seeds get dispersed?
2. Why is it important that the seeds of a plant or tree be dispersed? What would happen if they were not?
3. Explain how good-tasting fruits like the avocado evolved over time.
4. Can you think of other seeds that are dispersed by animals?
5. What types of seeds are dispersed by humans?
6. What are some other ways in which seeds are dispersed? Can you think of examples for each method?
7. This story describes ways in which predators are encouraged to spread the seeds of a plant. Can you think of defense mechanisms that protect plants from predators?
8. What do you think a seed dispersed by wind would look like? by water? by animals? by popping? by sticking?

Going Further

Seed Traps are a useful way to collect and study seeds. The Dragonfly site offers information on how to construct a seed trap from simple household items, and how to use it to determine the distance seeds travel and the method of transport they use.

Designer Seeds from Access Excellence challenges older students to create a seed that will travel the farthest. Seed dispersal mechanisms and elements of experimental design are addressed in the lesson.

Go toPoison Ivy for a Science Update that looks at the amazing diversity of chemical substances found in plants, and the defense mechanisms that plants and animals use to protect themselves from predators.