How Plants Prevent Overcrowding
Nature is full of tricks that help living things survive. Since most plants produce seeds to keep their species alive, they have also developed techniques to make seeds more effective. Plants that grow really close to one another have to compete for space, nutrients, water and sunlight. If all plant seeds just dropped to the ground and started growing, they would not grow as well as if they were farther apart.
Since plants can’t walk around to scatter their seeds, they have to used other methods to spread them.
Now that we know some of the reasons plants spread their seeds, it’s time to learn how they do it. There are five basic ways that seeds are spread. Some are pretty simple; others are very ingenious. Let’s take a look at each of the methods.
Gravity is the simplest type of seed dispersal. Simply put, the seeds just fall from the plant and land in the soil below it. A good example of gravity dispersal is the marigold flower.
When they are ready, these seeds drop to the ground. As you can see, the seeds have a fuzzy top on them. These tops act like the feathers on an arrow to keep the tip of the seeds pointing down. This allows them to penetrate into the top layer of the soil and gives them a head start on the growing process.
Gravity isn’t a very effective way to spread seeds, and most plants use other tricks in addition to gravity.
Wind dispersal is little more than an improvement on gravity. Seeds from these types of plants have special adaptations that allow the wind to move them a little farther away from the parent plant. There are two types of wind dispersal adaptations, the floaters, and the fliers.
These plants have lightweight seeds with fluffy tops that catch the wind and lift the seeds into the air to drift wherever the wind takes them.
The common dandelion is a good example of this type of seed dispersal. These seeds have an umbrella shaped top that works like a parachute.
Because this method of seed dispersal works so well, many of the plants that have this type of seeds are considered to be weeds.
These seeds have a special wing-like adaptation that causes the seeds to spin in a circle as they fall, like the blades of a helicopter. This keeps the seed in the air longer and allows the wind to carry it further from the parent plant than it would if it just fell to the ground.
Maple seeds grow in pairs, but when they are mature, they separate, and a strong wind can pull them from the stem. They will then flutter to the ground in the hope of finding a good spot to germinate and grow.
Some seeds will land in good soil, but others will end up in spots like parking lots or sidewalks where they have no chance of growing into trees. Since there is no guarantee that the seeds will land in a good spot, plants that use the wind to spread seeds produce a lot of them.
The more seeds they have, the better the chance that some will be able to germinate and grow into a strong and healthy plant.
Activity: Make Your Own Simple Samara
Seeds with wings, like maple seeds, are called samaras. In this activity, we’re going to make our own samaras and see how well they fly.
On the last page of this study guide, you will find patterns you can use to make your samara. Follow the instructions below as they relate to the diagram on the right.
- Cut along all the solid lines on the diagram to the right.
- Fold flap A forward on the dotted line and flap B to the back.
- Fold flaps C and D both forward along the dotted lines.
- Fold along the line E upward to give weight at the bottom.
- Use a paperclip to hold the bottom fold in place and provide some additional weight.
Hold the samara by the bottom near the paper clip and gently throw it into the air. You can also drop it from the top of stairs. If it’s OK with your parents or teachers, try dropping it out a window.
Ballistic seed dispersal
This method involves a mechanical action that physically throws or shoots the seeds away from a seed pod. The seeds are usually ejected by an elastic contraction of fruit tissue. This shoots the seeds sort of like the pellets from a shotgun.
Just like all the other methods of seed dispersal, some plants are better at ballistic dispersal than others. Some common examples of this type of plants are garden peas.
The pods of peas and other similar plants, have two layers of cells that run in different directions. As these cells dry and shrink, they pull in different directions. The result is that the pods break open and twist. This twisting actions pops the seeds lose and pushes them away from the pod.
Some pods break open quite gently and the seeds don’t go very far. One African tree that’s related to peas can shoot its seeds 150 feet or more.
Some common plants that use this type of seed dispersal are wisteria and mesquite. The jewel weed (also known as touch-me-not) shown here, has a seed pod that opens violently, spreading seeds in all directions.
Mother Nature's Screwdriver
Seeds from this plant, called the heron’s bill, have developed a system that uses uneven cell movement in a different way. When these seeds are ripe, they separate from each other and immediately begin to twist.
This action moves the tip of the seed down and in contact with the soil. As the twisting continues, the seed is actually screwed into the ground, planting it below the surface as effectively as if a gardener had planted it.
Coconut Seed on Beach
It’s not uncommon for seeds to end up in the water and float to new locations. Some plants have adapted special seed for this purpose, others just take advantage of an unexpected opportunity.
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Coconuts have a husk that surrounds the seed. This husk is very buoyant and a coconut seed can float across hundreds of miles of ocean before washing ashore on a new island to germinate and grow into a tree.
Not long ago, a mud slide created a temporary dam across a stream in central Utah. The water backed up and formed a reservoir. When the slide was cleared out, the water receded, but the following spring all along the high water mark sunflowers sprouted in abundance. The sunflower seeds had floated on the water and settled to the ground when the water level dropped. The result was a line of yellow flowers that showed exactly how high the water had risen.
The final, and possibly most effective, way that plants spread their seeds are the use of animals. There are two ways plants use animals to move their seeds. One is by making them good to eat; the other is by using spines or stickers to cling to fur or clothing.
Edible Fruits, Nuts and Other Seeds
Some animals like to gather seeds and store them in secret caches for later eating. Sometimes they forget where they put the seeds or just don’t get around to eating them. These seeds can then germinate and grow into new plants.
Other seeds are surrounded by good tasting fruit, like apples and peaches. Animals eat the fruit, but the seeds are covered with a hard seed coat that doesn’t digest. When the animal defecates (poops), the seed drops to the ground surrounded by a ready supply of good fertilizer.
Many seeds have barbs or hooks on them that snag fur or fabric. These seeds grab hold of animals that walk past and get tangled in their hair or a person’s clothing. When the animal or human gets annoyed enough to remove the seed, it is deposited in a new spot, spreading the seeds all over the landscape.
Because they can be painful and irritating, many of these plants are considered nuisance weeds. Some examples are cockleburs and foxtail.
An interesting benefit from the troublesome cocklebur, shown on the right, is that its spines are what inspired the invention of velcro.
Activity: Sock Walk
Pull a large white sock over the outside of your shoe. Now go for a walk around the neighborhood. Walk through some patches of grass and weeds to get a good cross-section sample.
Now carefully remove the sock and take it back to your classroom or home. Spread some newspaper on a table and place the sock on top. Take a close look at what types of seeds hitched a ride on your sock.
See if you can identify what plant the seeds came from. If you want, you can plant them and see what grows.
Note: In the interest of keeping costs down, only one sock, or since they come in pairs, two socks need to be used for any sized group.
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