Blowing in the Wind

(How Plants

Disperse Seeds)

Why so many seeds? Have you ever noticed that most plants produce dozens of seeds? The large amount of seeds helps assure that at least some of the seeds will grow into adult plants. Many times however, most of the seeds do not survive. Seeds might be eaten by an animal. They might dry out and not grow. Some of the seeds might never find the right conditions in which to grow. But some seeds do grow. What makes growing possible?

Why is seed dispersal important? People plant some seeds, but most plants don't rely on people. Plants rely on animals and wind and water to help scatter their seeds.

Trees reproduce in a very different way than animals and humans. Trees and other plants produce new plants by making seeds. These seeds must get to a place so that they can start to make a new tree. Trees have four ways to spread their seeds. This process is known as seed dispersal.

Why do plants and trees disperse their seeds?

If the seeds simply fell and grew beneath the parent plants there would be too many seedlings competing for the same resources. They would be too overcrowded and would be starved of nutrients. So, it is necessary for plants to spread their seeds around. But, the seeds are dispersed, or spread, over a wide area where they stand a better chance of finding the right condition to grow.

How are seeds dispersed?

There are five basic ways that seeds are dispersed.

1: Wind dispersal2: Water dispersal

3: Animal dispersal4: Explosions

5: Fire

Different types plants have developed different adaptations for

at least one of these dispersal methods.

How does WIND disperse seeds?

Some seeds are carried to a new place by the wind. These seeds are very light. Some like the seeds of the orchid are almost as fine as dust.

orchid

Many have very light hairy growths which act like little parachutes. Dandelion seeds are carried by the wind.

The seeds of the thistle are also carried away by the wind.

The fruits of the sycamore have winged seeds that are also carried away by the wind. The wind causes the ripe fruits of the poppy to sway. This causes the seeds to fall out.

The seeds are very light and

are carried away on the

wind.

Depending on the wind speed and distance above the ground, helicopter seeds can be carried considerable distances away from the parent plant.

Wind-dispersed seeds & fruits in different

plant families: Helicopters: A. Box Elder C. Big-Leaf Maple E: Evergreen Ash F. Tipu Tree. Flutterer/Spinners: B. Empress Tree D. Tree of Heaven G. Jacaranda

How can WATER disperse seeds?

Fruits which float, such as those of the water lily and the coconut palm, are carried by water. Coconuts can travel for thousands of kilometers across oceans.

How do ANIMALS disperse seeds?

Some plants have juicy fruit that animals like to eat. The animal eats the fruit but only the juicy part is digested. The stones and pits pass through the animal's digestive system and are excreted (pooped out) to form new plants. This can be far away from the parent plant.

Blackberry seeds are dispersed in this way. Birds also like to eat

fruit and they help to disperse seeds to other areas through their droppings.

Mistletoe has sticky fruits which are attractive to birds. The sticky seeds stick to the bird's beak. They then rub

their beaks clean on the bark of trees. The sticky seeds are left on the bark to grow into new mistletoe plants. Squirrels collect nuts like acorns and bury them for winter food, but they often forget where they have buried them or don’t come back for them. These grow into new trees.

Some fruits like that of the burdock plant have seeds with hooks. These catch on the fur of animals and are carried away. Humans also help to disperse seeds. The hooks or thorns of “hitchhiking” plants attach themselves to clothing and then are often pulled off and dropped on the ground farther down the road.

Why do some seed pods EXPLODE?

Some plants have pods that explode when ripe and shoot out the seeds. Lupines, gorse and broom scatter their seeds in this way.

Lupine

Pea and bean plants also keep their seeds in a pod. When the seeds are ripe and the pod has dried, the pod bursts open and the peas and

beans are scattered. These seed pods are often referred to as “poppers”.

How can FIRE help seeds disperse?

To survive fire some plants have adaptive traits that allow them to reproduce or regenerate. When fire occurs, animals have the ability to fly, run away or burrow deep into the ground. Plants cannot do this and so have adapted other ways of surviving. The way a plant stores its seeds and disperses them is an example of a fire adaptive strategy.

The high heat from the fire (it is important the fire reaches the right temperature) is very important to the seed’s dispersal. Also important is how often the fires occur. A number of species of pine have cones that only open after a fire. These are called serotinus.

Jack pines have cones that are held closed by a resin (sticky sap) sensitive to high temperatures. These cones will

not open to release their seeds until a certain temperature is reached.

Some plants may not come into flower for many years. If fires happen too often, the plant is destroyed before it has had the chance to produce seeds.

A plant called Old Man Baksia is killed by fire but uses the fire to trigger seed dispersal. However,

it takes about 15 years for this plant to reach flowering age! Some plants store their seeds in the soil until a fire reaches the right heat for germination to be triggered. Fires are often deliberately lit in the bush to trigger germination from soil seed banks and to kill weeds.

The important thing to remember is that flowering plants reproduce themselves by producing seeds. However, if the seeds are not dispersed, many seedlings will begin to grow too close to the parent plant. This results in the seedlings competing with the parent plant for light, space, water and nutrients. All of these are important for plants to be able to grow. The adaptation mechanisms for seed dispersal give plants a better chance for survival.