seeds dispersel

5/27/2018 Seeds Dispersel

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Before they can grow into new plants, seeds need to leave the seed pod. If all the seeds a plant producedlanded just underneath the parent plant, they would be too crowded, and the established large plant mightnot leave them enough light or water for them all to develop properly. When you plant seeds too thickly ina pot, you can see that they grow tall and leggy, and each plant is weak and spindly. The variousmethods of seed dispersal are designed to ensure that as many seeds as possible have a good chanceof growing up to produce seeds of their own.

Sometimes, the pod or fruit containing the seeds is carried away from the parent plant; sometimes,individual seeds are spread to a new location. The size and shape of the seedpod or the seeds influenceshow they are dispersed. The main methods plants use to disperse their seeds to places with bettergrowing conditions than directly under the parent plant include gravity, animals, force, wind and water.Often, a plant will spread its seeds by a combination of these methods. For instance, a fruit falling to theground by gravity might then be carried away by animals, or a seed blown by the wind might land in waterand be transported somewhere else before germinating.

On some plants native to Australia and South Africa, the seedpods may need the heat of the natural bushfires occurring in these areas to open and release their seeds, which are then dispersed by other means.

Dispersal of Seeds by Gravity

Large, round, heavy fruits just drop straight off the tree onto the ground when they are ripe.

Calabash Coconuts ApplesIf they have a tough outer shell, they may roll some distance from the parent plant. The higher up the treethey are, and the larger they are, the further they can roll.

If they have a soft skin, they may break open when they hit the ground and the individual seeds may bescattered.

On lower-growing plants, many fruits and seeds are also heavy enough to fall to the ground without anyother help.

When they reach the ground, some seeds are taken further from the parent plant by animals or water.

Seeds dispersed by gravity include:


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Passionfruit Hedychium Cerinthe Commelina Canna Salvia

Dispersal of Seeds by Animals

Animals of all sorts and sizes help plants to disperse their seeds. The method they use depends on thetype of seed.

Seeds with attractive fruit or seeds

To attract the animals and birds and encourage them to act as seed carriers,plants often surround their seeds with a brightly-coloured and sweet-tasting pulp.

Raspberry Sea Grape BilberryIn the deserts of North Africa, elephants eat the fruits that have fallen from the trees and deposit theseeds in their droppings several miles away. In South American jungles, monkeys eat figs and other fruit,carrying some away in their stomachs and dropping others onto the ground. In Britain, foxes eatraspberries, squirrels eat nuts, blackbirds eat our strawberries, mice eat grass seeds, and in South Africa,even ants carry seeds into their nests, eat the tasty outer covering and leave the seeds to grow safelyunderground.

As well as eating them, some animals collect the fruits or seeds and bury them to eat later, but forgetabout them and the seeds germinate in their new location. Sometimes, as in the case of Mistletoe, theseeds are covered in a sticky slime which the birds rub off on a new tree. Even humans carry seeds faraway for plants - by taking an apple on a picnic, for example, and throwing the core, with its seeds, intothe bushes.


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Examples of seeds spread by this method include:

Date Monstera Tamarind Lablab Diospiros SunflowerSeeds with clinging hooks or spines

When animals take fruits or seeds for food, they act as willing transporters of the plant's seeds.Sometimes, the plants make use of animals to carry their seeds without giving them any reward.

Entelia Lesser Burdock Sea HollyMany plants produce fruits or individual seeds covered in hooks or spines which attach the seed to theanimals's fur or feathers - or, in the case of humans, to our clothes or bags. The seeds are then carried asufficient distance from the parent plant to give them space to grow. Eventually, the seed may fall off, orbe rubbed off by the animal.

The most well-known plant of this type that we have in the UK is probably Goose Grass or Sticky Weed,which children throw at one another in a game, but in other countries there are larger such hitch-hikerswhich can damage animals when they become lodged between the animal's toes and cause infection andlameness. There's more information about species in North and South America that produce largehooked or prickly seedpodshere.
http://waynesword.palomar.edu/ww0801.htm#Dispersalhttp://waynesword.palomar.edu/ww0801.htm#Dispersal


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Examples of seeds spread by this method include:

Rambutan Bixa Trollius Cynoglossum Orlaya Eryngium

Dispersal of Seeds by Force

Some plants provide their fruits (seedpods) with a mechanism that ejects the seeds from the pod byforce.

Broom Eurphorbia GeraniumThere are several methods plants use to fling their seeds out of the seedpod. All of them rely on the effectof evaporation of water in the seedpod, so this method of seed dispersal usually takes place in the sun. Itis one of the favourite methods used by members of the Papilionaceae (formerly Leguminosae). If youwalk over heathland near Gorse bushes on a hot sunny day, you can hear the explosions of theseedpods as they burst open and send their seeds out. Lupins, too, have a similar system. The side ofthe seedpod facing the sun dries out more quickly than the side in the shade, causing the pod to buckleand pop open. The seedpods then curl up like animal horns, sending the seeds flying.

Geraniums also use the heat of the sun to shoot their seeds from the pods. The seeds are in a ring at thebase of the style, each one with its own cover which is attached to the tip of the style by a thin woodystrip. When the seeds are ripe, the seed covers split apart, and the strip rips up the style but is stoppedsuddenly because the strip remains attached to the style. The seeds are catapulted out at high speed.

Oxalis seeds are covered by an elastic coating. When this dries out, it splits suddenly, shooting theindividual seeds out of the pod.

Euphorbia is another plant whose seedpods crack open with a loud explosion. When the three joints inthe seedpod dry out, they split open suddenly, and the seeds are shot out. Ceanothus berries alsooperate this system when the fleshy outer covering has dried out.


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Plants that disperse their seeds by this method include:

Viola Ceanothus Oxalis Lupin Erodium Lathyrus

Dispersal of Seeds by the Wind

Wind is one of the main agencies of seed dispersal.The way it transports them depends on the type of seed and where it grows.

Seeds that can fly or glide

Hornbeam Sycamore AshSome tall trees produce seeds with stiff wings covering the seed that enable them to fly long distances.The wings are twisted and balanced so that the seed spins around as it is carried along by the wind.

These natural adaptations for using the wind to transport the weight of the seed must be technicallyaccurate, as the wings of modern planes and helicopters are designed in the same way.

These wings usually support one seed each, but may start off as a two-winged pod that later splits in twoto release the seeds. Some seeds have only one wing (e.g. Lime or Ash). This type of attachment is quiteheavy, and this system only works well in a good wind, and from a tall tree.

Sometimes seeds have thin wings as an extension of the seed that enable them to glide in the wind. Theydon't need so much wind as the seeds that fly, but they are not so heavy. The largest of this type of seedis 6" across, from a climber called Alsomitra growing in the tropical forests of Asia.

Some seeds have long, feathery tails which help them to fly, like the tail of a kite.


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Examples of seeds spread by the use of wings and tails are:

Puya Tecoma Embothrium Pandorea Pulsatilla ClematisSeeds that drift in the wind

Seeds that have almost weightless additions that enable them to be carried long distancesby the slightest breeze are familiar all over the world.

Willow Herb Bulrush DandelionThese are more flimsy additions to seeds which help them to be transported long distances by the wind.These additional features are usually various sorts of fluff which are almost weightless but increase thevolume of the seed, so that it can be picked up by the slightest breeze and carried over long distances.

Sometimes, the seed is attached to fine hairs which open out when the seed is shed to form a ball.Thistles produce seeds with this type of fluff, and thistledown is often seen blowing across motorways onits journey to colonise new sites. Many members of the Daisy family provide their seeds with a flat disk offine hairs to produce a parachute to keep the seed aloft. Bulrushes produce many millions of dust-likeseeds, each of which has its own tuft of fluff to give it a bigger area to be caught by the wind.


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Examples of seeds spread by this method are:

Amberboa Thistle Liatris Felicia Fleabane DandelionSeeds that are released from their pod by the wind

Many more plants just need the wind to bend their stalks so that the seeds spill out of the seed pod.

Columbine Poppy Evening PrimroseSome seed pods face downwards, but very many have their opening at the top, and these need the windto bend their stalks enough to allow the seeds to fall out. This often means that the seeds will not falldirectly under the parent plant, because the stalk holding the seedpod is bent at an angle, so the seedsfall a little way from the parent.

Very many popular garden plants and wildflowers, too, scatter their seeds this way, so it must be anefficient method of spreading seeds.


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Some seeds dispersed by this method are:

Agrostemma Nemophila Dianthus Nigella Calceolaria Lilium

Dispersal of Seeds by Water

Some plants make use of water to disperse their seeds.

Brooklime Yellow Water Lily MangroveAlthough seeds of plants that grow in water are obviously spread by water, there are many other ways inwhich water plays a part in dispersing seeds.

Plants which grow beside water often rely on water to transport their seeds for them. They may producelight seeds which float, or there may be fluff that helps buoyancy. Two tall trees, Willow and Silver Birch,are often found in the middle of moorland, far from any other trees, but along the course of a stream.They can colonise such isolated places because they both have very small, light seeds, which can becarried by the wind or by water. Foxgloves and Harebells often grow beside streams. They both have lightseed that floats.

Trees found on tropical beaches often have their seeds carried there by the sea. They have woody,waterproof coverings which enable them to float in the salty water for long periods. Coconuts are well-known travellers, as is the famous Coco-de-mer, found only on the Seychelles, but familiar before itsorigin was known from the giant seeds washed up on other tropical beaches.

Mangroves are another familiar tree of tropical beaches. Their seeds are unusual in that they can begingermination while still on the parent plant, and they drop into the ocean when about a foot long. These'sticks' float upright in the sea, waiting to be flung onto the beach to continue germinating.


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Seeds of some tropical trees can even be carried along by ocean currents to land on shores half a worldaway. Sometimes Sea Beans, the seeds of Entada gigas, are carried from their homes beside rivers inAfrica, Australia or South America, across the ocean to land on European shores.

Seeds spread by this method include:

Yellow Flag Water Mint Foxglove Willow Coconut Sea Bean