Dispersal of Seeds by Animals

Animals of all sorts and sizes help plants to disperse their seeds. The method they use depends on the type 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 Bilberry

In the deserts of North Africa, elephants eat the fruits that have fallen from the trees and deposit the seeds 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 eat raspberries, 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 safely underground.

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

Examples of seeds spread by this method include:

Date Monstera Tamarind Lablab Diospiros Sunflower

Seeds 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 Holly

Many plants produce fruits or individual seeds covered in hooks or spines which attach the seed to the animals's fur or feathers - or, in the case of humans, to our clothes or bags. The seeds are then carried a sufficient distance from the parent plant to give them space to grow. Eventually, the seed may fall off, or be 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-hikers which can damage animals when they become lodged between the animal's toes and cause infection and lameness. There's more information about species in North and South America that produce large hooked or prickly seedpods here.

Examples of seeds spread by this method include:

Rambutan Bixa Trollius Cynoglossum Orlaya Eryngium

Dispersal of Seeds by Water

Some plants make use of water to disperse their seeds.

Brooklime Yellow Water Lily Mangrove

Although seeds of plants that grow in water are obviously spread by water, there are many other ways in which water plays a part in dispersing seeds.

Plants which grow beside water often rely on water to transport their seeds for them. They may produce light 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 be carried by the wind or by water. Foxgloves and Harebells often grow beside streams. They both have light seed 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 its origin 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 begin germination 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.

Seeds of some tropical trees can even be carried along by ocean currents to land on shores half a world away. Sometimes Sea Beans, the seeds of Entada gigas, are carried from their homes beside rivers in Africa, 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

Water Dispersal

Fruits which float such as those of the water lily and the coconut palm are carried by water.

Coconuts can travel for thousands of kilometres across seas and oceans.

The original coconut palms on South Sea islands grew from fruits which were carried there from the mainland by ocean

currents.

Mangroves in the swamp regions of countries such as Thailand are another example.

Blowing In The Wind

Seeds & Fruits Dispersed By Wind

Like an endless army of parachutists released from an airplane, seeds and fruits travel the wind currents and gentle breezes of the earth, possibly colonizing a distant mountain slope or fertile valley. Literally hundreds of species in many plant families have adopted this remarkable method of dispersal, including a variety of ubiquitous plants that we recognize as "weeds." The answer to why some weedy composites (such as the European dandelion) have worldwide distributions is truly "blowing in the wind" (to quote from the Peter, Paul and Mary song). Some of the ingenious adaptations for this method of wind dispersal include seeds that resemble parachutes, helicopters and gliders. In fact, one species (see opening photo) reportedly inspired the design of some early aircraft. An astronomer friend of Mr. Wolffia once observed a strange formation of flying objects through his telescope. He was focusing on a squadron of tiny parachute seeds high above his house. And the entire plant body of wolffia (the world's smallest flowering plant) may be transported by powerful cyclonic storms. In the southeastern United States there are records of wolffia plant bodies less than one millimeter long being carried by a tornado, and they have even been reported in the water of melted hailstones.

An Introduction To The Botany of Seeds

Seeds provide the vital genetic link and dispersal agent between successive

generations of plants. Angiosperm seeds are produced and packaged in botanical structures called fruits which develop from the "female" pistils of flowers. Immature seeds (called ovules) each contain a minute, single-celled egg enclosed within a 7-celled embryo sac. The haploid (1n) egg is fertilized by a haploid (1n) sperm resulting in a diploid (2n) zygote that divides by mitosis into a minute, multicellular embryo within the developing seed. A second sperm unites with 2 haploid polar nuclei inside a binucleate cell called the endosperm mother cell which divides into a mass of nutritive tissue inside the seed. In most seeds the embryo is embedded in this endosperm tissue which provides sustenance to the embryo during germination. In exalbuminous seeds (found in many plants such as the legumes), the endosperm tissue is already absorbed by the time you examine a mature seed within the pod, and the 2

white fleshy halves in the seed are really the cotyledons (components of the embryo). The 2 sperm involved in the double fertilization process originated within the pollen tube that penetrated the embryo sac. The pollen grain (and pollen tube) come from the "male" organs (called anthers) on the same plant or different parental plants in a remarkable process known as pollination. Pollination is also accomplished by the wind (or water), and it may also involve insects in some of nature's most fascinating relationships between a plant and an animal. This is especially true of the amazing fig trees and their symbiotic wasps.

One of the important functions of seeds and fruits is dispersal; a mechanism

to establish the embryo-bearing seeds in a suitable place away from their parental plants. There are 3 main mechanisms for seed and fruit dispersal: (1) Hitchhiking on animals, (2) Drifting in ocean or fresh water, and (3) Floating in the wind. This article concerns one of the most remarkable of all seed dispersal methods, riding the wind and air currents of the world.

Types Of Wind Dispersal:1. Gliders 2. Parachutes 3. Helicopters (Whirlybirds)     4. Flutterer/Spinners 5. Cottony Seeds & Fruits 6. UbiquitousTumbleweed

7. Miscellaneous

Wind-dispersed seeds & fruits in different plant families:

Helicopters: A. Box Elder (Acer negundo, Aceraceae); C. Big-Leaf Maple (Acer macrophyllum, Aceraceae); E: Evergreen Ash (Fraxinus uhdei, Oleaceae); F. Tipu Tree (Tipuana tipu, Fabaceae).

Flutterer/Spinners: B. Empress Tree (Paulownia tomentosa, Scrophulariaceae); D. Tree Of Heaven (Ailanthus altissima, Simaroubaceae); G. Jacaranda (Jacaranda mimosifolia, Bignoniaceae).

Note: The tree of heaven seed (D) actually spins along its longitudinal axis like a rolling pin.

1. Gliders

The remarkable winged seed of the tropical Asian climbing gourd Alsomitra macrocarpa. The entire seed has a wingspan of 5 inches (13 cm) and is capable of gliding through the air of the rain forest in wide circles. This seed reportedly inspired the design of early aircraft and gliders.

Seed courtesy of The Cucurbit Network P.O. Box 560483, Miami, Florida 33256, USA

Gliders include seeds with 2 lateral wings that resemble the wings of an

airplane. They become airborne when released from their fruit and sail through the air like a true glider. One of the best examples of this method is Alsomitra macrocarpa, a tropical vine in the Gourd Family (Cucurbitaceae) native to the Sunda Islands of the Malay Archipelago. Football-sized gourds hang from the vine high in the forest canopy, each packed with hundreds of winged seeds. The seeds have two papery, membranous wings, with combined wingspans of up to 5 inches (13 cm). They reportedly inspired the wing design of some early aircraft, gliders and kites. Although the seeds vary in shape, some of the most symmetrical ones superficially resemble the shape of the "flying wing" aircraft or a modern Stealth Bomber. According to Peter Loewer (Seeds: The Definitive Guide to Growing, History, and Lore, 1995), the aerodynamic seeds spiral downward in 20 foot (6 meter) circles, although a gust of wind would probably carry them much farther away.

2. Parachutes

An individual parachute of western salsify (Tragopogon dubius) showing an umbrella-like, plumose crown of hairs (pappus) above a slender one-seeded fruit (called an achene). These fragile units can become airborne with the slightest gust of wind, and can literally sail across valleys and over mountain slopes.

Western salsify or goatsbeard (Tragopogon dubius) showing dense, puff-like cluster of numerous parachute seeds (one-seeded achenes). Each achene has an umbrella-like crown of plumose hairs and may literally be carried into the atmosphere by strong ascending air currents.

A population explosion of western salsify(Tragopogon dubius) near Mono Lake, on the east side of the Sierra Nevada of Central California. This ubiquitous species is actually native to Europe and Asia.

Parachutes include seeds or achenes (one-seeded fruits) with an elevated,

umbrella-like crown of intricately-branched hairs at the top, often produced in globose heads or puff-like clusters. The slightest gust of wind catches the elaborate crown of plumose hairs, raising and propelling the seed into the air like a parachute. This is the classic mechanism of dispersal for the Eurasian dandelion (Taraxacum officinale) and includes numerous weedy and native members of the Sunflower Family (Asteraceae). A giant Eurasian version of the dandelion called salsify or goat's beard (Tragopogon dubius), is one of the most successful wind-travelers in North America. Its seeds have literally blown across mountain ranges, colonizing vast fields of open land in the western United States. Three weedy species of salsify (T. dubius, T. pratensis and T. porrifolius) have been introduced into the western United States, 2 with yellow dandelion-type flowers and one with purple flowers. The latter, purple-flowered species (T. porrifolius) has a large, edible tap root with a flavor resembling oysters, hence the name "oyster plant."

Inflorescence and mature, seed-bearing head of the Eurasian dandelion (Taraxacum officinale). The slightest gust of wind catches the elaborate crown of plumose hairs, raising and propelling each seed-bearing achene into the air like a parachute. This successful weed thrives in a wide range of climates and has become naturalized throughout North America.

In some parachutes, the crown of silky hairs arises directly from the top of the

seed (not on an umbrella-like stalk). Again, the Sunflower Family (world's largest plant family with about 24,000 described species) contains many weedy representatives with this type of parachute seed. One of the most troublesome weeds of farm land in the western United States is wild or thistle artichoke (Cynara cardunculus). The large seed head of this weedy composite releases hundreds of parachute seeds which fly through the air and invade vast areas of grazing land with spiny, perennial bushes that literally take over. The large leaf stalks (resembling giant celery stalks) are edible and are sold under the name of "cardoon." Populations of wild artichoke often contain so much variation between spiny and non-spiny plants, that some experts believe that they belong to one variable species. In fact, some botanists believe that the cultivated artichoke (C. scolymus) may be a cultivated variety of the wild C. cardunculus. Incidentally, the delicious artichoke is really a cooked flower

head in which the outer bracts (phyllaries) and central basal portion (receptacle) are dipped in butter and eaten.

Brown puffs (Stebbinoseris heterocarpa), formerly Microseris heterocarpa, an interesting member of the sunflower family. A closely related species is called silver puffs (Uropappus lindleyi syn. Microseris lindleyi). In Stebbinoseris the pappus paleae are bifid at the apex. Unlike the weedy dandelions (Taraxacum) and salsify (Tragopogon), this is a native species in California.

ExplosionsSome plants have pods that explode when ripe and shoot out

the seeds.Lupins, gorse and broom scatter their seeds in this way.

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.

 

Fire

To survive fire some plants have adaptive traits that allow them to reproduce or regenerate.

An adaptive trait is a behaviour, physical feature or some other characteristic that helps a plant or animal survive and make

the most of its habitat.

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 intensity of the fire ( it is important the fire reaches the right temperature) is crucial to the seeds 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 sensitive to high temperatures. These cones will not open to release

their seeds until a critical temperature is reached.

Another type of pine called Lodgepole pine ( this is a western U.S. variety of tree) is both serotinus and free opening. This

means that when this pine grows in an area where there are frequent fires, the cones are serotinus, but when it grows in an area where fires are less frequent, the cones open and release

the seeds without the need for fire.

Plants in the Australian bush are very reliant on fire. Although some plants are killed off when a fire becomes extremely fierce and hot ( high intensity), this heat triggers the seed dispersal

mechanism.Some plants may not come into flower for many years and if the fires are too frequent, the plant is destroyed before it has

had the chance to produce seeds.In some cases if fires are frequent and do not reach the correct

temperature, the species will be killed and the seeds not dispersed ( the right heat is crucial to the seeds dispersal

mechanism).

An example is a plant called Old Man Baksia. This plant 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. A

low intensity fire will kill it without triggering germination. If a fire is high intensity but frequent, the plant dies without seed

dispersal.

Some species store their seeds in the soil until a fire reaches the right heat for germination to be triggered. Again if the fires are low intensity the parent plant may die but germination not triggered. Fires are often deliberately lit in the bush to trigger

germination from soil seed banks and to kill weeds.Unfortunately, sometimes these fires get out of control and

cause major damage.

Wind Dispersal

Some seeds are carried to a new place by the wind. These seeds are very light.

The seeds of the orchid are almost as fine as dust.Many have hairy growths which act like little parachutes and

carry the seeds far away from the parent plant.The seeds of the dandelion are carried by the wind.

The seeds of the thistle are also carried away by the wind.Both the dandelion and the thistle have hairy growths at the top of the seed to carry the seeds far away from the parent

plant.These seeds are also very light in weight.

Have you ever heard the saying"As light as a thistle down "?Well this is where it comes from.

The fruits of the sycamore have winged seeds and these 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.

Animal DispersalSome plants have juicy fruit that animals like to eat.

The animal eats the fruit but only the juicy part is digested.The stones and pips pass through the animal's digestive

system and are excreted to form new plants. This can be far away from the parent plant.

Blackberry, cherry and apple 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 - mistletoe is a parasitic plant.

Squirrels collect nuts like acorns and bury them for winter food, but they often forget where they have buried them and 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.