Seed Dispersal- on the Wind

Roulette wheel illustrating randomness of seed dispersal by the windUsing the wind as a means of seed dispersal requires plants and trees to produce large quantities of seeds.

Producing large quantities of seeds, dispersed by the wind, increases the probability of a single seed ‘getting lucky’ and landing in a suitable location in which to germinate.

Plants and trees use the wind in different ways to disperse their seeds including gliding, parachuting, ‘autorotating’, fluttering/spinning and tumbling.

We discuss each of these five ways below:

1) Seed dispersal- gliders

‘Gliders’ are seeds that fly.

One of the best examples of gliding seeds can be seen in the Javan Cucumber Vines (Alsomitra macrocarpa) of Malaya. These vines climb trees and suspend their fruits below branches.

Alsomitra macrocarpa treeCopyright;wildborneo.com.my

Javan Cucumber fruits have hard outer skins called ‘gourds’. Each gourd is packed with dozens of winged seeds.

Alsomitra macrocarpa gourd containing seeds

After a gourd ripens its hard outer skin disintegrates and winged seeds fly out the bottom.Juavan cucumber (Alsomitra macrocarpa) ripens and seed glides out

The seeds often fly long distances before coming to rest on the forest floor.

Alsomitra macrocarpa seed held between a person's fingers

Javan Cucumber seeds are thought to have inspired the wing design of some early aircraft including this glider built in Austria in 1906.

glider-etrich-1906-

2) Seed dispersal- parachutes

This method of seed dispersal involves seeds, attached to ‘parachutes’, floating on the wind.

Parachutists  Annual Randy Oler Memorial Operation Toy Drop at Fort Bragg N.C

The ‘parachute’ of the Western Salsify weed (Tragopogon dubius) comprise feathery hairs (called a ‘pappus’) attached to an elongated seed.

Tragopogon dubius - Western Salsify seed and flower

A Western Salsify seed takes to the air in the slightest of breezes and can sail across valleys and mountains.

Tragopogon dubius seed head from Hungary

The dandelion (Taraxacum officinale) is another common weed which disperses on the wind using a ‘parachute’.

Dandelion showing flower and seed headImage credit:Greg Hume

A dandelion seed is attached to a stalk; it is the stalk which is attached to the pappus.

Dandelion seed showing seed stalk and pappus
Wind dispersed seeds do not always have stalks. The seed of the globe artichoke…Cynara scolymus

Cynara cardunculus globe artichoke flower
is attached directly to the ‘pappus’.

Cynara cardunculus seed attached directly to the pappus
The ‘pappi’ (plural of pappus) of this bulrush (Typha latifolia) look like cotton wool …

Typha latifolia seeds being blown off bulrush head

and at maturity are easily blown off the plant!

  • But where are the seeds?

 Boy blowing seeds off a Typha latifolia seed head

If you look closely at a pappus of this bull rush you will see a tiny brown fruit containing a seed attached to a central bristle.

cattail fruitCopyright;Backyardnature.net

The tiny fruit is called an ‘achene’. An ‘achene’ is a small, one-seeded fruit that does not open (unlike the fruit of Javan Cucumber Vines) to release its seed.

typha latifolia diagram showing bristles and achene

Sometimes seeds of the same species vary in size according to where they grow. Some species of Asters (Asteraceae) growing on small islands off the coast of Australia….

Different species of asters

produce large seeds attached to small ‘pappi’. There is no evolutionary advantage in plants which inhabit small islands growing seeds capable of ‘parachuting’ long distances if it means that they fall into the sea.

In contrast flowers of the same species living on mainland Australia produce smaller seeds with larger pappi which means they can disperse long distances.

  • Are these Asteraceae seeds and pappi all the same size or are there small variations which could affect how far they disperse?

Vittadinia muelleri (Asteraceae) seeds germinating Royal Tasmanian Botanical Society

3) Seed dispersal- ‘autorotation’

This method of seed dispersal involves winged seeds rotating like blades of a helicopter. The seeds of the Sugar Maple tree are one such example.

MD 500E light utility helicopter

Fruits grow among the leaves of the Sugar Maple tree (Acer saccharum).

seeds grow under leaves of Acer saccharum

Two fruits are joined together in a pair to form a horseshoe shape. Both fruits hang off the tree on a single stalk. A single seed forms inside each fruit.

Acer saccarum seeds

This type of fruit is called a ‘samara’. A ‘samara’ is a fruit with at least one thin, flattened and papery wing.

Ripening of samaras occurs at the same time as winds start to blow stronger in autumn. The pair of fruits splits apart in autumnal winds and single samaras drop off the trees.

maple seed
The samaras spin round and round as they descend. This method of descent is called ‘autorotation’. Autorotation’ slows down the rate of descent; a slow rate of descent allows winds to disperse the samaras further away from the parent trees.

Maple seed swirling round and round falling to ground in a time lapsed photo
The dispersal of seeds coincides with the stunning autumnal colors of maple trees.

colors on trees  in fall Natchez Trace Parkway Tennessee, Alabama,

After they fall to the ground maple samaras must be exposed to 90 days of temperatures below -18°C to break down their dry skin; once the dry skin has broken down the seeds are exposed and are then ready to germinate.

Germination does not take place until the following spring when the soil has warmed up and the danger of frost has passed.

maple-leaf-seed-in-snowwhtmtnspirit.wordpress.com

The White Ash (Fraxinus americana) is another species of tree with single winged fruits that ‘autorotate’.

Fraxinus americana seeds hanging underneath a white ash tree

Some autorotating seeds have evolved two wings; this image shows the unripened fruits of the Diptocarpus obtusifolius tree from South East Asia.

Diptocarpus obtusifolius winged fruit from south east asia

The autorotating fruits of the ‘Burmese Lacquer’ tree (Melanorrhoea usitata) from Myanmar each have five helicopter blades!

Gluta (Melanorrhoea) usitata

4) Seed dispersal-Flutters and spinners

Another method that plants use to disperse their seeds is by ‘fluttering’ and ‘spinning’.

These are the fruits of the Tree of Heaven (Ailanthus altissima) from China and known in Chinese as ‘chouchun’ 臭椿  (‘foul smelling tree’)

Ailanthus-altissima in Spain

The ‘samaras’ of the Tree of Heaven have single seeds located in the middle papery wings that flutter as they drop off the tree. The samaras are twisted at their tips making them spin as well as flutter.

Ailanthus altissima seeds

The hopseed bush (Dodonea viscosa) is a fluttering and spinning samara with a circular wing surrounding the seed.

 hoptree fruit Ptelea trifoliata with two wings hanging on tree

The seeds of Yellow Bell trees (Tecoma stans) have two delicate wings; these seeds also disperse by fluttering and spinning.

Tecoma stans seed

Towards the left of this image below you can see long green pods containing developing seeds of the Yellow Bell.

Tecoma stans yellow trumpet bush flowers, leaves and seeds

When the pods ripen and turn brown they spit open; the winged seeds then flutter and spin to the ground.

Tecoma stans pods ripening

Four winged fruits about one centimeter wide…

salt bush Atriplex canescens four winged seeds .
…flutter and spin off Saltbushes (Atriplex canescens) This image shows one such bush laden with fruit ready to disperse.

Atriplex canescens saltbush

Other flutterers and spinners include the enormous fruits of the Cuipo tree (Cavanillesia platanifolia). Cuipo trees grow in the tropical rain forests of Panama…

cuipo or quipo tree from Panama

and produce massive star shaped fruits 15 centimeters in width! At the end of the growing season the winged fruits flutter and spin through the air covering the ground beneath the tree’s huge canopy.

Ulmaceae seeds

5) Seed Dispersal-Tumbling

A ‘tumbleweed’ is the name given to several unrelated species of plant that disperse their seeds by ‘tumbling’.

A species of tumble weed called the Russian Thistle (Salsola tragus) is very common in the United States and Australia. The ‘tumbleweed’ structure of the Russian Thistle includes everything you see above ground including the leaves, branches and twigs. (but excluding the roots) The branches are curved in shape giving the plant a spherical appearance.

Salsola tragus

In summer Russian Thistles grow white and pink flowers. Each flower is supported by 3 modified leaves called ‘bracts’. Each bract has a spike at the end of its tip. A single fruit, hidden from view in this image, grows between the flowers and bracts.

Russian Thistle flowers and sepalsForest & Kim Starr, U.S. Geological Survey

After the flowers have died and withered away the fruits are exposed; in this image each fruit can be seen supported by three bracts.

Salsola tragus showing dried fruits

At maturity Russian Thistle plants turn brown and break off from their stems at ground level above the roots. They now become ‘tumbleweeds’ rolling across roads, fields and valleys. Their ‘tumbling’ motion is assisted by their spiky bracts which are incredibly effective at gripping to different surfaces.

Russian Thistle tumbling across a road

Russian Thistles grow in arid climates in which there are few trees or other obstacles to stop them tumbling. This image shows a group of ‘tumbleweeds’ coming to rest alongside a (hidden) fence at the side of a dirt road.

Russian thistle (Salsola tragus) banked up at side of a dirt track

As Russian Thistles tumble they disperse thousands of fruits; each fruit contains a single seed. The seeds are poisonous and have a bitter taste discouraging animals from eating them.

Fruits of Russian thistle on the ground
Seeds of Russian Thistles are much smaller than their fruits. You can see the small seeds in this image surrounded by the much larger fruits. A single Russian Thistle bush can produce anything up to 50 000 seeds!

Russian Thistle fruits and seeds Howard F. Schwartz, Colorado State University

  • Why are tumbleweeds liked so much in Hollywood movies?

Science fair projects

  • Research helicopters, flutters and spinners. Which of these seeds are flutterers/ spinners and which are helicopters? You can get more information about flutters and spinners from Wayne’s World

seed dispersal wind examples

 a-maple; b-sycamore; c-ime; d-hornbeam; e-elm; f -birch;  g-pine; h-fir; i-ash.

  • Investigate the pappi of different plant species. These drawing below, from the Journal of Popular Science dated 1881, show different pappi and their seeds.

pappus of various plants

a- willow herb Epilobium; b-two forms of seed of Thrincia hirta; c-Tamarix; d-willow Salix; e-cotton grass Eriophorum; f-bulrush Typha.

  • Make your own maple seed helicopters at Ornithopter.org and watch them fly!
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