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:
‘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.
Javan Cucumber fruits have hard outer skins called ‘gourds’. Each gourd is packed with dozens of winged seeds.
After a gourd ripens its hard outer skin disintegrates and winged seeds fly out the bottom.
The seeds often fly long distances before coming to rest on the forest floor.
Javan Cucumber seeds are thought to have inspired the wing design of some early aircraft including this glider built in Austria in 1906.
This method of seed dispersal involves seeds, attached to ‘parachutes’, floating on the wind.
The ‘parachute’ of the Western Salsify weed (Tragopogon dubius) comprise feathery hairs (called a ‘pappus’) attached to an elongated seed.
A Western Salsify seed takes to the air in the slightest of breezes and can sail across valleys and mountains.
The dandelion (Taraxacum officinale) is another common weed which disperses on the wind using a ‘parachute’.
A dandelion seed is attached to a stalk; it is the stalk which is attached to the pappus.
and at maturity are easily blown off the plant!
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.
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.
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….
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.
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.
Fruits grow among the leaves of the Sugar Maple tree (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.
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.
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.
The dispersal of seeds coincides with the stunning autumnal colors of maple trees.
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.
The White Ash (Fraxinus americana) is another species of tree with single winged fruits that ‘autorotate’.
Some autorotating seeds have evolved two wings; this image shows the unripened fruits of the Diptocarpus obtusifolius tree from South East Asia.
The autorotating fruits of the ‘Burmese Lacquer’ tree (Melanorrhoea usitata) from Myanmar each have five helicopter blades!
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’)
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.
The hopseed bush (Dodonea viscosa) is a fluttering and spinning samara with a circular wing surrounding the seed.
The seeds of Yellow Bell trees (Tecoma stans) have two delicate wings; these seeds also disperse by fluttering and spinning.
Towards the left of this image below you can see long green pods containing developing seeds of the Yellow Bell.
When the pods ripen and turn brown they spit open; the winged seeds then flutter and spin to the ground.
Four winged fruits about one centimeter wide…
Other flutterers and spinners include the enormous fruits of the Cuipo tree (Cavanillesia platanifolia). Cuipo trees grow in the tropical rain forests of 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.
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.
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.
After the flowers have died and withered away the fruits are exposed; in this image each fruit can be seen supported by three bracts.
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 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.
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.
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!
a-maple; b-sycamore; c-ime; d-hornbeam; e-elm; f -birch; g-pine; h-fir; i-ash.
a- willow herb Epilobium; b-two forms of seed of Thrincia hirta; c-Tamarix; d-willow Salix; e-cotton grass Eriophorum; f-bulrush Typha.