Seed dispersal-explosions

Using ballistic (or explosive) means to disperse seeds helps plant species compete effectively for space in crowded habitats. This ballistic means of seed dispersal (ballistichory) is hand grenades are similar to seeds that explode ballisticallya type of autochory.

Autochory means that seeds are dispersed by means of mechanisms originating within the plant instead of making use of external mechanisms to facilitate seed dispersal. External mechanisms that facilitate seed dispersal include wind (anemochory), water (hydrochory), gravity (barochory) and animals (zoochory).

There are two basic mechanisms by which plants “go ballistic” and launch their seeds out of their seedpods. These two mechanisms are often referred to as either “passive” or “active” forms of ballistichory. Both these mechanisms are discussed below, illustrated with a few select examples:

Passive ballistic dispersal

Ballistic seed dispersal of the “passive” variety is characterised by hydroscopic (water) movement out of the cells of seed pods.

Hydroscopic movement out of plant cell is a characteristic of ballistic plant dispersal

Hydroscopic movement takes place in plants after the tissue in the cells of seed pods or seed capsules dessicate (dry out) and shrink at maturity or during periods of hot, dry weather.

cells of legume pericarp shown as a cross section

As cells shrink following a loss of moisture content, contraction forces build up within the walls of the seed pod. These contraction forces can take the form of bending, twisting, coiling or squeezing. The direction and strength of this contraction force depends on the shape and architecture of the seed pods.

Take the common pea plant.(Pisum sativum)

As the seed pod ripens its gradually lose moisture, leading to a build up of tension in the pod walls.

Pericarp of pea pod drying outwhiteonricecouple.com/recipes/savoring-moment-life-sugar-snaps/

This tension takes the form of a build up of opposing forces along lines of weakness called “suture lines”.

suture line a pea pod highlighted

When the opposing forces become too great, explosive dehiscence takes place during which the pea pod splits open explosively to release the seeds.

Another notable example of explosive dehiscence following hydroscopic (water) movement out of cells is the explosive dehiscence of the Sandbox tree (Hura crepitans). The Sandbox tree is also known as the “dynamite tree” on account of the loud cracking sound its capsules make when explosive dehiscence is taking place.

When an immature sandbox fruit dries out the loss of water means that it can lose up to 64% of its immature weight.

Mature and immature fruit of Hura crepitans aka dynamite tree

The fibres in the dried fruit capsule walls  the pull against each other to create tension and when the tension is great enough the capsule will split open;  the walls of the fruit capsule (the carpels) then separate explosively, flinging the seeds out with great force. A dried capsule can fling seeds 100 meters (300 ft)  from the parent tree at a velocity of 70 meters per second!

Hura crepitans aka sandbox tree fruit matures and seeds launched as carpels split open

The Erodium cicutarium, commonly known as redstem filaree or redstem storksbill is a small, flowering plant native to the Mediterranean basin and introduced to North America in the eighteenth century.

Erodium-cicutarium-flowers and fruits

The fruits of Erodium cicutarium produce long, spiky, awns (bristles). These awns are naturally coil shaped but are held straight by the calyx. 

Erodium cicutarium fruits and fused awns

As the fruits mature there is a movement of water out of the awns. Tension and stress builds up. The naturally coil-shaped awns, previously held straight in the calyx of the fruit, revert to their natural helical shape. There is a release of tension as individual awns to separate….

Erodium cicutarium awn with seeds breaks free from fruit

….and are explosively launched some distance away from the parent plant.

explosive dispersal of Hura crepitans

Once on the ground the awns unwind and straighten when wet, or rewind back to their natural coiled shape when dry. The resulting motor action, both coiling and unwinding, is sufficient to allow the seeds….

motor action of Erodium cicutarium seeds

….to drill into the ground in a process of self burial.

Erodium cicutarium corkscrew action buries seed into ground

Whereas Pisum sativum,  Hura crepitans and Erodium cicutarium all require water movement out of seed pods/capsules for explosive dehiscence to take place, the seeds of Cardamine hirsuta can explosively dehisce even when the pod remains partially hydrated.

Hairy bittercress (Cardamine hirsuta)  is an edible herb common in many moist habitats around the world.

cardamine hirsuta hairy bittercress, common bittercress, snapweed, shotweed, flowers and seed pods

Cardamine hirsuta possesses a seed pod which stores elastic energy as it matures, even when the pod walls remain partially hydrated

Seed pod of Cardamine hirsuta

When explosive dehiscence in Cardamine hirsuta takes place, the walls of the seed pod explosively coil upwards along lines of weakness (suture lines).

explosive dispersal of seeds of Cardamine hirsuta

As explosive dehiscence is taking place the cells in the outer layer of the pod wall remain hydrated.

Cardamine hirsuta pod wall remains hydrated

The hydrated outer layer of the pod wall creates downwards pressure on the inside layer of the pod wall,  generating the optimum amount of tension and pressure for explosive dehiscence to take place.

In fact a C. hirsuta seed pod wall ready to explosively dehisce along lines of weakness possesses properties similar to those of a slap bracelet.

Slap bracelet

Another flower that uses explosive coiling to disperse its seeds is Geranium sanguineum,  a flowering plant native to Europe and temperate Asia.

Geranium sanguineum uses explosive seed dispersal

This image shows the dried out seed capsule containing the seeds…

Geranium sanguineum fruit before explosive dehiscence

…..while this image shows the fruit after explosive dehiscence.

Geranium sanguineum fruit following explosive seed release

A diagram of the structure of the dried out fruit capsule is summarized below.

Fruit structure of a geranium following explosive dehiscence

Whereas C. hirsuta and G. sanguineum both eject their seeds through a process of explosive coiling of their fruits,  Asystasia gangetica (Chinese violet) widespread in the tropics, ejects its seeds using a different mechanism.

Asystasia gangetica Chinese violet flower

Following a process of dessication and a build up of tension, the two segments of the seed capsule spring apart  along lines of weakness…..

Unripe friut of Asystasia gangetica showing suture lines

… and the seeds, resting on hooks called ‘jaculators’, are flung out of the pod.

 Asystasia gangetica (or Chinese violet) jaculators and seeds

The genus Viola, like this Canada violet (Viola canadensis) has yet a different mechanism for explosively dispersing its seeds.

 Canada violet (Viola canadensis) showing developing seed capsule

As the seed pod dries out the three valves (carpels) which comprise the fruit capsule open out….

Viola chamissoniana showing fruit capsule opening out to reveal seeds

….revealing the seeds nesting inside.

The process of dessication creates pressure along the valve walls, squeezing the seeds until they jump out.

Viola chamissoniana seeds ejected from three valves of seed capsule

Active Ballistic Dispersal

Whereas ‘passive’ ballistic seed dispersal occurs following a decrease in the amount of water present in the cells of seed capsules….

Hydroscopic movement out of a plant cell

…..’active’ ballistic seed dispersal takes place following an increase in the amount of water in plant cells.

turgor in which vacuole of plant cell is filled with water
Take the example of the squirting cucumber (Ecballium elaterium), a herb that grows abundantly in the Mediterranean region.

Ecballium elaterium squirting cucumber and flowers

Turgor pressure (the pressure and increased rigidity of cell walls created by the absorption of fluids into the cells in the fruits) builds up. As the fruit becomes laden with the weight of carrying additional liquid, it turns downwards against the fruit stem. At the same time the attachment point where the stem joins the fruit (the peduncle) becomes weaker.

Ecballium elaterium squirting cucumber is an example of explosive seed dispersal

The build up of pressure inside the fruit becomes so great that the stem is forced out of the fruit at the peduncle and the seeds, together with sticky liquid water  are ejected with considerable force.

Ecballium elaterium squirting seeds

This image shows the seeds lying on the ground after explosive dispersal out of the fruit.

dehisced-fruit-of-squirting-cucumber-with-black-seeds-on-sand

Arceuthobium, commonly known as dwarf mistletoe, is another genus of plant which explosively ejects its seeds using ‘active’ means.

This species, A. cyanocarpum,  has a male and a female plant, growing on the same tree branch.

Dwarf Mistletoe Arceuthobium cyanocarpum showing male and female plantshttp://parasiticplants.siu.edu/Viscaceae/cycle.html

As the fruit matures and becomes heavy and swollen with all the liquid contained inside, the stem holding the fruit (the ‘pedicel’) begins to droop…..

Fruits of Artceuthobium attached to pedicels

…..as can be seen clearly in this image of dwarf mistletoe species A. durangense.

Mature fruit of dwarf mistle toe Arceuthobium durangensehttp://parasiticplants.siu.edu/Viscaceae/cycle.html

Eventually, the pressure and weight of the liquid in the fruit forcibly detaches the fruit from the pedicel; the seed is then propelled out of the fruit at a velocity of up to 27 meters per second!

After breaking free, fruit of Arceuthobium discharges seed and liquid content of fruit

Although most of the dispersed seeds fall within a few meters of the discarded fruit, some travel as far as 16 meters.

High speed image of Arceuthobium dwarf mistletoe seed being propelled by liquid out of fruit

When the seed is ejected it is covered in a layer of ‘viscin’, a clear sticky substance.

Arceuthobium vaginatum dwarf mistletoe seeds covered in viscin

In the below image the gluey viscin has enabled a dwarf mistletoe seed to stick to a needle of a lodgepole pine tree.

Seed from species of Arceuthobium dwarf mistletoe sticking to needle of lodgepole pine tree

After the seed has been moistened by rain or dew, the sticky coat on the seed is either washed off or imbibed (absorbed) into the seed at which point it becomes very slippery.

Layer of viscin of A. vaginatum dwarf mistletoe washed off

The imbibed seed slides to the base of the needle ‘fascicle’ where germination can begin.

 Afascicle is a bundle of pine leaves growing together

This diagram summarizes the stages by which the parasitic dwarf mistletoe germinates, attaches itself to and grows on a Lodgepole pine tree.

Stages through which a dwarf mistletoe seed disperse, germinates and attaches itself to the bark of a lodgepole pine treeapsnet.org

Further Reading

Ballistic seed dispersal of jewelweed at Abundantnature.com

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