Plant dispersal diversity
After days of anticipation a fresh breeze rises through a sunny field. Tiny fluffy parachutes rise, and there's a wave of excitement as dandelion seeds finally break free from their parent plants. With a bit of luck they’ll produce their own intrepid seeds in a few months, perhaps miles away, and in doing so defy a grumpy gardener who should have pulled its roots out properly.
Wind dispersal may be one of the first methods of seed dispersal we think of, but there are many other ways in which plants disperse their propagules. These are structures with the capacity to give rise to a new plant, for example a seed, a spore, or a part of the vegetative body capable of independent growth if detached from the parent (Biology Online, 2015). Types of propagule dispersal can be split into two main categories: autochory, where the plant itself carries out the dispersal of its propagules, and allochory, where the plant exploits different means of propagule transport such as animal vectors.
Autochory
Self-dispersal mechanisms include:
barochory – transport via gravity
blastochory – dispersal via runners
herpochory – transport via active creeping
ballochory – self seeding
(Schulze et al., 2005)
Barochory is a very simple seed dispersal mechanism, whereby plants, such as apple trees, simply distribute seeds by allowing them to drop to the ground. The fruits containing the seeds are often large, heavy and relatively round so they can then roll away. Seeds may later succumb to another form of dispersal, perhaps via water when it rolls into a river or sea, or an animal if it’s eaten whole and excreted somewhere else.
Plants such as grasses can perform vegetative reproduction – a process by which new organisms grow directly from plant stems called runners, rather than developing from seeds or spores (see image below). This is a type of asexual reproduction, which results in individuals with the same genetic code. Runners that lie underground are called rhizomes and those that shoot along the ground are called stolons. This method of reproductive enables plants to produce large quantities of offspring quickly and easily.
Herpochory is a curious dispersal mechanism which involves seeds effectively crawling along the ground. This has probably made some farmers somewhat surprised to find their oat seeds have escaped from their husks and crept along the ground after being left for some time. This movement is achieved by hairs or bristles that twist in different ways as they react to the surrounding level of humidity – in other words they are hygroscopic. The tails of oat seeds untwist when they’re damp and twist again when they’re dry.
On a warm summers day in the country you may here the friendly crackling of a gorse bush (Ulex europaeus). Their seeds suddenly ‘pop’ out of their seedpods which have dried and twisted in the sun and consequently split open. These seeds can be fired a few metres away from the parent, demonstrating a prime example of ballochory.
Allochory
Allochory (dispersal via a vector) includes:
anemochory – transport via wind
hydrochory – transport in water
zoochory – dispersal via animals
hemerochory – dispersal by man
(Schulze et al., 2005)
Anemochory is well demonstrated by the many species of tumbleweeds – the above ground structure of a plant which detaches is dry weather and rolls over ground in windy conditions, thereby spreading its seeds or spores (see the image below). The infamous Salsola tragus is the tumbleweed that’s closely associated with classic Wild West films, yet it’s not native to North America, in fact it’s from Russia. Because of its efficient reproductive strategy it can now be found in nearly all American states.
The coconut, which is a type of fruit called a drupe, not a nut, is a classic sea voyager. It’s hard, woody shell protects the delicate embryo inside from salty water which can damage cells via osmosis. It’s able to float so well because it has a large volume and plenty of air inside thanks to the cavity just under the outer coat. This air-filled cavity is filled with light, stringy fibres called coir. Coconuts can float hundreds and sometimes thousands of kilometres between tropical islands.
Most dog owners will probably have experienced the woes of brushing burrs out of dog fur. These are seeds which have the ideal adaptation of hooks or teeth that latch onto animals and effectively hitch a ride (see the image below). But have you heard of seed dispersal by ants? This specialist form of zoochory is called myrmecochory. Attached to these types of seeds are external appendages (called elaiosomes) which are rich in nutrients and are therefore a scrumptious meal for ants. Seeds are carried back to ants’ nests for consumption of this nutrient-rich treat, but the seeds are left undamaged – they end up with a favourable place to germinate and escape above-ground seed predation.
Being animals ourselves, one could argue that hemerochory is a type of zoochory. With the great distances we travel nowadays, hemerochory plays a huge role in the wide dispersal of some plants, even hitching rides to different countries on our shoes, cars and potentially skin. A result of this large-scale dispersal can unfortunately be the introduction of foreign plants which can become invasive in new environments and damage native species’ populations. And we of course spread millions of seeds each year through agriculture.
In conclusion, there’s an incredible diversity of dispersal methods out there used to distribute plant offspring far and wide, from crawling oats to exploding gorse. These help the next generation of plants to survive and prosper. In general, the further away offspring can get from its parent plant the less likely it will suffer from density-dependant seedling predators and pathogens, and potentially the less competition it has with its parent. The need for plants to disperse their offspring has resulted in truly innovative and fascinating dispersal solutions, demonstrating the creativeness of nature.
Photos sourced from Wikimedia Commons: http://commons.wikimedia.org/wiki/Category:Images
1. Taken April 2012 by Sage Ross: A cloud of dandelion seeds against a dark background, produce by shaking a handful of dandelion clocks above the field of view in my back yard 2. Taken February 2005 by Macleay Grass Man: Chloris gayana stolon, Austral Eden, New South Wales, Australia 3. Taken December 2013 by EriKolaborator: Nicelyous 4. Taken October 2007 by Huw Williams: Burrs in Warren County, Indiana.
Baldwin, Cradock and Joy, (1833), The Westminster Review, p.440
Biology Online, (2015), Propagule, [online], available at: http://www.biology-online.org/dictionary/Propagule Accessed 24 January 2015
Encyclopeadia Britannica, (2015), Seed and fruit: self-dispersal, [online], available at: http://www.britannica.com/EBchecked/topic/532368/seed-and-fruit/75925/Self-dispersal Accessed 21 January 2015
Janick, J., Paull, R., (2008), Encyclopedia of fruits and nuts, CABI, p.107-108
Schulze, S., Beck, E., Müller-Hohenstein, K., (2005), Plant Ecology, Springer Science and Business Media, p. 542-543
