Plant Delivery Systems: Understanding Seed Dispersal
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For plants, successful reproduction is crucial for the survival and continuation of their species, and this starts with effective seed dispersal. While plants are unable to move, they have evolved some ingenious techniques to ensure that their seeds are dispersed as far and wide as possible.
In this article, we will explore the amazing diversity of seed dispersal methods used by plants, from explosive seed pods to sticky burrs, and learn how each strategy has contributed to the evolution and survival of different species.
Types of Seed Dispersal Mechanisms
Not all plants disperse their seeds in the same way. Many of you are probably familiar with animals helping plants to spread their seeds but there are many other ways, including wind, water, and even fire!
Wind (Anemochory)
You’ve probably seen plant seeds blowing about in the wind. Some people even pick up a dandelion when it’s in its white, puffy seed stage and blow the seeds while making a wish. But did you know that when you do that, you’re actually helping the plant reproduce?
Some plants, like the dandelion, rely on the wind to blow their seeds to a new location where they can grow. The milkweed plant has fluffy growth attached to the seed that allows it to travel up to 400 miles (643 km) from its parent plant!
Other types of plants, like the maple, have winged seeds that fall to the ground like a helicopter. While they don’t travel as far, they still might travel the distance of a soccer pitch!
You’ll notice that, with plants that rely on wind dispersal, there’s usually a much greater seed production to ensure as many seeds as possible are spread over a wide area.
Water (Hydrochory)
Some plants, like the water lily, will grow very light seeds that are able to float on the surface of the water, eventually sinking down where they will grow into a new plant. This strategy isn’t the only method of water seed dispersal. It’s used by a variety of plants that live in and around water.
If you think of a tropical coastline, you might imagine swaying palm trees, but how did they get there? Did you know that they may have traveled hundreds or even thousands of miles across the ocean as a seed before being established? That’s because palm seeds are so light that they can travel over the sea to a location far away from their parent plant.
Not only this, but plants that transport their seeds on water create very robust little travelers. Some species of mangrove plants produce seeds that are not only impermeable to salt water but can thrive in the ocean for up to 12 months!
Animals (Endozoochory)
Many plants rely on animals to help spread their seeds but in order for this to happen, the animals need to be attracted to the seeds. In a lot of cases, seeds are encased in fruit which is attractive to the animal as a food source. As it eats the fruit, the seeds will pass through its digestive system intact and when the animal excretes them, they’re in a new location and hopefully ready to germinate.
Some trees produce seeds that are not within a fruit; things like oak trees and horse chestnut trees, for example. If these seeds were to be eaten, they’d be destroyed, but animals like squirrels will cache them and not always remember to go back to them, giving the seeds a chance to grow into an adult tree.
Explosive Release (Ballochory)
If I asked you to imagine an exploding plant, I’m sure you’d probably think I was crazy. But the strange reality is that some plants will literally burst to spread their seeds.
Plants like this usually produce their seeds in a pod; like peas, for example. When the time comes, the pod will burst open, scattering the seeds across the ground. This might not sound like a very effective method as one would assume that the seeds don’t travel very far. But that isn’t the case; the sandbox tree, for example, will explode its pods, and the seeds can travel up to 330 feet (100 meters) from the parent tree.
How these plants explode happens in a variety of ways. Some plants will use the pressure from dried-out fruits to shoot their seeds, while others use a build-up of water, like the squirting cucumber.
Gravity (Barochory)
Perhaps not the most complex method, but certainly one that works as many tree and plant species have survived using it, gravity is another common way of dispersing seeds.
This is something we see in fruit trees such as plum and apple and is the simple act of the fruit growing heavier until gravity causes it to fall from the tree. There is the possibility of the fruit rolling away to a new growing location.
Alternatively, an animal may pick up the fruit and eat it or move it to a new location far away from the parent tree.
Attachment
Earlier, I talked about how animals may help in the dispersal of seeds by eating fruits and moving larger seeds around. However, there are some seeds that don’t even need to be collected as food. For example, the burdock plant has sticky burrs that will attach to an animal’s fur. As the animal moves around, the seeds drop off and hopefully land in a nutrient rich, light location so they can sprout.
There are some plants that use deception to attach their seeds to passing animals by spitting them at a target.
In some cases, this method can be detrimental to animals. For example, foxtail seeds commonly get stuck in their ears or nose and can cause an injury.
You might have even helped a plant disperse its seeds yourself. Have you ever been out in nature only to find that, after brushing past a goosegrass bush, you’re covered in tiny green balls? Those are seeds and the plant is hoping you’ll dispatch them to a new location.
Fire
Many plants live in locations where fires may break out. In this case, they have an advantage over other plants and trees as their seeds are designed just for this type of natural disaster.
While other plants may be killed off during a forest fire, things like the pine tree will survive. Its seeds are encased in a cone that’s held together by resin. Amazingly, this resin will only melt and release the seeds when the temperature gets hot enough to kill all other plant life in the area.
But what’s the benefit of this?
Well, with all other plants and trees out of the way, these seeds have priority access to all the nutrients within the soil, the sunlight, rain, and other elements that help them to thrive.
The problem is that these seeds must be robust and long-lived since they may have to wait years between fires for an opportunity to spread.
What is the Importance of Seed Dispersal?
If plants grow in very close proximity to one another, there would be some serious competition to survive. Plants and trees need water, light, and nutrients to survive, but these resources are limited. By spreading seeds further afield, plants will be able to make the most of available resources without having to compete.
What’s more, in order to ensure the survival of any given species, the more widespread it can be, the better. On top of this, having the ability to spread seeds over greater distances means greater biodiversity in any given area.
These things combined mean that the plant species will be able to thrive and survive.
How do Seeds Germinate?
Isn’t it amazing to think that a single seed, sometimes something very tiny, can grow into a fully-fledged plant or tree? But before that happens, the seed needs to germinate which means that it bursts forth its very first sprout. Before this, the seed is in a state of dormancy.
In order for a seed to germinate, the conditions have to be just right. You’ll notice that there are seeds all over the place in nature; fallen fruits, acorns on the ground, dandelion seeds blowing through the air. But not all of them germinate and that’s because they haven’t experienced the correct conditions.
The first step in bringing a seed out of dormancy is adding moisture. For gardeners, this means watering their seeds but in nature, it means that the seed must be exposed to a source of moisture, such as rainfall. Once it receives this, enzymes are released, causing the seed to grow and eventually, those first shoots will emerge.
However, other factors are at play to make the perfect conditions for seed germination. These include the right temperature, being placed in the soil at the correct depth, and access to oxygen.
Plants with Unique Seed Dispersal Methods
It’s clear by now that plants have some rather innovative methods of dispersing their seeds. Let’s take a look at some of the most unique methods.
1. Squirting Cucumber
Just by looking at its name, it isn’t hard to guess how the squirting cucumber goes about dispersing its seeds. That’s correct; it literally squirts them out but there’s a science behind it.
Inside the plant, which is native to Europe, Africa, and some parts of temperate Asia, moisture builds up and this causes internal pressure. There comes a point when the seed pods are no longer able to withstand the pressure, and the plant squirts out the contents.
The seeds are forced out with such pressure that they can travel up to 20 feet (6 meters) from the parent plant.
2. Coconut Tree
We often refer to coconuts as…well, nuts. But did you know they’re actually a type of seed? And, they’re one of the biggest seeds in the world. But this isn’t the only thing that makes them interesting.
Each tree can produce up to 200 seeds per year and they’re incredibly robust allowing them to float in the ocean for many years until they are washed up and reach a suitable growing location. This is thanks to their exocarp which prevents them from absorbing salt water. What’s more, the drier the seed, the better it floats. While it’s floating, the seed even has its own food supply in the form of flesh called endosperm.
Coconut seeds can travel for many miles, and it’s thought that this is one of the reasons the trees are so abundant all over the world. From one starting location, seeds were spread globally.
3. Common Dandelion
If I asked you to think of a plant that used wind as a method of seed dispersal, I’d bet my bottom dollar that you would say the common dandelion. We’re all used to seeing these seeds flying around. While the seeds typically only fly a few meters from their parent plants, studies have shown that under the right conditions, one seed could travel as far as a 0.6 miles (1 km)!
Dandelion seeds are very small but they’re attached to a cloud-like parachute via a narrow stem and this makes them incredibly aerodynamic.
Amazingly, dandelions seem to know exactly when to release their seeds, and it’s all to do with moisture levels within the plant. When they’re higher, the plant senses that this isn’t a good time to release the seeds, as wind levels are likely lower.
4. Himalayan Balsam
The Himalayan balsam is, unsurprisingly, native to the Himalayan region, but after introduction to most of the Northern Hemisphere, it’s now considered to be an invasive species in many areas.
The seed pods of these flowering plants are not very large, measuring just 0.8 – 1.2 inches (2 – 3 cm). However, if they are disturbed, they go off with a bang, exploding and spreading their seeds as far as 23 feet (7 meters) from the parent plant!
Despite their small size, these plants produce a whopping number of seeds, with a single plant producing as many as 2500. This is one of the reasons that European authorities have now listed this plant as an Invasive Alien Species.
5. Coco de Mer
Native to the Seychelles, the Coco de Mer is a rare tree species and is thought to have the largest seed of any plant. The seeds can weigh as much as 39 lbs (18 kg) and may grow to 1.5 feet (0.5 meters). What’s more, they have a rather suggestive appearance which has given them another reason for their fame.
Sadly, the coco de mer is listed as endangered with only 8000 trees left in the wild. Locals are making efforts to save it but this is a slow-growing species that can take as long as 50 years to sexually mature.
It’s thought that the reason the seeds grow so large is to ensure the survival of the species. Millions of years ago, the islands were connected to the mainland and animals could disperse the seeds. However, after the land shifted, the trees needed to evolve to disperse their own seeds.
6. Hairy Bittercress
Hairy bittercress is a type of weed and it’s one that is very hard to get rid of thanks to how determined it is. It might be small but each plant can disperse an astonishing number of seeds.
This weed uses an explosive seed dispersal technique. Generally speaking, the seeds will fly around 3 feet (0.9 meters) from the plant, so it’s not the longest distance we have seen. However, if these lightweight seeds are caught by a gust of wind, they’ll travel much further.
If you’re ever walking around near hairy bittercress, be on your guard since stepping on the plant can cause the seed pods to explode unexpectedly with lots of popping sounds.
7. Javan Cucumber
In the forests of Indonesia, we find the Javan cucumber, which is remarkable if for no other reason than its massive seed pods. The pods can grow to be the size of a football, and each one can contain hundreds of individual seeds.
The seeds peel away from the tree and can travel hundreds of feet to their growing location thanks to two papery wings. These wings are much larger than the average tree seed and can be up to 5 inches (13 cm) across. There is some speculation that they inspired the design of early aircraft.
As the seeds fall from the tree, they move in huge circles, which can be as wide as 6 feet (1.8 meters). And if they are caught by a breeze, they’ll go even further. Once they land, the wings rot away, and the seeds begin to sprout, forming a skyward-facing vine that is the beginning of a new plant.
8. Chinese Plantain
As you can tell from its name, the Chinese plantain is native to China but also grows in other Asian countries like Japan and Korea. This plant uses attachment for seed dispersal and, when the seeds come into contact with water, they produce their own glue known as mucilage. This causes them to stick to everything; including your shoes!
But despite being a potential nuisance to remove from your hiking boots or clothing, this plant has many other uses. It’s known for its medicinal purposes and is a common ingredient in stews and herbal wines.
9. Sandbox Tree
The sandbox tree is an all-round fascinating tree species. If it isn’t the long spikes along the trunk that ooze toxic sap, it’s the flowers on the ends of spikes on the male trees. What’s more, the sandbox tree is aggressive in its seed dispersal and uses explosive methods.
That’s right, these trees burst open their pods and in some cases, can spread their seeds as far as 328 feet (100 meters)! The pods only explode when they are ripe but they’re said to make an impressive bang upon bursting.
When the pods burst, they’ve been known to cause injuries to nearby humans so if I were you, I’d give these trees a wide berth!
10. Burdock
Do you ever use velcro to fasten your clothing, shoes, bags, etc? If you do then you’re using a design that was inspired by the burdock plant; that’s right, it’s nature’s very own velcro!
These plants have sticky burrs that attach to passing animals and will even stick to human clothing if you get too close. The burrs of the burdock plant have tiny hook-like structures that allow them to attach securely to their host.
Native to Europe, but found in many global locations, the idea is that the burdock plant attaches its seeds which are then spread over a much wider area. The problem is that the burrs can be so sticky that smaller animals like birds and bats have been known to become stuck to the plant, unable to get away.
11. Sycamore
I remember my mom picking up sycamore seeds when I was a kid and showing me how they’d fall to the ground like a helicopter. In fact, I’m pretty sure we used to call them helicopters.
These seeds, which have V-shaped ‘wings’, travel around 150 yards (137 meters) and fall from the tree with the help of gravity before blowing in the wind.
The seeds will land in the soil and become embedded and what’s interesting is that sycamore seeds can germinate in all conditions. Even in winter, when there is little light, it’s possible for these hardy seeds to sprout.
12. Common Milkweed
Milkweed seed pods take a few weeks to develop, but eventually, they’ll burst open to reveal tiny seeds that have fluffy, feathery tendrils growing off them. They look beautiful, but the feathers aren’t there for aesthetic reasons. They provide a viable way for the seeds to be transported in the wind.
Amazingly, these wide fluffy growths can keep the seeds drifting through the wind in fall for up to 400 miles (644 km)! Once the seed lands, rainfall causes it to embed in the soil but it remains dormant until spring when it germinates.
However, where seed reproduction is not successful, the milkweed plant has another tactic; it grows horizontal roots to get a wider spread. These roots can grow up to 10 feet (3 meters) and above the ground it might look as though there are several plants in succession when in reality, they’re all from one parent.
13. Jackfruit
The jackfruit is a type of fig or mulberry tree that produces large fruits that can get as big as 39 inches (100 cm)! They grow on thick branches but are eventually too heavy for the tree, and gravity does its job of bringing them down to earth.
Once on the ground, the fruits, which contain seeds, are broken apart and eaten by various animals, particularly mammals such as monkeys and elephants. These mammals then disperse the seeds for new jackfruit trees to grow. The problem is that jackfruit trees can produce as many as 500 seeds a year and this far outweighs the number of local mammals able to spread the seeds.
Originally, the trees were found in India, but they have now become an invasive species in areas like Brazil.
14. Bur Oak
The bur oak is found in North America and bears the largest acorns found on this continent. It’s a very slow growing tree but can eventually reach around 100 ft (30 meters) in height.
It takes around 35 years before the bur oak will begin to seed, and when it produces those large acorns, it relies on animals to disperse them. It’s good news then that many species see the acorns as an important source of nutrition, including deer, rabbits, and blue jays.
The seeds are quick to germinate and often show no dormant period at all. However, trees may alternate between a sparse and fruitful production of seeds each year.
15. Banksias
A species of Australian plant, the banksias has a number of adaptations which allow it to survive in conditions where other plants may not. For example, very dry soil and areas where nutrients are low.
On top of this, and because of unfavorable conditions, the banksias have adapted to only release their seeds during the event of a bushfire. With only 1000 plants known to exist, the seeds need to germinate quickly, but the heavy rainfall that often comes after the fires ensures this.
Banksias plants are easily able to survive bushfires thanks to their thick bark, meaning that as many as half the population survives each fire.
