With March, the month of sowing, fast approaching, it can be interesting to study one of nature’s most fascinating phenomena: the dispersal of seeds in nature. After all, a plant doesn’t produce seeds simply for the sake of doing so: it’s the plant’s main way of not only reproducing, but also moving. If all the seeds fall at the foot of the plant, they risk being too far away from the mother plant to germinate. We need to find a way to send them further away… and plants have developed many techniques to do just that.
Wind to the Rescue
Many plants rely on the wind to disperse their seeds. Poppy seeds, for example, remain in their salt-shaker-shaped capsule until a windy day “shakes the cage” and blows them away. Seeds can also be equipped with wings to facilitate transport. Who hasn’t heard of the helicopter effect of maple samaras, which spin around and are carried far from the mother tree? Even more effective, the “wings” can become feathers, making the seed almost as light as air and ensuring even greater distance travel. The dandelion, with its “parachute”, is an expert in this field.
Other plants opt for seeds so small that they are easily carried away by the wind, even without wings. Orchids, for example, have seeds so fine that they are as light as the breeze and can travel several kilometers. Fern spores, although technically not seeds, are even lighter and travel even further. Ferns have already been found germinating on the freshly cooled lava of a new volcanic island more than 3,000 kilometers from the nearest fern population!
Water as a Dispersal Factor
For aquatic or riparian plants, water is often the dispersal factor. Their seeds have the ability to float, sometimes for months, until a wave or tide deposits them in a suitable location. This is how the coconut palm has come to tour the tropical world, because its nuts float so well. And note that the coconut palm doesn’t grow straight up, but leans over the sea, ensuring that its seeds fall into the water when they fall from the tree. Incidentally, this tropical fruit also grows inland… but only when planted by man, as without water, its large seeds are unable to move, falling foolishly to the foot of the mother plant where, lacking sunlight, they rot.
An Explosive Effect
Some plants have a rather surprising technique for dispersal: they are explosive capsules. When the seeds are ripe, the capsule suddenly twists or bursts, launching the seeds a good distance. Impatiens, whose capsules explode when touched, much to the delight of children, is in this category.
Count on Animals
Many plants have learned to rely on other, motile beings for their dispersal: animals (mammals, birds, reptiles, etc.). Most of these plants have edible seeds or fruit: the animal collects their seeds and, with a bit of luck, goes elsewhere to eat them, thus ensuring their dispersal. But don’t let the animal digest the whole seed, or the plant will be no further ahead. Often, then, the fruit is edible, but the seeds are not, or only slightly. So the animal eats the fruit and throws the seed on the ground, or swallows the seed, which travels intact through its digestive system to be evacuated elsewhere.
In fact, many seeds cannot germinate without passing through the digestive system of a bird or mammal. Many fruits even warn their hosts that they are ready to be eaten by radically changing color. Cherries, for example, turn red to tell birds and other animals that it’s time to come and eat them!
Forgetful Animals
Some seeds, however, place their trust in the weak memory of animals. The seed inside the walnut (walnut fruit) is entirely digestible, and so is the seed in the center of the acorn (oak fruit), but both rely on the greedy squirrel to disperse them. Squirrels like to stock up for winter. It eats some of the nuts and acorns it finds, but buries the rest elsewhere… and sometimes forgets to come and get them, thus ensuring the dispersal of walnut and oak trees.
And who doesn’t know the seeds that have learned to cling to animals to travel? The “toques” (burdocks), with their hooked teeth, cling to the hair of animals – and our clothes! – and travel freely. Since they are irritating, however, the animal scratches to remove them… and the seeds sometimes fall several kilometers from their place of origin. Some aquatic plants also have hooked seeds that cling to the feathers of waterfowl as they travel from one lake to another.
Over-Reliance on a Distributor…
But there’s a problem with depending too much on one animal to ensure dispersal. What if the animal disappeared?
That’s what happened to a number of tropical fruit trees in the Americas. Botanists exploring South and Central America were confounded by the discovery of several trees with very large, fleshy and apparently delicious fruits, obviously designed to be eaten and dispersed by a very large animal, but which no longer seemed to find any takers. What’s more, all these trees were on the verge of extinction, as they were no longer able to reproduce properly. The conclusion? Humans have so successfully hunted the giant Laidback, once abundant throughout the region, that it has disappeared, leaving its favorite trees with no means of dispersal. The isolated trees found here and there were the last of their kind. Fortunately, the new invader, the human being, has adopted some of these fruits, which are now found only in cultivation. This is the case of the avocado (Persea americana), for example.
Seeds Dispersed by Humans
Other plants, too, have come to depend on man for their dispersal. Wheat, oats and other cereals, as well as many garden flowers, hardly ever spread of their own accord: they rely on man for their reproduction. Seed merchants collect their seeds and sell them, in bags, sacks or boxes, to gardeners and farmers who sow them. As a result, these plants can travel thousands of kilometers (most of our garden seeds, for example, are produced in Central America).
Some of these plants are so dependent on man for their dispersal that they are no longer able to multiply on their own. Maize (Zea mays), considered the most “evolved” of all cereals (at least from the human point of view), is so far removed from its ancestral form that its seeds no longer fall from the cob and can only germinate if separated from it. So, if humans were to disappear, so would corn.
A Miracle in Your Garden
So, when you hold a bag of seeds in your hands, as well as marvelling at the idea that in just a few months these little seeds will produce beautiful vegetables or fruit, think also of the journey they had to make to get from Asia, South America or Europe to your garden. It’s a daily miracle!
Larry Hodgson published thousands of articles and 65 books over the course of his career, in both French and English. His son, Mathieu, has made it his mission to make his father’s writings accessible to the public. This text was originally published in Le Soleil newspaper on February 26, 2006.
I always think of a seed’s ability to travel when I look through my neighborhood and see the Rose of Sharon bushes in my yard–and this neighbor’s yard, and that neighbor’s yard, and so on. I wonder whose bushes they were first? 🙂
Nature is wonderful. Every little seed is something of a miracle.