Ill.: MAKY.OREL, Wikimedia Commons
In February and March, gardeners of all ages start to visit garden centers in droves. And to dig through plant catalogs. And to attend Seedy Saturdays. Not yet looking for already up-and-growing vegetables, annuals and perennials for their garden (in most climates, it’s still far too early to be planting out), but to obtain seeds.
They know that growing plants from seeds is economical, easy and fascinating … but how many of them have ever stopped to ask the question: “By what miracle does a small seed—hard, dry and apparently lifeless—transform itself into a living plant?”
And it is a miracle, one of the most impressive miracles in all of nature.
The seed, in fact, is an intermediate form between the adult plant and the seedling. The plant generally produces seeds at the end of its growing season: in many cases, just before dying! The seed can be as tiny as a dust particle or as big as a coconut, but, in every case, it contains a germ of life which makes it possible for the plant to renew itself … but later, often much later, when conditions are more appropriate.
Seeds Get Around
The seed also serves as a means of plant locomotion.
After all, an adult plant is anchored in the ground and simply can’t move about in search of better growing conditions. The seeds are much more mobile.
Some, very light or equipped with “parachutes,” get carried away by the wind.
Others, because of their fleshy and juicy envelope, attract birds and animals which transport them far from the mother plant … then drop the seeds to the ground somewhere new once the fruit is eaten. That’s how humans introduced the wild apple (Malus domestica), a plant native to Central Asia, throughout Europe, Asia and North America.
The seeds of aquatic or shore plants often evolved the ability to float and thus use water currents for transportation.
Still other plants, the burdock (Arctium lappa) for example, have seeds that cling to passing animals (and people) to benefit from a free if somewhat reluctant transportation service.
Some plants even use mimicry to get around. For example, the seeds of quite a number of plants look so much like ant eggs that ants pick them up and transport them to their nest … and, by a “happy coincidence,” many these plants germinate best in anthills!
There are even plants that carry out “ballistic seed dispersal.” Their seed capsules explode when touched—by an animal, a raindrop or even a leaf, shooting their seeds far and wide. You need to look no further than the common garden impatiens to see a good example of this surprising means of transport.
A Long Rest, Then… Full Speed Ahead!
If necessary, most seeds can live for several years, waiting for the conditions they need to start growing to be reunited. In the Alaskan tundra, 10,000-year-old seeds were found that turned out to be still viable and some weeds—yes, weeds: wouldn’t you know it!—have seeds that can remain viable for 80 years and more. To be honest, though, for most seeds, reasonable viability drops to near zero after 3 to 10 years, at least under normal conditions. In a special seed vault, under carefully controlled conditions, make that 1,000 years!
When all the conditions are met—heat, humidity, light, air, etc. —, the seed quickly transforms into a seedling. In the northern hemisphere, seeds usually germinate in the spring, even though most were produced in late summer or fall. In fact, many temperate-climate plants, especially trees, shrubs and perennials, can only germinate if they go through an extended period of cold winter weather. There’s no use trying to sow maple or apple seeds on the tropical beaches of Rio de Janeiro … unless you subject them to a “cold treatment” period in the refrigerator beforehand!
A Container and Its Content
You can think of seeds as being made up of two elements: a container made up of a seed coat—often with multiple layers—, usually tough and hard, and a living content: the embryo.
The purpose of the seed coat is to protect the embryonic plant from cold, drought, unacceptable conditions, etc. It’s often designed to be perfectly impermeable to the elements … at least, that is, until all the conditions necessary for germination—the right temperature, sufficient humidity, light, etc.—are reunited and then it finally allows water and air to infiltrate, which in turn stimulates the growth of the germ of life the seed contains … and germination begins.
But what’s inside the seed? A germ, an embryonic plant already composed of a tiny stem, root and shoot, and also energy reserves contained in thick leaves called cotyledons. Part of these reserves will be used to keep the embryo alive during the period of relative inactivity when the seed is waiting for the right conditions to germinate. The bulk of the reserves, however, is devoted to germination, the sudden growth spurt where the embryo must make its way to the light in record time.
And it’s only when exposed to light that the embryo will truly become a plant: under the effects of the sun, the stem stretches upward, the cotyledons unfold and turn green and leaves develop … the seed is said to have germinated. Indeed, once the first true leaves appear and the envelope has dropped off, the cotyledons themselves deplete the last of their energy reserves and disappear and the seedling becomes fully independent: a living, growing plant.
Also, the next time you sow seeds, be they of tomatoes, sunflowers, carrots or marigolds, remember that you are participating in your own way in the miracle of renewing life on earth.