Clever travelers seeking places to put down roots
You probably know more about seeds than you think you do. You know how a lot of them taste - and not just sunflower seeds. Corn, beans, and peas are all seeds. So are the wheat, barley, oats, and rye used to make bread and cereal. Rice is a seed, too, not to mention peanuts (which are actually related to peas) and other true nuts. Sesame seeds top hamburger buns. And don't forget those most important seeds, the ones from the cacao tree that are used to make chocolate.
Not all seeds are good to eat. But the huge variety of seed sizes, colors, shapes, and designs are all intended to help them do a very important job: find a good place to take root and grow.
Suppose you were to design a tree. You'd want to make sure it had seeds that created more trees. Which would you choose: a few really big, tough seeds, or lots and lots of tiny, delicate seeds?
Actually, both systems work. Some palm trees produce a few very big seeds, coconuts. If you've ever tried to open one, you know how hard its shell is. Other trees, like the cottonwood, create thousands of tiny seeds that are carried everywhere on the wind.
The size of the seed doesn't tell you the size of the plant. Giant sequoia trees, some of the tallest living things on Earth, start from seeds that are 1/4 inch long. They can grow more than 200 feet tall. The milkweed's seed is twice the size of a sequoia's, but grows into a plant only a few feet high.
Size is related to something else, though. The smaller the seed, the more of them there are likely to be. While a palm tree may produce only a few big coconuts, orchids create millions of seeds so small it takes 800,000 of them to weigh one ounce.
Now suppose you were to design a plant. You'd want to make sure its seeds spread way out so they have plenty of room to grow. With its roots stuck in the ground, a plant can't exactly run around throwing out seeds. What can it do?
Some plants use the wind to spread their seeds. When you blow on a dandelion and its seeds scatter through the air, floating on their little white parachutes, you're helping the dandelion spread its seeds. Maple trees produce twin seeds with long, flat wings that spin like helicopter blades on their way to the ground.
Plants like the wild geranium, witch hazel, and pansy don't rely on the wind. They shoot their seeds out. Their seed pods explode, sometimes with a loud pop or crack, and the seeds go flying. The pods become explosive as they dry out. Some parts dry out faster than others, creating tension that makes the pod burst.
Coconut palms, and other plants that grow near water, often have seeds that can float. They let streams, lakes, and oceans carry their seeds, sometimes for miles.
Have you ever gotten cockleburs stuck to your socks? That's another way seeds travel. Seeds from goose grass, burdocks, needle grass, and many other plants have sticky surfaces that cling to clothes and animal fur. This way they can ride long distances before they fall or are pulled off. They stick to birds and insects to travel by air as well.
Animals also help plant seeds in another way. When squirrels gather acorns, they sometimes bury them for safekeeping. They don't always remember to dig them up, however. So some acorns just remain safely planted.
Ever spit out watermelon seeds at a picnic? Some seeds come in tasty "packages" that help them get around.
When we eat apples, oranges, melons, grapes, and other fruits, the seeds usually are thrown in the garbage or compost pile. But animals just toss the seeds aside or spit them out, scattering them all over the place.
Once the seeds arrive in their new locations, the conditions have to be just right for them to sprout, or germinate. They can remain dormant (inactive) for weeks, months, years, sometimes even centuries, waiting for the right conditions.
In one case, lotus seeds more than 10,000 years old germinated under the right conditions.
Temperature is key
Every seed has a "baby plant" or embryo in it. Water starts the embryo growing, but it also needs air, and sometimes light. If the soil around it is packed too tightly, or there is too much moisture in the soil, the seed won't get enough oxygen.
And temperature can also be important. Seeds in colder climates must wait out the winter and don't sprout until the soil warms up. Lettuce and spinach will germinate at 55 to 65 degrees F. Basil and peppers stay dormant until the soil reaches 70 to 80 degrees F.
When conditions are right, the seed sends out a new plant. The new plant always grows upward, and the roots grow downward.
As it grows, it will eventually produce leaves, flowers, perhaps fruit, and, of course, seeds.
Try these experiments
Water can be absorbed through a seed's outer shell, but many seeds also have a tiny hole, called a micropyle, where water enters to start germination. You can see a bean's micropyle with a magnifying glass, but it's too small to see on many seeds. Here's a way to find it:
Ask a grown-up to help you boil some water in a pot. When it comes to a boil, turn off the heat. Now put in some seeds: sunflower seeds, dried beans, maybe an unshelled Brazil nut. Watch: Air will bubble out through the seeds' micropyles as water enters.
Seeds germinate at different temperatures. Here's a way to test this.
Buy some parakeet seed from a pet store. Get two plates, and put a damp paper towel on each. Sprinkle seeds on the towels, then cover them with an overturned bowl. Put one plate in the refrigerator (about 40 degrees F.). Leave the other at room temperature (about 70 degrees F.). In a few days, check the seeds. Which ones sprouted? Try it again with radish seeds for a different result.
How seeds get around
Answers: (1) E; (2) C; (3) A; (4) F; (5) B; (6) D.
(c) Copyright 1999. The Christian Science Publishing Society