Apparently, the thicker layers are just called "layers" or "scales." The scales are actually the swollen bases of leaves that grow up from the onion and through the surface of the ground. The scales function to protect the bud of the onion. Also, when the leaves stop growing and even wither, the scales continue to nourish the bulb and the bud deep within, especially with its large supply of water.
The outer layer of the onion that gives each onion its yellow, red, or white color, is called a "tunic." The base of the onion where all the hairy things are poking out is called the "plate." Those hairy things are the beginnings of the roots, and the plate is what holds the onion together.
The thin skin or membrane in between the layers is called the "skin" or "membrane." Unfortunately, I haven't been able to find any explanation of the purpose of these thinner layers. If anyone viewing this entry knows what the thin membranes do for the onion, please ring in. Just type what you know in the Comments field of this entry, and I and everyone who visits this page will be forever in your debt.
In the meantime, the best I can offer are these photos:
(Photo by Arthur H. Harris, from the Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso)
The above is an onion cut in half lengthwise, from stem to tip. When you cut an onion cross-wise (see below), it looks very symmetrical. But lengthwise, it appears to be more complex.
And here's a red onion cut in half cross-wise, with the red tunic of each layer plainly visible:
(Photo from Tom's Domain's recipe for tomato shrimp soup)
And finally, here are a few interesting tidbits I learned about onions while looking for the answers to my questions:
- Onions have been around for at least 4,000 years. It is believed that the workers who built the pyramids were fed radishes and onions.
- The Egyptians believed that the strong scent of onions was capable of reviving the dead, specifically by restoring the dead person's breath. King Ramses IV's mummy was found with small onions in his eye socket.
- In the Middle Ages, people treasured onions so, they often paid their rent in onions or gave onions to each other as gifts.
- Onions are still very popular throughout the world, but in no other country are they as tremendously popular as in Libya, whose citizens eat 66.8 pounds of onions per year.
- Here's what happens when you cut an onion and why tears form in your eyes:
- Cells containing enzymes and acids are broken open.
- The enzymes break down the acids, and when the acids hit the air, they turn into a gas.
- When the gas reaches the moisture in your eyes, the gas turns into sulfuric acid. This is a pretty nasty acid; it is behind the smell of rotting eggs, for example.
- The acid stings the nerve endings in your eyes, making them produce tears in order to flush away the irritant.
- To reduce the stinging effect of onions, you can try a lot of different things, including:
- Cutting the onion under running tap water (though this may be too slippery to accomplish this safely)
- Wetting the onion and your hands first so that the gas reacts with the water on the onion and your hands, not the moisture in your eyes
- Chilling the onion for a while before cutting it, or putting the knife in the freezer for a couple of minutes first. This lowers the temperature so that when you cut open the onion, the enzymes are not as likely to react with the acids.
- Using a very sharp knife. A sharper knife will rupture fewer cells, and thus release fewer enzymes and acids.
- If you plug your nose and take a bite of an apple, then take a bite of an onion, I'm willing to bet the two won't taste any different to you. That's due to the magic of your nose (which should rightly be discussed in another entry).
- After you've taken a bite of onion, to combat the onion smell on your breath or on your hands, rinse with lemon juice and water, or eat an apple, or eat a few sprigs of parsely.
New Mexico State Molecular Biology Program, Crop Plant Resources, Onion: Allium cepa L.
Food and Agriculture Organization of the United Nations, Ecoport, Allium cepa
Chapter 2: Microscopy Laboratory: Exercise 2A, from Botany: How to prepare an onion cell slide, available through Wikibooks
National Onion Association, Trivia About Onions
You Grow Girl, All About Bulbs, includes a helpful explanation of the differences between bulbs, tubers, rhizomes, and corms, in layperson's terms
For a great article about the different varieties of onions and how to cook them well, read Mike Sodaro, "The Onion's Layers of Flavors," Food Product Design, November 2005