Why Does Cutting an Onion Make You Cry? What a Closer Look Revealed

Hi, I’m Ken Kuwako, your Science Trainer. Every day is an experiment.

Why does cutting an onion make you cry? You run into onions in the kitchen almost every day, but they’re actually pretty fascinating plants. This time, I took a good, close look at one.

Onions Are Monocots

Onions are monocots. That means that when the seed sprouts, it sends up just one seed leaf. Rice, corn, and lilies are all part of this group, and onions are also close cousins of green onions and garlic.

The Roots Are Fibrous

Monocots have fibrous roots, which are lots of thin roots that spread out in all directions. Onion roots fit the description perfectly.

Dicots work differently. They have one thick main root (the taproot), with thinner side roots branching off from it.

What We Eat Is Actually Part of a Leaf

Surprisingly, the part we usually eat is made of leaf bases near the stem. Technically, it’s called a bulb. The bases of the leaves swell up as they store the nutrients made through photosynthesis.

So where’s the real stem? It’s the flat, tough disc at the bottom of the onion, called the basal plate. The roots grow from here, and the layers of leaves stack up on top of it. Slice an onion lengthwise and you can clearly see the tightly packed layers of leaves.

Onion flowers grow as a round ball made up of lots of tiny blossoms. They look a lot like the flower heads of green onions, which is hard to imagine when all you’ve ever seen is the onion on your cutting board.

The Leaf Veins Run in Parallel

Onion leaves have parallel veins, meaning the lines running through the leaf all point the same way, almost perfectly straight. This is another hallmark of monocots, and together with the fibrous roots, it’s a handy clue for telling them apart.

Here’s a summary of the monocot features we spotted in our observations:
・One seed leaf
・Fibrous roots
・Parallel leaf veins

So Why Does Cutting One Make You Cry?

When you cut an onion, its cells break open and the substances inside mix together. Enzymes then get to work and release a gas that irritates your eyes and nose into the air. It stings the surface of your eyes, and that’s what brings on the tears. This is thought to be the onion’s chemical defense system, a way to protect itself from insects and animals.

There are a few tricks to cut down on the tears, like using a sharp knife or chilling the onion before cutting. Both help because fewer cells get crushed, so less gas is released. Give them a try and see how well they work!

Science Experiments You Can Do with Onions

Onions are a perfect vegetable for learning science. Here are some experiments and observations you can try:
・Look at the thin skin under a microscope to see the cells and their nuclei
・Observe the root tips to watch cells dividing
・Dye a piece of cloth with the brown outer skin
・Soak an onion in water and record how its roots grow each day

探検家になろう！タマネギの薄皮一枚に隠された驚異のミクロワールド観察術（細胞の観察）

That onion sitting in your kitchen can become a ready-made science project all on its own.

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Science is more wonderful and fun than you might think! This site collects fun experiments you can do at home, along with tips to make them work. Feel free to look around and search for whatever sparks your curiosity!
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