Does a leaf need carbon dioxide?

Two plants stand in the sun, each in a clear box. Only one makes starch, the stored food. Which?

Two plants sit in the dark first. They use up their starch.

Step 1 of 3

Two plants sit in the dark first. They use up their starch.
Each stands in the sun, in a clear box. Each lid is taped shut. Box X holds a dish of caustic soda. Box Y holds a dish of plain water.
After the iodine test, a leaf from box Y shows starch. A leaf from box X does not.

The idea

In this test, only the plant in box Y makes starch.

Caustic soda takes carbon dioxide out of the air. The air in box X is left with very little.

Everything else is the same. Only the dish differs.

Caustic soda takes in carbon dioxide from the air. A leaf with no carbon dioxide makes no starch. So the leaf needs carbon dioxide.

Caustic soda burns skin and eyes. Only a teacher handles it. The class watches.

Now try it. Look at each set-up. Tap what the iodine test will show.

Loading lab…