Which gas do plants take in from the air?

Plants take in carbon dioxide through pores in their leaves called stomata, use light to strip the carbon out of it, and release the leftover oxygen. Nearly all the oxygen in the air you are breathing was put there by something photosynthesising.

The consequence people find hardest to believe is where a tree comes from. The bulk of a trunk is carbon, and that carbon arrived as gas. A tree is built mostly out of air, with water and a modest amount of mineral from the soil. Jan Baptist van Helmont worked out something close to this in the 1600s by growing a willow in a weighed pot of soil for five years: the tree gained about 74 kilograms, and the soil had lost a couple of hundred grams.

One common misconception is worth correcting. Plants also breathe - they consume oxygen and give off carbon dioxide, day and night, like any other living thing. It is simply that while the light is on, photosynthesis runs far faster than respiration, so the net traffic goes the other way.