A student seals the whole root system of a plant inside an airtight bag but leaves the shoot in normal air, and the plant keeps photosynthesising. Where is its carbon dioxide coming from, and by what route?

Explanation

The gas photosynthesis needs is a normal part of the atmosphere and reaches the inside of a leaf through the stomata, the small pores in its surface. Roots take up soil water and the minerals dissolved in it, and they were never a source of this gas, so a bag around them cuts off no supply of it; the waterproof waxy covering of a leaf is not a route for gases either.