Explain why the leaf discs moved to the surface of the solution during the investigation.

The leaf discs initially sink because the air has been removed from their internal air spaces. When the lamp is turned on, photosynthesis begins. A product of photosynthesis is oxygen gas. This oxygen gas starts to fill the air spaces within the spongy mesophyll layer of the leaf disc. As more and more oxygen gets trapped, the overall density of the leaf disc decreases, and its buoyancy increases. Eventually, enough oxygen is produced to make the disc less dense than the surrounding sodium hydrogencarbonate solution, causing it to float to the surface.