We use the halfway mark as a point of comparison rather than at the point at which all the disks are floating because If the experiment were to be repeated, we would require a set of reference points.
The net rates of photosynthesis can be compared at the point where half the initial number of disks are floating. The time it takes for half of the disks to float reduces as the net rate of photosynthesis rises, showing an inverse relationship between the value and the net rate.
As photosynthesis develops, oxygen is delivered into the leaf's interior, changing the buoyancy and raising the disks. The rate at which the disks increase is an indirect indicator of the net rate of photosynthesis because cellular respiration, which uses oxygen, is occurring concurrently.
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