Small disks were cut from spinach leaves that had been treated to remove any air from inside the leaf. The disks were placed in a solution that allowed them to carry out photosynthesis. At first, all the disks sank to the bottom of the container. These disks were then used for two different investigations. Investigation 1 The disks were divided into five groups. Each group was exposed to light of a different intensity, measured in watts per meter squared (W/m2). Some of the disks began to float. The results of the first investigation are shown in the graph below. The substance produced inside the leaf disks that caused them to float to the surface of the solution is: *