Why do chloroplasts clump in the middle of a plant cell?

In biology cl we took an elodea leaf and looked at it through a microscope and the chloroplasts were spread out. When we added salt water to the leaf the chloroplasts clumped together in the middle of the cell. My teacher did not know why this happened but explained that the vacuoles decreased in size. Are the chloroplasts attracted to each other and were the vacuoles keeping them separated before getting smaller?