In Experiment 1, researchers grew seedlings with or without a neighboring plant and found that neighbors reduced root growth. In Experiment 2, they grew seedlings alone but added either filtered solution collected from neighbor-grown soil, filtered solution from plant-free soil, or neighbor-grown solution treated to remove small organic molecules. Root growth was again measured. The second experiment was designed to test whether soluble root-secreted compounds, rather than physical crowding, could account for the first result.