A seedling is placed in a solution containing potassium ions, with its roots immersed, and is then left for 48 hours. The table shows the relative concentrations of the potassium ions. At the beginning of the experiment: concentration of potassium ions in the solution = 20, in the root hair = 15. After 48 hours: concentration of potassium ions in the solution = 18, in the root hair = 35. By what process did the potassium ions enter the cell?