In a hypotonic solution, more water molecules are able to enter the cell than those diffusing out. Thus, a plant cell in a hypotonic solution swells up and may even burst.
When the water concentrations of both the cell's cytoplasm and the surrounding medium are the same, the solution is called isotonic. In this case, nothing happens to the plant cell.
In a hypertonic solution, the water concentration of the medium is lower than that inside the cell. More water molecules diffuse out the cell than those entering, causing the cell to shrink and shrivel.