Plants began to develop large shoots only after they developed this adaptation. I will try to illustrate how the endodermis regulates solute movement and consequently regulates water movement at an elementary level. Keep in mind that molecules move from areas of high concentration to areas of low concentration. This includes water. The sucrose molecules can't go anywhere, but the water molecules can cross the membrane.
Sucrose solution is in the left half of the tube.
Pure water is in the right half. The membrane will allow water molecules to pass through but sugar is blocked. There is a greater concentration of water molecules on the right side of the membrane. Thus, there will be a net movement of water molecules from the right to the left side of the membrane. The movement of water molecules from an area of high concentration to one of low concentration results in a volume increase in the left side of the tube.