The way osmosis works is usually through an "SSM" a selectively semi-permeable membrane. If you have different solutions on different sides of the membrane, and one solution has larger molecules than the other (say if you have pure water one side and a sugar solution the other), then the smaller molecules will pass more readily through the membrane than the larger ones. In such circumstances the fluid travels in one direction,. As Tyto Alba says, plants take advantage of this when distributing water from the roots to the rest of the organism.
