What is the process by which plants absorb liquid through roots called?

Osmosis is the spontaneous passage or diffusion of water or other solvents through a semipermeable membrane (one that blocks the passage of dissolved substances - i.e., solutes). The process, important in biology, was first thoroughly studied in 1877 by a German plant physiologist, Wilhelm Pfeffer. The general term osmose (now osmosis) was introduced in 1854 by a British chemist, Thomas Graham.

The outermost tissue of a root is composed of epidermal cells, which have long, fine extensions called root hairs to maximize surface area. Water enters the root hairs by osmosis, and then travels from the root hairs through the cells of the cortex until it reaches vascular tissue, which provides a conduction path up the plant to the leaves.