Environment Planet Earth Turgor Pressure in Tree Cells How osmosis and turgidity support trees By Steve Nix Steve Nix Writer University of Georgia Steve Nix is a member of the Society of American Foresters and a former forest resources analyst for the state of Alabama. Learn about our editorial process Updated January 24, 2018 Kovcs Zsolt / EyeEm / Getty Images Share Twitter Pinterest Email Planet Earth Outdoors Weather Conservation Turgor pressure, also called turgidity when occurring in trees and most plants are the pressure of cell contents exerted against the plant cell wall including tree leaf and stem cells. A turgid plant cell contains more water and minerals in solution than flaccid (deflated) plant cells and exerts a greater osmotic pressure on its cell membrane and walls. So, turgor is a force exerted outward on a plant cell by the water contained within the sturdy cell wall. Water and its solutions fill up tree cells up to its optimum expansion capability determined by the cell wall. This force consequently gives a succulent plant rigidity and helps non-woody plants to keep erect. Woody-stemmed plants have extra structural support in the form of wood cells and bark. When you actually see a mature woody-stemmed plant like a tree's leaf wilt due to low turgor pressure, major damage may have been done and tree health compromised. Extreme turgidity can result in the bursting of a cell but is rare in nature. The tree cell wall is designed to handle pressures beyond the cell membrane. Turgor and Osmosis in Trees Turgor pressure is not the mechanism that rises solutions from roots to leaves. Trying to describe this simply, the process of osmosis creates tree and plant turgidity by the osmotic propensity of moving a heavy water volume of weak solution from the roots toward a low water volume of high solution in the leaves and branches. A solution, in this case, is simply a water mixture of solutes in the leaves being concentrated and high and the water-holding solutes entering the root being diluted and low. In this particular botanical example, water is the solvent with a mixture of dissolved concentrations of various nutritive substances called the solute. As the tree's liquid reaches a static or equal solution mixture from root to crown, turgor pressure becomes optimal and pressure increase stops. The Important Tree Cell Wall and Membrane A tree's cell wall is a tough, flexible "wicker basket" that is rigid but flexible and has the ability to stretch and expand as the cell membrane inside expands. It surrounds the delicate cell membrane and provides these cells with structural support and protection. The cell wall will also act as a filter but the major function of the cell wall is to act as pressure support for the cell and its contents. The tree's cellular membrane is a protective and functional cell layer that separates tree cell contents from the outside environment but is permeable to the organic molecules and minerals necessary to support tree life. Osmosis through the cell membrane controls the movement of substances in and out of tree cells. The basic function of the cell membrane is devoted to the protection of the cell contents from outside invasions of foreign materials.