A plant's endodermis is a single cylindrical layer of cells that does not permit water to flow between the cells. Not all plants have an endodermis, but the structure plays an important role in transporting water from the ground via the roots up through the rest of the plant. The endodermis thus allows trees and other large plants to grow much taller than otherwise would be possible.
Structure of Endodermis
The most important property of the endodermis that it can regulate the concentration of solutions inside the root. Because there is no space between cells for water to travel, any water or ions that traverse the endodermis must go through two cell membranes. The endodermis therefore helps control the passage of water and ions from the root's cortex (the outer layer) and its stele (the inner layer). This control is critical to a plant's ability to transport water upward via root pressure.
Endodermis and Root Pressure
The stele contains a plant's xylem, tissue that moves water up the plant. Water flows upward in two ways. Plants without an endodermis use transpirational pull. Water evaporation in mesophyll tissue in the leaves creates surface tension in the xylem, prompting water to move up the plant. The endodermis increases a plant's ability to transport water upward by creating high concentrations of dissolved ions inside the root. Water with lower concentrations of ions outside the root then diffuse into the root, creating higher water pressure at the bottom of the plant and moving water up the plant.
Increased Possibilities for Plants
The additional water-transport capability allows plants with endodermis tissue to grow taller, because they are able to bring water and other nutrients to greater heights. Height carries several evolutionary advantages, including greater access to sunlight and increased protection from predators.
Because it forms a cellular seal around a plant's vascular tissue, the endodermis also helps prevent the entry of harmful organisms and chemicals into the plant.