Plant roots usually lie beneath the soil; their purpose is to absorb water and nutrients, anchor the plant to the ground and store plant nutrients. The cortex of roots surrounds a central cylinder of vascular tissue called the stele that moves water and nutrients up into the plant. The tap root is the main vertical root that has both root hairs and lateral branches.
The cortex that encloses and protects the stele in the middle of the root is composed of two layers of cells in the shape of cylinders. The outer layer, the hypodermis, and the inner layer, the endodermis. are formed of tightly packed cells.
The stele, enclosed within the cortex, contains the pericycle and vascular tissues, xylem and phloem.
The pericycle is a cylinder of thin-walled, loosely-packed parenchyma cells just inside the endodermis. The pericycle contains one or several parenchyma cells that constantly divide. Parenchyma cells, the bulk of non-woody plants, store water and nutrients.
Xylem are vascular bundles that move water and nutrients up into the plants. Xylem work in two ways: as water evaporates from leaves through a process called transpiration, it forms a negative pressure that draws the water upward from the roots; if high concentrations of water in roots exceed the water in the surrounding soil, the water pushes upward.
Phloem are plant cells that carry sucrose and other organic nutrients to all parts of the plant.
The tips of roots are covered with a protective cap called the calyptra that secretes mucilage, a thick, gooey substance. The mucilage is a lubricant that helps the root penetrate the soil. Just behind the calyptra, small, densely packed cells with thin walls and large centers have the ability to divide and grow longer. These cells, forming what is called meristematic tissue, replace the cells of the cap that are worn off by friction as the root pushes through the soil. The meristematic tissue is called the root's growing point.
The meristemic tissue in the growing point also provides new cells that make the plant grow longer, pushing the cap deeper into the soil. Growing roots, needing more water and nutrients, fors lateral branches. Each of these lateral branches has a cap and a growing point of meristemic tissue and root hairs. Tiny root hairs grow near the ends of the main root and the lateral branches.
Some grasses and wildflowers lack a primary root and instead form thick mats of fibrous roots. Adventitious roots grow from the stems and branches of some plants.