Dicot RootDicot Root
Roots anchor the plant, absorb minerals and water, conduct water and nutrients, and store food. Many dicots have a tap root system, consisting of one large, vertical root that produces many smaller secondary roots. Some tap roots, such as carrots, turnips, sugar beets, and sweet potatoes, are modified roots that store larger amounts of foods. Most absorbtion of water and minerals occurs near the tip where the root hairs are located.
The procambium forms a central vascular cylinder, or stele, where the xylem and phloem develop. In most dicots, the xylem cells radiate from the center stele in two or more spokes, with phloem developing in the wedges in between the spokes. The xylem forms a cross in the center and the phloem surrounds it. The stele is still surrounded by the parenchymal cells of the cortex.
The actual increase in the size of roots is mostly due to cell growth. Most of this growth occurs when the vacuole and cytoplasm take in more water as the cell wall becomes temporarily more plastic. As a plant cell stops growing, additional material is applied to be inside of the existing wall, making the wall more rigid. An established root may sprout lateral roots, or secondary roots, which arise from the outermost layer of the stele.
The protoderm, the outermost primary meristem, give rise to the epidermis, a single layer of cells covering the root. Water and minerals that enter the plant from the soil must cross the epidermis. The root hairs enhance this process by greatly increasing the surface area of epidermal cells. The innermost layer of the cortex is called the endodermis, a cylinder one cell thick that forms the boundary between the cortex and the stele. It regulates the passage of subtances from the soil solution into the vascular tissue of the stele. Just inside the endodermis is the pericycle, a layer of cells that may become meristematic and begin dividing again.