Roots are those underground portions of a plant that store food, absorb water and minerals from the soil, and anchor the plant in the earth.
Unlike the shoot apical meristem, the root apical meristem produces cells in two dimensions. It harbors two pools of stem cells around an organizing center called the quiescent center (QC) cells and together produces most of the cells in an adult root. The QC cells are characterized by their low mitotic activity. Evidence suggests that the QC maintains the surrounding stem cells by preventing their differentiation, via signal(s) that are yet to be discovered. This allows a constant supply of new cells in the meristem required for continuous root growth. Recent findings indicate that QC can also act as a reservoir of stem cells to replenish whatever is lost or damaged.
The tip of a root is covered by a thimble-like root cap, which protects the delicate apical meristem as the root pushes through the abrasive soil during primary growth. The root cap also secretes a polysaccharide slime that lubricates the soil around the tip of the root. Growth occurs just behind the tip in three overlapping zones of cells at successive stages of primary growth. These are :
- Zone of cell division
- Zone of elongation
- Zone of differentiation or zone of maturation
The zone of cell division includes the root apical meristem and its derivatives. New root cells are produced in this region, including cells of the root cap. Typically, a few millimeters behind the tip of the root is the zone of elongation, where most of the growth occurs as root cells elongate—sometimes to more than ten times their original length. Cell elongation in this zone pushes the tip farther into the soil. Meanwhile, the root apical meristem keeps adding cells to the younger end of the zone of elongation. Even before the root cells finish lengthening, many begin specializing in structure and function. In the zone of differentiation, or zone of maturation, cells complete their differentiation and become distinct cell types.
The primary growth of a root produces its epidermis, ground tissue, and vascular tissue. Water and minerals absorbed from the soil must enter through the root’s epidermis. Root hairs, which account for much of this absorption, enhance this process by greatly increasing the surface area of the epidermis.
In angiosperm roots, the stele is a vascular cylinder, consisting of a solid core of xylem and phloem. In most eudicot roots, the xylem has a star like appearance in cross section and the phloem occupies the indentations between the arms of the xylem “star.” In many monocot roots, the vascular tissue consists of a central core of parenchyma cells surrounded by a ring of xylem and a ring of phloem.
The ground tissue of roots, consisting mostly of parenchyma cells, fills the cortex, the region between the vascular cylinder and epidermis. Cells within the ground tissue store carbohydrates and absorb water and minerals from the soil. The innermost layer of the cortex is called the endodermis, a cylinder one cell thick that forms the boundary with the vascular cylinder. The endodermis is a selective barrier that regulates passage of substances from the soil into the vascular cylinder. Lateral roots arise from the pericycle, the outermost cell layer in the vascular cylinder, which is adjacent to and just inside the endodermis. A lateral root pushes through the cortex and epidermis until it emerges from the established root.