Question

Explain the primary growth of roots in terms of the different zones of growth. Describe the different meristems that give rise to the primary tissues of roots.

Answer 1

Answer:

Part-1: Different zones of primary growth of roots

The increase in length of the the root is referred as primary growth of root. It is the result of cell division in the shoot apical meristem. Secondary growth is characterized by an increase in thickness or girth of the plant. It is caused by cell division in the lateral meristem.

The root tip has three main zones of growth: a zone of cell division, a zone of elongation, and a zone of maturation.

Zone of cell division

The apical meristem cells of root and the actively dividing cells behind it form the zone of cell division. Cells of the root apical meristem segregate into three primary meristems. Cells in the center of the root tip become the procambium. The cells which are just out- side the procambium become ground meristems. The cells which are located on the periphery of the apical meristem become protoderm.

Zone of cell elongation

The zone of elongation is where the newly-formed cells increase in length as their vacuoles fill with water, thereby lengthening the root. This "hydraulic" elongation pushes the root cap and apical meristem through the soil as much as several centimeters a day.

Zone of cell maturation

Above the zone of elongation, cells do not increase in length but they may differentiate further in the zone of maturation. This is where cells take on an identity about what function they will serve within the plant's root system. Cells become parenchyma cells (to store and move nutrients), or they may become sclerencyma cells (part of the cell wall). Epidermal cells in this zone give rise to root hairs, and the procambium, ground meristem, and protoderm complete their differentiation here. Cells differentiate based on the amount of division and amount of maturation within the cell wall. In this zone beginning of first root hair attached to the root takes place.

The young root hairs secret acids to initiate soil mineral exchange. The acids, or hydrogen ions, are secreted onto soil particles by the fine, young roots hairs in the zone of maturation are exchanged with the calcium, magnesium and iron mineral ions found in the soil.

Part-2: Different meristems that give rise to the primary tissues of roots

The root is composed of three concentric circles of primary tissues. The central core is vascular Tissue. This central core of cells is derived from the procambium and is called the stele. This is surrounded by a ring of ground tissue (cortex) which is covered by an epidermis.

A meristem is a tissue in plants that consists of meristematic cells which are capable of cell division. Meristems give rise to various tissues and organs of a plant and are responsible for growth. The meristem is divided into three types on the basis of the positions are apical meristem, intercalary meristem and lateral meristems.

Apical meristem is found at the tips of roots, stem and branches. It is responsible for the increase in the length of the plant. Intercalary meristem is present in the nodal region in in monocotyledons, e.g grasses. It is derived from the apical meristem and is responsible for the elongation of internodes. Lateral meristem is present along the longitudinal axis of stem and root. Vascular cambium and cork cambium are examples for lateral meristem. It produces secondary permanent tissues which result in the thickening of stem and root.

There are two types of apical meristem tissue. Shoot apical meristem (SAM), which gives rise to organs like the leaves and flowers. Root apical meristem (RAM), which provides the meristematic cells for future root growth.

The root apical meristem or root apex, is a small region at the tip of a root in which all cells are capable of repeated division and from which all primary root tissues are derived. The root apical meristem produces the three primary meristems (procambium, ground meristem and protoderm) and the root cap.

There are two types of meristem organizations. In closed meristems, cell lineages of the epidermis, cortex and stele converge at the QC, setting a conspicuous boundary between these tissues and the root cap. In open meristems this convergence does not occur, leaving the boundary between the distal tissues and the root cap unclear.

The root apical meristem in seed plants is composed of five major tissues epidermis, ground tissue (cortex and endodermis), stele (pericycle and vascular tissue), root cap and quiescent centre (QC). The epidermis, cortex, endodermis and pericycle exhibit radial symmetry, with the QC located at their base, surrounded by the root cap. The radial tissues and the root cap are generated by asymmetric cell divisions of stem cells around the QC. After division, cells proximal to the QC stay undifferentiated, whereas the cells distal to the QC undergo further cell divisions and differentiate into the cell type of the vertical lineage. This formative cell division constitutes different tissues with distinct biological functions.