Root Apical Meristem

The root apical meristem is a type of meristem located at the tip of a root. It is responsible for the growth of the root in length and the formation of lateral roots.

The meristem consists of undifferentiated cells that continuously divide and differentiate into specialized cells, contributing to the development of the root’s primary tissues. Different theories, such as the Apical Cell Theory and Histogen Theory, have been proposed to explain the organization of the root apical meristem.

Apical Meristems

The apices of the stem and root are called growing points. Meristematic tissues occupy them. These meristems are called apical meristems. The sole activity of apical meristems derives from all the primary permanent tissues of the plant body.

The apical meristem consists of the upper pro meristem and lower primary meristem. The cells are differentiated into three fundamental units in the primary meristematic region.

These are protoderm (gives epidermal tissue system), ground meristem (gives ground tissue system) and procambium (gives vascular tissue system). Different organs of the plant body are initiated by pro meristem and built up by primary meristem.

Root Apical Meristem

Root Apex:

The root apex is present opposite to the shoot apex. The root apex is more simple than the shoot apex. As in shoot apex, root apex is also occupied by pro meristem followed by primary meristem. A root cap protects the root apex.

As there is a root cap in roots, the root apical meristem is said to be sub-terminal. The root apex initials are more clear than the shoot apex initials. The meristematic cells divide and add new cells to the body of the root and the terminal root cap.

The root apex shows uniform growth due to the lack of lateral appendages and nodes and intermodal differentiation.

A. Types of Root Apices:

Root apices of dicots differ from monocots in some aspects.

Root Apices of Dicots:

According to the number of rows of initials, the root apices of dicots have been classified into three types.

1. Ranunculus Type: In some members (e.g., Ranunculaceae, Amentiferae and some Leguminosae), a single row of initials is present at the tip region. It gives rise to various regions of the root along with the rootcap.
2. Casuarinn Type: In this type, there are two rows of initials at the root tip (Eg: Casuarinaceae; Proteaceae, and some Leguminosae). The outer row gives rise to the epidermis, cortex, and root cap. The inner row gives rise to the vascular cylinder of the root.
3. Common Type: In most dicotyledonous families, three rows of initials Outer row gives rise to the epidermis and root cap, the middle row gives rise to the cortex, and the inner row gives rise to the vascular cylinder of the root. It is similar to the closed type of Guttenberg.

Root Apices in Monocots:

In monocots, four types of root apices have been recognized. The first, second, and third types are similar to those of dicot roots.

In the fourth type, four rows of initials give rise to different root regions. From the outside, the outermost row is called calyptrogen. It gives rise to root cap. The second, third, and fourth rows give the epidermis, cortex, and central cylinder, respectively.

Theories regarding apical root organization

Regarding root apical organization following, different theories have been put forward.

A. Apical Cell Theory:

It was proposed by Nageli (1858). He observed a single tetrahedral apical cell in the root apex of vascular cryptogams. The upper three sides of the apical cell divide and give rise to the body of the root, whereas the base plane gives rise to the root cap.

This theory applies to only vascular cryptogams but not spermatophytes. Because in spermatophytes, groups of initials are present in the root apex region.

B. Histogen Theory:

Histogen theory is proposed by Hanstein (1868) and supported by Strassburger. The histogen theory, as applied to the root apical meristem, speaks of four histogen in the meristem. They are respectively

i. Dermatogen: It is an outermost layer. It gives rise to the root epidermis.

ii. Periblem: It is a middle layer. It gives rise to the cortex.

iii. Plerome: It is the innermost layer. It gives rise to stele

iv. Calyptrogen: It gives rise to root cap.

C. Korper and Kappe Theory:

It was postulated by Sehuepp (1917). This theory is comparable with the tunica corpus theory shoot apex. This theory is mainly based on differences in the planes of cell division. The central portion of the root apex is known as korper, whereas the peripheral portion is called kappe.

According to this theory, the cells of the root apex divide into two distinct planes. In the Korper (body) region, after the transverse division of a cell, the upper cell undergoes vertical division. The sequence of cell divisions is a “1” pattern.

In the Kappe (cap) region, the lower is after the transverse division of a cell. The cell undergoes vertical division. The sequence of cell divisions is a “T” pattern. In Korper the bar of the “T faces towards the apex, whereas in the Kappe region, it faces away from the apex.

D. Quiescent Centre:

Clowes (1958) studied the root tips of Zea mays and observed an inactive center between the root cap and the active meristematic region. He called it the quiescent center. Afterward, several workers reported the presence of quiescent centers in various taxa.

It is a biconvex structure that is made up of thousands of dormant cells. In this region, the cells divide very rarely. Compared to surrounding cells in this zone, the cells contain little DNA, RNA, and proteins. They possess only a few cell organelles.

This peculiar zone may be the site for auxin synthesis. Further, it is believed that it acts as a reservoir of cells. The quiescent center is more resistant to injury and irradiations than the meristematic region. If root growth is stopped or the root tip is damaged, this zone restores the growth.

FAQ

References:

• University Botany (University of California)
• Plant Structure, Function, and Development (J. A. Romberger· Z. Hejnowicz. 1. F. Hill)
• Esau’s Plant Anatomy