Apical Meristem

An apical meristem is a type of meristem, which is a region of actively dividing cells found in plants. The apical meristematic tissue specifically refers to the apical meristem tissue found at the tips of roots and shoots and are crucial for the production of primary tissues and thus contribute to the elongation of plant organs. This process is known as primary growth. Examples of apical meristems include the root apex or root apical meristem, and the shoot apex or shoot apical meristem.

1.0Characteristics Of Apical Meristem

• Undifferentiated Cells: The cells within the apical meristem retain their undifferentiated state, indicating that they have not undergone specialization into specific cell types.
• Rapid Cell Division: Apical meristems exhibit high rates of mitotic activity, resulting in rapid cell division. This continual division process contributes to the generation of new cells.
• Location at Tips: Apical meristems are strategically positioned at the tips of roots (known as the root apical meristem) and shoots (termed the shoot apical meristem). This placement enables them to play a vital role in extending the plant's body structure.
• Protective Structures: To safeguard the actively dividing cells within the apical meristem, specialized protective structures are present. In roots, the apical meristem is shielded by a root cap, while in shoots, it is safeguarded by leaf primordia.

2.0Apical Meristem Function

• Primary Growth: Apical meristems are responsible for primary growth in plants. This type of growth involves the elongation of roots and shoots, leading to an increase in length of the plant body. The apical meristem continuously produces new cells that contribute to this growth.
• Cell Division and Differentiation: The apical meristem is a region of actively dividing cells. As cells divide, they differentiate into various specialized cell types, contributing to the formation of different tissues and organs in the plant. This process allows for the development of roots, stems, leaves, and other plant structures.
• Formation of New Organs: The apical meristem gives rise to new organs such as leaves, stems, and flowers. Meristematic cells differentiate into the specific cell types required for the formation of these organs. In roots, the apical meristem gives rise to the root cap, a protective structure that covers the tip of the root. 
• Apical Dominance: In shoots, the apical meristem exerts apical dominance, which means it inhibits the growth of lateral buds located below it. This phenomenon ensures that the main shoot continues to grow taller, while lateral branches develop later or in response to specific environmental cues.

3.0Types of Apical Meristem

Apical meristems are located at the tips of roots and shoots in plants and are responsible for primary growth, which involves the lengthening of plant structures. There are two main types of apical meristems:

1. Shoot apical meristem (SAM): Located at the tips of stems and branches, the shoot apical meristem is responsible for primary growth in the upward direction. It gives rise to new leaves, stems, and flowers. 
2. Root apical meristem (RAM): Situated at the tips of roots, apical meristem of root is responsible for primary growth in the downward direction. It generates new cells that differentiate into various root tissues, allowing the root to penetrate the soil, absorb water and nutrients, and provide anchorage to the plant.

4.0Apical Meristem Diagram

5.0Theories Regarding to Apical Meristem 

Haberlandt divided eumeristem (a type of primary meristem, specifically apical meristem) into three regions according to their functional roles:

• Protoderm: The outermost layer of eumeristem, protodermis responsible for the formation of the epidermal tissue system (E.T.S.). Activities of protoderm lead to the development of structures such as epidermis, stomata, and stem hairs (trichomes).
• Procambium:  Composed of elongated cells, procambium plays a crucial role in generating the vascular tissue system (V.T.S.). The vascular bundles, comprising xylem and phloem, originate from the activity of procambium.
• Ground Meristem: Cells in this region possess thin walls and are isodiametric in shape. Ground meristem contributes to the formation of the ground tissue system (G.T.S.). Various components of the ground tissue system, including hypodermis, general cortex, endodermis, pericycle, pith-rays (medullary rays), and pith (medulla), are formed through the activity of ground meristem.

Histogen Theory 

The theory proposed by Hanstein in 1870 provides a foundational understanding of root apex organization, delineating three distinct histogens within the apical meristem: dermatogen, periblem, and plerome.

• Dermatogen: The outermost histogen, composed of a single layer of cells. Its primary function is the formation of the uniseriate epidermis.
• Periblem: Situated just beneath the dermatogen, this region contributes to the formation of the cortex, including the hypodermis, general cortex, and endodermis.
• Plerome: Found at the innermost layer, responsible for the formation of the stele through cell division. It gives rise to structures such as the pericycle, vascular bundles, pith rays (medullary rays), and pith (medulla).

Note : While this theory correctly describes the organization of the root apex, it's important to note that it does not extend to the shoot apex of higher plants. In many gymnosperms and angiosperms, the shoot apex does not exhibit the differentiation into three distinct histogens as observed in the root apex.

Additionally, in monocotyledonous plants, a fourth histogen known as the calyptrogen is present within the root apex. The calyptrogen is responsible for the production of the root cap, a structure crucial for root protection and navigation through soil. In dicotyledonous plants, however, the root cap and epiblema/epidermis are produced by the dermatogen.

Tunica Corpus Theory 

Schmidt (1924) proposed a significant theory regarding the organization of the shoot apex in angiosperms, which remains widely acknowledged. This theory is primarily centered around the concept of planes of division. According to Schmidt's theory, the shoot apex of angiosperms comprises two distinct zones:

Tunica:

• The tunica constitutes the peripheral layer of the shoot apex. Within this layer, cells undergo anticlinal division exclusively in a single plane.
• Epidermal tissues originate from the cells of the tunica.
• Typically, the tunica is single-layered, although in certain instances, it may be multilayered. In such cases, the outermost layer forms the epidermis, while the remaining layers contribute to the formation of other types of tissues in conjunction with the corpus.

Corpus:

• The corpus refers to the mass of cells located beneath the tunica layer.
• Cells within the corpus exhibit division in all directions, leading to an increase in volume.
• Generally, cells within the corpus are larger compared to those in the tunica layer.
• The function of the corpus includes the formation of both the ground tissue system and the vascular tissue system or formation of cortex or stele.