Apical Meristem | Definition, Theories, Functions

Apical Meristem is a particular area of plant cell present at the tip or apex of different parts of the plant-like root, shoot, bud, stem, and leaf. This apical tissue is responsible for division and growth in plant cells. The main function of apical meristem is to increase the length of the plant. It is defined according to the location of the apical meristem. Apical meristem grows the entire life span because these cells go under unlimited division. It is an important type of Metsitematic Tissue.

These are present at the growing points or apices of stems, roots, and branches. The activity of the apical meristems adds to the length of plants or their parts. Apical meristems have two distinct zones. There are

  1. Promeristem zone : It consists of a group of dividng cells called the apical initials.
  2. Meristematic zone: It is further distinguished into three meristems
    • Protoderm: It is the outer layer which give rise to the epidermal tissue system (epiderms).
    • Procambium: It is the layer below protoderm wihch gives rise to primary vacular tissue.
    • Ground Meristem: It gives rise to cortex and pith.

The terms shoot apex and root apex are more convenient to use instead of apical meristem of root and apical meristem of root respectively.

Theroies of Apical Meristem

There are different theories regarding the structure, development, and differentiation of apical meristem such as:

1. Apical Cell Theory

Apical cell theory was put forward by Holfmmeister in 1857 and later supported by Nageli in 1878. According to this theory, a single superficial apical cell continuously divides to form the primary body of the plant. Apical cell theory was applicable to lower plants only.

2. The Histogen Theory ( Hanstein, 1870)

According to this theory, the apex of the stem and root have small masses of similar actively dividing cells called histogens which later differentiated into dermatogen, periblem, and plerome. In monocot roots, there are four histogens: dermatogen, periblem, plerome, and calyptrogen ( future root cap). This theory is applicable for the differentiation of shoot apex and root apex.

  • Dermatogen: It is the meristem which later gives rise to epidermis of stem. In dicot root, in later gives rise to epiblema and root cap but in monocot root, it later gices rise to only epiblema.
  • Periblem: It is the middle region of meristem which later gives rise ot hypodermis, cortex and endodermis.
  • Plerome: It is the central region of meriste which later gices rise to central stele consisting of pericycle, vascular tissue, medullary rays and pith.

3. Tunica Corpus Theory ( Schmidt, 1924)

Tunica corpus theory was purposed by Schmidt in 1924 and later supported by Foster in 1949. According to this theory, there are only two growth zones viz the outer tunica and the central corpus. The shoot apex is dome-shaped in outline and protected by young leaf (leaf primordia).

The tunica consists of the outer join one or more layers of cells. These are the form of sheets forming the lateral limits and the roof of the apical portion. These cells are small in size and divide in one plane only i.e. anticlinallly ( against the slope ).

The Corpus is multilayered. It forms the central portion of the apex. It is surrounded by the tunica and its cells are larger in size. The cells of the Corpus divide into all planes ( anticlinally, periclinally, and irregularly ). This theory is applicable for the differentiation of shoot apex.

Functions of Apical Meristem

  1. It’s main purpose is to divide the cells continouslly.
  2. It generates new cells in plants.
  3. It is responsible for growth of a plant.
  4. It helps in photosysthesis and exchange built up gases.
  5. It sense conditions of soil around the root.
  6. It passes nutrients and other minerals to other parts to upper part of plant.
  7. It regenrates cells lost to abrasion.

Types of Apical Meristem

There are two types of apical meristem according to their location and functions. One is shot apical meristem i.e. Shoot Apex and another is Root Apex i.e. Root Apical Meristem.

1. Shoot Apex ( Shoot Apical Meristem )

According to the Tunica-Corpus Theory of Schmidt (1924), leaf primordial protect the dome-shaped shoot apex which can be differentiated into outer mantle like tunica and inner cellular mass called corpus.

  1. Tunica: it is an outer zone which gives rise to epidermis or protoderm. The cells are small. Cell division occurs at right angle to the surface of apex ( anticilinal) finally giving rise to surface growth.
  2. Corpus: It is a lower multilayered zone which finally gices rise to procambium and ground meristem. the cells are comparatively larger. They divide in different planes. Procambium forms vascular tissue and ground meristem gices rise to ground tissue.

According to Histogen Theory of Hantein (1870), the shoot apex is differentiated into three regions

  • Dermatogen: It is the single outermost layer of the cells which gices rise to epidrmis of stem.
  • Periblem: It is the region internal to dermatogens which gices rise to cortex and endodermis.
  • Plerome: It is eht central meristematic region of stem apex. It lies internal to the periblem. It gices rises to pericycle, vascular bundles, medulary rays and pith.

Haberlandt (1914) proposed the nomenclature of protoderm for dermatogen, ground meristem for periblemand procambium for pleurome.

2. Root Apex ( Root Apical Meristem)

It is found at the tip of main root and its branches. It is covered by protective root capl it is further differentiated into fiver regions:

  • Protoderm: It forms epidermis or epiblema and root cap in dicots.
  • Calyptrogen: This region is found in monocots and is responsible for the formation of root cap.
  • Procambium: It gives rise to vascular tissues.
  • Ground Meristem: It takes part in the formation of pith, cortex endodermis and pericycle.
  • Quiescent Centre: IN this region, cells divide at a very slow rate. It occupies central part of root apex.

Hanstein in Hostogen Theory gave the idea that as in shoot apex, root apex has also three histogens:

  • Dermatogen: It forms epiblema and root cap in dicots.
  • Periblem: It forms cortex and endodermis.
  • Plerome: It forms pericycle vascular tissues, pith, etc.

This is all about Apical Meristem. In this article we described Introduction, Types, Functions and theory of Apical Meristem. You can learn more about Shoot Apex and Root Apex in another article. If you have any confusions related to this topic then feel free to comment down. If you think there is a lot to write for this topic then mail us uncovered section of this topic.

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