Meristem and its Types | plant anatomy

Meristem and its Types

The Tissues: 

A Tissue is a group of cells that are alike in origin, structure and function. The study of tissue is called Histology. A plant is made up of different types of tissues.

There are two principal groups:

1. Meristematic tissues

2. Permanent tissues

Meristematic tissues:

                                                                     The word meristem (Greek: Meristos= divisible) has been coined by Nageli (1858). A meristematic tissue is a group of cells that are in a continuous state of division or retain their power of division. In plants the growth is not diffused as in animals. It is restricted to only certain specific regions. They are called as meristematic regions in which meristematic tissues are located. The meristematic tissues divide and redivide and give rise to different organs of plant body.

Characters of Meristematic Cells

  • The cells may be rounded, oval, polygonal or rectangular in shape.
  • They are arranged very closely without any inter cellular spaces.
  • Their cell wall is thin, elastic and essentially made up of cellulose.
  • They have generally dense cytoplasm with prominent nucleus.
  • The vacuoles may be very few and small or sometimes may be absent.
  • They do not store reserve food material.
  • The cells may divide in a particular plane or in all directions depending upon the position of the meristem in plant body.
  • The cells undergo rapid and repeated divisions and produce new tissues, hence it is also called formative tissue.

Meristems and growth of the plant body:

                                                                                                                                The plant begins its life from single diploid zygote. After successive systematic divisions it gives rise to an embryo. In young embryo all the cells have capacity to divide. During maturation of embryo, cell differentiation begins. Thus we find different sets of initials that give rise to root system and shoot systern of seedling. The young seedling is an independent plant. It is basically composed of embryonic tissues and adult tissues. The embryonic tissues persist in the plant throughout its life. They are confined to apices of plant body, hence are called as apical meristems (Primary meristems).

The meristematic cells divide continuously and add new cells to the seedling. These cells differentiate into different mature tissues of the primary plant body. After certain period of elongation, in 8ymnosperms and dicots the plant body increases in its diameter by the activity of newly formed vascular cambium & cork cambium. They are called secondary (lateral in position) meristems. The secondary meristems divide and give rise to secondary permanent tissues. After occurrence of secondary growth, the plant body is called secondary plant body.

Classification of Meristem

                                                                           Meristem has been classified into several types on the basis of position, origin, function and division.

A) Based on origin and stage of development:

                                                                                                                 On the basis of their origin and development the meristems are classified as:

(a) Promeristem

b) Primary meristem and   

(c) Secondary meristem

Promeristem:

                                  A group of young and undifferentiated, actively dividing apical initials derived from the tissue of the embryo is called Promeristem. It is also called as embryonic meristem or primordial meristem. It consists of a few apical initials. t is present at the extreme tip of a young stem. The cells are isodiametric. They have thin primary walls with early stages of pits. They have dense and active protoplasm. The promeristem lays down the foundation of different organs of primary plant body in early stage of growth and development. These cells divide and give rise to a meristematic zone below it, called primary meristem.

Primary Meristem:

                                              The meristems which are derived from the embryonic meristems are known as primary meristems. The primary meristems are concerned with the formation of primary permanent tissues of primary plant body. In early stage of growth, different organs of the plant body are initiated by promeristem. Further growth and differentiation of various tissues and organization takes place by the activity of primary meristems. Shoot and root apical meristems, intercalary meristems and leaf primordia are examples of primary meristems. The cambium strips of vascular bundles are also primary in origin.

Secondary Meristem:

                                  Secondary meristems are not present in primary plant body. As the name indicates the meristems which are developed, after certain period of vegetative growth of the plant body, from the permanent tissues at the time of secondary growth are called as secondary meristems. In gymnosperms and dicots, the secondary growth takes place by the activity of lateral meristems (Secondary meristems) such as vascular cambium (in stelar region) and cork cambium (in extra stelar region).

In vascular cambium, intra fascicular cambium is primary in origin where as inter fascicular cambium is secondary in origin. Inter fascicular cambium develops from medullary ray cells. The cork cambium either originates from epidermis or cortex (ex: – Stem) or from pericycle (Eg: – root). Secondary meristems add secondary permanent tissues to the primary plant body and give protection and repair wounded regions.

B) Based on position in plant body:

                                                                                                              Depending upon the position in the plant body, the meristems are classified into three distinct types.

Apical Meristems:

                                           The meristems which are present universally at the apices of stem, branches and roots are called apical meristems. It is responsible for increase in the length of the plant, it is called as primary growth. They take part in the formation and elongation of plant organs. In higher plants apical meristem consists of group of cells, where as in cryptogams single cell forms apical meristem. The apical meristem is distinguished into two zones, upper promeristem and lower primary meristem.

Intercalary Meristems:

                                                              If the meristematic tissues are intercalated in between permanent tissues they are called intercalary meristems. It is believed that little portions of apical meristems get detached and remain embedded in between permanent tissues. They either present at the base of internodes, or at the base of node or at the base of leaf and leaf sheath (Eg: Equisetum, Pinus & grasses). The inter-nodal regions of stem increase by the activity of intercalary meristems. They are short lived and are converted into permanent tissues. The stem part becomes very weak due to the presence of intercalary meristem.

 Lateral Meristems:

                            As the name indicates these are present along the sides of plant body. The vascular cambium and cork-cambium are examples of lateral meristems. As they differentiate from primary permanent tissues at the time of secondary growth, they are called secondary meristems. The meristematic cells divide in periclinal plane only. They produce secondary permanent tissues. These meristems are responsible for the increase of girth of the plant body. Example: vascular cambium and cork cambium.



Figure 1: Different types of meristems on the basis of position in plant body

C) Based on function:

                                                                    Based on function the apical meristem is distinguished into three zones. These zones have distinct functions and give rise to various kinds of primary tissues of the plant body. The zones are named as protoderm, ground meristem and procambium.

Protoderm:

                   It is the outermost single layered structure. The cells always divide in anticlinal plane. This layer produces outer protective covering of plant body known as epidermal tissue system.

Ground Meristem:

                             It lies beneath the protoderm. It consists of several layers of meristematic cells. These cells divide in all planes and give rise to ground tissue system (cortex, endodermis, pericycle, medullary rays and pith). Or. It gives rise to all tissues except epidermis and vascular strands.

Procambium:

                                 Thin walled elongated procambial strands appear in groups in ground meristem near the central region at an early stage of differentiation. These strands give rise to vascular tissue system. Example: xylem and phloem.

D) Based on plane of cell division:

                                               Number of meristematic cells and their plane of division varies from group to group in plant kingdom. The plane of cell division is also different depending upon the position within the plant body. According to plane of cell division three kinds of meristems are recognized.

Rib meristem or File meristem:

                                                                            In this type of meristem, the cell division is in one plane. For example: development of cortex and pith. The cells divide and produce row of cells. This type of meristem is found mainly in filamentous Algae. It also plays an important role in the formation of young roots, cortex and pith of young stems in higher plants.

Plate Meristem:

                                      In this type of meristem, the cells divide in two planes. The surface area of an organ is increased. This type of meristem takes part in the formation of flat structures such as leaf formation without increasing thickness. For Example, growth shown by flat blades of angiosperm leaves.

The Plate meristem and Rib meristem are growth forms that occur in the ground meristem mainly.

Mass Meristem:

                           In mass meristem the cells divide in all planes to form bulk of tissue. This type of meristem takes part in the formation of cortex, pith, endosperm and sporangia.

 

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