Short essay on the classification of Meristems

I. Meristems based on the stage or method of development: Promeristem:

It is the first formed meristem representing the foundation stage of formation of new developing organ.

It consists of young initials and their derivatives. It is present at the root and stem apices. It is also known as primordial meristem or embryonic meristem.

II. Meristems based on origin:

On the basis of origin, meristems are of two types: Primary meristem and Secondary meristem

Primary meristem: It is derived directly from promeristem. It builds up the primary part of the plant body. It occurs at the apices of stem, roots and primordia of leaves etc. It continues to divide and the derivatives differentiate into permanent tissues.

Secondary meristem: It develops from primary permanent tissues which regain the power of division i.e. it becomes meristematic at a later stage.

The secondary meristems add new cells to the primary body effecting secondary growth in thickness. Cork combium, inter fascicular cambium, accessory combium etc. are examples of secondary meristem.

III. Meristems based on position:

On the basis of position in the plant body, meristems are classified as apical meristem, intercalary meristem and lateral meristem.

Apical meristem: The meristem that occurs at the apices of the root, stem and leaves is known as apical meristem.

It comprises either of a single apical cell or group of apical cells. The activity of this meristem results in longitudinal growth (height) of the plant. Apical meristem includes promeristem and primary meristem.

The apical meristem exhibits a dome shaped structure with clear demarcation in outer layers (Tunica) and the inner mass (corpus).

The apical meristem is composed of a small mass of cells which are alike and constitute the promeristem. The cells of the promeristem differentiate into three regions viz. dermatogen, periblem and plerome which grow and give rise to primary permanent tissues.

(i) Dermatogen:

This is the single outer-most layer of cells. These divide and give rise to the epidermis of the stem. In roots the cells of the dermatogen form a mas of small tissues called calyptrogen. The calyptrogen is also meristematic and give rise to root cap. In dicot root dermatogen forms epiblema.

(ii) Periblem:

This is located internal to the dermatogen and forms the cortex of the stem and root, (ii) (II) Plerome:

This lies internal to the periblem. It is the central region where cells show a tendency to elongate. These elongated cells form pro-combium that gives rise to the vascular tissues (xylem and phloem) constituting the central cylinder or stele q the stem.

Intercalary meristem:

It is placed between two permanent tissues. Actually it is a part of the apical meristem which gets separated from the apex during growth. Such meristems are found in monocot stems and leaf sheaths. They help in increasing the length of the organ.

Lateral meristem:

These meristems occur laterally on roots and stems. The cells are rectangular and always divide in tangential plane. The cork cambium and fascicular combium are examples of lateral meristems. The new cells formed by division differentiate into secondary tissues. These tissues lead to increase in thickness.

IV. Meristem based on function:

Meristem may be of three types on the basis of physiological aspects: protoderm, pro-combium and fundamental or ground meristem. The primary meristem at the apex of the axis is gradually differentiated into the following zones.

Protoderm:

It is the outermost layer which by radial division gives rise to a single layered epidermis. Sometimes by tangential division, it forms multiple epidermises.

Procambium:

Elongated tapering cells of the growing region constitute the procambium. It produces primary varcular tissues i.e. xylem, phloem and combium. Procambium strands are scattered or occur in a ring, in monocot stems and dicot stems respectively.

Fundamental or Ground Meristem:

The remainder of the meristematic tissues lying in between protoderm and procambium constitutes fundamental or ground meristem. The cells of this region are large, thin walled and isodiametric. It forms hypodermis, cortex, endodermis, pericycle, pith and medullary rays.

V. Meristems based on plane of division:

There are three types of meristem based on the plane of division.

Mass or block meristem:

The cells of this meristem divide in all planes to form an irregular mass. Hence it leads to formation of structures like endosperm, young pith and cortex etc.

Plate meristem:

The cells divide in two planes (Periclinal followed by several anticlinal divisions) and thus form plate-like structures. Plate meristem results in the formation of uniseriate epidermis and multi seriate leaf blade.

Rib or file meristem:

Cells of this meristem divide anticlinally to form a column of cells. It results in increasing the length of various organs. Cortex and pith in stem are formed from this meristem. _t

VI. Special type: Wound meristem.

These are the surrounding tissues of wound. They multiply to heal up the wound.

Induced meristem:

They are developed at the cutting regions of plant body due to the application of hormones (Auxin). They help in initiation of root primordia as seen in rose plants.

Hypertrophy:

This type of meristem occurs in diseased tissue which multiplies abnormally resulting in formation of gull.

Peristemoid:

This type of meristem is represented by stomata initial, archegonial initial, hair initial etc.