The tissues in which the cells are undifferentiated and capable of division are called meristem.  The activity of meristem Causes growth in plant. Meristem adds new cells continuously. The apices of roots, shoots and their lateral branches have meristems at their tips. It increases length or the respective axis. Other meristems are found in the mature portions of the plant body. These men stems increase in thickness of the respective parts. The activity of the meristems is not uniform in all parts of the plant.

The entire plant body is capable of growing in lower plants. But the entire plant body is not capable of growing in higher plants. Their growth is limited to certain regions known called growing points. The growing points consist of groups of cells which are capable of division. These growing points are formed of meristem. These meristematic cells are located in the stem and root.

Structure of meristematic cells

The cells in the meristem are thin walled. They are densely filled with granular contents. They are without vacuoles and isodiametric in shapes. There are two types of cells in meristem

(a) Initiating cells: These cells retain the meristematic activity

throughout the Growth of the plant.

(b) Derivative cells: These cells derive form meristems. They mature into different types of cells. Some differentiating cells may retain the potential meristematic power at maturity like parenchyma. But other loses the potential of meristematic activity permanently. They change into mature cells like sieve tube elements, tracheal elements and selerenchyma.

Classification of meristem

There are  ^Following  types of meristems:

1. Apical Meristems: The meristems present at the tips of roots and shoot are called apical meristems. Apical meristems are involved in the extension of plant body. the perpetual growls zones are present at the-apices of roots and stems. They increase the number of cells at the tips of roots and stem. Thus they play important role in primary growth. The cells derived from apical meristems are grouped into three types:

• Protoderm: This is the surface layer . Its cells are differentiated into epidermal systems.

• Procambium: They are also called provascular tissues. Their cells form different parts of vascular systems at ma urity.
• Ground meristem: These cells of these tissues form differ of ground tissues at maturity.

2. Intercalary MtriAems: The meristem situated at the bases of internodes is ailed intercalary meristem. These are the parts of apical meri.,tem. They get separated from apex meristem by permanent tissues. They play important role in the production of leaves and flower. These are of temporary nature.
3. Lateral Meristems: The cylinders of dividing cells present in the vascular and cork tissue of the plants are called lateral meristems. Lateral meristems are present in clients and gymnosperms. Vascular and cork cambium are the example of lateral meristem. They play an important role in the increase in diameter of stem and root. So they are involved in secondary growth.

Patterns of Divisions

The divisions of the meristematic cells are classified into the hallowing types on the basis of the plane of division.

(a)     Periclinal: In this case, cell division is parallel to the axis These divisions are also called tangential.

(b)       Anticlinal: In this case. cell division occurs at right Angle to the
surface. The division is called radial. when it i- parallel to the radius of the axis. It is called transverse when it is parallel to the transverse plane of axis.

SHOOT APEX

Shoot apex has apical meristems. The apical meristems of the shoot is more complex than The Root The Activity of the Shoot apical meristern increases the length of the plant body It also forms leases and lateral branches. Shoot apex has i‘‘ o tspes of arrangements:

(a ) Lower Vascular Plants The Apical meristem of the Shoot is more  Complex the the Shoot The Activity of The Shoot Apical Meristem Complex Then the Root The Activity of the Shoot Apcial meristem increases the length of the plant body It is also froms leaves and lateral branches Shoot apex has two Type of arrangements

(b) Higher Vascular Plants The Shoot Apex consists of a Set of meristematic cells in gymnosperms and angiosperms. These cells differentiate into different zones on the basis of their mcristematic activity

Organization of cells in apical meristem

There arc two concepts about the organization of cells in the .apical meristems.

1. Histogen theory: This theory was put forward by Hanstein. According to this theory the apical meristematic tissues arc divided into three Zones

(a) Dermatogens:  This zone consist the cells of the surface layer. It gives rise to the epidermis.

(b) Periblem: This zone comprise; the cells of the outer region Iaying under the surface laver. They give rise to the cortex or outer portion of the stem.

(c) Plerome: This zone comprises the central mass of cells. It gives rise to the vascular system and the pith

2. Tunica-Corpus theory This theory was  Given  by  Schmidt  Histogen theory was discarded due to lack of cytological proof Therefore, Tunica corpus theory was put forward. According to this concept, the dividing cells in the apical meristem are arranged in two zones:

Tunica: The outer zone is called tunica. It is composed of one or two layers of superficial cells. The cells of tunica divide anticlinally. It forms two layer thicknesses. But single layered tunica is present in monocot plants. Dicots have two layered arnica. The epidermis arise form the outer layer of Tunica

Corpus: The central zone is called corpus. The mass of corpus cells is covered by arnica cells. The cells of corpus divide in all planes. Inner tissues like cortex and vascular tissues are derived from both tunica and corpus.

3. Histogenic layer theory: This theory “as put forward by Dermen in I 947 He proposed the concept of primary histogenic layers. According to him. there are three basic histogenic layers  in  angiosperms He named then as L-I-L-II.L-III  gives rise to vascular Tissues and Pith

4. Cyto histological zonation theory: This theory was proposed by Foster. He divides ides the apex into zones on the basis of planes of divisions, size of cell and nucleus, differential staining and orientation of cell division
5. Leaf PrimordiaThe leaf primordia originate in the peripheral part of the apical meristem. The leaf primordia are formed by the periclinal divisions in the restricted cells of the second layer of tunica. These divisions produce protuberances. These protuberances are . called leaf primordia. Further divisions increase the thickness of protuberance. Leaf primordia may be !brined from tuniea or corpus layer depending on their thickness. In monocot, leaf primordia are formed from arnica. In dicot, they are formed from both tunica and corpusLateral BranchesAxillary bud or bud primordium is formed some distance from the apical meristem. First division of lateral branch occurs in the second leaf thrill the apex. This diyision is periclinal. It is followed by periclinal and anticlinal division. Thus a group of distinct meristematic cells are formed. The cells above these meristematic cells become vacuolated and get mature. It separates the bud primordia from the apical Meristematic cells. The newly formed cells of bud primordium are arranged like apical meristem. The bud primordium develops traces. These traces connect the central vascular cylinder with the bud primordia. The differentiation of traces continues from primordium to central cylinder. The bud continues to grow by the activity of its meristematic cells and form lateral branchROOT APEXRoot apex grows by the activity of apical meristem of root. The roots grow in the soil. Therefore. their meristetnatic cells require proper protection from injury. Root cap provides this protection. The organization of the meristematic cells in the root apex has different pattern than the stern. Root does not produce nodes and internodes. There the activity of the root meristem remains uniform there may be single root initial as in many lower vascular plants. Or there may be group of initials as in most of angiosperms and gymnospr Ins. The initials of root have no organized pattern. ‘Therefore. stem like organization is absent in root apex. Cells are grouped only on the basis of their position. Root initials divides to form protomeristem. The initial retain their position on root apex. ‘They form a zone called quiscent zone. The cells immediately present behind the apical meristem are called protomeristem Protomeristem cells are differentiated into following types of cells type of pattern.2. Close type root apex: In this case, the initials are predestined to form different structures in root. The initials are plerome, periblem, calyptrogen and dermatocalyptrogen. Plerome gi‘e rise to central cylinder. Periblem give rise to cortex and calyptrogen give rise to root cap. Dermatocalyptrogen give rise to epidermis