Introduction to Meristematic tissue:
The type of plant tissue which is composed of immature cells and are always in a state of active cell division producing new daughter cells continuously is called meristematic tissue or meristem. They are found at the growing tips of roots, stems and leaves. Cells of meristematic tissue helps to increase the number of cells whereby the overall plant growth occurs.
Structure of Meristematic tissue
- Cells look isodiametric, oval or polygonal.
- Cells compactly arranged without intercellular spaces.
- Cells filled up with dense granular cytoplasm and mostly without vacuoles.
- Each cell with a single prominent nucleus.
- Cell wall thin, homogeneous and made of cellulose.
Classification of Meristematic tissue
(I) Classification according to origin:
(A) Primary meristematic tissue
• Remains meristematic from the embryonic condition throughout entire plant life at the growing apices of roots, stems, primordial of leaves.
• Vascular cambium is also primary in nature.
(B) Secondary meristematic tissue
• Gradually differentiated from primary meristems.
• Examples include phellogen or cork cambium and interfascicular cambium.
(II) Classification according to position:
(A) Apical meristematic tissue
• Present at the apices of stems and roots of vascular plants forming the growing points.
• Includes primary meristem.
(B) Intercalary meristematic tissue
• Merely parts of apical meristems that have become separated from the apex and get get surrounded by permanent tissues.
• Main function is increase in length of the axis.
• Found in pulvini of grasses and leaf blades near the basal parts.
(C) Lateral meristematic tissue
• Present laterally or parallel to the sides of root and stem.
• Main role is to increase in diameter of plant.
• Cambium and cork cambium are examples.
(III) Classification according to function:
(A) Protoderm-Developing into epidermis.
(B) Procambrium- Developing into vascular tissue.
(C) Fundamental or ground meristem- Developing into ground tissue and pith.
(IV) Classification according to plane of cell division:
(A) Mass meristem
• Cell division occurs in all planes.
• Results in increase in volume.
• Example-growth of young embryos, endosperm.
(B) Plate meristem
• Cell division occurs in two planes.
• Results in increase of plate like area.
• Example-growth of flat leaf blade, one layered epidermis.
(C) Rib or file meristem
• Cell division occurs in one plane producing rows of cells.
• Results in increase in length of plant organs.
• Example-formation of cortex and pith of young stems and roots.