Syllabus Reference: Characteristics and Classification of Meristematic Tissues; Root and shoot apical meristems; [cite_start]Simple and complex tissues, Organs: Structure and function of dicot and monocot root, stem and leaf [cite: 547-549].
1. Meristematic Tissues
Definition: A group of immature cells that are in a continuous state of division or retain the power of division.
Characteristics of Meristems:
- Cells are small, isodiametric, and thin-walled.
- Dense cytoplasm with large prominent nuclei.
- Vacuoles are absent or very small.
- Intercellular spaces are absent (compactly arranged).
- High metabolic rate.
Classification of Meristems:
- Based on Position:
- Apical Meristem: At tips of roots and shoots. Responsible for linear growth (primary growth).
- Intercalary Meristem: At bases of leaves or internodes (e.g., Grasses). Responsible for elongation of internodes.
- Lateral Meristem: On sides (e.g., Cambium). Responsible for increase in girth (secondary growth).
- Based on Origin:
- Promeristem: The very first embryonic meristem.
- Primary Meristem: Derived from promeristem (e.g., Apical meristem).
- Secondary Meristem: Derived from permanent tissues by dedifferentiation (e.g., Cork cambium).
Shoot & Root Apical Meristem Theories:
- Tunica-Corpus Theory (Schmidt): For Shoot Apex.
Tunica: Outer layer(s), divides anticlinally (increases surface area).
Corpus: Inner mass, divides in all planes (increases volume). - Quiescent Centre Theory (Clowes): For Root Apex.
A region of low mitotic activity (Quiescent Centre) exists at the tip of the root, acting as a reservoir of cells.
2. Permanent Tissues
Cells derived from meristems that have lost the ability to divide and have attained a specific form and function.
A. Simple Permanent Tissues (One type of cell)
| Type | Structure | Function |
|---|---|---|
| Parenchyma | Living, thin-walled (cellulose), isodiametric, intercellular spaces present. | Storage, Photosynthesis (Chlorenchyma), Buoyancy (Aerenchyma). |
| Collenchyma | Living, thickened at corners (pectin + cellulose), no spaces. | Mechanical support to young stems/petioles (flexible strength). |
| Sclerenchyma | Dead at maturity, thick lignified walls. Two types: Fibers and Sclereids. | Mechanical strength, protection (e.g., seed coats, grit in pear). |
B. Complex Permanent Tissues (More than one type of cell)
1. Xylem (Water conducting):
- Tracheids: Dead, elongated, tapering ends. Main element in Gymnosperms.
- Vessels: Dead, tube-like, perforated end walls. Main element in Angiosperms.
- Xylem Fibers: Support.
- Xylem Parenchyma: Living, storage.
2. Phloem (Food conducting):
- Sieve Tubes: Living (but nucleus absent at maturity), perforated sieve plates.
- Companion Cells: Living, nucleated, control sieve tubes. (Absent in Gymnosperms).
- Phloem Parenchyma: Storage.
- Phloem Fibers: Mechanical support (Bast fibers).
3. Anatomy of Organs
Exam Tip: Always distinguish between Monocot and Dicot anatomy using the "Vascular Bundle" arrangement.
Root: Radial bundles (Xylem/Phloem separate).
Stem: Conjoint bundles (Xylem/Phloem together).
Root: Radial bundles (Xylem/Phloem separate).
Stem: Conjoint bundles (Xylem/Phloem together).
A. Root Anatomy
- Dicot Root:
- Epiblema (with root hairs), Cortex, Endodermis (Casparian strips).
- Vascular Bundles: Radial, Exarch (Protoxylem out, Metaxylem in). Number: Diarch to Hexarch (2-6).
- Pith: Small or absent.
- Monocot Root:
- Similar structure but Polyarch (many bundles, >6).
- Pith: Large and well-developed.
B. Stem Anatomy
- Dicot Stem:
- Vascular Bundles arranged in a Ring.
- Bundles: Conjoint, Collateral, Open (Cambium present).
- Hypodermis: Collenchymatous.
- Monocot Stem:
- Vascular Bundles Scattered (Atactostele).
- Bundles: Conjoint, Collateral, Closed (No Cambium).
- Bundle Sheath: Sclerenchymatous sheath surrounds bundles.
- Hypodermis: Sclerenchymatous.
C. Leaf Anatomy
- Dicot Leaf (Dorsiventral): Differentiated Mesophyll (Palisade parenchyma on upper side, Spongy parenchyma on lower). Stomata mostly on lower epidermis.
- Monocot Leaf (Isobilateral): Undifferentiated Mesophyll. Stomata on both surfaces. Bulliform cells present (for rolling/unrolling).