Knowlet

Unit 1: Meristematic and Permanent Tissues

Table of Contents

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:

  1. 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).
  2. 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.
[Image of longitudinal section of shoot apical meristem]

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).
[Image of parenchyma collenchyma sclerenchyma diagram]

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).

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).
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