Unit 1: Meristematic and Permanent Tissues

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).