Organisms & Populations

Plant Anatomy investigates the internal structural organization and tissue differentiation patterns that empower flowering plants (Angiosperms) to carry out water transport, mechanical support, and photosynthesis. In NCERT terminology, a tissue is defined as a cluster of cells sharing a common origin and performing a coordinated function.

1. Fundamental Classification of Plant Tissues

Based on whether the constituent cells possess the capacity to divide, plant tissues are categorized into two primary divisions:

• Meristematic Tissues (Active Division): Localized regions of continuous cellular proliferation. Based on their anatomical position:
  • Apical Meristems: Positioned at the tips of root and shoot axes; responsible for linear elongation and formation of primary plant body.
  • Intercalary Meristems: Localized between mature permanent tissues (e.g., at bases of grass internodes); regenerate plant parts removed by grazing herbivores. Both apical and intercalary meristems are primary meristems.
  • Lateral Meristems (Secondary Meristems): Cylindrical meristems occurring in mature regions of roots and shoots (e.g., Fascicular vascular cambium, Interfascicular cambium, and Cork cambium/Phellogen). They mediate secondary growth (girth increase).
• Permanent (Mature) Tissues: Cells derived from meristems that undergo structural and functional differentiation and lose the power of division. They are subdivided into:
  • Simple Tissues: Homogeneous cells of one type—Parenchyma (isodiametric, thin cellulosic walls, living, storage & photosynthesis), Collenchyma (living mechanical tissue with localized pectin/cellulose wall thickenings at corners, provides tensile flexibility to petiole/young stem without breaking), and Sclerenchyma (dead at maturity with thick lignified secondary walls, fibers & sclereids for rigid support).
  • Complex Tissues: Heterogeneous assemblies of multiple cell types working harmoniously—Xylem (Tracheids, Vessels, Xylem Fibres, Xylem Parenchyma) and Phloem (Sieve Tube Elements, Companion Cells, Phloem Parenchyma, Phloem Fibres). In gymnosperms, vessels are absent; sieve cells and albuminous cells substitute for sieve tubes and companion cells.

2. Critical Histological Structures & Diagnostic NCERT Pointers

Exam questions heavily test structural differences across plant axes:

• Casparian Strip: The innermost layer of root cortex, the endodermis, features tangential and radial walls deposited with water-impermeable waxy suberin. This blocks the apoplastic water pathway, compelling water and solutes to cross through the selective plasma membrane of endodermal passage cells into the vascular stele (symplast route).
• Stomatal Apparatus: Formed by the stomatal aperture, two specialized guard cells, and adjacent subsidiary cells. In dicots, guard cells are kidney-shaped/bean-shaped with thick inner walls and thin outer walls; in monocot grasses, guard cells are characteristically dumbbell-shaped. Stomatal opening is triggered by active $K^+$ ion accumulation and endosmosis resulting in guard cell turgidity.
• Xylem Arrangement: In stems, the primary xylem is endarch (protoxylem lies towards pith/center, metaxylem towards periphery); in roots, it is exarch (protoxylem directed towards periphery, metaxylem towards center).