Anatomy of Flowering Plants

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).