Anatomy of Flowering Plants

A tissue is a group of cells with a common origin and similar function. Plant tissues are of two types:

Meristematic tissues — actively dividing; found at apices (apical meristem), nodes (intercalary meristem), and lateral surfaces (lateral meristem — vascular cambium and cork cambium).

Permanent tissues — derived from meristematic tissues; no longer divide.

• Simple permanent tissues:
  — Parenchyma — thin-walled, isodiametric; stores food and water; performs photosynthesis in mesophyll.
  — Collenchyma — unevenly thickened walls; provides mechanical support to young stems; e.g., hypodermis of dicot stems.
  — Sclerenchyma — dead cells with uniformly thick, lignified walls; fibres and sclereids; e.g., stone cells in coconut shell, pear.
• Complex permanent tissues:
  — Xylem — tracheids, vessels (tracheae), xylem parenchyma, xylem fibres; conducts water and minerals upward.
  — Phloem — sieve tubes, companion cells, phloem parenchyma, phloem fibres; conducts food (sugars) bidirectionally.

From the outside inward, root anatomy has: Epidermis → Cortex → Endodermis → Pericycle → Vascular bundles → Pith.

Endodermis has Casparian strips (suberin deposits on radial and transverse walls) — regulates water entry into the vascular cylinder.

Stem has: Epidermis → Hypodermis → Cortex → Endodermis → Pericycle → Vascular bundles → Pith.

Stomata consist of two guard cells (kidney-shaped in dicots, dumbbell-shaped in monocots) surrounding the stomatal pore. Guard cells have chloroplasts and regulate opening/closing by changes in turgor.

Secondary growth increases girth in dicot stems and roots, mediated by two lateral meristems:

Vascular cambium — forms between xylem and phloem; produces secondary xylem (wood) toward the inside and secondary phloem toward the outside:

• Spring wood (early wood) — formed in spring; larger vessels; less dense; light coloured.
• Autumn wood (late wood) — formed in autumn; smaller vessels; dense; dark coloured.
• Spring + Autumn wood together = one annual ring; used to determine the age of a tree.
• Heartwood (duramen) — old, non-functional, dark-coloured, innermost xylem; contains tannins, resins.
• Sapwood (alburnum) — outer functional xylem; conducts water.

Cork cambium (phellogen) — arises in the cortex; produces cork (phellem) outward and secondary cortex (phelloderm) inward. Cork is impermeable to water. Bark = everything external to vascular cambium (secondary phloem + periderm).