Anatomy of Flowering Plants

🎯 Key Points

• Three tissue systems: epidermal (outer protection), ground (bulk filling — parenchyma/collenchyma/sclerenchyma), vascular (xylem + phloem for transport)
• Meristematic tissues: apical (root/shoot tip elongation), lateral (girth — vascular + cork cambium), intercalary (base of internodes in monocots)
• Simple permanent tissues: parenchyma (storage/photosynthesis), collenchyma (support, living), sclerenchyma (fibres + sclereids, dead, lignified)
• Xylem: tracheids + vessels (water conduction) + xylem fibres + xylem parenchyma
• Phloem: sieve tubes + companion cells (food transport) + phloem fibres + phloem parenchyma
• Dicot root: tetrarch/hexarch exarch xylem, monocot root: polyarch xylem (>6 bundles), Casparian strip in endodermis
• Dicot stem: vascular bundles in ring, open (cambium present), distinct cortex/pith; Monocot stem: scattered bundles, closed (no cambium), hypodermis sclerenchymatous
• Secondary growth: vascular cambium → secondary xylem inward (wood) + secondary phloem outward; cork cambium → bark
• Annual rings: one per year; heartwood (dark, old, tyloses) vs sapwood (light, functional)

Meristematic Tissues

• Apical: tips of root and shoot; responsible for primary growth (lengthening)
• Lateral: vascular cambium (wood/bark) + phellogen/cork cambium (bark); secondary growth in dicots
• Intercalary: at base of internodes and leaf bases; regrowth after grazing in grasses

Simple Permanent Tissues

• Parenchyma: living, thin-walled, isodiametric; storage, photosynthesis (chlorenchyma), gas exchange (aerenchyma)
• Collenchyma: living, unevenly thickened at corners; mechanical support in young dicot stems and petioles
• Sclerenchyma: dead, heavily lignified; fibres (long, narrow) and sclereids/stone cells (varied shapes); coconut husk fibres, pear grit

Complex Tissues: Xylem & Phloem

• Xylem: Tracheids (gymnosperm + angiosperm; pitted, dead); Vessels (angiosperm; perforation plates, more efficient); Xylem parenchyma (living, stores starch); Xylem fibres (support)
• Phloem: Sieve tube elements (angiosperm; enucleate at maturity, have sieve plates); Companion cells (nucleated, metabolically support sieve tubes); Phloem parenchyma; Phloem fibres (bast fibres — jute, flax)
• Gymnosperm phloem: lacks companion cells, has albuminous cells instead

Anatomy of Root, Stem, Leaf

• Dicot root layers (outside→in): epidermis → cortex → endodermis (Casparian strip) → pericycle → xylem (exarch, tetrarch) and phloem alternating
• Monocot root: same basic plan but polyarch (many xylem bundles), large pith, no secondary growth
• Dicot stem: epidermis → hypodermis (collenchyma) → cortex → endodermis → pericycle → vascular bundles in ring (open, conjoint collateral) → pith
• Monocot stem: epidermis → hypodermis (sclerenchyma) → ground tissue → scattered conjoint closed vascular bundles → no distinct cortex/pith
• Dorsiventral (dicot) leaf: distinct palisade (upper) and spongy mesophyll; isobilateral (monocot) leaf: similar on both sides

The Three Tissue Systems

• Epidermal tissue system: the outermost covering — epidermis (usually one cell thick), a waxy cuticle on aerial parts that checks water loss, stomata (guard cells regulating gas exchange and transpiration), and epidermal outgrowths: multicellular trichomes on the shoot (often secretory, reduce water loss) and unicellular root hairs that absorb water and minerals
• Ground tissue system: all tissues except the epidermis and vascular bundles — cortex, endodermis, pericycle, pith, and medullary rays; mostly parenchyma, with collenchyma and sclerenchyma for support and storage
• Vascular tissue system: the xylem and phloem, organised into vascular bundles that are conjoint (xylem and phloem on the same radius) in most angiosperms, and either open (cambium present, secondary growth possible) or closed (no cambium)

Dicot Stem vs Monocot Stem

Dicot Root vs Monocot Root

• Common to both: epidermis (epiblema, bearing root hairs) → cortex → endodermis with Casparian strips → pericycle (origin of lateral roots) → radial vascular bundles (xylem and phloem on separate radii)

Secondary Growth in the Dicot Stem

• The vascular cambium becomes a complete ring: fascicular cambium within the bundles joins the interfascicular cambium formed from medullary ray cells; it cuts off secondary xylem (wood) inward and secondary phloem outward, with far more xylem produced than phloem
• Spring/early wood has wide vessels (favourable season) and autumn/late wood has narrow vessels (darker); one spring plus one autumn wood layer makes an annual ring, used to estimate a tree's age (dendrochronology)
• Heartwood: central, dark, dead, non-conducting, plugged with tyloses and tannins, giving mechanical support; sapwood: outer, lighter, actively conducts water
• The cork cambium (phellogen) arises in the cortex and forms cork (phellem) outward — suberised, dead, waterproof — and secondary cortex (phelloderm) inward; phellem + phellogen + phelloderm together make the periderm, and lenticels in the cork permit gas exchange