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

Epidermal Tissue System

• The epidermis is the outermost single layer covering the entire plant, usually with a waxy cuticle on aerial parts.
• Stomata regulate transpiration and gaseous exchange; each is bounded by two bean-shaped guard cells (dumb-bell-shaped in grasses).
• Guard cells have inner walls thick and outer walls thin, and unlike other epidermal cells they contain chloroplasts.
• Trichomes are epidermal hairs on the shoot that may be branched or unbranched and help reduce water loss.
• Root hairs are unicellular elongations of epidermal cells that absorb water and minerals from soil.

Anatomy of Dorsiventral and Isobilateral Leaves

• A dorsiventral (dicot) leaf has mesophyll differentiated into upper columnar palisade parenchyma and lower loosely arranged spongy parenchyma.
• An isobilateral (monocot) leaf has undifferentiated mesophyll and stomata on both surfaces, so it is amphistomatic.
• In grasses, certain adaxial epidermal cells enlarge into bulliform cells that fold the leaf inward to reduce water loss.
• Vascular bundles are surrounded by a bundle sheath of thick-walled cells.
• Veins vary in size because of the reticulate venation of dicot leaves; the midrib is the most prominent vein.

Annual Rings and Wood Types

• The cambium is more active in spring, producing wide, light spring wood (early wood) with wider vessels.
• In autumn the cambium is less active, producing narrow, dark autumn wood (late wood) with narrow vessels.
• One ring of spring wood plus autumn wood forms an annual ring; counting them estimates the age of a tree (dendrochronology).
• Heartwood is the dark, dead inner wood filled with tannins and resins that gives mechanical support; it does not conduct water.
• Sapwood is the lighter peripheral wood that conducts water and minerals.