Anatomy of Flowering Plants Class 11 Notes

Meristematic Tissues

Meristematic tissue consists of cells that divide continuously and are the basis of growth. Cells are small, thin-walled, with dense cytoplasm and large nuclei; they lack vacuoles. Meristems are classified by position.

• Apical meristems: at root and shoot tips — responsible for primary (length) growth.
• Intercalary meristems: at the base of internodes (e.g. grass) — also contribute to primary growth.
• Lateral meristems: on the sides of stem and root — vascular cambium and cork cambium — responsible for secondary (girth) growth.

Simple Permanent Tissues

Complex Permanent Tissues — Xylem and Phloem

Tissue Systems and Dicot vs Monocot Roots

Every plant organ has three tissue systems arranged concentrically: epidermal tissue system (outermost — epidermis with cuticle and stomata), ground tissue system (all tissue between epidermis and vascular bundles — cortex, endodermis, pericycle, pith) and vascular tissue system (xylem and phloem).

Dicot Root (cross-section)

• Epiblema (epidermis) with root hairs.
• Cortex — multilayered parenchyma.
• Endodermis — innermost cortical layer with Casparian strips (suberin deposits forcing water through the symplast, controlling entry into the stele).
• Pericycle — gives rise to lateral roots and vascular cambium in secondary growth.
• Vascular bundles: 2–6 (diarch to hexarch), arranged radially. Xylem is exarch (protoxylem towards periphery, metaxylem towards centre) — a key diagnostic feature.
• Pith is small or absent.

Monocot Root (cross-section)

• Similar ground plan to dicot root but many vascular bundles (polyarch — more than 6).
• Large, well-developed pith at the centre.
• Metaxylem towards centre, protoxylem towards periphery (exarch), same as dicot.

Dicot vs Monocot Stem Anatomy

Dicot Stem

• Epidermis with trichomes and cuticle.
• Hypodermis of collenchyma (mechanical support).
• Cortex: general parenchyma, inner with starch sheath.
• Endodermis with Casparian strips.
• Pericycle (semilunar bundles of sclerenchyma in sunflower).
• Vascular bundles — conjoint, collateral, open (vascular cambium present between xylem and phloem — allows secondary growth), arranged in a ring.
• Pith is well-developed.

Monocot Stem

• Hypodermis is sclerenchymatous.
• Vascular bundles — scattered throughout the ground tissue (no ring arrangement).
• Bundles are conjoint, collateral, closed (no vascular cambium — no secondary growth).
• Each bundle has a sclerenchymatous bundle sheath.
• Protoxylem with a water cavity (protoxylem lacuna or cavity).
• Ground tissue is not differentiated into cortex and pith.

Leaf Anatomy — Dorsiventral vs Isobilateral

A leaf has three main parts in cross-section: upper epidermis (with thick cuticle), mesophyll (ground tissue performing photosynthesis) and lower epidermis (with more stomata). Vascular bundles (veins) run through the mesophyll.

• Dorsiventral leaf (dicots): upper epidermis on dorsal side, lower on ventral. Mesophyll differentiated into palisade (closely packed, columnar, many chloroplasts — upper) and spongy (loosely arranged, large air spaces — lower). Stomata mainly on lower epidermis.
• Isobilateral leaf (monocots): both surfaces are similar. Mesophyll is undifferentiated — all parenchymatous. Stomata are equally distributed on both sides (amphistomatic).
• Guard cells in dicots are bean-shaped; in monocots (grasses) they are dumbbell-shaped.

Secondary Growth

Secondary growth increases the girth (thickness) of the stem and root. It is absent in monocots (no cambium). It occurs by two lateral meristems: vascular cambium and cork cambium (phellogen).

Vascular Cambium

• In dicot stems, the fascicular cambium (between xylem and phloem in open bundles) and interfascicular cambium (from medullary rays) join to form a complete ring.
• The ring produces secondary xylem inward and secondary phloem outward — secondary xylem accumulates far more, so trunk girth increases.
• Annual rings: in temperate regions, spring wood (large vessels, light) and autumn/fall wood (small vessels, dense) form distinct rings. Each ring ≈ one year.

Cork Cambium (Phellogen) and Bark

• Phellogen (cork cambium) forms from cortical cells. It produces cork (phellem) outward and secondary cortex (phelloderm) inward — together called periderm.
• Bark = all tissues outside the vascular cambium — secondary phloem + periderm. Older bark peels off as the trunk grows.
• Heartwood (centrally, older, dark, non-functional, filled with tannins and resins) vs sapwood (outer, lighter, functional xylem — conducts water).

Solved Examples

Example: Why is the vascular bundle in a monocot stem described as 'closed' while that in a dicot stem is 'open'?

Solution: An 'open' vascular bundle contains a strip of vascular cambium between the xylem and phloem — this fascicular cambium can produce secondary xylem and phloem, allowing secondary growth. A 'closed' bundle lacks this cambium, so no secondary growth is possible. Monocot stems have closed bundles and therefore never increase in girth.

Example: How does the Casparian strip in the endodermis affect the pathway of water from soil to xylem?

Solution: Water enters root hairs and moves through the cortex via apoplast (cell walls) and symplast (cytoplasm). At the endodermis, the Casparian strip (suberin) blocks the apoplastic route, forcing all water to cross the plasma membrane and enter the symplast. This allows selective uptake of minerals and prevents uncontrolled flow of solutes into the stele — the endodermis acts as a checkpoint.