CBSE Class 11 Biology Chapter 6: Anatomy of Flowering Plants Notes

Classification of Plant Tissues

Plants are composed of different types of tissues. Broadly, tissues are classified into meristematic tissues (actively dividing cells) and permanent tissues (mature cells that have lost the capacity to divide).

Meristematic tissues are further divided based on their position into:

• Apical meristems: Located at root and shoot tips, responsible for primary growth (increase in length).
• Intercalary meristems: Positioned between mature tissues (e.g., at the base of leaves in grasses), helping in regenerating parts eaten by herbivores.
• Lateral meristems: Occur in mature regions of roots and shoots (e.g., vascular cambium and cork cambium), responsible for secondary growth (increase in thickness/girth).

Permanent tissues are grouped into:

• Simple permanent tissues: Homogenous cells with a single cell type performing one function. These include parenchyma (thin-walled, storage/photosynthesis), collenchyma (thickened corners, mechanical support to young parts), and sclerenchyma (dead, highly lignified walls, extreme mechanical strength, e.g., fibres and sclereids).
• Complex permanent tissues: Heterogenous cells working together as a unit. These include xylem (water conduction via tracheids, vessels, xylem fibres, and xylem parenchyma) and phloem (food translocation via sieve tubes, companion cells, phloem parenchyma, and phloem fibres).

Important Anatomical Terms

Meristematic Tissue

A group of young, immature cells that are continuously dividing to form new cells.

Casparian Strips

A band of water-impermeable, waxy material called suberin deposited on the radial and tangential walls of endodermal cells in roots.

Cambium

A strip of lateral meristematic tissue responsible for secondary growth in dicotyledonous plants.

Radial Vascular Bundle

A vascular bundle arrangement where xylem and phloem lie on different radii, alternating with each other, characteristic of roots.

Conjoint Vascular Bundle

An arrangement where xylem and phloem are located on the same radius, characteristic of stems and leaves.

Bulliform Cells

Large, empty, colorless epidermal cells found along the veins of monocot leaves that regulate leaf rolling to minimize transpiration.

Comparison: Dicot vs Monocot Stem Anatomy

Mechanism of Secondary Growth in Dicot Stems

1. Formation of Cambial Ring — Cells of the intrafascicular cambium join with newly activated interfascicular cambium (derived from medullary rays) to form a continuous ring of cambium.
2. Activity of the Cambial Ring — The cambial ring cuts off new cells on both sides. Cells cut off toward the pith mature into secondary xylem (more active), and those toward the cortex mature into secondary phloem.
3. Development of Cork Cambium (Phellogen) — As the stem increases in girth due to secondary xylem, outer cortical and epidermal layers break. A secondary meristem called phellogen (cork cambium) develops in the outer cortex.
4. Formation of Periderm — Phellogen cuts off cells on the outer side (phellem or cork, which is suberized and dead) and on the inner side (phelloderm or secondary cortex). These three layers collectively constitute the periderm.

Must-Remember Anatomical Concepts

• Xylem elements include tracheids, vessels, fibres, and parenchyma. Vessels are completely absent in most gymnosperms.
• Phloem components include sieve tube elements, companion cells, phloem fibres, and phloem parenchyma. Monocot stems lack phloem parenchyma.
• Endodermis in dicot stems is rich in starch grains and is therefore referred to as the starch sheath.
• In dicot roots, the vascular bundles are radial and exarch (protoxylem points outwards, metaxylem inwards).
• In dicot stems, the vascular bundles are conjoint and endarch (protoxylem points inwards towards the center, metaxylem outwards).
• Spring wood (early wood) has wider vessels and lower density because the cambium is highly active in spring. Autumn wood (late wood) has narrower vessels and higher density because the cambium is less active in winter.
• Heartwood is central, dark brown, non-functional (blocked by tyloses), filled with resins and tannins, and resistant to microbial decay. Sapwood is peripheral, light-colored, and active in conducting water.

Exam Traps & Board Marking Cues

• Radial vs Conjoint Bundles: Always draw radial bundles for roots (xylem and phloem alternate on different radii) and conjoint bundles for stems/leaves (xylem and phloem on the same radius). Drawing conjoint bundles in a root section is a common board exam trap that will cost you full marks.
• Endarch vs Exarch: Remember Root = Exarch (Protoxylem points outwards, metaxylem inwards) and Stem = Endarch (Protoxylem points inwards, metaxylem outwards). A simple mnemonic is 'Stem-In' (Endarch).
• Monocot Stem Bundles: Never draw cambium in monocot vascular bundles; they are strictly closed and cannot undergo secondary growth.

Quick Revision Checks

• Why are xylem vessels absent in gymnosperms, and what serves their function? Gymnosperms lack true xylem vessels (except Gnetales). Instead, tracheids are the primary water-conducting elements that perform this function.
• State the structural and functional difference between heartwood and sapwood. Heartwood is the dark, non-conducting central wood filled with tannins and resins, providing structural support. Sapwood is the outer, light-colored, active conducting wood responsible for water and mineral transport.
• What is the function of bulliform cells in grass leaves? When water is abundant, bulliform cells absorb water and become turgid, keeping the leaf surface exposed. Under water stress, they lose turgor, causing the leaves to roll inward to minimize transpiration.
• Explain the role of Casparian strips in roots. Casparian strips (composed of suberin) are located in the endodermis. They block the apoplastic pathway of water, forcing water and minerals to cross the selectively permeable plasma membrane via the symplastic pathway.

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