Biology: Class 11 — Chapter 6: Anatomy of Flowering Plants

Forms the outer-most covering of the whole plant body. It comprises:

• Epidermis: Outermost layer of the primary plant body, made of elongated, compactly arranged parenchymatous cells. Usually single-layered and covered with a waxy thick layer called cuticle (absent in roots) to prevent water loss.
• Stomata: Structures present in the epidermis of leaves that regulate transpiration and gaseous exchange. Each stoma is composed of two bean-shaped guard cells (dumb-bell shaped in grasses) enclosing a stomatal pore. Guard cells contain chloroplasts and regulate opening/closing. Specialized neighboring cells are called subsidiary cells. The stomatal aperture, guard cells, and subsidiary cells together form the stomatal apparatus.
• Epidermal appendages: Includes unicellular root hairs (absorb water/minerals) and multicellular trichomes on stems (prevent water loss, can be secretory).

All tissues except epidermis and vascular bundles form the ground tissue. It consists of simple tissues:

• Parenchyma, Collenchyma, and Sclerenchyma: Present in cortex, pericycle, pith, and medullary rays of primary stems and roots.
• Mesophyll: Ground tissue in leaves, consisting of thin-walled, chloroplast-containing cells.

Consists of complex tissues, xylem and phloem, which together form vascular bundles.

• Open vascular bundles: Present in dicotyledonous stems; possess cambium between xylem and phloem, enabling secondary growth.
• Closed vascular bundles: Present in monocotyledons; lack cambium and do not form secondary tissues.
• Radial arrangement: Xylem and phloem arranged alternately along different radii (e.g., in roots).
• Conjoint arrangement: Xylem and phloem jointly situated along the same radius (common in stems and leaves). Phloem is usually located on the outer side of xylem.

Internal tissue organization (e.g., sunflower root):

• Epiblema: Outermost layer, many cells protrude as unicellular root hairs.
• Cortex: Several layers of thin-walled parenchyma cells with intercellular spaces.
• Endodermis: Innermost layer of cortex, single layer of barrel-shaped cells. Tangential and radial walls have water-impermeable waxy suberin deposition called casparian strips.
• Pericycle: Thick-walled parenchymatous cells next to endodermis; initiates lateral roots and vascular cambium.
• Pith: Small or inconspicuous.
• Conjunctive tissue: Parenchymatous cells between xylem and phloem.
• Stele: All tissues on the inner side of endodermis (pericycle, vascular bundles, pith).

Similar to dicot root anatomy but with key differences:

• Has more than six (polyarch) xylem bundles (dicot roots have 2 to 4).
• Pith is large and well-developed.
• Do not undergo any secondary growth.

Transverse section of a young dicot stem shows:

• Epidermis: Outermost protective layer, covered with cuticle, may bear trichomes and stomata.
• Cortex: Divided into three sub-zones: hypodermis (collenchymatous, provides mechanical strength), cortical layers (parenchymatous), and endodermis (rich in starch grains, called starch sheath).
• Pericycle: Semi-lunar patches of sclerenchyma on the inner side of endodermis.
• Medullary rays: Radially placed parenchymatous cells between vascular bundles.
• Vascular Bundles: Arranged in a ring (characteristic of dicot stems). Each bundle is conjoint, open, and has endarch protoxylem.
• Pith: Central portion of stem, made of rounded parenchymatous cells with large intercellular spaces.

Characterized by:

• Sclerenchymatous hypodermis.
• Numerous scattered vascular bundles, each surrounded by a sclerenchymatous bundle sheath.
• Conjoint and closed vascular bundles; peripheral bundles are smaller than central ones.
• Phloem parenchyma is absent; water-containing cavities are present within vascular bundles.

Vertical section shows three main parts:

• Epidermis: Covers both upper (adaxial epidermis) and lower (abaxial epidermis) surfaces. Abaxial epidermis generally bears more stomata.
• Mesophyll: Photosynthetic tissue between epidermises, containing chloroplasts. Differentiated into:
  • Palisade parenchyma: Adaxially placed, elongated cells arranged vertically.
  • Spongy parenchyma: Loosely arranged, oval/round cells with large air cavities below palisade cells.
• Vascular system: Vascular bundles seen in veins and midrib, surrounded by thick-walled bundle sheath cells.

Anatomy is similar to dorsiventral leaf but with distinct differences:

• Stomata are present on both surfaces of the epidermis.
• Mesophyll is not differentiated into palisade and spongy parenchyma.
• In grasses, certain adaxial epidermal cells modify into large, empty, colourless bulliform cells. When turgid, they expose the leaf surface; when flaccid due to water stress, they make leaves curl inwards to minimize water loss.
• Parallel venation is reflected in near-similar sizes of vascular bundles.