Chapter 6: Anatomy of Flowering Plants - Overview
Introduction
Anatomy is the study of the internal structure of plants. Plants have cells as the basic unit; cells are organised into tissues, and tissues are organised into organs. Within angiosperms, monocots and dicots are anatomically different. Internal structures also show adaptations to diverse environments.
6.1 The Tissue System
Based on structure and location, there are three types of tissue systems:
6.1.1 Epidermal Tissue System
Forms the outer-most covering of the plant body.
- Epidermis: Outermost layer, usually single-layered with parenchymatous cells. Often covered by a waxy cuticle (absent in roots) to prevent water loss.
- Stomata: Structures in leaves for transpiration and gas exchange.
- Composed of two bean-shaped guard cells (dumb-bell shaped in grasses).
- Surrounded by specialised epidermal cells called subsidiary cells.
- Stomatal Apparatus: Stomatal aperture + guard cells + subsidiary cells.
- Epidermal Appendages:
- Root Hairs: Unicellular elongations involved in water/mineral absorption.
- Trichomes: Multicellular hairs on stems; prevent water loss and may be secretory.
6.1.2 The Ground Tissue System
All tissues except epidermis and vascular bundles. Consists of simple tissues (parenchyma, collenchyma, sclerenchyma).
- Location: Cortex, pericycle, pith, and medullary rays in stems and roots.
- Mesophyll: Thin-walled chloroplast-containing ground tissue in leaves.
6.1.3 The Vascular Tissue System
Consists of complex tissues: Xylem and Phloem.
Types of Vascular Bundles:
- Open: Cambium present between xylem and phloem. Capable of secondary growth (Dicot stems).
- Closed: Cambium absent. No secondary growth (Monocots).
- Radial: Xylem and phloem arranged in an alternate manner on different radii (Roots).
- Conjoint: Xylem and phloem situated along the same radius (Stems and Leaves). Usually, phloem is on the outer side.
6.2 Anatomy of Dicot vs. Monocot Plants
6.2.1 & 6.2.2 The Root
| Feature | Dicot Root (e.g., Sunflower) | Monocot Root |
|---|---|---|
| Cortex | Thin-walled parenchyma with intercellular spaces. | Similar to dicot. |
| Endodermis | Single layer; barrel-shaped cells; presence of waxy Casparian strips. | Present. |
| Pericycle | Thick-walled parenchyma. Lateral roots and vascular cambium initiate here. | Present. |
| Vascular Bundles | Usually 2 to 4 xylem/phloem patches. Later develops a cambium ring. | Polyarch (usually more than six xylem bundles). |
| Pith | Small or inconspicuous. | Large and well-developed. |
6.2.3 & 6.2.4 The Stem
Dicotyledonous Stem
- Epidermis: Protective outer layer with cuticle and trichomes.
- Cortex: Divided into three sub-zones:
- Hypodermis: Collenchymatous (mechanical strength).
- Cortical Layers: Parenchymatous.
- Endodermis: Rich in starch grains (referred to as starch sheath).
- Pericycle: Sclerenchymatous "semi-lunar patches" above phloem.
- Vascular Bundles: Arranged in a ring (characteristic of dicots). Conjoint, open, endarch protoxylem.
Monocotyledonous Stem
- Hypodermis: Sclerenchymatous.
- Vascular Bundles: Scattered. Conjoint and closed. Surrounded by sclerenchymatous bundle sheath.
- Distinction: Peripheral bundles are smaller than central ones. Phloem parenchyma is absent. Water-containing cavities present within vascular bundles.
6.2.5 & 6.2.6 The Leaf
Dorsiventral (Dicot) Leaf
- Epidermis: Abaxial (lower) epidermis has more stomata than adaxial (upper).
- Mesophyll: Differentiated into two types:
- Palisade Parenchyma: Adaxially placed, elongated cells, parallel arrangement.
- Spongy Parenchyma: Oval/round, loosely arranged below palisade cells with air cavities.
- Vascular System: Vascular bundles surrounded by thick-walled bundle sheath cells. Size of bundles varies with vein size (reticulate venation).
Isobilateral (Monocot) Leaf
- Stomata: Present on both epidermal surfaces.
- Mesophyll: Not differentiated into palisade and spongy parenchyma.
- Bulliform Cells: In grasses, adaxial epidermal cells modify into large, empty, colourless cells.
Function: When turgid, leaf surface is exposed; when flaccid (water stress), leaves curl inwards to minimise water loss. - Vascular System: Bundles are of near similar sizes (parallel venation).