Anatomy of Flowering Plants

Anatomy of flowering plants includes stems for structural support and transport of water and nutrients, roots for anchorage and nutrient absorption, leaves for photosynthesis, and reproductive structures such as flowers containing essential organs like stamens and pistils for sexual reproduction. The plant tissues are broadly classified into meristematic (apical, lateral, and intercalary) and permanent (simple and complex). The main functions of tissues include the assimilation of food and its storage, transportation of water, minerals, and photosynthates, and mechanical support to the plant.

The Tissue System

Plants consist of cells as the basic unit. These cells are organized into tissues, which serve specific functions. These tissues, in turn, are arranged into organs with specialized roles. Different organs in the plant have different internal structures. Tissues are classified based on their location in the plant body. Plants tissues are of two types:

• Meristematic Tissue:
  • Apical Meristem: Found at the tips, produces primary tissues (dermal, vascular, ground).
  • Intercalary Meristem: Present in grasses, located between matured tissues.
  • Lateral Meristem: Generates secondary tissues, such as cambium.
• Permanent Tissue: Cells that no longer divide after achieving their specialized structure and function.
  • Simple Tissue: Composed of a single cell type with uniform structure and function.
  • Complex Tissue: Consists of multiple cell types working in coordination.

Characteristic	Simple Tissue	Complex Tissue
Composition	Consists of a single type of cells.	Comprises multiple types of cells.
Structure	Uniform structure throughout.	Varied structure with different cell types.
Function	Performs a specific function.	Coordinates multiple functions.
Examples	Parenchyma (in plants), Simple Squamous Epithelium (in animals).	Xylem, Phloem, and Epidermis (in plants), Blood (in animals).
Location in Plants	Found in various plant organs.	Mainly in vascular and supporting tissues.
Cell Types	Consists of one type of cell (homogeneous).	Composed of several types of cells (heterogeneous).

There are three types of tissue systems based on their structure and location. These are:

• The Epidermal tissue system
• The ground or fundamental tissue system
• The vascular or conducting tissue system.

Epidermal Tissue System

The epidermal tissue system forms the outermost layer of the plant. It consists of epidermal cells, stomata and the epidermal appendages – the trichomes and hairs.

Epidermis

• It forms the outermost covering of the plant.
• It is made up of elongated, compactly arranged cells, which form a single continuous layer.
• These cells are parenchymatous with a large vacuole and small amount of cytoplasm lining the cell wall.
• A waxy thick layer known as cuticle covers the outer surface of epidermis. It prevents the loss of water and is absent in roots.

Stomata

• Stomata are present in the epidermis of leaves and regulate the process of transpiration and gaseous exchange.
• Each stoma is composed of two beanshaped cells known as guard cells.
• The guard cells possess chloroplasts and control the opening and closing of the stomatal pore by changing their shape. The outer wall of the guard cell is are thin and the inner wall is highly thickened. In grasses, the guard cells are dumb-bell shaped.

Epidermal Appendages

Epidermal cells have the following features:

• Epidermal cells of plants bear hairs.
• In roots, these hairs are unicellular extensions of epidermal cells.
• Root hairs aid in the absorption of water and minerals from the soil.
• Epidermal hairs on stems are called trichomes.
• Trichomes in the shoot system are typically multicellular.
• They can be branched or unbranched, and may be soft or stiff.
• Functions of Trichomes are:
  • Prevent water loss due to transpiration.
  • Provide protection against herbivores.
  • Some trichomes may have secretory functions.

The Ground Tissue System

Ground tissue is formed of all types of tissue except epidermis and vascular bundles. It consists of simple tissues such as parenchyma, collenchyma and sclerenchyma. The ground tissue system forms the main bulk of the plant. It is divided into three zones – cortex, pericycle and pith. In the primary stems and roots parenchymatous cells are present in cortex, pith, pericycle, and medullary rays. In the leaves the ground tissue is called mesophyll and consists of thin-walled chloroplast containing cells.

The Vascular Tissue System

The vascular system consists of complex tissues, the phloem and the xylem. The xylem and phloem together constitute vascular bundles. In dicotyledonous stems, cambium is present between phloem and xylem and have the ability to form secondary xylem and phloem tissues, and are called open vascular bundles. In the monocotyledons, the vascular bundles have no cambium present in them and therefore, lack secondary tissues. They are called closed vascular bundles. The xylem and pholem form different pattern in the leaves and roots. These arrangement are as follows:

• Within a vascular bundle when Xylem and phloem are arranged in an alternate manner along the different radii, the arrangement is called radial. For example Roots.
• Within a vascular bundle when Xylem and phloem are arranged jointly along the same radius of vascular bundles, the arrangement is called cojoint. Usually the phloem is located only on the outer side of xylem.For example in shoot and leaves.

Anatomy of Dicotyledonous and Monocotyledonous Plants

Dicotyledonous (dicots) and monocotyledonous (monocots) plants show anatomical differences. Dicots have two cotyledons, reticulate venation, and a taproot system with vascular bundles arranged in a ring and monocots have one cotyledon, parallel venation, and a fibrous root system with scattered vascular bundles. The tissue organisation of roots, stems and leaves are discussed in detail below.

Dicotyledonous Root

The internal tissue organisation of dicotyledonous root is as follows:

• The outermost layer of the root is known as epiblema. The cells of epiblema protrude in the form of unicellular root hairs.
• The cortex consists of several layers of thin-walled parenchyma cells with intercellular spaces.
• Innermost layer of the cortex is called endodermis which characterized by barrel-shaped cells with casparian strips made of water-impermeable suberin. It lacks intercellular spaces.
• Pericycle is present next to the endodermis which consists of a few layers of thick-walled parenchymatous cells. It act as site for initiation of lateral roots and vascular cambium during secondary growth.
• Pith is small or inconspicuous.
• The parenchymatous cells which lie between the xylem and the phloem are called conjuctive tissue.
• Usually there are two to four xylem and phloem patches. and cambium ring develops between xylem and phloem during growth.
• Tissues on the inner side of the endodermis, including pericycle, vascular bundles, and pith, constitute the stele.

Monocotyledonous Root

The internal tissue organisation of monocotyledonous root is as follows:

• The anatomy of the monocot root is similar to the dicot root, both have epidermis, cortex, endodermis, pericycle, vascular bundles and pith.
• In monocot root there are more than six (polyarch) xylem bundles as compared to dicot root which have fewer xylem bundles.
• Pith is large and well developed.
• Monocotyledonous roots do not undergo secondary growth.

Dicotyledonous Stem

The internal tissue organisation of dicotyledonous stem is as follows:

• The epidermis forms the protective outer layer of the stem and is covered with a thin layer of cuticle.
• It consists of trichomes (hair-like structures) and a few stomata for gas exchange.
• Multiple layers between epidermis and pericycle constitute the cortex. It consists of three sub-zones:
  • Outer hypodermis: Consists of few layers of collenchymatous cells just below the epidermis and provides mechanical strength to the young stem.
  • Middle parenchymatous cells: It consists of round thin-walled parenchymatous cells with small intercellular spaces.
  • Endodermis (Starch Sheath): It is the innermost layer of the cortex and cells are rich in starch grains, also called as starch sheath.
• Pericycle is present on the inner side of endodermis and above the phloem in the form of semi-lunar patches of sclerenchyma.
• Medullary Ray is present between the vascular bundles which consists of radially placed parenchymatous cells.
• Vascular bundles arranged in a “ring”, it is the main characteristics of dicot stem. Vascular bundles are conjoint, open, and have endarch protoxylem.
• Pith forms the central portion of the stem and consists of large number of rounded, parenchymatous cells with large intercellular spaces. It provides structural support and storage.

Monocotyledonous Stem

The internal tissue organisation of monocotyledonous stem is as follows:

• Monocot consists of sclerenchymatous hypodermis below the epidermis for mechanical strength.
• There are numerous vascular bundle, scattered throughout the stem. Each vascular bundle is surrounded by a sclerenchymatous bundle sheath.
• Vascular bundles are conjoint and closed. Peripheral vascular bundles are generally smaller than the centrally located ones.
• Phloem parenchyma is absent in the vascular bundles.
• Water-containing cavities are present within the vascular bundles.

Dorsiventral (Dicotyledonous) Leaf

• Epidermis covers both upper (adaxial) and lower (abaxial) surfaces of the leaf and have a small cuticle. Abaxial epidermis has more stomata than the adaxial epidermis.
• Mesophyll is present between the upper and the lower epidermis. It consists of chloroplast-containing parenchyma cells. It contain two type of cells:
  • Palisade parenchyma (adaxially placed): It consists of elongated cells, which are arranged vertically and parallel to each other.
  • Spongy parenchyma ( placed below palisade cells): It consist of oval or round loosely arranged cells that extends to the lower epidermis. Between these cells there are large spaces and air cavities.
• Vascular system consists of vascular bundles which are surrounded by a layer of thick walled bundle sheath cells. It is easily visible in the veins and the midrib of the leaves. The size of the vascular bundles are dependent on the size of the veins.
• The veins shows varying thickness in the reticulate venation of the dicot leaves.

Isobilateral (Monocotyledonous) Leaf

Most of the anatomy of isobilateral leaf is similar to that of the dorsiventral leaf. it has following different features:

• The stomata are present on both the surfaces of the epidermis.
• The mesophyll is not differentiated into palisade and spongy parenchyma.
• Monocot leaves exhibit parallel venation.
• Bulliform Cells in Grasses: Certain adaxial epidermal cells along veins modify into large, empty, colorless cells known as bulliform cells. When turgid (filled with water), bulliform cells expose the leaf surface; when flaccid (due to water stress), they cause leaves to curl inward, minimizing water loss.

This adaptation with bulliform cells and parallel venation is a strategy in grasses to respond to water availability and minimize water loss, important for their survival in various environmental conditions.

Secondary Growth

Secondary growth is characterized by an increase in the girth or thickness of a plant stem or root. Unlike primary growth, which occurs at the tips of shoots and roots, secondary growth involves lateral expansion. It has the following features:

• It is common in dicots and gymnosperms and results in the development of woody tissues.
• Two lateral meristems, the vascular cambium and cork cambium, play key roles in secondary growth.
• The vascular cambium produces secondary xylem (wood) inward and secondary phloem outward. It increases the mechanical support and allow water and nutrient transport.
• The cork cambium produces cork cells that replace the epidermis.It forms the protective bark.
• Annual rings visible in a tree’s cross-section reflect growth patterns influenced by favorable and unfavorable environmental conditions.
• Secondary growth is essential for the development of durable and resilient plant structures.

Revision of Anatomy of Flowering Plants

A brief revision of chapter:

FAQs – Anatomy of Flowering Plants

1. What is a Meristematic Tissue and What are Its Types?

Meristematic tissue consists of actively dividing cells, responsible for growth and development. It is divided as: Apical Meristem: Found at tips, produces primary tissues, lateral Meristem: Generates secondary tissues, e.g., vascular cambium and intercalary Meristem: Present in grasses, between mature tissues.

2. Describe Permanent Tissues and Their Types.

Permanent tissue consists of that have stopped dividing and have become specialized for specific functions in plants. It is of two types:Simple Tissues which have same structure and function and Complex Tissues which consist of different types of cell types.

3. What do You Mean By Tissue System?

Tissue system is defined as a group of tissues working together to perform specific functions in a plant. Based on their structure and location it is divided as the Epidermal tissue system, the ground or fundamental tissue system, the vascular or conducting tissue system.

4. What is Cambium? What is Its Function?

Cambium is meristematic tissue in plants, that produces secondary xylem and phloem. It contributes to the growth in girth (secondary growth) and produces cork cells to form protective bark.

5. What are the Anatomy of Plants?

Anatomy of plant is the study of plant internal structures and tissues. It also studies the arrangement and organization of cells, tissues, and organs in plant.