Anatomy of Flowering Plants – NEET High Quality Notes

We Need To Cover These Topics-

S. No.	Main Topic	Sub-Topics
1	Tissues	• Meristematic Tissues (Apical, Intercalary, Lateral) • Permanent Tissues: – Simple Tissues: Parenchyma, Collenchyma, Sclerenchyma – Complex Tissues: Xylem, Phloem
2	Tissue Systems	• Epidermal Tissue System (Epidermis, Stomata, Trichomes) • Ground Tissue System • Vascular Tissue System (Xylem and Phloem Arrangement)
3	Anatomy of Dicotyledonous Root	• Epidermis • Cortex • Endodermis • Pericycle • Vascular Bundles • Pith • Radial Vascular Bundles with Exarch Xylem
4	Anatomy of Monocotyledonous Root	• Similar to dicot root • Large pith • Polyarch xylem • Well-developed cortex
5	Anatomy of Dicotyledonous Stem	• Epidermis • Hypodermis • Cortex • Endodermis • Pericycle • Vascular Bundles (Open, Collateral, and Arranged in Ring) • Pith
6	Anatomy of Monocotyledonous Stem	• Scattered Vascular Bundles (Closed, Collateral) • Ground Tissue not differentiated • Sclerenchymatous hypodermis
7	Anatomy of Leaf (Dorsiventral and Isobilateral)	• Epidermis (Upper & Lower) • Mesophyll (Palisade & Spongy in Dorsiventral) • Vascular Bundles • Bulliform cells (in Isobilateral Leaf)
8	Secondary Growth in Dicot Stem	• Vascular Cambium Activity (Formation of Secondary Xylem & Phloem) • Formation of Annual Rings • Periderm Formation (Cork Cambium, Cork, Phelloderm)
9	Secondary Growth in Roots	• Similar to stem • Vascular Cambium from Conjunctive Tissue • Periderm Formation
10	Heartwood and Sapwood	• Characteristics and Differences • Significance in Support and Conduction
11	Lenticels	• Structure and Function • Role in Gaseous Exchange

1. Introduction to Plant Tissues

In plants, a tissue is a group of cells that work together to perform a specific function. These cells are similar in structure and origin. Based on their dividing capacity, plant tissues are classified into two main types: meristematic tissues and permanent tissues. Understanding these tissues is very important in the study of plant anatomy, growth, and development, especially for NEET-level competitive exams.

1. Meristematic Tissues

Meristematic tissues are the regions in plants where continuous cell division takes place. These tissues are responsible for the growth of plants. The cells of meristematic tissue are very active, with dense cytoplasm, a large nucleus, and thin cell walls. They lack vacuoles as they are constantly dividing.

Characteristics of Meristematic Tissue:

Cells are small and undifferentiated.
Have thin cell walls made of cellulose.
Nucleus is large and prominent.
No intercellular spaces between cells.
Vacuoles are absent or very small.

Types of Meristematic Tissues:

(a) Apical Meristem:

Apical meristem is present at the tips of roots and shoots. It is responsible for the increase in the length of the plant. This type of growth is known as primary growth. The apical meristem helps in the formation of primary tissues like root, stem, and leaves.

Functions:

Increase in plant height and root depth.
Formation of leaves and flowers.

(b) Intercalary Meristem:

Intercalary meristem is found at the base of leaves or internodes (stem regions between two nodes). It is active in regions where elongation is needed, such as in grasses, where fast regrowth is necessary after grazing or cutting.

Functions:

Helps in the elongation of stems and leaves.
Allows quick regrowth in some plants (especially monocots).

(c) Lateral Meristem:

Lateral meristem is found along the sides of stems and roots. It is responsible for secondary growth, which increases the thickness or girth of the plant. The most important lateral meristems are vascular cambium and cork cambium.

Functions:

Increase in the diameter of roots and stems.
Formation of secondary tissues (wood, bark).

2. Permanent Tissues

Permanent tissues are formed from meristematic tissues once the cells lose their ability to divide. These tissues perform specific functions in the plant. Once the cells attain maturity, they become specialized and are known as permanent cells.

Permanent tissues are of two types:

Simple Permanent Tissues
Complex Permanent Tissues

A. Simple Permanent Tissues

Simple permanent tissues are made up of similar types of cells that perform the same function. These are further divided into three types:

(i) Parenchyma:

Parenchyma is the most common type of simple permanent tissue. These are living cells with thin cell walls. They are generally isodiametric (almost equal in all directions), with large central vacuoles.

Structure:

Living, oval, round, or elongated cells.
Thin cell walls made of cellulose.
Large intercellular spaces.

Functions:

Storage of food and water.
Photosynthesis (when chloroplasts are present, it is called chlorenchyma).
Aeration (when cells have air spaces, it is called aerenchyma).

Special Types:

Chlorenchyma: Contains chloroplasts; found in green parts for photosynthesis.
Aerenchyma: Contains large air cavities; helps in buoyancy in aquatic plants.

(ii) Collenchyma:

Collenchyma is a simple permanent tissue that provides mechanical support to growing parts of the plant like young stems and petioles. The cells are living and have unevenly thickened walls due to pectin and cellulose deposits.

Structure:

Elongated and living cells.
Uneven thickening at corners.
No intercellular spaces.

Functions:

Provides mechanical strength.
Provides flexibility to plant parts.

Location:

Found in leaf stalks (petioles), below the epidermis of stems and leaves.

(iii) Sclerenchyma:

Sclerenchyma is a dead tissue that provides maximum mechanical strength. The cell walls are uniformly thick due to lignin deposition.

Structure:

Long, narrow, and dead cells.
Thick cell walls with lignin.
No intercellular space.
Lumen (central cavity) is very small or absent.

Types:

Fibres: Long and tapering cells found in stems, leaves.
Sclereids (stone cells): Irregularly shaped; found in nutshells, seed coats, and pulp of fruits like pear.

Functions:

Provides mechanical support.
Protects seeds and fruits.

B. Complex Permanent Tissues

Complex permanent tissues are made of more than one type of cell. These different types of cells work together as a unit to perform a common function. Complex tissues are mainly involved in the conduction of water, minerals, and food.

The two types of complex tissues are:

Xylem
Phloem

(i) Xylem:

Xylem is a conducting tissue that transports water and minerals from the roots to other parts of the plant. It also provides mechanical support. Most of the xylem elements are dead except for xylem parenchyma.

Components of Xylem:

Tracheids: Elongated, thick-walled dead cells with tapering ends. Water flows through pits in their walls.
Vessels: Cylindrical, tube-like dead cells arranged end to end; present mainly in angiosperms; allow efficient water conduction.
Xylem Fibres: Dead and thick-walled; provide mechanical strength.
Xylem Parenchyma: Only living component; stores food and helps in the lateral conduction of water.

Function of Xylem:

Conduction of water and minerals.
Provides mechanical strength.

Primary Xylem: Formed during primary growth.

Protoxylem: First formed, has smaller vessels.
Metaxylem: Later formed, has larger vessels.

Secondary Xylem: Formed during secondary growth; also called wood.

(ii) Phloem:

Phloem is the tissue that transports food (mainly sucrose) from leaves to all parts of the plant. Unlike xylem, most of the phloem elements are living.

Components of Phloem:

Sieve Tube Elements: Main conducting elements; elongated cells joined end to end; have sieve plates with pores.
Companion Cells: Associated with sieve tubes; help in the function of sieve tubes.
Phloem Parenchyma: Stores food and other substances.
Phloem Fibres: Only dead cells in phloem; provide support.

Function of Phloem:

Transport of food from source (leaves) to sink (growing parts, roots, fruits).

Primary Phloem: First formed phloem; includes protophloem and metaphloem.

Protophloem: Early formed; may get crushed as the plant matures.
Metaphloem: Later formed; remains functional for longer.

Secondary Phloem: Formed from vascular cambium during secondary growth.

Summary Table – Key Differences Between Xylem and Phloem

Feature	Xylem	Phloem
Conducts	Water and minerals	Food (sucrose, etc.)
Direction	Upward only	Upward and downward
Main Cells	Tracheids and vessels	Sieve tubes and companion cells
Cell Type	Mostly dead	Mostly living
Provides Support	Yes	Limited

Conclusion

Plant tissues are crucial for plant structure, function, and survival. Meristematic tissues are the driving force for plant growth, while permanent tissues perform a variety of essential functions like storage, support, conduction, and photosynthesis. Understanding the types, structures, and functions of these tissues helps in analyzing the anatomy of flowering plants and is a fundamental concept in NEET and other competitive exams. Students are advised to study diagrams, compare the characteristics, and understand the structure-function relationships to master this topic.

2. 🌿 Tissue Systems in Plants

Plants have complex body structures that perform many different functions like protection, transport, support, food storage, and photosynthesis. To perform all these roles efficiently, plant tissues are organized into tissue systems. A tissue system is a group of tissues that work together to perform a specific function or set of related functions.

In higher plants, there are three main tissue systems:

Epidermal Tissue System
Ground Tissue System
Vascular Tissue System

These tissue systems originate from specific meristems and are spread throughout the plant body from the root to the shoot. Let’s now understand each of these in detail.

🌱 1. Epidermal Tissue System

The epidermal tissue system is the outermost protective layer of the plant body. It covers all parts of the plant – roots, stems, leaves, flowers, fruits, and seeds. The cells of this system form a continuous layer without any intercellular spaces and play an important role in protecting internal tissues from mechanical injury, water loss, and pathogen attack.

🔹 Structure and Features of Epidermis

The epidermis is usually made of a single layer of compactly arranged cells. These cells are mostly parenchymatous (thin-walled), living, and without chloroplasts (except in guard cells). The outer walls of these cells are thick and often covered with a waxy cuticle, especially in aerial parts of the plant. This cuticle helps prevent water loss due to transpiration.

In roots, the epidermal cells may develop into root hairs, which increase the surface area for water and mineral absorption from the soil.

🔹 Stomata

Stomata are tiny pores found in the epidermis of leaves, young stems, and flowers. They are responsible for gaseous exchange and transpiration. Each stoma is surrounded by two specialized cells called guard cells that regulate the opening and closing of the stomatal pore. In dicot plants, the guard cells are kidney-shaped, while in monocot plants, they are dumbbell-shaped.

Guard cells contain chloroplasts, and their turgidity determines whether the stomatal pore is open or closed. When guard cells are full of water (turgid), the pore opens. When they lose water (flaccid), the pore closes. This mechanism helps the plant conserve water and regulate gas exchange.

Stomata play a vital role in photosynthesis, respiration, and transpiration. Their distribution varies in different plant species and parts.

🔹 Trichomes and Root Hairs

Trichomes are hair-like outgrowths found on the epidermis of stems, leaves, and even floral parts. They may be unicellular or multicellular, branched or unbranched, and sometimes secretory in function. Trichomes help in:

Reducing water loss by reflecting sunlight
Providing defense against insects and herbivores
Secreting sticky or toxic substances

On roots, the epidermal cells develop long extensions called root hairs. These are unicellular, non-lignified, and highly efficient in absorbing water and minerals from the soil.

🍃 2. Ground Tissue System

The ground tissue system forms the bulk of the plant body and is found in all organs like roots, stems, and leaves. It includes all tissues except the epidermis and vascular tissues. The main functions of the ground tissue system are photosynthesis, storage, support, and healing.

This tissue system is mainly composed of three types of simple permanent tissues:

Parenchyma
Collenchyma
Sclerenchyma

🔹 Components of Ground Tissue

✅ Parenchyma

Parenchyma is the most common and basic type of ground tissue. It is made up of living cells with thin cell walls and large intercellular spaces. These cells are usually isodiametric (equal in all directions) and loosely packed.

Functions of parenchyma:

Storage of food (in roots, stems, fruits)
Photosynthesis (when it contains chloroplasts – called chlorenchyma)
Aeration (in aquatic plants – with air cavities called aerenchyma)
Healing and regeneration of injured tissues

✅ Collenchyma

Collenchyma consists of living cells with unevenly thickened cell walls, especially at the corners. It provides mechanical support and flexibility to young stems, petioles, and leaves.

Collenchyma is generally located beneath the epidermis and contains cellulose, hemicellulose, and pectin in the walls. These tissues allow bending of parts without breaking.

Functions of collenchyma:

Mechanical support to growing parts
Flexibility to stems and leaves

✅ Sclerenchyma

Sclerenchyma is made up of dead cells with very thick lignified walls and no intercellular spaces. The walls have pits, and the lumen (central cavity) is very small. Sclerenchyma provides maximum strength and rigidity.

There are two types of sclerenchyma cells:

Fibres: Long, narrow, and pointed cells (found in stems, bark, etc.)
Sclereids (stone cells): Irregular-shaped, short cells (found in seed coats, nutshells, pear fruit)

Functions of sclerenchyma:

Mechanical strength to mature parts of the plant
Protection to seeds and hard structures

🔹 Arrangement of Ground Tissues in Different Plant Parts

In Dicot Stem:

Hypodermis: Collenchyma
Cortex: Parenchyma
Endodermis: Inner boundary of cortex with starch grains
Pericycle: Next to endodermis
Pith: Large central region

In Monocot Stem:

Ground tissue is not differentiated into cortex, endodermis, and pith. It appears as a continuous mass.

In Roots:

Parenchyma forms most of the ground tissue
Cortex, endodermis, and pericycle are clearly differentiated

🌾 3. Vascular Tissue System

The vascular tissue system is concerned with the transport of water, minerals, and food throughout the plant. It consists of complex permanent tissues – xylem and phloem. These tissues are organized in units called vascular bundles.

The arrangement of vascular bundles varies in different plant parts and types (monocot vs dicot).

🔹 Xylem

Xylem transports water and minerals from the roots to the aerial parts of the plant. It is a complex tissue, made up of four types of cells:

Tracheids – elongated, dead, lignified cells with tapering ends and pits; conduct water in gymnosperms
Vessels – cylindrical, tube-like, dead cells with perforated ends; main conducting elements in angiosperms
Xylem Parenchyma – living cells that store food and assist in lateral transport
Xylem Fibres – dead, thick-walled cells that provide support

In angiosperms, xylem is called wood when it becomes secondary in nature.

🔹 Phloem

Phloem transports organic food materials (mainly sucrose) from leaves to other parts of the plant. It is also a complex tissue consisting of:

Sieve Tube Elements – elongated, living cells without nuclei; arranged end to end; have sieve plates with pores
Companion Cells – closely associated with sieve tubes; help in food loading and unloading
Phloem Parenchyma – living, helps in storage and lateral transport
Phloem Fibres – dead, provide mechanical strength

In mature phloem, sieve tubes and companion cells are essential for food transport.

🌿 Arrangement of Vascular Tissues – Vascular Bundles

Vascular bundles are the structural units of the vascular system. They are the groupings of xylem and phloem. Their arrangement differs in various plant types.

🔸 Types of Vascular Bundles

Radial Bundle:
- Xylem and phloem are arranged in different radii
- Found in roots
- Usually exarch (xylem protoxylem towards periphery)
Conjoint Bundle:
- Xylem and phloem are together on the same radius
- Found in stems and leaves

Types of conjoint bundles:

Collateral: Xylem on inner side, phloem on outer side (common in dicot stems)
- Open: With cambium (can undergo secondary growth) – dicot stem
- Closed: No cambium (no secondary growth) – monocot stem
Bicollateral: Phloem on both outer and inner sides of xylem (e.g., cucumber, pumpkin)
Concentric: One tissue surrounds the other
- Amphivasal: Xylem surrounds phloem (e.g., Dracaena)
- Amphicribral: Phloem surrounds xylem (e.g., ferns)

🔁 Comparison of Tissue Systems

Tissue System	Main Components	Functions	Found In
Epidermal	Epidermis, stomata, trichomes	Protection, water loss control, gas exchange	Outer covering of all organs
Ground	Parenchyma, collenchyma, sclerenchyma	Storage, support, photosynthesis	Interior of plant body
Vascular	Xylem, phloem	Transport of water, minerals, food	Stems, roots, leaves

🧠 Summary Points for Quick Revision

Tissue systems are classified into epidermal, ground, and vascular.
Epidermal tissue system forms the outer layer, includes stomata, trichomes, and root hairs.
Ground tissue system includes parenchyma (storage/photosynthesis), collenchyma (flexibility), and sclerenchyma (rigidity).
Vascular tissue system is made of xylem (water transport) and phloem (food transport).
Vascular bundles may be radial, collateral, bicollateral, or concentric, and their arrangement differs in stems and roots of monocots and dicots.

3. 🌱 Anatomy of Dicotyledonous Root

The internal structure of a dicotyledonous (dicot) root shows how various tissues are organized to perform vital functions like absorption of water and minerals, conduction, support, and food storage. The dicot root is a part of the primary root system that develops from the radicle of the embryo. It grows downward into the soil and helps anchor the plant firmly. Internally, the dicot root has a very clear and distinct arrangement of tissues in concentric layers.

Let’s study the anatomy of the dicot root step-by-step, starting from the outermost layer to the innermost region.

🌿 1. Epidermis (Also called Epiblema or Piliferous Layer)

The epidermis is the outermost layer of the dicot root and is also known as the epiblema or piliferous layer. This layer is made up of a single row of thin-walled, compactly arranged living cells. The cells of the epidermis do not have cuticles because water absorption is the primary function of this layer.

Many of the epidermal cells grow into long, slender, unicellular extensions called root hairs. These root hairs penetrate into soil particles and increase the absorbing surface area of the root. They help absorb water and dissolved minerals from the soil. These root hairs are short-lived and constantly replaced by new ones as the root grows.

Since the outer epidermis serves mainly to absorb water and minerals rather than protect against desiccation, it lacks stomata and cuticle. This is a major difference from the epidermis of aerial parts like stems and leaves.

🌾 2. Cortex

Beneath the epidermis lies the cortex, which is a thick region composed of several layers of parenchyma cells. These parenchymatous cells are rounded or oval and are loosely arranged with intercellular spaces. This loose arrangement facilitates the free movement of water and dissolved substances from the root hairs to the vascular tissue inside.

The cortex stores some food and also helps in the transport of water laterally toward the vascular bundles. These cells are generally thin-walled, living, and have large vacuoles. They may sometimes contain starch and other storage materials.

The cortex is non-photosynthetic because its cells do not have chloroplasts. Its major function is conduction of water and storage of nutrients. In some roots, aerenchyma (air spaces) may develop in the cortex to help in gaseous exchange.

🌰 3. Endodermis

The endodermis is the innermost layer of the cortex. It acts as a barrier between the cortex and the central vascular cylinder. The cells of the endodermis are closely packed, barrel-shaped, and arranged in a single layer without intercellular spaces.

A unique feature of endodermal cells is the presence of Casparian strips or bands. These are band-like thickenings of suberin and lignin found on the radial and transverse walls of the cells. These waterproof bands block the apoplastic pathway, forcing water and dissolved substances to pass through the cell membranes of the endodermal cells (symplastic route). This regulation ensures that only selected substances enter the vascular system.

Some cells of the endodermis are thin-walled and lack Casparian strips. These are known as passage cells, and they allow the easy movement of water and minerals into the vascular cylinder, especially in young roots.

🌳 4. Pericycle

Just inside the endodermis lies the pericycle, which is a thin layer of parenchymatous cells arranged in a single row. These cells are living and capable of division. The pericycle plays a very important role in the growth and development of the root.

Key functions of the pericycle include:

Giving rise to lateral roots or secondary roots
Participating in the formation of vascular cambium during secondary growth
Contributing to the formation of the cork cambium (phellogen)

Thus, the pericycle is responsible for the growth and branching of the root system. It lies between the endodermis and the vascular bundles.

🌴 5. Vascular Bundles

The central part of the dicot root is occupied by vascular tissues arranged in a specific pattern. The vascular system includes xylem and phloem, which are arranged alternately on different radii in the form of radial vascular bundles. This means that the xylem and phloem are not located together, but in separate groups, one after the other, around a central axis.

In dicot roots, the number of xylem and phloem bundles is usually between 2 to 6. This is called a diarch to hexarch condition. These vascular bundles are arranged in a radial pattern, with exarch xylem.

The xylem is responsible for transporting water and minerals absorbed from the soil to different parts of the plant. It consists of tracheids, vessels, xylem parenchyma, and xylem fibers. The protoxylem (first-formed xylem) lies towards the periphery, and the metaxylem (later-formed) lies towards the center. This condition is termed exarch.

The phloem is responsible for the transport of organic nutrients, mainly sugars, from the leaves to other parts of the plant. It consists of sieve tube elements, companion cells, phloem parenchyma, and phloem fibers.

Between the xylem and phloem lies a region called conjunctive tissue, made up of parenchyma. In dicot roots, this conjunctive tissue is parenchymatous and helps in storing food and also supports vascular tissues.

In older dicot roots, a strip of vascular cambium may develop from the pericycle and conjunctive parenchyma, which contributes to secondary growth by producing secondary xylem and phloem.

🌸 6. Pith

The pith is the central core region of the dicot root and is generally very small or absent. When present, it consists of parenchyma cells, which may be rounded or oval-shaped, with large intercellular spaces.

In most dicot roots, the pith is inconspicuous because the metaxylem elements (vessels and tracheids) occupy the center. The pith does not play a major role in conduction but may help in food storage and support.

In contrast to dicot stems where the pith is well-developed, the pith in dicot roots is reduced due to the convergence of vascular tissues at the center.

🌿 7. Radial Vascular Bundles with Exarch Xylem

The vascular bundles in dicot roots are radial in arrangement. This means that xylem and phloem are located on alternate radii, not side-by-side. This radial arrangement is typical of roots, whereas stems usually have conjoint vascular bundles (xylem and phloem together on the same radius).

Another key feature is that the xylem is exarch, which means the protoxylem lies towards the outside (near pericycle) and the metaxylem lies towards the center. This pattern of xylem development is exactly the opposite of stems, where xylem is endarch (protoxylem towards the center).

The exarch condition ensures that water conduction starts efficiently from the root tip upwards as the outer protoxylem matures first. This pattern helps the plant adapt to its underground environment where water absorption is crucial for survival.

🧠 Summary of the Anatomy of Dicot Root

Region	Description
Epidermis	Outermost layer; thin-walled; bears root hairs for absorption
Cortex	Several layers of parenchyma with intercellular spaces; conducts water
Endodermis	Innermost layer of cortex; contains Casparian strips; controls movement
Pericycle	Single layer; gives rise to lateral roots, cambium, cork cambium
Vascular Bundles	Radial arrangement of xylem and phloem in alternate positions
Xylem	Exarch; protoxylem outside, metaxylem inside; conducts water
Phloem	Transports food; lies between arms of xylem
Pith	Small or absent; made of parenchyma

✍️ Final Thoughts

Understanding the internal structure of the dicot root is essential to appreciate how the plant maintains water and mineral balance and supports growth. Each part of the root — from the epidermis that absorbs water, to the vascular bundles that conduct fluids, to the pericycle that gives rise to lateral roots — plays a unique role in ensuring the survival and growth of the plant.

The radial arrangement of vascular tissues and the presence of exarch xylem are two key characteristics that distinguish root anatomy from stem anatomy. The knowledge of this structure not only helps in plant physiology and pathology but also in areas like agriculture, horticulture, and botany-based entrance exams.

4. 🌾 Anatomy of Monocotyledonous Root

Roots are essential parts of plants that help in absorbing water and minerals, anchoring the plant, storing food, and sometimes even performing additional functions like aeration and support. Based on the seed type, flowering plants are divided into monocotyledons and dicotyledons. The monocot root (found in monocot plants like maize, rice, wheat, sugarcane, etc.) shows some similarities and a few distinct differences compared to dicot roots.

Just like dicot roots, monocot roots also show a well-organized arrangement of tissues in concentric layers. However, there are a few important features that help us distinguish between the anatomy of monocot and dicot roots. Let’s explore the internal structure of monocot roots in a step-by-step manner — from the outermost to the innermost layers.

🌱 General Structure – Similar to Dicot Root

The overall organization of tissues in a monocot root is quite similar to a dicot root. The root is composed of three main tissue systems:

Dermal tissue system (outermost protective covering – epidermis)
Ground tissue system (cortex, endodermis, pericycle, pith)
Vascular tissue system (xylem and phloem arranged in a radial pattern)

In both types of roots, the arrangement follows the same order: epidermis → cortex → endodermis → pericycle → vascular bundles → pith. However, the number of xylem and phloem bundles, the size of pith, and the degree of development of each tissue varies.

🌿 1. Epidermis (Piliferous Layer)

The epidermis or piliferous layer is the outermost layer of the monocot root. It is made up of a single layer of thin-walled, compactly arranged cells. This layer does not have cuticles because its main function is to absorb water and minerals from the soil.

Some epidermal cells develop into root hairs — long, tubular outgrowths that greatly increase the surface area for absorption. These hairs extend into the soil particles and are the primary sites of water and mineral intake. They are short-lived and are replaced regularly as the root grows.

Just like in dicot roots, the epidermis does not have stomata or trichomes and is completely adapted for absorption.

🌴 2. Cortex – Well-developed and Multilayered

Below the epidermis lies the cortex, which is well-developed in monocot roots. It consists of many layers of parenchyma cells. These cells are round to oval, thin-walled, and loosely packed, with abundant intercellular spaces.

This loose arrangement allows the easy movement of water and solutes from the epidermis to the central vascular cylinder. Since these cells are alive and large, they also help in temporary storage of food, especially in roots that act as storage organs.

The cortex in monocot roots is usually broader and thicker than in dicot roots. It plays a major role in transport, storage, and aeration. Chloroplasts are generally absent in cortical cells because their main function is not photosynthesis.

🌻 3. Endodermis – Innermost Layer of Cortex

The endodermis is the innermost layer of the cortex and forms a single, continuous layer of compactly arranged cells. These cells are barrel-shaped and arranged without intercellular spaces.

One of the most important features of the endodermis is the presence of Casparian strips or bands. These are ribbon-like thickenings made of suberin and lignin found on the radial and transverse walls of the endodermal cells. The Casparian strips are impermeable to water and solutes, thus forcing substances to move through the living cytoplasm of the cells (symplastic pathway), allowing controlled entry into the vascular region.

In between the normal endodermal cells, there are some cells without Casparian thickenings, called passage cells. These are thin-walled, remain unsuberised, and help in easy movement of water and minerals toward the xylem.

🌱 4. Pericycle – Source of Lateral Roots

Inside the endodermis lies the pericycle, which is a single-layered thin layer of parenchyma cells. These cells are living, and unlike dicot roots, they do not give rise to vascular cambium, because monocot roots do not undergo secondary growth.

However, the pericycle still plays an important role:

It gives rise to lateral roots (which emerge through the cortex and epidermis)
It may help in structural support and storage

In monocot roots, the pericycle may also become multi-layered in some mature roots and contribute to the thickening of the root to some extent, although not through cambial activity.

🌾 5. Vascular Bundles – Radial and Polyarch

The central part of the monocot root is occupied by a radially arranged vascular system, consisting of xylem and phloem bundles that alternate with each other. This arrangement is called radial, as the xylem and phloem lie on different radii.

The most distinctive feature of the monocot root is that it has polyarch vascular bundles. This means that the number of xylem and phloem bundles is more than six, usually 8 to 16 or even more. In contrast, dicot roots generally have only 2 to 6 bundles (diarch to hexarch condition).

In monocot roots:

Xylem is arranged in the form of many radiating arms
Between two xylem patches lies a phloem patch

The xylem is exarch, meaning that protoxylem lies towards the periphery (outer side) and metaxylem lies towards the center (inner side). This pattern ensures that the oldest xylem elements are located outside and the newer ones are formed toward the center.

The phloem lies between the xylem arms and conducts organic nutrients from the leaves and storage organs to other parts of the plant.

There is no vascular cambium between xylem and phloem in monocot roots, which is why secondary growth does not occur in them. Therefore, monocot roots usually do not increase in thickness by cambial activity like dicot roots do.

🌰 6. Conjunctive Tissue

The region lying between the xylem and phloem is called conjunctive tissue. In monocot roots, the conjunctive tissue is generally made up of parenchyma, but in some mature roots, it may become sclerenchymatous, providing extra mechanical strength.

Since there is no cambium, this conjunctive tissue does not become secondary vascular tissue. It primarily serves the function of:

Connecting xylem and phloem
Providing strength and support
Helping in lateral conduction

🌸 7. Pith – Large and Prominent

Another major difference between monocot and dicot roots is the size of the pith. In monocot roots, the pith is large, prominent, and well-developed. It occupies a significant central portion of the root.

The pith is made up of parenchyma cells which are thin-walled, round or oval, and often contain stored starch and other food materials. These cells may also help in internal transport of water and nutrients, as well as provide structural support.

In contrast, dicot roots have very small or even absent pith, as the metaxylem occupies the central part. Thus, a large pith is a diagnostic feature of monocot roots.

🧠 Summary – Key Differences Between Monocot and Dicot Roots

Feature	Monocot Root	Dicot Root
Vascular Bundles	Polyarch (>6)	Diarch to Hexarch (2–6)
Cambium	Absent (No secondary growth)	Present (Secondary growth occurs)
Pith	Large and well-developed	Small or absent
Cortex	Well-developed and broad	Comparatively narrow
Conjunctive Tissue	Mostly parenchymatous	Parenchymatous, becomes meristematic
Xylem	Exarch	Exarch
Arrangement	Radial (xylem and phloem alternate)	Radial (same as monocot)

✨ Functional Adaptations of Monocot Roots

Monocotyledonous plants, especially grasses, cereals, and aquatic monocots, often grow in environments where extensive branching of roots and rapid absorption of water and minerals is crucial. The polyarch condition with many xylem strands allows efficient water conduction. The large cortex helps in lateral movement of water and may also store nutrients.

The broad and prominent pith may assist in storing food and helping in internal transport. While they lack cambium and secondary growth, monocot roots can still thicken by enlarging cortical cells or by forming additional layers of pericycle and parenchyma, making them structurally stable even without true secondary thickening.

🔁 Complete Flow from Outside to Inside

Here’s the complete order of tissues from the outermost layer to the innermost part in a monocot root:

Epidermis (Epiblema) – Protective outer layer with root hairs for absorption
Cortex – Several layers of loosely packed parenchyma; helps in transport and storage
Endodermis – Innermost cortex with Casparian strips; controls inward flow of substances
Passage Cells – Unsuberised cells in the endodermis for water movement
Pericycle – Just inside the endodermis; gives rise to lateral roots
Vascular Bundles – Radial, polyarch arrangement of xylem and phloem
Conjunctive Tissue – Connects vascular tissues; parenchymatous
Pith – Large central region filled with parenchyma for storage

✅ Final Takeaway

The anatomy of monocot roots is structured in such a way that it allows efficient absorption, transport, and support. The radial and polyarch arrangement of xylem and phloem, the broad cortex, the large pith, and the absence of secondary growth are key characteristics. Understanding these anatomical features helps not only in identifying monocot roots under the microscope but also in understanding how they function in different environments.

5. 🌿 Anatomy of Dicotyledonous Stem

The stem is the main aerial axis of the plant that bears branches, leaves, flowers, and fruits. It plays important roles in support, conduction, transport, and storage. When observed under the microscope, the internal structure of a young dicot stem shows a well-organized tissue system arranged in concentric layers. This type of structure helps the stem perform its functions efficiently.

A dicot stem is found in plants with two cotyledons, such as sunflower, mustard, pea, bean, etc. Internally, it is composed of different types of tissues that are arranged in a fixed order from the outside to the center. Let us understand the anatomy of dicot stem layer-by-layer, beginning from the outermost part and moving inward.

🌱 1. Epidermis – Protective Outer Layer

The epidermis is the outermost protective layer of the stem. It consists of a single layer of compactly arranged living cells. These cells are barrel-shaped or rectangular, with no intercellular spaces. The outer walls of the epidermal cells are thickened and often covered with a waxy layer called the cuticle.

The cuticle protects the plant from excess water loss and entry of pathogens. It also helps in reducing transpiration. In some plants, the epidermis may have multicellular hairs or trichomes, which serve protective functions like reducing overheating and discouraging insects.

In many young stems, stomata (tiny openings) may also be present in the epidermis. These stomata are used for gaseous exchange between the plant and the atmosphere. The epidermis acts as the first line of defense and protects the inner tissues.

🌿 2. Hypodermis – Supportive Tissue Below Epidermis

Beneath the epidermis lies the hypodermis, which provides mechanical support to the young stem. In dicot stems, the hypodermis is made up of collenchyma cells. These cells are living, elongated, and have unevenly thickened cell walls due to the presence of cellulose and pectin.

The collenchyma cells are usually arranged in 2 to 3 layers just below the epidermis. The thickening of walls occurs mainly at the corners of the cells, giving them extra strength without losing flexibility. This allows the young stem to bend without breaking and helps in withstanding mechanical stress like wind and movement.

The hypodermis is absent in the internodes of older stems, but in younger stems, it plays an essential role in supporting the growing stem.

🍃 3. Cortex – Region Between Hypodermis and Endodermis

The cortex is the broad region that lies between the hypodermis and the endodermis. It is composed mainly of parenchyma cells, which are thin-walled, isodiametric (equal in all directions), and loosely packed. This loose arrangement allows the movement of air and substances between cells.

These parenchymatous cortical cells may contain starch, tannins, resin ducts, or other inclusions, depending on the plant species. Their main function is storage of food and nutrients, and they may also help in lateral transport of water and solutes.

In some dicot stems, the outer layers of cortex (below hypodermis) may contain chloroplasts, and this part is called chlorenchyma, which performs photosynthesis.

The cortex region may also act as a temporary storage tissue and plays a role in healing and regeneration after injury.

🌸 4. Endodermis – Innermost Layer of the Cortex

The endodermis is the inner boundary of the cortex and is present as a single layer of barrel-shaped cells. These cells are usually tightly packed without intercellular spaces. The endodermal cells often contain starch grains, which is why this layer is sometimes referred to as the starch sheath.

In stems, the endodermis is not as distinct as it is in roots. Also, in stems, Casparian strips (found in root endodermis) are usually absent or poorly developed. However, the endodermis still functions as a boundary between the cortex and the central vascular region.

The main functions of the endodermis include:

Regulating the flow of materials into vascular tissues
Providing protection to inner tissues
Storing food materials like starch

🌴 5. Pericycle – Tissue Just Inside the Endodermis

The pericycle lies just beneath the endodermis and surrounds the vascular bundles. In dicot stems, the pericycle is usually made up of a few layers of sclerenchyma (dead, thick-walled cells) and sometimes mixed with parenchyma. The sclerenchymatous pericycle is usually located above the phloem of the vascular bundles.

This layer plays an important role in:

Providing mechanical support and protection to vascular tissues
Sometimes participating in the formation of lateral branches
Forming part of the vascular cambium in plants undergoing secondary growth

In older stems, the pericycle may give rise to cork cambium (phellogen), which later develops into bark.

🌾 6. Vascular Bundles – Open, Collateral, and Arranged in Ring

The vascular bundles in dicot stems are the main conducting tissues, made up of xylem and phloem. In dicot stems, these bundles are arranged in a circular ring inside the pericycle, around the central pith. This ring arrangement is a typical feature of dicot stems.

🔹 Collateral Vascular Bundles

Each vascular bundle is collateral, which means that the xylem and phloem lie on the same radius, with xylem located towards the center and phloem towards the periphery. This arrangement allows for efficient conduction of water and food throughout the plant.

🔹 Open Vascular Bundles

Dicot vascular bundles are open, meaning they contain a strip of cambium between xylem and phloem. The cambium is meristematic (can divide), and it plays a vital role in secondary growth. It produces secondary xylem (wood) on the inner side and secondary phloem on the outer side. This results in the thickening of the stem as the plant grows older.

The cambium also allows for the formation of growth rings seen in trees and shrubs.

🔹 Arrangement in a Ring

The vascular bundles are arranged in the form of a complete ring surrounding the pith. This ring-like structure provides strength and mechanical stability, helping the plant stand upright.

The xylem is made up of:

Tracheids – long, tapering, dead cells with pits
Vessels – wide, cylindrical tubes that conduct water
Xylem parenchyma – living cells for storage
Xylem fibers – dead, supportive fibers

The phloem consists of:

Sieve tubes – long conducting tubes
Companion cells – regulate sieve tubes
Phloem parenchyma – storage
Phloem fibers – thick-walled, support-giving cells

Together, xylem and phloem conduct water, minerals, and organic nutrients throughout the plant.

🌰 7. Pith – Central Region

The pith (also called medulla) is the central part of the dicot stem. It is made up of large, thin-walled parenchyma cells that are loosely arranged with intercellular spaces. These cells store starch, oils, resins, and other food materials.

In young stems, the pith is large and well-developed. It helps in:

Storage of food and water
Transport of substances through parenchyma
Mechanical support in some cases

In woody plants, the pith may reduce in size as the plant matures due to the accumulation of secondary xylem and phloem.

🧠 Summary of Internal Structure of Dicot Stem

Region	Description
Epidermis	Single layer, outer protective layer with cuticle and sometimes trichomes
Hypodermis	2–3 layers of collenchyma for mechanical support
Cortex	Parenchyma cells with storage function and sometimes chlorenchyma
Endodermis	Innermost cortex layer; contains starch grains (starch sheath)
Pericycle	Sclerenchyma/parenchyma; protects vascular tissue and may form cambium
Vascular Bundles	Collateral, open, arranged in a ring with cambium for secondary growth
Pith	Large, central parenchymatous region for storage and conduction

✨ Key Features of Dicot Stem Anatomy (Quick Revision)

Epidermis is protective with a cuticle and may have stomata or trichomes.
Hypodermis is made of collenchyma, giving strength and flexibility.
Cortex is parenchymatous, helps in storage, and may be photosynthetic.
Endodermis is a single layer with starch grains (acts as a food reservoir).
Pericycle may include sclerenchyma and is important for support and secondary growth.
Vascular bundles are collateral, open (have cambium), and arranged in a circular ring.
Pith is large and made of soft parenchyma that stores food and helps in transport.

📚 Final Thoughts

The internal anatomy of a dicot stem is highly organized and specialized for various essential plant functions. Every layer of tissue contributes to the growth, support, protection, and transport systems of the plant. The presence of cambium, ring-shaped vascular bundles, distinct cortex, and well-defined pith make dicot stems unique.

Understanding this topic is not just important for theory-based NEET questions but also helpful in solving diagram-based questions, comparisons, and application-level problems. Once you clearly understand the structure-function relationship, it becomes easier to tackle any question from this topic.

6. 🌾 Anatomy of Monocotyledonous Stem

The stem of a monocotyledonous plant (commonly called a monocot stem) shows unique internal features. It is designed to provide support, allow the conduction of food and water, and store nutrients. Monocot plants include grasses, maize (corn), sugarcane, wheat, rice, bamboo, and banana. Their stem shows a different anatomical arrangement compared to dicot stems.

Let us now study the internal structure of the monocot stem, layer by layer, starting from the outer surface and going inward.

🌱 1. Outer Layer – Epidermis

The epidermis is the outermost protective layer of the monocot stem. It is made up of a single layer of compactly arranged cells that are flat or rectangular in shape. These epidermal cells are living, and the outer walls are often covered with a thick, waxy cuticle. This cuticle prevents excess water loss through evaporation and protects the plant from mechanical damage, heat, and pathogen attack.

The epidermis acts like a barrier between the plant and the environment. It does not contain chloroplasts, hence does not perform photosynthesis. In some monocots, stomata and multicellular hairs may be present on the epidermis, though they are more common in leaves than in stems.

The main function of the epidermis is protection, and it forms the outermost boundary of the stem.

🌴 2. Hypodermis – Sclerenchymatous Mechanical Layer

Below the epidermis lies the hypodermis, which in monocot stems is sclerenchymatous. This means it is composed of thick-walled, dead sclerenchyma cells. These cells have lignified walls (impregnated with lignin), making them very strong and rigid.

The hypodermis is arranged in 2–3 layers, and since it is made of dead cells, it provides mechanical strength rather than flexibility. This is an important distinction from dicot stems, where the hypodermis is collenchymatous and living.

The sclerenchymatous hypodermis acts as a supportive tissue, helping the monocot stem withstand bending, pressure, and wind. It is especially important in monocot plants that are tall, slender, or grass-like, providing them the necessary stiffness to remain upright.

🌿 3. Ground Tissue – Undifferentiated and Uniform

The ground tissue system in monocot stems is not differentiated into cortex, endodermis, pericycle, and pith. Instead, it appears as one continuous and uniform mass of parenchyma cells. This is a major difference from dicot stems, where the ground tissue is well-organized and divided into distinct regions.

The parenchyma cells in the ground tissue are:

Thin-walled
Round or oval
Living
Have intercellular spaces

Some of the ground tissue cells may contain starch, which serves as food storage. In a few cases, chloroplasts may be present, allowing some amount of photosynthesis.

The entire central region of the stem, from below the hypodermis to the very center, is filled with this parenchymatous ground tissue. There is no separate pith, so the ground tissue acts as filler material, storage tissue, and conduction tissue.

The lack of clear boundaries like endodermis and cortex gives the monocot stem its simple and compact design.

🌾 4. Vascular Bundles – Scattered, Collateral, Closed

The vascular bundles (which consist of xylem and phloem) in the monocot stem show three key features:

They are scattered throughout the ground tissue
Each bundle is collateral
They are closed, meaning there is no cambium between xylem and phloem

Let’s explore each of these in more detail.

🔹 a) Scattered Arrangement

In monocot stems, the vascular bundles are randomly scattered in the ground tissue. This is different from dicot stems where vascular bundles are arranged in a ring around the pith. In monocots, you can find vascular bundles both near the periphery and deep inside the central region.

This scattered pattern gives monocot stems a spotted appearance under the microscope. The number and size of vascular bundles vary — those near the periphery are smaller and more numerous, while those toward the center are larger and fewer.

This distribution helps in:

Providing mechanical support throughout the stem
Even conduction of water, minerals, and food
Allowing the stem to resist bending and breaking

🔹 b) Collateral Bundles

Each vascular bundle in the monocot stem is collateral, meaning xylem and phloem are placed side-by-side on the same radius. In this arrangement:

Xylem is located towards the center (inner side)
Phloem lies towards the periphery (outer side)

There is no cambium between them, which means no secondary growth can occur in monocot stems. This is why monocots do not show increase in girth or thickness with age, unlike dicots.

🔹 c) Closed Vascular Bundles

The vascular bundles are closed, meaning they lack cambium. Cambium is the lateral meristem that divides to produce secondary xylem and phloem. Since monocots don’t have cambium, they cannot produce secondary tissues, and thus they do not undergo secondary thickening.

This means the vascular system remains the same throughout the plant’s life, and no additional rings of conducting tissue are added. However, monocots like palm trees grow tall by expanding parenchyma and increasing vascular bundle size, not by cambial growth.

🔹 d) Xylem and Phloem Arrangement

The xylem contains:

Vessels (wide tubes for water conduction)
Tracheids (less prominent in monocots)
Xylem parenchyma (living cells for storage)
Xylem fibers (supportive cells)

The xylem is endarch, meaning the protoxylem lies towards the center, and the metaxylem towards the periphery — the reverse of root xylem.

The phloem contains:

Sieve tubes (main conducting elements)
Companion cells (help in controlling sieve tubes)
Phloem parenchyma (may be absent in monocots)
Phloem fibers (supportive tissue)

Each vascular bundle is surrounded by a bundle sheath made of sclerenchyma cells, which provides extra protection and support to the vascular tissues.

🌰 5. Absence of Cambium and Secondary Growth

Monocot stems do not have vascular cambium, so they cannot produce secondary xylem or phloem. This is a major anatomical difference between monocot and dicot stems.

Because of this:

No growth rings are seen in monocot plants
No increase in diameter with age (except in some exceptional monocots like palm)
Monocot stems remain soft, herbaceous, or sometimes woody without cambium

Some monocots may show a thickened stem due to enlargement of parenchyma or formation of specialized secondary tissues, but this is not true secondary growth.

🌿 6. Summary of Monocot Stem Anatomy

Feature	Monocot Stem Description
Epidermis	Single outer layer, with cuticle, sometimes stomata/hairs
Hypodermis	Made of 2–3 layers of sclerenchyma, gives rigidity
Ground Tissue	Uniform, undifferentiated, made of parenchyma
Vascular Bundles	Scattered, numerous, vary in size from outer to inner regions
Type of Bundles	Collateral (xylem & phloem side by side)
Cambium	Absent, hence bundles are closed
Xylem Orientation	Endarch (protoxylem towards center, metaxylem outside)
Bundle Sheath	Sclerenchymatous layer around each vascular bundle
Secondary Growth	Absent due to lack of cambium

🔍 Key Differences Between Dicot and Monocot Stem

Characteristic	Dicot Stem	Monocot Stem
Vascular Bundles	Arranged in a ring	Scattered throughout ground tissue
Bundle Type	Open (with cambium)	Closed (no cambium)
Secondary Growth	Present (due to cambium)	Absent
Hypodermis	Collenchymatous (living, flexible)	Sclerenchymatous (dead, rigid)
Ground Tissue	Differentiated (cortex, pith, etc.)	Undifferentiated
Pith	Well-defined	Not distinct
Bundle Sheath	Not always distinct	Clearly present (sclerenchymatous)

✨ Final Thoughts

The monocot stem has a compact and efficient design adapted to the needs of grass-type and herbaceous plants. The sclerenchymatous hypodermis gives mechanical support, the scattered vascular bundles allow even distribution of transport, and the lack of secondary growth makes the stem light and flexible.

These features are essential for plants that grow quickly and do not need thick, woody stems. Understanding the internal anatomy of monocot stems helps students appreciate how structure supports function in plant physiology.

7. 🍃 Anatomy of Leaf (Dorsiventral and Isobilateral)

The leaf is one of the most important parts of a plant. It performs photosynthesis, which is the process by which plants make their own food using sunlight, carbon dioxide, and water. To carry out this function efficiently, the internal structure of a leaf is specially arranged. The study of this internal arrangement is called leaf anatomy.

There are two main types of leaves based on their internal structure:

Dorsiventral leaf (also called bifacial leaf) – usually found in dicot plants
Isobilateral leaf – usually found in monocot plants

Even though the basic parts are the same in both types of leaves, their arrangement and structure differ to suit the plant’s environment and mode of life.

Let us now study both types in detail, one by one.

🌿 1. Dorsiventral Leaf (Dicot Leaf)

A dorsiventral leaf has a distinct upper surface (adaxial) and lower surface (abaxial). The two sides look different both externally and internally. The upper side is usually darker green and receives more sunlight, while the lower side is lighter and contains more stomata (tiny pores for gas exchange). These types of leaves are horizontal in position.

Let’s understand its anatomy layer by layer:

🔹 (a) Epidermis (Upper and Lower)

The epidermis is the outermost layer of the leaf. It is present on both the upper and lower surfaces.

The upper epidermis lies on the top (adaxial) side of the leaf.
The lower epidermis lies on the bottom (abaxial) side.

Both layers are made up of a single layer of compactly arranged, flat cells. These cells are usually colorless, without chloroplasts, and are covered with a cuticle — a waxy layer that prevents water loss.

The main functions of the epidermis are:

Protection of inner tissues
Prevention of water loss due to cuticle
Regulation of gas exchange through stomata

Stomata are more numerous in the lower epidermis in dorsiventral leaves. Each stoma is guarded by two guard cells that control the opening and closing of the pore. In dicot leaves, guard cells are usually kidney-shaped.

🔹 (b) Mesophyll – Photosynthetic Tissue

The tissue present between the upper and lower epidermis is called mesophyll. It is the main site for photosynthesis and is made up of parenchyma cells containing chloroplasts.

In dorsiventral leaves, the mesophyll is differentiated into two distinct regions:

Palisade parenchyma
Spongy parenchyma

▪️ Palisade Parenchyma:

Located just below the upper epidermis.
Made up of elongated, tightly packed cells arranged vertically.
Contains a large number of chloroplasts.
Main site for photosynthesis because it receives more sunlight.
Usually arranged in 1 to 3 layers.

▪️ Spongy Parenchyma:

Located below the palisade parenchyma, towards the lower epidermis.
Made up of loosely arranged cells with large intercellular spaces.
Fewer chloroplasts than palisade cells.
Allows for gas exchange (CO₂ and O₂) and temporary storage of gases.
Air spaces connect to stomatal openings, helping in transpiration and respiration.

Thus, the mesophyll in dorsiventral leaves is clearly divided into upper palisade and lower spongy tissues.

🔹 (c) Vascular Bundles – Veins of the Leaf

The vascular bundles of the leaf are called veins. These bundles are found scattered in the mesophyll and appear as lines (veins) when viewed from the surface.

Each vascular bundle consists of:

Xylem – for upward conduction of water and minerals.
Phloem – for downward conduction of food (mainly sugars).

In dorsiventral leaves:

Xylem is present towards the upper side (adaxial side).
Phloem is present towards the lower side (abaxial side).

These vascular bundles are surrounded by a bundle sheath, made of parenchyma or sometimes sclerenchyma cells for protection and support.

Veins provide:

Mechanical strength
Transport of water and food
Support for flatness and expansion of the leaf blade

🌾 2. Isobilateral Leaf (Monocot Leaf)

An isobilateral leaf is one where the upper and lower surfaces are similar in structure and appearance. These leaves are usually found in monocot plants like maize, sugarcane, wheat, rice, etc. They are generally vertically oriented, so both surfaces receive almost equal sunlight.

Let’s understand the internal structure of an isobilateral leaf.

🔹 (a) Epidermis (Upper and Lower)

Just like dorsiventral leaves, isobilateral leaves also have epidermis on both surfaces — upper (adaxial) and lower (abaxial).

Made of flat, tightly packed cells.
Covered with a thick cuticle to prevent water loss.
Stomata are present equally on both sides (unlike dorsiventral where they are more on lower side).
Guard cells are dumbbell-shaped in monocots.

In some parts of the upper epidermis, there are special large, bubble-like cells called bulliform cells.

🌟 Bulliform Cells:

Also called motor cells.
Present in groups in the upper epidermis of monocot leaves.
These are large, thin-walled, and empty when they lose water.
When the plant faces water shortage or high temperature, bulliform cells lose turgidity and collapse, causing the leaf to roll up.
This reduces the surface area exposed to sunlight and helps in reducing water loss.

When water is again available, these cells regain water and the leaf unfolds. Thus, bulliform cells help the plant in water conservation.

🔹 (b) Mesophyll – Not Differentiated

In isobilateral leaves, the mesophyll is not divided into palisade and spongy parenchyma. Instead, it is made of similar-looking parenchyma cells throughout.

These mesophyll cells are:

Loosely arranged
Contain numerous chloroplasts
Have small intercellular spaces

Photosynthesis takes place throughout the mesophyll. Since both surfaces of the leaf receive light, there is no need for separate palisade and spongy layers.

🔹 (c) Vascular Bundles – Parallel and Uniform

In monocot leaves, the vascular bundles are arranged in parallel lines (parallel venation), running from base to tip of the leaf. The midrib contains the largest vascular bundle, and other smaller bundles are arranged parallel on both sides.

Each vascular bundle has:

Xylem towards the upper side
Phloem towards the lower side

The bundles are surrounded by a bundle sheath, made of sclerenchyma or parenchyma. This sheath protects the vascular tissue and helps in mechanical support.

In isobilateral leaves:

The vascular bundles are larger near the midrib
They become smaller as they move towards the margins

Some vascular bundles may also be embedded with mechanical tissue, giving the leaf stiffness.

🧠 Summary Table – Dorsiventral vs Isobilateral Leaf

Feature	Dorsiventral Leaf (Dicot)	Isobilateral Leaf (Monocot)
Leaf Orientation	Horizontal	Vertical
Epidermis	Upper and lower, more stomata on lower	Upper and lower, stomata on both
Bulliform Cells	Absent	Present on upper epidermis
Mesophyll	Differentiated into palisade & spongy	Not differentiated
Palisade Parenchyma	Present below upper epidermis	Absent
Spongy Parenchyma	Present above lower epidermis	Absent
Vascular Bundles	Net venation, irregular arrangement	Parallel venation, arranged in rows
Xylem & Phloem Position	Xylem above, phloem below	Xylem above, phloem below
Stomatal Type	Kidney-shaped guard cells	Dumbbell-shaped guard cells

🌼 Functional Importance of Leaf Anatomy

Understanding the internal structure of leaves helps us know how plants:

Make food through photosynthesis
Exchange gases
Regulate water through transpiration
Adapt to their environments (e.g., bulliform cells in dry areas)

Dorsiventral leaves are suited for maximum sunlight capture on one side, while isobilateral leaves are adapted for equal exposure on both sides.

8. Secondary Growth in Dicot Stem

🌿 What is Secondary Growth ?

In most dicot plants, the stem and root become thicker with age. This increase in girth or diameter is called secondary growth. It happens due to the activity of lateral meristems, especially the vascular cambium and cork cambium (phellogen).

Unlike primary growth (which increases length and is done by apical meristems), secondary growth adds layers of tissues to the sides of stems and roots. It helps dicot plants become woody, strong, and capable of living for many years.

Secondary growth mainly occurs in:

Dicotyledonous stems
Dicot roots
Gymnosperms

Monocots usually do not show secondary growth, with a few exceptions like Dracaena and Yucca.

🌱 1. Vascular Cambium Activity – Formation of Secondary Xylem and Phloem

The vascular cambium is a cylinder of meristematic tissue located between the primary xylem and primary phloem. It is laterally positioned (runs along the length of the stem) and is responsible for the formation of secondary vascular tissues – secondary xylem and secondary phloem.

🌀 How Does Vascular Cambium Form ?

In young dicot stems:

The vascular bundles are arranged in a ring.
Each bundle has open type vascular bundles (with a strip of cambium between xylem and phloem).
These strips of cambium are called intrafascicular cambium.

As the plant matures:

Some parenchyma cells between the vascular bundles (medullary rays) also become meristematic.
These form the interfascicular cambium.

Intrafascicular + Interfascicular cambium join to form a complete circular ring of vascular cambium.

Now this vascular cambium ring becomes active and starts dividing.

🔄 Activity of Vascular Cambium

Once active, the vascular cambium produces new tissues on both sides:

Secondary Xylem (wood) is formed towards the inner side.
Secondary Phloem (inner bark) is formed towards the outer side.

🌳 Key Points:

Secondary xylem builds up in large quantity, making the stem woody and thick.
Secondary phloem is produced in smaller amounts, and the older phloem layers are crushed or displaced outward.
The pressure caused by growing secondary xylem eventually breaks the cortex and epidermis.
The older layers of secondary xylem become non-functional but remain as a support tissue.

🌲 2. Formation of Annual Rings

In regions with seasonal changes (like India), secondary growth doesn’t occur continuously. It varies with seasons, resulting in the formation of concentric rings called annual rings.

🕰 What Are Annual Rings ?

Each year, the vascular cambium produces:

Spring wood (early wood) during the favorable season (spring/summer) – thin-walled, large cells, rapid growth.
Autumn wood (late wood) during the less favorable season (autumn/winter) – thick-walled, small cells, slow growth.

The contrast between spring wood and autumn wood forms a dark ring. Each ring represents one year of growth.

Thus, the number of annual rings in a tree trunk indicates its age. This method of age determination is called Dendrochronology.

📌 Characteristics of Annual Rings:

Visible in transverse section (cross-section) of the stem.
More pronounced in temperate climates.
Not very clear in tropical plants (where growth is continuous).

Annual rings serve as natural record keepers, providing clues about past climate, rainfall, and environmental conditions.

🌳 3. Periderm Formation – Cork Cambium, Cork, and Phelloderm

As secondary growth proceeds, the epidermis and cortex are stretched and ruptured due to the increasing girth of the stem. To replace the damaged outer layers, the plant forms a new protective covering called the periderm.

Periderm consists of:

Cork Cambium (Phellogen)
Cork (Phellem)
Secondary Cortex (Phelloderm)

These three layers together form the periderm, which replaces the epidermis in mature woody plants.

🌿 (a) Cork Cambium (Phellogen)

The cork cambium is a secondary lateral meristem that arises from the outer cortex or sometimes from the hypodermis. It is made of narrow, rectangular, meristematic cells.

The phellogen divides and produces:

Cork cells (phellem) towards the outside
Secondary cortex (phelloderm) towards the inside

The cork cambium is short-lived, but it is active every year, forming new layers of protective tissue.

🌰 (b) Cork (Phellem)

The cork is formed from the outer divisions of phellogen. Cork cells are:

Dead at maturity
Thick-walled
Impermeable to water and gases
Walls are impregnated with suberin, a waxy substance

Cork provides:

Protection from physical damage
Waterproofing
Insulation against temperature

This is the layer commonly known as bark in trees. Cork is commercially used for bottle stoppers, flooring, and insulation.

🍃 (c) Phelloderm (Secondary Cortex)

The phelloderm is produced on the inner side of cork cambium. It is made of:

Living parenchyma cells
Thin-walled
May contain chloroplasts, starch, and other inclusions

Phelloderm is similar to cortical parenchyma and helps in:

Photosynthesis (if green)
Storage of food
Healing of wounds

Together, cork, cork cambium, and phelloderm form the periderm, which replaces the epidermis during secondary growth.

🌲 Bark – What Is It ?

The term bark refers to all the tissues outside the vascular cambium, including:

Secondary phloem
Periderm (cork, cork cambium, phelloderm)

There are two types of bark:

Early bark (soft) – Formed early in the growing season.
Late bark (hard) – Formed later; thick and dark.

Sometimes, bark peels off in sheets (e.g., guava), while in others, it remains rough (e.g., mango, neem).

🌿 Lenticels – Tiny Pores in Bark

As cork is impermeable, the plant needs special structures for gaseous exchange. These are called lenticels.

🍃 Characteristics:

Appear as raised spots or slits on the stem surface.
Made of loosely arranged cells (complementary tissue).
Allow oxygen and carbon dioxide to pass through bark.
Visible on the stems of woody plants and fruits (e.g., apples).

Lenticels play an important role in plant respiration, especially when stomata are absent in stems.

🧠 Summary of Secondary Growth in Dicot Stem

Feature	Description
Vascular Cambium	Lateral meristem between xylem & phloem; forms secondary xylem & phloem
Secondary Xylem	Formed towards inside; becomes wood
Secondary Phloem	Formed outside; older layers crushed
Annual Rings	Alternate spring & autumn wood; shows plant age
Periderm	Protective outer layer (cork, cork cambium, phelloderm)
Cork	Dead, suberized cells; prevent water loss
Phelloderm	Living parenchyma; photosynthesis & storage
Lenticels	Openings in bark for gaseous exchange
Secondary Growth	Increase in stem/root diameter in dicots

📌 Differences Between Primary and Secondary Growth

Feature	Primary Growth	Secondary Growth
Type of Growth	Increase in length	Increase in thickness/girth
Responsible Meristem	Apical meristem	Lateral meristem (cambium, cork cambium)
Found In	All vascular plants	Mostly in dicots and gymnosperms
Growth Pattern	Continuous	Seasonal (in temperate plants)
Vascular Bundles	No new formation	New secondary xylem and phloem added

🌳 Importance of Secondary Growth

Provides mechanical strength to tall plants
Helps form wood used in construction and furniture
Creates bark, which protects the inner tissues
Allows plant to live longer and grow larger
Stores water and nutrients in parenchymatous cells

📝 Final Thoughts

Secondary growth in dicot stems is a vital biological process that ensures the long life, strength, and survival of many plants. It helps trees form thick, woody trunks, and also contributes to wood and bark that humans use for multiple purposes.

9. Secondary Growth in Roots

🌿 Introduction to Secondary Growth in Roots

In most dicot plants, not only the stem but also the roots grow thicker as the plant matures. This increase in diameter or girth is called secondary growth. It is essential for:

Providing strength
Improving water conduction
Supporting large and tall plants

Just like in stems, this thickening happens due to the activity of lateral meristems, specifically:

Vascular Cambium – gives rise to new xylem and phloem
Cork Cambium (Phellogen) – gives rise to new outer protective layers

Although the overall process of secondary growth in roots is similar to that in stems, there are some key differences, mainly in how the vascular cambium is formed. Let us understand this entire process step by step.

🌾 Primary Structure of a Young Dicot Root (Before Secondary Growth)

To understand secondary growth, we must first look at the internal structure of a young dicot root.

From outer to inner, the major tissues are:

Epidermis (also called epiblema) – outermost protective layer
Cortex – parenchymatous, stores food and helps in transport
Endodermis – inner boundary of cortex, has a Casparian strip
Pericycle – layer just inside the endodermis
Conjunctive tissue – parenchyma between xylem and phloem
Radial vascular bundles – xylem and phloem are separate and arranged alternately in a ring (no cambium present)
Pith – centrally located, small or absent

Now let’s see how secondary growth modifies this structure.

🔄 Step-by-Step Process of Secondary Growth in Dicot Root

1️⃣ Formation of Vascular Cambium

The vascular bundles in young dicot roots are radial – i.e., xylem and phloem are placed on alternate radii, not side by side as in stems. Also, there is no cambium present initially between xylem and phloem.

So, the first step in secondary growth is the formation of vascular cambium, which happens as follows:

Certain parenchyma cells of the conjunctive tissue, which lies between xylem and phloem, become meristematic (i.e., they start dividing).
These meristematic cells form strip-like layers between the xylem and phloem.
These strips connect with the pericycle cells (above the phloem poles) that also become meristematic.
Slowly, a complete ring of vascular cambium is formed.

🔹 This vascular cambium originates from two tissues:

Conjunctive tissue (between xylem and phloem)
Pericycle (at the phloem poles)

Now that a full ring of cambium is formed, it begins its secondary growth activity.

2️⃣ Cambial Activity – Formation of Secondary Tissues

Once the cambium ring is established, it becomes active and starts dividing:

Secondary xylem is formed towards the inner side
Secondary phloem is formed towards the outer side

🔹 Important points:

More secondary xylem is produced than phloem
Due to increased secondary xylem, the primary xylem gets pushed towards the center
The central pith may get crushed or disappear
The root becomes solid and woody

The arrangement of tissues begins to resemble that of a dicot stem, even though it started out differently.

3️⃣ Disruption of Primary Tissues

As the secondary tissues grow:

The outer tissues (epidermis, cortex) cannot stretch indefinitely
These tissues rupture, especially the epidermis
A new protective layer must form

That’s where periderm comes into play.

🌳 Formation of Periderm – Replacing Outer Tissues

As the cortex and epidermis break, the plant forms a new secondary protective layer called periderm. The periderm is made of three parts:

a) Cork Cambium (Phellogen)

Arises from the outermost layer of pericycle or cortical cells.
It is a lateral meristem.
Phellogen divides to form:
- Cork (phellem) towards the outer side
- Phelloderm (secondary cortex) towards the inner side

b) Cork (Phellem)

Formed by outer divisions of phellogen
Cells are:
- Dead at maturity
- Thick-walled and suberized
Cork is impermeable to water and gases
Protects inner tissues from:
- Drying out
- Infections
- Mechanical injury

c) Phelloderm

Formed by inner divisions of phellogen
Made of living parenchyma cells
Helps in storage and sometimes photosynthesis if green

This entire set of layers – cork, cork cambium, and phelloderm – is called the periderm.

🌿 Lenticels in Roots

Since cork is impermeable, roots and stems form lenticels – small openings in the bark.

Formed at intervals in the cork
Allow exchange of gases
Appear as raised spots or slits on surface
Also found in fruits (e.g., apples)

Though more common in stems, roots also develop lenticels in woody plants.

🌱 Summary of Secondary Growth in Roots

Let’s recap the major events:

Vascular cambium forms from:
- Conjunctive parenchyma
- Pericycle at phloem poles
Cambium produces:
- Secondary xylem (inwards)
- Secondary phloem (outwards)
Primary tissues like cortex and epidermis rupture
Cork cambium (phellogen) arises from pericycle
Phellogen gives rise to:
- Cork (phellem) – outer side
- Phelloderm – inner side
Formation of periderm as a new protective covering
Lenticels may form in the cork for gas exchange

🌿 Differences: Secondary Growth in Stem vs Root

Feature	Stem	Root
Origin of Vascular Cambium	Intrafascicular + Interfascicular cambium	Conjunctive tissue + Pericycle
Type of Vascular Bundle	Collateral (xylem & phloem together)	Radial (xylem and phloem on alternate sides)
Cambium Position	Between xylem and phloem in a bundle	Between xylem and phloem of different bundles
Secondary Xylem/Phloem	Towards inner/outer sides respectively	Same (xylem inside, phloem outside)
Pith	Prominent in stem	Usually crushed in root
Cortex	Crushed in both cases	Same

🧠 Functional Importance of Secondary Growth in Roots

Secondary growth is essential for:

Strengthening the root system
Better support for tall trees and large shrubs
Increased absorption and transport capacity
Storing starch and other substances in parenchyma
Replacing broken outer tissues with protective periderm

🔍 Real-Life Examples

In trees like mango, banyan, and neem, you can observe thick woody roots – a sign of active secondary growth.
Old roots become almost as hard and woody as stems due to years of secondary tissue deposition.
Root barks are used medicinally, e.g., sarpgandha, ashwagandha, because of active tissues formed during secondary growth.

📝 Final Takeaway for NEET

Secondary growth in roots is a frequently asked topic in NEET. You should remember:

How vascular cambium originates from conjunctive tissue and pericycle
What tissues are formed by cambium and cork cambium
Differences from stem secondary growth
Terms like phellogen, phellem, phelloderm and their roles
Diagrams showing stages of secondary growth in root

Understanding this topic helps you:

Answer theory-based MCQs
Label diagrams of root cross-sections
Score well in plant anatomy questions

📌 Diagram (for Practice)

You can draw or practice a labeled T.S. of a mature dicot root showing:

Epidermis (ruptured)
Periderm (cork, phellogen, phelloderm)
Secondary phloem
Vascular cambium ring
Secondary xylem (inside)
Remnants of primary xylem in the center

10. 🌳 Heartwood and Sapwood

🌿 Introduction

As trees grow older and undergo secondary growth, their stems increase in thickness. During this process, new layers of secondary xylem are formed every year by the activity of the vascular cambium. These layers of xylem accumulate year after year, forming the bulk of the woody stem or trunk.

Over time, the xylem in the central region of the stem stops conducting water and undergoes various chemical and physical changes. This central, older, non-functional xylem becomes known as the heartwood, while the newer, outer xylem that is still active in conduction is called the sapwood.

Understanding heartwood and sapwood is important because they not only provide mechanical support and conduction, but also show how trees grow, age, and adapt. Let’s explore each of them in detail.

🌲 What is Sapwood ?

Sapwood, also known as alburnum, is the outer region of the secondary xylem in a woody stem. It lies just inside the vascular cambium and forms the younger portion of the wood.

🔹 Characteristics of Sapwood:

Lighter in color because it is still alive and functional.
Contains living xylem cells that are actively involved in the transport of water and minerals.
Made up of tracheids, vessels, xylem parenchyma, and xylem fibres.
The cells are permeable, and their walls are thin and soft.
Can store starch, oils, and other nutrients in xylem parenchyma cells.
Contains functional vascular tissue that connects roots to leaves.
Resin, gum, and other deposits are absent in sapwood.

🧠 Function of Sapwood:

Its main job is to conduct water and minerals from the roots to the leaves.
It also provides some storage of nutrients and food materials.
Although it’s not as hard as heartwood, it still supports the stem structure to some extent.

🌳 What is Heartwood ?

Heartwood, also known as duramen, is the central, innermost part of the stem in a mature tree. It is formed from older layers of xylem that are no longer active in conduction.

🔹 Characteristics of Heartwood:

Darker in color due to accumulation of tannins, resins, oils, gums, and other substances.
Made of dead xylem cells, mainly tracheids and fibres.
Chemically modified, and cell lumens are often blocked, making it non-functional in conduction.
These chemical changes make heartwood hard, dense, and durable.
The structure becomes stronger due to lignification and deposition of substances.
Does not participate in water conduction anymore.
The tissues become dry and more compact than sapwood.

🧠 Function of Heartwood:

Heartwood provides mechanical strength and rigidity to the tree.
It supports the weight of the branches and crown.
It protects the inner region of the stem from insects, fungi, and decay due to its chemical composition.
It also acts as a storage region for plant waste substances like gums and resins.

🔄 Transition from Sapwood to Heartwood

As a tree ages:

New secondary xylem is continuously added to the outer layers.
The inner layers of sapwood gradually become non-functional.
These inner layers lose the ability to conduct water, and the cells die.
Over time, they become filled with various substances, and the region is transformed into heartwood.

This transformation is slow and natural, ensuring that the tree maintains conduction through outer sapwood while strengthening itself through inner heartwood.

🔍 Microscopic and Chemical Differences

Feature	Sapwood	Heartwood
Function	Conducts water and minerals	Provides support
Color	Light-colored	Dark-colored
Living/Dead Cells	Mostly living	All cells dead
Conductivity	Conductive	Non-conductive
Chemical Composition	Low in resins/tannins	Rich in tannins, gums, oils, etc.
Hardness	Softer	Harder and denser
Resistance to Decay	Less	More due to antimicrobial compounds
Position in Stem	Outer secondary xylem	Central xylem region

🧠 Significance of Sapwood

1. Water Conduction:

Sapwood is responsible for moving water and dissolved minerals from roots to leaves. It contains open xylem vessels and tracheids that allow the flow of fluid through capillary action and transpiration pull.

2. Storage:

Some cells in the sapwood, especially the xylem parenchyma, store starch, lipids, and other food materials. This provides energy for wound healing and growth.

3. Plant Healing and Repair:

If a plant part is damaged, the nearby living tissues in sapwood help in repair and regeneration.

4. Temperature Regulation:

By maintaining fluid flow, sapwood helps in cooling the plant through transpiration and protects the stem from overheating.

🌳 Significance of Heartwood

1. Mechanical Support:

Heartwood forms the core of the trunk. It is dense, hard, and strong, supporting the entire weight of the tree, branches, and leaves.

2. Long-Term Durability:

The chemical compounds in heartwood (like tannins and resins) make it resistant to decay, insect attacks, and microbial damage.

3. Natural Protection:

Heartwood acts as a barrier against fungal invasion and physical damage. It seals off the inner part of the stem and strengthens the tree base.

4. Commercial Use:

Heartwood is preferred in the timber industry. It is used for:

Furniture
Construction
Flooring
Carving
Because it is more durable, resistant, and visually appealing.

🌴 Examples of Trees with Distinct Heartwood

Teak (Tectona grandis) – Golden-brown heartwood, high in oil content, very durable
Rosewood (Dalbergia spp.) – Dark red or brown heartwood, heavy and aromatic
Neem (Azadirachta indica) – Yellowish-brown heartwood, used in furniture and pest control
Shisham (Dalbergia sissoo) – Strong hardwood used in carvings and agricultural tools
Pine – Heartwood is less distinct, used in light furniture

🪵 Commercial Importance of Heartwood and Sapwood

Timber Industry:

Heartwood is preferred due to its strength, polish, and durability.
Sapwood is often removed before using wood commercially because:
- It may decay faster.
- It is more vulnerable to insects and fungi.
- It stains easily.

Furniture and Flooring:

Heartwood gives a polished, attractive appearance.
It is used for making doors, tables, chairs, wardrobes, etc.

Fuel and Charcoal:

Both sapwood and heartwood are used as firewood, but heartwood gives better charcoal due to its density.

🌿 Is Heartwood Essential for the Tree ?

Interestingly, heartwood is not vital for the survival of the tree. A tree can survive even if its heartwood is damaged or hollow, as long as the sapwood is intact and conducting water properly.

However, the mechanical strength of the tree depends heavily on heartwood. A tree with hollow heartwood may be weaker and more prone to falling during storms.

📘 Summary of Key Points

Sapwood is the outer, younger part of secondary xylem that is active in conduction.
Heartwood is the inner, older part of secondary xylem that is non-conductive but provides support.
Sapwood is lighter, softer, and contains living cells.
Heartwood is darker, denser, filled with resins, oils, and tannins.
Heartwood provides long-term strength and protection, while sapwood ensures water and nutrient flow.
With time, sapwood turns into heartwood as it loses function and becomes chemically modified.
Annual rings, observed in tree trunks, show the accumulation of xylem (both sapwood and heartwood).

🌿 Final Words

Understanding heartwood and sapwood is essential to appreciate how trees:

Grow over time
Maintain balance between strength and function
Survive for hundreds of years
Provide humans with wood, medicine, and ecological support

11. 🌿 Lenticels

🌱 Introduction

Plants, like all living organisms, need to exchange gases with their surroundings. They take in oxygen for respiration and carbon dioxide for photosynthesis, while also releasing water vapor and other gases. In young green stems and leaves, this gas exchange is mainly done by stomata — small openings mostly present on the lower surface of leaves.

But what about woody stems and old branches, where stomata are absent or no longer functional? How do these parts of the plant “breathe”? This is where lenticels come into play.

Lenticels are small pores or openings found on the surface of woody stems, roots, and some fruits. They allow the plant to continue gaseous exchange even after the outer surface becomes covered with thick protective layers like bark or cork.

Let’s understand everything about lenticels — their structure, functions, and importance.

🌳 What Are Lenticels ?

Lenticels are lens-shaped or slit-like openings that appear on the bark of woody stems and roots. They often appear as raised, rough, or slightly cracked areas on the stem surface.

These structures are formed during secondary growth in dicot stems and roots, replacing the role of stomata in older tissues. While stomata are usually found on leaves and are controlled by guard cells, lenticels are permanent openings that remain open all the time.

They are named from the word “lens”, due to their typical shape.

🔬 Structure of Lenticels

Lenticels form in the periderm — the protective outer covering that replaces the epidermis in woody plants during secondary growth. The periderm consists of:

Cork (phellem) – protective outer dead layer
Cork cambium (phellogen) – meristematic layer that produces cork and phelloderm
Phelloderm – inner living parenchyma cells

When cork cambium becomes active, it sometimes forms loosely arranged, thin-walled parenchyma cells in certain areas. These loosely packed cells form a spongy tissue called complementary tissue. This tissue pushes against the outer cork, rupturing it and forming a small opening — the lenticel.

🔹 Key Structural Features:

Complementary tissue: Loosely arranged parenchyma cells with many air spaces.
Suberized cork cells: Sometimes mixed with complementary tissue, limiting water loss.
No guard cells: Unlike stomata, lenticels do not have specialized guard cells.
Permanent openings: Lenticels remain open and are not regulated by any mechanism.
Can be raised or sunken: They may appear as raised dots, slits, or patches on the bark.

🔍 Appearance:

On young stems, lenticels are often light-colored dots.
On mature trees, they may appear as dark cracks or slit-like structures.
In fruits like apples or pears, lenticels appear as tiny brown spots on the skin.

🌿 How Are Lenticels Formed ?

During secondary growth in dicot stems:

The vascular cambium forms secondary xylem and phloem.
The epidermis ruptures due to increase in stem thickness.
A new protective layer called periderm forms from cork cambium.
At some locations, the cork cambium produces complementary tissue instead of tightly packed cork cells.
These regions swell outward, breaking the cork and forming a pore or opening – a lenticel.

Lenticel formation often begins just beneath a stoma, but it eventually replaces the function of stomata in those areas.

🌬️ Function of Lenticels

The main function of lenticels is to allow the exchange of gases between the internal tissues of the plant and the external atmosphere.

This includes:

Oxygen intake for cellular respiration.
Release of carbon dioxide produced during respiration.
Release of water vapor in some cases (though minor compared to stomata).

🔹 Key Functions:

Gaseous Exchange:
- Supply of oxygen to living cells of cortex and phloem.
- Removal of carbon dioxide generated from metabolic activities.
Maintaining Life in Non-Photosynthetic Tissues:
- Older stems and roots do not photosynthesize.
- Still, their cells respire, so lenticels allow aerobic respiration.
Compensating for Loss of Stomata:
- In woody parts where stomata disappear, lenticels become the only pathway for gas exchange.
Helping in Healing and Regrowth:
- Oxygen provided through lenticels helps in cell regeneration and tissue healing after injury.

🌲 Lenticels in Roots

Yes, roots also have lenticels, especially in woody plants. These lenticels are:

Found in the periderm of roots.
Allow oxygen to reach deep tissues.
Play a role in aerobic respiration of root cells.

In some wetland plants, lenticels are very prominent and large, helping in gaseous exchange in flooded conditions.

🍎 Lenticels in Fruits

You might have seen small brown or black dots on apples, pears, or mangoes. These are lenticels.

In fruits, lenticels:

Help the fruit tissues in breathing.
Are sometimes blocked or damaged, causing discoloration or disease.
Allow moisture to evaporate from the fruit, affecting shelf life.

Overactive lenticels may also result in cracking or uneven texture in some fruits.

🔍 Differences Between Stomata and Lenticels

Feature	Stomata	Lenticels
Found On	Leaves, young stems	Bark of stems, roots, fruits
Structure	Guard cells with pore	Loose parenchyma (complementary tissue)
Regulation	Open/close based on need	Remain open permanently
Visibility	Usually microscopic	Visible to naked eye
Formation	From epidermal cells	From cork cambium
Function	Photosynthesis, transpiration, respiration	Gaseous exchange in woody parts

🧠 Importance of Lenticels in Plant Life

1. Essential for Respiration

Plants need oxygen for aerobic respiration.
Internal tissues like secondary phloem and cortex are active and need a continuous oxygen supply.
Lenticels ensure that even non-green, woody parts can “breathe”.

2. Survival in Harsh Conditions

In thick-stemmed trees, where bark is hard and waterproof, lenticels are the only pores.
Help in gas exchange even during rain, drought, or frost.

3. Support in Secondary Growth

As plants undergo secondary growth, their structure changes.
Lenticels evolve as a new adaptation to continue gas exchange.

4. Role in Plant Breathing Balance

Together with stomata, lenticels balance the plant’s respiration.
Help maintain oxygen and carbon dioxide levels inside plant tissues.

🌳 Variations in Lenticel Structure

Lenticels can vary in:

Shape: round, oval, elongated, or slit-like.
Size: small dots (e.g., apple) or long cracks (e.g., cherry tree).
Color: white, brown, black, or red.
Structure: compact or loosely packed complementary tissue.

These differences depend on:

Type of plant
Age of the tissue
Environmental conditions

🍃 Examples of Plants with Lenticels

Mango – rough patches on old bark are lenticels.
Guava – lenticels are very visible on thin bark.
Cherry tree – horizontal slits on bark are lenticels.
Apple – tiny brown dots on fruit skin are lenticels.
Pear – same as apples; visible and sometimes sunken lenticels.

🧪 Experimental Observation

In lab experiments, you can observe:

Take a twig from a tree, and you’ll find raised dots or lines — those are lenticels.
If you cover lenticels with wax, the branch may die due to lack of oxygen — proving their importance in gas exchange.

📝 Summary of Key Points

Lenticels are openings in the bark of stems, roots, and fruits.
They are formed due to secondary growth.
Made of loosely packed complementary tissue, allowing gas exchange.
They remain open permanently and do not have guard cells.
Lenticels replace stomata in woody regions.
Allow oxygen in and carbon dioxide out, enabling aerobic respiration in internal tissues.
Found in dicot stems, roots, and some fruits.
Appear as raised dots, patches, or slits on the surface.
Essential for the survival of woody plants.

🔍 Common NEET Questions on Lenticels

Where are lenticels found?
→ On bark of woody stems, roots, and fruits.
What is their main function?
→ Gaseous exchange in parts lacking stomata.
Are lenticels regulated like stomata?
→ No, lenticels are always open.
How are lenticels formed?
→ From cork cambium by producing loosely packed complementary tissue.
What replaces stomata in old stems?
→ Lenticels.

🌟 Final Thoughts

Though small and often unnoticed, lenticels are crucial for a plant’s survival. They act like the “lungs” of woody parts — constantly bringing in oxygen and removing carbon dioxide, even when the plant is covered in hard bark.

Understanding lenticels not only helps in NEET preparation but also gives us insight into how beautifully nature has adapted to changing plant needs — replacing stomata with lenticels as plants mature.