Observing the external structure (morphology) of higher plants reveals fascinating similarities and variations. Similarly, the internal structure of plants also shows both commonalities and differences.

The study of the internal structure and organization of plant tissues and organs is called Anatomy.

Plants are fundamentally composed of cells, which are the basic units. These cells are organized into groups called tissues. Different tissues are further organized into organs (like roots, stems, and leaves).

Different plant organs have distinct internal structures. Within angiosperms, monocots and dicots also show significant anatomical differences.

The internal structures of plants are often adapted to diverse environmental conditions.

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The Tissues

A tissue is defined as a group of cells that have a common origin and typically perform a specific, shared function.

Plant tissues are broadly classified into two main types based on the division capability of their cells:

1. Meristematic tissues: Cells are actively dividing.
2. Permanent tissues: Cells have lost the ability to divide and have become specialized.

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Meristematic Tissues

Plant growth, particularly in length and girth, occurs in specialized regions containing actively dividing cells called meristems (from Greek 'meristos', meaning divided).

Based on their location, meristems are classified as:

1. Apical Meristems: Located at the tips (apices) of roots and shoots. They are responsible for increasing the length of the plant (primary growth).
   • Root apical meristem: Found at the root tip.
   • Shoot apical meristem: Found at the stem tip.

   As the shoot elongates and leaves form, some cells from the shoot apical meristem remain behind in the leaf axils, forming axillary buds. These buds can develop into branches or flowers.

2. Intercalary Meristems: Found located between mature tissues, often at the base of leaves or internodes (e.g., in grasses). They are responsible for regenerating parts removed by grazing or adding to the length of internodes.
3. Lateral Meristems: Occur in the mature regions of roots and shoots, particularly in plants that develop woody growth. They appear later in the plant's life than primary meristems and are responsible for increasing the girth or diameter of the plant parts (secondary growth). They are cylindrical in shape. Examples include:
   • Fascicular vascular cambium: Found within the vascular bundles.
   • Interfascicular cambium: Forms between vascular bundles.
   • Cork-cambium (Phellogen): Forms in the cortex or outer layers.

   These lateral meristems produce secondary tissues.

Apical and intercalary meristems are called primary meristems because they appear early in the plant's life and contribute to the formation of the primary plant body (resulting from primary growth).

Cells produced by both primary and secondary meristems eventually stop dividing, become specialized in structure and function, and differentiate into permanent tissues.

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Permanent Tissues

Cells in permanent tissues have lost the ability to divide and have specific roles. Permanent tissues are classified based on the types of cells they contain:

• Simple tissues: Composed of only one type of cells that are structurally and functionally similar.
• Complex tissues: Composed of more than one type of cells that work together as a unit to perform a specific function.

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Simple Tissues

Simple permanent tissues consist of a single type of cell. The three main types in plants are:

1. Parenchyma: Forms the bulk of plant organs.
   • Cell Shape: Generally isodiametric (equal in diameter in all directions), but can be spherical, oval, round, polygonal, or elongated.
   • Cell Wall: Thin-walled, made of cellulose.
   • Cell Arrangement: May be closely packed or have small intercellular spaces.
   • Functions: Photosynthesis (when chloroplasts are present), storage of food, secretion.
2. Collenchyma: Provides mechanical support to growing parts.
   • Location: Found in layers below the epidermis, especially in dicot stems and petioles, either as continuous layers or patches.
   • Cell Shape: Oval, spherical, or polygonal.
   • Cell Wall: Characteristically thickened at the corners due to the deposition of cellulose, hemicellulose, and pectin.
   • Intercellular Spaces: Generally absent.
   • Other Features: May contain chloroplasts and perform photosynthesis.
   • Functions: Provides mechanical support to growing stems and petioles.
3. Sclerenchyma: Provides mechanical support and rigidity.
   • Cell Shape: Long and narrow (fibres) or spherical, oval, cylindrical (sclereids).
   • Cell Wall: Thick and lignified, often with pits.
   • Protoplast: Usually dead cells without protoplasm at maturity.
   • Types:
     • Fibres: Elongated, thick-walled, pointed cells, often found in groups.
     • Sclereids: Spherical, oval, or cylindrical, highly thickened cells with very narrow cavities (lumen). Found in fruit walls of nuts, pulp of fruits (guava, pear, sapota), seed coats of legumes, and tea leaves.
   • Function: Provides mechanical support to plant organs and makes tissues hard.

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Complex Tissues

Complex permanent tissues are composed of more than one type of cell working together. The two main complex tissues in plants are xylem and phloem.

1. Xylem:
   • Primary Function: Acts as the main water and mineral conducting tissue from roots to stem and leaves. Also provides mechanical strength.
   • Components: Composed of four different types of elements in angiosperms:
     • Tracheids: Elongated, tube-like cells with thick, lignified, tapering walls. They are dead (without protoplasm) at maturity. Inner wall thickenings vary in form.
     • Vessels: Long cylindrical tubes formed by many interconnected cells called vessel members. Vessel members have lignified walls and a large central cavity, and are also dead at maturity. Interconnected via perforations in common walls. The presence of vessels is a characteristic of angiosperms (gymnosperms lack vessels).
     • Xylem fibres: Thick-walled with obliterated central lumens. May be septate or aseptate. Provide mechanical support.
     • Xylem parenchyma: Living, thin-walled cells (cellulose walls). Store food (starch, fat) and other substances (tannins). Involved in radial conduction of water (via ray parenchymatous cells).
   • Primary Xylem: Differentiated into protoxylem (first formed, narrower vessels/tracheids) and metaxylem (later formed, wider vessels/tracheids).
     • Endarch: Protoxylem towards the pith (centre), metaxylem towards the periphery (found in stems).
     • Exarch: Protoxylem towards the periphery, metaxylem towards the centre (found in roots).
2. Phloem:
   • Primary Function: Transports food materials (sugars, mainly) from leaves (where they are produced) to other parts of the plant.
   • Components (in angiosperms):
     • Sieve tube elements: Long, tube-like cells arranged longitudinally. End walls are perforated to form sieve plates, allowing food transport. Mature sieve elements have peripheral cytoplasm and a large vacuole but lack a nucleus. Their function is controlled by companion cells.
     • Companion cells: Specialized parenchymatous cells closely associated with sieve tube elements. Connected by pit fields in common walls. Help maintain pressure gradient in sieve tubes.

     (Gymnosperms have albuminous cells and sieve cells instead of sieve tubes and companion cells).

     • Phloem parenchyma: Elongated, cylindrical cells with dense cytoplasm and nucleus. Cellulose walls with pits. Store food (resins, latex, mucilage). Absent in most monocots.
     • Phloem fibres (Bast fibres): Sclerenchymatous cells, generally absent in primary phloem but found in secondary phloem. Elongated, unbranched, pointed, thick-walled, and dead at maturity. Provide mechanical support. Commercially used fibres (jute, flax, hemp) are phloem fibres.
   • Primary Phloem: Differentiated into protophloem (first formed, narrow sieve tubes) and metaphloem (later formed, bigger sieve tubes).

The Tissue System

Tissues are organized into larger units based on their location and function within the plant body. There are three main types of tissue systems:

1. Epidermal tissue system: The outermost protective covering.
2. Ground or fundamental tissue system: All tissues except the epidermis and vascular tissues.
3. Vascular or conducting tissue system: Composed of xylem and phloem.

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Epidermal Tissue System

Forms the outermost protective layer covering the entire plant body (except for root cap).

Components:

• Epidermal cells: Elongated, compactly arranged parenchymatous cells, usually forming a single, continuous layer called the epidermis. Have a small amount of cytoplasm and a large vacuole.
• Cuticle: A waxy, protective layer covering the outer surface of the epidermis in shoots. Helps prevent water loss by transpiration. Absent in roots.
• Stomata: Pores present in the epidermis (mainly of leaves). Regulate gaseous exchange (CO$_2$ uptake) and transpiration (water vapour loss). Each stoma is surrounded by two specialized cells called guard cells.
• Guard cells are bean-shaped in dicots and dumb-bell shaped in grasses.
• Their outer walls (away from the pore) are thin, while inner walls (towards the pore) are thick.
• Guard cells contain chloroplasts and control the opening and closing of the stomatal pore.
• Subsidiary cells: Sometimes, specialized epidermal cells surrounding the guard cells are present, called subsidiary cells.

The stomatal aperture (pore), guard cells, and subsidiary cells together form the stomatal apparatus.

• Epidermal appendages: Outgrowths from the epidermis.
  • Root hairs: Unicellular elongations of epidermal cells in roots, increasing surface area for water and mineral absorption.
  • Trichomes: Epidermal hairs on the shoot system. Usually multicellular, can be branched or unbranched, soft or stiff, and sometimes secretory. Help reduce water loss through transpiration.

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The Ground Tissue System

Includes all tissues of the plant body except the epidermal tissue system and the vascular bundles.

Composition: Primarily consists of simple tissues like parenchyma, collenchyma, and sclerenchyma.

Regions:

• In primary roots and stems, the ground tissue is typically divided into:
  • Cortex: Located between the epidermis and vascular bundles. Often composed of parenchymatous cells.
  • Pericycle: Layer(s) located just inside the endodermis, surrounding the vascular tissues.
  • Pith: The central region, usually composed of parenchymatous cells.
  • Medullary rays: Radially arranged parenchymatous cells between vascular bundles in stems.
• In leaves, the ground tissue (between the upper and lower epidermis) is called mesophyll, which consists of thin-walled, chloroplast-containing parenchymatous cells for photosynthesis.

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The Vascular Tissue System

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

Arrangement of Xylem and Phloem in Vascular Bundles:

• Open vascular bundles: Presence of cambium between the xylem and phloem. Allows for secondary growth (production of secondary xylem and phloem) (found in dicot stems).
• Closed vascular bundles: Absence of cambium. No secondary growth occurs (found in monocot stems).
• Radial arrangement: Xylem and phloem are arranged alternately on different radii within the vascular bundle (found in roots).
• Conjoint arrangement: Xylem and phloem are located on the same radius within the vascular bundle.
  • Common in stems and leaves.
  • Usually, the phloem is located on the outer side of the xylem.

Anatomy Of Dicotyledonous And Monocotyledonous Plants

Studying transverse sections (T.S.) of mature root, stem, and leaf zones is helpful for understanding the internal tissue organization and anatomical differences between dicots and monocots.

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Dicotyledonous Root

Transverse section of a dicot root (e.g., Sunflower root) shows the following layers from outside to inside (Figure 6.6 a):

1. Epiblema: The outermost layer (epidermis). Many cells extend as unicellular root hairs.
2. Cortex: Consists of several layers of thin-walled parenchyma cells with intercellular spaces.
3. Endodermis: The innermost layer of the cortex. Composed of a single layer of barrel-shaped cells without intercellular spaces. Characterized by the presence of water-impermeable, waxy material called suberin deposited in tangential and radial walls, forming Casparian strips.
4. Pericycle: Layer(s) of thick-walled parenchymatous cells located just inside the endodermis. Important for initiating lateral roots and vascular cambium during secondary growth.
5. Vascular Bundles: Xylem and phloem are arranged radially, separated by parenchymatous conjunctive tissue. Typically, there are two to four (diarch to tetrarch) xylem and phloem patches.
6. Pith: The central region, small or inconspicuous.

The tissues inside the endodermis (pericycle, vascular bundles, pith) collectively form the stele.

Secondary growth in dicot roots involves the formation of a cambium ring between xylem and phloem.

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Monocotyledonous Root

The anatomy of a monocot root is similar to a dicot root in having epidermis (epiblema) with root hairs, cortex, endodermis with Casparian strips, pericycle, vascular bundles, and pith (Figure 6.6 b).

Key differences compared to dicot root:

• Vascular Bundles: Usually have more than six (polyarch) xylem bundles.
• Pith: Large and well-developed.
• Secondary Growth: Monocotyledonous roots do not undergo secondary growth.

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Dicotyledonous Stem

Transverse section of a young dicot stem shows the following arrangement of tissues from outside to inside (Figure 6.7 a):

1. Epidermis: Outermost protective layer, covered by a thin cuticle, may bear trichomes and some stomata.
2. Cortex: Multiple layers of cells between the epidermis and pericycle, differentiated into three sub-zones:
   • Hypodermis: Outer few layers, typically collenchymatous, providing mechanical strength to the young stem.
   • Cortical layers: Below hypodermis, composed of rounded, thin-walled parenchymatous cells with intercellular spaces.
   • Endodermis: Innermost layer of the cortex, cells rich in starch grains (also called starch sheath).
3. Pericycle: Present on the inner side of the endodermis, above the phloem, often as semi-lunar patches of sclerenchyma.
4. Medullary rays: A few layers of radially placed parenchymatous cells between the vascular bundles.
5. Vascular Bundles: A large number arranged in a ring, which is characteristic of dicot stems. Each bundle is conjoint, open (with cambium), and has endarch protoxylem (protoxylem towards the pith).
6. Pith: Large central portion, composed of rounded parenchymatous cells with large intercellular spaces.

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Monocotyledonous Stem

Transverse section of a monocot stem shows key differences from a dicot stem (Figure 6.7b):

• Hypodermis: Made of sclerenchyma.
• Vascular Bundles: Numerous, scattered within the ground tissue (not arranged in a ring). Each is surrounded by a sclerenchymatous bundle sheath. Peripheral bundles are generally smaller than central ones. Vascular bundles are conjoint and closed (no cambium).
• Ground Tissue: A large, conspicuous, continuous mass of parenchymatous tissue filling the region from the hypodermis to the centre. It is not differentiated into cortex, endodermis, pericycle, pith, or medullary rays.
• Phloem Parenchyma: Absent in monocot stems.
• Water-containing cavities: Often present within the vascular bundles.

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Dorsiventral (Dicotyledonous) Leaf

Transverse section of a dorsiventral leaf (showing distinct upper and lower surfaces) reveals three main parts (Figure 6.8 a):

1. Epidermis: Covers both the upper (adaxial) and lower (abaxial) surfaces. Both are covered by a conspicuous cuticle. The abaxial epidermis usually has more stomata than the adaxial epidermis (which may even lack stomata).
2. Mesophyll: The tissue located between the upper and lower epidermis. Composed of parenchyma cells containing chloroplasts (photosynthetic). It is differentiated into two types:
   • Palisade parenchyma: Located on the adaxial side, composed of elongated cells arranged vertically and parallel to each other.
   • Spongy parenchyma: Located below the palisade parenchyma, composed of oval or round, loosely arranged cells with numerous large intercellular spaces and air cavities.
3. Vascular System: Includes the vascular bundles found in the veins and the midrib. The size of the vascular bundles correlates with the size of the veins (larger in midrib, smaller in finer veins). Vascular bundles are surrounded by a layer of thick-walled bundle sheath cells. Xylem is typically located towards the adaxial (upper) side, and phloem towards the abaxial (lower) side within the vascular bundle.

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Isobilateral (Monocotyledonous) Leaf

The anatomy of an isobilateral leaf (where both surfaces appear similar) has some similarities to the dorsiventral leaf but shows key differences (Figure 6.8 b):

• Stomata: Present on both the adaxial and abaxial epidermal surfaces.
• Mesophyll: Not differentiated into palisade and spongy parenchyma. It consists of uniformly arranged parenchymatous cells.
• Vascular Bundles: In monocot leaves with parallel venation, the vascular bundles are of nearly similar sizes (except the main veins), and are surrounded by bundle sheath cells.
• Bulliform cells: In grasses (a type of monocot), certain adaxial epidermal cells along the veins are modified into large, empty, colourless cells called bulliform cells. These cells help in rolling and unrolling of leaves in response to water availability. When turgid, they expose the leaf surface; when flaccid due to water stress, they cause the leaf to curl inwards, reducing water loss.

Secondary Growth

Plant growth that results in an increase in length (due to apical meristems) is called primary growth.

In most dicotyledonous plants (and gymnosperms), there is an additional increase in girth or diameter of the stem and root. This increase is called secondary growth.

Secondary growth is carried out by the activity of two lateral meristems: the vascular cambium and the cork cambium.

Secondary growth does not occur in monocotyledons.

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Vascular Cambium

The vascular cambium is the meristematic layer responsible for producing secondary xylem (towards the inside) and secondary phloem (towards the outside).

In a young dicot stem, vascular cambium is initially present in patches as a single layer located between the primary xylem and primary phloem. Later, this forms a complete ring called the cambial ring.

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Formation Of Cambial Ring

In dicot stems, the cambium located within the vascular bundles (between the primary xylem and phloem) is called intrafascicular cambium (fascicular cambium).

The cells of the medullary rays that are adjacent to the intrafascicular cambium become meristematic and form the interfascicular cambium.

The joining of intrafascicular and interfascicular cambia results in the formation of a continuous cambial ring.

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Activity Of The Cambial Ring

Once formed, the cambial ring becomes active and starts dividing, cutting off new cells both towards the pith (inside) and towards the periphery (outside).

• Cells cut off towards the pith mature into secondary xylem.
• Cells cut off towards the periphery mature into secondary phloem.

The vascular cambium is typically more active on the inner side (producing more secondary xylem) than on the outer side (producing less secondary phloem).

This differential activity leads to a greater accumulation of secondary xylem, which forms a compact mass. The primary and secondary phloem tissues are gradually crushed due to this continuous production of secondary xylem.

The primary xylem, located at or near the center, remains relatively intact.

At some points, the cambium produces narrow bands of parenchyma cells that extend radially through the secondary xylem and secondary phloem. These are called secondary medullary rays (Figure 6.9).

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Spring Wood And Autumn Wood

The activity of the vascular cambium is influenced by environmental and physiological factors, especially in temperate regions with distinct seasons.

• Spring Wood (Early Wood): In spring, the cambium is highly active. It produces a large quantity of xylary elements with wider vessels. This wood is lighter in colour and has a lower density.
• Autumn Wood (Late Wood): In winter, the cambial activity decreases. It produces fewer xylary elements with narrower vessels. This wood is darker and has a higher density.

The distinct layers of spring wood and autumn wood produced in a year form a concentric ring known as an annual ring.

Annual rings are visible in a cross-section of a woody stem and can be used to estimate the age of a tree (dendrochronology).

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Heartwood And Sapwood

In older trees, the central or innermost layers of the secondary xylem often become dark brown.

This is due to the deposition of various organic compounds like tannins, resins, oils, gums, aromatic substances, and essential oils in the central region.

These deposits make the central wood hard, durable, and resistant to microbial and insect attacks.

• Heartwood: This dark, central region of secondary xylem is called heartwood. It consists of dead elements with highly lignified walls. Its primary function is to provide mechanical support to the stem; it does not conduct water.
• Sapwood: The peripheral region of the secondary xylem remains lighter in colour. This is the sapwood. It consists of living cells (xylem parenchyma) and is responsible for the conduction of water and minerals from the root to the leaves.

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Cork Cambium

As the stem increases in girth due to vascular cambium activity, the outer protective layers (cortex and epidermis) get stretched and eventually break. A new meristematic tissue, the cork cambium or phellogen, develops to replace them and provide new protection.

Origin: Usually develops in the cortex region (can also originate from epidermis or pericycle).

Structure: Typically a couple of layers thick, composed of narrow, thin-walled, nearly rectangular cells.

Activity: Phellogen actively divides, cutting off cells on both sides:

• Outer cells differentiate into cork or phellem. Phellem cells are dead and have suberin deposited in their cell walls, making them impervious to water and gases.
• Inner cells differentiate into the secondary cortex or phelloderm. Phelloderm cells are parenchymatous and living.

Periderm: The cork cambium (phellogen), cork (phellem), and secondary cortex (phelloderm) are collectively known as the periderm.

Bark: The continuous activity of the cork cambium produces layers that build pressure on the tissues outside the phellogen, causing these outer layers to die and slough off. Bark is a non-technical term referring to all tissues located exterior to the vascular cambium. This includes the secondary phloem and the periderm.

Bark formation varies seasonally, leading to early (soft) bark and late (hard) bark.

Lenticels: At certain points, the phellogen produces parenchymatous cells instead of cork cells on the outer side. These cells are loosely arranged (complementary cells) and rupture the epidermis, forming lens-shaped openings called lenticels (Figure 6.10 a). Lenticels facilitate the exchange of gases between the internal tissues and the atmosphere. They are common in woody trees.

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Secondary Growth In Roots

Secondary growth also occurs in the roots of most dicotyledonous plants and gymnosperms, increasing their girth (Figure 6.11).

Origin of Cambium: In the dicot root, the vascular cambium is entirely secondary in origin.

• It originates from the tissue located just below the phloem bundles and a portion of the pericycle tissue located above the protoxylem.
• This forms a continuous but initially wavy ring, which later becomes circular.

Activity: The subsequent steps of secondary growth (activity of vascular cambium and cork cambium) are similar to those in the dicot stem, resulting in the production of secondary xylem and phloem, and periderm.

As mentioned earlier, secondary growth does not occur in monocotyledonous roots or stems.

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