Plant Tissues and Anatomy
The Tissues
In multicellular organisms, cells performing similar functions are organised into groups called tissues. Plant tissues are groups of cells having a common origin and usually performing a common function.
Plant tissues can be broadly classified into two main types based on whether the cells are capable of dividing or not:
- Meristematic Tissues: Composed of actively dividing cells.
- Permanent Tissues: Composed of cells that have lost the ability to divide and have differentiated to perform specific functions.
Meristematic Tissues
Meristematic tissues contain actively dividing cells. These cells are typically small, isodiametric, with thin cell walls, dense cytoplasm, and prominent nuclei. They lack vacuoles.
Types of Meristems (Based on Position)
- Apical Meristems: Present at the tips of roots and shoots. They are responsible for primary growth, leading to an increase in the length of the plant axis.
- Root apical meristem: Present at the tip of the root.
- Shoot apical meristem: Present at the tip of the shoot.
- Intercalary Meristems: Present between permanent tissues, typically at the base of leaves or internodes. They are also responsible for primary growth, allowing for elongation (e.g., in grasses, helping them regenerate parts removed by herbivores). They are remnants of apical meristems.
- Lateral Meristems: Present along the lateral sides of stems and roots in dicotyledonous plants and gymnosperms. They are responsible for secondary growth, leading to an increase in the thickness or girth of the plant organ. They are cylinders of meristematic tissue.
- Fascicular vascular cambium: Present within the vascular bundles.
- Interfascicular cambium: Present between the vascular bundles.
- Cork cambium (phellogen): Present in the cortex region.
Note: Apical and intercalary meristems are called primary meristems because they appear early in plant life and contribute to the formation of the primary plant body. Lateral meristems are called secondary meristems because they appear later and are responsible for secondary growth.
*(Image shows a plant diagram indicating root apical meristem, shoot apical meristem, intercalary meristem in stem/leaf base, and lateral meristems (cambium) in mature stem/root girth)*
Permanent Tissues
Permanent tissues are composed of cells that have differentiated and lost the ability to divide. They are derived from meristematic tissues. The cells of permanent tissue may be living or dead, and thin-walled or thick-walled.
Permanent tissues are classified into two main types:
- Simple Tissues: Made up of only one type of cell.
- Complex Tissues: Made up of more than one type of cell, working together as a unit.
Simple Tissues
Simple tissues are homogeneous, composed of only one type of cell.
Parenchyma
- Structure: Consists of living, thin-walled cells. Cells are usually isodiametric (spherical, oval, round, polygonal, or elongated). Cell walls are thin and made of cellulose. Cells are closely packed or may have small intercellular spaces.
- Function: Primary functions include storage of food and water. They also perform secretion. In some cases, they are involved in photosynthesis (chloroplasts present, called chlorenchyma) or buoyancy (large air spaces, called aerenchyma).
- Location: Found in almost all parts of the plant body: cortex, pith, mesophyll of leaves, ground tissue in stems and roots, and in xylem and phloem.
Collenchyma
- Structure: Consists of living, elongated cells with irregularly thickened corners due to deposition of cellulose, hemicellulose, and pectin. Intercellular spaces are generally absent.
- Function: Provides mechanical support to the growing parts of the plant, such as young stem, petiole of leaf.
- Location: Found in layers below the epidermis in most dicotyledonous plants (e.g., hypodermis of dicot stem). Absent in monocots.
Sclerenchyma
- Structure: Consists of dead cells with thick, lignified cell walls. The cell walls are uniformly thickened, often leaving a very narrow lumen (cell cavity). Cells may have pits.
- Function: Provides mechanical support to mature parts of the plant, giving them rigidity and strength.
- Location: Found in cell walls of nuts, pulp of fruits like guava, pear and sapota, seed coats of legumes, and leaves of tea. Also forms the sclerenchymatous bundle sheath in monocots.
- Types:
- Fibres: Long, narrow, thick-walled, lignified cells with pointed ends. Found in various parts like xylem, phloem, pericycle.
- Sclereids: Short, isodiametric or irregular shaped, thick-walled cells. Found in the pulp of fruits, seed coats, and nutshells.
Complex Tissues
Complex tissues are made up of more than one type of cells which work together as a unit to perform a common function. The main complex tissues are Xylem and Phloem. These are also known as vascular tissues, and together they form the vascular bundles.
Xylem
Xylem is the water and mineral conducting tissue in plants. It also provides mechanical strength.
Xylem is composed of four different kinds of elements:
- Tracheids: Elongated, dead cells with tapering ends and lignified cell walls. Have pits but lack perforation plates. Found in gymnosperms and pteridophytes as the main water-conducting elements.
- Vessels: Long cylindrical tube-like structures made up of many cells called vessel members. Vessel members are dead and have lignified walls with perforations (perforation plates) in their end walls, forming a continuous pipe. Found mainly in angiosperms (flowering plants), where they are the main water-conducting elements.
- Xylem Parenchyma: Living, thin-walled cells found associated with xylem. Store food materials (starch, fats) and tannins. Involved in radial conduction of water.
- Xylem Fibres: Thick-walled, lignified, tapering dead cells. Provide mechanical support. They can be septate or aseptate.
Primary xylem differentiates from procambium. Secondary xylem differentiates from vascular cambium (during secondary growth).
Primary xylem is differentiated into Protoxylem (first formed primary xylem, with narrower vessels/tracheids) and Metaxylem (later formed primary xylem, with wider vessels/tracheids).
- Endarch: Protoxylem lies towards the pith, and metaxylem lies towards the periphery. Found in stems.
- Exarch: Protoxylem lies towards the periphery, and metaxylem lies towards the centre. Found in roots.
*(Image shows illustrations of tracheids, vessel members (forming a vessel), xylem parenchyma cells, and xylem fibres)*
Phloem
Phloem is the tissue responsible for the transport of organic nutrients (sugars, primarily sucrose) synthesised during photosynthesis, from the leaves to other parts of the plant.
In angiosperms, phloem is composed of four different kinds of elements:
- Sieve Tube Elements: Living, elongated cells arranged end to end forming sieve tubes. Their end walls are perforated, forming sieve plates, which allow passage of food material. They lack a nucleus at maturity.
- Companion Cells: Living, specialised parenchyma cells closely associated with sieve tube elements. They are connected to sieve tube elements by pit fields in the common wall. Companion cells have a nucleus and control the activity of the sieve tube elements.
- Phloem Parenchyma: Living, thin-walled parenchyma cells in the phloem. Store food material and other substances like resins, latex, and mucilage. Absent in most monocot stems.
- Phloem Fibres (Bast fibres): Sclerenchymatous fibres found in phloem. These are generally absent in primary phloem but present in secondary phloem. Provide mechanical strength. Examples: jute, flax, hemp are commercially important phloem fibres.
Gymnosperms have albuminous cells and sieve cells instead of companion cells and sieve tube elements.
*(Image shows illustrations of a sieve tube element with sieve plate, an adjacent companion cell, phloem parenchyma cells, and phloem fibres)*
| Feature | Xylem | Phloem |
|---|---|---|
| Main Function | Transport of water and minerals (mostly unidirectional, from roots to leaves); Mechanical support | Transport of organic solutes (food, bidirectional) |
| Conduction Pathway | Lumen of tracheids and vessels | Sieve tubes |
| Major Conducting Cells | Tracheids and Vessels (dead) | Sieve tube elements (living, but anucleate) |
| Associated Cells | Xylem parenchyma (living), Xylem fibres (dead) | Companion cells (living), Phloem parenchyma (living), Phloem fibres (dead) |
| End walls | Solid or with pits (tracheids); Perforated (vessels) | Perforated (sieve plates) |
| Movement | Mainly upward | Bidirectional (source to sink) |
The Tissue System
The tissues in a plant are organised into distinct groups based on their structure and function. These groups form tissue systems. There are three main tissue systems:
- Epidermal Tissue System
- The Ground Tissue System
- The Vascular Tissue System
Epidermal Tissue System
The epidermal tissue system forms the outermost covering of the whole plant body. It is derived from the protoderm (a primary meristem).
It consists of:
- Epidermis
- Stomata
- Epidermal appendages (trichomes and hairs)
Epidermis
- It is the outermost layer of cells in the primary plant body.
- Made up of a single layer of parenchymatous cells, which are usually elongated and compactly arranged, forming a continuous layer.
- Cells are living and have a small amount of cytoplasm and a large vacuole.
- The outer surface of the epidermis is often covered by a waxy layer called the cuticle. Cuticle helps prevent water loss (transpiration). It is usually thicker on the upper surface of leaves than the lower surface. Cuticle is absent in roots.
- Function: Provides protection to the underlying tissues from mechanical injury, pathogens, and water loss.
Stomata
- Stomata are pores found in the epidermis, mainly on the leaves.
- They are involved in gas exchange (intake of $CO_2$ for photosynthesis and release of $O_2$) and transpiration (loss of water vapour).
- Each stoma is surrounded by two bean-shaped cells called guard cells.
- In grasses (monocots), guard cells are dumbbell-shaped.
- Guard cells contain chloroplasts (epidermal cells usually lack them).
- The outer walls of guard cells are thin, and the inner walls (towards the stomatal pore) are thick.
- The guard cells regulate the opening and closing of the stomatal pore. Changes in turgidity of guard cells cause the stomatal pore to open or close.
- Sometimes, a few specialised epidermal cells near the guard cells are called subsidiary cells. The stomatal apparatus consists of the stomatal pore, guard cells, and surrounding subsidiary cells (if present).
*(Image shows a stoma with open pore, two guard cells, and surrounding epidermal and subsidiary cells)*
Epidermal Appendages
- These are outgrowths of the epidermis.
- Trichomes: Hairs on the stem. Can be branched or unbranched, soft or stiff, secretory or non-secretory.
Functions: Prevent water loss (reduce transpiration), provide protection against herbivores, secrete substances.
- Root Hairs: Unicellular elongations of epidermal cells in the root.
Function: Greatly increase the surface area for absorption of water and minerals from the soil.
The Ground Tissue System
All tissues lying inwards to the epidermis and exclusive of the vascular bundles constitute the ground tissue system. It is derived from the ground meristem.
It includes:
- Cortex: The region between the epidermis and the vascular bundles. In stems, it is typically differentiated into hypodermis, cortical layers, and endodermis. In roots, it is composed of general parenchyma.
- Endodermis: The innermost layer of the cortex. It is a layer of cells characterised by the presence of Casparian strips (suberin deposits) in their radial and tangential walls, which regulate water movement into the vascular cylinder.
- Pericycle: Located just inside the endodermis and outside the vascular bundles. It is the site of origin of lateral roots and vascular cambium (during secondary growth in roots).
- Pith (Medulla): The central region of the stem or root, consisting of parenchyma cells. It may be large or small, or sometimes absent.
- Medullary Rays: Parenchymatous cells located between vascular bundles in stems, extending from the pith to the cortex. Involved in radial transport of substances.
The ground tissue system is mainly composed of parenchyma cells, but collenchyma (in the hypodermis of dicot stems) and sclerenchyma may also be present.
Functions of ground tissue: Storage of food, support, photosynthesis (in mesophyll), etc., depending on the region.
The Vascular Tissue System
The vascular tissue system consists of xylem and phloem, which are organised into vascular bundles.
Functions: Conduction of water, minerals, and food, and providing mechanical support.
Types of Vascular Bundles
- Radial: Xylem and phloem are arranged in an alternate manner on different radii.
Example: Found in roots.
- Conjoint: Xylem and phloem are located on the same radius.
Example: Found in stems and leaves.
- Conjoint Collateral: Xylem is located towards the centre (pith), and phloem is located towards the periphery (epidermis). This is the most common type.
- Conjoint Bicollateral: Phloem is present on both sides of the xylem. (e.g., in Cucurbitaceae).
Open vs. Closed Vascular Bundles (in Conjoint type)
- Open: A strip of cambium is present between the xylem and phloem.
Significance: The presence of cambium allows for secondary growth (increase in girth).
Example: Found in dicot stems and gymnosperm stems.
- Closed: Cambium is absent between xylem and phloem.
Significance: These bundles do not undergo secondary growth.
Example: Found in monocot stems.
*(Image shows cross-section diagrams illustrating radial arrangement (root) and conjoint collateral arrangement (stem), showing cambium in open type and its absence in closed type)*
These three tissue systems form the body of the plant, arranged in specific patterns that differ between roots, stems, and leaves, and also between dicots and monocots.
Anatomy of Dicotyledonous and Monocotyledonous Plants
The internal structure (anatomy) of dicotyledonous (dicots) and monocotyledonous (monocots) plants shows distinct differences, particularly in their roots, stems, and leaves. Studying the transverse sections of these organs reveals these differences.
Dicotyledonous Root
Anatomy of a typical dicot root (e.g., sunflower root):
- Epidermis: Outermost layer, often called epiblema. Some cells protrude to form unicellular root hairs.
- Cortex: Several layers of thin-walled parenchyma cells with intercellular spaces.
- Endodermis: Innermost layer of the cortex. Single layer of barrel-shaped cells. Characterised by Casparian strips (suberin deposition on radial and tangential walls), which regulate water and mineral movement into the vascular cylinder (symplast pathway enforced).
- Pericycle: Layer next to the endodermis, consisting of thick-walled parenchymatous cells. It is the site of origin for lateral roots and vascular cambium during secondary growth.
- Vascular Bundles: Radial arrangement (xylem and phloem on different radii). Usually 2 to 6 bundles of xylem and phloem (diarch to hexarch condition). Xylem is exarch (protoxylem towards periphery, metaxylem towards centre).
- Pith: Small or inconspicuous, sometimes absent.
- Conjunctive Tissue: Parenchymatous cells present between xylem and phloem bundles. Gives rise to vascular cambium during secondary growth.
*(Image shows a cross-section of dicot root highlighting epiblema, cortex, endodermis with Casparian strips, pericycle, radial vascular bundles (few, exarch xylem), and small pith)*
Monocotyledonous Root
Anatomy of a typical monocot root (e.g., maize root):
- Epidermis (Epiblema): Outermost layer, with unicellular root hairs.
- Cortex: Several layers of parenchymatous cells with intercellular spaces.
- Endodermis: Innermost layer of cortex, with Casparian strips and passage cells (cells opposite the protoxylem, lacking Casparian strips, allowing water passage).
- Pericycle: Layer next to endodermis. Gives rise to lateral roots. Vascular cambium does not originate from pericycle in monocots as they generally lack secondary growth.
- Vascular Bundles: Radial arrangement. Usually more than 6 bundles of xylem and phloem (polyarch condition). Xylem is exarch (like dicot root).
- Pith: Large, well-developed, and prominent.
- Conjunctive Tissue: Parenchymatous tissue between xylem and phloem, but it does not form vascular cambium.
*(Image shows a cross-section of monocot root highlighting epiblema, cortex, endodermis, pericycle, radial vascular bundles (many, exarch xylem), and large pith)*
| Feature | Dicot Root | Monocot Root |
|---|---|---|
| Number of Xylem/Phloem Bundles | Fewer (2-6) | Many (more than 6) |
| Pith | Small or inconspicuous/absent | Large and well developed |
| Secondary Growth | Present | Absent |
| Casparian Strips | Present in Endodermis | Present in Endodermis |
| Passage Cells | Generally absent | Present in Endodermis (opposite protoxylem) |
Dicotyledonous Stem
Anatomy of a typical dicot stem (e.g., sunflower stem):
- Epidermis: Outermost layer. Covered by a cuticle. May have trichomes and a few stomata.
- Cortex: Located between the epidermis and the pericycle. Differentiated into three sub-zones:
- Hypodermis: 2-3 layers of collenchymatous cells just below the epidermis. Provides mechanical strength to young stem.
- Cortical layers: Several layers of parenchymatous cells with intercellular spaces. Involved in storage.
- Endodermis: Innermost layer of the cortex. Cells are rich in starch grains, also called the starch sheath. Casparian strips are present but less prominent than in roots.
- Pericycle: Located below the endodermis and above the phloem. Made up of sclerenchymatous cells, often in patches (semilunar patches of sclerenchyma) in dicot stems.
- Vascular Bundles: Conjoint, collateral, and open (cambium present between xylem and phloem). Arranged in a ring. Xylem is endarch (protoxylem towards pith, metaxylem towards periphery).
- Medullary Rays: Rows of parenchymatous cells extending between the vascular bundles, from the pith to the cortex. Involved in radial transport.
- Pith: Large, well-developed central parenchymatous region with intercellular spaces. Stores food.
*(Image shows a cross-section of dicot stem highlighting epidermis, hypodermis (collenchyma), cortical layers (parenchyma), endodermis (starch sheath), pericycle (sclerenchyma patches), vascular bundles (in a ring, conjoint, open, endarch xylem), medullary rays, and pith)*
Monocotyledonous Stem
Anatomy of a typical monocot stem (e.g., maize stem):
- Epidermis: Outermost layer. Covered by a cuticle. May have stomata.
- Hypodermis: Composed of sclerenchymatous cells (unlike collenchyma in dicots).
- Ground Tissue: The remaining tissue is a large, undifferentiated parenchymatous ground tissue with abundant intercellular spaces. There is no distinct cortex, endodermis, pericycle, or pith as seen in dicot stems.
- Vascular Bundles: Conjoint, collateral, and closed (cambium absent). They are scattered throughout the ground tissue, not arranged in a ring.
- Vascular bundles are surrounded by a sclerenchymatous bundle sheath.
- Vascular bundles are typically larger towards the centre and smaller towards the periphery.
- Xylem is endarch (like dicot stem).
- Water-containing cavities are often present within the vascular bundles.
*(Image shows a cross-section of monocot stem highlighting epidermis, sclerenchymatous hypodermis, scattered conjoint closed vascular bundles with sclerenchymatous bundle sheath, and undifferentiated parenchymatous ground tissue)*
| Feature | Dicot Stem | Monocot Stem |
|---|---|---|
| Vascular Bundles arrangement | Arranged in a ring | Scattered throughout ground tissue |
| Vascular Bundles type | Conjoint, collateral, open (cambium present) | Conjoint, collateral, closed (cambium absent) |
| Bundle Sheath | Absent around vascular bundles | Present (sclerenchymatous) around vascular bundles |
| Ground Tissue | Differentiated into cortex, endodermis, pericycle, pith, medullary rays | Undifferentiated ground tissue |
| Hypodermis | Collenchymatous | Sclerenchymatous |
| Secondary Growth | Present | Absent |
Dorsiventral (Dicotyledonous) Leaf
Dorsiventral leaves (e.g., mango leaf) are typically found in dicot plants. They have distinct upper (adaxial) and lower (abaxial) surfaces.
Anatomy in transverse section:
- Epidermis: Covers both the upper (adaxial) and lower (abaxial) surfaces. A cuticle is present.
- Lower epidermis usually has more stomata than the upper epidermis.
- The upper epidermis may have fewer or no stomata.
- Mesophyll: The tissue between the upper and lower epidermis. It is the primary site of photosynthesis and contains chloroplasts. It is differentiated into two types of parenchyma:
- Palisade parenchyma: Located on the adaxial side, below the upper epidermis. Consists of elongated cells arranged vertically and parallel to each other. Rich in chloroplasts.
- Spongy parenchyma: Located below the palisade parenchyma, extending to the lower epidermis. Consists of irregularly shaped cells with large intercellular spaces and air cavities.
- Vascular Bundles: Present in the veins and midrib. They contain xylem (usually towards the upper epidermis) and phloem (usually towards the lower epidermis).
- Vascular bundles are surrounded by a layer of thick-walled cells called the bundle sheath cells.
- The size of the vascular bundles varies according to the size of the veins; reticulate venation in dicots results in veins of varying thickness.
*(Image shows a cross-section of a dorsiventral leaf highlighting upper and lower epidermis, cuticle, stomata (more on lower), palisade and spongy mesophyll, vascular bundle with bundle sheath)*
Isobilateral (Monocotyledonous) Leaf
Isobilateral leaves (e.g., grass leaf) are typically found in monocot plants. They have similar upper and lower surfaces.
Anatomy in transverse section:
- Epidermis: Covers both surfaces. A cuticle is present.
- Stomata are present on both the upper and lower epidermis (amphistomatic).
- Some cells in the upper epidermis along the veins are large, empty, and colourless. These are called bulliform cells. They help in rolling and unrolling of leaves in response to water availability.
- Mesophyll: Undifferentiated parenchyma tissue between the upper and lower epidermis. It is not differentiated into palisade and spongy parenchyma. Cells are typically isodiametric with intercellular spaces.
- Vascular Bundles: Present in the veins. They contain xylem (towards the upper epidermis) and phloem (towards the lower epidermis).
- Vascular bundles are surrounded by bundle sheath cells.
- The size of the vascular bundles is mostly uniform, except for the main veins; parallel venation in monocots results in veins of similar size.
*(Image shows a cross-section of an isobilateral leaf highlighting upper and lower epidermis, stomata on both, undifferentiated mesophyll, vascular bundle with bundle sheath, and bulliform cells)*
| Feature | Dorsiventral (Dicot) Leaf | Isobilateral (Monocot) Leaf |
|---|---|---|
| Surface Distinctiveness | Dorsiventral (distinct upper/lower) | Isobilateral (similar upper/lower) |
| Stomata Distribution | More on lower epidermis | Present on both upper and lower epidermis |
| Mesophyll Differentiation | Differentiated into palisade and spongy parenchyma | Undifferentiated parenchyma |
| Veins (Vascular Bundles) Size | Varying sizes (reticulate venation) | Mostly uniform size (parallel venation) |
| Bulliform Cells | Absent | Present in upper epidermis of grasses |
Secondary Growth
Primary growth in plants increases the length of the stem and root. Secondary growth is the increase in girth or diameter of the stem and root. It occurs in dicotyledonous plants and gymnosperms but is generally absent in monocotyledonous plants.
Secondary growth is brought about by the activity of two lateral meristems:
- Vascular cambium
- Cork cambium (Phellogen)
*(Image shows a stem cross-section illustrating primary structure (primary xylem, primary phloem, pith, cortex) and how secondary growth adds secondary xylem and phloem, increasing girth)*
Vascular Cambium
The vascular cambium is the meristematic layer responsible for cutting off vascular tissues (xylem and phloem) during secondary growth.
Origin of Vascular Cambium
In dicot stems, the vascular cambium originates from two regions:
- Intrafascicular cambium (Fascicular cambium): This is the primary vascular cambium present *within* the vascular bundles, located between the primary xylem and primary phloem. It is a remnant of the procambium.
- Interfascicular cambium: This is a secondary meristem that forms *between* the vascular bundles. It originates from the dedifferentiation of parenchymatous cells of the medullary rays that are adjacent to the intrafascicular cambium.
Formation of Cambial Ring
The intrafascicular cambium and the newly formed interfascicular cambium join together to form a continuous ring called the cambial ring. This ring is located between the primary xylem (towards the inside) and primary phloem (towards the outside).
*(Image shows a sector of a dicot stem cross-section, illustrating vascular bundles with fascicular cambium, and medullary ray cells between bundles dedifferentiating to form interfascicular cambium, connecting to form a ring)*
Activity of the Cambial Ring
The cambial ring becomes active and starts dividing. The cells of the cambial ring divide in tangential direction (periclinal divisions) and produce new cells both towards the inside and towards the outside.
- Cells cut off towards the pith (inside) mature into secondary xylem.
- Cells cut off towards the periphery (outside) mature into secondary phloem.
The cambium is generally more active on the inner side, so the amount of secondary xylem produced is much more than the amount of secondary phloem.
The continuous formation of secondary xylem and phloem pushes the primary xylem and primary phloem. Primary xylem remains more or less in the centre (or pushed slightly towards the pith), while primary phloem gets crushed and disintegrates due to the continuous formation of secondary phloem.
At some places, the cambium forms a narrow band of parenchymatous cells passing through the secondary xylem and secondary phloem in radial directions. These are called secondary medullary rays. They are involved in radial transport of water and nutrients.
*(Image shows a close-up of the cambial ring, showing cells dividing inwards (secondary xylem) and outwards (secondary phloem), pushing primary tissues)*
Spring Wood and Autumn Wood
The activity of the vascular cambium is influenced by environmental factors, particularly in temperate regions. This results in the formation of distinct rings of secondary xylem, known as annual rings or growth rings.
- Spring wood (or early wood): Formed during the spring season when the cambium is very active. It produces a large number of xylem elements with wider vessels. The wood formed is lighter in colour and lower in density.
- Autumn wood (or late wood): Formed during the winter season when the cambium is less active. It produces fewer xylem elements with narrower vessels. The wood formed is darker in colour and higher in density.
A ring of spring wood and a ring of autumn wood together constitute an annual ring. Annual rings are prominent in trees of temperate regions and can be used to estimate the age of the tree (dendrochronology).
*(Image shows a segment of a tree trunk cross-section with visible concentric rings, highlighting the difference between spring wood (lighter, wider vessels) and autumn wood (darker, narrower vessels))*
Heartwood and Sapwood
In old trees, the central part of the secondary xylem becomes dark and hard. This region is called heartwood.
- Heartwood: The inner, older portion of the secondary xylem. It is dark brown due to the deposition of organic compounds like tannins, resins, oils, gums, aromatic substances, and essential oils in the lumens of the vessels and tracheids. The cells are dead and highly lignified.
Function: Provides mechanical support to the stem. It is not involved in water conduction.
It is durable and resistant to insect attacks, making it valuable timber.
- Sapwood: The outer, lighter coloured portion of the secondary xylem. It consists of the younger secondary xylem.
Function: Involved in the conduction of water and minerals from the root to the leaf.
*(Image shows a tree trunk cross-section highlighting the central dark heartwood and the surrounding lighter sapwood)*
Cork Cambium (Phellogen)
As the stem increases in girth due to the activity of vascular cambium, the outer cortical and epidermal layers get stretched and eventually break. To provide protection to the outer layers, another lateral meristem, the cork cambium or phellogen, develops.
Origin and Activity of Cork Cambium
- Cork cambium usually develops from the cells of the cortex, but can also originate from epidermis or pericycle.
- It is a couple of layers thick.
- Phellogen cuts off cells on both sides:
- Cells cut off towards the outside differentiate into cork or phellem. These are dead cells with suberised walls, impermeable to water.
- Cells cut off towards the inside differentiate into secondary cortex or phelloderm. These are living parenchymatous cells.
The phellogen, phellem, and phelloderm together constitute the periderm. Periderm replaces the epidermis in older stems and roots as the protective outer layer.
Due to the activity of the cork cambium, pressure builds up on the remaining primary tissues (like primary phloem and crushed cortex) outside the periderm, eventually causing them to die and slough off.
Bark
Bark is a non-technical term referring to all tissues external to the vascular cambium. This includes secondary phloem, primary phloem (if present and not crushed), cortex, pericycle, and periderm.
- Early/Soft bark: Bark formed early in the season.
- Late/Hard bark: Bark formed towards the end of the season.
Lenticels
- At certain regions, the phellogen cuts off parenchymatous cells instead of cork cells towards the outside. These parenchymatous cells are loosely arranged with intercellular spaces.
- These lens-shaped openings on the surface of the bark are called lenticels.
- They provide aeration to the internal tissues of the stem or root.
*(Image shows a cross-section of an old stem segment highlighting vascular cambium, secondary xylem/phloem, cork cambium (phellogen), phellem (cork), phelloderm (secondary cortex), and a lenticel interrupting the periderm)*
Secondary Growth in Roots
Secondary growth also occurs in dicot roots, increasing their diameter. It is similar in process to stem secondary growth but with some differences in initiation and arrangement.
- Origin of Vascular Cambium: In dicot roots, the vascular cambium originates from two regions:
- A portion of the conjunctive tissue located just below the phloem bundles.
- A portion of the pericycle tissue located opposite the protoxylem points.
- The cambium initially forms a wavy ring because it originates from discrete patches. As it becomes active, it produces more secondary xylem towards the inside (pushing the pericycle outwards) and secondary phloem towards the outside, gradually forming a continuous, circular cambial ring.
- The activity of the cambium produces secondary xylem towards the centre and secondary phloem towards the periphery, just like in the stem.
- The primary xylem and primary phloem get embedded in the secondary tissues.
- Cork Cambium (Phellogen): Develops from the pericycle (unlike in stem where it often originates from the cortex). It produces phellem (cork) outwards and phelloderm (secondary cortex) inwards, forming the periderm. The epidermis and cortex of the root are sloughed off.
*(Image shows sequential cross-sections of a dicot root illustrating primary structure, initiation of wavy cambium from conjunctive and pericycle tissue, formation of continuous cambial ring, and production of secondary xylem/phloem with periderm formation)*