NSEJS Biology Anatomy in Plants: Plant Tissues, Meristem & Secondary Growth

Plant anatomy is an important topic in NSEJS Biology that focuses on understanding the internal structure, organisation and functions of plants. 

It explains how cells are arranged into tissues and how different tissue systems help plants grow, transport materials and survive. Concepts like plant tissues, meristematic tissues, permanent tissues and secondary growth form the foundation for understanding plant development. 

A clear understanding of these topics helps students analyse plant structures and answer application-based questions in competitive exams. This article covers the important concepts of plant anatomy, including tissue types, meristem functions and the process of secondary growth. 

Tissue

A tissue is defined as a group of cells that have a common origin and function, working together to perform a specific activity. Tissues organise into organs, which form organ systems, eventually building a complete organism. 

Plant tissues are distinct from animal tissues, as some plant tissues continuously divide, while others attain specific forms and functions throughout their life. In plants, this continuous division is not restricted to specific cell groups in the same way as in animals. (Think of Meristematic tissue as a student in a phase of division and potential, while Permanent tissue is like a person with a decided, specialised career path.)

Types of Plant Tissues

Plant tissues are broadly classified into two main types based on their capacity for cell division:

1. Meristematic Tissue: Composed of actively dividing cells.

2. Permanent Tissue: Composed of cells that have lost the ability to divide.

Meristematic Tissue

Meristematic tissues are regions in plants characterised by active cell division.

Characteristics of Meristematic Cells:

• Actively dividing cells.

• Dense cytoplasm.

• Large nucleus.

• Small or absent vacuoles.

• No intercellular spaces (cells are compactly arranged).

• Thin cellulosic cell walls.

Classification of Meristematic Tissues (Based on Position):

Meristematic tissues are categorised into three types based on their location in the plant body:

1. Apical Meristem

2. Intercalary Meristem

3. Lateral Meristem

1. Apical Meristem

• Location: Found at the apices (tips) of roots and shoots.

• Function: Primarily responsible for primary growth, leading to an increase in the length of the plant. It produces primary tissues.

• Types:

• Root Apical Meristem (RAM): Located at the tip of the root, protected by a root cap.

• Shoot Apical Meristem (SAM): Located at the apex of the shoot, produces leaf primordia.

• Axillary Buds: Formed from cells "left behind" during shoot apical meristem growth, capable of forming branches or flowers.

2. Intercalary Meristem

• Location: Found between mature tissues, often at the base of leaves or internodes.

• Function: Also contributes to primary growth, specifically the elongation of internodes. It helps in the regrowth of parts removed by grazing herbivores.

• Both apical and intercalary meristems are responsible for increasing the length of the plant.

3. Lateral Meristem

• Location: Found along the lateral sides of the stem and root, arranged parallel.

• Function: Primarily responsible for secondary growth, which leads to an increase in the girth or width of the plant stem and root.

• Also known as: Secondary meristem.

• Examples: Vascular cambium and cork cambium.

Permanent Tissue

Permanent tissues are formed from meristematic tissues. Their cells have lost the ability to divide and have attained a specific shape, size, and function, performing specialised roles. 

(Plants, being immobile and exposed to environmental extremes, require robust mechanical support and protection, which is met by specialised permanent tissues like the cell wall and bark.)

Types of Permanent Tissues:

Permanent tissues are classified into two main types:

1. Simple Permanent Tissue: Composed of only one type of cell.

2. Complex Permanent Tissue: Composed of more than one type of cell working together as a unit.

Simple Permanent Tissue

Simple permanent tissues consist of cells that are structurally and functionally similar.

1. Parenchyma

• Description: The most abundant tissue in plants, forming the major part of most plant organs.

• Cell Characteristics:

• Living cells.

• Isodiametric but can also be oval, spherical, polygonal, or elongated.

• Thin cellulosic cell walls.

• Intercellular spaces are usually present.

• Large central vacuole.

• Functions:

• Photosynthesis (when chloroplasts are present).

• Storage of food, water, and waste products.

• Secretion.

• Modifications:

• Chlorenchyma: Parenchyma with chloroplasts for photosynthesis.

• Aerenchyma: Parenchyma with large air cavities for buoyancy in aquatic plants.

2. Collenchyma

• Description: Provides mechanical support to growing parts of the plant.

• Cell Characteristics:

• Living cells.

• Often oval, spherical, or polygonal.

• Characterised by thickening at the corners of the cell walls due to deposition of cellulose, hemicellulose, and pectin.

• No intercellular spaces.

• Functions:

• Provides mechanical support to growing parts (e.g., young stems, petioles).

• Allows flexibility, preventing tearing.

3. Sclerenchyma

• Description: Provides rigid mechanical support and protection.

• Cell Characteristics:

• Dead cells (at maturity), lack protoplasts.

• Long, narrow cells with thickened and lignified cell walls.

• Have pits for substance transport.

• Very narrow lumen.

• Functions:

• Provides mechanical strength and rigidity.

• Protects the plant from external stresses.

• Types:

• Fibres: Long, elongated cells with pointed ends, provide general mechanical support.

• Sclereids (Stone Cells): Spherical, oval, or cylindrical, highly thickened, lignified cell walls. Provide hardness and grittiness (e.g., in guava, sapota, pear pulp, seed coats).

Complex Permanent Tissue

Complex permanent tissues are composed of more than one type of cell, all working together as a single unit to perform a common function, primarily transport of water, minerals, and food. The two main types are Xylem and Phloem.

1. Xylem

• Function: Conducts water and minerals from roots to aerial parts.

• Transport Direction: Unidirectional (upwards).

• Components (Xylem Elements):

• Tracheids: Elongated, tube-like, dead cells with tapering ends and pits.

• Vessels: Cylindrical, tube-like structures formed by end-to-end cells, dead at maturity, with perforations for efficient water conduction (main in flowering plants).

• Xylem Parenchyma: Living cells, store food, help in radial conduction.

• Xylem Fibres: Dead, sclerenchymatous cells, provide mechanical strength.

Arrangement of Primary Xylem (Protoxylem & Metaxylem):

• Protoxylem: First-formed primary xylem, smaller elements.

• Metaxylem: Later-formed primary xylem, larger elements.

2. Phloem

• Function: Transports food materials (sugars) from leaves to other parts.

• Transport Direction: Bidirectional.

• Components (Phloem Elements):

• Sieve Tube Elements: Long, tube-like, lack a nucleus at maturity, have sieve plates for food passage.

• Companion Cells: Specialised parenchyma cells, living and nucleated, control sieve tube activities via plasmodesmata.

• Phloem Parenchyma: Living parenchyma cells, store food.

• Phloem Fibres (Bast Fibres): Sclerenchymatous, dead cells, provide mechanical support.

Tissue Systems

Plant tissues are organised into three main tissue systems:

1. Epidermal Tissue System

2. Ground Tissue System

3. Vascular Tissue System

1. Epidermal Tissue System

• Description: Forms the outermost covering of the plant body.

• Components:

• Epidermis: Outermost layer, typically single-layered (multi-layered in xerophytes), compactly arranged parenchymatous cells for protection.

• Cuticle: Waxy layer covering the epidermis (absent in roots), prevents water loss.

• Stomata: Small pores in the epidermis, regulate transpiration and gas exchange. Guard cells regulate opening and closing.

• Epidermal Appendages: Trichomes (stem hairs) and Root Hairs (unicellular elongations for absorbing water and minerals).

2. Ground Tissue System

• Description: Includes all tissues except the epidermis and vascular tissues.

• Composition: Primarily parenchyma, collenchyma, and sclerenchyma cells.

• Location: Fills internal regions (e.g., cortex, pericycle, pith, medullary rays, mesophyll).

3. Vascular Tissue System

• Description: Consists of xylem and phloem, organised into vascular bundles.

• Function: Responsible for transportation of water, minerals, and food.

Transpiration

Transpiration is the process where water evaporates from the plant body. Though it involves water loss, it is an important force essential for the conduction of various substances within the plant.

Stomata

Stomata, found on leaf surfaces, regulate gas exchange and transpiration.

Stoma and Guard Cells

• A stoma is surrounded by two guard cells that regulate its opening and closing.

• Guard cells have a thin outer wall and a thick inner wall. When water fills them, the thin outer wall expands more easily, causing them to bow outwards and open the stoma. Loss of water causes them to become flaccid, closing it. (The thin layer expands like a balloon inflating.)

• Guard cells uniquely possess chloroplasts and perform photosynthesis.

Subsidiary Cells & Stomatal Apparatus

• Subsidiary cells are modified epidermal cells surrounding the guard cells.

• The stomatal apparatus consists of the stoma, guard cells, and subsidiary cells together.

Secondary Growth

Primary growth increases the length of roots and stems due to apical meristems. Secondary growth refers to the increase in girth (diameter) of the plant body, characteristic of dicot plants. This growth involves lateral meristems: the Vascular Cambium and Cork Cambium (Phellogen), which form through the dedifferentiation of permanent tissues.

Vascular Cambium and Cambial Ring Formation

In young dicot stems, vascular cambium initially exists in patches as intrafascicular cambium between primary xylem and phloem. Cells of the medullary rays become meristematic (dedifferentiation) to form interfascicular cambium. These connect to form a complete cambial ring.

Activity of the Cambial Ring

The cambial ring actively cuts off cells:

• Towards the pith (inner side): Differentiate into secondary xylem.

• Towards the periphery (outer side): Differentiate into secondary phloem.
  The cambium is usually more active on the inner side, producing significantly more secondary xylem. This crushes primary and secondary phloem, while primary xylem remains intact. The cambium also forms medullary rays for radial conduction.

Cork Cambium (Phellogen) and Periderm

As the stem girth increases, outer cortical regions and epidermis rupture. A new protective meristematic tissue, the cork cambium (phellogen), develops (usually in the cortex).

• Phellogen produces cells towards the outside which differentiate into cork (phellem). Cork cells are impervious to water (due to suberin).

• Cells towards the inside differentiate into secondary cortex (phelloderm), consisting of living parenchymatous cells.
  The collective term for phellem, phellogen, and phelloderm is periderm, forming a new protective layer.

Lenticels

At certain regions, phellogen produces parenchymatous cells (complementary cells) instead of cork, rupturing the epidermis to form lens-shaped openings called lenticels. They permit gaseous exchange between the atmosphere and internal stem tissues.

Understanding plant anatomy is essential for mastering important NSEJS Biology concepts related to plant growth and development. Plant tissues, meristematic regions, permanent tissues and secondary growth explain how plants maintain structure, transport materials and adapt to their surroundings. A strong foundation in these topics helps students solve conceptual and application-based questions effectively. Regular revision and practice can improve accuracy and confidence in competitive biology examinations like NSEJS.