Plant Tissues

Plant Tissues

Tissues are groups of cells that work together to perform specific functions within an organism. In plants, tissues are collections of specialized plant cells that collaborate to carry out particular tasks essential for the survival and growth of the plant. These tissues combine to form different structures like leaves, stems, roots, flowers, and fruits, enabling plants to perform various functions necessary for their existence.

Classification of Plant Tissues

There are two main types of plant tissues: meristematic tissues and permanent (non-meristematic) tissues. They are further classified into subtypes.

Meristematic Tissues

Meristematic tissues are regions of actively dividing and rapidly growing cells found in plants. They serve as the source of new cells and play a fundamental role in plant growth and development. Meristematic tissues are responsible for the continuous growth of plants in length and girth, enabling them to increase in size and complexity. These tissues are particularly prominent in growing parts of the plant, such as the tips of stems and roots.

Key characteristics of meristematic tissues include:

a) Cell Division: Meristematic tissues consist of cells that are actively undergoing cell division that leads to the production of new cells, allowing the plant to grow and develop.

b) Undifferentiated Cells: Meristematic cells are undifferentiated, meaning they have not yet specialized into specific cell types. As they divide, some of these cells remain in the meristem, while others undergo differentiation to become specialized cells for various plant tissues.

c) Rapid Growth: The cells produced by meristems are initially small and have thin cell walls, which allows them to expand and elongate quickly.

d) Location: Meristematic tissues are primarily found in specific regions of the plant, such as the apical meristems (tips of stems and roots) and lateral meristems (sides of stems and roots). Intercalary meristems are also found in some plants, particularly and are located at the base of leaf blades and nodes.

Apical Meristem

Apical meristem is a specialized type of plant tissue located at the growing tips of stems and roots. It plays a crucial role in the primary growth of plants, which involves the increase in the length of the plant body. The apical meristem is responsible for the formation of new cells through a process called mitosis, where a single cell divides into two identical daughter cells. These new cells can further differentiate into various types of plant tissues, ultimately leading to the growth and development of the plant.

Key features of apical meristems:

a) Location: Apical meristems are found at the tips of both roots and stems. In stems, they're commonly referred to as shoot apical meristems, while in roots, they're known as root apical meristems.

b) Growth in Length: The primary function of apical meristems is to promote the elongation of stems and roots. This lengthening occurs through continuous cell division at the meristem, leading to the addition of new cells to the plant body.

c) Cell Differentiation: As cells produced by the apical meristem undergo division, they also differentiate into specialized cell types that will contribute to various plant tissues. These tissues include dermal, ground, and vascular tissues, which collectively form the structural components of the plant.

d) Protection: The delicate apical meristem is often protected by young leaves or root caps. These protective structures help shield the meristem from damage as the plant grows and interacts with its environment.

e) Developmental Zones: Apical meristems can be divided into distinct zones based on cell activity. The zone of cell division contains actively dividing cells. The zone of elongation is where newly formed cells start to elongate, contributing to the increase in the length of the stem or root. The zone of maturation is where cells differentiate into specific tissue types.

Lateral Meristem

Lateral meristem is a specialised type of plant tissue that is located along the lateral sides of stems and roots. Unlike apical meristems, which are responsible for increasing the length of plant structures, lateral meristems are responsible for increasing the girth or thickness of stems and roots. This process is known as secondary growth and is particularly important in woody plants, allowing them to become thicker over time.

Key features of lateral meristems:

a) Location: Lateral meristems are found within the mature tissues of stems and roots, specifically along their lateral sides. They are responsible for the increase in diameter of the plant body.

b) Secondary Growth: Lateral meristems are responsible for secondary growth, which occurs after primary growth (elongation) and typically starts in older plants. This type of growth leads to the thickening of stems and roots, contributing to the overall structural stability of the plant.

c) Two Types: There are two main types of lateral meristems: the vascular cambium and the cork cambium.

• Vascular Cambium: The vascular cambium is responsible for producing secondary xylem (wood) and secondary phloem, which add layers of tissue to the inside of the stem. The secondary xylem provides structural support and contributes to water transport, while the secondary phloem transports nutrients.
• Cork Cambium: The cork cambium produces cork cells, which form a protective outer layer that provides protection against physical damage, pathogens, and water loss in older roots.

d) Growth Rings: The activity of lateral meristems leads to the formation of growth rings in woody stems. Each year, a new layer of secondary xylem is added to the stem, creating visible rings. These rings can be used to estimate the age of the tree and provide information about its growth history.

e) Bark Formation: The accumulation of cork cells produced by the cork cambium contributes to the formation of bark.

f) Environmental Factors: The activity of lateral meristems can be influenced by environmental factors such as temperature and day length. This is why the width of growth rings can vary from year to year in response to changing conditions.

Intercalary Meristem

Intercalary meristem is a specialised type of plant tissue located at the base of leaves or internodes in some plants. Unlike apical and lateral meristems, which are commonly found at the tips of stems and roots or along their sides, intercalary meristems are specific to certain regions of the plant where they contribute to growth and development in distinct ways.

Key features of intercalary meristem:

a) Location: Intercalary meristems are typically found at specific regions within the stem, often at the base of leaves or internodes (the segments of the stem between nodes). They can also occur at the base of leaf blades in some plants.

b) Role in Growth: The primary function of intercalary meristem is to contribute to the elongation of plant structures. This type of meristem is responsible for the increase in length between nodes (internodal elongation) or the elongation of leaf blades.

c) Grass Growth: Intercalary meristems are well-known for their role in the growth of grasses. In grasses, the base of each leaf blade contains an intercalary meristem. This allows the grass to grow rapidly from its base, making it well-suited for grazing animals.

d) Regenerative Ability: Intercalary meristems contribute to the plant's regenerative ability. For example, if a grass leaf is cut or grazed by an animal, the intercalary meristem at its base can continue to grow and elongate, allowing the grass to quickly recover its lost tissue.

e) Inhibition by Apical Meristem: In some cases, the activity of intercalary meristems can be inhibited by the presence of active apical meristems at the tips of stems. When the apical meristem is actively growing, it can suppress the growth of intercalary meristems, directing the plant's resources toward elongating the main stem rather than individual leaves.

Permanent Tissues

Permanent tissue is a type of plant tissue that is derived from meristematic tissue through a process called differentiation. Unlike meristematic tissues, which consist of actively dividing cells, permanent tissues are composed of cells that have ceased division and have taken on specific roles, shapes, and functions within the plant. Permanent tissues contribute to the overall structure, support, and functioning of the plant.

Key characteristics of permanent tissues:

a) Origin: Permanent tissues originate from meristematic tissues, which are responsible for cell division and growth. As meristematic cells divide, some of them undergo differentiation, transitioning into specialised cell types with specific functions.

b) Lack of Division: Once cells undergo differentiation and become part of permanent tissues, they lose the ability to divide further. This is in contrast to meristematic cells, which continually divide to contribute to the plant's growth.

c) Types of Permanent Tissues: Permanent tissues are categorized into two main types based on their complexity: simple permanent tissues and complex permanent tissues.

• Simple Permanent Tissues: These tissues are composed of cells that have similar structures and functions. Simple permanent tissues are further divided into three types: parenchyma, collenchyma, and sclerenchyma.
• Complex Permanent Tissues: These tissues are composed of multiple cell types that work together to perform specific functions. The two main types of complex permanent tissues are the xylem and phloem.

I. Simple Permanent Tissues

Simple permanent tissues are plant tissues composed of a single type of cell that serves specific functions within the plant. These tissues play essential roles in various plant processes and contribute to the overall structure and function of the plant. There are three main types of simple permanent tissues: parenchyma, collenchyma, and sclerenchyma.

1. Parenchyma

a) Structure: Parenchyma cells are relatively simple in structure, with thin and flexible primary cell walls. They have a large central vacuole and a prominent nucleus. Parenchyma cells are closely packed with intercellular spaces between them.

b) Intercellular Spaces: Parenchyma cells are loosely packed, leaving large intercellular spaces between them. This arrangement facilitates gas exchange and movement of water and nutrients.

c) Metabolically Active: Parenchyma cells are metabolically active and can undergo division, growth, and differentiation as needed.

d) Function: Parenchyma tissue serves multiple functions, including photosynthesis, storage of nutrients (such as starch), gas exchange, and wound healing. Parenchyma cells are found in various plant organs, including leaves, stems, roots, and fruits.

e) Example: Mesophyll cells in leaves are parenchyma cells responsible for photosynthesis.

2. Collenchyma

a) Structure: Collenchyma cells have unevenly thickened primary cell walls, which provide mechanical support and flexibility. The thickening occurs at the corners of the cells, creating a flexible framework.

b) Living Cells: Unlike some other types of plant tissues, collenchyma cells remain alive even after they have matured. This feature enables them to adapt to mechanical stresses and environmental changes.

c) Intercellular Spaces: Collenchyma cells have little to no intercellular spaces, which means they are densely packed together. This close arrangement enhances their supportive function.

d) Function: Collenchyma tissue provides structural support to young and growing plant parts, such as stems and leaves. It allows these parts to elongate while maintaining strength and flexibility.

e) Example: The strings of celery are made up of collenchyma cells that provide support to the stem.

3. Sclerenchyma

a) Structure: Sclerenchyma cells have thick, lignified secondary cell walls that provide rigidity and strength. These walls make the cells rigid and inelastic. There are two main types of sclerenchyma cells: sclereids and fibres.

b) Lack of Intercellular Spaces: Similar to collenchyma, sclerenchyma also lacks intercellular spaces. This dense packing of cells further enhances the tissue's structural integrity.

c) Cell Vitality: Unlike other living plant cells, sclerenchyma cells are dead at maturity. However, their tough cell walls continue to contribute to the plant's structural support.

d) Types of Sclerenchyma:

• Fibers: Long, slender cells that form bundles, providing tensile strength to plant tissues. Examples include flax and jute fibres used for textiles.
• Sclereids: Short, irregularly shaped cells that may have various functions such as protection or support. They are found in seed coats, nutshells, and fruit pits.

e) Function: Sclerenchyma tissue offers mechanical support and protection to mature plant structures. Sclereids are responsible for providing protection, while fibres contribute to the strength of plant tissues.

f) Example: The gritty texture of pear flesh is due to the presence of sclereids.

II. Complex Permanent Tissues

Complex permanent tissues consist of different types of cells that work together to perform specific functions vital for the survival and growth of plants. These tissues are involved in essential processes such as the conduction of water, minerals, and organic nutrients throughout the plant. The two main types of complex permanent tissues are xylem and phloem.

The xylem and phloem together form vascular bundles that run through the plant, ensuring the efficient transport of water, nutrients, and organic compounds. These complex permanent tissues play a crucial role in maintaining the plant's physiological processes, growth, and overall survival.

1. Xylem

The xylem is a complex tissue in plants that plays a crucial role in the transport of water, minerals and dissolved nutrients from the roots to other parts of the plant, including the stems and leaves. It is responsible for maintaining the plant's water balance, providing structural support, and facilitating the movement of important substances necessary for growth and metabolism. Xylem tissue consists of several distinct cell types that work together to ensure the efficient conduction of water and nutrients throughout the plant.

Components of Xylem

a) Tracheids: Tracheids are elongated, tapered cells with lignified secondary cell walls. They have pits, which are areas of thinner cell walls that allow water movement between adjacent tracheids. Tracheids are present in various types of plants, including both angiosperms and gymnosperms.

b) Vessels: Vessels are wider, shorter cells formed by the alignment of vessel elements. Vessel elements are specialized cells with perforated end walls called perforation plates. These plates allow uninterrupted water flow through the vessels. Vessels are found mainly in angiosperms, contributing to their efficient water-conducting capabilities.

c) Xylem Parenchyma: These are living cells found interspersed among tracheids and vessels. Xylem parenchyma cells store starches, oils, and other nutrients, and they also play a role in lateral water movement within the xylem tissue. They help maintain the vitality of the surrounding cells and participate in tissue repair.

d) Xylem Fibres: Xylem fibres are elongated cells with thick walls composed of lignin, making them rigid and supportive. They contribute to the mechanical strength of the plant, especially in woody plants. Xylem fibres help prevent collapse of the xylem vessels and provide structural integrity.

Function of Xylem

The primary function of xylem tissue is to transport water and dissolved minerals from the roots to the aerial parts of the plant. This process is essential for various physiological processes, including photosynthesis, nutrient uptake, and maintenance of turgor pressure. The movement of water through the xylem is driven by a combination of transpiration, cohesion, and adhesion:

a) Transpiration: Water loss through small openings called stomata on the leaves creates negative pressure within the leaves. This negative pressure, also known as tension, pulls water upwards from the roots through the xylem vessels.

b) Cohesion and Adhesion: Water molecules in the xylem are cohesive, meaning they are attracted to each other. Additionally, water molecules adhere to the walls of the xylem cells. This cohesion and adhesion create a continuous column of water within the xylem vessels, allowing water to be pulled up the plant against gravity.

2. Phloem

Phloem is another complex tissue in plants that has a crucial role in the transportation of organic nutrients, primarily sugars produced through photosynthesis, from the leaves to other parts of the plant, including the roots, stems, and developing fruits. Unlike the xylem, which conducts water and minerals in one direction (roots to shoots), phloem is responsible for the bi-directional transport of nutrients, allowing the distribution of essential resources to different parts of the plant as needed.

Components of Phloem

a) Sieve Tubes: Sieve tubes are the main conducting cells of the phloem and are formed by the alignment of sieve elements. These cells have perforated end walls called sieve plates that allow the movement of nutrients between adjacent cells. Sieve tubes lack most cellular organelles, including a nucleus, in their mature state, which allows for efficient nutrient transport.

b) Companion Cells: Companion cells are closely associated with sieve tubes and provide metabolic support to them. They have active nuclei and a full complement of organelles, which help regulate and maintain the functions of sieve tubes. Companion cells are responsible for loading sugars into sieve tubes at the source (usually leaves) and unloading them at the sink (areas of the plant where sugars are utilized or stored).

c) Phloem Fibers: Similar to xylem fibres, phloem fibres are elongated cells with thick walls made of lignin. They provide mechanical support and contribute to the structural integrity of the phloem tissue.

d) Phloem Parenchyma: These are living, specialized cells interspersed within the phloem tissue. Phloem parenchyma cells store and transport nutrients like starches and oils. They also assist in the overall functioning of the phloem by maintaining the metabolic needs of adjacent cells.

Function of Phloem

The main function of phloem tissue is the transportation of photosynthetically produced sugars (primarily sucrose) from the sites of production (source) to areas of growth, storage, or utilization (sink) in the plant. This translocation of nutrients is driven by a process known as "pressure flow" or "mass flow." It involves the active transport of sugars from source cells into sieve tubes and the subsequent movement of these sugars along the concentration gradient from source to sink. This movement is facilitated by the continuous flow of water within the plant and the difference in osmotic pressure between the source and the sink.

Phloem also plays a role in long-distance signalling within plants, as well as in the transport of hormones and other signalling molecules. The ability of phloem to transport materials in both directions allows the plant to allocate resources efficiently based on its physiological needs.