The plant tissue system includes permanent and meristematic tissues, each with distinct roles. Plant tissues are organised into systems such as dermal, vascular, and ground tissues. Dermal tissues form the outer protective layer of the plant, while vascular tissues facilitate the transport of water, nutrients, and sugars. Ground tissues fill the interior of the plant and perform various metabolic functions, including support and storage.
Plant tissues can be further categorised into permanent tissues, which are mature and differentiated, and meristematic tissues, which are regions of actively dividing cells responsible for growth and development.
Types of Plant Tissues
- Meristematic Tissues
- Permanent Tissues
Meristematic Tissues
These cells undergo continuous division, contributing to the increase in both the length and girth of the plant. Upon maturation, these tissues can stretch, elongate, and differentiate into other types of tissues. Meristematic tissues can be categorised into three types: apical meristems, lateral meristems, and intercalary meristems.
Three types of Meristematic tissues are given below:
- Apical Meristem: These tissues are found at the tips of shoots and roots, which help in the increase of plant length.
- Lateral Meristem: These tissues are found along the sides of stems and roots. This meristem helps in the structural development of the plant.
- Intercalary Meristem: These tissues are located at certain points along the stems or internodes of plants. These meristems play an important role in plant growth and development, contributing to the elongation of the bases of leaves or internodes.
Properties of Meristematic Tissues
- Meristematic tissues consist of cells that are actively dividing, contributing to plant growth and development.
- Cells in meristematic tissues are undifferentiated and have the potential to differentiate into specialised cell types.
- Meristematic tissues are primarily located in regions of the plant where growth occurs, such as the tips of roots and shoots.
- These tissues enable rapid growth and elongation of plant structures, allowing for the formation of new roots, stems, leaves, and reproductive organs.
- Meristematic cells are pluripotent, meaning they can develop into different types of cells depending on environmental cues and developmental signals.
- Cells in meristematic tissues are generally small and densely packed, with thin cell walls and abundant cytoplasm to support active cell division.
- Meristematic cells generally lack large vacuoles and fully developed chloroplasts
Permanent Tissues
These tissues are also known as non-meristematic tissues, consisting of cells that have undergone differentiation and have attained a permanent form and function.
1. Simple Permanent Tissues
Simple permanent tissues are composed of only one type of cell that is similar in origin, structure, and function. Their primary role is to provide support, strength, storage, and protection to the plant.
Three kinds of simple permanent tissues are given below:
- Parenchyma: These consist of living cells, polygonal in shape with a large vacuole in the centre. The parenchyma tissues perform the following functions, including storage, photosynthesis, and buoyancy (flotation of the plant on water). Additionally, they have large intercellular spaces in between them.
- Collenchyma: These tissues consist of living cells and are elongated in shape, and have no intercellular space between them. These provide mechanical support and flexibility; they allow different parts of the plant easy-bending properties.
- Sclerenchyma: This tissue is made up of dead cells that have lignin deposits on their cell wall. These provide the plant with mechanical strength, thus aiding the plant to survive severe strains.
2. Complex Permanent Tissues
Complex permanent tissues are plant tissues made up of more than one type of cell, but all the cells work together as a functional unit. Their primary role is the transport of water, minerals, and food within the plant.
Two types of complex tissues are given below:
- Xylem: This tissue is vascular and mechanical in form. Its primary function is the transportation of water and minerals upwards from the roots to other parts of the plant. Also, the xylem functions in only one direction, i.e. vertical. Its secondary function is to provide structural support. The xylem is composed of four components- tracheid, vessels, xylem parenchyma, and xylem fibres.
- Phloem: Unlike the xylem, the phloem is bidirectional, i.e. it can function upwards as well as downwards. Phloem transports the food prepared via photosynthesis from the leaves to the whole plant body.
Properties of Permanent Tissues
- Permanent tissues consist of mature, specialised cells that have ceased to divide and have adopted specific functions.
- Permanent tissues comprise various cell types adapted to perform specific functions such as support, storage, and photosynthesis.
- Some permanent tissues, like sclerenchyma and collenchyma, provide structural support to plant organs, aiding in upright growth and resistance to mechanical stress.
- Parenchyma cells, a type of permanent tissue, are involved in essential metabolic processes such as photosynthesis, respiration, and storage of nutrients.
- Permanent tissues exhibit adaptations to environmental conditions and plant requirements, such as the ability to store water or withstand mechanical forces.
- Permanent tissues are distributed throughout the plant body, serving specific functions in different organs and tissues.
- Cells in permanent tissues have stable cell walls and structures suited to their specialised functions, allowing for long-term maintenance and integrity of plant tissues.
Vascular Cambium
- The vascular cambium is a meristematic tissue layer found in the stems and roots of many vascular plants, and is responsible for secondary growth or increase in girth.
- It is generally located between the xylem and phloem in the stem and vascular bundles of the root.
- The primary function of the vascular cambium is to produce secondary xylem (wood) and secondary phloem (inner bark), thereby contributing to secondary growth.
- In addition to the vascular cambium, several plants have a cork cambium (phellogen) that produces cork cells towards the exterior, forming the protective outer bark layer.
Plant Tissue Systems and Their Functions
- Protection: Plant tissues form a protective outer covering that safeguards the plant from physical injury, harmful microorganisms, and excessive water loss.
- Transport: Specialised tissues help in the movement of water, minerals, and food (sugars) throughout the plant body, ensuring proper nourishment.
- Support: Tissue systems provide structural strength, helping plants maintain their shape and stand upright.
- Photosynthesis: Certain tissues, especially in leaves, contain chlorophyll and carry out photosynthesis to produce food and energy for the plant.
- Storage: Plants store food, water, and other essential substances in different tissues such as roots, stems, and leaves.
- Growth and Development: Tissue systems support growth by enabling cell division and differentiation, especially in meristematic tissues.
- Gas Exchange: Plant tissues regulate the exchange of gases like oxygen and carbon dioxide, which are necessary for photosynthesis and respiration.
- Defence: Some tissues produce protective structures (like trichomes) and chemical substances (secondary metabolites) that help defend against herbivores and pathogens.
- Reproduction: Tissue systems play an important role in the formation and development of reproductive structures such as flowers, fruits, and seeds.