Detailed Notes -1 – Chapter 6 – Tissues

Are Plants and Animals Made of Same Types of Tissues?

1. Structural Differences:

   • Plants: Plants have a fixed, stationary structure. They don’t move and require structural support to remain upright. This support is provided by a large quantity of supportive tissue, which mainly consists of dead cells, such as sclerenchyma cells in the stem.
   • Animals: Animals are mobile organisms that actively move in search of food, mates, and shelter. Due to their mobility, animals consume more energy compared to plants. Most of the tissues in animals are living, contributing to their ability to move and respond to stimuli.

2. Pattern of Growth:

   • Plants: Plant growth is limited to specific regions and is characterized by the presence of meristematic tissues, which actively divide throughout the plant’s life. These meristematic tissues are localized in certain regions, such as the tips of roots and shoots.
   • Animals: In animals, cell growth is more uniform, and there is no distinct demarcation between dividing and non-dividing regions. Animals do not possess specialized regions for continuous growth like plants do with meristematic tissues.

3. Organizational Complexity:

   • Plants: The structural organization of organs and organ systems in plants is less specialized compared to animals. While plants have organ systems such as roots, stems, and leaves, the differentiation and complexity within these systems are typically less pronounced than in animals.
   • Animals: Complex animals exhibit a highly specialized and localized organization of organs and organ systems. This complexity is essential for functions like digestion, respiration, and circulation. Animals have evolved diverse organ systems tailored to their specific modes of life and feeding methods.

4. Adaptations for Locomotion and Sedentary Existence:

   • Plants: Plants are adapted for a sedentary existence, rooted in one place. Their structures, such as roots for anchorage and uptake of water and nutrients, stems for support and transport, and leaves for photosynthesis, reflect their stationary lifestyle.
   • Animals: Animals, on the other hand, are adapted for active locomotion. They possess specialized structures such as muscles, skeletons, and nervous systems that enable movement, seeking of resources, and response to environmental stimuli. These adaptations are crucial for their survival in dynamic and often competitive environments.

6.2.2 PERMANENT TISSUE

1. Cells formed by Meristematic Tissue:

   • Meristematic tissue is responsible for cell division and growth in plants. Cells produced by meristematic tissue are initially undifferentiated and capable of continuous division.

2. Differentiation:

   • As cells mature, they undergo a process called differentiation. This is where they take on specific characteristics and functions. Differentiation involves changes in cell structure, shape, and function to suit particular roles within the plant.

3. Formation of Permanent Tissue:

   • During differentiation, cells adopt specific roles and lose their ability to divide further. These specialized cells collectively form permanent tissue, which makes up the bulk of plant organs and structures.

4. Characteristics of Permanent Tissue:

   • Permanent tissue is characterized by its distinct shape, size, and function within the plant. Unlike meristematic tissue, which is primarily involved in growth, permanent tissue performs specialized functions such as support, storage, and transport.

5. Types of Permanent Tissues:

   • Differentiation leads to the development of various types of permanent tissues, each adapted to specific functions within the plant. Examples include:
     • Parenchyma: Thin-walled cells with large central vacuoles, involved in photosynthesis, storage, and secretion.
     • Collenchyma: Cells with unevenly thickened cell walls, providing flexible support to growing plant parts.
     • Sclerenchyma: Cells with thick, rigid cell walls containing lignin, providing mechanical support and protection.
     • Xylem: Conductive tissue responsible for water and mineral transport.
     • Phloem: Conductive tissue involved in the transport of organic nutrients like sugars.

6.2.2 (i) SIMPLE PERMANENT TISSUE 

1. Parenchyma:

   • Characteristics: Consists of relatively unspecialized living cells with thin cell walls. Cells are usually loosely arranged, creating large intercellular spaces.
   • Functions: Mainly involved in storing food. In some cases, contains chlorophyll and performs photosynthesis, known as chlorenchyma. In aquatic plants, parenchyma may have large air cavities to aid in flotation, known as aerenchyma.

2. Collenchyma:

   • Characteristics: Consists of living, elongated cells with irregularly thickened cell walls, especially at the corners. Cells are found below the epidermis of leaf stalks.
   • Functions: Provides flexibility to various plant parts, allowing bending without breaking. Also offers mechanical support to the plant.

3. Sclerenchyma:

   • Characteristics: Composed of dead cells with thickened, lignified cell walls. Cells are long and narrow, with extremely thick walls that may leave no internal space.
   • Functions: Provides rigidity and strength to plant parts, making them hard and stiff. Found in structures like the husk of a coconut, stems around vascular bundles, veins of leaves, and hard coverings of seeds and nuts.

Activity ______________ 6.3 

1. Observation of Epidermis:

   • The outermost layer of cells observed after breaking and peeling a freshly plucked leaf of Rhoeo is identified as the epidermis.
   • The epidermis is typically a single layer of cells, but in plants living in very dry habitats, it may be thicker to provide protection against water loss.
   • Epidermal cells often secrete a waxy, water-resistant layer on their outer surface, aiding in protection against water loss, mechanical injury, and invasion by parasitic fungi.

2. Stomata and Guard Cells:

   • Small pores observed in the epidermis are called stomata, which are necessary for gas exchange with the atmosphere.
   • Stomata are surrounded by two kidney-shaped cells called guard cells, which regulate the opening and closing of the stomatal pore.
   • Transpiration, the loss of water in the form of water vapor, also occurs through stomata.

3. Root Epidermis and Absorptive Surface:

   • Epidermal cells of roots, responsible for water absorption, often bear long hair-like structures that increase the total absorptive surface area.

4. Role of Cutin in Desert Plants:

   • In some plants like desert plants, the epidermis has a thick waxy coating of cutin on its outer surface, providing waterproofing.
   • This thick waxy coating helps reduce water loss through transpiration and protects the plant from desiccation in arid environments.

5. Changes in Protective Tissue as Plants Age:

   • As plants grow older, the outer protective tissue undergoes changes.
   • A strip of secondary meristem located in the cortex forms layers of cells, constituting cork.
   • Cork cells are dead and compactly arranged without intercellular spaces, with suberin in their walls, making them impervious to gases and water.

6.2.2 (ii) COMPLEX PERMANENT TISSUE

1. Complex Tissues:

   • Complex tissues are made up of more than one type of cells, all working together to perform a common function.
   • Unlike simple permanent tissues, which are composed of one type of cells, complex tissues exhibit cellular diversity.

2. Vascular Tissues – Xylem and Phloem:

   • Xylem and phloem are examples of complex tissues and together constitute the vascular bundle in plants.
   • Vascular tissue, including xylem and phloem, is a critical feature that enables plants to transport water, minerals, and nutrients, facilitating their survival in terrestrial environments.

3. Xylem:

   • Components: Xylem consists of tracheids, vessels, xylem parenchyma, and xylem fibers.
   • Characteristics: Tracheids and vessels have thick walls, and many are dead cells when mature. They are tubular structures, facilitating the vertical transport of water and minerals.
   • Function: Xylem primarily transports water and minerals from the roots to other parts of the plant. Xylem parenchyma stores food, while xylem fibers provide support.

4. Phloem:

   • Components: Phloem is composed of sieve cells, sieve tubes, companion cells, phloem fibers, and phloem parenchyma.
   • Characteristics: Sieve tubes are tubular cells with perforated walls, facilitating the movement of food substances.
   • Function: Phloem transports organic nutrients, such as sugars, from photosynthetic tissues (e.g., leaves) to other parts of the plant for growth, storage, and metabolism. Companion cells provide metabolic support to sieve tubes, while phloem fibers offer structural support.

5. Cell Viability:

   • In xylem, tracheids and vessels are often dead cells when mature, whereas in phloem, except for phloem fibers, most cells are living.