1. Are Plants and Animals Made of Same Types of Tissues?

From the previous chapter, we recall that all living organisms are made of cells. In unicellular organisms, a single cell performs all basic functions (movement, intake of food, respiration, excretion). But in multicellular organisms, there are millions of cells, most of which are specialised to carry out specific functions.

A group of cells that are similar in structure and/or work together to achieve a particular function forms a tissue.

Plants and animals have very different lifestyles, which translates into different tissue requirements:

• Plants: They are stationary (fixed in one place). Since they have to stand upright, they need a large quantity of supportive tissue. The supportive tissue generally has dead cells (dead cells can provide mechanical strength as easily as live ones, and need less maintenance).
• Animals: They move around in search of food, mates, and shelter. They consume more energy compared to plants. Most of the tissues they contain are living.
• Growth Pattern: The growth in plants is limited to certain regions, while in animals, cell growth is more uniform. Hence, plants have dividing tissues localized in specific regions, unlike animals.

2. Plant Tissues

2.1 Meristematic Tissue (Dividing Tissue)

The growth of plants occurs only in certain specific regions. This is because the dividing tissue, also known as meristematic tissue, is located only at these points.

Depending on the region where they are present, meristematic tissues are classified as:

• Apical Meristem: Present at the growing tips of stems and roots. It increases the length of the stem and the root.
• Lateral Meristem (Cambium): Responsible for increasing the girth (thickness) of the stem or root.
• Intercalary Meristem: Seen in some plants, it is located near the node.

Characteristics of Meristematic Cells: Cells of meristematic tissue are very active, have dense cytoplasm, thin cellulose walls, and prominent nuclei. They lack vacuoles (because they don't need to store food or waste, their only job is to divide).

2.2 Permanent Tissue

What happens to the cells formed by meristematic tissue? They take up a specific role and lose the ability to divide. This process of taking up a permanent shape, size, and function is called differentiation. This leads to the development of permanent tissues.

A. Simple Permanent Tissue

A few layers of cells beneath the epidermis are generally simple permanent tissue. They are made of one type of cells.

• Parenchyma: The most common simple permanent tissue. It consists of relatively unspecialised cells with thin cell walls. They are living cells. They are usually loosely arranged, thus large spaces between cells (intercellular spaces) are found in this tissue. It provides support to plants and also stores food.
  • Chlorenchyma: In some situations, parenchyma contains chlorophyll and performs photosynthesis.
  • Aerenchyma: In aquatic plants, large air cavities are present in parenchyma to help them float.
• Collenchyma: The flexibility in plants is due to another permanent tissue, collenchyma. It allows bending of various parts of a plant like tendrils and stems of climbers without breaking. It provides mechanical support. The cells are living, elongated, and irregularly thickened at the corners. There is very little intercellular space.
• Sclerenchyma: It is the tissue which makes the plant hard and stiff. Example: the husk of a coconut. It is made of dead cells. They are long and narrow as the walls are thickened due to lignin (a chemical substance acting as cement). Often these walls are so thick that there is no internal space inside the cell.

The Epidermis (Outer Protective Layer)

The outermost layer of cells is called the epidermis. The epidermis is usually made of a single layer of cells. In dry habitats, it may be thicker to protect against water loss. It secretes a waxy, water-resistant layer on its outer surface (cuticle).

• Stomata: Small pores in the epidermis of the leaf. They are enclosed by two kidney-shaped cells called guard cells. They are necessary for exchanging gases with the atmosphere and for transpiration (loss of water in the form of water vapour).
• Root Hair: Epidermal cells of the roots bear long hair-like parts that greatly increase the total absorptive surface area for water absorption.
• Bark (Cork): As plants grow older, the outer protective tissue undergoes certain changes. A strip of secondary meristem replaces the epidermis of the stem, forming a multi-layered thick cork/bark. Cork cells are dead, compactly arranged without intercellular spaces, and have a chemical called suberin in their walls that makes them impervious to gases and water.

B. Complex Permanent Tissue

Complex tissues are made of more than one type of cells. All these cells coordinate to perform a common function. Xylem and phloem are examples of such complex tissues. They are both conducting tissues and constitute a vascular bundle.

• Xylem: Conducts water and minerals from roots vertically upwards. It consists of:
  1. Tracheids (tubular structures)
  2. Vessels (tubular structures)
  3. Xylem parenchyma (stores food)
  4. Xylem fibres (mainly supportive)
  Most of these cells are dead at maturity. The thick walls and tubular structure allow them to transport water vertically.
• Phloem: Transports food from leaves to other parts of the plant. Except for phloem fibres, other phloem cells are living cells. Phloem consists of four types of elements:
  1. Sieve tubes (tubular cells with perforated walls)
  2. Companion cells
  3. Phloem fibres
  4. Phloem parenchyma

3. Animal Tissues

When we breathe, we can actually feel the movement of our chest. How do these body parts move? We have specialised cells called muscle cells for this. Animal tissues are broadly classified into four types:

3.1 Epithelial Tissue

The covering or protective tissues in the animal body are epithelial tissues. Epithelium covers most organs and cavities within the body. It forms a barrier to keep different body systems separate. (e.g., Skin, the lining of the mouth, the lining of blood vessels, lung alveoli).

Epithelial cells are tightly packed and form a continuous sheet. They have almost no intercellular spaces. All epithelium is usually separated from the underlying tissue by an extracellular fibrous basement membrane.

• Simple Squamous Epithelium: Extremely thin and flat and form a delicate lining (like tiles on a floor). Found in cells lining blood vessels or lung alveoli where transportation of substances occurs through a selectively permeable surface. Also found in the lining of the mouth and oesophagus.
• Stratified Squamous Epithelium: Skin epithelial cells are arranged in many layers (strata) to prevent wear and tear.
• Columnar Epithelium: Tall, pillar-like cells. Found where absorption and secretion occur, as in the inner lining of the intestine. In the respiratory tract, this tissue also has hair-like projections on the outer surfaces of epithelial cells called cilia (Ciliated Columnar Epithelium). The cilia can move, and their movement pushes the mucus forward to clear it.
• Cuboidal Epithelium: Cube-shaped cells. Found in the lining of kidney tubules and ducts of salivary glands, where it provides mechanical support.
• Glandular Epithelium: Sometimes a portion of the epithelial tissue folds inward, and a multicellular gland is formed. This secretes substances at the epithelial surface.

3.2 Connective Tissue

The cells of connective tissue are loosely spaced and embedded in an intercellular matrix. The matrix may be jelly-like, fluid, dense, or rigid, depending on the function of the particular connective tissue.

• Blood: Has a fluid (liquid) matrix called plasma, in which red blood corpuscles (RBCs), white blood corpuscles (WBCs), and platelets are suspended. Plasma contains proteins, salts, and hormones. Blood flows and transports gases, digested food, hormones, and waste materials to different parts of the body.
• Bone: Forms the framework that supports the body. It anchors muscles and supports main organs. It is a strong and non-flexible tissue. Bone cells are embedded in a hard matrix that is composed of calcium and phosphorus compounds.
• Ligament: Connects bone to bone. This tissue is very elastic and has considerable strength. Ligaments contain very little matrix.
• Tendon: Connects muscle to bone. Tendons are fibrous tissues with great strength but limited flexibility.
• Cartilage: Has widely spaced cells. The solid matrix is composed of proteins and sugars. Cartilage smoothens bone surfaces at joints and is also present in the nose, ear, trachea, and larynx. (We can fold the cartilage of the ears, but we cannot bend the bones in our arms!).
• Areolar Tissue: Found between the skin and muscles, around blood vessels and nerves, and in the bone marrow. It fills the space inside the organs, supports internal organs, and helps in repair of tissues.
• Adipose Tissue: Fat-storing tissue found below the skin and between internal organs. The cells of this tissue are filled with fat globules. Storage of fat also lets it act as an insulator.

3.3 Muscular Tissue

Muscular tissue consists of elongated cells, also called muscle fibres. This tissue is responsible for movement in our body. Muscles contain special proteins called contractile proteins, which contract and relax to cause movement.

• Striated Muscles (Skeletal / Voluntary Muscles): We can move these muscles by conscious will (like in our limbs). Under the microscope, these muscles show alternate light and dark bands or striations. The cells of this tissue are long, cylindrical, unbranched, and multinucleate (having many nuclei).
• Smooth Muscles (Unstriated / Involuntary Muscles): Movement of food in the alimentary canal or the contraction of blood vessels are involuntary movements. Found in the iris of the eye, ureters, and bronchi of the lungs. The cells are long with pointed ends (spindle-shaped) and uninucleate (having a single nucleus). They do not show striations.
• Cardiac Muscles: The muscles of the heart show rhythmic contraction and relaxation throughout life. These involuntary heart muscles are called cardiac muscles. Heart muscle cells are cylindrical, branched, and uninucleate.

3.4 Nervous Tissue

All cells possess the ability to respond to stimuli. However, cells of the nervous tissue are highly specialised for being stimulated and then transmitting the stimulus very rapidly from one place to another within the body. The brain, spinal cord, and nerves are all composed of nervous tissue.

• The cells of this tissue are called nerve cells or neurons.
• A neuron consists of a cell body with a nucleus and cytoplasm, from which long thin hair-like parts arise.
• Usually, each neuron has a single long part called the axon, and many short, branched parts called dendrites.
• An individual nerve cell may be up to a metre long! Many nerve fibres bound together by connective tissue make up a nerve.
• Nerve impulses allow us to move our muscles when we want to. The functional combination of nerve and muscle tissue is fundamental to most animals, enabling rapid response to stimuli.