Chapter 3 Important Questions: Tissues in Action

Meristematic tissue is a group of actively dividing cells in plants, responsible for growth. These cells are small, have thin walls, dense cytoplasm, a large prominent nucleus, and lack vacuoles. They are found at specific regions such as root tips, shoot tips, and along the stem circumference.

Totipotency is the ability of some mature plant cells to undifferentiate, divide, and redifferentiate to develop into a new complete plant under specific conditions. Cells exhibiting this property are called totipotent cells. F. C. Steward demonstrated this using phloem cells of carrot in 1958.

Differentiation is the process by which meristematic tissue cells lose their ability to divide and undergo changes in structure and function to become specialised permanent tissues. These specialised tissues perform specific functions such as support, transport, or storage in the plant body.

Tendons are strong, flexible bands of connective tissue that connect muscles to bones, transmitting the force generated by muscle contraction to produce movement at joints. Ligaments, on the other hand, are connective tissues that connect bones to other bones at joints, providing stability and limiting excessive movement to prevent dislocation. Both are made of tough connective tissue but serve different roles in the musculoskeletal system.

Cardiac muscles are found only in the heart and are specially adapted for continuous, rhythmic contractions throughout life. They possess a high number of mitochondria and receive an abundant, uninterrupted blood supply, ensuring a constant supply of oxygen and energy. Their fibres are cylindrical, branched, with a single nucleus and faint striations, enabling tireless functioning unlike skeletal muscles which fatigue with prolonged use.

Companion cells are specialised parenchyma cells associated with sieve tubes in phloem. They regulate the cellular functions of sieve tube cells, which lack a nucleus at maturity. Their primary function is to monitor and control the loading and unloading of sugars into and out of the sieve tubes. Thus, companion cells maintain the metabolic activities of sieve tubes and ensure efficient food transport throughout the plant.

Intercalary meristem is located at the base of internodes or just above the nodes of certain plants. It enables the plant to regrow after being cut or grazed by animals. For example, when grass is mowed or grazed, the intercalary meristem at the nodes allows it to grow back quickly. Similarly, when a hedge is cut, new bushy branches appear due to the activity of intercalary meristem at the nodes.

Sclerenchyma cells have very thick walls due to the deposition of lignin, a substance that makes the cells extremely hard, strong, and rigid. Most sclerenchyma cells are dead at maturity, and their thick lignified walls remain to provide structural support. This tissue is found in stems, leaf veins, seed coats, and nut shells such as coconut husk and walnut shell, where mechanical strength is required. Because living parenchyma has thin walls, it cannot provide this kind of rigidity.

Plants have three types of meristematic tissues: (1) Apical meristem is located at the tips of roots and shoots. It consists of actively dividing cells that increase the length of the plant by adding new cells at the growing tips, as demonstrated by the onion root tip experiment. (2) Lateral meristem is arranged in a ring along the circumference of the stem. It divides to produce new cells both inward and outward, increasing the girth or diameter of the stem. The annual growth rings visible on cut tree trunks are a result of lateral meristem activity. (3) Intercalary meristem is found at the base of internodes or just above the nodes. It allows plants like grasses to regenerate after grazing or mowing. Together, these three meristems account for growth in length, girth, and branching in plants.

Xylem and phloem are complex permanent tissues because they are composed of more than one type of cell. Xylem transports water and minerals from roots to all parts of the plant and also provides mechanical strength. It consists of tracheids, vessels (tubular, thick-walled, primarily sclerenchymatous), xylem parenchyma (the only living component), and xylem fibres. Phloem transports food prepared in leaves to other parts of the plant. It consists of sieve tubes (long tubular cells with perforated walls), companion cells (specialised parenchyma that regulate sieve tube functions), phloem parenchyma (store food, resin, tannins, and latex), and phloem fibres (sclerenchymatous, provide strength). A key difference is that xylem is mostly composed of dead cells while phloem is mostly made up of living cells.

Epithelial tissue lines the outer surface and internal organs of animals, and its structure is closely related to its function: (1) Simple squamous epithelium consists of a single layer of thin, flat cells ideal for exchange of gases and nutrients, as seen in the lungs and blood vessels. (2) Stratified epithelium has multiple layers of cells providing protection against wear and tear, found in skin, mouth, and oesophagus. (3) Cuboidal or columnar epithelium found in glands is specialised for secretion of enzymes and hormones. (4) Ciliated epithelium with hair-like cilia is found in nostrils, taste buds, and the inner ear, serving sensory functions. (5) Columnar epithelium with microvilli lining the small intestine increases surface area for absorption. In all cases, closely packed cells with minimal intercellular space prevent germ entry and control substance movement.

The human body has four main types of joints: (1) Ball and socket joint — found at the shoulder and hip, where the rounded head of one bone fits into a cup-shaped socket of another. It allows movement in all directions — forward, backward, sideways, and circular. (2) Hinge joint — found at the elbow and knee. Like a door hinge, it allows movement in only one plane, i.e., bending and straightening. The kneecap protects the knee hinge joint. (3) Pivot joint — found where the skull meets the backbone. It allows rotational movement, enabling the head to turn side to side. (4) Fixed joints — found in the skull, where flat bones are fused together. No movement is possible, providing maximum protection to the brain. Each type of joint is structurally adapted to allow the movement required for that part of the body.

A neuron or nerve cell is the structural and functional unit of nervous tissue, specialised for receiving, processing, and transmitting messages. Each neuron has three main parts: (1) Cell body — contains the nucleus and controls all cellular activities of the neuron. (2) Dendrites — are short, branched extensions that receive signals from other neurons or sensory receptors and bring them to the cell body. (3) Axon — is a long fibre that carries nerve impulses away from the cell body toward axon terminals, which then transmit the message to the next neuron or effector organ such as a muscle or gland. The nervous tissue forms the body's control and coordination network. For example, when you touch something hot, sensory neurons carry the signal to the brain, which processes it and sends a motor signal through motor neurons to muscles, causing you to withdraw your hand rapidly.

Experiment by F. C. Steward (1958): F. C. Steward and his team aimed to demonstrate that even specialised, mature plant cells retain the ability to develop into a complete plant, a property called totipotency. Procedure: Small 2 mg fragments were cut from the cross-section of a carrot root. These fragments were cultured in a liquid nutrient medium containing simple sugars and hormones. The medium was kept stirring so that single cells sheared off into the liquid. These free single cells were allowed to grow under appropriate controlled laboratory conditions. The single cells began to divide to form an embryonic plant structure. This was then transferred to agar medium and later planted in soil, where it developed into a complete adult carrot plant. Observations: The phloem cells of carrot, which had already differentiated (become specialised), were able to dedifferentiate (regain the ability to divide), then redifferentiate to form roots, shoots, and ultimately a whole new plant. Conclusions: Some mature plant cells retain totipotency, i.e., the ability to undifferentiate, divide, and redifferentiate into a complete organism when given the right conditions, nutrients, and growth hormones. This shows that every cell contains the complete genetic information needed to build an entire organism. Significance: (1) This discovery formed the basis of plant tissue culture, which is widely used in agriculture for rapid multiplication of superior plant varieties. (2) It enabled the development of disease-free plants in controlled environments. (3) It contributed to genetic engineering, where genes are introduced into plant cells to create improved or disease-resistant crop varieties. (4) The study of crown gall disease caused by Agrobacterium tumefaciens extended this knowledge, as scientists used this bacterium as a tool to introduce useful genes into plants for producing phytochemicals and improved crops.

Experiment — Onion Root Tip (Activity 3.1): Two glass jars were filled with water and an onion bulb was placed on each jar so that the roots dipped into the water. The length of roots was measured on days 1, 2, and 3 in both jars. On day 3, the root tips of the onion in Jar B were cut by about 1 cm, while those in Jar A were left intact. Root lengths were then measured for four more days. Observations: Roots in Jar A continued to grow steadily in length throughout the experiment. Roots in Jar B stopped growing after their tips were cut and showed no further increase in length. This is also clearly shown in the graphical representation (Fig. 3.2). Inference: This experiment demonstrates that roots grow only from their tips. The root tips contain groups of actively and continuously dividing cells, forming the apical meristem. The apical meristem is located at the tips of both roots and shoots. It is responsible for increasing the length of the plant by continuously adding new cells at the growing tips. Characteristics of Meristematic Cells: Meristematic cells are small in size with thin cell walls. They have a large and prominent nucleus with dense cytoplasm containing many organelles. Vacuoles are generally absent, as vacuoles store water and would reduce the metabolic activity needed for rapid division. The cells are tightly packed with little or no intercellular space. These features allow rapid and continuous cell division. Process of Differentiation: As meristematic cells continuously divide, not all new cells remain meristematic. Some cells lose the ability to divide and undergo structural and functional changes to become specialised for specific roles such as support, conduction, or storage. This process is called differentiation. The differentiated cells form permanent tissues, which may be simple (parenchyma, collenchyma, sclerenchyma) composed of one cell type, or complex (xylem, phloem) composed of more than one cell type. These permanent tissues are organised into three tissue systems — the dermal, ground, and vascular tissue systems — enabling the plant to efficiently carry out all its life processes.