Chapter 3: Tissues
in Action

Answer: Meristematic cells lack vacuoles because:

• Space for division: vacuoles store water and waste and occupy large portions of a cell's volume. Meristematic cells need maximum space for the nucleus and cytoplasm, which are actively involved in cell division.
• Dense cytoplasm needed: mitosis requires dense cytoplasm rich in ribosomes, mitochondria and other organelles to provide energy and materials for rapid, continuous division — vacuoles would dilute the cytoplasm.
• Turgor not needed: vacuoles provide turgor pressure in mature plant cells for firmness. Meristematic cells are tightly packed and don't need turgor for support — that function is served by permanent tissues.
• Metabolic priority: all resources must go towards DNA replication and cell division, not storage. Even a small vacuole would be a metabolic burden on cells dividing continuously.

• Apical growth stops: the apical meristem at the shoot tip is removed, so the stem stops growing in length from that point.
• New branches appear: cutting removes apical dominance (the suppression of lateral bud growth by the apical bud). Lateral buds at the nodes, previously inhibited, are released and begin to grow, producing new branches.
• Bushy appearance: multiple branches develop instead of one main stem, giving the plant a bushier, wider appearance — this is why gardeners prune plant tips to encourage bushy growth.
• Intercalary meristems active: in grasses and bamboo, intercalary meristems at nodes allow continued growth even after tips are cut or grazed.

Answer: In a T.S. of sunflower stem (Fig. 3.7), the following tissues can be identified:

• Epidermis: outermost single layer of flat, rectangular cells with a cuticle, providing protection.
• Collenchyma: just below the epidermis, cells with unevenly thickened corners, providing flexible support.
• Parenchyma (ground tissue): large, loosely packed cells with thin walls and intercellular spaces — constitutes the bulk of the stem and stores food.
• Sclerenchyma: thick lignified walls, dead cells, providing mechanical strength.
• Vascular bundles (xylem + phloem): xylem is towards the centre of each bundle, phloem towards the outside — transporting water/minerals and food respectively.
• Lateral meristem: actively dividing cells in a ring, responsible for the increase in girth.

Differences: the cells vary in wall thickness (thin in parenchyma, unevenly thick in collenchyma, uniformly thick and lignified in sclerenchyma), cell size (xylem vessels are large and hollow), living state (xylem vessels/fibres are dead; phloem cells are living), and arrangement (epidermis is a single compact layer; ground tissue is loosely packed).

Coconut husk fibres — hard and brittle: made of sclerenchyma tissue. Sclerenchyma cells have extremely thick walls due to lignin deposition — a hard polymer that makes wood hard. Most sclerenchyma cells are dead at maturity, and lignin is rigid and non-elastic, making the fibres hard and brittle — they break rather than bend.

Coriander leaf stalks — soft and flexible: contain collenchyma tissue. Collenchyma cells are living, with unevenly thickened corners due to pectin deposition. Pectin gives flexibility — like rubber, it bends without breaking — and these cells also have intercellular spaces and are not lignified, so they stay soft and pliable.

Summary: the difference lies in wall composition — lignin (sclerenchyma) means hard and brittle; pectin (collenchyma) means flexible and soft.

Advantageous for desert plants: desert plants lose water rapidly through transpiration via stomata. A thick, waxy cuticle (made of cutin) on the epidermis is impermeable to water, drastically reducing water loss by evaporation from the leaf surface — preventing desiccation and letting the plant survive in hot, dry conditions with minimal water.

Disadvantageous for underwater plants: aquatic plants are surrounded by water and need to absorb dissolved minerals, oxygen and carbon dioxide directly through their surfaces. A thick waxy cuticle would block this absorption, preventing the plant from taking in what it needs — aquatic plants have thin or no cuticle to allow free exchange of gases and nutrients with the surrounding water.

Answer: Water movement through xylem against gravity is maintained by the cohesion-tension theory, involving both dead xylem cells and living leaf cells:

• Dead xylem cells provide the channel: xylem vessels and tracheids are dead, hollow, thick-walled, lignified tubes arranged end-to-end, forming a continuous water column from roots to leaves — their dead, empty nature provides unobstructed, low-resistance pathways.
• Transpiration pull from living leaf cells: living mesophyll cells in leaves contact the atmosphere through stomata. As water evaporates from these cells (transpiration), they lose water, become more concentrated, and draw water from adjacent xylem cells — creating negative pressure (tension/suction) at the top of the xylem column.
• Cohesion of water molecules: water molecules are strongly attracted to each other (cohesion) via hydrogen bonds, so the tension at the top is transmitted down the entire water column, pulling water upward continuously — like lifting a chain by pulling its top end.
• Root pressure: living root cells actively absorb minerals by osmosis, drawing water in and creating root pressure that pushes water up into the stem.

Together, transpiration pull (from living leaf cells) + cohesion of water + dead xylem tubes as channels allow water to travel up to the tops of 100-metre tall trees, against gravity.

Answer: Stomata serve multiple critical functions, so their absence would cause:

• No gaseous exchange: CO₂ cannot enter leaves for photosynthesis and O₂ cannot exit — without CO₂ the plant cannot photosynthesise and will eventually die from lack of food production.
• No transpiration: water vapour cannot exit through stomata, so the transpiration pull that moves water up the xylem would be lost — leaves and upper parts would wilt and die from a severe reduction in water and mineral transport.
• Heat regulation fails: transpiration also cools the plant by evaporative cooling — without stomata, the plant would overheat in sunlight.
• Waste accumulation: stomata help eliminate volatile waste products, which would accumulate without them.

Overall effect: the plant would be unable to photosynthesise effectively, water transport would fail, and the plant would die — no stomata means no life for the plant under typical conditions.

Answer: Classical and folk dance poses involve virtually every joint in the body:

Answer: The correct option is they have thin walls, dense cytoplasm and a large prominent nucleus. Meristematic cells must divide rapidly and continuously. Thin cell walls allow easy cell-plate formation during division; dense cytoplasm packed with organelles (ribosomes, mitochondria) provides the energy and materials for division; and a large, prominent nucleus contains the genetic information needed for replication. Thick walls, large vacuoles, and being differentiated are all properties of permanent tissues, not meristematic ones.

Answer: Phloem. Phloem transports food (sugars, amino acids) made in leaves by photosynthesis to all other parts of the plant, including roots, flowers and fruits — a process called translocation. Xylem transports water and minerals upward from roots, epidermis is a protective tissue, and sclerenchyma is a supporting tissue — only phloem handles food transport from source (leaves) to sink (roots, growing parts).

Answer: To allow quick exchange of materials across them. Epithelial tissues lining blood vessels (squamous epithelium) and lung alveoli need to allow rapid diffusion of O₂, CO₂, nutrients and waste products. A thin layer means materials only need to cross a very short distance, making exchange faster and more efficient — the rate of diffusion is inversely proportional to distance. Multiple cell layers (as in skin) would slow down exchange drastically, and are used only where protection, not exchange, is the primary need.

Straight-leg jump: knees and ankles stay rigid (locked in extended position), and the hips remain relatively straight. On landing, there is no joint flexion to absorb the impact — all landing force is transmitted directly through stiff joints to the skeleton. This feels hard, jarring, and painful, with a high risk of injury (especially to the knees).

Normal jump: on takeoff and landing, knees and ankles flex naturally, and the hips flex slightly too. The joints act as shock absorbers, distributing the impact force progressively through cartilage, tendons and muscles — landing feels soft and controlled.

Key observation: joints (especially hinge joints at the knee and ankle) function as dynamic shock absorbers. Bending them during landing distributes the impact over a longer time and distance, reducing peak stress on bones and joints — the biomechanical principle behind bent-knee landings in sports and gymnastics.

Answer: Hinge joint. Both knees and ankles are hinge joints — they allow movement in one plane only (bending/flexion and straightening/extension), like a door hinge. The knee is one of the largest hinge joints, protected by the kneecap. Ball and socket joints (shoulder, hip) allow multi-directional movement, and pivot joints (neck, elbow-forearm) allow rotational movement.

Assertion: Epithelium is well-suited for gas exchange in the lungs. Reason: it consists of multiple layers of tall cells that slow down diffusion.

Answer: A is true, but R is false. The assertion is correct — squamous (thin, flat) epithelium in the lungs is well-suited for gas exchange. But the reason is false: lung epithelium is a single layer of thin, flat cells (squamous epithelium), not multiple layers of tall cells. Multiple layers would actually hinder diffusion — it's precisely the thinness and single-layer nature that makes it suitable for rapid gas exchange.

Assertion: Cardiac muscle can contract continuously without fatigue. Reason: cardiac muscle cells have a high number of mitochondria and an abundant blood supply.

Answer: Both A and R are true, and R is the correct explanation of A. Cardiac muscle contracts rhythmically for a lifetime without fatigue because cardiac cells are packed with mitochondria (which produce ATP through aerobic respiration), and the heart receives a rich blood supply via the coronary arteries, ensuring continuous delivery of oxygen and glucose.

Assertion: Tendons connect bone to bone and allow joint movement. Reason: tendons are made of tough connective tissue that transmits force from muscle to bone.

Answer: A is false, but R is true. The assertion is incorrect — tendons connect muscle to bone (not bone to bone); it is ligaments that connect bone to bone. The reason is true — tendons are indeed tough connective tissue that transmit muscle force to bone.

Assertion: In a hinge joint, movement occurs primarily in one plane. Reason: the bone ends are shaped to allow sliding in all directions.

Answer: A is true, but R is false. The assertion is correct — hinge joints (elbow, knee, ankle) move primarily in one plane (flexion/extension). But the reason is false: the bone ends in a hinge joint are shaped to restrict movement to one plane, not to allow sliding in all directions. Ball and socket joints have rounded ends that allow multi-directional movement.

The graph shows the teak tree's diameter increasing steadily with age, in an approximately linear relationship. At age 5, diameter = 4 cm; at age 40, diameter = 40 cm — roughly 1 cm of diameter per year on average, though the growth rate appears to slow slightly in later years (only 4 cm increase from age 20–25, versus 8 cm from age 5–10), possibly due to environmental conditions or competition.

The number of annual rings formed equals the tree's age in years — the diameter and the number of annual rings are directly proportional, since each ring represents one year of lateral (secondary) growth by the lateral meristem. The data shows a 1:1 ratio of age to annual rings, with diameter roughly doubling for every doubling of age in the productive years (8 cm at 10 years, 24 cm at 20 years).

The lateral meristem (also called the vascular cambium in secondary growth) is responsible for the increase in girth. It's located along the circumference of the stem, in a ring between the xylem and phloem, and divides to produce new xylem cells inward (forming wood/secondary xylem with annual rings) and new phloem cells outward, increasing the stem's diameter year by year.

Bark consists of cork (dead protective cells), cork cambium (lateral meristematic cells) and phloem. Debarking removes protection — exposing inner tissues to mechanical damage, desiccation, pathogens, insects, and extreme temperatures — and, if phloem is also removed, food transport, since food synthesised in leaves can no longer reach roots and other parts below the debarked region.

Below the bark lies xylem (wood), containing vessels and tracheids for water and mineral transport, and the vascular cambium (lateral meristem) lies just beneath it too. Further damage to the trunk would affect the xylem and cambium, stopping water transport and the tree's ability to grow in girth.

If xylem is damaged, water and mineral transport from roots to leaves is severely compromised — leaves cannot receive water for photosynthesis, leading to wilting, reduced photosynthesis, and eventual death. Damage to the cambium means the tree can no longer produce new xylem or phloem — it cannot heal itself or grow in girth.

Assumptions: the debarking removed both cork and phloem; the tree is a dicot with secondary growth; and debarking was complete around the circumference (ring-barking). If only the cork was removed, food transport continues; a monocot tree would be structured differently (no vascular cambium); and if debarking was only partial, some phloem would remain functional and the tree could survive with reduced function.

Tissue responsible: Collenchyma. Collenchyma cells are living, with unevenly thickened corners due to pectin deposition. Pectin is flexible like rubber, allowing the tissue to bend without snapping. Collenchyma is found in the cortex of young stems and leaf stalks/petioles, providing mechanical support and flexibility at the same time.

Impact if replaced by sclerenchyma: sclerenchyma has thick, lignified cell walls and is made of dead cells. Lignin is rigid and inflexible, so if collenchyma were replaced by sclerenchyma:

• The stem would become rigid and hard, like wood.
• During strong winds, instead of bending and recovering, the stem would snap and break under stress, since sclerenchyma cannot bend.
• Young plants need flexibility to withstand mechanical stresses — rigidity is only useful once the plant is older and larger, which is why mature woody stems have sclerenchyma as secondary xylem.
• The sapling would also have difficulty growing taller, since a rigid stem could not elongate — sclerenchyma cells are dead and cannot divide or expand.

Type B cuttings had nodes (the regions on the stem where buds and leaves arise), while Type A cuttings had only internodes (the regions between nodes). Nodes contain intercalary meristematic tissue and axillary buds — actively dividing cells capable of producing new shoots and roots — while Type A lacked nodes and therefore lacked the meristematic tissue needed to regenerate a new plant.

Type B contained at least one node with an intact axillary bud and intercalary meristem. Type A consisted only of internode segments — mature, permanent tissue without meristematic cells. This presence or absence of a node is what determined whether regeneration was possible.

The observable measurement was sprouting — whether new shoot growth emerged from the cutting after a few weeks. Additional measurements could include the number of new shoots, their length, root development, and eventual establishment of a new plant.

Length of cutting, number of nodes in Type B cuttings, soil type, watering frequency and amount, temperature and light conditions, and the initial health/age of the cuttings — without keeping these constant, any observed difference could be due to factors other than the presence or absence of a node.

Rohan is partially correct — his definition applies to simple tissues: parenchyma, collenchyma, and sclerenchyma each consist of one type of cell performing one primary function (storage, flexible support, rigid support respectively) — similar cells with a similar function makes a tissue in these cases.

Rajiv is correct for complex tissues: xylem and phloem are complex tissues because they're made of more than one type of cell.

• Xylem contains: tracheids and vessels (dead tubular cells for water conduction), xylem parenchyma (living, for storage), and xylem fibres (for mechanical strength).
• Phloem contains: sieve tubes (for food transport), companion cells (regulate sieve tubes), phloem parenchyma (storage), and phloem fibres (mechanical support).

In complex tissues, different types of cells with different structures perform different sub-functions, all contributing to the overall function of the tissue (conduction). A better definition, then, is: a tissue is a group of cells — similar or different in structure — that work together to perform a specific function.

Tissue: Sclerenchyma (specifically phloem fibres — coir fibres from coconut husk). These cells have extremely thick, lignified secondary cell walls, making them hard, rigid, and tensile. Most sclerenchyma cells are dead at maturity — their walls provide the mechanical strength. Lignin resists compression, tension, and microbial decomposition, making it ideal for tough fibrous materials like mats, rope (coir), and coconut fibre products.

Why parenchyma cannot serve the same purpose:

• Living cells: parenchyma consists of living cells with thin primary walls. Living cells decay after the plant dies — parenchyma-based material would rot and disintegrate quickly, making it unsuitable for durable mats.
• Thin walls: parenchyma cell walls are thin and lack lignin, providing little mechanical strength or resistance to wear and tear.
• No tensile strength: parenchyma cells are loosely packed with intercellular spaces — they cannot bear tension or resist pulling forces, which mats require.
• Compressible: parenchyma cells collapse easily under pressure, whereas sclerenchyma fibres are rigid and do not compress.

Vibha's statement is incorrect. Meristematic tissues are found in three locations:

• Apical meristem: root tips and shoot tips — responsible for increase in length.
• Lateral meristem: along the circumference of the stem (vascular cambium, cork cambium) — responsible for increase in girth.
• Intercalary meristem: at the base of internodes/just above nodes in grasses and other monocots — responsible for regrowth after cutting or grazing.

Question Neha can ask Vibha: "If meristematic cells are only at root and shoot tips, then how does the stem of a tree increase in thickness (girth) over the years? And when grass is mowed, it grows back quickly — where are the meristematic cells that allow this, since the tips are cut off?" These questions would guide Vibha to discover lateral and intercalary meristems respectively.

(i) The plant cell will have a larger vacuole. In mature plant cells, the central vacuole typically occupies 50–90% of the cell's volume, filled with cell sap — it stores water, minerals, pigments (anthocyanins giving flower colour), and waste materials, and maintains turgor pressure. Animal cells have small or no permanent vacuoles — only tiny, temporary vacuoles for food digestion (food vacuoles) or waste expulsion (contractile vacuoles in protists).

(ii) Assumptions made: (a) the plant cell is a mature leaf or storage cell, not a meristematic cell (which also lacks a large vacuole); (b) the animal cell is a typical somatic cell, not a fat cell/adipocyte, whose large lipid droplet could be comparable in size; and (c) both cells are in a healthy, turgid state. If these assumptions change — a meristematic plant cell might have a smaller vacuole than an animal cell, and an adipocyte might have a larger "vacuole-like" lipid droplet than a typical plant cell.

Answer: This statement is an oversimplification. Critical questions to examine it:

• Does parenchyma perform only one function, or multiple functions in different locations?
• Does xylem only transport water, or does it also provide mechanical support?
• Does epidermis only protect, or does it also absorb (root hair cells), exchange gases (stomata), or aid transpiration?
• Do tissues change function under different conditions or at different stages of plant development?

Tissue examples that disprove the statement:

• Parenchyma: stores food, performs photosynthesis (chlorenchyma in mesophyll), and forms air spaces for buoyancy in aquatic plants (aerenchyma) — three functions in different contexts.
• Epidermis: provides protection, and root hairs absorb water and minerals, and stomata perform gaseous exchange and transpiration — multiple functions.
• Xylem: transports water and minerals, and also provides mechanical support through thick-walled dead cells — two major functions.
• Leaf parenchyma: photosynthesis, gas exchange, storage, and wound healing when needed.

Conclusion: the textbook statement is an oversimplification. Many plant tissues, especially parenchyma and epidermis, perform multiple functions. A more accurate statement would be: "Each plant tissue has a primary specialised function, but may also contribute to secondary functions depending on its location and context."