NCERT Solutions Class 9 Science Chapter 3 — Tissues in Action

The difference is due to the type of simple permanent tissue present in each.

Coconut husk fibres — hard and brittle:

• Coconut husk contains sclerenchyma tissue
• Sclerenchyma cells have thick, lignified walls and are mostly dead at maturity
• Lignin is a hard, woody substance deposited in the cell walls, making them rigid and inflexible
• This rigidity is useful for providing structural strength but results in brittleness — the fibres snap rather than bend

Coriander leaf stalks — soft and flexible:

• Coriander leaf stalks contain collenchyma tissue
• Collenchyma cells are living cells with unevenly thickened corners due to pectin deposition
• Pectin gives flexibility (like rubber), allowing the cells to stretch and compress without breaking
• This is why young stems and leaf stalks can bend without snapping

The cuticle is a waxy layer of cutin secreted by the epidermis. It is impermeable to water and gases.

Why thick cuticle is advantageous in the desert:

• Desert plants face high temperatures and very low humidity
• A thick cuticle reduces water loss by preventing transpiration through the epidermal surface
• It also protects against mechanical injury and invasion by parasites
• This allows the plant to conserve the limited water available in the desert environment
• Example: Cactus has a very thick cuticle to minimise water loss

Why thick cuticle is disadvantageous for an aquatic plant:

• Aquatic plants are submerged in water and must absorb dissolved gases (CO₂ and O₂) directly from the water through their surface
• A thick cuticle would block the absorption of CO₂ needed for photosynthesis
• It would also prevent the exchange of O₂, needed for respiration, between the plant and water
• Aquatic plants therefore have no or very thin cuticle, sometimes with large air spaces (aerenchyma) to aid gas exchange
• Example: Water hyacinth and hydrilla have minimal or no cuticle

Water moves upward against gravity through xylem by a combination of three forces working together:

1. Role of dead xylem cells:

• Dead xylem cells (tracheids and vessels) form a continuous, uninterrupted hollow tube from roots to leaves
• Their thick, lignified walls provide structural support, preventing the tubes from collapsing under the tension of water being pulled upward
• Being hollow with no living contents, they allow water to flow through freely

2. Role of living cells in leaves (transpiration pull):

• Living cells in leaves constantly lose water vapour through stomata (transpiration)
• This creates a lower water concentration in leaf cells, which draws water from adjacent xylem cells by osmosis
• The loss of water from the top of the xylem column creates a negative pressure (tension) that pulls the water column upward
• This is called the transpiration pull — the main driving force for upward water movement

3. Root pressure:

• Root cells actively absorb water from the soil, creating a positive root pressure that pushes water up from below
• Together, root pressure from below and transpiration pull from above maintain a continuous column of water in the xylem — even in trees 100 metres tall

If there were no stomata, the following consequences would occur:

1. Photosynthesis would stop:

• CO₂ enters the leaf through stomata. Without stomata, no CO₂ could enter
• Without CO₂, photosynthesis cannot occur and the plant cannot produce food
• Eventually, the plant would starve and die

2. Gas exchange would cease:

• O₂ produced during photosynthesis could not escape, and O₂ needed for respiration could not enter
• CO₂ produced during respiration could not be released, causing toxic build-up

3. Transpiration and water transport would stop:

• Stomata are the main route for water vapour to escape (transpiration)
• Without transpiration, the transpiration pull would not be generated
• Water and minerals from the roots would not be transported to the leaves
• This would eventually cause wilting and the death of the plant

Overall result: The plant would be unable to photosynthesise, respire properly or transport water. It would die within a short time.

Dance poses involve multiple joints working simultaneously. The following joints can be identified in typical dance poses:

Classical dances like Bharatanatyam involve extreme bending of knees (hinge joints) and wide shoulder and hip positions (ball and socket joints), demonstrating the full range of joint movements in the human body.

Answer: (iii) They have thin walls, dense cytoplasm and large prominent nucleus.

Explanation:

• Thin cell walls — allow cell division without needing to break down thick rigid walls; thin walls are easier to expand during growth and division
• Dense cytoplasm — contains a high concentration of organelles (ribosomes, mitochondria) needed for rapid metabolic activity and division
• Large, prominent nucleus — contains all the genetic material; a large nucleus with active chromatin is essential for directing cell division (mitosis)
• Absence of vacuoles — vacuoles take up space and dilute the cytoplasm; their absence keeps the cytoplasm concentrated and ready for division

Why other options are wrong:
(i) Thick walls are a feature of sclerenchyma (dead, non-dividing cells), not meristematic cells.
(ii) Large vacuoles are characteristic of mature permanent cells. Meristematic cells lack vacuoles.
(iv) Functionally differentiated cells are permanent tissues that have lost the ability to divide.

Learn more: Meristematic Tissue — Complete Guide

Answer: (ii) Phloem

Explanation:

• Phloem is the conducting tissue responsible for transporting food (mainly sugars produced during photosynthesis in leaves) to all parts of the plant, including roots, stems and other non-photosynthetic organs
• Phloem consists of sieve tubes (transport channels), companion cells (regulate loading/unloading of sugars), phloem parenchyma (stores food) and phloem fibres (provides support)
• This movement of food from leaves (source) to roots (sink) is called translocation

Why other options are wrong:
(i) Xylem transports water and minerals from roots to leaves — it does not carry food.
(iii) Epidermis forms the outer protective covering and does not transport food.
(iv) Sclerenchyma provides mechanical support (dead fibres with thick walls) and has no transport function.

Answer: (iii) To allow quick exchange of materials across them.

Explanation:

• Epithelial tissue lining the lungs, blood vessels and intestines must allow rapid diffusion of substances such as O₂, CO₂, nutrients and waste products
• The rate of diffusion is inversely proportional to the thickness of the tissue — the thinner the barrier, the faster the exchange
• A single layer of flat, thin cells (simple squamous epithelium) provides minimal diffusion distance, enabling very fast material exchange
• In the lungs, O₂ and CO₂ diffuse across a single-cell-thick epithelium in milliseconds
• In the small intestine, a single layer of tall columnar cells maximises nutrient absorption

Why other options are wrong:
(i) Food storage is a function of parenchyma (plants) or fat/adipose tissue (animals), not epithelium.
(ii) Maximum strength requires multiple layers (stratified epithelium found in skin), not single layers.
(iv) Reducing friction is a secondary function; the primary reason for thin lining is rapid exchange.

The two jumps differ significantly in how the joints are used:

Key observations:

• In the straight-leg jump, the ankle and knee are stiff, meaning the hinge joints are not used. The jump is shorter, less powerful, and the landing is jarring (more force travels directly to the skeleton)
• In the normal jump, bending the knee and ankle uses the hinge joints efficiently, stores elastic energy in tendons and muscles during the crouch, releases it powerfully during the jump, and absorbs the impact force on landing
• This shows that hinge joints at the knee and ankle are essential for effective jumping and safe landing — they act as natural shock absorbers

Answer: (ii) Hinge joint

Explanation:

• The hinge joint functions like a door hinge — it allows movement in only one plane (bending and straightening)
• At the knee, the lower leg can only move backward (flexion) and forward (extension) relative to the upper leg; a small bone called the kneecap (patella) protects this joint
• At the ankle, the foot can only point downward (plantar flexion) and upward (dorsiflexion)
• Neither the knee nor ankle allows rotation or sideways movement — confirming they are hinge joints

Why other options are wrong:
(i) Ball and socket joint (shoulder, hip) allows multidirectional movement including full circular motion.
(iii) Pivot joint (base of skull/atlas vertebra) allows rotational movement (turning the head side to side).

A. Epithelium and gas exchange in the lungs

Answer: (iii) — A is true, but R is false.

• Assertion is TRUE: The epithelium of the lungs (alveoli) is indeed well-suited for gas exchange — O₂ and CO₂ diffuse rapidly across it
• Reason is FALSE: The lung epithelium does NOT consist of multiple layers of tall cells. It is a single layer of very thin, flat cells (simple squamous epithelium) — this minimal thickness is precisely what makes diffusion rapid. Multiple layers of tall cells would SLOW diffusion, not aid it

B. Cardiac muscle and continuous contraction

Answer: (i) — Both A and R are true, and R is the correct explanation of A.

• Assertion is TRUE: Cardiac muscle contracts rhythmically and continuously throughout life without fatigue
• Reason is TRUE and correctly explains A: Cardiac cells contain an exceptionally high number of mitochondria (up to 35% of cell volume), providing abundant ATP for constant energy supply. The rich blood supply from the coronary arteries ensures continuous delivery of glucose and oxygen and rapid removal of metabolic wastes, preventing fatigue

C. Tendons and bone-to-bone connection

Answer: (iv) — A is false, but R is true.

• Assertion is FALSE: Tendons do NOT connect bone to bone. Tendons connect muscle to bone. It is ligaments that connect bone to bone
• Reason is TRUE: Tendons are indeed made of tough connective tissue (dense regular collagen fibres) that transmit the contractile force of a muscle to the bone, bringing about movement at a joint

D. Hinge joint and single-plane movement

Answer: (iii) — A is true, but R is false.

• Assertion is TRUE: In a hinge joint (knee, elbow, ankle), movement does indeed occur primarily in one plane — the joint bends and straightens in a single direction
• Reason is FALSE: The bone ends of a hinge joint are NOT shaped to allow sliding in all directions. Rather, the convex surface of one bone fits into the concave surface of the other in a way that specifically restricts movement to one plane. Sliding in all directions would describe a ball and socket joint, not a hinge joint

(i) Interpretation of graph — diameter over time:

• As the age of the teak tree increases, the diameter (DBH) also increases steadily
• The relationship is approximately linear (proportional) — doubling the age roughly doubles the diameter
• At age 5, diameter = 4 cm; at age 40, diameter = 40 cm — a 10-fold increase in diameter for an 8-fold increase in age
• This steady increase indicates consistent growth in girth throughout the life of the tree
• Any year with unusually wide or narrow rings would indicate favourable or unfavourable growing conditions respectively (more rainfall = wider ring; drought = narrower ring)

(ii) Relationship between diameter and annual rings:

• The number of annual rings is exactly equal to the age of the tree in years (both values are the same in the data)
• This confirms that one annual ring is formed every year
• The diameter of the tree increases by approximately 1 cm for every annual ring (or every year), showing a direct proportional relationship between the number of annual rings and the diameter
• Conclusion: Counting annual rings accurately estimates both the age and indicates the rate of girth increase of the tree

(iii) Tissue responsible for girth increase:

• The tissue responsible for the increase in girth (diameter) of the stem is the lateral meristem
• Location: The lateral meristem is located along the circumference (periphery) of the stem, arranged in a ring-like pattern
• These cells divide and produce new cells both towards the inside (forming xylem/wood) and outside (forming phloem/bark), thereby increasing the diameter of the stem in a concentric manner
• Each year, the lateral meristem produces a new layer of wood (xylem), which appears as one annual ring on the cross-section of the trunk

(i) Functions hampered by debarking:

• Protection is lost: The bark (outer layers of cork cells) protects the tree against mechanical injury, pathogens, insects and water loss. Without bark, the tree is exposed to infection and physical damage
• Food transport is disrupted: The bark contains the phloem tissue just beneath the outer cork layer. When bark is removed, phloem is damaged, disrupting the downward transport of food (sugars) from leaves to roots
• Water regulation is affected: Cork cells are impermeable and prevent excessive water loss from the trunk. Removal of bark causes desiccation (drying out) of underlying tissues

(ii) Plant tissue affected by further damage:

• After the bark (which includes the epidermis/cork and phloem), further damage would affect the xylem tissue lying deeper in the trunk
• Xylem forms the bulk of the wood in the trunk and is responsible for water and mineral transport from roots to leaves
• Damage to xylem would also structurally weaken the tree, as xylem provides mechanical support

(iii) Function hampered if tissues beneath bark are damaged:

• If the xylem beneath the bark is severely damaged, water and mineral transport from roots to leaves stops
• Without water, leaves cannot perform photosynthesis and the tree will eventually die
• The tree would also lose structural support and could fall

(iv) Assumptions made:

• The phloem in this tree lies just beneath the bark (outer bark = cork; inner bark = phloem). If the tree had thick cork such that phloem was not damaged by debarking, then food transport might not be immediately affected
• It is assumed that the entire circumference of the tree is debarked (ring barking), not just one side. If only partial debarking occurred, remaining phloem on the other side might still transport some food
• If the assumption changes (e.g., only partial debarking), the tree may survive temporarily as the remaining phloem and bark provide partial function

Tissue responsible for flexibility:

The tissue responsible for the flexibility of the young mango sapling's stem is collenchyma.

• Collenchyma consists of living cells with unevenly thickened corners due to deposition of pectin (a flexible, rubber-like substance)
• This unevenly thickened but not lignified wall allows the cells to stretch and compress, giving the stem the ability to bend without breaking
• It is predominantly found in young stems, leaf stalks and tendrils — all parts that need mechanical support combined with flexibility

Impact if collenchyma were replaced by sclerenchyma:

• Sclerenchyma cells have thick, lignified walls and are dead at maturity
• If collenchyma were replaced by sclerenchyma, the mango sapling's stem would become rigid and inflexible
• During monsoon winds, instead of bending, the stem would snap and break, as sclerenchyma cannot stretch or compress
• The sapling would be highly vulnerable to wind damage and would likely not survive strong winds
• In older, woody trees, sclerenchyma provides hardness — but in young, still-growing saplings, this rigidity would be fatal rather than beneficial

(i) Why type B sprouted but type A did not:

• Type B cuttings sprouted because they contained nodes (the points on the stem where leaves arise)
• Nodes contain intercalary meristem — actively dividing cells capable of producing new growth
• Type A cuttings lacked nodes; they were plain internodal sections containing only permanent tissues that have lost the ability to divide
• Without meristematic cells, type A cuttings could not produce new shoots, roots or leaves

(ii) Difference present in type B compared to type A:

• Type B cuttings had at least one node (the junction between two internodes), whereas type A cuttings were cut from the internode only (between two nodes) and had no nodes
• The node is the critical structure as it contains intercalary meristematic cells that can regenerate the whole plant

(iii) Observation or measurement made:

• The measurement made to determine the effect was sprouting — whether new shoots and roots emerged from the cuttings after a few weeks
• Type B showed visible green shoots emerging from the nodes; type A showed no growth
• Additional measurements could include: number of shoots produced, length of roots formed, fresh weight gain over time

(iv) Parameters to be kept same (controlled variables):

• Type and quality of soil — same potting mix for both
• Amount of water — same irrigation schedule
• Light conditions — same intensity and duration of sunlight
• Temperature — both kept in same environment
• Length and size of cuttings — same number of cells/same length to ensure fair comparison
• Time period of observation — both observed for the same number of days
• Application of rooting hormone (if any) — must be same for both

Rajiv is correct. Rohan's statement is partially right — it applies accurately to simple tissues but needs modification for complex tissues.

Simple tissues — Rohan's statement holds true:

• Simple tissues are composed of only one type of cell
• Example: Parenchyma — all cells are similar (thin-walled, loosely packed living cells) and they together perform storage, photosynthesis or buoyancy
• Example: Collenchyma — all cells are similar (living, pectin-thickened corners) and together provide flexible support
• Example: Sclerenchyma — all cells are similar (dead, lignified thick walls) and together provide hardness and rigidity
• For all three, Rohan's definition applies: same type of cell, same function

Complex tissues — Rajiv's counter-argument applies:

• Complex tissues are composed of more than one type of cell, yet they work together as a unit to perform a common function
• Xylem contains four types of cells: tracheids, vessels (tubular dead cells for water transport), xylem parenchyma (living, for storage) and xylem fibres (dead, for support). These are structurally different cells working together for water transport and mechanical support
• Phloem contains: sieve tubes (tubular living cells for food transport), companion cells (regulate loading/unloading of sugars), phloem parenchyma (stores food) and phloem fibres (dead, for support). Again, different cell types with a shared goal of food transport
• So in complex tissues, the cells are not similar to each other — different cell types collaborate to accomplish the same overall function

Tissue responsible — Sclerenchyma:

• The tough, fibrous nature of coconut husk is due to sclerenchyma tissue
• Sclerenchyma cells have extremely thick, heavily lignified cell walls due to deposition of lignin — a complex organic polymer that is one of the hardest natural substances
• The cells are dead at maturity — the living contents have broken down, and the walls persist as strong hollow tubes that provide structural integrity
• These fibres are long, narrow and tightly bundled, making them resistant to stretching, tearing and compression
• This is why sclerenchyma fibres (called coir in coconut) are ideal for making tough products like mats, ropes and brushes

Why parenchyma cannot serve the same purpose:

• Parenchyma cells are living cells with thin cell walls and large central vacuoles
• Their thin walls provide no structural strength or rigidity — the cells are easily squashed or torn
• Parenchyma is primarily a storage tissue (stores starch, water, gases in aquatic plants) — not a mechanical support tissue
• Unlike lignified sclerenchyma fibres, parenchyma cells would collapse and decompose quickly, making them completely unsuitable for manufacturing fibrous mats or ropes
• Additionally, being living cells, parenchyma would dry out, shrink and decay after harvest, losing any structural integrity it may have had

Vibha's statement is incomplete and partially incorrect.

While it is correct that meristematic cells are found at root and shoot apices (apical meristem), this is not the only location. The statement ignores two other important types of meristematic tissue:

Three types of meristematic tissue and their locations:

• Apical meristem — located at root tips and shoot tips; responsible for increase in length. (This is what Vibha mentioned — correct but incomplete)
• Lateral meristem — located along the circumference (periphery) of stems in dicot plants; responsible for increase in girth (diameter)
• Intercalary meristem — located at the base of internodes or just above nodes; responsible for regrowth after grazing or mowing. Found prominently in grasses

Question Neha can ask Vibha:

This question would guide Vibha to realise that grasses regrow because of intercalary meristem at the nodes — which is not at the apex but at the base of the internodes. Similarly, how do tree trunks grow thicker every year? — through lateral meristem along the circumference. These examples demonstrate that Vibha's claim is incomplete.

(i) The plant cell will have a larger vacuole.

Reasons:

• A mature plant cell has a single large central vacuole that can occupy up to 80–90% of the cell's total volume
• The plant cell's central vacuole performs critical functions:
  • Water storage — stores large quantities of water to maintain cell turgidity and shape
  • Turgor pressure — the pressure exerted by the vacuole pushes the cytoplasm against the cell wall, keeping the cell firm and erect. This is how plants stay upright without a skeleton
  • Storage of nutrients, pigments, waste products and defensive chemicals
• Animal cells have only small, temporary vacuoles (food vacuoles or contractile vacuoles) that are much smaller than the plant cell's central vacuole
• Animal cells do not need a large vacuole for structural support because they have a skeleton; they also have many small lysosomes that perform similar digestive functions

(ii) Assumptions made:

• The plant cell is a mature cell — young, actively dividing meristematic plant cells do not have a large vacuole (they lack vacuoles entirely)
• The animal cell is also a mature, non-specialised cell — some animal cells (like fat cells or phagocytes) can have relatively large vacuoles for storage or digestion
• Both cells are not undergoing active division — dividing cells in both plants and animals have reduced vacuoles
• If the plant cell was a meristematic cell (young and dividing) and the animal cell was a mature resting cell, the comparison might be different

The statement is oversimplified and needs critical examination.

Critical questions to examine the statement:

• Does parenchyma perform only one function — or does it also perform photosynthesis in addition to storage?
• Does xylem only transport water — or does it also provide mechanical support to the plant?
• Do epidermal cells only protect — or do root hair cells also absorb water, and stomatal guard cells also control gas exchange?
• In aquatic plants, does parenchyma only store food — or does the specialised aerenchyma also provide buoyancy?

Examples that disprove the statement: