NCERT Solutions for Class 9 Science Chapter 3 Tissues in Action (PDF) – 2027

Life processes: every life process takes place in cells. In a multicellular body, cells of a similar type group into tissues and each tissue does one job — muscle tissue brings about movement, nervous tissue carries messages, xylem carries water and phloem carries food. Studying cells and tissues shows where each life process happens and why that structure suits it.

Human welfare: knowing how normal tissues work helps us understand and treat diseases (for example, bone marrow stem cell transplants for leukaemia and thalassaemia) and improve crops through plant tissue culture, in which whole plants are grown from a few cells.

They differ because plants and animals live in different ways — plants stay fixed and make their own food, animals move about and take food in.

At every level, the structure is shaped by the one job that level has to do — and each higher level exists to combine jobs that a lower level cannot combine.

Cells of a similar type that perform a similar function group together to form a tissue, because this division of labour makes the body efficient.

In a multicellular organism no single cell can do every job. So groups of similar cells specialise — muscle tissue for movement, nervous tissue for carrying messages, xylem for carrying water and phloem for carrying food. Each tissue can then be built for its own job, and the body can carry out complex life processes.

Only tissues whose cells keep the ability to divide can go on growing.

In plants, meristematic tissue — at the tips of roots and shoots, in a ring in the stem and at the nodes — keeps dividing throughout life, so a plant can grow in length and girth all its life. Other cells differentiate into permanent tissues, which lose the ability to divide. In animals, most tissues stop growing once the body reaches its adult size.

Meristematic tissues — tissues made of actively dividing cells.

• Seedling growing tall and roots growing deep (length): apical meristem at the tips of shoots and roots.
• Stem becoming thicker (girth): lateral meristem.
• Grass growing again after grazing (regrowth): intercalary meristem.

The roots in Jar A keep getting longer day after day; the roots in Jar B grow at the same rate until their tips are cut, and then they stop completely.

Similar to Fig. 3.2? Yes — your readings should show the same pattern as the graph. The readings in Fig. 3.2 (length of root in cm from the base of the bulb) are:

Inference: a root grows only from its tip. Remove the tip and the whole root stops elongating, even though the rest of the root is alive and healthy.

A ring of actively dividing cells inside the stem — the lateral meristem.

These cells are arranged in a cylinder running along the length of the stem. When they divide, the new cells are added inside and outside the ring in a concentric manner, so the stem thickens from within while keeping its round shape.

The stem stops growing in length, but the plant does not stop growing — new branches come out from the nodes (Fig. 3.5).

Cutting the tip removes the shoot apical meristem, so the stem cannot grow taller. The nodes, however, still have actively dividing cells (the book calls this the intercalary meristem), so buds at the nodes grow out as new branches. That is why a trimmed hedge becomes bushy.

Because a large vacuole would take up the space and the resources that a rapidly dividing cell needs for dividing.

A meristematic cell must divide again and again, so it stays small, with a large nucleus and dense cytoplasm full of organelles that make new materials and release energy. A large vacuole is a storage structure; it forms later, when the cell stops dividing and becomes a permanent tissue such as parenchyma.

No — in the T.S. of the sunflower stem (Fig. 3.7) the cells are clearly not all alike, and you can pick out several distinct tissues arranged in a definite order from outside inwards.

The reason: one cell shape cannot do five jobs. A cell that conducts water must be hollow and dead; a cell that stores food must be alive and roomy; a cell that gives strength must be thick-walled. So the stem is built as a set of specialised tissues, each with the shape its job requires — division of labour once again.

The epidermis — the single, outermost layer of flat, rectangular, tightly packed cells that covers every part of the plant body — together with the waxy cuticle of cutin laid over it.

All four questions are answered by the three simple permanent tissues — parenchyma, collenchyma and sclerenchyma — and by which of them is present where.

Water goes up through the xylem; food comes out of the leaves through the phloem. Both are complex permanent tissues, made of more than one type of cell working together.

Water: water evaporates from the leaves through the stomata (transpiration). This creates a pull that draws the continuous column of water up through the hollow tracheids and vessels of the xylem, from the roots to the leaves.
Food: food made in the leaves moves through the sieve tubes of the phloem to all other parts of the plant; companion cells monitor the loading and unloading of sugars in the sieve tubes.

Different tissues: the coconut husk is sclerenchyma, the coriander leaf stalk is collenchyma (with parenchyma).

Because the cuticle blocks the movement of water and gases — and a desert plant needs that block, while a submerged plant is killed by it.

In the desert: water is scarce, so a thick waxy cuticle reduces water loss from the surface; it also protects against heat and blowing sand.
Underwater: water is never short, and a submerged plant takes in water, minerals and dissolved gases through its whole surface. A thick cuticle would block this absorption and gas exchange, so there it is harmful.

The dead cells supply the pipe; the living cells of the leaf supply the pull. Neither works without the other.

Step by step:

1. In the leaf, living mesophyll cells lose water vapour through the stomata — transpiration.
2. Losing water makes those cells draw water from the nearest xylem vessel.
3. Water molecules stick to one another (cohesion), so the column of water in the xylem does not break. Pulling at the top therefore pulls the whole column.
4. This tension — the transpiration pull — is transmitted right down to the root, where water enters through the root hair and replaces what was lost.

The plant would seal itself in: gas exchange would nearly stop, transpiration would stop, and with it the upward pull that carries water and minerals to the leaves.

Four different questions, four different animal tissues — and in every case the structure of the tissue is the answer.

Here is the completed Table 3.3, with the observations the book gives and the questions each one should make you ask.

All three experiences point to the same tissue: blood, a connective tissue in which cells float in a fluid matrix called plasma.

Platelets — the smallest of the formed elements — collect at the site of the injury and start the clot that seals it.

What happens at a cut, in order:

1. The vessel wall is torn and blood escapes.
2. Platelets reach the injured spot and stick to the torn edges and to one another, forming a temporary plug.
3. They set off the chemical reactions of clotting, which lay down a mesh of fibres across the wound.
4. Blood cells are trapped in that mesh; the mass hardens and bleeding stops.

Here is Table 3.4 completed. Each action lets you feel one connective tissue through your own skin.

Yes — a great many. Movement of food along the intestine and the beating of the heart go on without any conscious control at all.

Bones make up about 12 – 15 per cent of an adult’s body weight; muscle makes up about 40 – 50 per cent in adult males and about 30 – 40 per cent in adult females. Together, bones and muscles make up roughly 40 – 65 per cent of body weight.

Worked example for a student weighing 50 kg (use your own reading from the weighing scale in step 1):

Why they differ: bone and muscle mass depend on age (bone mass builds up till early adulthood and declines later), gender (adult males have more muscle on average), physical activity (regular exercise builds muscle and denser bones), nutrition (protein for muscle; calcium, phosphorus and vitamin D for bone), body size and composition, and health.

Contribution to body weight: bones make up about 12 – 15% and muscles about 30 – 50% of body weight, so together they are roughly 42 – 65% of it. The rest is fat, skin, blood, organs and body fluids.

Here is Table 3.5 filled in. The book gives the elbow row (No, No, Yes) as your model; the rest follow from the type of joint at each place.

Because the type of joint is different at different places, and a joint is simply the junction between two or more bones shaped in a particular way.

The muscles do — by contracting and pulling the bones through tendons. Joints only permit movement; they cannot produce it.

The chain of events, in order:

1. The nervous system sends an impulse to the muscle. (The musculoskeletal system works under the control of the nervous system.)
2. The muscle contracts — it gets shorter and thicker.
3. The tendon, a tough flexible band joining the muscle to the bone, transmits that pulling force to the bone.
4. The bone turns about the joint, and the joint decides the direction and the limit of that turn.
5. The ligaments holding bone to bone stop the movement going past a safe limit; the cartilage at the bone ends cushions the contact.

The skull is connected to the backbone through a pivot joint. It allows the head to turn from side to side, like a doorknob turning in its socket — which is why the neck can move so freely.

Almost every joint in the body appears somewhere in Fig. 3.17. Look at what each dancer is doing and name the joint by the movement, not by the pose.

How it moves: the ribs are attached to the backbone at the back and to the breastbone (sternum) in front, and they are joined by flexible cartilage. This flexibility lets the rib cage move.

Why it needs to move: during breathing the rib cage expands and contracts. This increases and decreases the space in the chest, so that air moves into and out of the lungs.

That a mature carrot phloem cell is totipotent — although it has already differentiated into a food-conducting cell, it still carries the complete set of instructions to build an entire plant, and it can be made to use them again.

Steward’s 1958 experiment shows this in three steps:

So the characteristics are: the cell is living (a dead xylem cell could never do this), it has retained its full nucleus and complete genetic information, and its differentiated state is reversible given the right nutrients, hormones and conditions.

Highest biomass: combination 2 — light present, air present, liquid medium + nutrients (20% increase).
Lowest biomass: combinations 1 and 3, where the fresh weight actually reduced below the starting weight.

Reason: the cells need air (oxygen for respiration, which releases energy for division), light, and a liquid medium that surrounds every cell with nutrients. Only combination 2 had all three. Without air or without light, the cells used up their own stored food, so their fresh weight fell.

No. Animal cells can be grown in culture, but a differentiated animal cell will not regenerate a whole animal the way a carrot phloem cell regenerates a whole plant.

Two clear ones:

1. Micropropagation — mass production of identical, disease-free planting material. A few grams of tissue from one superior plant can be cultured to raise thousands of plantlets in a small laboratory space, at any time of year, all genetically identical to the parent. This is how banana, sugarcane, potato and orchid planting material is produced commercially.
2. Industrial production of plant chemicals. Cultured cells can be grown in large vessels to make valuable phytochemicals — medicines, pigments, flavours — without growing, harvesting and destroying whole plants, and without depending on the season.

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

Thin walls, dense cytoplasm with many organelles and a large nucleus let these cells grow and divide quickly, again and again. Thick walls, large vacuoles and differentiated cells are features of permanent tissues, which do not divide.

(ii) Phloem.

Food is made in the leaves and has to travel downwards to the roots, which cannot photosynthesise. That job belongs to the sieve tubes of the phloem, helped by companion cells that load and unload the sugars.

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

Oxygen, digested food and other materials cross this lining by diffusion, which is fastest over a short distance. A lining only one or a few cells thick gives the shortest path, so the exchange is quick.

In the normal jump all three joints bend deeply before take-off and bend again on landing; in the straight-leg jump they stay almost locked, and you jump much lower and land with a jolt.

(ii) Hinge.

The knee and the ankle both bend and straighten in one direction only, like a door turning on its hinge. At the knee a small bone, the kneecap, sits in front of the joint and protects it.

A — (iii): the lung lining is a single layer of thin, flat cells, which makes diffusion fast; multiple layers of tall cells would slow it down.
B — (i): many mitochondria and a rich blood supply keep cardiac muscle supplied with energy and oxygen, so it does not tire.
C — (iv): tendons connect muscle to bone, not bone to bone (ligaments do that); the reason is a true statement.
D — (iii): a hinge joint moves in one plane; bone ends that allow sliding in all directions do not describe a hinge joint.

Plot age on the x-axis and put both DBH (cm) and the number of annual rings on the same y-axis, since both run from 4 to 40.

(i) The diameter keeps increasing with age, from 4 cm at 5 years to 40 cm at 40 years. It increased fastest between 10 and 20 years (1.6 cm per year); at other times it grew by about 0.8 cm per year.

(ii) One ring forms each year, so the number of rings equals the age. The diameter increases as the number of rings increases, though not in exact proportion, since rings formed in favourable years are wider.

(iii) The lateral meristem — a ring of actively dividing cells inside the stem, along its circumference. It adds new cells inside and outside, increasing the girth.

(i) Protection is lost — the bark protects the trunk from water loss, microbes, insects and injury. Food transport from leaves to roots is also interrupted, because the phloem lies just beneath the bark and is usually stripped with it; girth growth stops at that place.
(ii) The lateral meristem, and then the xylem.
(iii) The upward transport of water and minerals by the xylem (and the strength of the trunk). If the phloem is removed all round the trunk, the roots get no food and the whole tree may die.

(iv) Assumptions, and what changes if they are altered.

Collenchyma is responsible for the flexibility.

Collenchyma consists of living cells whose walls are unevenly thickened at the corners by the deposition of pectin — a chemical the book compares to rubber. The extra material at the corners gives strength, while the thin regions in between let the wall deform. So the stem can bend under the wind and spring back when the gust passes.

If it were replaced by sclerenchyma:

Prediction: the sapling would stand very stiffly in still air, but in the first strong monsoon gust the stem would crack or break outright — the way a dry twig snaps while a fresh twig bends. Its height growth would also be held back, because a lignified wall cannot stretch as the stem elongates.

(i) Cutting B has a node with a bud, which contains meristematic (actively dividing) tissue that can produce a new shoot and roots. Cutting A is a piece of internode made only of permanent tissues, whose cells cannot divide, so it could not sprout.
(ii) Type B has a node (with a bud); type A has no node.
(iii) Whether each cutting sprouted — for example, the number of cuttings that sprouted, and the length of the new shoots and roots after a few weeks.
(iv) The same variety and age of sugarcane, the same length and thickness of cuttings, the same soil, water, sunlight and temperature, the same planting depth, and the same number of cuttings of each type.

Rajiv is right, and Rohan’s statement fits only simple tissues. A tissue is a group of cells that work together for a common function — they need not all be the same kind of cell.

A better definition, covering both cases: a tissue is a group of cells that are organised together and work as a unit to carry out a specific function — similar in structure in a simple tissue, of several kinds in a complex tissue.

Sclerenchyma. Coconut husk fibre is sclerenchyma, and every one of its features is a strength feature.

Why not living parenchyma: parenchyma cells have thin walls and are loosely packed with intercellular spaces, so the tissue is soft, not fibrous. Their firmness comes from the water they hold; once dry they shrivel, and being living they would soon rot. So they cannot give lasting strength.

The statement is incorrect. Apical meristem is only one of three kinds. Meristematic cells are also present along the circumference of the stem (lateral meristem) and at the nodes and the base of internodes (intercalary meristem).

Questions Neha can ask Vibha — each one points at a fact her claim cannot explain:

1. “If meristem is only at the apices, then why does the trunk of a tree get thicker every year, and what forms the annual growth rings we count to find its age?”
2. “When a lawn is mowed, the shoot tips of every blade of grass are cut off. Why does the grass grow back?”

(i) The plant cell. A mature plant cell has one large central vacuole that can occupy most of the cell volume, pushing the cytoplasm and the nucleus into a thin layer against the wall. An animal cell has either no vacuole or only a few very small ones.

Reasons: the plant cell has a rigid cell wall; its large vacuole, filled with water, presses the cytoplasm against the wall and keeps the cell firm (turgid), and it also stores water, food and wastes. An animal cell has no cell wall and must be able to change shape, so a large vacuole would be of no use to it.

(ii) Assumptions: both cells are mature, living, ordinary cells — the plant cell is not a dividing meristematic cell (which has no vacuole) or a dead cell, and it is well supplied with water.

The statement is an over-simplification. Specialisation makes one function dominant, but most plant tissues do more than one job.

Questions worth asking:

1. Does any tissue in the chapter have more than one function listed for it?
2. Does the same tissue behave differently in different organs, or in different plants?
3. Does the function change with the age or the condition of the plant?
4. Can one function be carried out by more than one tissue — that is, is the mapping tissue → function one-to-one at all?
5. If a tissue really did only one thing, what should we expect to see when it is damaged? Does the plant lose only that one ability?

Examples that answer them:

Reducing the risk: eat a balanced diet with enough calcium, phosphorus, vitamin D (from sunlight) and protein; exercise regularly and warm up before games; keep a correct posture and lift with bent knees; practise yoga; wear proper footwear and rest enough between hard sessions.

What you will feel: a thick, firm cord under the skin just above the heel that you can feel moving as you point your toes down and pull them up. That is the tendon joining your calf muscle to the heel bone.

Other tendons you can find on yourself:

Sample answer (kabaddi): kabaddi involves running, sudden stops, crouching and grappling. Running and jumping put load on the bones, which then lay down more matrix and become denser and stronger. Repeated strong contractions make the muscles thicker and stronger, so they tire less quickly. Quick changes of direction strengthen the muscles and ligaments around the knees and ankles, which protects the joints, and the heart and lungs also become fitter.

Each garden practice works on a particular tissue. Match the practice to the tissue and the reason becomes obvious.

Sample answer — part (i):

Sample answer — part (ii): banana and palash leaves are used to make leaf plates (pattal) because they are large, waterproof and do not tear; banana leaves also stay fresh for days. Neem leaves are kept with stored grain and clothes to keep insects away.

Sample answer — a movement analysis you can start from:

Not in the simple way it happens in plants. Most differentiated animal cells lose the ability to form a whole organism. Only special cells such as stem cells keep some of this ability, and a whole animal can be obtained only by special laboratory methods (for example, placing the nucleus of a body cell into an egg cell whose own nucleus has been removed).

Advantages: damaged tissues and organs could be replaced, without rejection if grown from the patient’s own cells; endangered animals could be conserved and good livestock multiplied.

Challenges: the process is difficult and often fails; animal cells need precise signals and a blood supply; uncontrolled cell division can lead to cancer; and there are ethical questions about consent, animal welfare and cost.