Class 9 Science · Chapter 3

3. Tissues in Action

Complete solutions to Revise, Reflect, Refine. CBSE Class 9 Science (NCERT Exploration).

Revise, Reflect, Refine

1Meristematic tissues divide repeatedly. What property of their cells allows them to do this?

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

These features keep the cells active and ready to divide again and again.

2If a plant is unable to transport food from leaves to roots, which tissue is malfunctioning?

Answer: (ii) Phloem

Phloem carries food from the leaves to other parts of the plant. Xylem carries water and minerals.

3Why are the epithelial tissues that line an animal’s internal organs usually only one or a few cells thick?

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

A thin layer makes diffusion fast, for example gas exchange in the lungs.

4You can perform these two jumps (Fig. 3.21): Straight-leg jump — keep knees and ankles stiff. Normal jump — bend knees and ankles naturally. How did your ankle, knee and hip positions differ between the two jumps?

Straight-leg jump: the ankles, knees and hips stay straight and stiff. Only the toes push the body up. The jump is low, and the landing gives a hard jerk to the legs.

Normal jump: the ankles, knees and hips bend before take-off. They straighten quickly to push the body up. They bend again on landing. The jump is higher, and the landing is soft.

This shows that joints and muscles work together to produce movement and absorb shock.

5Which type of joint is involved when you bend your knees and ankles?

Answer: (ii) Hinge

The knee bends mainly in one direction, like a door on its hinge.

6Assertion and Reason: choose (i) Both true, R explains A; (ii) Both true, R does not explain A; (iii) A true, R false; (iv) A false, R true.

A. A: Epithelium is well-suited for gas exchange in the lungs. R: It consists of multiple layers of tall cells that slow down diffusion.

Answer: (iii) The lung epithelium is a single layer of thin, flat cells. This makes diffusion fast.

B. A: Cardiac muscle can contract continuously without fatigue. R: Cardiac muscle cells have a high number of mitochondria and an abundant blood supply.

Answer: (i) Many mitochondria and good blood supply give a constant supply of energy.

C. A: Tendons connect bone to bone and allow joint movement. R: Tendons are made of tough connective tissue that transmits force from muscle to bone.

Answer: (iv) Tendons connect muscle to bone. Ligaments connect bone to bone.

D. A: In a hinge joint, movement occurs primarily in one plane. R: The bone ends are shaped to allow sliding in all directions.

Answer: (iii) The bone ends of a hinge joint allow movement in one direction only.

7Plot a graph between the age of a teak tree (x-axis) and the diameter of the tree along with the number of annual rings (y-axis), using the data in Table 3.7.

Age of teak tree vs diameter (DBH) and number of annual rings.

(i) Analyse the graph in terms of the diameter of the stem over time and share the interpretation.

The diameter increases with the age of the tree. It grows from 4 cm at 5 years to 40 cm at 40 years. The growth is fastest between 10 and 20 years (8 cm to 24 cm). After that it grows more slowly. So the tree keeps growing in girth throughout its life.

(ii) What is the relation between the diameter of the teak tree to the annual rings formed?

One annual ring is formed every year. So the number of rings equals the age of the tree. As the number of rings increases, the diameter also increases. Both rise together.

(iii) Which specialised tissue is responsible for the girth of the stem and where is it located?

The lateral meristem (cambium) increases the girth. It is located along the circumference of the stem.

8A tree was severely debarked by an elephant (Fig. 3.22). Answer the following.

(i) Which function(s) of the tree is/are hampered by debarking?

The bark protects the tree. After debarking, the tree loses protection from water loss, insects, germs and injury. Transport of food may also be affected.

(ii) Which plant tissue would be affected by further damage to the tree trunk even after debarking?

The phloem and the lateral meristem (cambium) lying just below the bark. Deeper damage would also affect the xylem.

(iii) Which function of the tree would be hampered if the tissues beneath the bark were severely damaged?

Food made in the leaves cannot reach the roots through phloem. The roots will starve and slowly die. Growth in girth will also stop. If xylem is damaged, water cannot reach the leaves.

(iv) What assumptions are you making to answer the questions above? How would the answer change if your assumptions are also changed?

Assumption: the bark is removed in a complete ring around the trunk and the tissues below are damaged.

If only a small patch of bark is removed, food can still travel through the remaining phloem. The tree may then heal and survive.

9A young mango sapling’s stem bends flexibly during monsoon winds and does not break. Which tissue is responsible for this flexibility? Predict and explain the impact if the existing tissue was replaced by sclerenchyma.

Collenchyma gives flexibility. Its cells are living and have unevenly thickened walls. So the young stem can bend without breaking.

If it is replaced by sclerenchyma, the stem will become hard and stiff. Sclerenchyma cells are dead and have thick lignified walls. The stem cannot bend in strong winds. It may crack or break.

10Sohan used two types of sugarcane cuttings, type ‘A’ and type ‘B’ (Fig. 3.23). Type ‘B’ cuttings sprouted, whereas type ‘A’ cuttings did not.

(i) Why were the type ‘B’ cuttings able to grow as sugarcane but type ‘A’ could not?

Type B has a node with a bud and meristematic tissue. These cells can divide and form new shoots and roots. Type A has only an internode, so it has no bud to sprout.

(ii) What difference was present in type ‘B’ compared to type ‘A’?

Type B has a node (with bud). Type A is only an internode piece without a node.

(iii) What observation or measurement was made to determine whether this change had an effect?

Whether the cuttings sprouted or not. The number of cuttings that sprouted, and the growth of shoots and roots after a few weeks.

(iv) What parameters should be kept the same for both types of cuttings to ensure a fair comparison?

Length and thickness of cuttings, age of the plant, soil, water, sunlight, temperature, depth of planting and the number of days of observation.

11Rohan says, “A tissue is a group of similar cells performing similar functions”. Rajiv says, “this is true in case of simple tissues but little different in case of complex tissues”. Provide your explanation.

Rajiv is more correct.

Simple tissues, like parenchyma, have only one type of cell. So Rohan’s statement fits them.

Complex tissues, like xylem and phloem, have more than one type of cell. For example, xylem has tracheids, vessels, xylem parenchyma and xylem fibres. These different cells work together for one common function, transport of water.

So a better definition is: a tissue is a group of cells working together to perform a specific function.

12Coconut husk fibres are used for mats which are tough and fibrous. Which tissue has structural features suitable for providing this strength? Explain why living parenchyma couldn’t serve the same purpose.

Sclerenchyma. Its cells are long, narrow and dead. Their walls are thick due to lignin. There are no gaps between cells. This makes the husk hard and strong.

Parenchyma cells are living, thin-walled and loosely packed. They mainly store food. So they are soft and cannot give such strength.

13Vibha claims, “Meristematic cells are located only at the root and shoot apices”. What do you think about this statement? What question can Neha ask Vibha to help her understand further?

The statement is not correct. Apical meristem is at the root and shoot tips. But lateral meristem is along the circumference of the stem, and intercalary meristem is at the nodes or base of leaves.

Neha can ask: “If meristem is only at the tips, how does a tree trunk become thicker every year?” or “How does grass grow again after the top is cut?”

14A plant cell and an animal cell are of the same size.

(i) Which cell will have a larger vacuole? Give reasons.

The plant cell. A mature plant cell has a single large central vacuole. It stores water and keeps the cell firm. Animal cells have small vacuoles or none.

(ii) What assumptions are you making to answer the question above?

The plant cell is a mature cell, like a parenchyma cell. It is not a young meristematic cell, which has small or no vacuoles.

15A textbook states, “Each plant tissue performs only one specific function”. What questions would you ask to critically examine this statement? What examples of tissues would you take?

Questions to ask:

Does any tissue perform more than one function?
Can a tissue change its function in different parts of the plant?
Do complex tissues do more than transport?

Examples to examine:

Parenchyma stores food. In leaves it has chloroplasts and makes food. In water plants it has large air spaces and helps them float.
Xylem carries water and minerals. It also gives mechanical support.
Collenchyma gives flexibility and support. In young stems it may also make food.

These examples show that the statement is not fully correct.

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