Revise, Reflect, Refine
1. Meristematic tissues divide repeatedly. What property of their cells allows them to do this?
(i) They have thick walls for protection.
(ii) They contain large vacuoles that store nutrients.
(iii) They have thin walls, dense cytoplasm and large prominent nucleus.
(iv) They are functionally differentiated cells.
Answer: Correct option: (iii) They have thin walls, dense cytoplasm and large prominent nucleus.
Explanation: Meristematic cells divide continuously because they have thin cell walls, dense cytoplasm, and a large nucleus, which support active cell division. They usually lack vacuoles and are not yet specialised.
2. If a plant is unable to transport food from leaves to roots which tissue is malfunctioning?
(i) Xylem
(ii) Phloem
(iii) Epidermis
(iv) Sclerenchyma
Answer: Correct option: (ii) Phloem
Explanation: Phloem is the tissue responsible for transporting food (sugars) from leaves to other parts of the plant, including roots. If this transport is not happening, it means the phloem is not functioning properly.
3. Why are the epithelial tissues that line an animal’s internal organs usually only one or a few cells thick?
(i) To store food efficiently.
(ii) To provide maximum strength.
(iii) To allow quick exchange of materials across them.
(iv) To reduce friction.
Answer: Correct option: (iii) To allow quick exchange of materials across them.
Explanation: Epithelial tissues lining internal organs are thin (one or few cells thick) so that substances like gases, nutrients, and wastes can diffuse quickly across them. This helps in efficient exchange in organs like lungs and blood vessels.
4. You 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?
Answer: In the straight-leg jump, the ankles, knees, and hips remain straight and stiff, so there is very little bending at these joints. This makes the movement less flexible and more rigid.
In the normal jump, the ankles, knees, and hips bend naturally. The knees and hips bend forward, and the ankles also flex, which helps in pushing the body upward more efficiently.
Thus, the normal jump involves bending of joints for flexibility and better movement, while the straight-leg jump keeps the joints straight and less flexible.
5. Which type of joint is involved when you bend your knees and ankles?
(i) Ball and socket
(ii) Hinge
(iii) Pivot
Answer: Correct option: (ii) Hinge
Explanation: The knee joint is a hinge joint, which allows movement in one direction (bending and straightening). The ankle mainly works in a similar way during bending movements.
6. In each of the following cases (A, B, C and D), choose the correct option as given below:
(i) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(ii) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
(iii) (A) is true, but (R) is false.
(iv) (A) is false, but (R) is true.
A. Assertion: Epithelium is well-suited for gas exchange in the lungs.
Reason: It consists of multiple layers of tall cells that slow down diffusion.
B. Assertion: Cardiac muscle can contract continuously without fatigue.
Reason: Cardiac muscle cells have a high number of mitochondria and an abundant blood supply.
C. 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.
D. 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.
Assertion is true (epithelium helps in gas exchange).
Reason is false (it is actually thin, not multilayered, to allow diffusion).
Correct option: (iii)
B.
Assertion is true (cardiac muscles work continuously without fatigue).
Reason is true and correctly explains it (more mitochondria and good blood supply).
Correct option: (i)
C.
Assertion is false (tendons connect muscle to bone, not bone to bone).
Reason is true (they transmit force from muscle to bone).
Correct option: (iv)
D.
Assertion is true (hinge joints move in one plane).
Reason is false (they do not allow movement in all directions).
Correct option: (iii)
7. Plot a graph between the age of a tree (in years) on the x-axis and the diameter of the tree (in cm) along with the number of annual rings formed over time on the y-axis, using the data given in the Table 3.7.
(i) Analyse the graph in terms of the diameter of the stem over time and share the interpretation.
(ii) What is the relation between the diameter of the teak tree to the annual rings formed?
(iii) Which specialised tissue is responsible for the girth of the stem and where is it located?
Answer:
(i) Analysis of stem diameter over time:
The diameter of the teak tree increases steadily with age, but not at a uniform rate. It grows slowly in the early years (only 4 cm at age 5, 8 cm at age 10), then shows a rapid jump between ages 10 and 20 (from 8 cm to 24 cm — a 16 cm increase in just 10 years). After age 20, the rate of increase slows down again (28 → 32 → 40 cm). This suggests that the tree undergoes the fastest girth growth during its middle years, likely when environmental conditions are most favourable and the lateral meristem is most active.
(ii) Relation between diameter and annual rings:
The number of annual rings is exactly equal to the age of the tree in years (5 years = 5 rings, 40 years = 40 rings). This means one new ring is added every year. The diameter also increases with age, so as more rings are added, the diameter increases — showing that diameter and annual rings both increase together as the tree ages. In fact, counting annual rings is a reliable method to estimate a tree’s age.
(iii) Specialised tissue responsible for girth:
The lateral meristem (also called the vascular cambium) is responsible for the increase in girth of the stem. It is located along the circumference (periphery) of the stem, arranged in a ring. Its cells divide in a concentric manner — producing new cells both inward and outward — which causes the stem to increase in diameter each year, forming one new annual ring per growing season.
8. In a forest, it was observed that one of the trees was severely debarked by an elephant to meet its food requirements, as the bark is a rich source of nutrients (Fig. 3.22). Based on your learning, answer the following:
(i) Which function(s) of the tree is/are hampered by debarking?
Answer: The bark consists of the epidermis (outermost protective layer) and the cork/bark tissue. Removing it hampers these functions: protection against mechanical injury, water loss, and invasion by pathogens and insects is lost; transpiration regulation is disrupted since the cuticle is removed; and most importantly, the phloem (which lies just beneath the bark) is damaged, so the transport of food (sugars) from leaves down to the roots is interrupted. This can starve the roots and eventually kill the tree.
(ii) Which plant tissue would be affected by further damage to the tree trunk even after debarking?
Answer: Just beneath the bark lies the lateral meristem (vascular cambium). If the trunk is damaged further after debarking, this meristematic tissue would be affected. Since the lateral meristem is responsible for producing new cells that increase the girth of the stem and for regenerating the bark over time, damage to it would permanently stop the tree from healing or growing in girth. The xylem tissue deeper inside could also be damaged, disrupting water and mineral transport from roots to leaves.
(iii) Which function of the tree would be hampered if the tissues beneath the bark were severely damaged?
Answer: The tissues beneath the bark include the lateral meristem, phloem, and xylem. If these are severely damaged:
- Transport of food from leaves to roots (via phloem) would completely stop.
- Transport of water and minerals from roots to leaves (via xylem) would be disrupted.
- The tree would lose its ability to grow in girth and regenerate new tissue.
- Ultimately, the tree would die as it can neither feed its roots nor supply water to its leaves.
(iv) What assumptions are you making to answer the questions above? How would the answer change if your assumptions are also changed?
Answer:
Assumptions:
- Bark includes phloem tissue.
- Debarking removes phloem but xylem is initially intact.
- Damage is severe and continuous around the trunk (girdling).
If assumptions change:
- If debarking is partial, the tree may survive because some phloem remains functional.
- If xylem is not damaged, water transport continues, so the tree may live longer.
- If damage is minor, repair and regeneration may occur.
9. Aamrapali observed that a young mango sapling’s stem bends flexibly during monsoon winds and does not break. Which tissue is responsible for this flexibility? Predict and provide your explanation of the impact if the existing tissue was replaced by sclerenchyma.
Answer: Tissue responsible for flexibility:
The tissue responsible for the flexibility of the young mango sapling’s stem is collenchyma. It consists of living cells with unevenly thickened corners due to deposition of pectin, a chemical that gives flexibility like rubber. This allows the stem to bend during monsoon winds without breaking.
Impact if collenchyma was replaced by sclerenchyma:
If collenchyma were replaced by sclerenchyma, the following impacts would occur:
- Stem would become hard and rigid — Sclerenchyma cells have thick walls due to lignin deposition, making them hard and strong like wood. Most sclerenchyma cells are dead at maturity.
- Stem would lose flexibility — Unlike collenchyma, sclerenchyma cannot bend, so the stem would no longer be able to withstand monsoon winds.
- Stem would break easily — Under wind pressure, the rigid stem would snap instead of bending, just like a dry twig breaks when forced to bend.
- Growth would be affected — Sclerenchyma is found in mature parts like coconut husk and walnut shell, not in young growing stems. Replacing collenchyma with it would interfere with the sapling’s normal growth.
Conclusion: Collenchyma is ideal for young, flexible, growing plant parts. Replacing it with sclerenchyma would make the sapling rigid, brittle, and unable to survive strong winds.
10. Sohan designed an experiment for the regeneration of sugarcane, where he used cuttings to grow sugarcane. He used two types of cuttings, type ‘A’ and type ‘B’ (Fig. 3.23). After a few weeks, type ‘B’ cuttings sprouted and developed into sugarcane plants, whereas the type ‘A’ cuttings did not sprout.
(i) Why were the type ‘B’ cuttings able to grow as sugarcane but type ‘A’ could not?
Answer: Type ‘B’ cuttings were able to grow because they had nodes (buds) present. These buds contain meristematic tissue, which helps in growth and regeneration.
Type ‘A’ cuttings lacked buds, so no new growth could occur.
(ii) What difference was present in type ‘B’ compared to type ‘A’?
Answer:
- Type ‘B’ had nodes with buds (eyes).
- Type ‘A’ had no nodes or buds.
(iii) What observation or measurement was made to determine whether this change had an effect?
Answer:
- The observation was whether new shoots (sprouting) developed.
- Type ‘B’ showed sprouting and growth into new plants, while type ‘A’ did not.
(iv) What parameters should be kept the same for both types of cuttings to ensure a fair comparison?
Answer: To make the experiment fair, the following should be the same:
- Soil type
- Water supply
- Sunlight exposure
- Size/length of cuttings
- Environmental conditions (temperature, humidity)
11. During the discussion in class, Rohan gives a statement that, “A tissue is a group of similar cells performing similar functions”. But Rajiv counter argues that, “this is true in case of simple tissues but little different in case of complex tissues”. Provide your explanation in view of the discussion in class.
Answer: Rohan’s statement is partly correct, but Rajiv’s argument makes it more complete.
A tissue is generally defined as a group of similar cells performing similar functions, and this is true in case of simple tissues. Simple tissues (like parenchyma, collenchyma and sclerenchyma) are made up of only one type of similar cells, and all these cells perform the same function.
However, in case of complex tissues, the situation is different. Complex tissues (such as xylem and phloem) are made up of different types of cells, not similar ones. These different cells work together to perform a common function, like transport of water or food in plants.
Conclusion: Rohan’s definition is correct for simple tissues, but Rajiv is also right because complex tissues consist of different kinds of cells working together. Hence, the definition of tissue is slightly modified in the case of complex tissues.
12. Coconut 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.
Answer: The tissue responsible for the strength of coconut husk fibres is sclerenchyma tissue.
Sclerenchyma has thick, lignified cell walls which make the cells very hard and strong. Most of these cells are dead, and they provide mechanical strength and rigidity to plant parts. This is why coconut husk fibres are tough, durable and suitable for making mats.
On the other hand, parenchyma is made up of living cells with thin cell walls. These cells are mainly involved in storage and photosynthesis and are loosely packed. Because of their thin walls and soft nature, they cannot provide strength or toughness like sclerenchyma.
Conclusion: Sclerenchyma provides strength due to its thick, lignified walls, whereas living parenchyma cannot serve this purpose because it is soft and has thin walls.
13. Vibha claims to her friend Neha that, “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 if the statement is incorrect?
Answer: Vibha’s statement is incorrect.
Meristematic cells are not located only at the root and shoot apices (apical meristem). Although apical meristems help in the increase in length of the plant, there are other types of meristematic tissues also present in plants.
- Lateral meristem is present along the sides of stems and helps in increase in girth (thickness).
- Intercalary meristem is found at the base of nodes or internodes and helps in regrowth of plants like grass after cutting.
So, meristematic tissues are found in different regions, not just at the apices.
Question Neha can ask:
- “If meristematic cells are only at the tips, then how do plants increase in thickness or regrow after being cut?”
- This question will help Vibha understand that other meristematic tissues (lateral and intercalary) also exist.
14. A plant cell and an animal cell are of the same size.
(i) Which cell will have a larger vacuole? Give reasons.
Answer: The plant cell will have a larger vacuole.
In plant cells, there is usually a large central vacuole that occupies most of the cell space. It helps in storage of water, food and wastes, and also maintains turgidity (firmness) of the plant.
In contrast, animal cells either have small vacuoles or none, because they do not need to maintain rigidity like plants.
(ii) What assumptions are you making to answer the question above?
Answer:
- We are assuming that both cells are normal, mature cells.
- The plant cell has a typical large central vacuole, while the animal cell has small or no vacuole.
- Both cells are compared under normal conditions and are not specialised differently.
15. A textbook states, “Each plant tissue performs only one specific function”. What questions would you ask to critically examine the correctness of this statement? What examples of tissues would you take to find out the answers to these questions?
Answer: To examine the statement “Each plant tissue performs only one specific function”, we should question whether tissues really perform only one role or multiple roles.
Questions to ask:
- Do some plant tissues perform more than one function?
- Can a single tissue be involved in both support and transport or storage?
- Are all cells in a tissue doing exactly the same function, or do they have different roles?
Examples to check:
- Parenchyma: Mainly stores food, but also performs photosynthesis in green parts and helps in floating in aquatic plants.
- Xylem: Transports water and minerals, but also provides mechanical support to the plant.
- Phloem: Transports food, but also stores food materials (phloem parenchyma).
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