Vardaan Learning Institute
NCERT SOLUTIONS • CHAPTER 3
Tissues In Action (Revise, Reflect, Refine)
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.
Solution
Correct Option: (iii) They have thin walls, dense cytoplasm and large prominent nucleus.
Reason:
Meristematic cells are actively dividing, undifferentiated cells. Their thin primary cellulosic walls, abundant dense cytoplasm, large prominent nucleus, and absence of large central vacuoles allow rapid nuclear and cellular division without cellular rigidity or storage barriers.
If a plant is unable to transport food from leaves to roots which tissue is malfunctioning?
(i) Xylem
(ii) Phloem
(iii) Epidermis
(iv) Sclerenchyma
Solution
Correct Option: (ii) Phloem
Reason:
Phloem is the complex permanent vascular tissue responsible for the translocation of food (sucrose and organic nutrients) synthesized in leaves to all other parts of the plant, including roots. Xylem transports water and minerals.
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.
Solution
Correct Option: (iii) To allow quick exchange of materials across them.
Reason:
Simple epithelium (single layer of cells, e.g., in blood capillaries, alveoli, and intestinal lining) provides a minimal diffusion barrier, enabling rapid passive exchange of gases, nutrients, and waste products across internal organ boundaries.
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?
Solution
Differences in Joint Positions and Mechanics:
- Straight-Leg Jump (Stiff Joints):
- Ankles, knees, and hips remain locked in extension ($180^\circ$).
- Prevents quadriceps and calf muscle contraction and stretch-recoil storage in tendons.
- Landings lack shock absorption, transferring harsh impact forces directly to the spine and articular cartilage.
- Normal Jump (Flexed Joints):
- Hips, knees, and ankles flex deeply during crouch (knee flexion $\approx 90^\circ$, ankle dorsiflexion).
- Quadriceps, hamstrings, and gastrocnemius muscles contract powerfully to convert mechanical energy into upward propel force.
- Joint flexion upon landing acts as a hydraulic spring, allowing tendons and cartilage to safely dissipate impact energy.
Which type of joint is involved when you bend your knees and ankles?
(i) Ball and socket
(ii) Hinge
(iii) Pivot
Solution
Correct Option: (ii) Hinge
Reason:
Knee and ankle joints operate primarily as hinge joints, allowing back-and-forth movement in a single plane (flexion and extension), similar to a door hinge.
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.
Solution
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.
Answer: (iii) (A) is true, but (R) is false.
Explanation: Epithelium in lung alveoli is extremely thin simple squamous (single layer of flat cells) to facilitate rapid gas exchange. It is not multi-layered or tall.
B. Assertion: Cardiac muscle can contract continuously without fatigue.
Reason: Cardiac muscle cells have a high number of mitochondria and an abundant blood supply.
Answer: (i) Both (A) and (R) are true, and (R) is the correct explanation of (A).
Explanation: High mitochondrial density supplies continuous ATP via aerobic respiration, preventing metabolic fatigue in heart muscle.
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.
Answer: (iv) (A) is false, but (R) is true.
Explanation: Tendons connect muscle to bone (not bone to bone). Ligaments connect bone 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: (iii) (A) is true, but (R) is false.
Explanation: Hinge joints restrict motion to one plane because one convex bone surface fits into a concave surface, restricting sliding in all directions.
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 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?
Solution
(i) Graph Analysis & Interpretation:
The graph shows a steady, positive upward slope. As the age of the teak tree increases from 5 to 40 years, the stem diameter (DBH) increases from 4 cm to 40 cm. This indicates continuous secondary growth in girth over time.
(ii) Relationship between Diameter and Annual Rings:
The number of annual rings formed is directly proportional to both the age of the tree and the progressive increase in trunk diameter ($1 \text{ annual ring} = 1 \text{ year of growth}$). Each ring represents one year's secondary xylem growth by cambial activity.
(iii) Responsible Tissue & Location:
Lateral Meristem (Vascular Cambium and Cork Cambium) is responsible for the growth in girth. It is located circumferentially between the xylem and phloem in the stem and root trunks.
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?
(ii) Which plant tissue would be affected by further damage to the tree trunk even after debarking?
(iii) Which function of the tree would be hampered if the tissues beneath the bark were severely damaged?
(iv) What assumptions are you making to answer the questions above? How would the answer change if your assumptions are also changed?
Solution
(i) Functions Hampered by Debarking:
- Protection: Loss of outer cork/epidermis exposes inner tissues to mechanical injury, desiccation, fungal/bacterial infections, and pest invasion.
- Translocation: Removal of inner bark damages active Phloem, disrupting food transport from leaves to roots.
(ii) Affected Tissue with Further Damage:
Further inward damage affects the Vascular Cambium and underlying Xylem.
(iii) Function Hampered by Sub-Bark Tissue Damage:
Damage to sub-bark vascular tissues halts upward water and mineral transport (Xylem) and downward sugar transport (Phloem), leading to root starvation and tree wilting/death (girdling effect).
(iv) Assumptions Made & Alternative Cases:
- Assumption: Debarking is a complete $360^\circ$ ring around the trunk.
- If assumption changes (partial debarking): Intact phloem strips remain, allowing the tree to survive and heal via wound cambium.
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.
Solution
1. Responsible Tissue: Collenchyma
2. Explanation & Impact of Sclerenchyma Replacement:
- Collenchyma consists of living cells with localized pectin/cellulose wall thickenings at corners, providing tensile strength along with flexibility and elasticity without breaking.
- If collenchyma were replaced by sclerenchyma (dead cells with heavily lignified, rigid, inflexible walls), the sapling stem would become extremely hard and brittle. During strong monsoon winds, the stem would be unable to bend and would snap/break off easily.
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?
(ii) What difference was present in type ‘B’ compared to type ‘A’?
(iii) What observation or measurement was made to determine whether this change had an effect?
(iv) What parameters should be kept the same for both types of cuttings to ensure a fair comparison?
Solution
(i) Reason for Growth in Type B:
Type B cuttings contained intact stem nodes with intercalary meristem and axillary buds capable of active cell division and sprouting into new shoots/roots. Type A cuttings lacked nodes (only internodal tissue).
(ii) Difference Present in Type B:
Presence of Nodes bearing Intercalary Meristems.
(iii) Observation / Measurement Made:
Presence of visible new green shoots/leaves, root sprouting, and shoot length (cm) over time.
(iv) Parameters for Fair Comparison:
Identical cutting length, soil moisture, sunlight exposure, temperature, and planting depth.
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.
Solution
Rajiv's counter-argument is scientifically accurate.
Explanation:
- Simple Tissues (Parenchyma, Collenchyma, Sclerenchyma): Composed of a single, structurally identical type of cell working together for a common function.
- Complex Tissues (Xylem and Phloem): Composed of multiple different types of cells (e.g., Xylem tracheids, vessels, parenchyma, fibres) that differ in structure yet coordinate together as a functional unit to perform long-distance transport.
- Thus, a broader definition of tissue is: "A group of similar or dissimilar cells having a common origin that work together to perform a specific function."
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.
Solution
1. Suitable Tissue: Sclerenchyma (Sclerenchymatous Fibres)
2. Structural Features: Long, narrow dead cells with extremely thick cell walls lignified with lignin deposition, devoid of protoplasm, giving high mechanical strength and tear resistance.
3. Why Living Parenchyma Cannot Serve the Purpose:
Parenchyma cells have thin cellulosic walls, large vacuoles, and abundant cytoplasm meant for storage and photosynthesis. Under mechanical pressure, thin parenchyma cells collapse and decompose easily when exposed to moisture.
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?
Solution
Vibha's statement is incomplete and incorrect. Root and shoot apices contain only Apical Meristem, but meristems also exist elsewhere in the plant.
Questions Neha can ask Vibha:
- "If meristems are only at the tips, how does a tree trunk increase in thickness (girth) over the years?" (Pointing to Lateral Meristem / Cambium).
- "Why does lawn grass regrow quickly after being mowed when its shoot tip has been cut off?" (Pointing to Intercalary Meristem).
A plant cell and an animal cell are of the same size.
(i) Which cell will have a larger vacuole? Give reasons.
(ii) What assumptions are you making to answer the question above?
Solution
(i) Larger Vacuole & Reasons:
The plant cell will have a significantly larger vacuole (occupying 50%–90% of cell volume).
Reasons: Mature plant cells require a large central vacuole filled with cell sap to maintain cellular turgidity, osmotic pressure, and store water/minerals. Animal cells have small, temporary vacuoles.
(ii) Assumptions Made:
- Assumed both cells are fully mature and functionally active.
- Assumed standard eukaryotic physiological conditions.
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?
Solution
1. Critical Questions to Ask:
- "Can a single tissue perform multiple physiological roles simultaneously?"
- "Do structural tissues also participate in metabolic storage and photosynthesis?"
2. Counter-Examples Proving Statement Incorrect:
- Parenchyma: Performs food storage, photosynthesis (Chlorenchyma), floating buoyancy (Aerenchyma), and wound healing/regeneration.
- Epidermis: Provides protection against mechanical injury, prevents water loss (cuticle), conducts gas exchange (stomata), and absorbs water/minerals (root hair).
- Xylem: Transports water/minerals AND provides mechanical support to the plant body.
