Assertion Reason Questions for Biology Chapter 15 Plant Growth and Development

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Directions: In the following questions, a statement of assertion is followed by a statement of reason.
Mark the correct choice as:
(a) If both Assertion and Reason are true and Reason is the correct explanation of Assertion.
(b) If both Assertion and Reason are true but Reason is not the correct explanation of Assertion.
(c) If Assertion is true but Reason is false.
(d) If both Assertion and Reason are false.

Q.1. Assertion: As a whole plant growth is indefinite.
Reason: Plants retain the capacity of continuous growth throughout their life.

Answer Answer: (a) The shoot tip tissues in plants are meristematic.

Q.2. Assertion: For the synthesis of protoplasm nutrients are required by plants which act as source of energy.
Reason: Water provides the medium for enzymatic activities needed for growth.

Answer Answer: (b) Nutrients are raw materials required for synthesis of protoplasm as well as source of energy. It should be rich in nitrogenous components to increase the synthesis of protoplasm and carbohydrates for energy and cell wall synthesis. All types of micronutrients and macronutrients should be available for proper growth. Water is required for cell elongation, maintenance of turgidity of growing cells and for providing medium for enzyme action. Even slight deficiency of water reduces growth. It may, however, promote differentiation. Water stress completely stops growth.

Q.3. Assertion: Both at the root apex and the shoot apex, the constantly dividing cells show the meristematic phase of growth.
Reason: The cells of this region are rich in protoplasm and lacks nuclei.

Answer Answer: (c) The root apex, shoot apex and many other regions consists meristematic tissue. By mitotic divisions of the meristematic cells new cells are produced. These cells have cellulosic cells walls, contains large nucleus and rich in protoplasm. It is called the phase of cell formation or cell division.

Q.4. Assertion: Primary growth of the plants leads to the elongation of the plants along their axis.
Reason: Root apical meristem and shoot apical meristem are responsible for primary growth of the plants.

Answer Answer: (a) Growth is unique in plants because plants retain the capacity for unlimited growth throughout their life. These ability of the plants is due to the presence of meristems at certain locations in their body. The cells of such meristems have the ability to divide and self-perpetuate. The root apical meristem and shoot apical meristem are indeterminate meristems, because these meristems continue thier activity throughout life of the plant. Root apical meristem and shoot apical meristem cause primary growth of plants and also causes the elongation of the plants along their axis.

Q.5. Assertion: Sigmoid growth curve consists of four parts.
Reason: Lag phase is also known as grand phase of growth.

Answer Answer: (c) An S-shaped or sigmoid curve is obtained, if total growth is plotted against time. It consists of four parts-lag phase, log phase, phase of diminishing growth and stationary phase. Growth is slow in the lag phase, rapid during log or exponential phase, slow again during the phase of diminishing growth. During the stationary phase growth stops completely. Log phase is also called as grand phase of growth due to fast growth in this phase.

Q.6. Assertion: Due to environment, the difference in shapes of leaves produced in air and those produced in water in buttercup represent the heterophyllous development.
Reason: The phenomenon of heterophylly is an example of plasticity.

Answer Answer: (b) During different phases of growth as well as in response to environment different structures are developed. To cope up with the changes in environment. The plant organs also change their structures, which is called plasticity. Heterophylly is the occurrence of different types of leaves on the same plant in different growth phases or under different environmental conditions. In case of environmental plasticity shown by aquatic butter cup Ranunculus flabellaris, the submerged leaves are highly dissected while the emerged leaves are broad and lobed.

Q.7. Assertion: The sum of growth and differentiation is development.
Reason: Development in plants is under the control of extrinstic factors only.

Answer Answer: (c) In the life of a plant, growth, differentiation and development are closely related events. Development is sum of growth and differentiation. Development in plants is under control of both intrinsic and extrinsic factors. Intrinsic factors include intracellular genetic factors and intercellular factors growth regulators. Extrinsic factors are light, temperature, water, oxygen and nutrition, etc.

Q.8. Assertion: Secondary roots and shoots are plagiogeotropic.
Reason: Plagiogeotropic roots are those which develop at an angle of 45° from the vertical axis.

Answer Answer: (a) Secondary roots and shoots are plagiogeotropic i.e., they grow to a position at an oblique angle (45°) to the gravitational force. Root and stem branches lie at an angle other than 90° to the direction of gravity.

Q.9. Assertion : Apical dominance is increased by removal of shoot tip.
Reason : Due to accumulation of auxin in lateral parts, growth is inhibited.

Answer Answer: (d) Removal of shoot tip increases the lateral dominance not apical dominance. Accumulation of auxin in the lateral part elongates the cells and increases growth.

Q.10. Assertion: F.W. Went isolated auxin from the times of coleoptiles of wheat seedlings.
Reason: Ethylene delays the senescence.

Answer Answer: (d) F.W. Went isolated auxin from tips of coleoptiles of oat seedlings. Ethylene promotes senescence and abscission of plant organs especially of leaves and flowers.

Q.11. Assertion: During Vietnam War, agent orange (a mixture of 2, 4-D and 2,4,5-T) was used .
Reason: 2,4-D and 2,4,5-T are used as herbicides.

Answer Answer: (b) The first selective herbicides to be discovered and used widely were 2,4-D and its derivatives and are very potent auxins. 2,4-D and 2,4,5-T destroy dicot weeds. They block their sieve elements and disturb mitosis. The plant is ultimately destroyed. Agent orange, which was used in the war in Vietnam as a defoliant is an effective mixture of free 2,4-D and the N-butyl ester of 2,4,5-T.

Q.12. Assertion: By suppressing the activity of lateral buds, auxins promote apical dominance.
Reason: In moriculture, periodic pruning of shoot tips is done to make mulberry plants bushy.

Answer Answer: (a) Auxin is produced by shoot tip that acts as a growth promoter for apical bud and inhibit the growth of lateral buds. When a terminal bud is removed, the nearest axillary buds begin to grow and the plant branches rapidly. In case of mulberry plants, pruning is done to remove apical dominance and causes more branching of the main body of the plant.

Q.13. Assertion : Auxins help to prevent fruit and leaf drop at early stages.
Reason : Auxins promote the abscission of older mature leaves and fruits.

Answer Answer: (b) Auxin delays abscission of young leaves and fruits. Its effect is through nonformation of abscission zone below a leaf or fruit. Abscission zone cuts off nutrients and water supply. However, auxin promotes the abscission of mature or older leaves and fruits.

Q.14. Assertion : The apical bud is the only source of auxins.
Reason : Removal of apical bud promotes lateral bud growth.

Answer Answer: (d) The apical bud is not the only source of auxins. Young developing leaves also produce auxins and it has been shown that auxins from this source may inhibit lateral bud growth. When the apical bud is removed, the lateral buds sprout. However, if a paste of auxin is painted on the cut end of the decapitated shoot, the lateral buds remain inhibited, as if the apical bud is present.

Q.15. Assertion : Gibberellins induce flowering in long day plants.
Reason : Genetically tall plant become dwarf by application of Gibberellin.

Answer Answer: (c) Application of GA can induce flowering in long day plants. It has no favourbale effect on size of flower and fruit in certain plants. The genetically dwarf plants can be made tall by application of GA.

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