Class 11 Biology - Plant Growth and Development

Exercises

1. Define:

(a) Growth (b) Differentiation (c) Development (d) Dedifferentiation (e) Redifferentiation (f) Determinate Growth (g) Meristem (h) Growth Rate

Answer: (a) Growth: Irreversible permanent increase in size of an organ or its parts or even of an individual cell. (b) Differentiation: The process by which cells derived from meristems mature to perform specific functions. (c) Development: All the changes that an organism goes through during its life cycle from germination to senescence. (d) Dedifferentiation: The phenomenon by which differentiated cells regain the capacity to divide under certain conditions. (e) Redifferentiation: The process where cells produced by dedifferentiated tissues lose their capacity to divide again and mature for specific functions. (f) Determinate Growth: Growth that stops after reaching a certain size or stage. (g) Meristem: A specialized region of active cell division that contributes to the growth of the plant. (h) Growth Rate: The increased growth per unit time.

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2. Why is not any one parameter good enough to demonstrate growth throughout the life of a flowering plant?

Answer: Growth is a multifaceted process that involves different parameters at different stages. For example, increase in cell number reflects growth in meristems, while increase in cell size (volume) is characteristic of the elongation phase. Increase in fresh or dry weight is common but can be affected by water content. Length increase is used for roots and pollen tubes, while surface area increase is used for leaves. No single parameter captures all these changes simultaneously.

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3. Describe briefly:

(a) Arithmetic growth (b) Geometric growth (c) Sigmoid growth curve (d) Absolute and relative growth rates

Answer: (a) Arithmetic Growth: Only one daughter cell continues to divide while the other differentiates. It results in a constant increase in size over time. (b) Geometric Growth: Both daughter cells continue to divide, leading to an exponential increase in growth initially. (c) Sigmoid Growth Curve: A typical S-shaped curve obtained in geometric growth, consisting of lag, log (exponential), and stationary phases. (d) Absolute vs. Relative Growth Rates: Absolute growth rate is total increase per unit time. Relative growth rate is the increase per unit initial parameter (e.g., area per unit initial area).

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4. List five main groups of natural plant growth regulators. Write a note on one.

Answer: The five groups are: Auxins, Gibberellins, Cytokinins, Abscisic acid (ABA), and Ethylene.

• Auxins: Naturally occurring auxins like IAA and IBA promote apical dominance, induce rooting in stem cuttings, prevent early fruit drop, and are used to induce parthenocarpy in tomatoes. Synthetic auxins like 2,4-D are used as selective herbicides to kill dicot weeds.

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5. Why is abscisic acid also known as stress hormone?

Answer: Abscisic acid (ABA) is called the stress hormone because it increases the tolerance of plants to various kinds of stresses. It stimulates the closure of stomata during drought to prevent water loss and inhibits seed germination during unfavorable conditions, helping the plant survive periods of high stress.

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6. ‘Both growth and differentiation in higher plants are open’. Comment.

Answer: Plant growth is "open" because plants retain the capacity for unlimited growth throughout their life due to meristems always adding new cells. Differentiation is "open" because the final structure of a cell arising from a meristem is not predetermined by its origin but by its final location in the plant body (e.g., cells at the tip become root cap cells, while peripheral ones become epidermis).

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7. ‘Both a short day plant and a long day plant can flower simultaneously in a given place’. Explain.

Answer: Flowering depends on the photoperiod (day length) receiving critical values. If the actual day length at a given time and place is intermediate between the critical requirements of both a short-day plant and a long-day plant, both can receive the necessary signal to flower simultaneously.

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8. Which one of the plant growth regulators would you use if you are asked to:

(a) induce rooting in a twig (b) quickly ripen a fruit (c) delay leaf senescence (d) induce growth in axillary buds (e) ‘bolt’ a rosette plant (f) induce immediate stomatal closure in leaves.

Answer: (a) Auxin (e.g., NAA or IBA) (b) Ethylene (c) Cytokinin (d) Cytokinin (e) Gibberellin ($GA_3$) (f) Abscisic Acid (ABA)

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9. Would a defoliated plant respond to photoperiodic cycle? Why?

Answer: No, a defoliated plant would not respond. The site of perception of light and dark duration is the leaves. The stimulus (likely a hormonal substance like florigen) is produced in the leaves and then translocated to the shoot apices to induce flowering.

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10. What would be expected to happen if:

(a) GA3 is applied to rice seedlings (b) dividing cells stop differentiating (c) a rotten fruit gets mixed with unripe fruits (d) you forget to add cytokinin to the culture medium.

Answer: (a) The rice seedlings will show rapid internode elongation (and potentially the symptoms of 'bakanae' disease). (b) It will result in a mass of undifferentiated cells called a callus. (c) The unripe fruits will ripen quickly due to the release of ethylene from the rotten fruit. (d) Cell division and shoot formation will be significantly inhibited in the tissue culture.