Chapter 13 : Plant Growth and Development

Class 11 Biology Chapter 13 Plant Growth and Development Questions and Answers :

1. Define growth, differentiation, development, dedifferentiation, redifferentiation , determinate growth, meristem and growth rate.

Answer :  Growth: Growth is one of the most conspicuous events in any living organism. It is an irreversible increase expressed in parameters such as size, area, length, height, volume, cell number, etc. It conspicuously involves increased protoplasmic material.

Differentiation: Differentiation refers to the process in which cells derived from root apical and shoot-apical meristems and cambium differentiate and mature to perform specific functions. During differentiation, cells undergo structural changes in their cell walls and protoplasm. For example, cells may lose their protoplasm and develop strong, elastic, lignocellulosic secondary cell walls to perform specialized functions, such as carrying water over long distances.

Development: Development is a comprehensive term that encompasses all changes an organism goes through during its life cycle, starting from germination of the seed to senescence. It includes processes such as growth, differentiation, maturation, and aging.

Dedifferentiation: Dedifferentiation occurs when living, differentiated cells that have lost the capacity to divide regain the ability to divide under certain conditions. An example of dedifferentiation is the formation of meristems, like interfascicular cambium and cork cambium, from fully differentiated parenchyma cells.

Redifferentiation: Redifferentiation is the process through which meristems or tissues that have undergone dedifferentiation regain the capacity to divide and produce cells. These newly formed cells, once again, lose their capacity to divide but mature to perform specific functions.

Determinate Growth: Determinate growth is a type of growth in which an organism or a part of an organism stops growing after reaching a certain size or stage of development. In other words, growth is limited and does not continue throughout the organism's life.

Meristem: Meristem is a region of actively dividing, undifferentiated plant cells found at the tips of roots and shoots. It is responsible for plant growth and can give rise to various specialized cell types. Meristematic cells are essential for plant development and regeneration.

Growth Rate: Growth rate refers to the speed or rate at which an organism or a specific part of an organism increases in size or mass over a given period of time. It is often expressed as a percentage or a specific measure (e.g., centimeters per year) and is used to quantify the pace of growth.

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

Answer :  A single parameter is inadequate to represent growth in a flowering plant throughout its life because growth is a complex process that involves multiple aspects, including changes in size, cell number, tissue differentiation, and development. A comprehensive understanding of growth requires considering these various parameters collectively, as they all contribute to the overall development and maturation of the plant.

3. Describe briefly:

(a) Arithmetic growth

(b) Geometric growth

(c) Sigmoid growth curve

(d) Absolute and relative growth rates

Answer :   (a) Arithmetic Growth: Arithmetic growth is a type of growth where an organism or a part of an organism increases in size or number of cells at a constant, linear rate over equal time intervals. In this growth pattern, only one of the daughter cells produced during mitotic cell division continues to divide, while the other differentiates and matures. It results in a straight-line curve when plotting the parameter of growth against time.

Mathematically, it can be expressed as  , where  is the length at time ' t ' ,  is the initial length,  is the growth rate or elongation per unit time.

(b) Geometric Growth: Geometric growth is a growth pattern in which the organism or part of an organism experiences slow initial growth (lag phase), followed by rapid and exponential growth (log or exponential phase). In geometric growth, both daughter cells resulting from mitotic cell division retain the ability to divide and continue doing so. This type of growth results in a sigmoid or S-shaped curve when plotting growth against time. The mathematical expression for exponential growth is  , where  is the final size,  is the initial size,  is the growth rate,  is the time of growth, and e is the base of natural logarithms.

(c) Sigmoid Growth Curve: A sigmoid growth curve is characteristic of living organisms growing in a natural environment. It typically starts with a slow initial growth phase, transitions into a rapid exponential growth phase, and eventually reaches a stationary phase when growth slows down due to limited nutrient supply or environmental factors. The resulting curve resembles an "S" shape and is observed in various cells, tissues, and organs of a plant.

(d) Absolute and Relative Growth Rates:

Absolute Growth Rate: The absolute growth rate is a measure of the total change in a parameter (e.g., size, weight, or number) over a specific time interval. It quantifies the actual increase and is expressed in absolute terms.

Relative Growth Rate: The relative growth rate is a measure of growth expressed on a common basis, often relative to the initial parameter. It allows for comparisons between different systems. The relative growth rate can be calculated as the growth rate divided by the initial parameter, providing a basis for assessing the efficiency of growth in different organisms or parts of an organism.

4. List five main groups of natural plant growth regulators. Write a note on discovery, physiological functions and agricultural/horticultural applications of any one of them.

Answer : Five main groups of natural plant growth regulators (PGRs) are:

Auxins (e.g., Indole-3-acetic acid or IAA)

Gibberellins (e.g., Gibberellic acid or )

Cytokinins (e.g., Kinetin)

Abscisic Acid (ABA)

Ethylene ()

Auxins: Auxins are a group of plant hormones that regulate various growth processes in plants. The discovery of auxins can be attributed to the work of Charles Darwin and his son Francis Darwin, who first observed phototropic responses in plants. The hormone auxin was later isolated and identified as indole-3-acetic acid (IAA) by Fritz Went in the 1920s.

Physiological Functions: Auxins are responsible for cell elongation, root initiation, apical dominance, and phototropism. They help plants bend toward a light source and promote the development of new roots.

Agricultural/Horticultural Applications: Auxins find extensive applications in agriculture and horticulture. They are used to promote root formation in cuttings, prevent premature fruit drop, and control the growth and branching of plants. For instance, auxin-based herbicides can selectively control weed growth by causing abnormal growth in weeds while leaving crops unharmed. Additionally, auxins are used in tissue culture techniques to initiate the growth of plantlets from small pieces of plant tissue, a valuable tool for propagation and plant breeding.

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

Answer : Abscisic acid (ABA) is known as the "stress hormone" in plants because it helps them respond to environmental stresses. ABA stimulates stomatal closure, reducing water loss during drought and extreme conditions. It also enhances a plant's tolerance to stress factors such as high salinity and extreme temperatures. ABA induces seed dormancy, enabling seeds to endure unfavorable conditions. This hormone plays a crucial role in plant survival and adaptation when faced with adverse environmental challenges.

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

Answer :  In higher plants, both growth and differentiation are open processes. Growth, primarily driven by cell division and elongation, is influenced by various intrinsic and extrinsic factors, allowing flexibility and adaptability to changing environmental conditions. Differentiation, the process by which cells become specialized in structure and function, also remains open to modification throughout a plant's life, enabling responses to environmental cues. This open nature of growth and differentiation ensures plants can adjust to their surroundings and optimize their development.

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

Answer : Short day and long day plants are categorized based on their response to day length or photoperiod. In a given location, when a short day plant and a long day plant are exposed to the same day length, conditions may exist where they both receive the critical day length required for flowering simultaneously. This can happen when the day length falls within the range that triggers flowering in both types, allowing them to flower at the same time.

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) To induce rooting in a twig : Auxins are typically used to promote root formation in cuttings .

(b) To quickly ripen a fruit : Ethylene is used to accelerate the ripening of fruits .

(c) To delay leaf senescence : Abscisic Acid (ABA) can help delay leaf senescence and maintain leaf quality .

(d) To induce growth in axillary buds : Cytokinins can promote the growth of axillary buds and shoots .

(e) To "bolt" a rosette plant : Gibberellins are often used to induce bolting, which is the rapid elongation of the stem in rosette plants that precedes flowering .

(f) To induce immediate stomatal closure in leaves : Abscisic Acid (ABA) is the primary plant growth regulator responsible for stimulating the immediate closure of stomata in response to stress, such as drought .

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

Answer : No, a defoliated plant would not respond to photoperiodic cycles as effectively as a healthy, fully foliated plant. Leaves play a crucial role in capturing and processing light for many photoperiodic responses, and their absence can limit the plant's ability to regulate these processes.

10. What would be expected to happen if :

(a) 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) If (gibberellic acid) is applied to rice seedlings, it is expected to promote stem elongation and growth. Gibberellic acid is a plant growth hormone that can stimulate cell division and elongation, resulting in taller and more vigorous rice plants.

(b) If dividing cells stop differentiating, it would lead to uncontrolled cell division, possibly resulting in the formation of a tumor or cancerous growth. Differentiation is the process by which cells become specialized for specific functions. When cells continue to divide without differentiating, it can disrupt normal tissue structure and function.

(c) If a rotten fruit gets mixed with unripe fruits, the ethylene gas produced by the rotting fruit can accelerate the ripening of the unripe fruits. Ethylene is a plant hormone that promotes fruit ripening. Mixing the two can lead to faster ripening of the unripe fruits, potentially causing them to become overripe or spoiled more quickly.

(d) If you forget to add cytokinin to the culture medium, it can negatively affect the growth and development of plant tissue in tissue culture. Cytokinins are plant hormones that promote cell division and shoot formation. Without cytokinins, the tissue may not proliferate as expected, and the culture may not develop shoots or roots properly. Cytokinins are essential for balanced growth and development in tissue culture .