GSEB Class 11 Biology Solutions Chapter 15 Plant Growth and Development

 

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

• Growth: This is an irreversible and lasting process, achieved by an increase in the size of an organ, organ parts, or even an individual cell.
• Differentiation: This is a process where cells from the apical meristem and cambium undergo structural changes in their cell walls and protoplasm, maturing to perform particular functions.
• Development: This refers to the various changes happening in an organism throughout its life cycle, from seeds germinating to aging.
• De-differentiation: This is the process where permanent plant cells gain back the ability to divide under specific conditions.
• Re-differentiation: This is the process where de-differentiated cells mature again and lose their dividing capacity.
• Determinate growth: This means limited growth. For example, animals and plant leaves stop growing after reaching maturity.
• Meristem: In plants, growth is confined to specialized areas where active cell divisions occur. Such a region is called a meristem.
• Growth rate: This can be described as the increased growth in plants per unit of time.

In simple words: These terms explain different ways plants grow and change throughout their lives, from getting bigger to specializing cells and stopping growth at certain points.

Exam Tip: When defining multiple terms, provide a clear, concise definition for each and try to differentiate between similar concepts like differentiation and de-differentiation.

 

Question 2. Why is not any one parameter good enough to demonstrate growth throughout the life of a flowering plant?
Answer: Growth at a cellular level is primarily a result of the increase in the amount of protoplasm. Because directly measuring protoplasm increase is difficult, people usually measure a quantity that is somewhat proportional to it. Therefore, growth is measured using various parameters, including increased fresh weight, dry weight, length, area, volume, and cell number.In simple words: It's hard to directly measure how much a plant's living material (protoplasm) grows. So, scientists use different ways to measure growth, like its weight, size, or how many cells it has, because no single method shows the whole picture.

Exam Tip: Remember that plant growth is complex and multi-faceted, requiring several indicators for a complete assessment. Mentioning protoplasm increase as the fundamental basis is key.

 

Question 3. Describe briefly:
(1) Arithmetic growth
(2) Geometric growth
(3) Sigmoid growth curve
(4) Absolute and relative growth rates.
Answer:
(1) Arithmetic growth: In arithmetic growth, after mitotic cell division, only one daughter cell continues to divide, while the other differentiates and matures. The simplest example of arithmetic growth is a root getting longer at a constant pace. If you plot the length of the organ against time, you get a straight line curve. Mathematically, it is shown as:
\( L_t = L_0 + rt \)
Where:
\( L_t \) = length at time 't'
\( L_0 \) = length at time 'zero'
\( r \) = growth rate/elongation per unit time.
(2) Geometrical growth: In most systems, the initial growth is slow (lag phase), and it increases quickly after that – at an exponential rate (log or exponential phase). Here, both progeny cells after a mitotic cell division keep their ability to divide and continue doing so. However, with limited nutrient supply, growth slows down, leading to a stationary phase.
(3) Sigmoid growth curve: If we plot the parameter of growth against time, we get a typical sigmoid or S-curve. An S-shaped curve shows a characteristic of living organisms growing in a natural environment. It is common for all cells, tissues, and organs of a plant. The exponential growth can be shown as:
\( W_1 = W_0e^{rt} \)
Where:
\( W_1 \) = final size (weight, height, number, etc.)
\( W_0 \) = initial size at the beginning of the period
\( r \) = growth rate
\( t \) = time of growth
\( e \) = base of natural logarithms
Here, \( r \) is the relative growth rate and also measures the plant's ability to produce new plant material, known as the efficiency index. Therefore, the final size of \( W_1 \) depends on the initial size, \( W_0 \).
(4) Absolute and relative growth rates: Quantitative comparisons between the growth of a living system can also be done in two ways:

• Absolute growth rate: This is the measurement and comparison of total growth per unit time.
• Relative growth rate: This is the growth of a given system per unit time expressed on a common basis, e.g., per unit initial parameter. For instance, two leaves of different sizes may show the same absolute increase in area over a given time, but one might show a much higher relative growth.

In simple words: This explains different patterns of plant growth. Arithmetic growth is steady, geometric growth starts slow then speeds up, and sigmoid growth shows an S-curve. Growth rate can be measured simply as total increase (absolute) or compared to the initial size (relative).

Exam Tip: Distinguish clearly between arithmetic and geometric growth, and remember the three phases of the sigmoid curve (lag, log, stationary). For growth rates, focus on the 'total increase' versus 'increase per initial unit'.

 

Question 4. List five main groups of natural plant growth regulators. Write a note on the discovery, physiological functions, and agricultural/horticultural applications of any one of them.
Answer: Plant growth regulators are chemical molecules secreted by plants that influence their physiological traits. The five main plant growth regulators are:

• Auxins
• Gibberellic acid
• Cytokinins
• Ethylene
• Abscisic acid

**Note on Abscisic acid (ABA):**
**Discovery:** During the mid-1960s, "inhibitor – B," "abscission II," and "dormin" were found by three separate researchers. These substances were later found to be chemically similar and were then called Abscisic acid (ABA).
**Physiological Functions:**

• It acts as an inhibitor to plant metabolism.
• It stimulates the closure of stomata during water stress.
• It promotes seed dormancy.
• It promotes the shedding of leaves, fruits, and flowers.

**Horticultural application:** It causes seed dormancy in stored seeds.In simple words: Plant growth regulators are natural chemicals that control how plants grow. There are five main types: auxins, gibberellic acid, cytokinins, ethylene, and abscisic acid. Abscisic acid (ABA) was discovered in the 1960s and helps plants deal with stress by closing leaf pores, making seeds sleep, and causing leaves and fruits to drop.

Exam Tip: When asked to describe one regulator, ensure you cover its discovery, key physiological roles, and at least one practical application in agriculture or horticulture.

 

Question 5. What do you understand by photoperiodism and vernalization? Describe their significance.
Answer:**Photoperiodism:** It has been observed that some plants need regular exposure to light to trigger flowering. It is also seen that such plants can measure how long they are exposed to light. For example, some plants need light exposure for a period longer than a specific critical duration, while others must be exposed to light for a period shorter than this critical duration before flowering starts. The first group of plants is called short-day plants, while the second group is called long-day plants. However, many plants show no such link between light duration and flowering response; these are called day-neutral plants. It is now known that not only the light period but also the dark period's duration is equally important. Therefore, flowering in certain plants depends not only on a mix of light and dark exposures but also on their relative durations. This response of plants, in terms of flowering, to the relative length of day and night is called photoperiodism. The leaves are where light/dark duration is perceived. It has been suggested that a hormonal substance called florigen is responsible for flowering. Florigen moves from leaves to shoot tips for the inductive photoperiod.
**Vernalization:** Some plants' flowering is either quantitatively or qualitatively dependent on exposure to low temperatures. This phenomenon is called vernalization. It prevents early reproductive development late in the growing season, allowing the plant enough time to reach maturity. Vernalization specifically refers to promoting flowering by a period of cold temperatures. Some important food plants, such as wheat, barley, and rye, have two types of varieties: winter and spring varieties. The 'spring' variety is normally planted in the spring and flowers and produces grain before the growing season ends. Winter varieties are planted in the fall. They germinate and spend the winter as small seedlings, restart growth in the spring, and are harvested around mid-summer. Another instance of vernalization is seen in biennial plants. Biennials are monocarpic plants that usually flower and die in the second season. Sugarbeet, cabbages, and carrots are some common biennials.
In simple words: Photoperiodism is how plants use the length of day and night to know when to flower, like short-day or long-day plants. Vernalization is when plants need cold temperatures to be able to flower, which helps them mature properly before they reproduce. Both are important natural triggers for plant life cycles.

Exam Tip: Clearly define both terms and highlight their biological significance, emphasizing how they help plants time their flowering and reproduction optimally with environmental conditions.

 

Question 6. Why is abscisic acid also known as stress hormone?
Answer: Abscisic acid is called a stress hormone because it triggers various responses in plants to stress conditions. It helps plants increase their tolerance to different types of stress. It causes the stomata to close during water stress. It also promotes seed dormancy, ensuring seeds germinate only when conditions are favorable. This helps seeds endure drying out. Additionally, it helps induce dormancy in plants as the growing season ends and promotes the shedding of leaves, fruits, and flowers.In simple words: Abscisic acid is called a stress hormone because it helps plants cope with difficult situations like lack of water. It makes leaves close their pores, keeps seeds from sprouting too early, and causes old leaves and fruits to drop, all to protect the plant.

Exam Tip: Focus on the protective roles of abscisic acid, such as stomatal closure, seed dormancy, and abscission, as these are direct responses to environmental stressors.

 

Question 7. “Both growth and differentiation in higher plants are “open”. Comment.
Answer: Plant growth is unique because plants maintain the capacity for unlimited growth throughout their entire life. This ability of plants is due to the presence of meristems at specific locations in their body. The cells of such meristems have the capacity to divide and self-perpetuate. However, the resulting cells soon lose their ability to divide and form the plant body. This type of growth, where new cells are constantly added to the plant body by meristem activity, is known as the open form of growth. We have stated that growth in plants is open, meaning it can be indeterminate or determinate. We can also say that even differentiation in plants is open because cells/tissues originating from the same meristem have different structures when they mature. The final mature structure of a cell/tissue is also determined by the cell's location. For example, cells situated away from root apical meristems differentiate as root-cap cells, while those pushed to the periphery mature as the epidermis.In simple words: Plant growth is "open" because plants can keep growing their whole lives thanks to special dividing cells called meristems. Differentiation is also "open" because even cells from the same starting point can develop into different structures depending on where they are in the plant.

Exam Tip: To answer this, emphasize the role of meristems for continuous growth and explain how cell position within the plant influences its final differentiated structure, illustrating the 'open' nature of both processes.

 

Question 8. "Both a short-day plant and a long-day plant produce flower simultaneously in a given place". Explain.
Answer: The flowering response in short-day plants and long-day plants relies on how long these plants are exposed to light. Both short-day and long-day plants can flower in the same location, provided they have received an adequate photoperiod. This means if the environmental conditions provide the specific light and dark durations required by each type, they can flower at the same time, even with opposite light needs.In simple words: Both short-day and long-day plants can flower at the same time and place if each plant gets the right amount of light and darkness it needs to trigger flowering. Their flowering depends on getting the correct light "signal."

Exam Tip: The key to explaining simultaneous flowering is that each plant type (short-day or long-day) must simply receive its *specific optimal* photoperiod, not that they respond identically to light duration.

 

Question 9. Which one of the plant growth regulators would you use if you are asked to:
(1) Induce rooting in a twig
(2) Quickly ripen a fruit
(3) Delay leaf senescence
(4) Induce growth in axillary buds
(5) 'bolt' a rosette plant
(6) Induce immediate stomatal closure in leaves
Answer:

1. **Auxins:** These are used to induce rooting in a twig.
2. **Ethylene:** This regulator helps to quickly ripen a fruit.
3. **Cytokinins:** These are used to delay leaf senescence (aging).
4. **Auxins (after decapitation):** In most higher plants, the growing apical bud prevents the growth of the lateral (axillary) buds, a process called apical dominance. Removing shoot tips (decapitation) usually leads to the growth of lateral buds. This technique is widely used in tea plantations and hedge-making.
5. **Gibberellins:** These are used to 'bolt' a rosette plant, meaning to cause rapid stem elongation.
6. **Abscisic acid:** This regulator helps to induce immediate stomatal closure in leaves.

In simple words: If you want a twig to grow roots, use auxins. To quickly ripen fruit, use ethylene. To keep leaves from getting old, use cytokinins. To make side buds grow, remove the top bud which releases auxins. To make a short plant grow tall fast, use gibberellins. To close the tiny holes on leaves (stomata), use abscisic acid.

Exam Tip: For each scenario, identify the specific plant growth regulator that directly performs the desired function. Understanding the primary roles of each hormone is crucial.

 

Question 10. Would a defoliated plant respond to the photoperiodic cycle? Why?
Answer: A defoliated plant will not respond to the photoperiodic cycle. It is suggested that the hormonal substance responsible for flowering is formed in the leaves, later moving to the shoot tips and changing them into flowering tips. Therefore, without leaves, light perception would not happen, meaning the plant would not respond to light signals.In simple words: A plant without leaves can't respond to the daily light cycle that tells it when to flower. This is because the leaves are where the plant senses light and makes the special chemicals needed for flowering. No leaves, no light signal.

Exam Tip: Remember that leaves are the primary site of photoperiod perception, as they contain the photoreceptors and are where florigen is thought to be produced. The absence of leaves breaks this crucial link.

 

Question 11. What would be expected to happen if:
(1) GA3 is applied to rice seedlings.
(2) dividing cells stop differentiating,
(3) a rotten fruit gets mixed with unripe fruits,
(4) you forget to add cytokinin to the culture medium.
Answer:
(1) If GA3 (Gibberellin) is applied to rice seedlings, it would lead to an increase in the length of the rice plants. This is because gibberellins promote stem elongation.
(2) If dividing cells stop differentiating, it would significantly affect them, causing major structural changes in both their cell walls and protoplasm. For instance, to form a tracheary element (a water-conducting cell), the cells would lose their protoplasm.
(3) If a rotten fruit gets mixed with unripe fruits, it would promote the senescence (aging) and abscission (shedding) of plant organs, especially leaves and flowers. This might also impact the rate of respiration in the surrounding unripe fruits, causing them to ripen quickly due to ethylene gas released by the rotten fruit.
(4) If you forget to add cytokinin to the culture medium, plant development and growth would be negatively affected. Essential functions like making new leaves, forming chloroplasts in leaves, lateral shoot growth, and adventitious shoot formation may cease or be significantly reduced.In simple words: (1) Rice seedlings would grow taller if given GA3. (2) If cells stop changing into specialized types, they won't develop properly, affecting their structure. (3) A rotten fruit would make other unripe fruits spoil or ripen too fast because it releases a ripening gas. (4) Without cytokinin in a growth medium, plants would struggle to grow new leaves, side shoots, or form chloroplasts.

Exam Tip: For "what if" questions, consider the specific function of the substance or process mentioned and predict the direct biological consequences of its presence or absence.