NCERT Solutions for Class 11 Biology Chapter 13: Plant Growth and Development

• Growth: An irreversible and permanent increase in the size of an organ or its parts or even of an individual cell. It is accompanied by metabolic processes.
• Differentiation: The process by which cells derived from root apical and shoot-apical meristems and cambium mature and change to perform specific functions.
• Development: The sum of all changes that an organism goes through during its life cycle, from germination of the seed to senescence. It is the sum of growth and differentiation.
• Dedifferentiation: The process where living differentiated cells, which have lost the capacity to divide, can regain the power of division under certain conditions.
• Redifferentiation: The process where dedifferentiated cells lose their ability to divide and mature to perform specific functions.
• Determinate Growth: A type of growth where growth stops after a certain period or after reaching a certain size. It is characteristic of organs like leaves, flowers, and fruits.
• Meristem: A region in a plant containing undifferentiated cells that are actively dividing and are responsible for the growth of the plant.
• Growth Rate: The increased growth per unit time. It can be expressed mathematically as an arithmetic or geometric rate.

It is not sufficient to use just one parameter to measure growth throughout a plant's life because different parameters are relevant at different stages and for different parts of the plant.

• Increase in fresh or dry weight: Fresh weight is unreliable due to water content. Dry weight is more accurate but requires destroying the plant.
• Increase in length or height: This works for stems and roots but is irrelevant for leaves or fruits, which grow in area or volume.
• Increase in surface area: This is good for measuring the growth of flat organs like leaves.
• Increase in cell number: While fundamental, it is impractical to count cells in a large plant.

Therefore, a combination of parameters is needed for an accurate description of growth.

(a) Arithmetic growth:

In this type, after mitosis, only one daughter cell continues to divide while the other differentiates. It results in linear growth. The formula is: $$L_t = L_0 + rt$$ Where L_t is length at time ‘t’, L₀ is length at time ‘zero’, and r is the growth rate.

(b) Geometric growth:

In this type, both daughter cells retain the ability to divide, leading to exponential growth. The formula is: $$W_1 = W_0e^{rt}$$ Where W₁ is final size, W₀ is initial size, r is the relative growth rate, and t is time.

(c) Sigmoid growth curve:

A characteristic S-shaped curve obtained when plotting growth against time in a resource-limited environment. It has three phases:

1. Lag Phase: Initial slow growth.
2. Log/Exponential Phase: Rapid, exponential growth.
3. Stationary Phase: Growth slows down as resources become limited.

(d) Absolute and relative growth rates:

• Absolute Growth Rate (AGR): The measurement of total growth per unit time.
• Relative Growth Rate (RGR): The growth per unit time, expressed relative to the initial size. It is a measure of efficiency.

The five main groups of plant growth regulators (PGRs) are:

1. Auxins
2. Gibberellins
3. Cytokinins
4. Ethylene
5. Abscisic Acid (ABA)

A detailed note on Auxins:

• Discovery: Discovered by F.W. Went from the observations of Charles and Francis Darwin on canary grass coleoptiles bending towards light. The first isolated auxin was Indole-3-acetic acid (IAA).
• Physiological Functions:
  • Promotes apical dominance (growth of the main stem).
  • Initiates rooting in stem cuttings.
  • Induces flowering (e.g., in pineapples).
  • Promotes parthenocarpy (fruit development without fertilization).
  • Controls cell elongation and tropic movements.
• Agricultural Applications:
  • Used for plant propagation via stem cuttings.
  • Synthetic auxins like 2,4-D are used as selective weedicides.
  • Prevents premature fruit and leaf drop.

Photoperiodism:

It is the physiological response of plants, especially flowering, to the relative lengths of day and night. Based on this, plants are categorized as:

• Long-Day Plants (LDP): Flower when the day length exceeds a critical duration (e.g., Spinach, Wheat).
• Short-Day Plants (SDP): Flower when the day length is shorter than a critical duration (e.g., Soybean, Tobacco).
• Day-Neutral Plants (DNP): Flowering is independent of day length (e.g., Tomato, Cucumber).

Significance: It ensures plants flower at the right time for pollination and seed dispersal, and it allows farmers to manipulate flowering in commercial crops.

Vernalisation:

It is the induction of flowering in plants by exposing them to a prolonged period of low temperature. The stimulus is perceived by the apical meristems.

Significance: It prevents premature flowering, allowing the plant to mature vegetatively first. It also enables growing biennial plants as annuals and is a key adaptation for plants in temperate climates.

Abscisic acid (ABA) is called the "stress hormone" because its synthesis in plants increases in response to environmental stresses, and it helps the plant cope with these conditions.

1. Drought Stress: ABA causes the rapid closure of stomata, which reduces water loss via transpiration.
2. Dormancy: It induces dormancy in seeds and buds, allowing them to survive unfavourable conditions like winter or drought.
3. General Inhibitor: It acts as a general plant growth inhibitor, slowing down metabolism during stressful periods to conserve energy.

This statement refers to the plant's ability to grow and produce new structures throughout its life, thanks to the presence of meristems.

• Open Growth: Plants exhibit indeterminate growth because meristems (apical and lateral) are regions of perpetually dividing cells. This allows them to continuously add new cells, increasing in height and girth throughout their life.
• Open Differentiation: The new cells produced by meristems are undifferentiated. These cells then mature to form various specialized tissues and organs. Since new cells are constantly being produced, the process of differentiation is also "open," meaning new leaves, branches, and roots can be formed continuously.

This is unlike animals, whose growth is determinate and body plan is fixed early on.

Yes, this is possible. Flowering depends on whether the photoperiod is above or below a specific critical photoperiod for each plant.

Let's take an example:

• A **Long-Day Plant (LDP)** has a critical photoperiod of 10 hours. It will flower if the day is longer than 10 hours.
• A **Short-Day Plant (SDP)** has a critical photoperiod of 14 hours. It will flower if the day is shorter than 14 hours.

If these two plants are grown in a place with a day length of 12 hours:

• The LDP will flower because 12 hours > 10 hours (its critical period).
• The SDP will also flower because 12 hours < 14 hours (its critical period).

Thus, both can flower simultaneously under the same photoperiodic condition.

• (a) induce rooting in a twig: Auxins (e.g., IBA, NAA).
• (b) quickly ripen a fruit: Ethylene (or Ethephon).
• (c) delay leaf senescence: Cytokinins.
• (d) induce growth in axillary buds: Cytokinins (to overcome apical dominance).
• (e) ‘bolt’ a rosette plant: Gibberellins (GA).
• (f) induce immediate stomatal closure in leaves: Abscisic Acid (ABA).

No, a defoliated plant (a plant with its leaves removed) would not respond to the photoperiodic cycle.

Reason:

The perception of the light/dark stimulus occurs in the leaves. It is in the leaves that a hormonal substance, hypothetically called 'florigen', is produced in response to the correct photoperiod. This hormone then travels to the shoot apices to induce flowering. Without leaves, the plant cannot perceive the stimulus, and therefore, cannot initiate the flowering process.

• (a) GA3 is applied to rice seedlings: The seedlings will show abnormal elongation of internodes, becoming excessively tall and weak. This is the 'bakanae' or foolish seedling disease.
• (b) dividing cells stop differentiating: The plant would fail to form specialized tissues and organs (like xylem, phloem, leaves). It would remain as an unorganized mass of cells (a callus), and would not be able to function or survive.
• (c) a rotten fruit gets mixed with unripe fruits: The rotten fruit releases large amounts of ethylene gas, which will hasten the ripening of the surrounding unripe fruits, causing them to spoil faster.
• (d) you forget to add cytokinin to the culture medium: In tissue culture, a cytokinin/auxin balance is needed. Without cytokinin, cell differentiation and shoot formation (organogenesis) will not occur. An undifferentiated mass of cells called a callus will form.

Determinate Growth: Growth is finite and stops after the plant part reaches a certain size. E.g., Growth of leaves, flowers, and fruits.

Indeterminate Growth: Growth is unlimited and continues throughout the plant's life due to the presence of meristems. E.g., Growth of stems and roots.

Plasticity is the ability of a plant to follow different developmental pathways and produce different structures in response to environmental conditions or different phases of life.

Example: In buttercup (*Ranunculus*), the leaves produced in water are finely dissected and different from the lobed leaves produced in the air. This is an example of environmental heterophylly.

Phytochrome is a pigment that exists in two forms: Pr (absorbs red light) and Pfr (absorbs far-red light). The Pfr form is physiologically active and controls flowering.

• The length of the dark period is crucial as it allows Pfr to slowly convert back to Pr.
• In short-day plants, high levels of Pfr at the end of the day inhibit flowering. A long night allows Pfr to decrease, triggering flowering.
• In long-day plants, high levels of Pfr promote flowering.

Physiological Roles of Gibberellins (GAs):

1. Stem Elongation: GAs cause a remarkable increase in the length of the plant axis.
2. Breaking Dormancy: They overcome the natural dormancy of buds and seeds, promoting germination.
3. Bolting: GAs promote bolting (internode elongation prior to flowering) in rosette plants like beet and cabbage.
4. Delayed Senescence: Spraying GAs can delay the ageing of fruits, allowing them to remain on the tree longer.

Agricultural/Horticultural Applications:

1. Increase Fruit Size: Used in grape cultivation to increase the length and size of grapes.
2. Malting Process: GAs speed up the malting process in the brewing industry.
3. Sugarcane Yield: Spraying sugarcane with gibberellins increases stem length, thereby increasing the sugar yield.
4. Seed Production: Spraying juvenile conifers with GAs hastens maturity, leading to early seed production.

1. What natural phenomenon is responsible for flowering in the second year?
   The phenomenon is Vernalisation, the requirement of a prolonged low-temperature period for flowering.
2. Which plant hormone could the farmer spray to mimic this?
   The farmer could spray Gibberellins (GA), as they can substitute for the cold treatment requirement.
3. What is the term for the rapid internode elongation before flowering?
   The term is Bolting.