NCERT Solutions for Class 11 Biology Chapter 13: Plant Growth and Development (NCERT 2026–27)
These Class 11 Biology Chapter 13 solutions cover Plant Growth and Development with complete, step-by-step answers to every NCERT exercise question. The chapter explains how a mature plant develops from a zygote through growth, differentiation and development, the phases and rates of growth, the five major plant growth regulators (PGRs)—auxins, gibberellins, cytokinins, ethylene and abscisic acid—and the roles of light (photoperiodism) and temperature (vernalisation) in flowering. All answers are written in CBSE exam-ready prose, updated for session 2026–27.
Development is the sum of two processes—growth and differentiation—and the chapter traces a plant from seed germination to senescence. Growth is an irreversible permanent increase in size, made possible by meristems that give plants indeterminate (open) growth. It is measured by parameters such as fresh/dry weight, length, area, volume and cell number, and passes through three phases—meristematic, elongation and maturation. Growth rate may be arithmetic (linear) or geometric (exponential, giving a sigmoid S-curve). Differentiated cells can dedifferentiate and then redifferentiate, and because differentiation is open, development shows plasticity (e.g. heterophylly). Finally, five groups of plant growth regulators—auxins, gibberellins, cytokinins, ethylene and abscisic acid—along with extrinsic factors such as light and temperature, govern flowering, dormancy, senescence and other developmental events.
Key Concepts & Definitions
Growth: an irreversible, permanent increase in the size of an organ, its parts or even a single cell, accompanied by metabolism that uses energy.
Meristem: a group of actively dividing, self-perpetuating cells; root and shoot apical meristems give primary growth, while lateral meristems (vascular cambium, cork cambium) give secondary growth.
Phases of growth: meristematic (constant division), elongation (vacuolation, cell enlargement) and maturation (maximal wall thickening, protoplasmic modification).
Arithmetic growth: only one daughter cell continues to divide after each mitosis, giving a linear plot: Lt = L0 + rt.
Geometric growth: both daughter cells keep dividing; growth is slow (lag), then exponential (log), then stationary, giving a sigmoid curve. Exponential phase: W1 = W0ert.
Differentiation, dedifferentiation, redifferentiation: maturation of meristem-derived cells; regaining division capacity by differentiated cells; and re-maturation after such division, respectively.
Plant growth regulators (PGRs): small molecules controlling growth—promoters (auxins, gibberellins, cytokinins) and inhibitors (abscisic acid, and largely ethylene).
Photoperiodism: response of flowering to the relative lengths of light and dark periods, perceived by the leaves.
Vernalisation: promotion of flowering by exposure of plants to low temperature for a period.
NCERT Exercises — Full Solutions
All questions are reproduced verbatim from the NCERT textbook (Reprint 2026–27); answers are original and written for exam preparation.
ANSWERGrowth: an irreversible, permanent increase in the size of an organ, its parts, or an individual cell, usually accompanied by metabolic processes that consume energy.Differentiation: the process by which cells derived from apical meristems and cambium mature and undergo structural changes (in cell wall and protoplasm) to perform specific functions, e.g. formation of tracheary elements.Development: the sum of all changes an organism passes through during its life cycle, from seed germination to senescence; broadly, development = growth + differentiation.Dedifferentiation: the phenomenon in which living differentiated cells that had lost the capacity to divide regain it under certain conditions, e.g. formation of interfascicular cambium and cork cambium from parenchyma.Redifferentiation: the maturation of cells produced by dedifferentiated tissues; these cells again lose the capacity to divide but mature to perform specific functions.Determinate growth: growth that stops after attaining a certain size or maturity (limited growth), as seen in leaves, flowers and fruits.Meristem: a localised group of actively dividing cells that retain the capacity to divide and self-perpetuate, responsible for growth in plants (apical and lateral meristems).Growth rate: the increased growth per unit time; it can be expressed mathematically and may be arithmetic or geometric.
2. Why is not any one parameter good enough to demonstrate growth throughout the life of a flowering plant?
ANSWERGrowth is shown by many different parameters—increase in fresh weight, dry weight, length, area, volume and cell number—and a single parameter cannot represent growth in all organs at all stages.Different organs grow in different ways: a pollen tube’s growth is best measured by its length, a dorsiventral leaf by its surface area, a maize root apex by an increase in cell number, and an enlarging watermelon cell by its volume.Because the most suitable measure changes with the organ and the phase of growth, no single parameter is adequate; hence growth has to be demonstrated by a combination of parameters chosen to suit each situation.
ANSWER(a) Arithmetic growth: following mitosis, only one daughter cell continues to divide while the other differentiates and matures. The simplest example is a root elongating at a constant rate. A plot of length against time gives a linear curve, expressed as Lt = L0 + rt, where Lt = length at time t, L0 = length at zero time, and r = growth rate.(b) Geometric growth: here both daughter cells produced by mitosis retain the ability to divide and continue to do so. The initial growth is slow (lag phase) and then increases exponentially (log phase). The exponential phase is expressed as W1 = W0ert, where W1 = final size, W0 = initial size, r = growth rate, t = time and e = base of natural logarithms; r is the relative growth rate or efficiency index.(c) Sigmoid growth curve: in a natural environment, geometric growth slows down due to limited nutrient supply, leading to a stationary phase. When the growth parameter is plotted against time, a typical S-shaped (sigmoid) curve is obtained with three phases—lag, log (exponential) and stationary. It is characteristic of living organisms growing under natural conditions and is typical of all cells, tissues and organs of a plant.(d) Absolute and relative growth rates: the measurement and comparison of total growth per unit time is the absolute growth rate. The growth of a system per unit time expressed on a common basis, e.g. per unit initial parameter, is the relative growth rate. For example, two leaves of different sizes may show the same absolute increase in area (say 5 cm2), but the smaller leaf shows a much higher relative growth rate.
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.
ANSWERThe five main groups of natural plant growth regulators are: auxins, gibberellins, cytokinins, ethylene and abscisic acid (ABA).Note on Auxins:Discovery: Auxins (Greek auxein = to grow) were first isolated from human urine. The story began with Charles Darwin and his son Francis Darwin, who observed that canary grass coleoptiles bend towards a unilateral light source (phototropism) and that the tip of the coleoptile is the site of a transmittable influence causing the bending. Auxin was later isolated by F. W. Went from the tips of oat coleoptiles.Physiological functions: Auxins (such as IAA and IBA, with synthetic forms like NAA and 2,4-D) are produced at growing apices of stems and roots and migrate to their regions of action. They initiate rooting in stem cuttings, promote flowering, prevent fruit and leaf drop at early stages while promoting abscission of older leaves and fruits, control apical dominance (inhibition of axillary buds by the apical bud), induce parthenocarpy, and control xylem differentiation and cell division.Agricultural/horticultural applications: auxins are widely used to induce rooting in stem cuttings for plant propagation, to promote flowering in pineapples, and as herbicides—2,4-D is used to kill dicotyledonous weeds (without affecting mature monocots) and to make weed-free lawns. Removing apical buds (decapitation) to promote bushy growth is used in tea plantations and hedge-making.
5. Why is abscisic acid also known as stress hormone?
ANSWERAbscisic acid (ABA) is called the stress hormone because it helps plants cope with adverse and stressful conditions.It stimulates the closure of stomata, which reduces transpirational water loss during water stress, and it increases the tolerance of plants to various kinds of stresses.ABA also induces seed dormancy, enabling seeds to withstand desiccation and other unfavourable conditions, and it inhibits seed germination and acts as a general growth inhibitor. Because all these roles help the plant survive stress, ABA is known as the stress hormone.
6. ‘Both growth and differentiation in higher plants are open’. Comment.
ANSWERGrowth is open because plants retain the capacity for unlimited (indeterminate) growth throughout their life, owing to meristems whose cells continuously divide and add new cells to the plant body; this is the open form of growth.Differentiation is also open because cells or tissues arising from the same meristem develop into different structures at maturity, and the final structure of a cell is determined by its position within the organ. For example, cells positioned away from the root apical meristem differentiate into root-cap cells, while those pushed to the periphery mature into epidermis.Since cells from a common origin can mature into different structures depending on position and conditions, both growth and differentiation in higher plants are described as open, which also makes development flexible (plastic).
7. ‘Both a short day plant and a long day plant can produce can flower simultaneously in a given place’. Explain.
ANSWERFlowering in many plants depends on photoperiod—the relative durations of light and dark periods. A short-day plant flowers when the day length is shorter than a critical period, whereas a long-day plant flowers when the day length is longer than a critical period.At a given place, the day length changes through the seasons. There can be a particular day length that is simultaneously shorter than the critical period of the long-day plant in the future and longer than the critical period of the short-day plant in a way that satisfies the requirement of both at the same time of year.Thus, at one location and on the same day, a short-day plant and a long-day plant can both meet their respective photoperiodic requirements and flower simultaneously, because each responds to its own critical day length being met by the local photoperiod.
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) Induce rooting in a twig: Auxins (e.g. IBA/NAA), which initiate rooting in stem cuttings.(b) Quickly ripen a fruit: Ethylene, which is highly effective in fruit ripening.(c) Delay leaf senescence: Cytokinins, which promote nutrient mobilisation and delay leaf senescence.(d) Induce growth in axillary buds: Cytokinins, which help overcome apical dominance and promote lateral (axillary) bud growth.(e) ‘Bolt’ a rosette plant: Gibberellins, which promote bolting (internode elongation just prior to flowering) in rosette plants.(f) Induce immediate stomatal closure: Abscisic acid (ABA), which stimulates the closure of stomata.
9. Would a defoliated plant respond to photoperiodic cycle? Why?
ANSWERNo, a fully defoliated plant would not respond to the photoperiodic cycle.The site of perception of the photoperiod (light/dark stimulus) is the leaf. The leaves perceive the photoperiodic stimulus and produce a flowering hormone-like substance that is then transmitted from the leaves to the shoot apex to induce flowering.If the plant is defoliated, there are no leaves to perceive the photoperiod or to generate and transmit the flowering signal to the apical bud. Therefore, the defoliated plant cannot respond to the photoperiodic cycle and will not flower.
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) GA3 applied to rice seedlings: the seedlings would show excessive elongation of the stem and grow abnormally tall and thin (lanky), resembling the ‘bakanae’ or foolish-seedling condition, because gibberellins increase the length of the axis.(b) Dividing cells stop differentiating: the cells would continue to divide but fail to mature into specific tissues and organs; without differentiation, no proper plant body with functional tissues (such as xylem, phloem, epidermis) would form, resulting in a mass of undifferentiated cells.(c) A rotten fruit mixed with unripe fruits: the rotten (over-ripe) fruit releases ethylene, which hastens ripening; the ethylene would diffuse to the surrounding unripe fruits and ripen them quickly, possibly leading to spoilage of the whole lot.(d) Cytokinin forgotten in the culture medium: in plant tissue culture, cytokinin (along with auxin) is needed for cell division and shoot formation. Without cytokinin, cell division and shoot/bud development would not proceed properly, and adventitious shoot formation and overcoming of apical dominance would be hampered—callus proliferation and shoot regeneration would fail.
Extra Practice Questions
Short Answer Type Questions
Q1. Why is plant growth said to be indeterminate?
ANSWERPlants retain the capacity for unlimited growth throughout their life because of meristems whose cells continuously divide and self-perpetuate, constantly adding new cells to the plant body. This open form of growth makes plant growth indeterminate.
Q2. What is meant by the ‘efficiency index’ in geometric growth?
ANSWERIn the exponential growth equation W1 = W0ert, the term r is the relative growth rate. It measures the ability of the plant to produce new plant material and is therefore called the efficiency index; the final size W1 depends on the initial size W0 and on r.
Q3. Distinguish between primary and secondary growth.
ANSWERPrimary growth is brought about by root and shoot apical meristems and causes elongation of the plant along its axis. Secondary growth is brought about by lateral meristems—vascular cambium and cork cambium—in dicots and gymnosperms, and causes an increase in the girth (thickness) of organs.
Q4. Why is ethylene considered both a promoter and an inhibitor of growth?
ANSWEREthylene promotes some processes such as fruit ripening, root growth, root-hair formation, internode elongation in deep-water rice, and flowering in pineapple. At the same time it promotes senescence and abscission of leaves and flowers, and largely inhibits growth activities. Hence it fits into either group but is mainly an inhibitor.
Q5. What is heterophylly? Give one example caused by environment.
ANSWERHeterophylly is the occurrence of different leaf forms on the same plant, an example of plasticity. In buttercup, leaves produced in air differ in shape from those produced under water, showing heterophyllous development due to the environment.
Long Answer Type Questions
Q1. Describe the three phases of growth seen at a root tip.
ANSWERGrowth at a root tip is divided into three phases. The meristematic phase occurs at the root apex, where constantly dividing cells are rich in protoplasm, have large conspicuous nuclei, and possess thin, cellulosic primary cell walls with abundant plasmodesmata. Proximal to this lies the phase of elongation, characterised by increased vacuolation, cell enlargement and deposition of new cell wall material. Further away, more proximal still, is the phase of maturation, where cells attain their maximal size with wall thickening and protoplasmic modifications; most of the differentiated tissues and cell types belong to this zone. These three zones together represent the orderly transition from cell division to a fully mature, functional cell.
Q2. Compare the physiological effects of gibberellins and cytokinins.
ANSWERGibberellins (acidic PGRs, e.g. GA3) cause elongation of the axis—used to lengthen grape stalks and to elongate and improve the shape of apples; they delay senescence, speed up malting in brewing, increase sugarcane yield by stem elongation, hasten maturity in conifers, and promote bolting in rosette plants. Cytokinins (e.g. zeatin, kinetin) are synthesised in regions of rapid cell division such as root apices, developing shoot buds and young fruits; they promote cytokinesis, formation of new leaves, chloroplasts, lateral shoot and adventitious shoot formation, help overcome apical dominance, and promote nutrient mobilisation that delays leaf senescence. Thus, gibberellins mainly drive internode elongation and fruit/stem growth, whereas cytokinins mainly drive cell division and lateral growth while delaying senescence; both act as growth promoters, often complementing each other.
Q3. Explain the role of intrinsic and extrinsic factors in plant growth and development.
ANSWERPlant growth and development are under the control of both intrinsic (internal) and extrinsic (external) factors. Intrinsic factors include intracellular genetic (genomic) control and intercellular chemical factors—the plant growth regulators (auxins, gibberellins, cytokinins, ethylene and ABA)—which are synthesised in various plant parts and control differentiation and developmental events; they may act synergistically or antagonistically. Extrinsic factors include light, temperature, water, oxygen, nutrition and gravity. Many extrinsic factors act through PGRs; for example, light and temperature regulate events such as vernalisation, flowering (photoperiodism), dormancy, seed germination and plant movements. Together, genetic control, PGRs and environmental factors coordinate the entire course of plant growth and development from germination to senescence.
MCQs & Assertion–Reason
1. Growth in plants is best described as:
(a) a reversible decrease in size (b) an irreversible permanent increase in size (c) change in colour only (d) loss of protoplasm
For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.
A-R 1. Assertion: Plant growth is generally indeterminate.
Reason: Meristems retain the capacity to divide and self-perpetuate, continually adding new cells to the plant body.
A-R 2. Assertion: A single parameter is sufficient to demonstrate growth in all plant organs.
Reason: Growth can be measured by fresh weight, dry weight, length, area, volume and cell number.
A-R 3. Assertion: Abscisic acid acts as an antagonist to gibberellins.
Reason: ABA inhibits seed germination and induces dormancy, while GAs promote growth.
A-R 4. Assertion: A defoliated plant cannot respond to a photoperiodic cycle.
Reason: The photoperiodic stimulus is perceived by the leaves.
A-R 5. Assertion: Ethylene is widely used to ripen fruits.
Reason: Ethylene enhances the respiration rate during fruit ripening (respiratory climactic).
Answer key: 1-(A), 2-(D), 3-(A), 4-(A), 5-(A).
Common Mistakes to Avoid
Watch out for these
Confusing arithmetic growth (linear, one daughter cell divides) with geometric growth (exponential, both daughter cells divide).
Mixing up dedifferentiation (regaining division capacity) with redifferentiation (re-maturing after such division).
Writing that ethylene is only a promoter—it is largely an inhibitor of growth though it promotes ripening.
Stating that the shoot apex perceives photoperiod—the stimulus is perceived by the leaves.
Saying kinetin occurs naturally in plants—it does not; the natural cytokinin is zeatin.
Forgetting that development = growth + differentiation, and that both are open in higher plants.
How to score full marks in this chapter
Memorise the five PGRs with one example each of discovery, function and application—these are frequently asked. Practise writing the growth equations (Lt = L0 + rt and W1 = W0ert) with the meaning of each symbol. For “which PGR” questions, link the action to the right hormone (rooting→auxin, ripening→ethylene, bolting→gibberellin, delaying senescence/axillary growth→cytokinin, stomatal closure→ABA). Always justify photoperiodism answers by mentioning that the leaf perceives the stimulus, and clearly distinguish differentiation, dedifferentiation and redifferentiation with examples.
Frequently Asked Questions
What is Class 11 Biology Chapter 13 about?
Chapter 13, Plant Growth and Development, explains growth, differentiation and development in plants—the phases and rates of growth (arithmetic, geometric, sigmoid), dedifferentiation and redifferentiation, plasticity, and the five plant growth regulators (auxins, gibberellins, cytokinins, ethylene, abscisic acid) along with the roles of light and temperature in flowering.
Why is abscisic acid called the stress hormone?
Because it helps plants tolerate stress—it closes stomata to reduce water loss, increases tolerance to various stresses, induces seed and bud dormancy, and inhibits germination, helping seeds withstand unfavourable conditions.
Which plant growth regulator ripens fruits?
Ethylene, a gaseous PGR, is highly effective in fruit ripening; it enhances the respiration rate during ripening (respiratory climactic) and is commonly supplied through the compound ethephon.
Are these Class 11 Biology Chapter 13 solutions free?
Yes. All solutions are free and follow the official NCERT Biology textbook for session 2026–27, with every exercise question answered in exam-ready format.
