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

The class 11 biology NCERT solutions chapter 13 Plant Growth and Development cover every back-exercise question, according to the latest 2026-27 CBSE syllabus, and help students prepare for the CBSE Boards, NEET and CUET. Each answer is worked step by step, from defining growth and differentiation to the sigmoid growth curve, the five plant growth regulators, photoperiodism and the flexible development that makes plant tissue culture possible.

This is the closing chapter of the Plant Physiology unit, and the hormone and growth ideas it introduces are tested heavily in both the Boards and NEET.

• CBSE Weightage: 4 to 5 marks, part of the Plant Physiology unit alongside photosynthesis and respiration.
• Topics covered: growth and its parameters, arithmetic and geometric growth, the sigmoid curve, absolute and relative growth rates, differentiation and dedifferentiation, the five plant growth regulators, apical dominance, abscisic acid, plasticity and photoperiodism.
• Exercise count: 10 back-exercise questions, mixing definitions, one numerical on relative growth rate and several applied hormone questions.

These class 11 biology NCERT solutions chapter 13 Plant Growth and Development are curated by subject experts, based on the 2026-27 NCERT textbook, and checked against the last five years of CBSE Board and NEET papers.

Why Plant Growth and Development Matters and What the Chapter Covers

Plant growth and development is the story of how a single germinating seed becomes a flowering adult. The chapter explains why growth is measured in so many different ways, how cells specialise and sometimes despecialise, and how five small hormone molecules split the work of building a plant between them. Understanding this chapter ties together everything students have learned about cells, tissues and physiology.

• Everyday role: rooting a cutting, ripening a fruit, keeping a hedge bushy and growing a weed-free lawn are all this chapter in action.
• Core skill: defining growth precisely, choosing the right parameter to measure it, and matching each of the five plant growth regulators to the job it does.
• Why it is central: the ideas of open growth and open differentiation introduced here explain why plant tissue culture works and are revisited in the reproduction and biotechnology chapters.

Almost every applied question in this chapter comes down to knowing which hormone owns which effect. That is why students who learn the five regulators and their signature actions early find the exercise straightforward. The class 11 biology NCERT solutions chapter 13 Plant Growth and Development below follow the NCERT order so students can check their working line by line.

Growth, Differentiation and the Eight Key Definitions

The first exercise question asks students to define eight terms taken straight from Sections 13.1 to 13.3. They describe one story in order: a cell divides, enlarges, takes on a job, and sometimes changes its mind. Each term should be defined by what happens to the cell, not by a synonym, and the single keyword in each definition is what the marker looks for.

The three-step chain runs in order: a meristematic cell divides, then differentiates into a mature cell; if that cell dedifferentiates it divides again, and its products then redifferentiate. The word irreversible is the whole definition of growth, since a wilted leaf that swells again after watering has not grown. Writing the chain in order proves students know the terms are related rather than three lookalike words, which is exactly what Q1 rewards.

Why No Single Parameter Can Measure Growth

Growth is measured indirectly. At the cellular level it is principally a consequence of an increase in the amount of protoplasm, but protoplasm cannot be measured directly, so every parameter is a proxy. NCERT lists the usual stand-ins: increase in fresh weight, dry weight, length, area, volume and cell number. The point of Q2 is to notice that this is a list, and to see why it has to be.

• Different directions: a pollen tube grows in one dimension, so length works; a dorsiventral leaf spreads in two, so surface area works and length fails.
• Different mechanisms: a single maize root apex can make more than 17,500 new cells per hour, so cell number fits, while a watermelon may swell up to 3,50,000 times by cell enlargement, where cell number records almost nothing.
• Some parameters mislead: fresh weight rises and falls with water status, and a germinating seed loses dry weight while it grows fast, because stored food is respired away.

The question phrasing carries two signals: throughout the life is a time signal, and of a flowering plant is an organ signal. No single measurable quantity stays proportional to protoplasm across root, leaf, pollen tube, fruit and whole plant, over germination, elongation and maturation. Growth is therefore demonstrated by a variety of parameters, each chosen to suit the organ and the stage being measured. Answering with three concrete pairings scores higher than a paragraph saying growth is complex.

Arithmetic, Geometric and the Sigmoid Growth Curve

Following mitotic division in a root or shoot apical meristem, growth can be arithmetic or geometric, depending on how many daughter cells keep the ability to divide. The difference between the two is a single question: how many daughters keep dividing? That one question sets both the formula and the shape of the curve.

• Arithmetic growth: only one daughter cell keeps dividing while the other matures, so the increment per unit time is constant. It follows L_t = L₀ + rt, the equation of a straight line, as a root elongating at a constant rate.
• Geometric growth: both daughter cells keep dividing, so the population doubles and the increment itself grows. It follows W₁ = W₀e^rt, an exponential, where r is the relative growth rate, also called the efficiency index.
• Sigmoid curve: plotting any growth parameter against time gives a typical S-curve with three phases, a slow lag phase, a rapid exponential (log) phase, and a stationary phase where limited nutrients flatten the curve.

The gap between the two is dramatic: over ten divisions, geometric growth from one cell gives 2¹⁰ = 1024 cells, while arithmetic growth over the same ten steps gives only 1 + 10 = 11 cells. The sigmoid curve is simply geometric growth meeting the real world, where nutrients eventually run short. It is characteristic of all cells, tissues and organs of a plant growing in a natural environment, which is why students should be able to label its lag, log and stationary phases from memory.

Absolute and Relative Growth Rates Worked Out

Quantitative comparisons between growing systems can be made in two ways, and part (d) of Q3 asks students to tell them apart. The absolute growth rate (AGR) is the total growth per unit time and ignores how big the organ was to begin with. The relative growth rate (RGR) is the growth per unit time expressed per unit initial size, so it compares the machinery rather than the stock.

Both leaves add the same 5 cm², so their absolute growth rates are identical, yet leaf A doubles itself while leaf B adds only a tenth, so their relative rates differ tenfold. Leaf A has the higher relative growth rate because the same absolute gain is divided by a much smaller initial area. The commonest error is quoting RGR in cm²: because RGR divides an area by an area, the units cancel and it is a pure number, written as 1 or 100%, never as 5 cm². Full working for Fig. 13.7 is shown in Exercise Q3.

The Five Plant Growth Regulators

The plant growth regulators (PGRs) are small, simple molecules of diverse chemical composition. NCERT groups them into five and splits them by function into promoters and inhibitors. Q4 asks students to list the five and then write a full note on any one, and the table below fixes the names, chemical classes and roles that every applied question later depends on.

The first three are growth promoters, driving cell division, enlargement, pattern formation, tropic growth, flowering and fruiting. Abscisic acid is an inhibitor, and ethylene sits across both groups but is largely an inhibitor of growth activities. Splitting the five this way shows the list is organised rather than memorised, and it sets up the applied questions Q8 and Q10, where each PGR must be matched to a specific effect.

Auxins and Abscisic Acid: Discovery and Roles

Auxin has the richest discovery story of the five. It started with Charles Darwin and his son Francis Darwin, who observed that canary grass coleoptiles bend towards a unilateral light source, a response called phototropism. Later experiments showed the tip of the coleoptile was the site of the transmittable influence that caused the bending, and F.W. Went finally isolated auxin from the tips of oat coleoptiles.

• Where auxin acts: made at growing stem and root apices, it migrates to its site of action, initiates rooting in cuttings, promotes flowering in pineapple, and enforces apical dominance, where the apical bud suppresses the lateral buds.
• Auxin applications: IBA and NAA root cuttings, 2,4-D kills dicot weeds in lawns without harming mature monocots, decapitation removes apical dominance in tea and hedges, and auxins induce parthenocarpy in tomato.
• Abscisic acid, the stress hormone: ABA stimulates the closure of stomata, cutting water loss during drought, and increases the tolerance of plants to various kinds of stresses, which is precisely why NCERT calls it the stress hormone.

ABA also acts as a general growth and metabolism inhibitor, inhibits seed germination, and induces dormancy so seeds withstand desiccation and other unfavourable factors. In most situations ABA acts as an antagonist to gibberellins: gibberellins say grow and germinate, while ABA says wait. Every other PGR is defined by what it builds, but ABA is defined by what it stops, and stopping is exactly what survival under stress requires. Students answering Q5 should lead with the stomata and stress tolerance, not with abscission, which explains ABA's first name and not its second.

Open Growth, Open Differentiation and Plasticity

Q6 asks students to comment on the statement that both growth and differentiation in higher plants are open. Open growth means the plant keeps producing new cells and organs for as long as it lives, because meristems are retained and stay active. Open differentiation means the fate of those cells is not fixed in advance, so the same kind of cell can mature into different structures depending on where it ends up.

• Growth is open: higher plants retain meristems that divide and self-perpetuate, so new cells never run out and growth stays indeterminate, unlike an animal that reaches a fixed adult size and stops.
• Differentiation is open by position: cells of the same origin form different structures depending on where they sit, so cells at the root apex form the root cap while cells behind them elongate and mature into vascular tissue.
• Differentiation is open by reversal: a differentiated cell can dedifferentiate and its products can redifferentiate, so interfascicular cambium arises from parenchyma and secondary xylem then arises from that cambium.

Plants also follow different pathways in response to environment or phase of life to form different structures, an ability called plasticity. Heterophylly in cotton, coriander and larkspur shows juvenile leaves differing in shape from adult leaves, while heterophylly in buttercup shows leaves formed in air differing from those formed in water on the same plant. Because differentiation is open, development in plants is flexible, which is exactly why a cutting can regenerate a whole plant and why plant tissue culture works while animal tissue culture is far harder.

Photoperiodism and the Site of Flowering

Some plants require a periodic exposure to light to induce flowering, a response called photoperiodism. The key idea is the critical duration, a threshold day length each species carries. A short day plant (SDP) flowers when the day length falls below its critical duration, while a long day plant (LDP) flowers when the day length rises above its critical duration. Q7 asks how both can flower together in one place, and Q9 asks whether a defoliated plant can respond at all.

• Flowering together: because the two species have different critical durations, a single day length can lie below the SDP's threshold and above the LDP's at the same time, so both flower simultaneously in the same place.
• Worked example: if an SDP has a critical duration of 14 hours and an LDP has one of 12 hours, a 13-hour day satisfies both, so the two flower together.
• Site of perception: the leaf, not the shoot apex, perceives the light and dark stimulus, and a hormonal substance then migrates from the leaves to the shoot apices where flowers form.

This is why a defoliated plant does not respond to a photoperiodic cycle: with its leaves removed, there is no organ left to perceive the inductive photoperiod, so the flowering signal is never produced or exported. The critical duration belongs to the species, not to a universal clock, and there is no single threshold that fits all plants. Students should note that it is really the length of the dark period that matters, and that the leaf is the receiver even though the shoot apex is where the flower appears.

Choosing the Right PGR for the Job

Questions 8 and 10 are applied: they name a task and ask which regulator does it, or name a change and ask what would follow. Each PGR owns a signature set of effects, so the trick is to name the function first and then apply it. The table below pairs each job in Q8 with its regulator.

The same logic answers Q10. Applying GA₃ to rice seedlings makes them grow abnormally tall, because gibberellins increase the length of the axis. If dividing cells stop differentiating, no specialised tissues form and the plant stays a mass of dividing cells. A rotten fruit mixed with unripe fruits ripens them fast, because the rotten fruit releases ethylene that hastens ripening. Forgetting to add cytokinin to a culture medium means cytokinesis and cell division fail, so the tissue does not proliferate. Naming the function before the outcome is what turns a guess into a full-mark answer.

Plant Growth and Development Exercise-wise Breakdown

The NCERT back exercise has 10 questions, mixing definitions, one numerical on relative growth rate, and several applied hormone questions. The table below maps the question blocks to their topics so students can revise by theme.

The definitions and the applied hormone questions carry most of the marks, and the one numerical on relative growth rate is a reliable scorer if students remember to cancel units. Learn the single keyword in each definition and the signature effect of each of the five regulators, then attach a real example. Every question in the class 11 biology NCERT solutions chapter 13 Plant Growth and Development PDF is solved with each step of reasoning shown, so students can compare their answers against the model working.

Common Mistakes Students Make in Plant Growth and Development

Most marks in this chapter are lost on loose definitions and swapped hormones, not on hard ideas. Each mistake below costs 1 to 2 marks, so watch for it at the exact step.

Student Feedback on Plant Growth and Development Solutions

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