Subtopics - Plant Growth and Development (NEET)
Six major content blocks: growth fundamentals, differentiation processes, five plant hormones with discovery and functions, photoperiodism and phytochrome, vernalization, and senescence with abscission.
1) Plant Growth
Covers the fundamental concepts of plant growth as an irreversible permanent change in size, form, weight and volume. Discusses three types of meristems based on position: apical meristems (shoot and root apex, increase in length), intercalary meristems (above nodes, e.g. Bambusa), and lateral meristems (cork cambium and vascular cambium, increase in girth). Growth proceeds through three phases: cell division (formative phase by mitosis), cell enlargement (vacuolation, turgor-driven), and cell maturation (differentiation into specialised tissues). The sigmoid growth curve shows lag phase (slow initial growth), log/exponential phase (grand period of growth, Sachs), and stationary phase (nutrient limitation, maturity). Growth measurement instruments include auxanometer (arch and Pfeffer types) and Bose's crescograph (10,000x magnification). External factors affecting growth include light (intensity causing etiolation in absence, quality effects, photoperiodism), temperature (5 C arctic minimum, 20-30 C temperate optimum, 35-40 C tropical maximum), water, oxygen, mineral salts, and pollutants. Internal factors include C/N ratio (high C/N causes wall thickening, low C/N favours protoplasm formation) and growth regulators.
2) Differentiation, Dedifferentiation and Redifferentiation
Covers the three developmental fates of cells derived from meristems. Differentiation is the process by which cells from root apical, shoot-apical meristems and cambium mature to perform specific functions, involving structural changes in cell walls and protoplasm (e.g., tracheary elements lose protoplasm and develop strong lignocellulosic secondary cell walls). Dedifferentiation is the phenomenon where living differentiated cells that have lost the capacity to divide regain it under certain conditions (e.g., formation of interfascicular cambium and cork cambium from fully differentiated parenchyma cells). Redifferentiation occurs when cells produced by dedifferentiation once again lose the capacity to divide and mature to perform specific functions. These three processes are fundamental to understanding plant plasticity, wound healing, and tissue culture applications.
3) Plant Growth Hormones
Comprehensive coverage of five major plant hormones plus additional growth regulators. Auxins (discovered through Darwin's coleoptile experiments, isolated by Went 1928 from Avena; IAA is universal auxin; functions include cell elongation, apical dominance, parthenocarpy, 2,4-D as weedicide, root differentiation via IBA; antiauxins PCIB, TIBA). Gibberellins (from Gibberella fujikuroi causing bakanae disease, Kurosawa 1926; GA3 first obtained; functions: bolting in rosette plants, reversal of dwarfism, alpha-amylase induction in aleurone layer, substitution for vernalization; anti-gibberellins: CCC, malic hydrazide). Cytokinins (kinetin from herring sperm DNA, Miller-Skoog 1955; zeatin first natural cytokinin from Zea mays by Letham 1964; functions: cell division, Richmond-Lang effect delaying senescence, counteract apical dominance; coconut milk factor). Ethylene (gaseous hormone from methionine; triple response bioassay on pea; climacteric ripening; ethaphon commercial source; epinasty). ABA (abscisin II by Addicott 1963; stress hormone; dormin; antitranspirant; antagonist of GA). Additional regulators: traumatic acid (wound hormone), morphactins (synthetic, fluorene ring), jasmonic acid, calines (rhizocaline, caulocaline, phyllocaline).
4) Physiology of Flowering: Photoperiodism
Covers photoperiodism as the effect of daily light/dark duration on plant flowering, first demonstrated by Garner and Allard (1920) with Maryland mammoth tobacco. Plants classified into: short day plants (SDP, flower when day shorter than critical period: Xanthium with 5-6 hr critical, Chrysanthemum, sugarcane, Dahlia), long day plants (LDP, flower when day exceeds critical period: Hyoscymus niger with 11 hr critical, spinach, wheat, radish), day-neutral plants (flower in all photoperiods: cucumber, cotton, sunflower, tomato), intermediate plants (flower only in 12-16 hr range: Mikania scandens), and additional categories (amphiphotoperiodic, short-long day, long-short day plants). Critical photoperiod concept clarifies that SDP and LDP are distinguished by whether flowering requires photoperiods shorter or longer than their critical value. Skotoperiodism reveals the dark period is actually critical: SDP needs long uninterrupted night (also called long night plant). Light break reaction prevents SDP flowering. Mechanism involves phytochrome (Pr at 660nm, Pfr at 730nm, interconvertible; Pfr inhibitory to SDP, stimulatory to LDP; isolated by Butter et al. 1959). Florigen complex proposed by Chailakhyan 1936 includes gibberellins (stem growth) and anthesins (flower formation). Photomorphogenesis explains light-reversal of etiolation.
5) Vernalization
Covers the method of inducing early flowering by chilling treatment. Term coined by Russian agronomist Lysenko (1929-30). Defined by Chourad (1960) as acquisition or acceleration of ability to flower by chilling. First noticed by Klipport (1857) with winter cereals (wheat, barley, oat, rye) that fail to flower when sown in spring but flower when sown in autumn. Stimulus perceived by meristematic cells (shoot tip, embryo tips, root apex, developing leaves). Requirements: low temperature (0-4 C), specific duration (varies by species), actively dividing cells, water, and oxygen. Process has two stages: thermostage (germinating seeds treated at 0-5 C with O2 and moisture, breaks dormancy) and photostage (correct photoperiod applied after vernalization for flowering). De-vernalization occurs if chilling is immediately followed by high temperature (~40 C). Vernalin is the proposed stimulus (Mechlers), believed to be a gibberellin or mixture of gibberellins. Vernalization reduces vegetative period, increases yield, resistance to cold and diseases, and allows multiple crops per year.
6) Senescence and Abscission
Covers the final developmental phases from maturity to death. Senescence is the latter part of development leading from maturity to complete loss of organization and function. Study of plant senescence is called phytogerontology. Four types: whole plant senescence (monocarpic plants: annuals like rice and wheat, biennials like cabbage, perennials like certain bamboos), shoot senescence (perennials with underground perennating structures: banana, gladiolus, ginger), sequential senescence (evergreens where tips remain meristematic while older parts senesce: Eucalyptus, Pinus, Mango), and simultaneous/synchronous senescence (temperate deciduous trees shed all leaves in autumn: Dalbergia, Elm, Mulberry, Poplar). Characteristics include decline in metabolic activities, decrease in RNA/DNA, decreased membrane semipermeability, increased hydrolytic enzymes, decreased auxin/cytokinin and increased ABA/ethylene. Abscission is shedding of leaves, fruits or flowers via formation of abscission layer where middle lamella is digested by cellulase and pectinases. ABA and ethylene promote abscission layer formation while auxin inhibits it.
Plant Growth and Development Download Notes & Weightage Plan
For each topic in the Plant Growth and Development chapter below, you get (2) the exact resources to download and how to use them, and (3) a simple importance & time plan so NEET students know what to do first and what to revise last.
Growth fundamentals: meristems, phases of growth, sigmoid curve, measurement instruments, and factors affecting growth including etiolation and C/N ratio.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Sigmoid curve phases and their characteristics are tested regularly. Auxanometer and crescograph as measurement tools. Etiolation as a phenomenon in absence of light. C/N ratio effects on growth type.
- High-risk Area: Confusing lag phase with stationary phase (both show slow growth but at different stages). Mixing up the three meristem types and their functions (apical = length, lateral = girth).
- Best Practice Style: Draw and label the sigmoid curve. Create a comparison table of meristem types. Memorise Bose crescograph magnification (10,000x) and auxanometer types.
Five major hormones (auxins, gibberellins, cytokinins, ethylene, ABA) with discovery, chemical nature, bioassay, functions, and additional regulators (traumatic acid, morphactins, calines).
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Hormone functions and discoverers constitute the highest-yield content. Bakanae disease (GA), Richmond-Lang effect (cytokinin), triple response (ethylene), stress hormone (ABA), apical dominance (auxin) are NEET staples.
- High-risk Area: Mixing up which hormone does what: bolting is GA not auxin, Richmond-Lang effect is cytokinin not GA, parthenocarpy can be induced by both auxin and GA (but auxin was first demonstrated). Confusing kinetin (synthetic from herring sperm DNA) with zeatin (first natural cytokinin).
- Best Practice Style: Build the master comparison table. Create flashcards for each hormone with discoverer and bioassay. Solve 20+ MCQs specifically on hormone identification and function matching.
Physiology of Flowering: Photoperiodism
Photoperiodism, plant classification by day length, critical photoperiod, skotoperiodism, phytochrome mechanism, florigen, and photomorphogenesis.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: SDP vs LDP examples are tested every year. Phytochrome Pr/Pfr interconversion and wavelengths (660nm red, 730nm far-red). Critical photoperiod concept. Skotoperiodism concept (dark period is critical). Xanthium and Hyoscymus both flower at 14hr but are SDP and LDP respectively.
- High-risk Area: The biggest trap is understanding that SDP requires long uninterrupted DARK period, not short day. Red light interruption of dark period converts Pr to Pfr and inhibits SDP flowering. Students also confuse that both Xanthium (SDP) and Hyoscymus (LDP) flower at 14 hr light.
- Best Practice Style: Draw the complete photoperiodism diagram showing SDP and LDP responses to different day lengths. Practice night-interruption experiment questions. Memorise the wavelengths: 660nm (Pr) and 730nm (Pfr).
Vernalization, Senescence and Abscission
Combines vernalization (low temperature flowering induction), senescence (four types of plant ageing), and abscission (organ shedding) as the concluding developmental processes.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Vernalization definition and its requirement of low temperature. GA can substitute for vernalization is a commonly tested fact. Senescence types with correct examples. Abscission layer formation and hormonal control.
- High-risk Area: Confusing vernalization site (meristematic cells, not leaves) with photoperiodism site (leaves). Mixing up senescence types: whole plant vs shoot senescence. Forgetting that de-vernalization occurs at 40 C, not just any warm temperature.
- Best Practice Style: Draw a flowchart: seed moistening to thermostage (0-5 C) to photostage (photoperiod) to flowering. Create a senescence types comparison table with 3 examples each.
Plant Growth and Development Chapter NEET Traps & Common Mistakes (Topic-Wise)
Each subtopic below is of the Plant Growth and Development chapter and shows what NEET students usually do wrong in NEET examination, a short example of the mistake, and how NEET frames the question to trick you with close options are given below.
Mistake Snapshot (What Students Do Wrong)
- Attributing auxin discovery to wrong scientist: Students confuse the chain: Darwin observed coleoptile bending, Boysen-Jensen proved chemical signal, Paal demonstrated asymmetric growth, and Went (1928) actually isolated auxin from Avena coleoptile tip. The discovery is attributed to Went, not Darwin.
- Confusing kinetin with zeatin: Kinetin is a SYNTHETIC cytokinin from autoclaved herring sperm DNA (Miller, Skoog 1955). Zeatin is the first NATURAL cytokinin from unripe maize grain by Letham (1964). Students often swap these or think kinetin is natural.
Q: 'Who first isolated auxin from plant tissue?' Options often include Darwin, Boysen-Jensen, Paal, Went. Answer: Went (1928) from Avena coleoptile tip. Darwin only observed the bending response.
How NEET Frames The Trap
NEET frames questions as 'first isolated', 'first observed', 'coined the term' to test whether students know the precise contribution of each scientist in the discovery chain.
Q. Kinetin was isolated from:
A. Unripe maize grain B. Autoclaved herring sperm DNA C. Coconut milk D. Gibberella fujikuroi culture
Trick: Option B is correct. Kinetin is a synthetic cytokinin obtained from autoclaved herring sperm DNA by Miller, Skoog and Strong (1955). Option A is zeatin (first natural cytokinin by Letham). Option C is coconut milk factor (contains natural cytokinins). Option D is the source of gibberellins.
Mistake Snapshot (What Students Do Wrong)
- Attributing bolting to auxin instead of gibberellin: Bolting (rapid stem elongation in rosette plants like cabbage and henbane before flowering) is specifically a gibberellin function. Students often confuse this with auxin-induced cell elongation, which is a different process.
- Attributing Richmond-Lang effect to gibberellin: The Richmond-Lang effect (delay of leaf senescence) is caused by cytokinins, not gibberellins. Richmond and Lang (1957) demonstrated this with detached Xanthium leaves. Gibberellins substitute for vernalization, not delay senescence.
Q: 'Which hormone induces bolting in rosette plants?' Students who pick auxin are wrong. Answer: Gibberellin. GA induces enormous internode elongation in cabbage and henbane prior to flowering.
How NEET Frames The Trap
NEET may combine two functions in one question: 'Hormone X causes bolting while Hormone Y delays senescence. Identify X and Y.' Students must clearly map bolting = GA and senescence delay = cytokinin.
Q. Richmond-Lang effect of delaying senescence in detached leaves is associated with:
A. Gibberellins B. Auxins C. Cytokinins D. Ethylene
Trick: Option C is correct. Cytokinins delay senescence by controlling protein synthesis and chlorophyll mobilization. Richmond and Lang (1957) demonstrated this on detached Xanthium leaves. Ethylene and ABA actually PROMOTE senescence. Gibberellins promote stem elongation and bolting.
Mistake Snapshot (What Students Do Wrong)
- Thinking SDP needs short day for flowering: The name 'short day plant' is misleading. SDP actually requires a long uninterrupted dark period (skotoperiodism). If the dark period is interrupted by even a brief flash of red light, flowering is inhibited. The critical factor is the DARK period, not the light period.
- Confusing SDP and LDP based on critical photoperiod values: Xanthium (SDP) has a critical photoperiod of 5-6 hours while Hyoscymus niger (LDP) has 11 hours. Yet both flower with 14 hours of light. The difference is whether the plant flowers BELOW (SDP) or ABOVE (LDP) its critical value.
Q: 'Both Xanthium and Hyoscymus flower at 14 hours of light. Why is Xanthium classified as SDP?' Because Xanthium flowers when photoperiod is LESS than its critical value (15.5 hr), while Hyoscymus requires photoperiod MORE than 11 hr.
How NEET Frames The Trap
NEET may present a scenario where both SDP and LDP flower under the same photoperiod and ask students to explain the classification, testing understanding of critical photoperiod concept.
Q. A short day plant kept in continuous darkness will:
A. Flower profusely because dark period is very long B. Not flower because it still needs a minimum light period C. Flower only if given a flash of far-red light D. Die before flowering can occur
Trick: Option B is correct. Even though SDP requires long dark periods, the plant still needs some light period for photosynthesis and florigen production in leaves. Continuous darkness means no photosynthesis, no florigen synthesis, and no flowering. The trap is thinking that since SDP = long night plant, unlimited darkness will induce maximum flowering.
Mistake Snapshot (What Students Do Wrong)
- Reversing Pr and Pfr absorption wavelengths: Pr absorbs RED light at 660nm and converts to Pfr. Pfr absorbs FAR-RED light at 730nm and converts back to Pr. Students often get the wavelengths reversed or confuse which form absorbs which wavelength.
- Forgetting Pfr is the biologically active form: Pfr (far-red absorbing form) is the biologically active form that triggers physiological responses. It promotes LDP flowering but inhibits SDP flowering. Students sometimes think Pr is the active form because it absorbs the commonly discussed red light.
Q: 'What happens when Pr absorbs red light (660nm)?' It converts to Pfr. If Pfr then absorbs far-red (730nm), it converts back to Pr. In darkness, Pfr slowly reverts to Pr thermally.
How NEET Frames The Trap
NEET tests this with night-interruption experiments: a flash of red light during long dark period converts Pr to Pfr, inhibiting SDP flowering. A subsequent far-red flash reverses this. Students must track the interconversions carefully.
Q. In a short day plant, interruption of the dark period with a flash of red light followed by far-red light will result in:
A. Flowering is inhibited because red light was given B. Flowering occurs because far-red light reversed the red light effect C. No effect on flowering since both lights cancel each other D. Flowering is delayed but not prevented
Trick: Option B is correct. Red light converts Pr to Pfr (inhibits SDP flowering), but the subsequent far-red light converts Pfr back to Pr, reversing the inhibition. The LAST light treatment determines the outcome. If last light is red = no flowering (Pfr dominates). If last light is far-red = flowering (Pr dominates).
Mistake Snapshot (What Students Do Wrong)
- Confusing perception sites: Photoperiodism stimulus is perceived by LEAVES (even one leaf is sufficient). Vernalization stimulus is perceived by MERISTEMATIC CELLS (shoot tip, embryo tips, root apex). Students frequently swap these sites.
- Thinking vernalization affects mature tissues: Vernalization only affects actively dividing meristematic cells. Mature differentiated tissues do not respond to chilling treatment. This is why vernalization is given to germinating seeds, not dry seeds or mature plants.
Q: 'The site of perception of vernalization stimulus is ___' Options: leaves, flowers, meristematic cells, roots. Answer: Meristematic cells (shoot tip, embryo tip). Leaves perceive photoperiodism, not vernalization.
How NEET Frames The Trap
NEET pairs these two concepts to test whether students can differentiate: 'Photoperiodic stimulus is perceived by ___ while vernalization stimulus is perceived by ___'.
Q. The site of perception of photoperiodic stimulus for flowering in plants is:
A. Shoot apex B. Lateral buds C. Leaves D. Root tips
Trick: Option C is correct. Photoperiodic stimulus is perceived by leaves (even a single leaf is sufficient). Defoliated plants do not flower. The trap is choosing shoot apex (which perceives vernalization) or confusing with where flowers actually form.