Big picture
A seedling does not become a mature plant simply by enlarging every cell equally. Cells divide, expand and specialise, while environmental signals influence when flowers or dormant stages appear. Growth is an irreversible increase in size, usually associated with increased dry mass, volume or cell number.
Development includes growth and the organised changes in form and function across the life cycle. A farmer's choice of sowing time, storage conditions or growth-regulator treatment depends on how internal programmes interact with light, temperature and water. The chapter connects visible plant changes with cellular processes and chemical regulation.
Learning goals
Explain growth regions, phases and measurable growth rates.
Compare differentiation, dedifferentiation and redifferentiation.
Describe the major plant-growth regulators with examples.
Interpret growth curves and simple experiments.
Explain photoperiodism, vernalisation and seed dormancy.
Core ideas
1. Open growth and its measurement
Plants often show indeterminate growth because meristems remain active throughout life. Apical meristems extend roots and shoots, while lateral meristems increase girth. Individual leaves, flowers and fruits usually have more limited growth even though the whole plant retains growing regions. Growth is therefore open at the organism level but not unlimited in every organ.
Choose a measurement appropriate to the organism or organ. Length suits a root, surface area suits a leaf, and dry mass helps avoid misleading changes caused only by water content. Cell number can measure proliferation, but growth can also occur through cell enlargement. For example, a soaked seed may gain fresh mass before substantial new dry matter is formed, showing why the chosen variable matters.
2. Growth phases
A growing root shows a meristematic phase near the tip, where cells divide actively; an elongation phase, where cells enlarge through vacuolation and wall expansion; and a maturation phase, where cells achieve specialised form. Root hairs arise in the maturation zone, not directly on the root cap.
A typical sigmoid growth curve has an initial lag phase, a rapid phase and a slowing phase approaching maturity. Plot time on the horizontal axis and size or mass on the vertical axis. The steepness represents growth rate, so the highest point on a curve is not necessarily the fastest-growing interval. For example, a nearly mature leaf can be large while adding very little area each day.
3. Arithmetic and geometric growth
In arithmetic growth, one daughter cell continues dividing while the other differentiates, producing an approximately constant absolute increment. The simple relationship is Lₜ = L₀ + rt, where r is growth per unit time. If a shoot is initially 8 cm and adds 1.5 cm daily for four days, Lₜ = 8 + 1.5 × 4 = 14 cm.
In geometric growth, both daughter cells continue dividing under ideal conditions, so increase is proportional to existing size. A common model is Wₜ = W₀eʳᵗ. For discrete doubling, a culture with 120 cells undergoing three doublings reaches 120 × 2³ = 960 cells. Real resources limit exponential growth, contributing to a sigmoid curve rather than indefinite increase.
Absolute growth rate measures total change per time; relative growth rate relates increase to initial size over a specified interval. A leaf growing from 4 to 6 cm² gains 2 cm², while another growing from 20 to 22 cm² also gains 2 cm². Their percentage increases are 2 ÷ 4 × 100 = 50% and 2 ÷ 20 × 100 = 10%. State the interval and definition because continuous relative-growth calculations may use logarithms instead.
4. Differentiation and developmental plasticity
Differentiation produces specialised cells, such as tracheary elements developing lignified walls. Dedifferentiation occurs when living differentiated cells regain division capacity, as in formation of interfascicular cambium. Redifferentiation produces specialised derivatives again, such as secondary tissues formed from cambial cells.
Plasticity is the ability to follow different developmental pathways in response to environment or life stage. Heterophylly means different leaf forms on the same plant. Cotton can show developmental differences between juvenile and mature leaves; aquatic buttercup can show different leaf forms under water and in air. These examples distinguish developmental change from an environmental response. Development is controlled by both intrinsic genetic factors and extrinsic conditions, not by hormones acting independently of genes.
5. Conditions and discovery of regulators
Growth requires water for turgor and metabolism, oxygen for respiration, mineral nutrients for cellular components and a suitable temperature range. Light influences photosynthesis and developmental signalling. To test one factor, keep seed type, age, water supply and other conditions constant, changing only the selected variable.
Plant growth regulators, PGRs, are small organic substances active at low concentrations. The main groups are auxins, gibberellins, cytokinins, ethylene and abscisic acid. Classic coleoptile experiments showed that a tip detects directional light and influences bending below. Agar-block experiments associated the transmitted growth effect with diffusible substances. In an experimental answer, distinguish observation, inference and control; bending alone does not prove every proposed molecular detail.
6. Auxins
Auxins, including indole-3-acetic acid, promote cell elongation in suitable tissues and concentrations. They contribute to apical dominance, in which the active shoot apex suppresses lateral-bud outgrowth. Removing the apex can therefore encourage branching, useful in pruning. The response involves interactions with other signals, not direct mechanical pressure from the tip.
Auxins promote adventitious rooting in stem cuttings and can delay premature leaf or fruit drop under suitable conditions. Some synthetic auxins, such as 2,4-D, are used as selective herbicides. Auxins can also induce parthenocarpic fruit development in suitable plants. A hormone's effect depends on concentration, tissue and developmental state: a dose that promotes shoot elongation may inhibit root elongation.
7. Gibberellins and cytokinins
Gibberellins, such as GA₃, promote stem elongation and bolting, rapid internode elongation before flowering in rosette plants. They can break dormancy in suitable seeds and stimulate hydrolytic enzyme production during germination. Malting uses this capacity to mobilise stored reserves. Grapevine applications can influence stalk elongation and fruit characteristics, but responses depend on crop and treatment.
Cytokinins promote cell division, often in interaction with auxin. They are associated with actively dividing tissues such as root tips and developing fruits. They promote lateral-bud development and can delay leaf senescence. In tissue culture, the auxin–cytokinin balance influences root and shoot formation; relative cytokinin predominance often favours shoots. This is a controlled culture relationship, not a rule that a hormone can create a whole organ in any tissue under any conditions.
8. Ethylene and abscisic acid
Ethylene is a gaseous regulator associated with fruit ripening, senescence and abscission. It can produce the seedling triple response: reduced elongation, increased radial growth and altered orientation. Ethephon releases ethylene and is used in appropriate agricultural treatments. A ripening fruit can influence nearby fruit because the signal diffuses through air.
Abscisic acid, ABA, promotes seed dormancy and responses to water stress, including stomatal closure. It can oppose growth-promoting effects of gibberellins in seed germination. Calling ABA only a harmful inhibitor is misleading: conserving water and preventing germination during unsuitable conditions can improve survival. Likewise, ethylene cannot be classified as solely inhibitory because its effects depend on the developmental process.
9. Photoperiodism, vernalisation and dormancy
Photoperiodism is a response to the relative lengths of light and darkness, often affecting flowering. Short-day plants flower when suitable long nights occur; long-day plants generally require sufficiently short nights. Day-neutral plants are less dependent on day length for flowering. Leaves perceive the photoperiodic signal, which influences the shoot apex. A brief night interruption can alter flowering in sensitive plants, showing that uninterrupted darkness matters.
Vernalisation is acquisition or acceleration of flowering competence after suitable low-temperature exposure. It helps some plants flower after winter rather than before it. It differs from photoperiodism because temperature rather than light–dark duration is the defining cue. Dormancy prevents immediate germination even when some external conditions appear favourable; seed coats, inhibitors or embryo state may contribute. Scarification, chilling or suitable hormonal treatment can release dormancy depending on the species. These flowering-response topics are included for teacher confirmation where rationalised course coverage differs.
A reliable approach to questions
Identify the measured variable and distinguish growth from broader development.
Choose absolute, relative or exponential reasoning before substituting numbers.
Match regulator, tissue, concentration and developmental context.
For experiments, specify a control and change one factor at a time.
Distinguish light-duration cues, chilling responses and dormancy mechanisms.
Connections that matter
Cell division supplies new cells while differentiation creates specialised tissues.
Photosynthesis and respiration provide material and energy for growth.
Plant anatomy shows products of meristem activity and redifferentiation.
Common mistakes
Equating fresh-mass gain with new dry matter ignores water uptake.
Calling equal absolute increases equal relative growth ignores initial size.
Assigning one universal effect to a PGR ignores tissue and dose.
Equating vernalisation with photoperiodism confuses temperature and light cues.
Revision checklist
I can interpret growth curves and calculate growth rates.
I can compare three differentiation-related processes.
I can explain each major regulator with an example.
I can design a controlled growth experiment.
I can distinguish dormancy, photoperiodism and vernalisation.