What this chapter is about
This chapter introduces the unique characteristics of plant growth, which differs fundamentally from animal growth. Plants grow throughout their life (indeterminate growth), and growth occurs only in specific regions containing meristematic tissue. A Class 11 student learns how growth is measured, what phases growth passes through, and the factors that influence it.
The chapter also covers plant development, which includes differentiation, dedifferentiation, and redifferentiation of cells. Students explore how plant hormones (phytohormones) regulate growth and coordinate developmental processes such as flowering, fruit formation, seed dormancy, and responses to environmental signals like light and gravity.
After studying this chapter, a student should be able to define and measure plant growth, describe the roles of the five major plant hormones, explain photoperiodism and vernalisation, and understand how internal and external factors work together to control plant development.
Key ideas
- Growth is irreversible permanent increase in size accompanied by increase in dry mass, cell number, and complexity; it occurs in meristematic regions (root tip, shoot tip, cambium).
- Growth can be measured by increase in fresh weight, dry weight, length, area, volume, or cell number; the simplest method is measuring increase in length over time.
- Growth occurs in three phases: the cell division phase (meristematic), the cell enlargement phase (elongation), and the cell maturation phase (differentiation).
- The growth rate of a plant organ can follow arithmetic growth (constant addition, e.g., root elongating at fixed length per day) or geometric growth (exponential increase when all daughter cells continue dividing).
- Plant hormones (phytohormones) are organic molecules produced in minute quantities that regulate growth and development; the five major groups are auxins, gibberellins, cytokinins, abscisic acid, and ethylene.
- Photoperiodism is the response of plants to the relative lengths of day and night, which controls flowering in short-day, long-day, and day-neutral plants.
- Vernalisation is the promotion of flowering by exposure to low temperatures for a certain period, important in winter crops like wheat.
- Seed dormancy prevents germination even under favourable conditions; it is broken by factors like light, temperature, or hormone balance.
Formulas and facts to remember
1. Arithmetic growth: L_t = L_0 + r × t, where L_t is final length, L_0 is initial length, r is elongation rate per unit time, and t is time. Growth adds a constant amount per unit time.
2. Exponential (geometric) growth: W_t = W_0 × e^(r × t), where W_t is final size (weight or number), W_0 is initial size, r is relative growth rate, and t is time. Growth doubles repeatedly.
3. Relative growth rate (RGR) = (W_2 − W_1) / (W_1 × (t_2 − t_1)), measuring growth per unit of existing material per unit time.
4. Auxins (e.g., indole-3-acetic acid, IAA) promote cell elongation, apical dominance, root initiation, and are used in rooting of cuttings and as herbicides at high concentrations.
5. Gibberellins (e.g., GA_3) promote stem elongation, bolting in rosette plants, breaking of seed dormancy, and delay senescence.
6. Cytokinins (e.g., zeatin, kinetin) promote cell division, delay leaf senescence, and help overcome apical dominance by promoting lateral bud growth.
7. Abscisic acid (ABA) inhibits growth, promotes stomatal closure during water stress, and induces seed dormancy; often called the stress hormone.
8. Ethylene (C₂H₄), a gaseous hormone, promotes fruit ripening, leaf abscission, and senescence; used commercially to ripen fruits like bananas.
Worked examples
### Example 1: Calculating arithmetic growth
A maize seedling root measures 2 cm on Monday morning. It grows at a constant rate of 0.8 cm per day. What will its length be after 5 days?
Solution
Use L_t = L_0 + r × t
L_0 = 2 cm, r = 0.8 cm/day, t = 5 days
L_t = 2 + (0.8 × 5) = 2 + 4 = 6 cm
The root will be 6 cm long after 5 days.
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### Example 2: Predicting flowering based on photoperiod
A farmer in Punjab wants to grow a short-day plant (e.g., chrysanthemum, which flowers when night length exceeds a critical duration of about 10 hours). In June, nights in Punjab are approximately 8 hours long, while in December, nights are about 13 hours long. In which season will the plant flower naturally?
Solution
Short-day plants require uninterrupted dark periods longer than a critical length to flower.
June night: 8 hours (less than 10 hours) → plant remains vegetative.
December night: 13 hours (more than 10 hours) → flowering is induced.
The chrysanthemum will flower naturally in winter (December).
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### Example 3: Role of hormones in fruit ripening
A fruit vendor stores unripe mangoes in a closed container along with a few ripe bananas. After three days, the mangoes ripen faster than those stored alone. Explain the hormonal basis.
Solution
Ripe bananas release ethylene gas, a plant hormone that promotes fruit ripening.
In a closed container, ethylene accumulates and diffuses to the unripe mangoes.
Ethylene triggers enzymes that break down cell walls and convert starch to sugars, accelerating ripening.
This is why commercial ripening rooms expose fruits to controlled ethylene concentrations.
Common mistakes
- Thinking growth occurs throughout the plant body → growth is localised to meristematic regions; mature cells do not divide.
- Confusing growth with development → growth is quantitative increase in size; development includes differentiation and morphogenesis.
- Believing all hormones only promote growth → ABA inhibits growth and ethylene can promote senescence and abscission.
- Assuming short-day plants need short days → they actually require long, uninterrupted nights (darkness is the critical factor).
- Mixing up apical dominance and lateral growth → auxin from the apical bud suppresses lateral buds; removing the apex or adding cytokinin promotes lateral branching.
Quick revision
- Plant growth is irreversible, localised (meristems), and measurable by length, weight, or cell number.
- Three phases of growth: division → elongation → differentiation.
- Five major hormones: auxins, gibberellins, cytokinins, ABA (inhibitor), ethylene (gaseous).
- Photoperiodism = flowering response to day-night ratio; vernalisation = flowering response to cold treatment.
- Ethylene ripens fruits; ABA closes stomata and induces dormancy; cytokinins delay senescence.
- Relative growth rate compares growth efficiency across different plants or organs.