Unit 8 Growth and development in plants and animals
Key unit competence: Analyse how plants and animals develop through different stages of their life cycles.
Key ideas
- Growth is a permanent increase in size or dry mass, by cell division, cell enlargement and cell differentiation. Development is the series of changes from zygote to mature organism.
- Dormancy is a period of no growth with very low metabolism. Seed dormancy is caused by an impermeable testa, an immature embryo, growth inhibitors (abscisic acid), or the need for light, cold or fire. It ensures germination in favourable conditions and spreads germination over time. It can be broken by scarification, soaking, cold (stratification), light or gibberellins.
- Germination: water is absorbed (imbibition), enzymes are activated; gibberellins stimulate amylase in the aleurone layer (cereals), starch is hydrolysed to maltose for the embryo. Epigeal (cotyledons above ground: bean) and hypogeal (cotyledons stay below: maize, pea).
- Primary growth is growth in length at apical meristems (zones of cell division, elongation and differentiation). Secondary growth is growth in thickness by the vascular cambium (new xylem inside, phloem outside: annual rings) and cork cambium (bark), mainly in dicot trees.
- Measuring growth: length, height, fresh mass (varies with water) and dry mass (most reliable, but destructive), number of cells. Growth curves are usually sigmoid. Annual plants complete their life cycle in one year; perennials grow for many years.
- Plant hormones (phytohormones): auxins (cell elongation, phototropism, apical dominance, rooting of cuttings, fruit growth; synthetic auxins as weedkillers), gibberellins (stem elongation, breaking dormancy, germination, seedless fruit), cytokinins (cell division, delay ageing of leaves), abscisic acid (dormancy, stomatal closure in drought, leaf fall), ethene (fruit ripening, leaf and fruit fall).
- Tropisms are directional growth responses (phototropism, gravitropism, hydrotropism, thigmotropism, chemotropism of pollen tubes); nastic movements are non-directional (Mimosa leaves folding, flowers opening).
- Photoperiodism: flowering controlled by day (night) length, detected by phytochrome in leaves. Short-day plants (long-night plants) flower when nights exceed a critical length (e.g. some soya, chrysanthemum); long-day plants flower when nights are short (e.g. wheat, spinach); day-neutral plants (e.g. tomato, maize varieties) are unaffected.
- Animal growth: metamorphosis is a big change in form during development. Complete metamorphosis (holometabolous): egg → larva → pupa → adult (butterfly, housefly, mosquito, bee). Incomplete metamorphosis (hemimetabolous): egg → nymph (several moults) → adult (grasshopper, cockroach, termite). Frogs: egg → tadpole → adult, controlled by thyroxine.
- Growth patterns: arthropods show intermittent (discontinuous) growth: size increases in steps after each moult (ecdysis), because the exoskeleton cannot stretch; controlled by ecdysone and juvenile hormone. Vertebrates show continuous growth with an internal skeleton, often a sigmoid curve, with a growth spurt at puberty in humans.
Common misconceptions
- "Growth is just getting bigger." No: growth is a permanent increase in size or dry mass; swelling with water is not true growth.
- "Insects grow continuously like humans." No: insects grow in steps after each moult (intermittent growth).
- "Short-day plants are controlled by the length of the day." No: they respond mainly to the length of the uninterrupted night.
- "Ethene makes fruit grow larger." No: ethene ripens fruits; auxins and gibberellins promote fruit growth.
Worked examples
Example 1. Explain why dry mass is a better measure of plant growth than fresh mass.
Solution
Fresh mass changes with the water content of the plant, which varies with time of day and watering, so it may rise or fall without real growth. Dry mass measures the organic matter actually made, giving a true measure of growth, although the plants must be killed (so many identical plants are needed).
Example 2. Explain how gibberellins bring about germination of a barley grain.
Solution
Water is absorbed; the embryo releases gibberellins, which diffuse to the aleurone layer; this layer makes amylase, which hydrolyses starch in the endosperm to maltose; sugars are absorbed by the embryo for respiration and growth, so the radicle and plumule emerge.
Practice questions
Define growth and development and state three ways of measuring growth.
Show answer
Growth: permanent increase in size or dry mass. Development: series of changes from zygote to adult. Measurements: height/length, fresh mass, dry mass, leaf area, cell number.
Common mistake: Saying fresh mass is the most reliable.
State three causes of seed dormancy and two ways to break it.
Show answer
Causes: impermeable testa, immature embryo, inhibitors (abscisic acid), need for light or cold. Breaking: scarification, soaking, cold treatment (stratification), gibberellins, light.
Common mistake: Confusing dormancy with death.
Distinguish between epigeal and hypogeal germination with examples.
Show answer
Epigeal: hypocotyl elongates and carries cotyledons above ground (bean). Hypogeal: epicotyl elongates and cotyledons stay below ground (maize, pea).
Common mistake: Reversing the examples.
State the effects of five plant hormones.
Show answer
Auxin: cell elongation, tropisms, apical dominance, rooting. Gibberellin: stem elongation, germination. Cytokinin: cell division, delays ageing. Abscisic acid: dormancy, stomatal closure. Ethene: fruit ripening, leaf fall.
Common mistake: Saying ethene causes growth in length.
Distinguish between primary and secondary growth.
Show answer
Primary growth: increase in length at apical meristems (root and shoot tips). Secondary growth: increase in thickness by lateral meristems (vascular cambium and cork cambium), forming wood and bark.
Common mistake: Saying monocots usually have secondary growth.
Compare complete and incomplete metamorphosis.
Show answer
Complete: egg, larva, pupa, adult; larva very different from adult (butterfly, fly). Incomplete: egg, nymph, adult; nymph resembles a small wingless adult (grasshopper, cockroach).
Common mistake: Including a pupa in incomplete metamorphosis.
Explain why arthropods show intermittent growth.
Show answer
Their rigid chitin exoskeleton cannot stretch; they grow only when they moult (ecdysis), swell before the new exoskeleton hardens, so size increases in steps.
Common mistake: Saying insects grow continuously.
Explain photoperiodism and give an example of a short-day and a long-day plant.
Show answer
Photoperiodism is the response of plants (especially flowering) to the length of day and night, detected by phytochrome. Short-day: chrysanthemum, some soya. Long-day: wheat, spinach.
Common mistake: Saying plants measure the day, not the night.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. A period of no growth with very low metabolism is: a. thyroxine 2. Metamorphosis of a tadpole into a frog is controlled by: b. dormancy 3. Gibberellins stimulate: c. seasonal secondary xylem 4. Annual rings in a tree trunk are formed by: d. stem elongation and germination Show answer
1 → b (dormancy) · 2 → a (thyroxine) · 3 → d (stem elongation and germination) · 4 → c (seasonal secondary xylem)
- Seeds and buds can be dormant.
- Lack of iodine delays it.
- They activate amylase in seeds.
- Rings show growth seasons.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
cytokinins · egg, larva, pupa, adult · intermittent (discontinuous) · egg, nymph, adult- Cell division in plants is promoted by .
- Insect growth is described as .
- Complete metamorphosis includes the stages .
Show answer
1: cytokinins · 2: intermittent (discontinuous) · 3: egg, larva, pupa, adult
- Used in tissue culture.
- Size increases after each moult.
- Example: butterfly.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Short-day plants flower when: a. phytochrome 2. A permanent increase in dry mass of an organism is: b. growth 3. Growth in thickness of a tree is due to the: c. vascular cambium 4. The pigment that detects day length in plants is: d. nights are longer than a critical length Show answer
1 → d (nights are longer than a critical length) · 2 → b (growth) · 3 → c (vascular cambium) · 4 → a (phytochrome)
- They are long-night plants.
- It involves cell division and enlargement.
- It forms secondary xylem and phloem.
- It is found in leaves.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
imbibition · auxin from the shoot tip · incomplete metamorphosis · abscisic acid from roots- Grasshoppers develop by .
- Absorption of water by a dry seed is .
- Apical dominance is caused by .
Show answer
1: incomplete metamorphosis · 2: imbibition · 3: auxin from the shoot tip
- Nymphs resemble small adults.
- The first stage of germination.
- Removing the tip lets side buds grow.