Lesson 13 of 13 · 11 min
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Must-know facts
17 facts
- 1Growth: irreversible, permanent increase in size, needing energy. Development = growth + differentiation.
- 2Open form of growth: meristems keep adding cells throughout life.
- 3Primary growth: root and shoot apical meristems (length). Secondary growth: vascular cambium and cork cambium (girth), in dicots and gymnosperms.
- 4Parameters: fresh weight, dry weight, length, area, volume, cell number. Maize root apical meristem: over 17,500 new cells per hour; watermelon cells: up to 3,50,000 times larger.
- 5Three phases of growth: meristematic, elongation, maturation.
- 6Arithmetic growth: one daughter divides; Lt = L0 + rt; straight line.
- 7Geometric growth: both daughters divide; lag, log (exponential), stationary; sigmoid curve; W1 = W0 e^rt; r = relative growth rate = efficiency index.
- 8Absolute growth rate: total growth per unit time. Relative growth rate: growth per unit time per unit initial size.
- 9Dedifferentiation examples: interfascicular cambium and cork cambium from parenchyma.
- 10Heterophylly (plasticity): cotton, coriander, larkspur (phase of life); buttercup (air against water).
- 11PGR chemistry: IAA indole; kinetin adenine derivative; ABA carotenoid derivative; GA₃ terpene; ethylene gas.
- 12Discoverers: Darwins (coleoptile tip, canary grass), Went (auxin, oat), Kurosawa (bakanae, Gibberella fujikuroi), Skoog and Miller (kinetin), three groups (inhibitor-B, abscission II, dormin = ABA), Cousins (ethylene, oranges and bananas).
- 13Auxin: rooting of cuttings, apical dominance, parthenocarpy in tomato, 2,4-D herbicide for dicot weeds, xylem differentiation.
- 14Gibberellin: grape stalk length, apple shape, delays senescence, malting, sugarcane stem (up to 20 tonnes per acre), bolting in rosette plants.
- 15Cytokinin: cytokinesis, overcomes apical dominance, delays leaf senescence; kinetin from herring sperm DNA, zeatin from corn kernels and coconut milk.
- 16Ethylene: fruit ripening, respiratory climacteric, apical hook, deep-water rice elongation, ethephon, female flowers in cucumber.
- 17ABA: stomatal closure, stress hormone, seed dormancy, inhibits germination, antagonist of GAs.
Common traps
Where marks are lost
Calling a piece of wood swelling in water an example of growth.
Growth is irreversible and driven by metabolism; swelling by soaking is reversible and involves no metabolism.
Thinking the zone of elongation is the meristem at the very tip.
The tip is the meristematic zone; elongation happens in the cells just behind it, which spread apart most in the parallel line test.
Using W1 = W0 e^rt for a root elongating at a constant rate.
Constant elongation is arithmetic growth: Lt = L0 + rt, a straight line. The exponential form applies to geometric growth.
Assuming the larger leaf has the higher relative growth rate when both gain the same area.
Same absolute gain means the smaller leaf has the higher relative rate, since the gain is a bigger fraction of its starting size.
Naming vascular cambium in the stele as a product of dedifferentiation.
NCERT's dedifferentiation examples are interfascicular cambium and cork cambium, formed from fully differentiated parenchyma.
Saying cytokinin maintains apical dominance.
Auxin from the apical bud maintains apical dominance; cytokinins help overcome it and promote lateral shoot growth.
Saying kinetin is a natural plant cytokinin.
Kinetin came from autoclaved herring sperm DNA and does not occur naturally in plants; zeatin is a natural one.
Believing 2,4-D kills all plants in a lawn.
2,4-D kills dicot weeds but does not affect mature monocots such as lawn grass.
Key terms
20 terms
- Growth
- An irreversible, permanent increase in size of a cell, organ or organism, usually needing energy.
- Meristem
- A group of cells that keep dividing and perpetuating themselves, adding new cells to the plant body.
- Open form of growth
- Growth in which meristems keep adding new cells to the body throughout life.
- Arithmetic growth
- Growth in which only one daughter cell of each division keeps dividing; length rises in a straight line with time.
- Geometric growth
- Growth in which both daughter cells keep dividing; gives lag, log and stationary phases.
- Sigmoid curve
- The S-shaped curve of growth against time, with lag, exponential and stationary phases.
- Efficiency index
- The relative growth rate r in W1 = W0 e^rt, measuring a plant's ability to produce new material.
- Absolute growth rate
- Total growth per unit time.
- Relative growth rate
- Growth per unit time expressed per unit of initial size.
- Differentiation
- Maturation of cells from meristems to perform specific functions, with changes to walls and protoplasm.
- Dedifferentiation
- A differentiated cell regaining the ability to divide, as when parenchyma forms cork cambium.
- Redifferentiation
- Cells from dedifferentiated tissue losing the ability to divide again and maturing for a function.
- Plasticity
- The ability of a plant to form different structures in response to environment or phase of life.
- Heterophylly
- Different leaf shapes on the same plant, by life phase (larkspur) or environment (buttercup).
- Apical dominance
- Suppression of lateral bud growth by the growing apical bud.
- Parthenocarpy
- Development of fruit without fertilisation; induced by auxins in tomato.
- Bolting
- Internode elongation just before flowering in rosette plants, promoted by gibberellins.
- Respiratory climacteric
- The rise in respiration rate during fruit ripening, enhanced by ethylene.
- Ethephon
- A compound that releases ethylene slowly inside the plant; the usual commercial source of ethylene.
- Stress hormone
- Abscisic acid, which closes stomata and raises tolerance to many stresses.