Growth in plants: meristems and phases
A tree keeps adding new branches and roots for as long as it lives, while a puppy stops growing taller once it is an adult. Plants grow all their life from special growing points.
Growth is a permanent, irreversible increase in size, volume or mass of a plant or its parts, with synthesis of new protoplasm. Plant growth is indeterminate because meristems keep dividing throughout life, in contrast to most animals.
In the figure : Three phases of growth in the root or shoot tip.
Rule 1: Open form of growth and meristems — Apical meristems at root and shoot tips make the plant longer (primary growth). Lateral meristems (vascular cambium and cork cambium) in dicots and gymnosperms make the plant thicker (secondary growth). Intercalary meristems at the base of leaves and internodes in grasses add length. New cells formed by meristems become specialised.
- Easy : A root tip lengthens because its apical meristem keeps dividing.
- Medium : The trunk of a tree becomes wider each year due to the cambium.
- Exam-type : Q: Which meristem is responsible for the increase in girth? A: Lateral meristem (cambium).
Rule 2: Phases of growth — In the meristematic phase, cells at the tip divide, with rich protoplasm, dense cytoplasm and prominent nuclei. In the elongation phase, cells just behind the tip enlarge: vacuoles form, the cell wall grows and new wall material is added. In the maturation phase, cells further back reach their final size and thickness and become specialised.
- Easy : The cells nearest the root tip are small and dividing; those farther back are bigger and specialised.
- Medium : A cell that has formed a large vacuole and is lengthening is in the elongation phase.
- Exam-type : Q: In which phase do cells acquire their final shape and structure? A: Maturation phase.
Plant growth is indeterminate and begins at meristems. The three zones from the tip are: division, elongation, maturation.
Saying that all cells in a plant divide. Only meristematic cells do; most cells have stopped dividing.
Memory clue: D-E-M — Divide, Enlarge, Mature: the order from the tip backward.
Spot the difference
| Phase | Place | Main event |
|---|---|---|
| Meristematic | At the tip | Cell division |
| Elongation | Behind the tip | Cell enlargement, vacuole formation |
| Maturation | Farther back | Specialisation |
Practice questions
- 1Why is plant growth called indeterminate?
- 2Where are apical meristems found?
- 3Name two lateral meristems.
- 4What is secondary growth?
- 5Where are intercalary meristems found?
- 6Name the three phases of growth.
- 7What happens to a cell in the elongation phase?
- 8A cell behind the elongation zone no longer enlarges. Which phase is it in?
Show answers (try them yourself first)
- 1Because meristems keep dividing throughout the life of the plant.
- 2At the tips of roots and shoots.
- 3Vascular cambium and cork cambium.
- 4Increase in girth due to lateral meristems.
- 5At the base of leaves or internodes, as in grasses.
- 6Meristematic, elongation and maturation.
- 7It enlarges, forms vacuoles and its wall grows.
- 8Maturation phase.
Growth rates and conditions for growth
A savings account that earns interest on its interest grows faster each year. Some cells grow in this way, so more cells mean even more new cells.
Growth rate is the increase in size or number per unit time. It can be arithmetic (constant increase) or geometric (increase proportional to existing size). Growth needs water, oxygen, nutrients, suitable temperature and light.
In the figure : Sigmoid growth curve: lag, log and stationary phases.
Rule 1: Arithmetic and geometric growth — In arithmetic growth, only one daughter cell keeps dividing, so a root elongating at a constant rate gives a straight line on a graph: Lt = L0 + rt (L0 is initial length, r is growth rate). In geometric growth, both daughter cells divide, so growth is exponential: W1 = W0 e^(rt). A plot of size against time is S-shaped (sigmoid), with lag, log (exponential) and stationary phases.
- Easy : A root growing 2 mm every day shows arithmetic growth: 10 mm, 12 mm, 14 mm.
- Medium : A culture that doubles in number every hour shows geometric growth: 100, 200, 400.
- Exam-type : Q: What is the shape of a typical growth curve? A: Sigmoid (S-shaped).
Rule 2: Absolute and relative growth — Absolute growth is the measurement of total growth per unit time. Relative growth is growth per unit time expressed as a fraction of the initial size. Two leaves may grow by the same absolute amount, but the smaller one has the higher relative growth rate.
- Easy : Two leaves each grow 5 cm2 in a day.
- Medium : If one started at 10 cm2 and the other at 50 cm2, relative growth is 50 per cent and 10 per cent.
- Exam-type : Q: Which leaf has the higher relative growth rate, the one of 10 cm2 or of 50 cm2? A: The one of 10 cm2.
Rule 3: Conditions for growth — Water gives turgor for cell enlargement and is the medium for enzyme activity. Oxygen is needed for respiration and energy. Nutrients (macro and micro) are needed for protoplasm and energy. Temperature must be in the optimum range for the species. Light, and the environment in general (gravity, for instance), also affect growth.
- Easy : Seeds do not germinate in dry soil.
- Medium : A waterlogged seed may fail to grow because of lack of oxygen.
- Exam-type : Q: Why is water essential for cell enlargement? A: It gives the turgor pressure that stretches the cell wall.
Arithmetic = straight line; geometric = exponential; whole curve = S-shaped. Needs: water, oxygen, nutrients, temperature, light.
Comparing absolute growth only. Relative growth shows which part grows faster for its size.
Memory clue: L-L-S — Lag, Log, Stationary: the S-curve in order.
Spot the difference
| Feature | Arithmetic | Geometric |
|---|---|---|
| Dividing cells | Only one of the daughter cells | Both daughter cells |
| Graph | Straight line | Exponential curve |
| Example | Root elongating at constant rate | Early growth of a fruit or culture |
Practice questions
- 1What is growth rate?
- 2Name the three phases of a sigmoid growth curve.
- 3Write the equation for arithmetic growth.
- 4What is meant by relative growth rate?
- 5List four conditions needed for growth.
- 6A root grows 3 mm per day from 12 mm. What is its length after 4 days (arithmetic)?
- 7Two seedlings each gain 6 cm in a week. One was 12 cm long, the other 30 cm. Which has the higher relative growth?
- 8Why does growth reach a stationary phase?
Show answers (try them yourself first)
- 1Increase in size or number per unit time.
- 2Lag, log and stationary.
- 3Lt = L0 + rt.
- 4Growth per unit time expressed as a fraction of initial size.
- 5Water, oxygen, nutrients, suitable temperature (also light).
- 612 + 3 x 4 = 24 mm.
- 7The 12 cm seedling (50 per cent against 20 per cent).
- 8Nutrients become limited and other conditions become unfavourable.
Differentiation, dedifferentiation and development
A heap of identical bricks can become a wall, an arch or a window frame. Similarly, the same type of new cells can mature into different cell types.
Differentiation is the process by which cells formed by meristems become specialised for different functions. Dedifferentiation is when mature cells regain the ability to divide. Redifferentiation is when such cells again mature into specialised cells. Development includes all changes from seed germination to senescence.
In the figure : Cells can differentiate, dedifferentiate and redifferentiate.
Rule 1: Differentiation and dedifferentiation — A meristem cell becomes, for example, a tracheary element, which loses its protoplasm and develops a thick, strong wall to carry water. A mature parenchyma cell that has lost its power of division can regain it, for example when interfascicular cambium or cork cambium forms. Cells that again lose the power of division and mature are said to have redifferentiated.
- Easy : Parenchyma cells in the stem can start dividing to form cork cambium.
- Medium : A callus formed in tissue culture arises by dedifferentiation.
- Exam-type : Q: What is dedifferentiation? A: Regaining of the capacity to divide by mature cells.
Rule 2: Development and plasticity — Development covers growth plus differentiation, and ends with senescence and death. Plants follow different pathways in response to the environment: this is plasticity. Heterophylly, for example in cotton, coriander and larkspur, means that juvenile and mature leaves differ in shape. In buttercup, leaves in air differ from those in water.
- Easy : A seedling's first leaves look different from the leaves of the adult plant.
- Medium : Buttercup leaves developing in water are narrow, while those in air are broad.
- Exam-type : Q: What is heterophylly? A: Different forms of leaves on the same plant in different stages or environments.
Growth + differentiation = development. Plasticity lets plants adapt form to the environment.
Confusing differentiation with development. Differentiation is one part of development.
Memory clue: De-differ goes backwards — Dedifferentiation turns a mature cell back to dividing; Redifferentiation moves forward again.
Spot the difference
| Term | Meaning |
|---|---|
| Differentiation | Cell becomes specialised |
| Dedifferentiation | Specialised cell regains division |
| Redifferentiation | Dedifferentiated cell matures again |
| Plasticity | Different forms in different conditions |
Practice questions
- 1What is differentiation?
- 2Give an example of a dedifferentiated tissue.
- 3What is heterophylly?
- 4Name two plants that show heterophylly.
- 5Define development in plants.
- 6What is plasticity?
- 7What happens to a tracheary element during differentiation?
- 8Distinguish dedifferentiation from redifferentiation.
Show answers (try them yourself first)
- 1Specialisation of cells formed by meristems.
- 2Interfascicular cambium or cork cambium (also callus).
- 3Occurrence of leaves of different shapes on the same plant.
- 4Cotton and coriander (or larkspur).
- 5All changes a plant goes through from seed germination to senescence.
- 6Ability of plants to follow different pathways of development in response to the environment.
- 7It loses protoplasm and gets a thick strong wall.
- 8In the first a mature cell regains division; in the second the cells formed mature again into specialised cells.
Plant growth regulators: auxins and gibberellins
Just as chemical messengers called hormones coordinate a human body, small amounts of chemicals made in plants guide growth, flowering and fruit ripening.
Plant growth regulators (PGRs) are small, simple organic molecules of diverse chemical composition that influence growth and development. They may be promoters (such as auxins, gibberellins, cytokinins) or inhibitors (such as abscisic acid); ethylene is a gaseous PGR with both roles.
In the figure : Five groups of plant growth regulators.
Rule 1: Auxins — Auxins (Greek auxein, to grow) were first noticed by Darwin when canary grass coleoptiles bent towards light, and the substance was isolated from oat seedling tips by F.W. Went. IAA and IBA are natural; NAA and 2,4-D are synthetic. Uses: root initiation in stem cuttings, flowering in pineapple, preventing fruit and leaf drop, parthenocarpy in tomato, weed removal (2,4-D kills broadleaved plants), and apical dominance (the terminal bud inhibits lateral buds).
- Easy : Rooting hormone powder contains an auxin that helps a cutting form roots.
- Medium : Removing the shoot tip releases lateral buds from inhibition, so a hedge becomes bushy when pruned.
- Exam-type : Q: Which auxin is used as a weedkiller? A: 2,4-D.
Rule 2: Gibberellins — Gibberellins were found when the 'bakanae' (foolish seedling) disease of rice, due to the fungus Gibberella, was studied by Kurosawa (1926). GA3 is most studied. Effects: increase in length of stem axis (grape stalks, sugarcane stems), elongating and shaping apple fruit, delay of senescence, speeding malting in brewing, hastening maturity of conifers, and bolting in cabbage and beet.
- Easy : Spraying GA3 on sugarcane increases the length of the stem and so the sugar yield.
- Medium : Bolting is rapid internode elongation just before flowering in a rosette plant.
- Exam-type : Q: Which PGR was discovered through bakanae disease? A: Gibberellin.
Auxin = rooting, apical dominance, weedkiller 2,4-D. Gibberellin = stem elongation, bolting, malting.
Saying that auxin helps lateral buds grow. The terminal bud's auxin inhibits lateral buds.
Memory clue: Auxin = Axis; GA = Gain length — Auxin keeps the tip dominant; Gibberellin makes stems longer.
Spot the difference
| PGR | Discovery / source | Main effect |
|---|---|---|
| Auxin | Darwin, Went | Rooting, apical dominance, parthenocarpy |
| Gibberellin | Kurosawa (bakanae) | Stem elongation, bolting |
Practice questions
- 1What are plant growth regulators?
- 2Name two natural auxins.
- 3Name a synthetic auxin used as a weedkiller.
- 4What is apical dominance?
- 5Who studied the bakanae disease of rice?
- 6State two uses of gibberellins.
- 7How do gardeners use auxins in cuttings?
- 8A gardener pinches off the tip of a shrub. Why do side branches grow?
Show answers (try them yourself first)
- 1Small organic molecules that influence plant growth and development.
- 2IAA and IBA.
- 32,4-D.
- 4Inhibition of lateral buds by the terminal bud.
- 5Kurosawa.
- 6Increase length of sugarcane stems or grape stalks; speed malting; bolting (any two).
- 7To stimulate root formation.
- 8Removing the tip removes auxin inhibition on lateral buds.
Cytokinins, ethylene, abscisic acid and external control
Green fruits stay hard on the tree and turn soft when a ripening gas builds up. A bag of ripe banana in a box with raw mangoes ripens them faster for the same reason.
Cytokinins promote cell division and delay ageing; ethylene is a gas that triggers ripening; abscisic acid (ABA) is the stress hormone that causes dormancy and stomatal closure. Light and temperature also control flowering through photoperiodism and vernalisation.
In the figure : Cytokinins, ethylene and ABA control different responses.
Rule 1: Cytokinins — Skoog and Miller isolated kinetin from autoclaved herring sperm DNA; zeatin was later found in corn kernels and coconut milk. Cytokinins are made where cell division is rapid: root tips, shoot buds, young fruits. They promote cell division, new leaves, chloroplast formation, lateral shoot growth and nutrient mobilisation; they delay leaf senescence and overcome apical dominance.
- Easy : Coconut milk contains zeatin and is used in tissue-culture media.
- Medium : Cytokinins keep a cut leaf green for longer.
- Exam-type : Q: Which PGR delays leaf senescence? A: Cytokinin.
Rule 2: Ethylene and ABA — Ethylene is a gaseous hormone made mainly by ripening fruits and ageing tissues. It hastens fruit ripening, shedding of leaves and flowers, breaks seed and bud dormancy, promotes elongation of petioles in deepwater rice and flowering in pineapple and mango. Ethephon, a liquid that releases ethylene, is used commercially. ABA inhibits growth, closes stomata in water stress, promotes seed dormancy and acts against gibberellins.
- Easy : A raw banana kept beside ripe ones ripens sooner.
- Medium : In drought, ABA makes stomata close and reduces water loss.
- Exam-type : Q: Which PGR is known as the stress hormone? A: Abscisic acid.
Rule 3: Light and temperature — Photoperiodism is the response of flowering to day length. Short-day plants flower when the day is shorter than a critical length, long-day plants when it is longer, and day-neutral plants are not affected. Vernalisation is the promotion of flowering by a period of low temperature (for example winter varieties of wheat, barley and rye, and biennials).
- Easy : Winter wheat sown in autumn flowers the next spring after the cold treatment.
- Medium : A short-day plant flowers when the nights are long enough.
- Exam-type : Q: What is vernalisation? A: Promotion of flowering by exposure to low temperature.
Cytokinin: division, delays ageing. Ethylene: ripening gas. ABA: stress, dormancy, closes stomata.
Saying that ABA and gibberellin work together on germination. ABA inhibits germination; GA promotes it.
Memory clue: E-C-A — Ethylene ripens, Cytokinin multiplies cells, ABA applies the brakes.
Spot the difference
| PGR | Main effect |
|---|---|
| Cytokinin | Cell division, delays senescence |
| Ethylene | Fruit ripening, abscission |
| ABA | Seed dormancy, stomatal closure |
Practice questions
- 1From which source was zeatin first obtained?
- 2Name the gaseous plant growth regulator.
- 3Which PGR is called the stress hormone?
- 4What is the commercial source of ethylene?
- 5What is vernalisation?
- 6What is photoperiodism?
- 7How does ABA help a plant in drought?
- 8Give one way in which gibberellin and ABA act oppositely.
Show answers (try them yourself first)
- 1Corn kernels and coconut milk.
- 2Ethylene.
- 3Abscisic acid.
- 4Ethephon.
- 5Promotion of flowering by cold treatment.
- 6Response of flowering to the length of day and night.
- 7It closes stomata and reduces water loss.
- 8GA breaks dormancy and promotes germination; ABA promotes dormancy.