Why a Banyan Tree Keeps Growing for Its Whole Life
See why growth is an irreversible increase and which meristems keep plants growing, read the lag, log and stationary phases of a sigmoid curve, compare arithmetic and geometric growth, calculate absolute and relative growth rates, and learn differentiation and plasticity.
What makes plant growth different from animal growth?
A person stops growing taller after adolescence, but a banyan tree keeps adding branches, leaves and roots for as long as it lives. That difference comes from tissues that never stop dividing.
This part covers growth and meristems, growth phases and curves, growth rates and conditions, and differentiation, development and plasticity.
This part covers growth and meristems, growth phases and curves, growth rates and conditions, and differentiation, development and plasticity.
What is growth, and which meristems give plants indeterminate growth?
Growth is an irreversible, permanent increase in the size of an organ, its parts or a cell, and plants can grow throughout life because meristems at their tips and along their sides keep dividing — a pattern called open or indeterminate growth.
Growth involves cell division, cell enlargement and the use of energy from metabolism. It can be measured as an increase in fresh or dry weight, length, area, volume or cell number.
Meristems:
- Apical meristems — at root and shoot tips; increase length, giving primary growth
- Lateral meristems — vascular cambium and cork cambium; increase girth, giving secondary growth in dicots and gymnosperms
- Intercalary meristems — at the bases of internodes and leaves, as in grasses
Open growth. Meristems keep adding new cells to the plant body, so the plant as a whole is never finished growing.
An everyday example. A lawn grows back after mowing because intercalary meristems near the bases of the grass leaves keep dividing.
The substance. An increase in size alone is not growth — a raisin swelling in water gets bigger, but drying would reverse it.
Growth involves cell division, cell enlargement and the use of energy from metabolism. It can be measured as an increase in fresh or dry weight, length, area, volume or cell number.
Meristems:
- Apical meristems — at root and shoot tips; increase length, giving primary growth
- Lateral meristems — vascular cambium and cork cambium; increase girth, giving secondary growth in dicots and gymnosperms
- Intercalary meristems — at the bases of internodes and leaves, as in grasses
Open growth. Meristems keep adding new cells to the plant body, so the plant as a whole is never finished growing.
An everyday example. A lawn grows back after mowing because intercalary meristems near the bases of the grass leaves keep dividing.
The substance. An increase in size alone is not growth — a raisin swelling in water gets bigger, but drying would reverse it.
What are the lag, log and stationary phases, and how does arithmetic growth differ from geometric growth?
Growth usually starts slowly in a lag phase, speeds up in an exponential or log phase, and levels off in a stationary phase, giving an S-shaped sigmoid curve; in arithmetic growth only one daughter cell keeps dividing, giving a straight line, while in geometric growth both daughters divide, giving an exponential rise.
Phases of the sigmoid curve:
- Lag phase — slow growth at first
- Log (exponential) phase — rapid growth
- Stationary phase — growth slows and stops as nutrients become limited
Arithmetic growth. After each division, one daughter cell continues dividing while the other matures, as in a root elongating at a constant rate. Length against time is a straight line:
Geometric growth. Both daughter cells keep dividing, so growth is exponential:
Here is final size, initial size, the relative growth rate and the base of natural logarithms.
Worked example — arithmetic growth. A root 2 cm long grows 0.5 cm per day. After 6 days:
An everyday example. Moong seeds sprouting in a damp cloth grow slowly at first, then quickly, then level off.
The substance. In geometric growth, the final size depends on the initial size — a bigger start gives a bigger result.
Phases of the sigmoid curve:
- Lag phase — slow growth at first
- Log (exponential) phase — rapid growth
- Stationary phase — growth slows and stops as nutrients become limited
Arithmetic growth. After each division, one daughter cell continues dividing while the other matures, as in a root elongating at a constant rate. Length against time is a straight line:
Geometric growth. Both daughter cells keep dividing, so growth is exponential:
Here is final size, initial size, the relative growth rate and the base of natural logarithms.
Worked example — arithmetic growth. A root 2 cm long grows 0.5 cm per day. After 6 days:
An everyday example. Moong seeds sprouting in a damp cloth grow slowly at first, then quickly, then level off.
The substance. In geometric growth, the final size depends on the initial size — a bigger start gives a bigger result.
How do absolute and relative growth rates differ, and what conditions does growth need?
Absolute growth rate is the total increase per unit time, while relative growth rate is that increase per unit of initial size; growth needs water, oxygen, nutrients, a suitable temperature and, for some stages, light and gravity.
Worked example — two leaves. In one day, leaf A grows from 5 cm to 10 cm, and leaf B from 50 cm to 55 cm.
Both have the same absolute growth rate, but leaf A has a ten times higher relative growth rate.
Conditions for growth:
- Water — keeps cells turgid for enlargement and provides the medium for enzyme activity
- Oxygen — releases metabolic energy through respiration
- Nutrients — supply raw materials and energy for building protoplasm
- Temperature — each plant has an optimum range; extremes harm growth
- Light and gravity — influence certain phases of growth
An everyday example. A tiny tulsi seedling can double its leaf area in a few days, while a large plant adding the same area barely changes in relative terms.
The substance. Relative growth rate is usually highest in young organs, which is why seedlings seem to shoot up.
Worked example — two leaves. In one day, leaf A grows from 5 cm to 10 cm, and leaf B from 50 cm to 55 cm.
Both have the same absolute growth rate, but leaf A has a ten times higher relative growth rate.
Conditions for growth:
- Water — keeps cells turgid for enlargement and provides the medium for enzyme activity
- Oxygen — releases metabolic energy through respiration
- Nutrients — supply raw materials and energy for building protoplasm
- Temperature — each plant has an optimum range; extremes harm growth
- Light and gravity — influence certain phases of growth
An everyday example. A tiny tulsi seedling can double its leaf area in a few days, while a large plant adding the same area barely changes in relative terms.
The substance. Relative growth rate is usually highest in young organs, which is why seedlings seem to shoot up.
How do differentiation, dedifferentiation and redifferentiation differ, and what is plasticity?
Differentiation is when meristem cells mature to do specific jobs, dedifferentiation is when mature cells regain the power to divide, and redifferentiation is when the cells they produce mature again; development covers all these changes, and plasticity is a plant's ability to form different structures in response to environment or life stage.
The three processes:
- Differentiation — for example, a cell becoming a tracheary element loses its protoplasm and builds a strong, lignified wall
- Dedifferentiation — mature parenchyma cells start dividing again to form interfascicular cambium and cork cambium
- Redifferentiation — cells from these new meristems mature into secondary xylem, secondary phloem and cork
Development is the sum of growth and differentiation, from germination to senescence.
Plasticity and heterophylly. One plant can bear different leaf forms:
- Cotton, coriander and larkspur — juvenile leaves differ in shape from mature leaves
- **Buttercup (Ranunculus) — leaves under water are finely divided; leaves in air are broader
An everyday example. A coriander plant in a kitchen garden bears rounded lower leaves and feathery upper leaves on the same stem.
The substance. Differentiation in plants is open** — cells from the same meristem can mature into different tissues depending on where they end up.
The three processes:
- Differentiation — for example, a cell becoming a tracheary element loses its protoplasm and builds a strong, lignified wall
- Dedifferentiation — mature parenchyma cells start dividing again to form interfascicular cambium and cork cambium
- Redifferentiation — cells from these new meristems mature into secondary xylem, secondary phloem and cork
Development is the sum of growth and differentiation, from germination to senescence.
Plasticity and heterophylly. One plant can bear different leaf forms:
- Cotton, coriander and larkspur — juvenile leaves differ in shape from mature leaves
- **Buttercup (Ranunculus) — leaves under water are finely divided; leaves in air are broader
An everyday example. A coriander plant in a kitchen garden bears rounded lower leaves and feathery upper leaves on the same stem.
The substance. Differentiation in plants is open** — cells from the same meristem can mature into different tissues depending on where they end up.
Exam tip
What earns full marks on plant growth?
Draw and label the sigmoid curve with its three phases, and write each growth formula with every symbol defined.
- Growth: irreversible increase; open growth from meristems
- Meristems: apical for length, lateral for girth, intercalary in grasses
- Curve: sigmoid, with lag, log and stationary phases
- Formulas: arithmetic ; geometric
- Rates: absolute is total increase; relative is increase per unit initial size
- Differentiation: dedifferentiation gives interfascicular and cork cambium; redifferentiation gives secondary tissues
- Heterophylly: cotton, coriander, larkspur, Ranunculus
The trap. Comparing two organs by absolute growth alone. Equal absolute growth can hide very different relative growth rates.
- Growth: irreversible increase; open growth from meristems
- Meristems: apical for length, lateral for girth, intercalary in grasses
- Curve: sigmoid, with lag, log and stationary phases
- Formulas: arithmetic ; geometric
- Rates: absolute is total increase; relative is increase per unit initial size
- Differentiation: dedifferentiation gives interfascicular and cork cambium; redifferentiation gives secondary tissues
- Heterophylly: cotton, coriander, larkspur, Ranunculus
The trap. Comparing two organs by absolute growth alone. Equal absolute growth can hide very different relative growth rates.
Did you know
How can a small piece of carrot grow into a whole new plant?
In a tissue culture laboratory, a tiny piece of carrot root is placed on a jelly-like medium containing sugar, minerals and plant hormones.
Its mature cells dedifferentiate and divide into a shapeless mass called a callus. With the right hormone mix, cells in the callus then redifferentiate into shoots and roots, and a complete plant develops.
This ability of a living plant cell to give rise to a whole plant is called totipotency, and nurseries use it to raise large numbers of identical banana and orchid plantlets.
Its mature cells dedifferentiate and divide into a shapeless mass called a callus. With the right hormone mix, cells in the callus then redifferentiate into shoots and roots, and a complete plant develops.
This ability of a living plant cell to give rise to a whole plant is called totipotency, and nurseries use it to raise large numbers of identical banana and orchid plantlets.
Exam relevance
How is plant growth tested in NEET?
Plant Growth and Development is part of the Plant Physiology unit of NEET Biology, and its first half mixes definitions with graphs and formulas.
What gets asked. Types of meristems, phases of the sigmoid curve, arithmetic versus geometric growth equations, absolute versus relative growth rate, conditions for growth, examples of dedifferentiation and redifferentiation, and heterophylly examples. Meristems and cambia link back to Anatomy of Flowering Plants, and totipotency returns in Strategies for Enhancement in Food Production.
Question types. Statement-based questions, graph-identification questions and match-the-column lists.
The trap that costs marks. Calling interfascicular cambium a product of redifferentiation — it forms by dedifferentiation.
What gets asked. Types of meristems, phases of the sigmoid curve, arithmetic versus geometric growth equations, absolute versus relative growth rate, conditions for growth, examples of dedifferentiation and redifferentiation, and heterophylly examples. Meristems and cambia link back to Anatomy of Flowering Plants, and totipotency returns in Strategies for Enhancement in Food Production.
Question types. Statement-based questions, graph-identification questions and match-the-column lists.
The trap that costs marks. Calling interfascicular cambium a product of redifferentiation — it forms by dedifferentiation.
Key takeaways
What must you be able to do from this part?
- Growth: irreversible increase in size; open growth from apical, lateral and intercalary meristems
- Growth curves: sigmoid curve with lag, log and stationary phases; arithmetic growth linear, geometric growth exponential
- Growth rates: absolute is total increase per unit time; relative divides by initial size; growth needs water, oxygen, nutrients and a suitable temperature
- Differentiation: dedifferentiation forms interfascicular and cork cambium; redifferentiation forms secondary tissues; heterophylly shows plasticity
Leaf P grows from cm to cm and leaf Q from cm to cm in a week. Compare their absolute and relative growth rates.
- Growth curves: sigmoid curve with lag, log and stationary phases; arithmetic growth linear, geometric growth exponential
- Growth rates: absolute is total increase per unit time; relative divides by initial size; growth needs water, oxygen, nutrients and a suitable temperature
- Differentiation: dedifferentiation forms interfascicular and cork cambium; redifferentiation forms secondary tissues; heterophylly shows plasticity
Leaf P grows from cm to cm and leaf Q from cm to cm in a week. Compare their absolute and relative growth rates.