Plant Growth

BiologyPlant Growth and DevelopmentFor NEET aspirants

Plant growth is an irreversible, permanent increase in the size of an organ, its parts or even a single cell. This page covers seed germination, why plant growth is indeterminate, how growth is measured, the three phases of growth, arithmetic and geometric growth rates, the conditions for growth, differentiation, dedifferentiation, redifferentiation and development with plasticity. It follows the NCERT Class 11 chapter Plant Growth and Development. NEET asks this topic as match-the-list, statement and sequence questions on growth phases, equations and examples.

On this page1Growth and development2Growth3Indeterminate growth4Measuring growth5Phases of growth6Growth rates7Conditions for growth8Differentiation9Development and plasticity10Exam essentials11Quick revision12Solved examples13Practice

Key Points at a Glance

★ Must learn Growth: an irreversible, permanent increase in size of an organ, its parts or a single cell.
Development = growth + differentiation; it covers all changes from seed germination to senescence.
★ Must learn Plant growth is indeterminate (open), because meristems keep adding new cells throughout life.
Growth is measured as fresh weight, dry weight, length, area, volume or cell number.
★ Must learn Three phases of growth: meristematic, elongation, maturation.
Arithmetic growth: $L_{t} = L_{0} + r t$ , a straight line; geometric growth: $W_{1} = W_{0} e^{r t}$ in the exponential phase, an S-shaped curve overall.
★ Must learn Sigmoid curve phases: lag, log (exponential), stationary.
Relative growth rate: growth per unit time per unit initial size; absolute growth rate: total growth per unit time.
★ Must learn Dedifferentiation: living differentiated cells regain the capacity to divide, e.g. interfascicular cambium and cork cambium.
Plasticity: different structures formed in response to environment or phase of life, e.g. heterophylly in larkspur and buttercup.

1. Growth and Development: An Overview

1.1 From zygote to mature plant

A plant passes through the stages of seed, seedling, plantlet and mature plant.
Roots, stems, leaves, flowers, fruits and seeds arise in an orderly sequence during the life of a plant.
All plant organs are made up of a variety of tissues.
Trees keep increasing in height or girth over a period of time.
Leaves, flowers and fruits of the same tree have limited dimensions; they appear and fall periodically, sometimes repeatedly.
In a plant, the vegetative phase precedes flowering.
All cells of a plant are descendants of the zygote, yet they differ in structure and function.
Development is the sum of two processes: growth and differentiation.
Development of a mature plant from a zygote (fertilised egg) follows a precise and highly ordered succession of events.
This forms a complex body that produces roots, leaves, branches, flowers, fruits and seeds, and eventually they die.
The factors that govern development are both intrinsic (internal) and extrinsic (external) to the plant.

1.2 Seed germination: the first step

★ Exam imp The first step in the process of plant growth is seed germination.
A seed germinates when favourable conditions for growth exist in the environment.
Without favourable conditions, seeds do not germinate; they enter a period of suspended growth or rest.
When favourable conditions return, the seeds resume metabolic activities and growth takes place.

Test yourself: hide labels

Figure 1: Germination and seedling development in bean. The hypocotyl grows and lifts the cotyledons above the soil line; the epicotyl then grows up and bears the first leaves.

Key idea

Development = growth + differentiation, and it begins with seed germination under favourable conditions.

2. Growth

★ Very important Growth: an irreversible, permanent increase in size of an organ or its parts, or even of an individual cell.

Growth is one of the most fundamental and conspicuous characteristics of a living being.
★ Exam imp Growth is generally accompanied by metabolic processes, both anabolic (building up) and catabolic (breaking down).
These metabolic processes occur at the expense of energy.
Example: the expansion of a leaf is growth.

Tips and Tricks

Check two things before calling an increase growth: it must be irreversible, and it is generally accompanied by metabolism. A piece of wood swelling in water fails on both counts. The swelling is reversible, and the dead wood carries out no metabolism, so it is not growth.

2.1 Plant growth generally is indeterminate

★ Exam imp Plants retain the capacity for unlimited growth throughout their life.
This ability is due to meristems present at certain locations in the plant body.
Meristem: a group of cells that can divide and self-perpetuate (keep producing more meristem cells).
The cells produced by a meristem soon lose the capacity to divide, and such cells make up the plant body.

Open form of growth: growth in which new cells are always being added to the plant body by the activity of the meristem.

The root apical meristem and the shoot apical meristem are responsible for primary growth.
Primary growth mainly elongates the plant along its axis.
★ Exam imp In dicotyledonous plants and gymnosperms, the lateral meristems (vascular cambium and cork-cambium) appear later in life.
Lateral meristems increase the girth of the organs in which they are active; this is secondary growth.
Root and shoot apical meristems, sometimes along with the intercalary meristem, contribute to elongation of the plant axes.
In plants, meristems are the sites of growth; growth is indeterminate in higher plants.

Meristem	Location	Type of growth	Result
Shoot apical meristem	Shoot tip	Primary	Elongation of shoot along its axis
Root apical meristem	Root tip	Primary	Elongation of root along its axis
Intercalary meristem	Within the axis	Primary	Adds to elongation of the axis
Vascular cambium (lateral)	Inside stem and root	Secondary	Increase in girth
Cork-cambium (lateral)	Outer region of the organ	Secondary	Increase in girth

Test yourself: hide labels

Figure 2: Locations of the apical meristems and the vascular cambium. Arrows show the direction of growth: the apical meristems lengthen the axis, while the vascular cambium increases girth.

Memory Trick

Apical for Axis, Lateral for Large girth. Apical meristems (root and shoot tips) lengthen the axis; lateral meristems (vascular cambium and cork-cambium) widen the organ.

Key idea

Meristems keep adding cells, so plant growth is open: apical meristems lengthen the plant and lateral meristems thicken it.

2.2 Growth is measurable

★ Exam imp At the cellular level, growth is principally a consequence of an increase in the amount of protoplasm.
Protoplasm is difficult to measure directly, so a quantity more or less proportional to it is measured.
Growth is therefore measured by a variety of parameters.

Parameters used to measure growth

Fresh weight and dry weight
Length, area and volume
Cell number

Memory Trick

Two weights, three sizes, one count. Fresh and dry weight; length, area and volume; and cell number. That gives all six parameters.

Example	How its growth is expressed
One maize root apical meristem	Increase in cell number: more than 17,500 new cells per hour
Cells in a watermelon	Increase in cell size: up to 3,50,000 times
Pollen tube	Increase in length
Dorsiventral leaf	Increase in surface area

Numbers to Remember

More than 17,500 new cells per hour: produced by one single maize root apical meristem.
Up to 3,50,000 times: increase in the size of cells in a watermelon.

Tips and Tricks

Match the example to what grows in it. Many cells forming (maize root tip) means cell number. Cells swelling (watermelon) means cell size. A thin tube growing (pollen tube) means length. A flat organ spreading (leaf) means area.

Key idea

Growth is an increase in protoplasm, measured indirectly through weight, size or cell number.

2.3 Phases of growth

The period of growth is generally divided into three phases: meristematic, elongation and maturation.
These phases are easiest to understand by looking at root tips.
Proximal: just next to a region, on the side away from the tip.

The three phases, from the apex backwards

Meristematic phase. Constantly dividing cells at the root apex and the shoot apex. They are rich in protoplasm and have large, conspicuous nuclei. Their cell walls are primary, thin and cellulosic, with abundant plasmodesmatal connections.
Elongation phase. Cells just proximal to the meristematic zone. They show increased vacuolation, cell enlargement and deposition of new cell wall.
Maturation phase. Further away from the apex, proximal to the elongation zone. Cells attain their maximal size in terms of wall thickening and protoplasmic modifications. Most tissues and cell types of a plant represent this phase.

Feature	Meristematic	Elongation	Maturation
Position	At the root and shoot apex	Just behind the meristematic zone	Further back, behind the elongation zone
Key features	Rich protoplasm, large nuclei	Vacuolation, enlargement	Maximal wall thickening
Cell wall	Primary, thin, cellulosic; many plasmodesmata	New wall deposited	Wall thickening at its maximum

Figure 3: Detection of zones of elongation by the parallel line technique. The root is marked with equally spaced lines (left). After growth (right), zones A, B, C and D just behind the apex have elongated most.

Parallel line technique: a young root is marked with equally spaced lines, and the spacing is checked after growth.
★ Exam imp The marks just behind the apex move farthest apart: zones A, B, C and D immediately behind the apex have elongated most.

Memory Trick

Make, Enlarge, Mature. Meristematic cells make new cells, elongating cells enlarge, and maturing cells mature. The order runs from the apex backwards.

Key idea

At a root tip, cells divide at the apex, enlarge just behind it, and mature further back.

2.4 Growth rates

Growth rate: the increased growth per unit time. It can be expressed mathematically.

An organism, or a part of it, can produce more cells in a variety of ways.
Growth is generally not sustained at a high rate throughout the life of a cell, tissue, organ or organism.
The increase in growth rate may be arithmetic or geometric.

Test yourself: hide labels

Figure 4: (a) Arithmetic and (b) geometric growth, and (c) stages of embryo development showing the geometric and arithmetic phases. Dark cells can divide; light cells have lost the capacity to divide.

(a) Arithmetic growth

★ Exam imp After mitotic cell division, only one daughter cell continues to divide; the other differentiates and matures.
The simplest example is a root elongating at a constant rate.
Plotting the length of the organ against time gives a linear curve (a straight line).

L_{t} = L_{0} + r t

$L_{t}$ = length at time $t$
$L_{0}$ = length at time zero
$r$ = growth rate, the elongation per unit time

Figure 5: Constant linear growth, a plot of length

L

against time

t

. The straight line shows arithmetic growth:

L_{t} = L_{0} + r t

, with intercept

L_{0}

and slope

r

(b) Geometric growth

★ Exam imp After mitotic cell division, both progeny cells retain the ability to divide and continue to do so.
In most systems, initial growth is slow, then increases rapidly at an exponential rate.
With a limited nutrient supply, growth slows down.
A plot of the growth parameter against time gives a typical sigmoid or S-curve.
A sigmoid curve is characteristic of a living organism growing in a natural environment.
It is typical for all cells, tissues and organs of a plant.

Phases of the sigmoid curve

Lag phase: initial growth is slow.
Log or exponential phase: growth increases rapidly, at an exponential rate.
Stationary phase: growth slows down, as the nutrient supply is limited.

Test yourself: hide labels

Figure 6: An idealised sigmoid (S-shaped) growth curve, typical of cells in culture and of many higher plants and plant organs: lag phase, exponential (log) phase, then stationary phase.

Exponential growth is expressed as:

W_{1} = W_{0} e^{r t}

$W_{1}$ = final size (weight, height, number, etc.)
$W_{0}$ = initial size at the beginning of the period
$r$ = growth rate; $t$ = time of growth
$e$ = base of natural logarithms

★ Exam imp Here $r$ is the relative growth rate.
$r$ also measures the ability of the plant to produce new plant material; this is called the efficiency index.
Hence the final size $W_{1}$ depends on the initial size $W_{0}$ .

Arithmetic growth

Only one daughter cell keeps dividing
$L_{t} = L_{0} + r t$
Straight-line graph
Example: root elongating at a constant rate

Geometric growth

Both daughter cells keep dividing
$W_{1} = W_{0} e^{r t}$
Sigmoid (S-shaped) graph
Typical of cells, tissues and organs in nature

Memory Trick

Sigmoid phases in order: Lag, Log, Stationary. Think of a runner: slow start, fast middle, then a steady finish.

NEET Focus

Do not mix up the different sets of three phases. The phases of growth at a root tip are meristematic, elongation and maturation. The phases of the sigmoid curve are lag, log (exponential) and stationary. The three principal phases of growth over a life are also stated as the lag, log and senescent phase.

2.5 Absolute and relative growth rates

Growth of living systems can be compared quantitatively in two ways.
★ Exam imp Absolute growth rate: measurement and comparison of total growth per unit time.
★ Exam imp Relative growth rate: growth of the system per unit time expressed on a common basis, e.g. per unit initial parameter.

Figure 7: Absolute and relative growth rates. Leaves A and B both gain

5 c m^{2}

in the same time (equal absolute growth rate), but A grows by 100% and B by only 10%, so A has the much higher relative growth rate. Areas are drawn to scale.

Leaf	Initial area	Final area	Absolute increase	Relative increase
A	$5 c m^{2}$	$10 c m^{2}$ ( $A^{1}$ )	$5 c m^{2}$	$5 \div 5 = 1$ , i.e. 100%
B	$50 c m^{2}$	$55 c m^{2}$ ( $B^{1}$ )	$5 c m^{2}$	$5 \div 50 = 0.1$ , i.e. 10%

Both leaves show the same absolute increase in area in the given time.
★ Exam imp Leaf A shows the much higher relative growth rate, because it gains the same area on a smaller initial size.

Equal absolute growth does not mean equal relative growth. The smaller leaf A doubles in area, so its relative growth rate is ten times that of leaf B.

Key idea

Arithmetic growth gives a straight line and geometric growth an S-curve; relative growth rate is growth per unit time per unit initial size.

2.6 Conditions for growth

Water, oxygen and nutrients are very essential elements for growth.

Conditions and their roles

Water. Plant cells grow in size by cell enlargement, which requires water. Turgidity of cells helps in extension growth. Growth and development are intimately linked to the water status of the plant. Water also provides the medium for enzymatic activities.
Oxygen. It helps release the metabolic energy essential for growth activities.
Nutrients. Macro and micro essential elements are required for the synthesis of protoplasm, and they act as a source of energy.
Temperature. Every plant has an optimum temperature range best suited for its growth; any deviation could be detrimental to its survival.
Environmental signals. Light and gravity also affect certain phases or stages of growth.

Memory Trick

Without Oxygen, Nothing Thrives; Light Guides. Water, Oxygen, Nutrients, Temperature, Light and Gravity: the six conditions for growth.

Key idea

Growth needs water for cell enlargement, oxygen for energy, nutrients for protoplasm, a suitable temperature, and signals such as light and gravity.

Quick Recall: tap to check

Is the swelling of a piece of wood in water growth?

No. The increase is reversible and involves no metabolism in the dead wood.

Which phase of growth has cells with large, conspicuous nuclei and thin cellulosic walls?

The meristematic phase.

What would happen if the meristem stopped dividing?

No new cells would be added by it, so the open form of growth in that organ would stop.

W_{1} = W_{0} e^{r t}

, what is

r

called?

The relative growth rate, also the efficiency index.

Leaves A (

5 c m^{2}

) and B (

50 c m^{2}

) each gain

5 c m^{2}

. Which has the higher relative growth rate?

Leaf A: 100% against 10% for leaf B.

In Figure 2, which part, shown by dotted lines in both the shoot and the root, increases girth?

The vascular cambium, a lateral meristem.

3. Differentiation, Dedifferentiation and Redifferentiation

3.1 Differentiation

Differentiation: the process by which cells derived from the root apical meristem, shoot apical meristem and cambium mature to perform specific functions.

When a cell loses the capacity to divide, it leads to differentiation.
During differentiation, cells undergo few to major structural changes in both their cell walls and protoplasm.
★ Exam imp Example: to form a tracheary element, the cells lose their protoplasm.
They also develop very strong, elastic, lignocellulosic secondary cell walls.
These walls let them carry water to long distances, even under extreme tension.
Differentiation produces structures that suit the function the cell finally has to perform.

3.2 Dedifferentiation

★ Very important Dedifferentiation: the regaining of the capacity to divide, under certain conditions, by living differentiated cells that had lost it.

★ Exam imp Example: formation of the meristems interfascicular cambium and cork cambium from fully differentiated parenchyma cells.

3.3 Redifferentiation

Redifferentiation: the maturing, to perform specific functions, of cells produced by dedifferentiated meristems; these cells once again lose the capacity to divide.

The sequence

Differentiation: meristem-derived cells mature and lose the capacity to divide.
Dedifferentiation: some living differentiated cells regain the capacity to divide.
Redifferentiation: the cells they produce lose the capacity to divide again and mature for specific functions.

Process	Capacity to divide	Example
Differentiation	Lost as cells mature	Formation of a tracheary element
Dedifferentiation	Regained by living differentiated cells	Interfascicular cambium and cork cambium from parenchyma
Redifferentiation	Lost again as new cells mature	Cells produced by these new meristems

Memory Trick

DE = Divide again; RE = REturn to a job. In dedifferentiation, cells regain division; in redifferentiation, the new cells return to a specialised function.

3.4 Growth and differentiation in plants are open

Growth in plants is open: it can be indeterminate or determinate.
Determinate growth: growth that stops after a limited size is reached, as in leaves, flowers and fruits.
★ Exam imp Differentiation in plants is also open: cells or tissues arising from the same meristem have different structures at maturity.
The final structure of a cell or tissue is also decided by the location of the cell.
Cells positioned away from the root apical meristem differentiate as root-cap cells.
Cells pushed to the periphery mature as epidermis.
General principles of differentiation are similar for cells, tissues and organs.
Because differentiation is open, development in plants is also flexible.

Key idea

Differentiated plant cells can divide again (dedifferentiation) and then specialise again (redifferentiation); position decides what a cell becomes.

Quick Recall: tap to check

What would you call parenchyma cells made to divide in plant tissue culture?

Dedifferentiated cells; they form a callus, a mass of undifferentiated cells.

Give two meristems formed by dedifferentiation.

Interfascicular cambium and cork cambium, from fully differentiated parenchyma cells.

Which tissues in a woody dicot are products of redifferentiation?

The tissues produced by the interfascicular cambium and cork cambium, such as secondary xylem, secondary phloem and cork.

Cells pushed to the periphery near a root apex mature into which tissue?

Epidermis.

4. Development

★ Very important Development: all the changes that an organism goes through during its life cycle, from germination of the seed to senescence.

Test yourself: hide labels

Figure 8: Sequence of the developmental process in a plant cell: meristematic cell, plasmatic growth and cell division, expansion (elongation), differentiation and maturation to a mature cell, then senescence and death. The same sequence applies to tissues and organs.

Sequence of the developmental process in a plant cell

Meristematic cell: undergoes plasmatic growth; cell division keeps producing meristematic cells.
Expansion (elongation) of the cell.
Differentiation and maturation into a mature cell.
Senescence (ageing) of the mature cell.
Death.

The same sequence also applies to tissues and organs.

Memory Trick

My Plant Eventually Develops, Matures, Senesces, Dies. Meristematic cell, Plasmatic growth, Expansion, Differentiation, Maturation, Senescence, Death.

4.1 Plasticity and heterophylly

★ Very important Plasticity: the ability of plants to follow different pathways, in response to the environment or the phases of life, to form different kinds of structures.

Heterophylly: the occurrence of leaves of different shapes on the same plant.
★ Exam imp Heterophylly is an example of plasticity.
In cotton, coriander and larkspur, leaves of the juvenile plant differ in shape from those of the mature plant.
In buttercup, leaves produced in air differ in shape from those produced in water.
★ Exam imp Heterophylly in buttercup is due to the environment; in larkspur it is due to the phase of life.

Plant	Cause of heterophylly	Leaves that differ
Cotton	Phase of life	Juvenile and mature leaves
Coriander	Phase of life	Juvenile and mature leaves
Larkspur	Phase of life	Juvenile and adult leaves
Buttercup	Environment	Leaves in air (terrestrial) and in water

Test yourself: hide labels

Figure 9: Heterophylly in (a) larkspur, where juvenile and adult leaves differ, and (b) buttercup, where leaves formed in air and in water differ. Both are examples of plasticity.

Memory Trick

Phase of life: Cotton, Coriander, Larkspur change their leaves as they age. Environment: Buttercup, B for below water.

4.2 Control of development

Growth, differentiation and development are very closely related events in the life of a plant.
★ Exam imp Broadly, development is the sum of growth and differentiation.
Development in plants is under the control of intrinsic and extrinsic factors.
Plants exhibit plasticity in development.

Factor	Type	Examples
Intrinsic	Intracellular	Genetic factors
Intrinsic	Intercellular	Chemicals such as plant growth regulators (PGRs)
Extrinsic	External	Light, temperature, water, oxygen, nutrition, etc.

Key idea

Development runs from germination to senescence; plasticity lets one plant form different leaves with age or habitat.

5. Exam Essentials

Pairs to Match

List I	List II
Seed germination	First step in plant growth
Vascular cambium and cork-cambium	Secondary growth; increase in girth
One maize root apical meristem	More than 17,500 new cells per hour
Watermelon cells	Increase in size up to 3,50,000 times
Pollen tube	Growth measured as length
Dorsiventral leaf	Growth measured as surface area
Meristematic phase	Large nuclei; thin, cellulosic primary walls
Elongation phase	Increased vacuolation; new cell wall deposition
Maturation phase	Maximal wall thickening and protoplasmic modifications
Arithmetic growth	$L_{t} = L_{0} + r t$ ; linear curve
Geometric growth	$W_{1} = W_{0} e^{r t}$ in the log phase; sigmoid curve overall
$r$ in exponential growth	Relative growth rate; efficiency index
Tracheary element	Loses protoplasm; lignocellulosic secondary walls
Interfascicular cambium	Dedifferentiation of parenchyma
Buttercup	Heterophylly due to environment

Exceptions

Swelling of wood in water is not growth: it is reversible and involves no metabolism.
Cells produced by a meristem soon lose the capacity to divide; only the meristem cells keep dividing and self-perpetuating.
In arithmetic growth only one daughter cell keeps dividing; in geometric growth both do.
Lateral meristems appear later in life, in dicotyledonous plants and gymnosperms.
A tracheary element loses its protoplasm when it differentiates.
Heterophylly in buttercup is due to the environment, unlike cotton, coriander and larkspur, where it is due to the phase of life.

NEET Focus

Two traps appear often. First, a statement may place the elongation zone distal to (beyond) the meristematic zone; it is actually proximal, on the side away from the tip. Second, a question may give two leaves with the same absolute gain; the smaller leaf always has the higher relative growth rate.

6. Quick Revision

Development is the sum of growth and differentiation, from seed germination to senescence.
Seed germination is the first step of plant growth; without favourable conditions, seeds rest.
Growth is an irreversible, permanent increase in size, generally accompanied by metabolism at the expense of energy.
Plant growth is indeterminate (open) because meristems keep adding new cells.
Apical meristems give primary growth (length); vascular cambium and cork-cambium give secondary growth (girth).
Growth is an increase in protoplasm, measured as fresh weight, dry weight, length, area, volume or cell number.
Maize root apical meristem: more than 17,500 new cells per hour; watermelon cells: up to 3,50,000 times larger.
Phases of growth: meristematic, elongation and maturation.
Arithmetic growth: one daughter cell keeps dividing; $L_{t} = L_{0} + r t$ ; a straight line.
Geometric growth: both daughter cells keep dividing; the log phase follows $W_{1} = W_{0} e^{r t}$ ; overall a sigmoid curve with lag, log and stationary phases.
$r$ is the relative growth rate (efficiency index); $W_{1}$ depends on $W_{0}$ .
Absolute growth rate is total growth per unit time; relative growth rate is growth per unit time per unit initial size.
Water, oxygen, nutrients, an optimum temperature, light and gravity affect growth.
Dedifferentiation: differentiated cells regain division (interfascicular cambium, cork cambium); redifferentiation follows.
Plasticity, such as heterophylly in larkspur (phase of life) and buttercup (environment), shows flexible development.

7. Solved Examples

Solved Example 1

Match List I with List II.
List I (Example): A. Maize root apical meristem; B. Watermelon cells; C. Pollen tube; D. Dorsiventral leaf
List II (Growth expressed as): I. Surface area; II. Length; III. Cell size; IV. Cell number
(A) A-IV, B-III, C-II, D-I
(B) A-III, B-IV, C-I, D-II
(C) A-IV, B-III, C-I, D-II
(D) A-II, B-I, C-IV, D-III

Solution:

Answer: (A). The maize root apical meristem forms more than 17,500 new cells per hour (cell number). Watermelon cells grow up to 3,50,000 times in size (cell size). A pollen tube grows in length, and a dorsiventral leaf grows in surface area.

Solved Example 2

Read the statements about the phases of growth.
A. Cells of the meristematic phase have thick, lignified secondary walls.
B. Increased vacuolation is a feature of the elongation phase.
C. Cells of the maturation phase attain maximal wall thickening.
D. Meristematic cells have abundant plasmodesmatal connections.
E. The elongation zone lies distal to the meristematic zone.
Choose the correct answer.
(A) A, B and C only
(B) B, C and D only
(C) B and E only
(D) C, D and E only

Solution:

Answer: (B). A is false: meristematic cell walls are primary, thin and cellulosic. B, C and D are true. E is false: the elongation zone is proximal to the meristematic zone, just next to it and away from the tip.

Solved Example 3

Arrange the stages of development of a plant cell in the correct sequence.
A. Expansion (elongation) B. Meristematic cell C. Senescence D. Plasmatic growth E. Maturation
(A) B, D, A, E, C
(B) B, A, D, E, C
(C) D, B, A, C, E
(D) B, D, E, A, C

Solution:

Answer: (A). A meristematic cell undergoes plasmatic growth, then expansion (elongation), then differentiation and maturation into a mature cell. Senescence follows, ending in death.

Solved Example 4

A root is 4 cm long and elongates at a constant rate of 0.5 cm per day. What is its length after 6 days, and which type of growth does it show?
(A) 7 cm, arithmetic
(B) 7 cm, geometric
(C) 10 cm, arithmetic
(D) 24 cm, geometric

Solution:

Answer: (A). Constant elongation is arithmetic growth, so $L_{t} = L_{0} + r t$ . Here $L_{t} = 4 + 0.5 \times 6 = 7$ cm.

Solved Example 5

Statement I: Dedifferentiation is the regaining of the capacity to divide by living differentiated cells.
Statement II: Interfascicular cambium is formed from fully differentiated parenchyma cells.
(A) Both Statement I and Statement II are correct
(B) Both Statement I and Statement II are incorrect
(C) Statement I is correct but Statement II is incorrect
(D) Statement I is incorrect but Statement II is correct

Solution:

Answer: (A). Both are correct. The formation of interfascicular cambium and cork cambium from parenchyma is the standard example of dedifferentiation.

Solved Example 6

Which one of the following pairs is NOT correctly matched?
(A) Larkspur: heterophylly due to phase of life
(B) Buttercup: heterophylly due to phase of life
(C) Coriander: juvenile and mature leaves differ
(D) Heterophylly: an example of plasticity

Solution:

Answer: (B). In buttercup, leaves formed in air and in water differ, so its heterophylly is due to the environment, not the phase of life.

Practice Questions

Match List I with List II.
List I: A. Lag phase; B. Exponential phase; C. Stationary phase; D. Efficiency index.
List II: I. Growth slows as nutrients become limited; II. Initial slow growth; III. Relative growth rate $r$ ; IV. Rapid growth as all progeny cells divide.
(A) A-II, B-IV, C-I, D-III
(B) A-IV, B-II, C-I, D-III
(C) A-II, B-IV, C-III, D-I
(D) A-I, B-IV, C-II, D-IIIAnswer: (A). Lag is the slow start, log is rapid division, stationary is the slowdown, and $r$ is the efficiency index.
Which of the following statements are correct?
A. Growth is reversible.
B. Growth at the cellular level is mainly an increase in protoplasm.
C. Oxygen helps release metabolic energy for growth.
D. Turgidity helps extension growth.
(A) A, B and C only
(B) B, C and D only
(C) A and D only
(D) A, B, C and DAnswer: (B). Growth is irreversible, so A is false; B, C and D are true.
Arrange in the correct sequence.
A. Redifferentiation; B. Differentiation; C. Dedifferentiation.
(A) B, C, A
(B) C, B, A
(C) A, B, C
(D) B, A, CAnswer: (A). Cells differentiate first, may dedifferentiate later, and their products then redifferentiate.
Which is NOT a parameter used to measure growth?
(A) Dry weight
(B) Cell number
(C) Volume
(D) Colour of the leafAnswer: (D). Growth is measured by fresh weight, dry weight, length, area, volume and cell number.
Two leaves of $10 c m^{2}$ and $40 c m^{2}$ each gain $4 c m^{2}$ in a week. Which has the higher relative growth rate, and by how much?Answer: The $10 c m^{2}$ leaf: 40% against 10%, so four times higher. Absolute growth is the same.
Statement I: In arithmetic growth, both daughter cells continue to divide.
Statement II: A plot of arithmetic growth against time is a straight line.
(A) Both correct
(B) Both incorrect
(C) I correct, II incorrect
(D) I incorrect, II correctAnswer: (D). Only one daughter cell keeps dividing in arithmetic growth; its graph is linear.
Define growth, differentiation, development, dedifferentiation, redifferentiation, determinate growth, meristem and growth rate.Answer: Growth: irreversible permanent increase in size. Differentiation: maturation of cells for specific functions. Development: all changes from germination to senescence. Dedifferentiation: differentiated cells regain division. Redifferentiation: their products mature again for specific functions. Determinate growth: growth that stops at a limited size, as in leaves, flowers and fruits. Meristem: cells that divide and self-perpetuate. Growth rate: increased growth per unit time.
Why is no single parameter good enough to demonstrate growth throughout the life of a flowering plant?Answer: Different parts grow in different ways: a root tip by cell number, fruit cells by size, a pollen tube by length and a leaf by area. So no one parameter fits every organ and every stage.
Describe briefly arithmetic growth, geometric growth, the sigmoid growth curve, and absolute and relative growth rates.Answer: Arithmetic: one daughter cell divides, $L_{t} = L_{0} + r t$ , linear. Geometric: both divide, $W_{1} = W_{0} e^{r t}$ . Sigmoid curve: lag, log and stationary phases. Absolute rate: total growth per unit time; relative rate: growth per unit time per unit initial size.
'Both growth and differentiation in higher plants are open.' Comment.Answer: Growth is open because meristems keep adding cells, so it can be indeterminate or determinate. Differentiation is open because cells from the same meristem mature into different structures depending on their position, such as root-cap cells and epidermis.
What would be expected to happen if dividing cells stop differentiating?Answer: They would keep dividing without maturing into specialised tissues, forming a mass of undifferentiated cells, like a callus, instead of organised organs.

Common Mistakes to Avoid

Watch out

Calling the swelling of wood in water growth; growth is irreversible and is generally accompanied by metabolism.
Placing the elongation zone distal to the meristematic zone; it is proximal, just behind the meristematic zone.
Writing that both daughter cells divide in arithmetic growth; only one does, while the other matures.
Mixing the growth phases (meristematic, elongation, maturation) with the sigmoid phases (lag, log, stationary).
Judging relative growth by absolute gain; leaves A and B gain the same $5 c m^{2}$ , but A has the higher relative rate.
Reversing dedifferentiation: it is the regaining, not the loss, of the capacity to divide.
Giving buttercup heterophylly as a phase-of-life example; it is due to the environment (air and water).
Treating $r$ in $W_{1} = W_{0} e^{r t}$ as absolute growth rate; it is the relative growth rate, or efficiency index.

Frequently Asked Questions

What is growth in plants?

Growth is an irreversible, permanent increase in the size of an organ, its parts or even a single cell. It is generally accompanied by anabolic and catabolic processes that use energy. The expansion of a leaf is growth, but the swelling of wood in water is not, because it is reversible and involves no metabolism.

Why is plant growth called indeterminate or open?

Plants keep the capacity for unlimited growth throughout life because of meristems at certain locations. Meristem cells divide and self-perpetuate, so new cells are always added to the plant body. This is called the open form of growth. Apical meristems add length and lateral meristems add girth.

What are the three phases of growth?

The phases are meristematic, elongation and maturation. Meristematic cells at the apex divide constantly and have large nuclei and thin walls. Cells just behind them elongate with increased vacuolation and new wall deposition. Further back, cells mature with maximal wall thickening and protoplasmic modifications.

What is the difference between arithmetic and geometric growth?

In arithmetic growth only one daughter cell keeps dividing, giving a straight line, $L_{t} = L_{0} + r t$ . In geometric growth both daughter cells keep dividing, giving an exponential rise, $W_{1} = W_{0} e^{r t}$ , and an S-shaped curve with lag, log and stationary phases when nutrients become limited.

What is the difference between absolute and relative growth rate?

Absolute growth rate is the total growth per unit time. Relative growth rate is the growth per unit time on a common basis, such as per unit initial size. If leaves of $5 c m^{2}$ and $50 c m^{2}$ each gain $5 c m^{2}$ , their absolute rates are equal, but the smaller leaf has a relative rate ten times higher.

What are dedifferentiation and redifferentiation?

Dedifferentiation is the regaining of the capacity to divide by living differentiated cells, as when parenchyma forms interfascicular cambium and cork cambium. Redifferentiation follows: the cells these meristems produce lose the capacity to divide again and mature to perform specific functions.

What is plasticity in plants? Give an example.

Plasticity is the ability of plants to follow different pathways in response to the environment or phases of life and form different structures. Heterophylly is an example: juvenile and mature leaves differ in cotton, coriander and larkspur, while buttercup forms different leaves in air and in water.

Which points of Plant Growth are most important for NEET?

Every line of the NCERT chapter can be asked. Common points are the definition of growth, open growth, the six growth parameters with the maize and watermelon numbers, the three phases of growth, the arithmetic and exponential equations, the sigmoid curve, relative growth rate, dedifferentiation examples and heterophylly.

Previous year questions on Plant Growth

5 questions from past papers, each with a step-by-step solution.

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