NCERT Textbook: Plant Growth & Development | Biology Class 11 - NEET PDF Download

 Page 1


166 BIOLOGY
You have already studied the organisation of a flowering plant in Chapter
5. Have you ever thought about where and how the structures like roots,
stems, leaves, flowers, fruits and seeds arise and that too in an orderly
sequence? You are, by now, aware of the terms seed, seedling, plantlet,
mature plant. You have also seen that trees continue to increase in height
or girth over a period of time. However, the leaves, flowers and fruits of the
same tree not only have limited dimensions but also appear and fall
periodically and some time repeatedly. Why does vegetative phase precede
flowering in a plant? All plant organs are made up of a variety of tissues; is
there any relationship between the structure of a cell, a tissue, an organ
and the function they perform? Can the structure and the function of these
be altered? All cells of a plant are descendents of the zygote. The question
is, then, why and how do they have different structural and functional
attributes? Development is the sum of two processes: growth and
differentiation. To begin with, it is essential and sufficient to know that the
development of a mature plant from a zygote (fertilised egg) follow a precise
and highly ordered succession of events. During this process a complex
body organisation is formed that produces roots, leaves, branches, flowers,
fruits, and seeds, and eventually they die (Figure 13.1). The first step in the
process of plant growth is seed germination. The seed germinates when
favourable conditions for growth exist in the environment. In absence of
such favourable conditions the seeds do not germinate and goes into a
period of suspended growth or rest. Once favourable conditions return,
the seeds resume metabolic activities and growth takes place.
In this chapter, you shall also study some of the factors which
govern and control these developmental processes. These factors are both
intrinsic (internal) and extrinsic (external) to the plant.
PLANT GROWTH AND DEVELOPMENT
CHAPTER  13
13.1 Growth
13.2 Differentiation,
Dedifferentiation
and
Redifferentiation
13.3 Development
13.4 Plant Growth
Regulators
2024-25
Page 2


166 BIOLOGY
You have already studied the organisation of a flowering plant in Chapter
5. Have you ever thought about where and how the structures like roots,
stems, leaves, flowers, fruits and seeds arise and that too in an orderly
sequence? You are, by now, aware of the terms seed, seedling, plantlet,
mature plant. You have also seen that trees continue to increase in height
or girth over a period of time. However, the leaves, flowers and fruits of the
same tree not only have limited dimensions but also appear and fall
periodically and some time repeatedly. Why does vegetative phase precede
flowering in a plant? All plant organs are made up of a variety of tissues; is
there any relationship between the structure of a cell, a tissue, an organ
and the function they perform? Can the structure and the function of these
be altered? All cells of a plant are descendents of the zygote. The question
is, then, why and how do they have different structural and functional
attributes? Development is the sum of two processes: growth and
differentiation. To begin with, it is essential and sufficient to know that the
development of a mature plant from a zygote (fertilised egg) follow a precise
and highly ordered succession of events. During this process a complex
body organisation is formed that produces roots, leaves, branches, flowers,
fruits, and seeds, and eventually they die (Figure 13.1). The first step in the
process of plant growth is seed germination. The seed germinates when
favourable conditions for growth exist in the environment. In absence of
such favourable conditions the seeds do not germinate and goes into a
period of suspended growth or rest. Once favourable conditions return,
the seeds resume metabolic activities and growth takes place.
In this chapter, you shall also study some of the factors which
govern and control these developmental processes. These factors are both
intrinsic (internal) and extrinsic (external) to the plant.
PLANT GROWTH AND DEVELOPMENT
CHAPTER  13
13.1 Growth
13.2 Differentiation,
Dedifferentiation
and
Redifferentiation
13.3 Development
13.4 Plant Growth
Regulators
2024-25
PLANT GROWTH AND DEVELOPMENT 167
13.1 GROWTH
Growth is regarded as one of the most fundamental and conspicuous
characteristics of a living being. What is growth? Growth can be defined
as an irreversible permanent increase in size of an organ or its parts or
even of an individual cell. Generally, growth is accompanied by metabolic
processes (both anabolic and catabolic), that occur at the expense of
energy. Therefore, for example, expansion of a leaf is growth. How would
you describe the swelling of piece of wood when placed in water?
13.1.1 Plant Growth Generally is Indeterminate
Plant growth is unique because plants retain the capacity for unlimited
growth throughout their life. This ability of the plants is due to the presence
of meristems at certain locations in their body. The cells of such meristems
have the capacity to divide and self-perpetuate. The product, however,
soon loses the capacity to divide and such cells make up the plant body.
This form of growth wherein new cells are always being added to the
plant body by the activity of the meristem is called the open form of growth.
What would happen if the meristem ceases to divide? Does this ever
happen?
In earlier classes, you have studied about the root apical meristem
and the shoot apical meristem. You know that they are responsible for
Seed coat
Epicotyl
hook
Cotyledons
Cotyledon
Soil line
Epicotyl
Hypocotyl
Hypocotyl
Figure 13.1 Germination and seedling development in bean
2024-25
Page 3


166 BIOLOGY
You have already studied the organisation of a flowering plant in Chapter
5. Have you ever thought about where and how the structures like roots,
stems, leaves, flowers, fruits and seeds arise and that too in an orderly
sequence? You are, by now, aware of the terms seed, seedling, plantlet,
mature plant. You have also seen that trees continue to increase in height
or girth over a period of time. However, the leaves, flowers and fruits of the
same tree not only have limited dimensions but also appear and fall
periodically and some time repeatedly. Why does vegetative phase precede
flowering in a plant? All plant organs are made up of a variety of tissues; is
there any relationship between the structure of a cell, a tissue, an organ
and the function they perform? Can the structure and the function of these
be altered? All cells of a plant are descendents of the zygote. The question
is, then, why and how do they have different structural and functional
attributes? Development is the sum of two processes: growth and
differentiation. To begin with, it is essential and sufficient to know that the
development of a mature plant from a zygote (fertilised egg) follow a precise
and highly ordered succession of events. During this process a complex
body organisation is formed that produces roots, leaves, branches, flowers,
fruits, and seeds, and eventually they die (Figure 13.1). The first step in the
process of plant growth is seed germination. The seed germinates when
favourable conditions for growth exist in the environment. In absence of
such favourable conditions the seeds do not germinate and goes into a
period of suspended growth or rest. Once favourable conditions return,
the seeds resume metabolic activities and growth takes place.
In this chapter, you shall also study some of the factors which
govern and control these developmental processes. These factors are both
intrinsic (internal) and extrinsic (external) to the plant.
PLANT GROWTH AND DEVELOPMENT
CHAPTER  13
13.1 Growth
13.2 Differentiation,
Dedifferentiation
and
Redifferentiation
13.3 Development
13.4 Plant Growth
Regulators
2024-25
PLANT GROWTH AND DEVELOPMENT 167
13.1 GROWTH
Growth is regarded as one of the most fundamental and conspicuous
characteristics of a living being. What is growth? Growth can be defined
as an irreversible permanent increase in size of an organ or its parts or
even of an individual cell. Generally, growth is accompanied by metabolic
processes (both anabolic and catabolic), that occur at the expense of
energy. Therefore, for example, expansion of a leaf is growth. How would
you describe the swelling of piece of wood when placed in water?
13.1.1 Plant Growth Generally is Indeterminate
Plant growth is unique because plants retain the capacity for unlimited
growth throughout their life. This ability of the plants is due to the presence
of meristems at certain locations in their body. The cells of such meristems
have the capacity to divide and self-perpetuate. The product, however,
soon loses the capacity to divide and such cells make up the plant body.
This form of growth wherein new cells are always being added to the
plant body by the activity of the meristem is called the open form of growth.
What would happen if the meristem ceases to divide? Does this ever
happen?
In earlier classes, you have studied about the root apical meristem
and the shoot apical meristem. You know that they are responsible for
Seed coat
Epicotyl
hook
Cotyledons
Cotyledon
Soil line
Epicotyl
Hypocotyl
Hypocotyl
Figure 13.1 Germination and seedling development in bean
2024-25
168 BIOLOGY
the primary growth of the plants and principally
contribute to the elongation of the plants along
their axis. You also know that in dicotyledonous
plants and gymnosperms, the lateral meristems,
vascular cambium and cork-cambium appear
later in life. These are the meristems that cause
the increase in the girth of the organs in which
they are active. This is known as secondary
growth of the plant (see Figure 13.2).
13.1.2 Growth is Measurable
Growth, at a cellular level, is principally a
consequence of increase in the amount of
protoplasm. Since increase in protoplasm is
difficult to measure directly, one generally
measures some quantity which is more or less
proportional to it. Growth is, therefore,
measured by a variety of parameters some of
which are: increase in fresh weight, dry weight,
length, area, volume and cell number. You may
find it amazing to know that one single maize
root apical mersitem can give rise to more than
17,500 new cells per hour, whereas cells in a
watermelon may increase in size by upto
3,50,000 times. In the former, growth is
expressed as increase in cell number; the latter
expresses growth as increase in size of the cell.
While the growth of a pollen tube is measured
in terms of its length, an increase in surface area
denotes the growth in a dorsiventral leaf.
13.1.3 Phases of Growth
The period of growth is generally divided into
three phases, namely, meristematic, elongation
and maturation (Figure 13.3). Let us
understand this by looking at the root tips. The
constantly dividing cells, both at the root apex
and the shoot apex, represent the meristematic
phase of growth. The cells in this region are rich
in protoplasm, possess large conspicuous
nuclei. Their cell walls are primary in nature,
thin and cellulosic with abundant
plasmodesmatal connections. The cells
proximal (just next, away from the tip) to the
Shoot apical
meristem
Vascular
cambium
Vascular
cambium
Root apical
meristem
Shoot
Root
Figure 13.2 Diagrammatic representation of
locations of root apical meristem,
shoot aplical meristem and
vascular cambium. Arrows exhibit
the direction of growth of cells and
organ
G
F
E
D
C
B
A
Figure 13.3 Detection of zones of elongation by
the parallel line technique. Zones
A, B, C, D immediately behind the
apex have elongated most.
2024-25
Page 4


166 BIOLOGY
You have already studied the organisation of a flowering plant in Chapter
5. Have you ever thought about where and how the structures like roots,
stems, leaves, flowers, fruits and seeds arise and that too in an orderly
sequence? You are, by now, aware of the terms seed, seedling, plantlet,
mature plant. You have also seen that trees continue to increase in height
or girth over a period of time. However, the leaves, flowers and fruits of the
same tree not only have limited dimensions but also appear and fall
periodically and some time repeatedly. Why does vegetative phase precede
flowering in a plant? All plant organs are made up of a variety of tissues; is
there any relationship between the structure of a cell, a tissue, an organ
and the function they perform? Can the structure and the function of these
be altered? All cells of a plant are descendents of the zygote. The question
is, then, why and how do they have different structural and functional
attributes? Development is the sum of two processes: growth and
differentiation. To begin with, it is essential and sufficient to know that the
development of a mature plant from a zygote (fertilised egg) follow a precise
and highly ordered succession of events. During this process a complex
body organisation is formed that produces roots, leaves, branches, flowers,
fruits, and seeds, and eventually they die (Figure 13.1). The first step in the
process of plant growth is seed germination. The seed germinates when
favourable conditions for growth exist in the environment. In absence of
such favourable conditions the seeds do not germinate and goes into a
period of suspended growth or rest. Once favourable conditions return,
the seeds resume metabolic activities and growth takes place.
In this chapter, you shall also study some of the factors which
govern and control these developmental processes. These factors are both
intrinsic (internal) and extrinsic (external) to the plant.
PLANT GROWTH AND DEVELOPMENT
CHAPTER  13
13.1 Growth
13.2 Differentiation,
Dedifferentiation
and
Redifferentiation
13.3 Development
13.4 Plant Growth
Regulators
2024-25
PLANT GROWTH AND DEVELOPMENT 167
13.1 GROWTH
Growth is regarded as one of the most fundamental and conspicuous
characteristics of a living being. What is growth? Growth can be defined
as an irreversible permanent increase in size of an organ or its parts or
even of an individual cell. Generally, growth is accompanied by metabolic
processes (both anabolic and catabolic), that occur at the expense of
energy. Therefore, for example, expansion of a leaf is growth. How would
you describe the swelling of piece of wood when placed in water?
13.1.1 Plant Growth Generally is Indeterminate
Plant growth is unique because plants retain the capacity for unlimited
growth throughout their life. This ability of the plants is due to the presence
of meristems at certain locations in their body. The cells of such meristems
have the capacity to divide and self-perpetuate. The product, however,
soon loses the capacity to divide and such cells make up the plant body.
This form of growth wherein new cells are always being added to the
plant body by the activity of the meristem is called the open form of growth.
What would happen if the meristem ceases to divide? Does this ever
happen?
In earlier classes, you have studied about the root apical meristem
and the shoot apical meristem. You know that they are responsible for
Seed coat
Epicotyl
hook
Cotyledons
Cotyledon
Soil line
Epicotyl
Hypocotyl
Hypocotyl
Figure 13.1 Germination and seedling development in bean
2024-25
168 BIOLOGY
the primary growth of the plants and principally
contribute to the elongation of the plants along
their axis. You also know that in dicotyledonous
plants and gymnosperms, the lateral meristems,
vascular cambium and cork-cambium appear
later in life. These are the meristems that cause
the increase in the girth of the organs in which
they are active. This is known as secondary
growth of the plant (see Figure 13.2).
13.1.2 Growth is Measurable
Growth, at a cellular level, is principally a
consequence of increase in the amount of
protoplasm. Since increase in protoplasm is
difficult to measure directly, one generally
measures some quantity which is more or less
proportional to it. Growth is, therefore,
measured by a variety of parameters some of
which are: increase in fresh weight, dry weight,
length, area, volume and cell number. You may
find it amazing to know that one single maize
root apical mersitem can give rise to more than
17,500 new cells per hour, whereas cells in a
watermelon may increase in size by upto
3,50,000 times. In the former, growth is
expressed as increase in cell number; the latter
expresses growth as increase in size of the cell.
While the growth of a pollen tube is measured
in terms of its length, an increase in surface area
denotes the growth in a dorsiventral leaf.
13.1.3 Phases of Growth
The period of growth is generally divided into
three phases, namely, meristematic, elongation
and maturation (Figure 13.3). Let us
understand this by looking at the root tips. The
constantly dividing cells, both at the root apex
and the shoot apex, represent the meristematic
phase of growth. The cells in this region are rich
in protoplasm, possess large conspicuous
nuclei. Their cell walls are primary in nature,
thin and cellulosic with abundant
plasmodesmatal connections. The cells
proximal (just next, away from the tip) to the
Shoot apical
meristem
Vascular
cambium
Vascular
cambium
Root apical
meristem
Shoot
Root
Figure 13.2 Diagrammatic representation of
locations of root apical meristem,
shoot aplical meristem and
vascular cambium. Arrows exhibit
the direction of growth of cells and
organ
G
F
E
D
C
B
A
Figure 13.3 Detection of zones of elongation by
the parallel line technique. Zones
A, B, C, D immediately behind the
apex have elongated most.
2024-25
PLANT GROWTH AND DEVELOPMENT 169
meristematic zone represent the phase of elongation. Increased
vacuolation, cell enlargement and new cell wall deposition are the
characteristics of the cells in this phase. Further away from the apex, i.e.,
more proximal to the phase of elongation, lies the portion of axis which is
undergoing the phase of maturation. The cells of this zone, attain their
maximal size in terms of wall thickening and protoplasmic modifications.
Most of the tissues and cell types you have studied in earlier classes
represent this phase.
13.1.4  Growth Rates
The increased growth per unit time is termed as growth rate. Thus, rate
of growth can be expressed mathematically. An organism, or a part of the
organism can produce more cells in a variety of ways.
Figure13.4 Diagrammatic representation of : (a) Arithmetic (b) Geometric growth and
(c) Stages during embryo development showing geometric and arithematic
phases
2024-25
Page 5


166 BIOLOGY
You have already studied the organisation of a flowering plant in Chapter
5. Have you ever thought about where and how the structures like roots,
stems, leaves, flowers, fruits and seeds arise and that too in an orderly
sequence? You are, by now, aware of the terms seed, seedling, plantlet,
mature plant. You have also seen that trees continue to increase in height
or girth over a period of time. However, the leaves, flowers and fruits of the
same tree not only have limited dimensions but also appear and fall
periodically and some time repeatedly. Why does vegetative phase precede
flowering in a plant? All plant organs are made up of a variety of tissues; is
there any relationship between the structure of a cell, a tissue, an organ
and the function they perform? Can the structure and the function of these
be altered? All cells of a plant are descendents of the zygote. The question
is, then, why and how do they have different structural and functional
attributes? Development is the sum of two processes: growth and
differentiation. To begin with, it is essential and sufficient to know that the
development of a mature plant from a zygote (fertilised egg) follow a precise
and highly ordered succession of events. During this process a complex
body organisation is formed that produces roots, leaves, branches, flowers,
fruits, and seeds, and eventually they die (Figure 13.1). The first step in the
process of plant growth is seed germination. The seed germinates when
favourable conditions for growth exist in the environment. In absence of
such favourable conditions the seeds do not germinate and goes into a
period of suspended growth or rest. Once favourable conditions return,
the seeds resume metabolic activities and growth takes place.
In this chapter, you shall also study some of the factors which
govern and control these developmental processes. These factors are both
intrinsic (internal) and extrinsic (external) to the plant.
PLANT GROWTH AND DEVELOPMENT
CHAPTER  13
13.1 Growth
13.2 Differentiation,
Dedifferentiation
and
Redifferentiation
13.3 Development
13.4 Plant Growth
Regulators
2024-25
PLANT GROWTH AND DEVELOPMENT 167
13.1 GROWTH
Growth is regarded as one of the most fundamental and conspicuous
characteristics of a living being. What is growth? Growth can be defined
as an irreversible permanent increase in size of an organ or its parts or
even of an individual cell. Generally, growth is accompanied by metabolic
processes (both anabolic and catabolic), that occur at the expense of
energy. Therefore, for example, expansion of a leaf is growth. How would
you describe the swelling of piece of wood when placed in water?
13.1.1 Plant Growth Generally is Indeterminate
Plant growth is unique because plants retain the capacity for unlimited
growth throughout their life. This ability of the plants is due to the presence
of meristems at certain locations in their body. The cells of such meristems
have the capacity to divide and self-perpetuate. The product, however,
soon loses the capacity to divide and such cells make up the plant body.
This form of growth wherein new cells are always being added to the
plant body by the activity of the meristem is called the open form of growth.
What would happen if the meristem ceases to divide? Does this ever
happen?
In earlier classes, you have studied about the root apical meristem
and the shoot apical meristem. You know that they are responsible for
Seed coat
Epicotyl
hook
Cotyledons
Cotyledon
Soil line
Epicotyl
Hypocotyl
Hypocotyl
Figure 13.1 Germination and seedling development in bean
2024-25
168 BIOLOGY
the primary growth of the plants and principally
contribute to the elongation of the plants along
their axis. You also know that in dicotyledonous
plants and gymnosperms, the lateral meristems,
vascular cambium and cork-cambium appear
later in life. These are the meristems that cause
the increase in the girth of the organs in which
they are active. This is known as secondary
growth of the plant (see Figure 13.2).
13.1.2 Growth is Measurable
Growth, at a cellular level, is principally a
consequence of increase in the amount of
protoplasm. Since increase in protoplasm is
difficult to measure directly, one generally
measures some quantity which is more or less
proportional to it. Growth is, therefore,
measured by a variety of parameters some of
which are: increase in fresh weight, dry weight,
length, area, volume and cell number. You may
find it amazing to know that one single maize
root apical mersitem can give rise to more than
17,500 new cells per hour, whereas cells in a
watermelon may increase in size by upto
3,50,000 times. In the former, growth is
expressed as increase in cell number; the latter
expresses growth as increase in size of the cell.
While the growth of a pollen tube is measured
in terms of its length, an increase in surface area
denotes the growth in a dorsiventral leaf.
13.1.3 Phases of Growth
The period of growth is generally divided into
three phases, namely, meristematic, elongation
and maturation (Figure 13.3). Let us
understand this by looking at the root tips. The
constantly dividing cells, both at the root apex
and the shoot apex, represent the meristematic
phase of growth. The cells in this region are rich
in protoplasm, possess large conspicuous
nuclei. Their cell walls are primary in nature,
thin and cellulosic with abundant
plasmodesmatal connections. The cells
proximal (just next, away from the tip) to the
Shoot apical
meristem
Vascular
cambium
Vascular
cambium
Root apical
meristem
Shoot
Root
Figure 13.2 Diagrammatic representation of
locations of root apical meristem,
shoot aplical meristem and
vascular cambium. Arrows exhibit
the direction of growth of cells and
organ
G
F
E
D
C
B
A
Figure 13.3 Detection of zones of elongation by
the parallel line technique. Zones
A, B, C, D immediately behind the
apex have elongated most.
2024-25
PLANT GROWTH AND DEVELOPMENT 169
meristematic zone represent the phase of elongation. Increased
vacuolation, cell enlargement and new cell wall deposition are the
characteristics of the cells in this phase. Further away from the apex, i.e.,
more proximal to the phase of elongation, lies the portion of axis which is
undergoing the phase of maturation. The cells of this zone, attain their
maximal size in terms of wall thickening and protoplasmic modifications.
Most of the tissues and cell types you have studied in earlier classes
represent this phase.
13.1.4  Growth Rates
The increased growth per unit time is termed as growth rate. Thus, rate
of growth can be expressed mathematically. An organism, or a part of the
organism can produce more cells in a variety of ways.
Figure13.4 Diagrammatic representation of : (a) Arithmetic (b) Geometric growth and
(c) Stages during embryo development showing geometric and arithematic
phases
2024-25
170 BIOLOGY
The growth rate shows an increase that may be
arithmetic or geometrical (Figure 13.4).
In arithmetic growth, following mitotic cell
division, only one daughter cell continues to divide
while the other differentiates and matures. The
simplest expression of arithmetic growth is
exemplified by a root elongating at a constant rate.
Look at Figure 13.5. On plotting the length of the
organ against time, a linear curve is obtained.
Mathematically, it is expressed as
L
t
 = L
0
 + rt
L
t
 = length at time ‘t’
L
0
 = length at time ‘zero’
r    = growth rate / elongation per unit time.
Let us now see what happens in geometrical
growth. In most systems, the initial growth is slow
(lag phase), and it increases rapidly thereafter – at
an exponential rate (log or exponential phase). Here,
both the progeny cells following mitotic cell division
retain the ability to divide and continue to do so.
However, with limited nutrient supply, the growth
slows down leading to a stationary phase. If we plot
the parameter of growth against time, we get a typical
sigmoid or S-curve (Figure 13.6). A sigmoid curve
is a characteristic of living organism growing in a
natural environment. It is typical for all cells, tissues
and organs of a plant. Can you think of more similar
examples? What kind of a curve can you expect in
a tree showing seasonal activities?
The exponential growth can be expressed as
W
1
 = W
0
 e
rt
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
Here, r is the relative growth rate and is also the
measure of the ability of the plant to produce new
plant material, referred to as efficiency index. Hence,
the final size of W
1
 depends on the initial size, W
0
.
Figure 13.5 Constant linear growth, a plot
of length L against time t
Figure 13.6 An idealised sigmoid growth
curve typical of cells in culture,
and many higher plants and
plant organs
Size/weight of the organ
Exponential phase
Lag phase
Time
Stationary phase
2024-25