growth and development in plants

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MODULE - 3
Growth and Development in Plants
Reproduction and
Heredity
20
Notes
GROWTH AND DEVELOPMENT IN
PLANTS
If you sow a seed in your garden or in a pot, after few days you would find a tiny
seedling coming out from the seed. As days pass, the tiny seedling grows in size,
the number of leaves increases, and later, it grows into a mature plant and produces
flowers and fruits. This is the process of growth and development. Besides growth
and development plants also show movement, but it is not as clearly visible as in
the case of animals. In this lesson you will learn about growth, development and
movements in plants.
OBJECTIVES
After studying this lesson, you will be able to:
z define the terms growth and development;
z differentiate between growth and development and explain growth curve;
z list the various stages of cellular growth;
z explain the various methods of measurement of plant growth;
z describe the factors affecting plant growth and importance of growth regulators;
z explain the role of growth regulators in dormancy and germination of seeds;
z differentiate among short-day plants, long-day plants and day-neutral plants;
z define the terms abscission and senescence;
z identify the effects of salt stress and water stress on plants;
z define the various types of movement like geotropism, phototropism, nastic and
turgor movements.
20.1 GROWTH AND DEVELOPMENT
You must have noticed that all living organisms grow in size. But have you ever
thought how a do they grow? Growth takes place due to cell division, which
increases the number of cells in the body. This process continues and we observe
increase in weight, size and volume of all plants and animals. This is called growth.
Growth in living organisms may be defined as an irreversible increase in
the number and size of a cell, organ or whole organism.
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Growth in living organisms is not uniform throughout the life span. Growth takes
place at a faster rate till the plants or animals attain maturity. Then it slows down
and at a particular time it stops. Later in life death occurs. All these changes that
occur in an organism starting from its beginning till its death may collectively be
termed as development. Development is associated with morphogenesis and
differentiation. Morphogenesis is the process of development of shape and
structure of an organism; and differentiation is the process of change in cells, tissues
Notes
or organs to carry out different functions.
Development is the whole series of qualitative and quantitative changes
such as growth, differentiation and maturation, which an organism
undergoes throughout its life cycle.
20.2 STAGES OF CELLULAR GROWTH
You have already learnt that growth of an organism is always associated with growth
in size and number of cells. The growth of an organ or an organism occurs in three
successive stages. They are
(i) Cell division : The number of cells increases due to mitosis (Fig. 20.1a).
(ii) Cell enlargement: The size of individual cell increases after cell division due
to increase in the volume of its protoplasm (Fig. 20.1b).
(iii) Cell differentiation: In this stage, structure of the cells changes to perform
specific functions. And similar type of cells having same functions form a group,
which is known as tissue.
(a) Cell Division
(b) Cell Enlargement
Fig. 20.1 Comparison of cell division and cell enlargement
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In lower organisms such as bacteria and algae the entire body grows. But in higher
organisms like ferns, pine and flowering plants, growth is restricted to the cells present
only in the growing regions, like shoot apex and root tip and close to the lateral
sides of the stem and root. Growth at the tips leads to elongation of body parts and
lateral (side ways) growth leads to increase in the thickness of stem and root.
20.3 GROWTH CURVE
Notes The rate of growth of a plant or plant part is not always the same during its life
span. Sometimes it is slow and at other times rapid. If we plot the increase in cell
number (growth rate) against time, a typical S-shaped curve is obtained. This is
called growth curve or sigmoid growth curve. (Fig 20.2)
This curve has three phases of growth.
(i) Lag Phase – This is the initial phase of growth when the rate of growth is
very slow.
(ii) Log Phase – It shows rapid growth and is maximum during the entire life span.
(iii) Stationary Phase – Here the rate of growth starts decreasing and finally it
stops.
Fruit and
leaf loss
Dry weight
Stationary
Rapid
growth
No
growth
Death
Slow
growth
Seed
germinating
and losing
weight
Log phase
Lagphase
Fig. 20.2 Sigmoid curve
The total time period during which the festest growth of the organ or organism
occurs is called grand period of growth.
20.4 MEASUREMENT OF GROWTH
After knowing the different phases of growth let us know how to measure growth
in plants. Growth in plants being a quantitative phenomenon can be measured in
relation to time. It can be measured in terms of
z
Increase in length or growth – in case of stem and root;
z
Increase in area or volume – in case of leaves and fruits;
z
Increase in the number of cells – in algae, yeast and bacteria.
Let us discuss some methods of measuring growth in length.
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20.4.1 Direct Method
We know that growth generally takes place at the apical region of plant. So growth
in length can be directly measured by means of an ordinary measuring scale at any
particular interval of time.
ACTIVITY FOR YOU
Reproduction and
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Notes
Aim
To use an ordinary scale to measure growth in length of the stem of a plant in your
garden.
What do you require?
Thread, a piece of stone and a measuring scale.
What to do?
z Tie the stone at one end of the thread;
z Take the length of the stem from above the soil surface with the help of the
thread;
z Mark the length of the stem on the thread with the help of a pen;
z Put the thread on the scale and note down the length;
z Record the length citing date of the activity;
z Repeat the procedure and at an interval of one week.
Is there any change in length?
Make a table.
No. of the week
Length in cms.
1
2
3
4
5
20.4.2 Auxanometer
For more accurate measurement of length, we can use the specially designed
equipment called auxanometer. (Fig 20.3). We can use it to measure the rate of
growth of shoot length of plants. A thread is tied to the tip of stem of a potted
plant and the thread is hung on the pulley of auxanometer. The other end of the
thread is tied to a weight. The pulley is fixed with a long needle, which slides over
a graduated arc. As the stem grows in length the weight pulls the thread down. The
movement of the needle is read on the scale of arc.
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ARC
Pointer
Notes
Pulley
Thread
Weight
Plotted
plant
Stand
Fig. 20.3 Auxanometer
INTEXT QUESTIONS 20.1
1. Distinguish between growth and development.
............................................................................................................................
2. What is differentiation?
............................................................................................................................
3. What role does it play in plant growth and development?
............................................................................................................................
20.5 FACTORS AFFECTING PLANT GROWTH
Generally plant growth is influenced by a number of factors both external and
internal.
20.5.1 External growth factors
External factors are those factors present in the environment that affect the growth
of the plants directly or indirectly. These factors are
(i) Light
(ii) Temperature
(iii) Water
(iv) Mineral nutrients
(i) Light
You have already learnt about the necessity of light for the process of photosynthesis.
Besides photosynthesis, light is also essential for seed germination, growth of
seedling, differentiation of various tissues and organs, and reproduction.
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When plants grow in dark, they become tall, yellowish and weak, and the
leaves are very small.
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(ii) Temperature
Some plants grow in cold climate and some in hot climate. The optimum
temperature required for growth of plants ranges between 28-30°C, but it may occur
in the temperature range of 4-45°C. All metabolic activities of plants are directly Notes
affected by variation of temperature. A very low temperature causes injuries to the
plant due to chilling and freezing, and very high temperature stops its growth.
(iii) Water
You have already learnt that a plant absorbs water by its roots, uses it in
photosynthesis and other biochemical processes and some of it is lost through
transpiration. For proper growth of plants a particular quantity of water is required.
Both deficiency and excess of water retards the growth of plants.
(iv) Mineral Nutrients
In the lesson 9 “Plant nutrition” we have already discussed the importance of mineral
nutrients for plant growth and development. All metabolic processes require
inorganic nutrients. Plant growth is adversely affected by the deficiency of nutrients.
20.5.2 Internal Growth Factors
In addition to the external factors as discussed above, there are some substances
produced in the plant body itself, which affects the growth of the plant. These are
called plant hormones or phytohormones or growth hormones.
A phytohormone is an organic substance produced in a small quantity in
one part of plant body and capable of moving to other parts to influence
the growth of that part.
The growth of the plants can also be influenced by certain synthetic chemicals
resembling plant hormones both in structure and functions. These are called growth
regulators. They are not produced by plants naturally.
Growth regulators are chemical substances, other than naturally produced
hormones, which promote, inhibit or modify growth and development in plants.
The naturally produced growth hormones are broadly grouped under five major
classes. They are
(i) Auxin
(iv) Ethylene
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(ii) Gibberellins
(iii) Cytokinins
(v) Abscissic acid
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Let us know details about these hormones.
(i) Auxin
Auxin is a growth promoter, generally produced by the growing apex of stem and
root of the plants. It helps in the elongation of shoot and root tips behind apical
meristem. The naturally produced auxins is Indole-3-Acetic Acid (IAA). They are
also produced by chemical synthesis, which show same physiological responses like
Notes
Auxin. Some of the synthetic auxin are Indole-3-butyric acid (IBA), 2,4Dichlorophenoxy Acetic Acid (2,4-D), and Naphthalene acetic acid (NAA).
The Greek word auxein means “to grow”. It was first isolated from human urine.
An experiment was performed by Fritz Went on oat seedling to see the effect of
auxins. When tip of oat coleoptile (early shoot) is removed, growth stops. Then
the removed tip is placed on a block of agar (gelatinous material from sea weeds)
for about an hour. This agar block is then placed on the cut end of the seedling.
It was observed that the growth of the seedling started again. It shows that there
is something that has passed from the cut tip into the agar block, which helps to
restart the growth. This was named Auxin, a plant hormone.
Functions of Auxin
(a)
It promotes cell elongation;
(b) It suppresses the growth of lateral bud. If the tip of a plant is removed, the
lateral branches begin to grow; In most of the plants apical bud suppresses
the development of lateral buds. This is called apical dominance.
(c) It delays fall of leaves. (leaf abscission)
(d) NAA (Naphthalene acetic acid) is used for preventing fruit drop in apples
before they are ripe.
(e) 2, 4-D (2, 4-dichlorophenoxy acetic acid) acts as a dicot weedicide.
(ii) Gibberellin
Gibberellin or Gibberellic Acid (GA) was initially isolated from a fungus Gibberella
fujikuroi. In plants, it is produced in embryos, roots, and young leaves and it
enhances growth.
Functions of Gibberellins
(a) It helps in elongation of stems in genetically dwarf plants. By using gibberellin
the height of the dwarf plants can be increased.
(b) It breaks dormancy of seeds and buds.
(c)
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It induces parthenocarpy. (Formation of seedless fruits without fertilization)
or provides stimulus received by pollination.
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Growth and Development in Plants
(iii) Cytokinins : They were extracted from coconut milk.
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Cytokinins are synthesized in root apex, endosperm of seeds, and young fruits where
cell division takes place continuously.
Functions of Cytokinins
(a) They stimulate cell division, cell enlargement and cell differentiation.
(b) They prevent aging of plant parts.
Notes
(c) They inhibit apical dominance and help in growth of lateral buds into branches.
(iv) Ethylene
Ethylene is a gaseous hormone. It is found in ripening fruits, young flowers and
young leaves.
Functions of Ethylene
(a) It induces ripening of fruits.
(b) It promotes senescence and abscission of leaf, and flowers.
(c) In cells it only increases the width not the length.
(v) Abscissic acid
Abscissic acid also known as Dormin is a naturally occurring growth inhibitor found
in wide variety of plants. It is synthesised in leaves.
Functions of Abscissic acid:
(a) It induces dormancy of buds and seeds as opposed to Gibberellin, which breaks
dormancy.
(b) It promotes the senescence of leaf, i.e., fall of leaves happen due to abscissic
acid.
(c) It inhibits seed germination and development.
(d) It causes closing of Stomata.
20.6 PRACTICAL APPLICATION OF GROWTH REGULATORS
We have already discussed that by using the various types of growth regulators we
can promote, inhibit or modify growth and development in plants. Now-a-days these
are widely used by horticulturists to boost their production. Some of the applications
are –
(i) With the help of auxins and gibberellins seedless varieties of fruits can be
produced. You might have seen seedless grapes and papayas in the market.
(ii) Early flowering in some plants is possible by applying growth regulators.
(iii) With the use of hormones some fruits can be ripened at an early stage.
(iv) Germination in seeds can be possible by applying auxins.
(v) Germination of potatoes and onions can be stopped in storage by application
of growth inhibitors.
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20.6.1 Differentiation, Dedifferentiation and Redifferentiation
Differentiation: Differentiation in plants is a permanent, localised qualitative change
in size, biochemistry, structure and function of cells, tissues or organs. It refers to
the processes by which distinct cell types arise from precursor cells and become
different from each other. For example:Vascular tissues, xylem and phloem, are
differentiated from meristematic cells, procambium, and vascular cambium and
Notes mature to perform specific functions. The hormones auxin and cytokinin are essential
for vascular tissue differentiation. During differentiation, cells undergo few to major
structural changes both in their cell walls and protoplasm. For example, to form
a tracheary element, the cells would lose their protoplasm. They also develop a
strong, elastic, secondary cell wall to carry water to long distances even under
extreme conditions.
Dedifferentiation: It is the reversal of cell development in plants, so that the
differentiation that had occurred previously is lost and the cell becomes more
generalized in structure. The living differentiated cells that have lost the capacity
to divide can regain the capacity of division under certain conditions. This
phenomenon is termed dedifferentiation. For example, formation of meristems,
interfascicular cambium and cork cambium from fully differentiated parenchyma
cells.
Redifferentiation: While undergoing dedifferentiation plant cells once again lose
their capacity to divide but mature to perform specific functions. This process is
called redifferentiation.
INTEXT QUESTIONS 20.2
1. Name the plant hormones concerned with the following:
(i) Elongation of cell ......................................................................
(ii) Shedding of leaves.....................................................................
(iii) Breaking seed dormancy ...........................................................
2. Mention two functions of Auxin
(i)
...................................................................................................
(ii)
...................................................................................................
3. What is the difference between dedifferentiation and redifferentiation?
............................................................................................................................
4. Which two hormones are essential for vascular tissue differentiation?
............................................................................................................................
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Growth and Development in Plants
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20.7 DORMANCY AND GERMINATION IN SEEDS
In the previous lesson-7 you have already learnt about formation of seeds in plants.
In developed seeds metabolic activities are generally very slow. But at the time of
germination, the metabolic activities in seeds increase and they grow into new plants
under favourable conditions of growth. This is called seed germination.
Seed germination is the return of metabolic activities and growth by the
seed tissue to give rise to a new plant by the development of the embryo.
Notes
Some seeds do not germinate immediately after dispersal even if suitable conditions
of growth are provided. In this period growth of the seeds remains suspended and
it is said to be in the rest or dormant stage. This phenomenon is called dormancy
of seeds. It may occur due to immature embryo, hard or impermeable seed coat,
and presence of inhibitors like abscissic acid.
20.7.1 Types of Seed Germination
In flowering plants two types of germination are found. They are:
(a) Epigeal germination; and (b) Hypogeal germination.
Shoot tip
Hypocotyl hook
Cotyledons
Seed coat
Hypocotyl
Yound leaves
Radicle
Epicotyl
Plumule
Roots
(a) Epigeal
Roots
(b) Hypogeal
Fig.20.4 Epigeal Germination and Hypogeal Germination
Vivipary (= producing baby plants)
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(a) Epigeal Germination
In epigeal (epi - above; geo - soil)germination hypocotyl elongates and cotyledons
come out above the soil surface. Examples : seeds of pumpkin, mustard, tamarind,
and french bean.
(b) Hypogeal Germination
In hypogeal (hypo = below, geo = earth) germination the epicotyl elongates and
Notes cotyledons remain below the soil surface. Examples : Most monocots seed like rice,
wheat, maize, and coconut.
Some plants, which grow in marshy places show a special type of
germination called Vivipary (Fig. 20.5). Here the seed germinates inside
the fruit while it is attached to the parent plant. The weight of the seed
increases because of germination and seedling separates from the plant and
falls down into the mud. Then roots develop to fix it in the soil. These plants
are called viviparous plants. For example, Rhizophora and Sonneratia.
Parent plant
New plant
Water
Fruit
Radicle
Hypocotyl
Hypocotyl
Soil
Root
Fig 20.5 Viviparous Germination
20.7.2 Mechanism of Seed Germination
In seed germination, the first step is the imbibition or absorption of water by seed.
Then the seed swells and the seed coat ruptures. Through the ruptured seed coat
the radicle comes out from one end of embryonic axis. This radicle gives rise to
root system. From the other end of embryonic axis the plumule elongates and
develops as the shoot of the plant.
20.7.3 Factors Affecting Seed Germination
Seed germination requires five factors : water, temperature, oxygen, light and
growth hormones.
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(a) Water : The seed must swell up to rupture its seed coat. A ripe seed contains
very low quantity of water. So for swelling to cause rupture of seed coats supply
of adequate water is essential. Biochemical reactions required for growth and
development of the seedling require water.
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(b) Temperature : For germination of seeds a particular temperature is required.
The degree of temperature required varies from species to species. Warmth
Notes
accelerates chemical reactions inside.
(c) Oxygen : Oxygen is required in breaking down reserve food of seed and release
energy for metabolism of growth of the embryo.
(d) Light : In most of the seeds light is not an essential factor for germination. But
in some cases like lettuce and tobacco light is absolutely essential.
(e) Hormone : Besides the above external factors, hormones also control germination
of seeds. Some roles played by hormones are as follows.
z
Gibberellins can induce germination in some cases even in complete
darkness.
z
Auxin, Cytokinins and Ethylene can break dormancy in many seeds and
initiate germination.
z
In some seeds Abscissic acid inhibits germination process.
20.8 PHOTOPERIODISM – RESPONSES DUE TO LIGHT EXPOSURE
DURATION
You must have observed plants like spinach, wheat, etc. which produce flowers in
summer; and dahlia, cosmos, etc. flower in winter. Why is it so? Because the plants
that flower in summer require longer duration of light per day than those flowering
in winter. Thus, we can say that duration of light plays an important role in flowering
of plants. This effect of duration of light on the growth of plants is known as
photoperiodism.
Photoperiodism is the response in growth, transpiration, photosynthesis, and
reproduction (flowering) of a plant to the specific duration of light, which falls on
it per day.
On the basis of day-length required by the plants for flowering, the plants are
classified into the following three categories:
(i)
Short-day Plants (SDP) : Some plants produce flowers when exposed to a
light period shorter than a required day-length. These are called Short-day
Plants. Chrysanthemum, Cosmos, Dahlia, Soyabean, are short-day plants.
(ii) Long-day Plants (LDP) : They produce flowers when exposed to a light
period longer than a fixed day-length. Gulmohar, radish, spinach, are long-day
plants.
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(iii) Day-neutral Plants (DNP) : In these plants flowering is not affected by length
of light period i.e. they produce flower in almost all photoperiods.Cucumber,
Tomato, and Sunflower, are day-neutral plants.
Though flowering is the best known example of photoperiodism, many other plant
processes are also controlled by duration of light. Bud dormancy, bulb formation
in onion, and tuber formation in potato are affected by period of light.
Notes
20.9 ROLE OF FLORIGEN AND PHYTOCHROME IN FLOWERING
After the discovery of effect of light on flowering, the scientist tried to find out
the hormone responsible for flowering in plants. It is hypothesized that a plant
hormone called Florigen is responsible for initiation of flowering in plants. Florigen
is a hypotheticals flowering stimulus synthesized in the leaves under favourable
photoperiod, which migrates to shoot apex where flowering occurs.
Have you ever thought how a plant comes to know about the presence or absence
of light in its environment? It is due the presence of a particular type of pigment
in the plants, called Phytochrome. It is also known as light absorbing pigment and
it makes the plants sensitive to light and participates in seed germination and
flowering. This pigment occurs in two different forms, one Pr and the other, Pfr.
While Pr absorbs red light Pfr absorbs far-red light (such rays are invisible). Both
these forms are inter- convertible. The Pr form absorbs red light and gets converted
into Pfr form and the Pfr form absorbs far-red light and gets converted into Pr form.
Red light
ZZZZZZZ
X
Pr YZZZZZZ
Z Pfr
Far-red light
Fig: 20.6 Inter-conversion of the phytochrome into Pr and Pfr
20.10 VERNALISATION—APPLICATION OF LOW TEMPERATURES
You have already learnt that temperature affects growth and development of plants.
For flowering in some plants, a particular temperature is required. Studies show
that if temperature is reduced to a particular point then flowering occurs at an early
stage. For example by applying a temperature ranging between 1-10° C to certain
variety of wheat, rice and cotton, growth of seedlings is accelerated and flowering
occurs earlier. This method of inducing early flowering in plants at low temperature
is called vernalisation.
Vernalisation is the process of accelerating the process of flowering by
subjecting or exposing the plant to low tempratue.
Practical Utility of Vernalisation
Vernalisation has some practical applications like:
(a) Plants whose life cycle is completed in two seasons (biennials) can produce
flower in one season if their seeds are pre-treated to a low temperature.
(b) Crops can be grown and harvested earlier i.e. biennials can be turned into
annuals.
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20.11 SENESCENCE /AGING OF PLANTS
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Like animals, plants also have fixed life span and after completing that perioid, they
die. Before death we can observe several degradation processes in their body. You
might have noticed yellowing of leaves, and fading of flower colour, in plants. It
is due to loss in structure and function of an organ or the whole plant. The
deteriorative processes which ultimately lead to complete loss of organization
Notes
and functioning of the plant or its parts is known as Senescence.
Senescence occurs due to the deposition of waste material. In some plants the whole
plant dies after flowering and producing seeds. This is called whole plant
senescence. Example-annual plants like rice, wheat, beans, and tomato. In many
other plants, parts above soil die each year and root system stays alive. This is called
organ or shoot-senescence.
Role of hormones in senescence: Abscissic acid and ethylene promote senescence
of leaves but cytokinin delays senescence and helps leaves remain green for long
period.
20.12 ABSCISSION – SHEDDING OFF
You might have noticed whenever a leaf becomes old it separates from the plant
body and falls down. Again ripe fruits and older flowers also become separated from
plants. This detachment of older plant parts or organs from the main plant body
is called abscission.
In plants, a layer of tissue generally forms an abscission zone at the base of the
petiole of a leaf or flower or fruit. The cells of this layer become soft and weak
due to destruction of middle lamella and cell wall. So the organ is easily detached
by wind or rain fall. Plant hormones like abscissic acid and ethylene promote leaf
abscission and auxin prevents it.
20.13 STRESS FACTORS
What happens if you do not supply water to a potted plant for four to five days?
You may observe that the leaves bend down and the plant wilts. Here due to lack
of water the usual life processes of plant are disturbed. We can say that the plant
is facing stress in its life. This may be called as biological stress. Not only water,
there are a number of factors responsible for causing stress in plants like temperature,
salt, shade, light, and pollutants.
Any change in the environmental conditions that may adversely affect the
growth or development in plants is called biological stress.
The effect that is produced in plant as a result of stress is called strain. In the above
example bending of leaves and wilting of plant are strains.
So the reaction of plant facing the stress is called strain
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20.13.1 Types of Stress
There are a variety of stresses to which plants are exposed. Some common stresses
are(a) Water stress; and
(b) Salt stress.
Let us know details about them.
Notes (a) Water Stress
Water stress includes both excess of water (flood) and scarcity of water (drought).
Deficiency of water in the plant makes the leaves yellow and they wilt. The various
processes in plants like photosynthesis and respiration are reduced, cell enlargement
is checked, cell size is deformed and cell rigidity lost due to deficiency of water.
Flooding or excess amount of water in soil reduces root and shoot growth, and
causes blackening of root tips and yellowing of leaves.
(b) Salt Stress
Salt stress occurs mainly due to the presence of excess amount of calcium and
sodium salts in plant body. It causes dehydration of cell, change in shape of cell
and disturbance in metabolic processes. Thus cell growth as well as growth and
development of plants are retarded.
20.14. PLANT MOVEMENTS
While doing any work our body parts move and also entire body moves from one
place to another as per our desire. This is called movement of our body. Plants also
show movement. But their movements are completely different from our body
movement. Except some unicellular plants, all other higher plants cannot move from
place to place as their roots are fixed in the soil. Still they show movement by folding
the buds, opening and closing the flowers, and bending towards sun light. These
movements in plants are very slow and we have to wait and observe them carefully
and patiently to notice these movements. Let us learn about various types of
movements shown by plants.
(a) Tropic Movement (directional response or growth movements)
Movement in plants or in any part of the plants towards or away from some
environmental factors is known as tropic (trope : turn) movement. You must have
observed the movement of plants in the direction of light, the downward movement
of roots in the soil, drooping of leaves of some sensitive plants by touch, etc. These
are examples of tropic movement.
(i) Phototropism : Induced by light e.g. bending of stems towards light.
(ii) Geotropism : Induced by gravity e.g. growth of roots towards gravity.
(iii) Thigmotropism : Movement caused by contact e.g., twining stem and tendril
and the drooping of leaves of sensitive plant by touch.
(iv) Hydrotropism : Induced by water i.e., growth of roots towards source of
water.
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(b) Nastic Movement
The nastic (nastein : bending) movements are the growth movements resulting due
to difference in the rate of growth on opposite sides of an organ e.g., opening
of petals, coiling of leaves, etc. When upper side of an organ grows faster than the
lower side, the movement is called epinasty. (e.g., downward curling of leaf,
opening of sepals of goldmohur flower. When the lower side grows more rapidly
than upper side, it is called as hyponasty. (e.g. upward curling of leaf blade)
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Notes
(c) Turgor Movements
These movements are due to change in the volume of water inside the cell. When
more water is present in the cell it is fully expanded and becomes rigid or hard.
Such a condition is called turgidity and the cell is said to be turgid. When less water
is present inside the cell, it is not fully expanded and remains soft. This is called
flaccid condition. The leaves bend in hot summer due to excessive transpiration on
account of loss of turgidity of cells of the leaf.
Some examples of turgor movements are :
(i)
Leaves or leaflets of some plants close on the fall of darkness (sleep
movement). Example - Portulaca, Acacia.
(ii) Closing of leaflets and drooping of leaves in response to a strong stimulus of
blowing wind or of touch. Example - Sensitive plant (Mimosa pudica)
(iii) Closing of leaves of Venus Flytrap to catch a landing insect.
(iv) Seed pods of some plants open on maturity, vigorously expelling their seed.
Example - Balsam (Gulmehandi).
INTEXT QUESTIONS 20.3
1. Distinguish between Phototropism and Geotropism
....................................................................................................
2. Give two examples of turgor movement
....................................................................................................
WHAT YOU HAVE LEARNT
z
Growth in living organisms results from increase in the number and size of a
cell, organ or whole organism.
z
Development is the whole series of qualitative and quantitative changes (growth,
differentiation, maturation), which an organism undergoes throughout its life cycle.
BIOLOGY
51
MODULE - 3
Reproduction and
Heredity
Growth and Development in Plants
z
Growth of cells occurs in three successive stages i.e., cell division, cell
enlargement, cell differentiation.
z
Plants show three phases of growth - Lag Phase, Log Phase, Stationary Phase
z
Auxanometer is a specially designed equipment used to measure the rate of
growth of shoot length of plants.
z
The external factors that affect the growth of the plant are light, temperature,
Water and mineral nutrients.
z
The internal factors responsible for plant growth are auxin, gibberellins,
cytokinins, ethylene, and abscissic acid. These are substances produced in a small
quantity in one part of plant body and capable of moving to other parts to
influence the growth of that part.
z
Seed germination is the return of metabolic activities and growth by the seed tissue
to give rise to a new plant. The germination in seeds is mainly affected by factors
like Water, temperature, oxygen, light, and hormone. Flowering plants show two
types of germination, epigeal germination; and hypogeal germination.
z
Photoperiodism is the biological response in growth, reproduction (flowering)
of a plant to the duration of light, which falls on it per day.
z
Florigen is a hypothetical plant hormone, which is responsible for initiation of
flowering in plants.
z
The method of accelerating the ability of flowering in plants by keeping them
at low temperature for sometime is called vernalisation
z
Senescence is a gradual process during which any plant part or the whole plant
completely loses its function and ultimately dies.
z
The process of detachment of any leaves, fruits, flower or any part of the plant
from the main body after getting older is called abscission.
z
Any change in the environmental conditions that may adversely affect the growth
or development in plants is called biological stress. This stress occurs mainly
due to temperature, water, salt, shade, light, and various pollutants.
Notes
TERMINAL EXERCISES
1. State the different stages of cellular growth.
2. Distinguish between growth and development.
3. What is a sigmoid growth curve? State the different phases of sigmoid curve.
4. Describe the various external factors that affect the growth of plants.
5. What is vernalisation?
6. Define the term Photoperiodism.
7. What is auxin? What is its role in the growth of plants?
8. State any two functions of Gibberellin?
52
BIOLOGY
Growth and Development in Plants
9. Explain the role of Cytokinins and Ethylene in growth and development of
plants.
MODULE - 3
Reproduction and
Heredity
10. Distinguish between epigeal germination and hypogeal germination.
11. What is meant by seed germination? Describe the various factors responsible
for seed germination.
12. What is senescence?
13. State any two practical utilities of growth hormones.
Notes
14. What is biological stress? Describe the different types of biological stress.
15. What is apical dominance? Name the hormone responsible for it.
16. What is meant by plant movement? Describe any two types of movement of
plants with example.
ANSWERS TO INTEXT QUESTIONS
20.1 1. Growth : Increase in number and size of a cell, organ organism.
Development : Series of qualitative & quantitative changes including
growth, differentiation and maturation.
2. Process of change in cells, tissues or organs in order to carry out different
functions.
3. Similar cells organise to form a group called tissue to perform a particular
function
20.2 1. (i) Auxin, (ii) Ethylene (iii) Abscissic acid
2. (i) Cell elongation (ii) Delays fall of leaves (iii) suppresses growth of lateral
bud (any two)
3. Dedifferentiation: Process by which precursor cells become distinct cell
types to perform a specific function.
Redifferentiation: Process by which the plant cells while undergoing
dedifferentiation lose their capacity to divide once again but mature to
perform specific functions.
4. Auxin and Cytokinin.
20.3 1. Movement induced by light – Phototropism
Movement induced by gravity – Geotropism
2. (i) Closure of leaves on fall of darkness
(ii) dropping of leaves on touch
(iii) closing leaves of venus fly trap to catch a landing insect (any two)
BIOLOGY
53
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