patterns of growth and development pdf

Patterns Of Growth And Development 2017: Waliggo David 0774963452

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PATTERNS OF GROWTH AND DEVELOPMENT


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Growth and development in plants and animals

Growth is the irreversible increase acquired in the course of development of an

organism.

Development is the progressive change that takes place in an animal from conception

to adulthood Growth occurs in three distinct processes:

i) Cell division (mitosis) where cells increase in number.

ii) Assimilation. This is the synthesis if new structures from materials absorbed leading to

cell expansion.

iii) Cell expansion and differentiation. Cells increase in size and these un differentiated

cell change their shape and form to serve a particular tension.

Growth of a cell is followed by development where complexity is attained.

Measuring Growth.

Growth is established by measuring some parameters of the organism e.g. weight, i.e.

dry weight, fresh weight, height etc.

a) Change in Weight.

This may involve measuring fresh weight or dry weight. This influenced by variation

in the fluid content of the body. This is why dry weight is the best after all moisture

has been driven off.

Fresh Weight

Fresh weight of living organism is taken using a scale. The method is quick and cheap.

It enables the organism to be studied continuously.

However, the large water content may interfere with results. The method is suitable for

small organisms.

- It can not be used on plants which are not potted.

Dry weight

Weight of an organism taken after all water has been removed from it by heating.

It gives accurate results than fresh weight.

However the animal has to be killed.

And not suitable for precious organisms e.g. Man.

- Some materials may be lost during heating.

b) Height and Length


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This involves taking the linear measurements using a tape measure or kymographs,

which record on a graph changes in height with time.

It is fast and easy and suitable for measuring growth in some structures that show

continuous increase.

It does not cater for growth in other dimension, especially in plants that show

Allometric growth (growth of parts of an organism at different rates in relation at

different rates in relation to the organism growth) e.g. In human.

In isometric growth the whole organism grows at the same rate e.g. In fish and

locusts.

c) Surface area

This is used for measuring growth of plant structure e.g. Leaves, that show change in

surface area with times. It involves obtaining a leaf and determining its surface

dimensions on the graph paper. The method is quick and best for organisms that show

allometric growth.

But, it can not be employed for the whole organism and surface area can be affected

by environmental conditions.

d) Change in Volume

This involves measuring the growth of the organism e.g. stem using a tape measure.

It useful in studying secondary growth in plants. It is also easy and quick.

Factors affecting growth

Internal and external factors affect growth:

a) Internal factors.

Presence of growth hormones e.g. Thyroxine (in animals) and moulting hormones (in

insects), growth hormone in plants e.g. IAA (indole acetic acid)

Genetic factors: These are controlled by genes.

b) External factors.

Temperature, light e.g. most fungi and bacteria grow in darkness, plants need light for

growth.

- Nutrients- these provide energy and material for growth.

- Presence of toxic substances may limit growth e.g. alcohol concentrations inhibit

growth of yeast.


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DEVELOPMENT IN ANIMALS

Embryological development is triggered by the act of fertilization.

It occurs in three stages.

Cleavage: Division of the zygote into daughter cells.

Gastrulation: Arrangement of cells into distinct layers.

Organogenesis: Formation of organ and systems.

The above occur in chordates e.g. Amphioxus and amphibians.

Cleavage

This involves the division of the zygote repeatedly by mitosis into small cells called

blastomeres.

The amount of York present determines the type of cleavage.

Symmetrical cleavage occurs where there is no or little York; hence blastomeres of

equal size are established.

Too much York at the vegetal pole leads to un equal cleavage i.e. smaller cells at the

animal pole and larger ones at the vegetal pole within the York. This causes formation

of the micromeres at animal pole and macromeres at vegetal pole.

Cleavage follows a pattern e.g. vertical and horizontal divisions keep alternating.

Cleavage results in formation of spherical a mass of cells which draw away from the

center leaving a fluid cavity in the middle. The mass of cells is called a blastula and

the cavity is called a blastocoel.

Micromeres

Blastocoel


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Yorky macrometers

.

GASTRULATION

This means formation of gut.

Gasturation involves a process of invagination at one end of the blastula i.e. pushing in

wards resulting in formation of two layered cup shaped gastrula.

The outer layer is the ectoderm that forms the skin and other structures while the inner

layer forms lining of gut and its associated structures.

during gastruration, the blastocoel is replaced by the new cavity called Arcenteron

(primary gut). The blastopore is the exterior opening of the archenteron (posterior end

of embryo.

Ectoderm.

Endoderm

Blastopore

Archenteron.

This cells of the gastrula become arranged in three primary cells called the 3 germ

layers i.e. Ectoderm, endoderm, and middle layer called mesoderm.

The mesoderm is formed by a two pouch like evagination of the archenteron on either

side. The rest of the archenteron becomes the endodermal lining of the gut.


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Neural plate

Future notochord.

Becomes mesoderm

Endoderm.

Archenteron

Archenteron.

Neural groove.

Mesodermal pouch.

Neural tube (CNS)

Notochord

Mesoderm

Blastocoel

Gut


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The three germ layers formed later develop into various tissues and organs by

organogenesis. i.e.

Germ layer. Tissue / organ formed in later

Development.

Ectoderm. Skins, scale, hair, feathers,

- Nerves CNS.

- Adrenal medulla.

Mesoderm - Striated and smooth muscles.

- Connective tissue (bone, blood,

cartilage).

- Heart, kidney, lymphatic system.

Endoderm. Lining of alimentary canal, liver,

pancreas, thyroid gland.

- Lining of trachea, bronchi and lungs.

EXTRA EMBRYONIC MEMBRANES.

The embryo of birds and mammals are protected by membranes which develop from

tissues outside the embryo itself.

Drawing showing extra embryonic membranes

cavity


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The embryo is surrounded by the Amnion which encloses the ammonic fluid. The

other protective layer is the allantois and the outer is the chorion. These are separated

by coelomic cavity. The allantois grows towards the chorion to form the allanto-

chorion.

The alanto-chorion develops into the placenta, while chorion develops finger like out

growth called the chorionic villi which projects into the blood spaces in the wall of the

mother’s uterus.

The stalk of the allantois becomes the umbilical artery and vein which convey fetal

blood to and from the placenta.

Placenta provides means by which foetus obtain oxygen. Important changes occur in

circulation at birth. Since supply of oxygen is taken over by lungs.

While still foetus, the umbilical vein conveys oxygenated blood to the posterior vena

cava which enters the right atrium of the heart.

The lungs are functionless and blood bypasses them by flowing through the foramen

ovale, a hole which connects right and left atria, and Ductus arteriosus, a vessel

linking the pulmonary artery and aorta.


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On taking the first breath, the foramen and ductus arteriosus closes, hence, all blood

returning to right atrium is sent to the lungs. Failure for the closure results in blue

baby, a condition where some blood bypasses the lungs leading to in adequate

oxygenation of the tissues.

METAMORPHOSIS

Is the process of change from larva to adult forms? It often involves a profound re-

organization of the body involving considerable breakdown of larval tissues.

Metamorphosis in insects

Insects have two types of life history.

i) Hemimetabolous insects (incomplete metamorphosis). In incomplete

metamophosis, eggs develop into the adult via the nymph stage, which lack wings and

not fully adults.

Moulting and growth occur between each nympal stage.

Complete metamorphosis (holometabolous)

This Occurs in insects like butterflies, moths, beetles and flies. Eggs develop into a

larva which is different from the adult, feeds and grows rapidly. After several

moultings the larva enters a dormant stage, the pupa (chrysalis) its immobile and

active through formation of organs occurs by dividing cells. Nutrients are obtained by

dissolving larval tissue. CNS and imaginal cells remain but other tissues are dissolved

and used to form adult structures

The adult emerges from the pupa during favorable conditions.

The process of metamorphosis is controlled by hormones.

The insect’s brain produces a peptide hormone called brain hormone which

stimulates glands in thorax to release a steroid hormone (moulting hormone).this

hormone Causes moulting (ecydsone hormone).

When larva molts it develops into a larger larva or pupa, depending on the

concentration of a 2nd

inhibitory hormone secreted by corpra allata (allatum) in the

brain. These secrete a hormone called juvenile hormone which maintain larval

characteristics preventing moulting into pupa.

At early larva stage, more Juvenile hormone (neotonin) is released, as it grows bigger,

the brain inhibits its release and instead the moulting results into pupa stage. As

illustrated below.


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ILLUSTRATION OF HORMONAL GROWTH IN INSECTS

Moulting hormone followed by juvenile hormone causes epidermis to produce larval

cuticle. Moulting hormone alone causes the epidermis to produce an adult cuticle.

BRAIN

Neurosecretory Corpus allatum

cell

Juvenile hormone

Thoracic gland

Moulting hormone

Moulting hormone alone causes molting hormone & Juvenile lead to persistence of larval cuticle

Adult cuticle larval cuticle

epidermis epidermis


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Continuous and discontinuous growth in animals

Growth curve.

If an organisms measurements (changes weight) etc are plotted against time, a growth

curve is obtained.

Growth tends to be slow first, then it speeds up, and finally it slows down as adult size

is reached, giving an S-shaped curve (sigmoid curve) or normal growth curve.

Such growth is said to be continuous and it is a characteristic of most animals.

By continuous growth in animals (sigmoid growth curve)

Discontinuous growth

The growth of arthropods shows periods of rapid growth alternating with those of very

slow or no growth. Growth in them is therefore said to be discontinuous or

intermittent. This gives a step like graph.

Growth only occurs when the exoskeleton is shed since it hard and inflexible, hence

prevent increase in size. Ecdysis is immediately followed by increase in size. The

periods of no growth between successive moults is called an instar.

Moult

Weight in mg instar

10 20 30 40

Time in days


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Arrows show sudden increase in weight

Rate of growth

This estimates increase in size that takes place during successive intervals of time

called growth increments. A plot of these increments and time gives a bell-shaped

curve, when growth rate increases steadily until it reaches a maximum and falls

gradually.

Growth rate curve

Percentage growth

Here increase in growth over a period of time is expressed as a percentage of the

growth that has already taken place.

If a child of 10kg becomes 12kg the absolute increase in weight is 2 kg. but percentage

increase is (12-10) x 100

10

= 20%

An adult boy may also increase from 50kg to 55kg, giving an absolute increase of 5kg,

but percentage growth will be (55-50) x 100 = 10%

50

A plot of percentage growth against time gives the curve below;

Where growth is fastest at the beginning of life and gradually slows down.

Daily growth

Increments (mm)


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NB. Development refers to the progressive changes that take place in an animal from

conception to adult-hood: growth and development always occur together by

morphogenesis. In humans, period of rapid growth occur in infancy and adolescence.

The human growth curve

Daily % of

previous days

height

0 5 10 15 20

Age in days


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GROWTH AND DEVELOPMENT IN PLANTS.

Growth is a continuous process in plants and occurs in roots and shoot in special

organs called Meristems.

A meristem is a group of un differentiated plant cells which are capable of dividing

repeatedly by mitosis.

Apical meristems: these occur on the stem and root tips.

Lateral meristems: Occur in the cambium and cork cambium.

Intercalary meristems: These occur at the base of internodes.

Plants have two types of growth.

i) Primary Growth:

This starts at germination and continues at the apical meristems.

Primary growth leads to increase in length of stem or root, forming primary xylem and

phloem.

ii) Secondary Growth.

This is the increase in girth (diameter) or root after division of cells in vascular

cambium to form secondary tissues. (Secondary Thickness)

Seed Germination

This is the emergence and development of an embryo into a seedling that establishes

itself as a new and independent plant.

In plant, development commences with growth of the zygote into simple embryo

within the seed. The embryo is differentiated into plumule (shoot) and root (radicle).

The embryo is surrounded by endosperm tissue. All the above are enclosed and

protected within the seed coat.

Some seeds e.g. the broad bean have large fleshy cotyledons with food and less

endosperm tissues. Others e.g. Sun flower have a lot of endosperm and small

cotyledons, which determines the type of germination.


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TYPES OF GERMINATION.

This is determined on whether or not cotyledons emerge above the ground.

In dicotyledons, the shoot axis below the cotyledons hypocotyls) elongates, hence

cotyledon are carried above the ground. This is called epigeal germination. In

monocots, the inter node above the cotyledons (epicotyls) elongates and cotyledon

remain below the ground hence called hypogeal germination

Epigeal Germination:

This is when the cotyledons appear above the ground due to rapid elongation of

hypocotyls (Portion of a stem below the cotyledon) e.g. in cotton, beans, tomatoes, sun

flower.

Drawing of an embryo

It’s a characteristic of seeds with small cotyledons. Once these are exposed to light,

they develop chlorophyll and start carrying out photosynthesis, but have large

endosperm that they developing seedlings depends on.

Hypogeal germination.

This is when cotyledon remain below the ground and the embryo emerges due to the

elongation of the epicotyls (portion of the stem above the cotyledons. e.g. in Maize


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and peas. The seeds have much stored food in large cotyledons and provide the

growing embryo of with nourishment’s till the green leaves appear.

Viviparity

Seeds may germinate inside the fruits, while still attached to parent plant and obtain

nourishment from the parent plant.

Conditions for seed germination

Water:

This activates enzymes that control the metabolic process like hydrolysis of starch to

glucose and other food reserves. The water dissolves stored food.

The water is absorbed by a process of imbibitions and enters the seed through the

micropyle by osmosis

Imbibition depends on.

The amount of protein and other food reserves in seeds,

The permeability of seeds and availability of water.

The role of water is to activate the biochemical reactions associated with germination

It also causes the embryo to release hormones that stimulate rapid production of

energy from food stores

Dormancy of some seeds is broken after water intake. The stimulation is due to rise in

gibberellins or reduction in inhibitors e.g. in lettuce plants

Water is needed in the translocation of soluble products of hydrolysis to growth region

of the embryo

Air: this is oxygen

Oxygen since it’s used to oxidize the stored food by the living cells

It’s therefore required for aerobic respiration to provide energy needed in storage and

growth centres of the seed.


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Temperature.

The optimum temperature depends on the type of plant. it influences the rate of

enzyme controlled reactions

Light.

Some seeds may need light for germination (positive photoblastic) and others and

others do not need light to germinate in light inhibits germination (negative

photoblastic). Neutral seeds are not affected by light or darkness.

soil structure. Light inhibits germination of some seeds until the seed is buried in

suitable media e.g soil;

Micro Organisms e.g.

Fungi: these may break dormancy.

Other factors may be internal e.g.

Enzymes: these enzymes help in the hydrolysis of stored food reserves for utilization

by the germinating seed.

Energy needed to maintain the activities of the developing and seed growing embryo.

Energy is obtained from oxidation of stored food.


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Relative change in dry mass of endosperm and embryo during germination action of

barley.

There are two centre of action in germinating seed, the storage centre and the growth

centre (embryo).

The main event in the storage centre is catabolic reaction, and enzyme synthesis.

Protein → amino acids

Polysaccharides → carbohydrate sugars.

starch maltose glucose

lipids fatty acids + glycerol

The soluble foods are translocated to the growth regions of the embryo. Sugars, fatty

acid and glycerol are used to provide substrate for respiration in storage and growth

centre. Also used in growth centre for anabolism. Glucose may be used for cellulose

synthesis and cell wall materials.

Amino acids (a.a) are for protein synthesis, and structural component enzyme

synthesis for protoplasm.

The sugars are oxidized to carbondi oxide + water to provide energy.


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There is loss of dry mass till the seedling produces green leaves & start making up

food.

Respiration in germinating seeds.

There is high respiration in tissues and embryo.

Viability of seeds:

Only viable seeds can germinate even in presence of the required external conditions.

SEED DORMANCY

Seed dormancy is the state where the seed fails to germinate though its viable under

conditions normally considered to be adequate for germination.

Seeds may not germinate if the water content in them is very low 5%- 10%

Addition of water may help to beak this dormancy.

Other causes of dormancy may be.

- Environmental factors. e.g. light, pH, light may be necessary for germination of

certain seeds e.g. Lettuce.

- Seed structure/barriers

- The seed coat may be too hard and impermeable to water or air. The dormancy can be

broken by action of bacteria/fungi, passage and feed

- This dormancy is broken by scalirification or injuring the coat using rough paper, pin

or peeling the coat (abrasion of hard coat)

- Also fire can weaken the coat and enable germination

- Physiology of seed.

- The seed may be mature, but the embryo may be immature and not fully developed

leading to dormancy. This is broken if embryo is allowed to mature or allow fruit to

ripen. Growth promoters e.g. gibberellins may be applied.

- Some seeds may require cold period to be broken (stratification).

Dormancy due to inhibitors e.g Abscisic acid

These prevent mitosis and enzyme reactions e.g. abscisic acid.

These natural chemical inhibitions prevent the seed from germinating.

The dormancy can be broken by soaking in water for long periods leading to leakage

of inhibitors or in solution like sulphuric acid or heating

Role of seed dormancy.


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Ensures seeds do not develop a fruit.

Allows seeds to germinate when embryo is mature.

Allows storage of food as food for animals i.e. seeds are at a low metabolic rate.

Helps in seed dispersal.

Enables seeds to germinate only under favorable conditions.

Permits plant to postpone development when conditions are unfavorable

Seed dormancy permits embryo development to be synchronized with critical aspects

of the plant habitat such as temperature or moisture

Dormancy facilitates dispersal and migration of genotypes into new habitats

Seed attains maximum protection to the young plant vat its most vulnerable stage of

development

BREAKING DORMANCY

This is done by

Cracking the seed coat to enable oxygen and water to reach the embryo

Some seeds in tough fruits need to be exposed to fire in order germinate

Some seeds germinate if they pass through the intestines of birds and mammals

or regurgitated by them. This wakens the seed coat and enables water to enter

them

Improved circumstances may stimulate germination of seeds for particular

plants that may be extinct, therefore can re appear

Some seeds need stratification (frozen for periods of temperature at low

temperature in order to germinate. This prevents seeds that grow in cold areas

from germinating until they have passed the winter, protecting their seedlings

from cold conditions

NB

Seed dormancy is an evolutionary factor in plants that ensures their survival in

unfavorable conditions, allowing them to germinate when the chances of the

young plants to survive are very high


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GROWTH OF THE EMBRYO.

This occurs by cell division, enlargement and differentiation.

During growth proteins, cellulose, increase while dry mass of stored food decreases.

The 1st sign of growth is emergence of the root from the radicle. Then the shoots

develop from plumule.

In grasses, the plumule is protected by a sheath called coleoptiles. This is positively

phototrophic and negatively geotropic. The root is protected by coleorhizza

The 1st leaf emerges and responds to light. This is followed by phyotochrome

controlled responses called photo morphogenesis.

During photo morphogenesis thee is change from etiolation to normal growth which

involves expansion of cotyledons and 1st foliage leaves, formation of chlorophyll

Germination and early growth of flowering plants.

Germination is on set of growth of the embryo. It requires water, oxygen and

temperature within a range. Light may be necessary.

The seed takes up water rapidly by inhibition and later by osmosis.

The water causes the seed contents to sell and also activates the hydrolytic enzyme

e.g. invertase, zymase). That hydrolyses the insoluble storage maternal into soluble

forms that can be assimilated. I.e. Proteins charged to amino acids etc carbohydrate

e.g. starch to glucose, fats to fatty acids and glycerol.

These soluble forms are oxidized to obtain energy for growth.

Glucose is used in formation of cellulose Cell wall, amino acids for enzymes

formation and structural protein formation.

Early growth results in plumule and radicle growing rapidly. The radicle grows down

wards and plumule upwards. E.g. in sunflowers, the cotyledons are carried up and out

of soil (Epigeal germination). In broad beans and wheat, the cotyledons remain below

the soil surface (hypogeal germination)

The radicle is always the first to rupture and develops hairs.


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Then the hypocotyls grows, the plumule on surface develop chlorophyll and

photosynthesis before true foliage leaves appear. Those below ground e.g. in maize

don’t.

PRIMARY GROWTH AND DIFFERENTIATION.

.

PRIMARY GROWTH

This is the very 1st growth, and may be the only growth in some plants. It’s as a result

of the action of apical and intercalary meristems. Some plants have secondary growth

and it’s due to the growth of the lateral meristem. e.g. trees and shrub. Herbaceous

plants (Herbs) have no secondary growth. This means that herbaceous plants cannot

survive for more than 5 years (3-5yrs) life span. Cells formed by primary growth have

short life span. Herbs cannot expand sideways (laterally) because they lack lateral

meristems. They only have apical meristems

Apical meristems are structurally small, cubed and have a thin cellulose cell wall and

dense cytoplasmic contents. They have small vacuoles unlike in parenchyma with

large vacuoles. Apical meristem also has undifferentiated plastids called proplastids.

This meristem has a tight package with no space between them.

Their cell called initials divide, one remains in the meristem and the other increase in

size and differentiates to be a permanent plant body.

This involves the expansion of cotyledons and 1st phase leaves, formation of

chlorophyll (greening). This is when photosynthesis begins and dry mass starts to

increase until the seeding is independent of food reserves.

Apical meristem responsible for plant growth.

Lateral meristem responsible for 20

growths.

Intercalary


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Occurs in between permanent tissue e.g. nodes of monocots. Allows increase in

length in other parts of the plant not necessarily the tip. This is advantageous if

the tip is eaten by herbivores

GROWTH OF THE PRIMARY PLANT BODY:

Growth is confined to meristems.

Meristems are a group of cells with the ability to divide by mitosis, produces daughter

cells which grow and form the rest of the plant body.

There are three types of meristems

Apical meristems.

Located in root and shoot apex.

Responsible for 10 growths, which gives one to the primary plant body. Apical

meristems increase length. Apical meristems are known to cause increase in length

due to cell division and cell expansion in apical meristems.

Lateral meristem (cambium).

These occur in older parts of the plant which are parallel with the long axis of organ

e.g. Cork cambium (phellogen), vascular cambium.

Phello is a Greek word meaning cork

Phelloderm layer of plant cells produced by inner surface of the cork cambium in

woody plants from which cork tissues develops.

It’s responsible for 20 of growth. Vascular cambium gives rise to 2

0 vascular tissues its

therefore a secondary meristem:

The phellogen gives rise to the periderm. This replaces the epidermis and includes

the cork.

Periderm is the outer layer of the plant tissues in woody roots and stems.

The periderm (protoderm ) develops into the epidermis.

The overall effect causes increase in girth.

Phellogen ( cork), phellogen and phelloderm form periderm

Ground meristem: produces parenchyma cells Primary growth of shoot.


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The apical meristem has three major regions i.e.

- Region of cell division, cell elongation and division. There also occur in the root.

Pericycle- Outer layer of plant tissue surrounding the inner core of roots and stems of

plant stele. It conducts moisture and nutrients around the plant.

The secondary meristems that occur are:-

Protoderm – This forms the epidermis.

Procambium – Which forms vascular tissues i.e. Pericycle, xylem, phloem, etc.

Procambium is undifferentiated plant tissue that develops into cambium and vascular

tissues.

Ground meristem – this produces the parenchyma. (Cortex and pith)

The cells are formed by cell division and in the region of elongation, these cells absorb

water by osmosis hence increase in size. Increase in length of stems and roots occur by

elongation of cells. The small vacuoles increase in size.

PLANT TISSUES

Similar kinds of cells are organized into structural and functional units called tissues.

There make up the plant as whole new cells are formed at growing plants of there

dividing cells. The growing points are called meristems, which occur at the at tips of

shoots and roots (apical meristems)

They are responsible for 10 of growth.

Lateral meristems are responsible for 20 growth of plant.


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Vascular plants have 3 tissue (systems).

Dermal system.

Vascular system.

Ground system

NB

Apical meristms give rise to embryonic tissues called primary meristems

Which undergo differentiation to become plant body

The 3 primary meristems are

Protoderm which differentiate into epidermis

Procarbium which differentiates into vascular tissues, primary xylem and phloem

Ground tissue which differentiates into ground tissues

DERMAL SYSTEM.

Consists of epidermis, forms covering of leaves, flowers, roots, fruits and seed.

Epidermis may contain stomata, has specialized guard cells e.g. leaves.

Some times epidermis is covered with waxy coating called cuticle – water proofing

layer.

Plants which undergo secondary growth have their epidermis replaced by the

peridermis. Which is made up of heavily water proofed cells (cork) that are dead at

maturity.

VASCULAR SYSTEM

Consists of xylem (for the conduction of water) and phloem (for conduction of food)

XYLEM

This consists of water conducting cells.

Tracheids and

Vessels.

Tracheids

These are elongated cells, tapered at ends; both lack cytoplasm and are dead at

maturity. Then walls have pits – areas when no 20 thickening occurs so that water

moves from cell to cell.

Vessels

Vessels are shorter than tracheids vessels in addition to pits, have perforation


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- Areas of cell that lack both 10 and 2

0 thickening through which water and nutrients

freely pass.

PHLOEM

They are the Food conducting tissue. They are living at maturity. Principle cells are

the sieve elements. Sieve elements contains cytoplasm at maturity but have no nucleus

and other organelles.

They have companion cells allocated with them, which contains nuclei and

manufacture and secrete substances into the sieve elements and removes wastes from

them.

GROUND TISSUE. (Meristems)

This consists of:-

i) Parenchyma.

ii) Collenchymas.

iii) Sclerenchyma.

Parenchyma occurs throughout the plant and is living, capable of cell division at

maturity. They carry out physiological functions e.g. Photosynthesis, storage,

secretion, wound healing. They occur in phloem and xylem.

Collenchyma.

These are the second type ground tissue, which are living at maturity. They have un

thickened 10 cell walls.

They function as support tissue in young growing portions of plant.

Sclerenchyma

This consists of cells that lack protoplast at maturity and have thick secondary walls

that contain lignin. Hence important in support and strengthening plant portions that

have finished growing.

PRIMARY GROWTH IN ROOTS.

The apex of stem or root can be distinguished into 3 – zones i.e. the extreme apex

(zone o cell division) behind it is the zone of cell elongation and further back is the

zone of differentiation.

In roots, the apical meristem is protected by the root cap; in both plant shoot and roots,

the tissues behind the zone of differentiation are called permanent tissues. They are

formed by apical or primary growth and make up the primary structure of the root.


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SECONDARY GROWTH

Secondary growth leads to increase in girth while Apical growth or primary growth

leads to increase in length. It occurs in the permanent zone. 20

growth occurring in the

meristematic cells i.e. cambium, secondary growth is achieved by presence of a ring of

cambium in dicotyledons, which separates the xylem and phloem.

Secondary growth leads to side way expansion e.g. in trees. Secondary thickening

originates from two lateral meristems.

vascular cambium

cork cambium

Vascular cambium divides to form xylem and phloem. Old phloem extends outwards

and dies. Secondary xylem changes with age. It becomes strengthened with lignin and

cellulose to form wood. All tissues beyond vascular bundles are bark and includes

secondary phloem and cork cambium.

Cork cambium has dividing cells and produces cork which is beneath the epidermis.

Mature cork cells are dead and have a thick impregnation of suberin (waxy) makes

cork water proof and prevents weathering of plant tissues.

Cells of cambium divide to form secondary xylem tissue on the inside and secondary

phloem to the outside.

In between adjacent vascular bundles secondary parenchyma is formed, that leads to

increase in girth.

More xylem is always formed than phloem hence cambium and phloem are always

pushed outwards during secondary growth. Concentric annual rings are formed

seasonally and can be used to determine the age of tree. Surface tissues of the plant


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also undergo secondary growth. Beneath the epidermis is a layer of cells called cork

cambium, which divide to give new cells. Those to the inside form the secondary

cortex, while those to the outside form corky cells. Their walls become impregnated

with suberin; a fatty material witch makes them impermeable to water and respiratory

gases – which form the bark (dead and living tissue) outside wood. Cork has a loose

package of mass of cells called lenticels, used for gaseous exchange

Figure showing lenticels

THE CONTROL OF GROWTH

Growth is controlled by internal and external factors.

Refer to previous notes on growth regulators in animals, for details of these factors in case you have

forgotten.

THE CONTROL OF PLANT GROWTH

Growth is regulated by plant growth hormones.

PLANT GROWTH SUBSTANCES

They are natural substances occurring in plants at low concentrations. they regulate

growth and development from seed formation to ageing, also co-ordinate responses

e.g. tropisms. They include these include Auxins, e.g. IAA. Gibberellins,Cytokinns

(kinins), Abscisic acid and, ethane (ethylene).

AUXINS

Indole Acetic acid (IAA)


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This is synthesized in the apical meristems. it promoes growth by increasing the rate

of cell elongation. Also causes apical dorminance; where its high concentration

prevents growth of lateral branches. near the apex.

it also involved in leaf fall 9 abscission), root development and fruit growth and

development. A synthetic auxin e.g. 2-4-D (2,4 dichlorophenol xyacetic acid) which is

a herbicide.

IAA stimulates growth of adventitious roots

Giberellins

This stimulates growth of shoots and leaves. It’s formed in young leaves at growing

tips. It can stimulate growth in young shoots (genetically inherited dwarfness).

Gibberellins stimulates seed germination, hence used to break dormancy

Cytokinns

These are growth promoters synthesized in roots and transported to all plant parts. it

stimulates cell division once combined with auxins. cytokinns slow down senescence

or ageing.

Abscisic Acid

This is a powerful growth inhibitor, working antagonistically to the growth promoters.

Its synthesized in chloroplasts in shoots (leaves). It’s known to stimulate stomata

closure. ABA were once known to cause abscission (falling off of plant parts) but its

not the case.

Ethene (Ethylene)

It’s released from ripening fruits, nodes, ageing flowers and stems, leaves. Ethene

causes seed dormancy, fruit ripening and leaf abscission.

More growth substances are yet to be discovered.

Action of Plant growth substances

These work like animal hormones by carrying information from one part place to

another and regulate responses to environmental stimuli, but are not secreted in special

organs as in animal hormones, but made by cells in many different parts of the plant.

They are not moved from their site of production to target cells as in animal hormones

i.e. they work in the very tissue where they are produced e.g. ethene

Synergism


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Two or more growth substances interact to give a great effect than individual actions

of the substances.

Antagonism

Two or more growth substances interact to reduce each other’s effect e.g. ABA and

gibberellins. ABA causes dormancy in buds while gibberellin breaks dormancy.

Plant growth substances may stimulate or inhibit growth in different plants e.g. ethene

promotes root, leaves and flower development in some plants but inhibits in other

plants. Growth and development in plants is brought about by the interaction of a

number of growth substances in a given balance of concentration

Plant movements

Sleep movements

Change in position of leaves and petals at night occur in circadian (daily) rhythms.

Tropisms

The most important plant movements it’s the movement of one part of the body in

response to stimuli. Growth towards a stimulus is said to be positive tropism and that

away from the stimuli is a negative tropism.

Gravitropisms

Movement away or from gravity

Phototropism

The plant growth response to directional light

Hygroscopic movements

Response to changes in moisture, shown by non living plant organs e.g. fruits which

explode when dry to disperse seeds.

Nastic movements

This is the Movement by plant organs in response to external stimuli e.g. touch,

temperature or light level. The movement is independent of the direction of the

stimuli. Nastic responses are caused by differential growth, but the curling of Mimosa

pudica is due to rapid change in cell turgidity.

How photo tropism causes elongation

Coleoptiles tips produce auxins in the same concentration in dark and light but their

distribution varies. when light strikes one side of a coleoptiles, a flavin receptor e.g.

FAD (coenzyme from vitamin B2 which absorbs light in the blue spectrum, triggers


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the redistribution of auxins so that more travel to the shaded part of coleoptiles ,hence

shaded part grows longer than on illuminated side.

the auxins are believed to trigger protein synthesis and hence increase elongation.

also believed to cause secretion of protons (hydrogen into cells increasing their

acidity, which weakens the bonds between cellulose micro fibrils, allowing cell wall to

expand when the cell takes in water. however, the above may only be true for plants

that have coleoptiles.

LIGHT AND PLANTS GROWTH.

Light affect s a number of processes in plant e.g.

Photosynthesis.

Phototrophic movement

Stomata opening and closure

Root elongation.

Synthesis of chlorophyll.

If the is grown in darkness. Etoilation occurs, no chlorophyll forms the stem is

elongated and leaves fall off.

Seeds of lettuce only germinate if exposed to light./

The most effective light for germination is of wave length (580-660) nm.(Or red light)

while alight of wave length (200-730)nm. (Or far red light) inhibits germination.

Exposing seeds to an alternation of far red and red light with cause germination if the

last flash is red light, and inhibition occurs of the last flash of light is far red light.

TEMPERATURE AND PLANT GROWTH

Temperature controls processes like germination, mobilization and cell division.

Flowering is also increased by temperature. Tropical plants require high temperatures

to flower.

Flowering can be induced in temperate plants by exposing germinating seeds to cold

treatment a phenomenon called VERNALIZATION.

It is used to induce early flowering in crop plants, its effective ness increased by the

plant is later exposed to long period of light.


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PHYTOCHROME SYSTEM

Light is absorbed by a substance called phytochrome which occurs in tips of growing

shoots. Phytochrome is a pale blue light sensitive protein occurring in the plant tissues

(Phytochrome has a blue pigment attached to a protein)

The phytochrome occurs in two inter convertable forms each at different absorption

peaks E.g. one absorbs red light at peak of 660nm, and the other absorbs far red light

with peak of 730nm, hence called phytochrome 665 (or P660) and phytochrome 730

(or P730) .

If P660 Absorbs Red light, its converted into P 730

When P730 also absorbs far red light, it is converted into P660.

However in darkness into P730 is converted to P660 but it is very slowly.

During blight light, more Red light it is available so more P660 is converted into

P730, hence becoming abundant (P730) than P660, at night P730 accumulated during

day light is converted into P660, though it’s slow.

P660 Red light P 730

Far red light.

Slow conversion at night

Day (fast conversion)

P660 P730

Night (slow conversion)

P730 is more active than P660 (Inactive) P730 is known to inhabit growth.


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IMPORTANCE OF THE PHYTOCHROME SYSTEM.

Both red and far red light can be stimulants for given process or in habit. Others e.g.

Far Red light stimulates stem elongation while red light inhabits.

Far red light stimulates growth of lateral roots, but Red light inhabits.

Far red light stimulates growth of lateral roots, but red light inhabits it.

The alternation of light and dark periods cause flowering by a phenomenon of

photoperiodism.

Absorption of a given light of a given wave length stimulates hormonal production,

hence the observed growth response.

PHOTO PERIODISM

Photoperiodism is the response of a plant to changes in day or night length or

The influence of the relative length of a day and light on plant and animals activities

e.g. flowering. The relative length of day and night varies with time of year hence

called photo period. All activities of plants like flowering fruit ripening, etc occur by a

biological clock as in animals.

If some plants are flashed with light at night, the night length if interrupted and do not

flower while other plants if light period is increased, flowering is stimulated.

In terms of light / dark responses, plants can be divided into:

1. Long day plant e.g. lettuce, cereals

2. Short day plant.e.g. straw berries

3. Day neutral plants. E.g. tomatoes, cucumbers.

LONG DAY PLANT (LDP)

These require light period to exceed a critical value to flower about 10 hours on

average. Long day plants require longer days, and shorter nights.

Reduction of light to less than period causes flowering not to occur e.g. radish, lettuce,

cereals. In long day plants, accumulation of P730, due to long exposure to light

stimulates flowering.

Long day plants will flower in short days when the long night period is interrupted

with red light.

Short dark interruptions don’t cancel the effect of long days


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In LDPs, Pfr promotes flowering while in short day plants (SDP) it5 inhibits flowering

Only long nights remove sufficient Pfr since its conversion to Pr is slow, hence requires

more night period.

Exposing short far red light to SDP does not cause flowering because time / length of

exposure is very important for SDP, just because the conversion of Pfr to Pr requires

along period

SHORT DAY PLANTS

These flower when the light period is shorter than a critical length in each 24 hour

cycle e.g. Cocklebur, it’s about 141/2 hours. Short day plants are actually long night

plants because, interruption of their long night by short light period or flashes prevent

flowering in them

DAY NEUTRAL PLANT

These are not affected by day light e.g Tomato, cotton.

However in both, short day and long day plants, it’s the length of dark period not light

period that determines the flowering.

Long day plants flower if nights are shorter than critical length.

Short day plants are induced to flower by night longer than critical length.

The action of photo periodism involves hormones

In short day plants, presence of Pfr stimulates a biological reaction that inhibits flowering

Only red light inhibits flowering of short day plants, the inhibition is removed when

plants are treated with far red light.

The far red reconverts P730 to P660.

Some short day plants flower when a sufficient proportion of phytochrome is in form

of P660. Short day plants are triggered to flower by light accumulation. Of P660 or

low concentrations of P730

Therefore high P730 (Pfr) inhibits flowering of these plants and once it’s converted

into P660, the inhibition is removed and flowers develop.


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I.e. Flowering of SDP is promoted by absence of P730, more than presence of P660

24 short day plants (long nights) long day plants (short nights)

Light Light

12.............…………………critical night……………………………………………critical night length

Flash of light flash of light

Darkness Darkness

0 no flowering flowering no flowering Flowering no flowering flowering

Key

Light

Darkness

NB

Gibberellins mimic the effect of red light and causes flowering if applied on the plants.

The effect of phytochrome involves a hormone florigen that is released in presence of

appropriate light conditions for plant as shown below.

LDP SDP


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In short day plants, florigen is secreted when Pfr is low and Pr is high.

In long day plants florigen is secreted when Pfr levels are high and Pr level is low

But florigen has never been isolated the above observations have never been

confirmed.

Trial questions

florigen


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ENDS


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patterns of growth and development pdf

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