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 20
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 20
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

- 28 -
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)

- 29 -
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

- 30 -
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

- 31 -
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.

- 32 -
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.

- 33 -
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

- 34 -
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.

- 35 -
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

- 36 -
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

- 37 -
ENDS

- 38 -

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