Biology

Key Unit Competence
Account for the processes of growth and development in plants and animals

Learning Objectives
By the end of this unit, I should be able to:
–– Describe dormancy as a state of inactivity to absolute minimum due to the morphological and physiological state of a plant structure
–– Explain how dormancy is maintained and broken
–– State the conditions required for germination
–– Outline the role of enzymes in the process of germination
–– State types of plant growth hormones and their functions
–– Identify the hypocotyl and coleoptile in a germinating seed
–– Describe the stages and types of germination
–– Recognize that a meristem is a growing point of the plant and the main meristematic regions of a tree
–– Describe current views about photoperiodic control of flowering
–– Observe structures of endospermic and non-endospermic seeds
–– Demonstrate how fruit and seed dispersal takes place
–– Demonstrate hypogeal and epigeal germination
–– Carry out an investigation to distinguish between primary and secondary growth
–– Appreciate the importance of fruit and seed dormancy and germination in the life cycle of plants
–– Carry out an experiment on the development of eggs at different temperatures
–– Describe the process of metamorphosis in arthropods and amphibians
–– Distinguish the various stages of development in frogs
–– Analyse complete and incomplete metamorphosis
–– Compare growth patterns in arthropods and vertebrates
–– Appreciate the demands of the terrestrial environment to the adaptation of amphibians

Introductory activity
Suggest morphological differences by which different organisms develop and grow to maturity.

13.1 Fruit, seed and bud dormancy
Activity 13.1
–– Put dry bean seeds and maize grains in boiling water for 10 minutes.
–– Use nails to make a longitudinal section of the bean and maize.
–– Compare the two sections.
–– From your point of view, do you think that all plants’ organs are always active?
Justify your point of view.
–– Produce a picture showing how plant organ (seed and bud) behaves in
relation to oxygen, temperature and water.

A seed is a plant organ that develops from the fertilized ovule. As the seed embryo develops from the zygote, the seed makes a food reserve of macronutrients including carbohydrates, proteins and lipids. The amount of reserved food type depends on the plant species. Legumes like peanuts and soybeans store proteins as well as fats while beans store more starch and proteins. A seed consists of a plant embryo surrounded by the food supply in cotyledon or endosperm and a protective coat called seed-coat or testa. The plant embryo is composed of the radicle or embryonic root, the hypocotyl which is the embryonic stem and the plumule, the embryonic leaves

Dormancy is a period of cells’ inactivity due to a very low metabolism to prevent growth when environmental conditions are unfavorable. The dormancy can be for fruit, bud or seed.

a. Bud dormancy
Bud dormancy is a suspension of most physiological activities and growth that can be reactivated. It may be a response to environmental conditions such as seasonality or extreme heat, drought, or cold. The exit from bud dormancy is marked by the resumed growth of the bud. Bud dormancy may proceed to dormancy of the whole plant.

b. Seed dormancy
Seeds exhibit dormancy to avoid growth during unfavorable conditions. During the last stages of its maturation, the seed dehydrates until it gets dry. The embryo which is surrounded by a food supply (cotyledons, endosperm or both), enters dormancy.
Most seeds are enough durable that can last a year or two until conditions are favorable for germinating. However, the length of time a dormant seed remain viable and capable of germinating varies from a few days to years depending on the plant species and environmental conditions. This may justify the reason why after a bushfire or other environmental disruption, vegetation reappears as their seeds
have accumulated and stayed for longer in the soil. Many plants are easily grown from seeds. Although its embryo is alive, a dormant seed cannot germinate until it is exposed to certain environmental conditions to break the dormancy. Thus, it germinates and starts to grow.

Conditions needed for seed germination
Germination is the process by which a seedling develops from a seed embryo. There are requirements for germination to be successful. Environmental conditions such as water, oxygen, and temperature trigger seed germination. For a seed to germinate, it must be found in place where all these conditions are available. If one condition is not available, germination becomes impossible. These conditions include:

a. Water
For a seed to germinate, it requires water. Mature seeds are very dry and must absorb water by imbibition to germinate. Water softens the seed coat for embryo sprouting and provides a medium for reactions during hydrolysis. Enzymes breakdown macronutrients such as starch, proteins and fats stored in the cotyledons and the endosperm to simple sugars such as glucose, amino acids which provides energy for the embryo to grow.

b. Oxygen
This is needed for needed for cellular respiration by oxidizing glucose to liberate ATP to provide energy for embryo development.

c. Temperature
Seeds germinate only if exposed to their optimum temperature varying between 5 to 40 0c depending on the seed species. Enzymes are sensitive to temperature because enzymes need a favorable temperature to work efficiently. Burying seeds too deeply in the soil prevents them from germinating as they are cut off from temperature and air.

Self-assessment 13.1
1. Define the term dormancy?
2. Explain how dormancy is maintained and broken?
3. Suggest the advantage of dormancy in plants?
4. What are conditions needed for seed germination?
5. Explain the role of enzymes during the process of seed germination.
6. Draw and label endospermic and non-endospermic seeds

13.2 Types and stages of germination.
Learning activity 13.2
Conduct the experiment below to compare the two types of germination. Requirements Two bean seeds or soybean seeds, two sorghum or maize grains, water and two plastic devices/containers

Procedure
–– Fill each container with soil
–– Put two grains of maize in the container labeled A and two seeds of bean in the container labeled B
–– Pour some water in each container simply to moisture the soil.
–– Put both containers A and B in a secured place.
–– Record your observations from day five to day twenty one with emphasis to developmental changes.
In conducive environment, a seed can germinate. Germination involves three main
stages: imbibition, radicle sprouting and plumule sprouting.

The dormant seed contains very little water, when placed in moist conditions, it absorbs water by osmosis. The intake of water by a dry material is called imbibition.

As the seed absorbs water and oxygen,it swells,the ambryo grows and the seed-coat cracks, and the radicle also called primary root emerges downward. This is seed coat rupture and radicle sprouting. The primary root is the fi rst organ to appear during embryo development. The plumule cells develop and the fi rst leaf forms upwards in the process known as plumule sprouting.

A germinating seed shows the primary root from which secondary roots start to
develop and the primary leaf which is surrounded by a protective sheath called
coleoptile. The Hypocotyl is the stem below the plumule while the epicotyl is the
stem above the plumule.

13.2.1. Types of seed germination

There are two types of seed germination: hypogeal and epigeal seed germination.

a. Epigeal germination

Epigeal germination is typical to non-endospermic seeds like beans, soybeans and coff ee seeds.In this type of germination, the cotyledons come above the soil surface into air and light, due to rapid growth and elongation of hypocotyl. The cotyledons are green and make food to be used by stem during growing season. They fi nally dry off and seedlings become independent. The example of epigeal germination is like the one found in bean seeds.

b. Hypogeal germination
In this type of germination, the cotyledons remain in soil or just above the surface.Here the epicotyl elongates, pushing plumule upwards. Cotyledons do not turngreen and gradually dry up and fall off . An example of this type of germination is found in pea, mango, and groundnut seeds.

Self-assessment 13.2
1. Use diagrams to demonstrate and distinguish the types of seed germination.
2. Describe the three stages of seed germination.

13.3 Primary and secondary growth
Learning activity 13.3
Move around the school garden or anywhere in your surroundings, then collect two plant species in which one is a monocotyledon and another is a dicotyledon.
1. By examining their physical characteristics, describe similarities and differences between collected plants.
2. Predict the reason why all plants grow in length.
3. From your observation, suggest/ predict the reasons justifying why monocotyledons do not grow in width while dicotyledons do.
Growth is a permanent increase in the size of an organism or of some parts of it. It is brought by cell division and the assimilation of new material within the cells which result from the division and the cell expansion which follows. Cell expansion is particularly noticeable in plants, where rapid enlargement can occur as a result of water taken up by osmosis. If the organism achieves its mature size, it starts
development, the formation of new structures or organs to perform specific functions. That is the production of reproductive organs and locomotive or protective organs. It is controlled by cell differentiation. Growth is either primaryor secondary.

a. Primary growth
Primary growth consists of the increase in length and formation of primary plant organs including roots, stem, leaves, flowers, and fruits. It occurs in most herbaceous plants. The roots elongate to penetrate the soil, and shoots elongate to reach the sunlight. Primary growth is controlled by apical meristems. Meristematic cells divide by mitosis. Some daughter cells absorb water and nutrients. As the cell takes
in water the cell walls stretches, the cell elongates and slightly enlarges. After this growth by elongation, cells differentiate and specialize for specific functions. The cells formed from apical meristems do not expand laterally and this limits their size. The herbaceous plants exhibiting such growth tend to be short lived. They are called annual plants because most of them do not live for more than one growing season
(a year) after which they enter dormancy and survive as seeds.

b. Secondary growth
It consists of getting wider or thicker, and occurs in roots and stems of perennial woody plants, all trees and shrubs. Woody plants grow taller than herbaceous plants and they live longer, more than two years some plants while some others may live for and over 30 years.
Secondary growth is controlled by lateral meristems: vascular cambium and cork cambium. Vascular cambium is located between primary phloem and primary xylem. Vascular cambium cells divide by mitosis. Some cells remain meristematic while other cells expand sideways and differentiate. They form the secondary phloem outwards and the secondary xylem inwards. As the secondary growth continues,

the outermost phloem cells die. Their death causes no matter to the whole plant as dead cells are continually replaced by new ones.

The secondary xylem cells are strengthened by the accumulation of lignin and cellulose. They form a wood also called secondary xylem providing the mechanical support to very taller trees. The cork cambium situated between the epidermis and secondary phloem produces the cork cells by mitosis. The cork cells are pushed toward the epidermis and accumulate a waxy substance called suberin making
the cork waterproof. The cork gradually replaces the epidermis. Like the epidermis, the cork protects the plant from dehydration and infection. The expansion of the internal tissues results into the continual cork shedding. Because of meristematic cells that continuously divide, perennial plants have unlimited growth.

Self-assessment 13.3
1. In the table list the differences between primary growth and secondary growth.
2. Describe briefly what the wood is and its major function.
3. What is the name given to a substance that makes the cork to resist plant dehydration?
4. Identify the importance of apical and lateral meristems in plant growth.

13.4 Determination of growth
Activity 13.4
In the learning activity 13.1 you have grown two types of seeds (bean and maize). Based on that experiment do the following to investigate the primary growth in a seedling.
–– Use a centimeter ruler, measure the height of each plant once each five days.
–– Use a thread and a centimeter ruler to measure the width.
–– Record your measurement in a tabular form as shown below.

1. On the basis of the period of time indicated in the above table, count the number of leaves. What do you notice?
2. Among the two given plants, predict the one with an increased volume of protoplasm and dry mass.

The growth rate of an organism is measured by recording the variation in length, in width and in mass through a period of time. The aspects that can allow the measurement of the growth of a plant are the following:
–– The increase in the dry mass
–– The increase in the volume of protoplasm
–– The increase in the length
–– The increase in the thickness
–– The ability to reproduce.
Experiments show that the growth rate is faster in young plants and starts to decrease as the plant gets older.

Self-assessment
1. A sign is hammered into a tree 2 m from the tree’s base. If the tree is 10 m tall and elongates 1m each year, how high will the sign be after 10 years? A mark is hammered into a tree 2 m from the tree’s base. If the tree is 10 m tall and elongates 1m each year, how high will the mark be after 10 years
2. What features would enable you to conclude that this organism has grown?
3. Would you expect a tropical tree to have distinct growth rings? Why or why not?
4. If a complete ring of bark is removed around a tree trunk (a process called girdling), the tree usually dies. Explain why?

hormone prevents the action of another they are antagonist.The plant hormones include five major groups:

a. Auxins
It is produced in growing regions of plant such as shoots, tips, and young leaves, and developing fruits. The most known auxin is Indol Acetic Acid (IAA). Artificially produced auxins are widely used to:
–– Promote the cell elongation in the region behind the apex of the stem
–– Promote root formation on stem and leaf cuttings
–– Increase number of fruit
–– Prevent dropping of fruit
–– Prevent sprouting of stored potatoes and onions

b. Gibberellin or gibberellic acid (GA)
Produced in all parts of plants, especially in immature seeds. Gibberellin has different functions:
–– It promotes the parthenocarpy (formation of fruits without fertilization).
–– It breaks down the bud dormancy
–– It promotes the seed germination.

c. Cytokinins
They are produced in developing roots, fruits and seeds, cytokinin and have the following roles:
–– Promote cell division
–– Promote the growth of lateral buds
–– Promote the growth of fruits
–– Are used to delay aging and death (senescence).
–– Work with gibberellins to break down the bud dormancy and to promote the seed germination

d. Abscissic acid (ABA)
It is produced in leaves:
–– Promotes the abscission i.e. falling of some organs of the plants
–– Promotes the bud and seed dormancy
–– Inhibits the stem growth during the stress
–– Promotes stomatal closure
–– Inhibits other hormones blocking thus the growth.

e. Ethene or ethylene (C₂H₄)
It is produced in fruits, flowers, leaves and roots and:
–– Promotes ripening of fruit
–– Promotes flowering in mangoes and pineapples
–– Promotes abscission (detachment of leaves).

Commercial application of synthetic phytohormones
Artificial auxins are widely used. For example, 2,4-D or 2,4-dichlorophenoxyacetic acid and MCPA 4-chloro-2-methylphenoxyacetic acid are used as weeds killer (selective herbicides). Synthetic auxins used in right concentration, cause excessive growth and very rapid metabolism of broad-leaved dicotyledons herbaceous plants that are weeds in cereals. As weeds grow faster, they soon die increasing thus the cereals crop yield.

Napthaleneacetic acid (NAA) or rooting powder is another kind of auxin sprayed on stem cuttings for stimulating the development of adventitious root. This is very important in vegetative propagation of plants of economic value. Synthetic gibberellin helps to increase fruit crop yield because when sprayed on non-fertilized flowers, they promote parthenocarpy (fruit formation without fertilization). It is
mostly used in production of seedless grapes such as seedless tomatoes or citrus. Synthetic cytokinin is sprayed on cut flowers, fruits and vegetables to keep them fresh and extend shelf-life.

Self-assessment 13.5
1. What are the plants hormones?
2. Describe the role played by each plant hormone.
3. Explain why some plants develop lateral shoots when the apex is cutoff.

13.6 Plant movements and photoperiodism
13.6.2. Plant movements
Activity 13.6.2
Take the container with the bean seedling in the classroom nearby the window and observe the changes within one week.
Like animals, plants move as response to changes in their environment (internal or external changes). Plant movements are grouped into two categories: tropisms and nastic movements.

1. Tropism
A tropism is a movement of parts of a plant in response to external stimulus. The movement is always a growth movement. External stimuli cause changes in the direction of the plant’s growth, such as bending, turning or curving. Tropic responses are described as positive or negative depending on whether growth is towards or away from the source of the stimulus respectively. According to the type of stimulus,
tropisms are classified as follow:

a. Phototropism
Phototropism is a directional growth depending on the direction of the light source. Growth towards a light source is a positive phototropism, while growth away from light is called negative phototropism. It is believed that light destroys auxin where it strikes the stem, causing an imbalance in which the side of the stem that receives less light has more auxin. This causes the plant to have elongated cells on the farthest
side from the light. Because more auxin is present, the cells on the darker side are able to elongate more than the cells on the lighted side, causing the plant to bend toward the light.

a. Illustrate the expected shape of the shoot and that of the root after a week.
b. Suggest names (of what) for the expectations in the above experiment.
c. Based on the above experiment, draw a diagram illustrating the shape and size of cells of both upward and downward side of the root.
d. Suggest a technique that can be used to minimize the effect of the stimulus
in this experiment.
2. a. What is the difference between ‘antagonistic’ and ‘synergistic’ when referring to plant growth substances?
b. What are the two plant growth substances that act antagonistically and which act synergistically?
3. Copy and complete the following table

Activity 13.6
Most of plants grow toward the sunlight direction. The few which have been observed respond to external stimuli like touching and temperature.
1. From your experience, brainstorm what will happen to the plant when:
–– it is exposed to the direction of the sunlight

–– its growing part is exposed to a physical material like a stone
–– a living organism touches on it (Mimosa pudica)
2. Use internet and textbooks to describe why some plants flourish during long daylight while others do not?
3. What names can be given to the above processes.

13.6.1. Photoperiodism
The light provides energy that plants need to make its own food. The duration of daylight affects the plant growth and plant development.
Photoperiodism is a plant physiological response to relative lengths of daylight and darkness. Photoperiodism affects many plant processes, including the formation of storage organs, flowering and bud dormancy.
Plants monitor changes in day length with a bluish, light-sensitive pigment called phytochrome. The alternation of darkness and light triggers the phytochrome to change from one chemical form to another. By detecting the type and the amount of phytochrome present, plants determine the length of darkness and light each day.
One of the effects of photoperiodism is that plants produce fruits at different times and are classified into three categories:
–– Short-day plants (SDP),
–– Long-day plant (LDP)
–– Day–neutral plants (DNP).

–– Short-day plants (SDP): they only flower when the days are short and the night are longer than a certain length. Examples strawberry, blueberry, goldenrods, cocklebur and soybeans, tobacco are short day plants for flowering.

–– Long-day plant (LDP): they produce flower when the period of daylight exceeds a critical minimum length. Radishes, asters, apple trees, squash trees, and beets.
–– Day–neutral plants (DNP) are not dependent on day length for flowering. They produce flower regardless of the length of the daylight. Day neutral plants for flowering include tomatoes, roses, corn, cucumber, carrot, cotton and beans. They can produce fruit throughout the entire growing season.

Self –assessment13.6.1
1. What role does phytochrome play in photoperiodism?
2. What is the difference between a short-day plant and a long-day plant?
3. What could happen if a short-day plant is grown in the long days of summer?

13.7 Metamorphosis and growth patterns in insects and amphibians
Activity 13.7
From a pond or swamp, collect frog eggs together with water. Keep some eggs in warm conditions at room temperature between 2 to 30 days.
1. visit them regularly and note the observation.
2. Use the diagram below and relate your observation from question (1) to the one given in the diagram.

1. Metamorphosis and growth patterns in animals
Metamorphosis is a process consisting of changes in body form of a young organism before it reaches its adult size and becomes sexually mature, for example, the change from tadpole to frog or from caterpillar to butterfly.

a. Metamorphosis and growth patterns in insects
All insects develop and grow by metamorphosis. Some insects show incomplete metamorphosis. In incomplete metamorphosis, an immature nymph is hatched from the egg that looks like the adult, but it is smaller, and its wings and reproductive organs are undeveloped. It molts several times with each molt, the wings become larger and more fully formed. The fi nal molt transforms the nymph into an adult
that can reproduce and fl y. Insects with incomplete metamorphosis are known as heterometaboles e.g. weevil, cockroach (Periplaneta americana), Grasshopper; mayfl ies; dragonfl ies and termites.

Complete metamorphosis

In complete metamorphosis eggs hatch into larvae which are morphologically, physiologically and behaviorally diff erent from adult (wormlike larva) or caterpillar. The caterpillar molts several times, when it reaches its full size, it prepares the pupa, or chrysalis, a hard, sometimes thorny and oval structure. Inside the pupa, the larval tissues break down and group of cells called imaginal disks develop into wings
and other tissues of the adult: imago stage, the pupa becomes sexually mature.

Examples: Butterflies, moths, mosquitoes, beetles, bees, housefly.

Importance of metamorphosis

In life cycle based on complete metamorphosis, the larval and adult stages often fulfill different functions, live in different habitat and eat different foods. Example: mosquitoes.
Metamorphosis also enhances insect survival by helping insects survive harsh period. Example: butterflies (caterpillars feed on leaves, but adult butterflies feed on nectar from flowers).

b. Metamorphosis and growth patterns in amphibians
Many amphibian species like the frog breed in water and their eggs are fertilized externally. The fertilized eggs hatch into swimming, tailed larvae called tadpoles. Tadpoles, which usually live in water, look somewhat like small fish. A tadpole has an oval body, gills for breathing, and a long, muscular tail with fins along the upper and lower edges for swimming.
Then the tadpole grows legs—the hind legs appear first—and resorbs its tail. It loses its gills and grows lungs, and the structure of the heart, digestive system, and skeleton changes. The horny beak and other mouthparts adapted for eating algae disappear and are replaced by the long, sticky, projectile tongue that helps adult frogs catch insects. Frogs reach reproductive age anywhere from several months to
several years after metamorphosing.

Self-assessment 13.7
1. What is metamorphosis?
2. Describe the changes that occur during metamorphosis in frog.
3. Discus reasons why complete metamorphosis may have greater adaptive value for an insect than incomplete metamorphosis.
4. Compare metamorphosis of a butterfly and that of a grasshopper.

End of unit assessment 13
1. What do you understand by:
a. Dormancy
b. Gravitropism
c. Chemonasty
d. Phytochrome
e. Short-day plants
2. a. What factors can allow to measure the growth of a plant?
b. State any four external factors that can affect the growth of a plant.
3. a. State any three characteristics of the phytohormones.
b. What is phytohormone (s) responsible for:
i. The falling of some plant organs during the stress
ii. The fruit ripening.
iii. Development of the lateral buds
iv. Stem growth and parthenocarpy
c. What is meant by parthenocarpy? Give one example of a plant that shows
this phenomenon
4. A seedling has been grown in an opaque box receiving the light from a single
direction as shown by the diagram below

a. What will happen on the coleoptile (seedling) as it grows?
b. Suggest a name to the phenomenon investigated in this experiment.