CBC SCIENCE REVISION NOTES FOR ALL GRADES

UNIT 3: PLANTS
CROPS
Crops are useful plants grown by farmers in the farm.
Types of Crops
-There are two types of crops
(a) Food crops
(b)Cash crops
a) FOOD CROPS
Grown mainly for food
Examples of food crops
a) Cereal
• Maize
• Rice
• Wheat
• Barley
• Sorghum
• Millet
b) Legumes
• Beans
• Peas
• Black beans
• Green grams
• Groundnuts
• Cow peas
• Soya beans
c) Vegetables
• Kales
• Cabbage
• Spinach

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d) Fruits
• Pawpaws
• Oranges
• Lemons
• Pineapples
• Mangoes
• Tangerines
e) Tuber crops
• Sweet potato
• Cassava
• Yams
• Irish potatoes
• Arrow roots
• Carrots

CASH CROPS

Grown for sale to get money.
They are processed in factories into new products.
Examples of cash crops
a) Beverage crops – for making drinks e.g. Tea, Coffee, Cocoa
b) Fibre crops – produce threads woven to make ropes, baskets, cloths,
sacks, mats, etc. e.g. Sisal, Cotton, Palm tree
c) Oil crops – produce oil e.g. Sunflower, Coconut
Oil Crops
-Plants that produce cooking oil
-These crops include:
(a) Coconut tree
(b) Sunflower plants
(c) Groundnuts
(d) Macadamia

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WEEDS
These are plants that grow where they are not wanted
Examples of weeds
Blackjack- has hooks that stick on clothes or fur
Sodom apple- flowers are purple and white in colour.
Fruits are yellow with short sharp thorns in the stem and leaves.
Pigweed- used as a vegetable
Wandering Jew
Mexican marigold – yellow flower, Unpleasant smell
Oxalis
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Control of Weeds
The removal of weeds from a farm is called weeding.
The removal of the unwanted parts in a plant is called pruning.
Ways/Methods of controlling weeds
a) Digging them out – is the best method
b) Slashing
c) Mulching
d) Uprooting
e) Using chemicals (herbicides)
Effects of Weeds on Crops
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Weeds are harmful to crops in the following ways:
Compete with crops for:
• nutrients in the soil
• carbon dioxide for photosynthesis
• sunlight for photosynthesis
• moisture content in the soil
b) Weeds harbour diseases and pests that damage the crops.
PLANTS
Plants are living things.
The grouping of plants together with common characteristics or features is
called the classification of plants.
Plants can be grouped into:
a) Green and non-green plants
b) Flowering and non-flowering plants.
1 a) Green Plants
They contain the green colouring matter called chlorophyll.
They make their own food through a process called photosynthesis. e.g.
Algae – grow in water bodies
Moss – grow in damp places e.g. walls, cliffs, stones, etc.
Conifers e.g pine, cedar, cypress.
b) Non-green Plants
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They do not contain green matter (chlorophyll)
They do not make their own food – They feed on organic matter (dead
decayed matter)
Non-green plants comprise of fungi and bacteria
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Teachers Arena
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.Examples of fungi
– Bracket tree
– Penicillium
– Puffballs
– Mushrooms
– Moulds
– Toadstools
– Lichen
– Ringworms
– Athlete’s foot
– Dandruffs
– Yeast
– Mucor
Penicillium – used to make medicine.
Yeast – used for baking.
2 a) Flowering Plants
They produce flowers.
Flowers bear fruits that contain seeds.
They are green in colour and make their own food.
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b) Non-flowering Plants
They do not produce flowers.
They are both green and non-green.
All non-green plants are non-flowering (fungi and bacteria)
They reproduce by means of spores and cones which germinate into new
plants.
Plants that produce by means of cones (hard seeds) are called coniferous
plants e.g. pine, cedar, cypress.
Examples of non-flowering plants / how they reproduce
Ferns – spores
Algae – Spores
Mosses – Spores
Fungi – Spores e.g. mushrooms, mucor, puffballs etc.
Coniferous plants – cones e.g. pine, cedar, cypress
EXTERNAL PARTS OF A PLANT
Parts of the plant are:
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FUNCTIONS OF THE PARTS
Roots
a) Support/hold/anchor the plant firmly in the soil
b) Absorption of water and mineral salts
c) Food storage
Points to Note:
Plants that store food in the roots are called root tubers.
Examples of plants that store food in the roots:
• Arrow roots
• Cassava
• Carrots
• Sweet potatoes
There are two main types of roots:
a) Tap root – extension of stem with side roots.
b) Fibrous roots-many similar roots.
i) Plants with tap roots include:-
Legumes,
Acacia,
Fruit trees, etc.
ii) Plants with fibrous roots include:-
Cereals,
Oats,
Grass,
Sisal,
Onions,
Sugarcane,
Coconuts, etc.
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Other types of roots include:
a) Aerial roots – for breathing
b) Prop roots – used in maize for support
Stem
– Transports water and mineral salts from the roots to the leaves
– Carries food made by the leaves to the roots for storage
– Holds or supports the upper parts of the plant in good position
– Protects the plant
– Some stems store food and water for the plant
N.B. Plants that store food in the stem are called stem tubers.
Examples of plants that store food in the stem are:
a) Cactus
b) Sugar cane
c) Irish potato
Leaves
a) Breathing – Exchange of gases through small tiny holes called stomata.
b) Photosynthesis – Process of making its own food
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Requirements of photosynthesis are:-
Chlorophyll – green colouring matter
Water
Carbon dioxide
Sunlight
c) Storage of food – Edible vegetables
d) Transpiration – Process in which plants lose excess water through small
holes called stomata.
Transpiration is high when it is hot, sunny, dry, windy.
It is low when it’s cold, wet, calm and rainy.
Flower
– Reproductive organ of a plant
It bears fruits which contains seeds that germinate into a new plant
Seeds germinate into new young plants called seedlings
What is a flower?
A flower is a reproductive part of a flowering plant.
Most plants have both male and female reproductive parts.
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Functions of the Parts
Flower stalk: It joins the flower to the plant i.e. on the stem or a branch.
It holds and supports the flower.
Sepals: They are green in colour. Sepals protect the inner parts of
the flower while it is growing in the bud.
A collection of sepals is called calyx. Sepals can assist in photosynthesis.
Petals: These are the outermost parts of the flower. They are
usually brightly coloured to attract insects and some birds that help in
pollination
A collection of petals is called a corolla.
Filament: It is a stalk that holds/supports the anthers.
Anther: It produces pollens
The pollen contains pollen grains which are the male sex cells of a flower.
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Stigma: It receives pollen grains during pollination from the anthers.
Style: It is a long and narrow tube that joins the stigma and the
ovary.
Ovary: It contains and encloses the ovules.
The ovary develops into a fruit after fertilization.
Ovules: Ovules are the female sex cells of a flower.
Ovules develop into seeds that germinate into new plants after fertilization.
Receptacle: It connects the flower stalk and the ‘ovary base’.
Stamen: The male part of a flower is the stamen. The stamen consists of
:-
– Anthers
– Filament
In a pawpaw plant, male and female reproductive parts are on separate
plants.
Pistil: The female part of a flower is the pistil. A group of pistils are called a
carpel. The pistil comprises:
• Ovary
• Stigma
• Ovules
• Style
Ovules are the female reproductive cells of a flower.
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The pistil consists of:-
The Nectary is at the base of the sepals. It produces sugary substances
called nectar which is food for insects such as bees and butterflies.
In the maize plant the female and the male reproductive parts are in
different positions on the same plant.
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POLLINATION
What is pollination?
It is the transfer of pollen grains from the anthers to the stigma.
TYPES OF POLLINATION
There are two types:
a) Self pollination
b) Cross pollination
What does the pollen grain contain?
– The male reproductive cells of a plant
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a) Self pollination
What is self pollination?
It is the transfer of pollen grains from the anther within the same flower or
to the stigma of the same flower or of another flower on the same plant.
b) Cross pollination
What is cross pollination? |
It is the transfer of pollen grains from the anther to the stigma from one
plant to another.
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AGENTS OF POLINATION
What are agents of pollination?
These are the things that help transfer pollen grain from the anther to the
stigma.
Agents of pollination are:
a). Insects – bees -butterfly
b). Wind
c). Birds -sun bird -humming bird -honey bird
d). Water
N.B. Birds and insects visit flowers to collect nectar. They are attracted by
coloured petals and the sweet smell of the petals (scent). They carry the pollen
grains from one flower to another.
Characteristics of wind-pollinated flowers
Wind-pollinated flowers:
• Are small in size.
• Have dull petals i.e. not brightly coloured.
• Have no scent (no sweet smell)
• Have no nectar.
• Have large anthers which are loosely attached to the
filament. This makes them shed the pollen grains when
the air moves slightly around.
• Produce a large amount of light and powdery pollen
grains which can be easily carried by the wind.
N.B. Large amounts of pollen grains are produced because a lot of them are
lost on the way to the next plant.
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• Have large hairy or feathery stigmas. The stigmas hang
outside the flower and trap any pollen grains that may
be floating in the air.
N.B. An example of a wind-pollinated flower is the maize flower.
Characteristics of insect-pollinated flowers
Insect pollinated flowers;
• Have flat and sticky stigmas that are found inside the
flowers to stick pollen grains deposited by an insect.
• Produce a small amount of heavy and sticky pollen
grains which can stick firmly to the bodies of visiting
insects.
• Have anthers which are not very large.
• Produce nectar which acts as food for the insects.
• Have a strong, sweet smell called scent that attracts the
insects.
• Have brightly coloured petals that attract the insects.
• Are usually large in size.
N.B. An example of an insect-pollinated flower is the sunflower
FERTILIZATION IN PLANTS
Fertilization is the fusion of the male cell in the pollen grain and the female
cell in the ovary to form a seed.
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Fusion is the joining of the two cells.
When pollen grain falls in the stigma, it germinates to form a pollen tube.
The pollen tube grows/develops down the style to the ovary.
The pollen tube carries the male reproductive cells (gametes) to the ovary.
In the ovary, the pollen grain fuses (joins) with ovules (female reproductive
cells). Fertilization is then said to have taken place.
After fertilization:
a) Ovules become seeds.
b) The ovary develops into a fruit.
c) The other parts of the flower wither and fall off.
Stages that lead to fertilization
What is found when pollen grains germinate?
A pollen tube is formed.
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Where does the pollen tube grow?
In the stigma and through the style
What is carried along the tube?
Pollen grains
After fertilization what forms the seed?
The ovules
What does the ovary develop into?
The fruit
PARTS OF A SEED
There are two types of seeds:
a) Monocot seed
b) Dicot seed
PARTS OF A MONOCOT SEED
An example of a monocot seed is Maize.
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Testa/Seed coat: It is the outermost skin of a seed that protects the
inner parts of the seed.
Embryo: This is the part which can grow into a new plant. It has two
parts:
a) Radicle – Develops into a root
b) Plumule – Embryo part that grows into a shoot (leafy part)
Endosperm: Stores food for the seed.
Fruit scar: Point where the maize grain was attached to the cob (ovary)
Style scar: Marks the point that was attached to the style (remains of the
style)
N.B. A maize seed has two scars.
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PARTS OF A DICOT SEED
An example of a dicot seed is a bean seed.
Testa: For protection of inner parts.
Radicle: Grows into a root
Plumule: Grows into a shoot
Microphyle: It is a tiny hole that allows air and water to enter into the
seed during germination.
Scar/Hilumo: Is the part at which the seed was attached to the ovary.
Cotyledon: Stores food that is used during germination.
Cotyledon is also called seed leaf.
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Illustrations of Fruits and seeds showing the scars
N.B. Flowering plants with one cotyledon are called monocotyledons
e..g maize, wheat, rice, millet, sorghum etc.
Flowering plants with two cotyledons are called Dicotyledons
e.g. beans, peas, soya beans, green grams, etc.
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Differences
GERMINATION
Germination is the process in which a seed develops into a young plant.
A young plant is called a seedling.
STEPS OF SEED GERMINATION
1. The seed absorbs water and oxygen 9air) through the
tiny hole called microphylle.
2. The water makes the seed swell up.
3. The seed coat bursts and splits open.
4. The radical comes out through the microphylle to form a
tiny roof that grows into the soil.
5. Shortly after,the plumule forms with tiny leaves.
6. A new plant is formed (seedling)
N.B. During the initial stages of germination the seed used the food
stored in the cotyledon or endosperm before leaves develop to carry
out photosynthesis.
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Conditions necessary for germination
• Water/moisture
• Air (oxygen)
• Warmth
What is the difference between a fruit and a seed?
– Fruit have two scars while seeds have only one scar.
The new plant is the seedling. During germination, the growing embryo
feeds from the cotyledon.
After germination, cotyledons turn green and start making food before
the tiny leaves develop.
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Experiments
Activity 1
What is necessary for germination?
– Air;
– Water;
– Warmth.
Seeds will germinate since oxygen, water and warmth is available.
Activity 2
What is present?
– Air
– Warmth.
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Dry cotton wool indicates water/moisture is lacking, hence seeds will not
germinate.
Activity 3
What is present?
-Water,
– Warmth.
Water is boiled and then cooled to remove the oxygen.
The layer of oil prevents the oxygen from dissolving into the water.
Seeds will NOT germinate since there is no oxygen.
Activity 4
Place the set up in a freezer or refrigerator.
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Ice cubes make the temperatures very low, hence there is no warmth.
Seeds will not germinate because there is no warmth.
INTERDEPENDENCE IN PLANTS
Interdependence is a situation whereby living things depend on each
other.
All living things depend upon each other in many ways.
TYPES OF INTERDEPENDENCE
i) Interdependence between plants
ii) Interdependence between plants and animals
iii) Interdependence between animals.
1. Interdependence between plants and other plants
i) Shade
ii) Support
iii) Habitat
Shade
1. Plants that cannot grow in direct sunlight grow under bigger plants such
as trees.
2. An example of a plant that grows under a shade is the fern plant.
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Support
1. Plants with weak and soft stems which cannot stand firmly by
themselves depend on other plants for support. They are called climbing
plants.
2. They have special features to climb on other plants e.g.the passion
plant has tendrils to climb on other plants while the bougainvillea has
hooks to climb.
3. Others like the garden pea twine around other plants.
Habitat
1. A habitat is the natural place where a plant grows or an animal lives.
2. Some plants grow on other plants e.g. lichen.
3. Others live and feed directly from the host. They are called parasitic
plants e.g. dodder plant
Others feed on dead and decaying plants. They are known as saprophytes
e.g. mushroom and toadstool.
2. Interdependence between plants and animals
Plants and animals depend on each other in the following ways:
1. For oxygen
2. For carbon dioxide
3. For food
4. For medicine
5. For pollination
6. For shelter
7. For nutrients
8. Animal wastes
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9. Decomposition on death
Food
1. Herbivorous animals feed on plants directly. Carnivores feed
on them.
2. Animals produce carbon dioxide which is used by plants
during photosynthesis to make food.
Oxygen
1. During photosynthesis, plants produce oxygen which is used
by animals during respiration.
Carbon dioxide
1. During respiration, animals give out carbon dioxide which is
used by plants to make their own food.
Medicines
1. Many plants are a source of medicine e.g. Aloe vera,
Mwarubaini (neem tree), ginger, garlic.
2. Penicillin and quinine are modern medicines extracted from
plants.
Pollination
1. Most plants depend on animals such as bees, butterflies and
birds like the sunbird for pollination.
2. Pollination allows reproduction in plants.
Animal waste
1. Animal droppings and dung add nutrients to the soil.
2. The nutrients are absorbed by the plants through the roots.
Shelter
1. Some animals depend on trees for shelter and protection from
rain, wind, heat and cold.
2. Birds build nests on trees, monkeys live on trees and termites
build their shelters on trees.
Nutrients
1. Animals get nutrients by eating. Insectivorous plants trap
insects and feed on them.
2. Plants get nutrients from dead and decaying animal waste.
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Decomposition
1. Living things die, decay and rot. The process of rotting and
decaying is known as decomposition.
2. After decomposition, nutrients are released to the soil and
then used by plants.
FOOD CHAIN
1. A food chain is a feeding relationship among living things.
2. The producers in a food chain are plants.
3. Herbivorous animals feed on plants and are called primary
consumers.
4. Carnivorous animals feed on herbivores and are called
secondary consumers.
5. Animals that feed on secondary consumers are called tertiary
consumers.
6. Finally, in a food chain we have decomposers who bring about
decomposition.
Living things in a food chain
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Examples of food chains
CROP PESTS
1. A pest is a harmful thing, person or animal.
2. Pests are grouped into two: – Field pests
– Storage pests
FIELD PESTS
They attack crops when they are in the field.
They include: – Weaver birds
– Army worms
-Cut worms
-Stalk borers
– Aphids
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– Locusts
STORAGE PESTS
They attack and destroy stored crop produce. They include:
• Rodents like rats, mice and squirrels
• Weevils
• White ants (termites)
Weevils
They bore holes in grains and eat them reducing their quality.
Rodents
They attack and destroy grains.
White ants
They eat stored grains and root tubers e.g. yams.
Crop pest Crops attacked Part attacked
1. Locust All growing crops Leaves
2. Aphids Beans, cabbage,
groundnuts. peas, sorghum,
carrot, tobacco
Stem and leaves
3. Army worms Cereals, cassava, grass Leaves
4. Stalk borer Sugarcane, maize, rice,
sorghum, millet
Stem
5. Cut worms Cabbages, tomatoes,
potatoes, maize, wheat, rice
Stems of young plants
6. Weaver birds Rice, maize, wheat, millet,
sorghum
Eat the grains
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EFFECTS OF CROP PESTS ON CROPS
1. Lower yields: The quantity of the harvest is reduced.
2. Reduced quality of produce: The produce is not attractive to the
buyer. This leads to a loss for the farmer.
3. Transmission of diseases to crops: Some plants transmit diseases to
crops e.g. aphids.
4. Transmission to consumers: Crops attacked by pests may cause
diseases to consumers
CONTROL MEASURES FOR CROP PESTS
Pests are reduced or eliminated by the following methods:
1. Scaring
2. Trapping
3. Hand-picking
4. Weeding
5. Spraying
6. Pruning
7. Biological method
Scaring: Birds and monkeys can be kept away by scaring, using
scarecrows or metals that make unpleasant noise.
Trapping: Traps are put where pests are commonly found.
Hand picking: Some pests can be hand-picked. Hand-picking is best
used in small pieces of land.
Weeding: This is the removal of unwanted plants. Some weeds host
pests.
Spraying: Pesticides are sprayed on crops to control pests.
Pruning: Remove the affected parts of a plant and destroy them.
Biological method: This is the use of other animals to feed on pests
e.g. Ladybirds feed on aphids which affect crops.
ADAPTATION OF PLANTS TO THEIR ENVIRONMENT
Adaptation is the ability/or a mechanism of a plant to survive in a particular
environment or habitat.
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PLANTS ADAPTED TO DIFFERENT ENVIRONMENTS
Xerophytes – Plants that grow and survive in dry areas.
Mesophytes – Plants that grow and survive under normal
conditions
Hydrophytes -Plants that grow and survive in watery areas.
Halophytes – Plants that grow in salty areas.
Adaptation of plants to survive in dry areas (Desert – Arid and Semi-
Arid)
• These plants suffer water shortage
• They are called Xerophytes
• They conserve water either by their structure or through their
behaviour.
• Examples include:
• Cactus
• Acacia
• Euphorbia
• Baobab tree
• Sisal
• Marram grass
• Prickly pear
• Blue gum tree
Cactus plant in desert
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Adaptation of Xerophytes
1.Some plants shed their leaves during dry season to reduce the rate of
transpiration. Plants that shed (lose) their leaves are called deciduous
plants.
They do not carry out photosynthesis during dry season hence they
become dormant i.e. inactive.
2. They have needle-like leaves. The surface area of the leaves is small,
hence it reduces the rate of transpiration.
3.They have sharp spines. This protects the plant from being eaten by
desert animals and also reduces water loss.
4.Some plants roll/fold or curl their leaves. This reduces the rate of
transpiration. They also trap moist air and make it available for the plant.
5. Some have normal or silvery (shiny) hairs. Normal hairs trap water
vapour for the plant. Shiny hair reflects sunlight and heat the leaves, hence
reduce
temperatures on the plant’s surface.This reduces the rate of
evapotranspiration.
6. Other plants have leaves covered by a thick waxy cuticle. The wax
reduces the amount of water loss through evaporation.
7.Some plants have fleshy, thick green stems instead of leaves for
photosynthesis. The thick stem does not allow much water to be lost e.g.
cacti plant.
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8.Others have fleshy stems that store large amounts of water in their
tissues. Plants with fleshy stems are called succulents.
9.Some close the stomata during the day when the rate of evaporation is
high and open during the night.
10. Some have fewer or sunken stomata to reduce water loss.
11. Some are deep-rooted to obtain underground water e.g. acacia.
Adaptation of plants to survive in water (wet areas)
These plants are called Hydrophytes
They face a big problem obtaining energy oxygen
Examples of hydrophytes are water lily, buttercup, water lettuce,
duckweed, aquatic ferns.
Water lily
Adaptation of Hydrophytes
1. They have thin cuticles since they do not have to reduce the
amount of water loss.
2. They have an increased number of stomata.
Most of the stomata are on the upper surface of the leaf and
remain open most of the time to allow absorption of gases
from air.
3. They have flexible stems that sway or bend with the currents
so that they are not broken.
4. They have large flat leaves to enable the plant float on water
e.g. water lily.
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5. The leaves have air sacs that enable the plant to float in water.
6. Some have hairs on the stems and leaves to prevent water
from standing and soaking into the plant.
7. Some have a waxy layer on the stem and leaves to prevent
water from entering into the plant.
8. Their roots are shallow and small to reduce the amount of
water absorption.
9. They have floating flowers above the water to facilitate
pollination.
NB: Mesophytes grow under normal soils and water conditions.
They require an average amount of water e.g. beans, maize, bananas,
potatoes.
SIGNS OF UNHEALTHY CROPS (ILL-HEALTH)
a) Stunted growth – also known as retarded growth. Plants develop at
a slower rate than expected.
b) Discolouration of plant parts. Other than the normal colour of the
parts,they gain a different colour e.g leaves turn yellow ,hence the
photosynthesis process is affected.
c) Curled/folded leaves.
d) Wilting. Plants may wither and die i.e. appear weak.
e) Presence of spots, streaks, dots or patches. (Streaks are thin lines)
f) Distortion or malformation – Abnormal shapes
EFFECTS OF CROP DISEASES
• Reduced yields – The quantity of harvest is less than
expected.
• Reduced quality of the produce i.e. the produce is not up to
the expected standards e.g. small maize grains, fruits or kales.
• Leads to high production cost. More expenses are incurred
when controlling the diseases.
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