Reproduction in Plants, Germination PDF

BIOLOGY MADE SIMPLE  After a week there is a swelling above the ring, reduced growth below the ring and the leaves are unaffected.  This is evidence that sugars are transported downwards in the phloem: - sugars cannot pass the cut - decrease wat...

BIOLOGY MADE SIMPLE  After a week there is a swelling above the ring, reduced growth below the ring and the leaves are unaffected.  This is evidence that sugars are transported downwards in the phloem: - sugars cannot pass the cut - decrease water potential; - water moves into cells; - damage triggers increased cell division; - to produce cells to store sugars; DIAGRAM: Ring barking Try this question: Suggest explanations for the following observation: Fruit growth is suppressed if a ring of bark between the fruit and mature leaves is removed. Answer Removal of bark removes phloem; responsible for transport of sugars to fruit; to enable fruit development/formation of food store in fruit; FORM 4: REPRODUCTION IN PLANTS FORM4: ASEXUAL REPRODUCTION IN PLANTS – VEGETATIVE REPRODUCTION REPRODUCTION — Reproduction is the process that makes more of the same kind of an organism. BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 65  BIOLOGY MADE SIMPLE — Reproduction is the production of offspring through a sexual or asexual process. Types of reproduction  There are 2 types of reproduction: asexual and sexual. Sexual reproduction occurs through fertilisation of gametes of two parents whereas asexual reproduction occurs by mitosis of vegetative structures of a single parent. 1. ASEXUAL REPRODUCTION:  Is the production of genetically identical offspring from one parent.  Is formation of a new organism, without involvement of gametes or fertilisation.  Asexual reproduction involve mitosis, a process by which a cell divides into two daughter cells, each of which has the same number of chromosomes as the original (parent) cell. The two cells further divide into four and so on. Examples of asexual reproduction  Binary fission in bacteria  Spores in fungi  Budding in yeast  Vegetative reproduction/propagation in plants. VEGETATIVE REPRODUCTION/PROPAGATION IN PLANTS  Vegetative reproduction is the production of new and identical plants from vegetative structures (outgrowths) or fragments of a single parent plant.  Through asexual reproduction, a plant produces clones (genetically identical offshoots) of itself, which then develop into independent plants.  Some plants, however, reproduce sexually by pollination, fertilisation and seeds.  Examples of plant vegetative structures (outgrowths) are rhizomes, tubers, cuttings, suckers, runners and bulbs.  Vegetative reproduction in plants can either be natural or artificial.  Natural vegetative reproduction occurs by means of rhizomes, tubers, suckers, runners, tubers, etc.  Artificial vegetative reproduction occurs by means of cuttings, grafting, etc. NATURAL VEGETATIVE REPRODUCTION  DEFINITION: Natural vegetative reproduction is the natural propagation of a new plant from vegetative structures without human intervention.  OCCURRENCE: Natural vegetative propagation occurs naturally in plants.  EXAMPLES: Natural vegetative propagation occurs by means of rhizomes, tubers, suckers, runners, bulbs, tubers, etc. a) Rhizomes  A rhizome is an underground stem that grows horizontal to the ground.  Grasses reproduce by means of rhizomes BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 66  BIOLOGY MADE SIMPLE  A rhizome produces new plants at each bud on the nodes along its length.  A rhizome stores food which enables the plant to grow the next season and perennate (survive from year to year). b) Suckers  A sucker is a shoot that grows from an underground root or stem.  Banana plants reproduce by means of suckers. New stems grow from the base of old ones, forming new plants. BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 67  BIOLOGY MADE SIMPLE c) Tubers  A tuber is a swollen underground stem or root.  The tuber is swollen with stored food in the form of starch.  Irish potatoes reproduce by means of tubers.  The parent tubers are allowed to produce short shoots from their eyes or lateral buds and the planted to produce new potato plants. BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 68  BIOLOGY MADE SIMPLE d) Runners — Runners aka stolons. — Runners are thin, long horizontal stems that spread above the ground from the main plant. — Entirely new plants develop from nodes located at intervals on the runners; each node gives rise to new roots and shoots. — Strawberry plants and runner grass are examples of plants that propagate by means of runners. DIAGRAM: Strawberry plant propagating by means of runners. PICTURE: Strawberry runners branching off parent plant. e) Bulbs BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 69  BIOLOGY MADE SIMPLE  Bulbs are spherical underground storage structures.  Bulbs are stems or buds that are enclosed with fleshy layered leaves.  The parent bulb produces smaller buds called lateral buds.  As the mature plant gets to the end of its life, the lateral buds develop into smaller bulbs that are attached to the base of the parent bulb.  The smaller bulbs then develop into new plants during the new growing season.  The new bulbs can also be physically separated and planted.  Examples of plants that reproduce by means of bulbs include onions, tulips and daffodils.  The propagation by bulbs can be summarised as follows: DIAGRAM: Vegetative propagation by bulbs. ARTIFICIAL METHODS OF VEGETATIVE REPRODUCTION  DEFINITION: Artificial vegetative reproduction is the artificial propagation of plants under the influence of man.  OCCURRENCE: Artificial vegetative reproduction occurs artificially in plants.  EXAMPLES: Artificial vegetative reproduction occurs by means of cuttings, grafting etc. (a) Cuttings  Cuttings: this is simply where a stem is cut from a plant and replanted.  Examples of plants that can reproduce by cuttings are sugar cane and sweet potato.  The cutting part buried underground quickly grows roots from nodes to become a new plant. BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 70  BIOLOGY MADE SIMPLE (b) Grafting  Grafting is insertion of shoot or bud onto a related plant so they grow as one plant.  Grafting joins plant parts from two related plants so they will heal and grow as one plant.  Two plant portions, rootstock and scion, of the same variety are joined. BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 71  BIOLOGY MADE SIMPLE  The lower part of the combined plant called the rootstock or stock provides the root system.  Definition: The stock or rootstock is a rooted stem into which a scion or a bud is grafted.  The upper part of the combined plant called the scion provides the shoot system.  Definition: The scion is a piece of young stem or bud which is inserted into a root stock.  Basically there are two major types of grafting namely stem grafting and bud grafting. (i) Stem grafting  This is when a shoot of a selected desired plant (known as the scion) is grafted onto the stem of another plant (known as the stock), so that the cambia of the two combine and grow as one plant.  The following are the stages in stem grafting: Both scion and stock are cut at a slanting angle, placed in close contact facing each other, and are then held together by binding around with tape. DIAGRAM: Grafting stock and scion of the same plant variety. (ii) Bud grafting (aka budding)  Bud grafting is also known as budding.  In bud grafting a leaf bud is transplanted from one tree to another of the same type.  Unlike stem grafting, which attaches the entire upper part of a plant, bud grafting (budding) only attaches the bud to a different plant.  The transplanted bud is the scion and the plant to which it is transplanted is the stock (rootstock).  The following are the stages in bud grafting: 1. Cut a leaf bud from one plant. This is your scion 2. Make a T-shaped pocket cut through the bark of the stock of another plant. 3. Transplant the bud (scion) into the T-shaped pocket cut on the stock. 4. Hold the bud (scion) and stock together by binding around with tape. BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 72  BIOLOGY MADE SIMPLE DIAGRAMS: Stages in budding (bud grafting). (source: https://www.wikihow.com/Do-Budding-in-Plants#/Image:Do-Budding-in-Plants-Step-4- Version-2.jpg) The requirements for grafting to be successful:  Grafting requires: a) that cambia of stock and scion must meet and match to fuse and grow together. (Grafting is therefore only suitable for dicots because they have cambium and not suitable for monocots because they do not have cambium). b) firm binding at junction to prevent tearing of joined tissue. c) that grafting is done in autumn to prevent death from excessive transpiration. d) waterproofing the cuts with wax or tape to prevent drying and infection. e) related / compatible plant species (e.g. lemons will graft with oranges because they are both citrus, but will not graft with apple since they are not related).  The stock and scion are joined and grow into one plant which has the following advantages: 1) Neither stock nor scion is altered genetically by grafting. Each provides its desirable characteristics separately to the new plant: 2) The specifically chosen root stock provides the vigour. BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 73  BIOLOGY MADE SIMPLE 3) The scion provides the quality of the product (e.g. flowers or fruits) on the shoot. ADVANTAGES OF VEGETATIVE REPRODUCTION IN PLANTS 1. Quick. Offspring mature more rapidly. 2. Vegetative structures store large amounts of food leading to rapid growth. 3. Only one parent is needed. This eliminates the need for special mechanisms such as pollination and pollinating agents. 4. All good characteristics are passed on. 5. Useful/desirable traits are preserved as genetically identical offspring are produced. 6. No dispersal so offspring grow in same favourable environment as parent. 7. Seedless plants / plants that do not produce viable seeds can be reproduced by vegetative reproduction e.g. bananas, sugar cane, potatoes and roses. DISADVANTAGES OF VEGETATIVE REPRODUCTION IN PLANTS 1. There is no mixing/blending of characteristics. 2. Little variation, therefore adaptation to environment is unlikely. 3. Offspring inherit poor/undesirable characteristics (e.g. poor resistance to diseases). 4. If a particular plant clone is susceptible to a certain disease, there is potential to lose the entire crop. 5. Lack of dispersal leads to competition for nutrients, water, light etc. 6. Colonisation of new areas is unlikely since there is no dispersal. FORM 4: SEXUAL REPRODUCTION IN PLANTS Form 4: Flowers & Pollination. SEXUAL REPRODUCTION IN PLANTS  Flowers are the reproductive organs of plants.  Most flowers develop seeds that grow into new plants. Structure and functions of a flower  You need to be able to describe the structure and functions of a named dicotyledonous flower. BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 74  BIOLOGY MADE SIMPLE DIAGRAM: Structure of an insect-pollinated dicot flower e.g. bean flower.  All of the petals of a flower are collectively known as the corolla.  Sepals are modified leaves collectively called the calyx. DIAGRAM: Wind-pollinated flower e.g. grass flower. BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 75  BIOLOGY MADE SIMPLE FUNCTIONS OF PARTS OF A FLOWER Flower part Function 1 Petal Large, coloured and scented to attract insects/pollinators. 2 Sepal Protects flower while in bud form. 3 Petiole Supports flower to make it easily seen by insects, and to be able to withstand (stalk/pedicel) wind. 4 Receptacle Connects the stalk to the flower and supports the weight of the flower or fruit when it develops. 5 Nectary Produces nectar, to attract insects. 6 Stamen Male reproductive part of flower. Made up of anther and filament. 7 Anther Produce pollen grains in pollen sacs. 8 Pollen grains Male sex cells. 9 Filament Supports the anther. 10 Carpel (pistil) Female reproductive part of flower. Made up of stigma, style and ovary. 11 Stigma A sticky / feathery surface on which pollen grains are deposited. 12 Style Links stigma to ovary. Passage through which pollen tube grows. 13 Ovary Produce ovules. 14 Ovules Female sex cells. Develop into seeds when fertilised. Differences between insect and wind pollinated flowers  (Characteristics of insect and wind pollinated flowers)  (Structural adaptation of flower parts to mode of pollination) Differences between insect- and wind-pollinated flowers Feature Insect-pollinated flower Wind-pollinated flower Petals  Large, brightly coloured and scented to  Small, dull, usually green or absent. attract insects.  Guidelines for guiding insects into flower.  No guidelines. Stamens Inside flower so insects must make contact. Exposed so that wind can easily blow pollen away. Filaments Short filaments. Long filaments. Anther Anther inside flower to make contact with Anther hangs outside flower so wind can insects. blow pollen away. Stigma  Enclosed to make contact with insects.  Exposed to catch pollen being blown by  Sticky, so that pollen attaches to insects. wind.  Feathery, to catch pollen blown from wind. Nectary Nectary present as reward for insects. Nectary absent. Pollen  Larger, sticky or spiky to attach to insects.  Smaller, smooth and light so easily carried Grains by wind.  Smaller amounts.  Larger amounts. Bracts Absent Sometimes present. (Modified leaves) BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 76  BIOLOGY MADE SIMPLE POLLINATION  This is the transfer of pollen grains from the anther to the stigma.  Two types of pollination are self-pollination and cross-pollination.  Self-pollination is the transfer of pollen grains from the anther of one flower to the stigma of the same flower.  Cross-pollination is the transfer of pollen grains from the anther of one flower to the stigma of a different flower, of the same species. Agents of pollination  Agents of pollination include wind, insects, birds and mammals.  Insect pollinators include bees, butterflies and mosquitoes. ADVANTAGES AND DISADVANTAGES SELF-POLLINATION 1. Advantages of self-pollination: a) Purity of plant variety is maintained (desirable characteristics are maintained). b) Flowers do not depend on agents of pollination. c) Wastage of pollen grains is avoided d) Less chances of failure of pollination. e) Creates new offspring when pollen grains from other plants are hard to find, especially in geographically isolated areas such as islands. f) Does not need to expend (waste) energy on attracting pollinators (Flowers need not produce scent, nectar or be colourful). 2. Disadvantages of self- pollination: a) Undesirable characteristics cannot be eliminated. b) New varieties of plants cannot be produced. c) Since all offspring are genetically the same, they may all be wiped out by the same disease or same environmental change. d) Continued self-pollination leads to weakening of progeny (Vigour and vitality of the plants decreases with prolonged self-pollination). e) Variability and adaptability to changed environment are reduced. ADVANTAGES AND DISADVANTAGES CROSS-POLLINATION 1. Advantages of cross pollination: a) Cross pollination leads to the production of new varieties. b) More viable seeds are produced. 2. Disadvantages of cross- pollination: a) Pollination may fail due to distance barrier. b) More wastage of pollen grains. c) Need to produce more pollen to avoid pollination failure due to distance and pollen wastage. d) Increases chances of the occurrence of some unwanted characteristics. e) Cannot occur without pollinating agents. BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 77  BIOLOGY MADE SIMPLE FORM 4: FERTILISATION & SEED DISPERSAL FERTILISATION IN PLANTS  The pollen grain contains the male nucleus.  When the pollen grain lands on the stigma of the same species, it absorbs nutrients and germinates forming a pollen tube.  This pollen tube is nourished by the cells of the style and grows through the style.  The pollen tube grows down the style, through the ovary wall, and through the micropyle of the ovule.  The pollen tube carries the male nucleus from the pollen grain to the female nucleus in the ovule.  Fertilisation (fusion of male and female nuclei) occurs in the ovule to form a diploid zygote.  If the ovary contains many ovules, each will be fertilised by a different male nucleus brought in by a different pollen tube. DIAGRAM: Pollen grain germination and fertilisation of ovule. BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 78  BIOLOGY MADE SIMPLE After fertilisation the following changes take place in a flower:  The zygote develops into an embryo (plumule and radicle) and cotyledons (food stores).  The integuments (wall of ovule) develop into testa (seed coat).  The fertilised ovules become seeds.  The ovary develops into a fruit.  The ovary wall develops into pericarp (fruit wall).  The style, dries up and falls off leaving a scar.  The corolla (petals), calyx (sepals) and stamens dry up and fall off.  In some the calyx (sepals) persists. SEED DISPERSAL  Seed dispersal is the scattering of seeds / fruits away from the parent plant.  The flowers produce seeds which can be dispersed by the wind, water, animals or mechanically providing a means of colonising new areas. 5. Wind dispersal  Wind dispersed fruits and seeds have the following features: a) Small and light in order to be carried by air currents. b) Hairy or feathery structures, called a pappus, which increase surface area to catch the wind like a parachute, e.g. dandelion fruit. The fruit counterbalances the pappus. c) Wing-like structures which catch the wind and increase surface area for buoyancy, e.g. sycamore fruit, jacaranda fruit and crowfruit. When the fruit drops off the tree it spins, slowing down its descent.  Hairy and wing-like extensions increase surface area of fruits and seeds such that they are easily carried by the wind.  If caught by the wind the seed will be carried away from the parent plant, reducing competition for nutrients, water and light. 6. Animal dispersal  There are two main modifications of fruits for animal dispersal: succulent / fleshy fruits and hooked fruits. a) Succulent / fleshy fruits attract animals because they are brightly coloured, scented and juicy and edible, e.g. tomatoes and guavas. BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 79  BIOLOGY MADE SIMPLE  When eaten, the seed pass through animal‘s faeces, which may be a long way from the parent plant. The faeces provide nutrients when the seeds germinate. Such seeds have hard, resistant seed coats which enable them to overcome acids and enzymes in the digestive system. b) Hooked fruits have hooks or spines which stick on animal fur or on clothes as it brushes past the parent plant, e.g. black jack and burr grass. Eventually the fruits are brushed off or fall on their own. This disperses the seeds away from the parent plant. 7. Self-dispersal mechanism  Also known as Explosive mechanism or Mechanical (ballistic) dispersal.  When dry some seed pods or pericarps (fruit walls) twist and explode to cast away their seeds from the parent plant e.g. the musasa seed pods and pea seed pods.  Pressure inside the seed pod forces it to open along lines of weakness, twisting, exploding and throwing seeds away from the parent plant. 4. Water dispersal  Certain fruits/seeds float on water and are carried by waves or currents in rivers or oceans to faraway places.  Fruits like coconut have fibrous mesocarp which is spongy to trap air, the trapped air make the fruit light and buoyant to float on water. (Mesocarp is the middle and fleshy layer of the pericarp or fruit wall).  Plants like water lily produce seeds whose seed coats trap air bubbles.  The air bubbles make the seeds float on water and are carried away.  The pericarp and seed coat are waterproof. BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 80  BIOLOGY MADE SIMPLE DIAGRAM: Methods of seed / fruit dispersal. The advantages of fruit or seed dispersal 1. Reduces overcrowding and competition for food and light with parent plant 2. Increases chances for seed germination 3. Enables plants to colonise new and favourable habitats. The disadvantages of fruit or seed dispersal 1. Dispersal depends on external agents such as wind, water or animals. 2. Can be wasteful: a great number of seed. Try this question Describe citing particular examples how seeds are adapted for: (a) Animal dispersal. (b) Wind dispersal. (c) Mechanical /ballistic/ self-dispersal. (d) Water dispersal. Solution (a) Animal dispersal adaptations  Fleshy fruits and hooked fruits.  Fleshy fruits: Brightly coloured, scented, juice, edible to attract animals. E.g. BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 81  BIOLOGY MADE SIMPLE guava and tomato fruits. Eaten by animals. Seeds are hard and have testas that are resistant to digestive enzymes and acids in animal digestive systems. Seeds passed out in faeces away from parent plant.  Hooked fruits: Hooks / spines stick to animal’s fur or clothes. the seeds are brushed off or fall on their own (b) Wind dispersal adaptations  Small / light for easy carriage by wind.  Hairy / parachute-like structure, e.g. dandelion fruit, and wing-like structures e.g. jacaranda, to increase surface area for buoyancy / catching wind. (c) Mechanical /ballistic/ self-dispersal adaptations:  Seed pods have lines of weakness e.g. musasa pods.  Seed pods twist when dry. Twisting force opens pod along lines of weakness.  Explosion occurs. Pressure from inside casts away seeds. (d) Water dispersal adaptations.  Fibrous and spongy mesocarp to trap air making fruit light to float on water and to be carried by waves, e.g. coconut fruit.  Seed coat traps air bubbles making the seeds buoyant / float on water, e.g. water lily seeds.  The pericarp and seed coat are waterproof to prevent absorption of water during transit e.g. coconut fruit and water lily seeds. FORM 4: SEED STRUCTURE & GERMINATION Monocots and dicots — Flowering plants are classified into two groups based on the number of cotyledons (seed leaves), as monocotyledonous plants (aka monocots) and the dicotyledonous plants (aka dicots). — A monocot seed has one cotyledon whereas a dicot seed has two cotyledons. (Mono- means one and di- means two). — A seed is a fertilised and mature ovule containing an embryo. BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 82  BIOLOGY MADE SIMPLE SEED STRUCTURE Monocot seed e.g. maize seed aka maize grain. Dicot seed e.g. bean seed, pea seed and groundnut seed. FUNCTIONS OF PARTS OF A SEED Seed part Description Function 1 Testa Tough outer seed coat. Protect seed from damage due to insects, bacteria and fungi. 2 Hilum A scar on the testa of a bean Marks point where the stalk once attached seed. the seed to the pod or ovary wall. 3 Radicle V-shaped swelling beside the Grows to form the root after germination. hilum in bean seed. BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 83  BIOLOGY MADE SIMPLE 4 Micropyle A very small pore between hilum Absorbs water needed for germination. and radicle. 5 Cotyledons Fleshy modified seed leaves. Act as food stores which supply the Monocot seed e.g. maize seed, nutrition needed for germination. has one cotyledon whereas dicot seed e.g. bean seed has two cotyledons. 6 Endosperm Starch, oil and protein reserve in Surrounds/protects and nourishes the cereal/monocot grains. embryo in monocots. 7 Embryo Consists of the plumule and the Grows to form the new plant after radicle. germination. 8 Plumule Embryonic shoot. Grows into the shoot system. 9 Radicle Embryonic root. Grows into the root system. 10 Coleoptile Plumule sheath in a monocot Pointed protective sheath which protects seed. the emerging shoot tip or plumule. 11 Coleorhiza Radicle sheath in a monocot Protective sheath that protects the root tip seed. or the radicle. Differences between monocot seed and dicot seed Monocot seed Dicot seed 1 Example maize seed. Example bean seed. 2 One cotyledon present in the embryo. Two lateral cotyledons present in the embryo axis. 3 Cotyledon is small and thin and lacks food Cotyledons are fleshy and stores more food. materials. 4 Endosperm is present and stores food. Endosperm is absent and lacks food. 5 Radicle is protected by coleorhiza. Coleorhiza is absent. 6 Plumule is protected by coleoptile. Coleoptile is absent. 7 Monocot seed is usually shield-shaped. Dicot seed is usually kidney-shaped. PRACTICAL ACTIVITY Aim: To investigate structural differences between monocotyledonous and dicotyledonous seeds Materials  Bean seeds and maize grains which have been soaked overnight.  Scalpel or razor blades.  Iodine solution.  Petri-dish.  Hand lens. Procedure BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 84  BIOLOGY MADE SIMPLE 1) Using a scalpel or razor blade make longitudinal sections (LS) of both the bean seed and the maize grain. 2) Observe the LS of the specimens using a hand lens. 3) Note any structural difference between the specimens. 4) Draw the LS of each specimen and label. 5) Put a drop of iodine solution on the cut surfaces of both specimens. 6) Note any differences in colouration with iodine on the surfaces of the two specimens. 7) On your diagrams indicate the distribution of the stain. 8) Account for the difference in distribution of the colouration with iodine in the two specimens. Observations Record your observations on: 1. the structural differences between monocot and dicot seeds using a table with appropriate headings, rows and columns. 2. the difference in distribution of colouration with iodine in the two seed specimens. Conclusion Make appropriate conclusions on; 1. differences between monocot and dicot seeds. 2. differences in the amount of starch in the two seed specimens. FORM 4: SEED GERMINATION Germination  Seed germination is the development of a seed into a seedling.  Germinating seeds use their food stores in cotyledons / endosperm until the seedlings can produce their own food by photosynthesis.  Generally seed germination occurs as follows: 1. At the beginning of germination water is absorbed into the seed through the micropyle and causes the seed to swell. 2. The cells of the cotyledons become turgid and active. 3. They begin to make use of the water and enzymes to dissolve and break down the food substances stored in the cotyledons. 4. The soluble food is transported to the growing plumule and radicle. 5. The plumule grows into a shoot while the radicle grows into a root. 6. The radicle emerges from the seed through micropyle, bursting the seed coat as it does so. BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 85  BIOLOGY MADE SIMPLE TYPES OF GERMINATION  The changes that occur during seed germination are slightly different depending on the type of seed.  The nature of germination varies in different seeds.  During germination the cotyledons may be brought above the soil surface. This type of germination is called epigeal germination and occurs in dicot seeds e.g. bean seed.  If during germination the cotyledons remain underground the type of germination is known as hypogeal and occurs in monocot seeds e.g. maize seed.  Germination in dicot seed e.g. bean seed is called epigeal germination. It occurs as follows: 1. Absorption of water through micropyle. 2. Bursting/splitting of testa. 3. Radicle emerges and grows down into soil to form a root system. 4. The hypocotyl (part of embryo below cotyledons) grows upwards pulling cotyledons out of the soil. The plumule emerges between the two cotyledons. 5. Cotyledons open up, turn green to form small leaves and begin to photosynthesize. 6. The plumule forms the first true two green leaves. 7. The food stored in the cotyledons is used for growth and cotyledons shrivel and fall off. 8. The radicle gives rise to the root system while the plumule forms the shoot. 9. At this point, the young plant is called a seedling. DIAGRAM: Stages in the germination of a bean seed.  Germination in dicot seed e.g. maize seed is called hypogeal germination. It occurs as follows: 1. The seeds absorb water and swells. 2. The radicle grows out from coleorhiza down into soil and forms the primary root. 3. The plumule grows upwards inside the coleoptile. 4. Coleoptile turns green upon being exposed to light and bursts open to expose the first leaves. 5. Adventitious roots grow from the plumule. 6. The food stored in the endosperm is used for growth. BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 86  BIOLOGY MADE SIMPLE 7. The cotyledons remain below the ground. 8. The radicle gives rise to the root system while the plumule forms the shoot. 9. At this point, the young plant is called a seedling. DIAGRAM: Stages in the germination of the maize seed. Difference between Epigeal and Hypogeal Germination  In epigeal germination, the cotyledons emerge out of the soil during germination whereas, in hypogeal germination, the cotyledons remain inside the soil.  This means the hypocotyl shows a greater elongation in epigeal germination while the hypocotyl is short in hypogeal germination.  Epigeal germination occurs in dicot seeds whereas hypogeal germination occurs in monocot seeds. The conditions necessary for germination  The conditions necessary for seed germination are water, oxygen and optimum temperature: CONDITION DESCRIPTION 1 Water  Water rehydrates the seed, softens the testa and allows all the chemical reactions involved in the growth of the embryo to take place: 1. Water activates the enzymes and provides the medium for enzymes to act and break down the stored food into soluble form. 2. Water hydrolyses and dissolves the food materials. 3. Water is also the medium of transport of dissolved food substances through the various cells to the growing region of the radicle and plumule. 4. Besides, water softens the seed coat which can subsequently burst and facilitate the emergence of the radicle. 2 Oxygen  Oxygen is needed for aerobic respiration (oxidation of food substances stored in the seed) so that energy or ATP can be released for cell division and growth of embryo. BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 87  BIOLOGY MADE SIMPLE 3 Optimum temperature  Most seeds require a suitable temperature before they can germinate.  Seeds will not germinate at very low or very high temperatures.  The optimum temperature for seeds to germinate is approximately 25°C. It varies from plant to plant.  At higher temperature the protoplasm is killed and the enzymes in the seed are denatured. (Protoplasm is the cytoplasm, organelles and nucleus of the cell).  At very low temperatures the enzymes become inactive.  Therefore, the protoplasm and the enzymes work best within the optimum temperature range.  The rate of germination increases with temperature until it reaches an optimum and decreases thereafter. Role of enzymes during germination  Germination is a process controlled by enzymes.  The functions of enzymes during germination include: 1. Controlling respiration that releases energy for germination. 2. Controlling hydrolysis of food stored in the cotyledons and endosperm to nourish the embryo during germination.  Enzymes play a vital role during germination in the hydrolysis / breakdown and oxidation of food.  Food is stored in seeds in the form of insoluble carbohydrates, lipids and proteins.  The insoluble food is converted into a soluble form by the enzymes.  Carbohydrates are broken down into glucose by amylase enzyme, lipids into fatty acids and glycerol by lipase, and proteins into amino acids by protease.  Enzymes are also necessary for the conversion of hydrolysed products to new plant tissues. Role of hormones in seed germination  Several hormones play a vital role in germination since they act as growth stimulators.  These include gibberellins and cytokinins.  These hormones also counteract the effect of germination inhibitors. Controlled experiments to investigate conditions necessary for germination Design of experiments to investigate factors affecting germination  Firstly outline the factors needed for seed germination:  There are 3 essential conditions required for a seed to germinate - a seed will not germinate until these external conditions are met.  They are: 1. Suitable temperature – for maximum enzyme activity (enzymes work best at an optimum temperature - this requires enzyme and substrate to move around at a faster rate with many successful collisions can occur). 2. Water – (to rehydrate seed and soften the testa and allow the plumule to emerge. 3. Oxygen - (required for aerobic respiration) needed for energy or ATP production.  The experimental design requires two treatments: The control where all factors are present and then the separate experiment which omits one of the factors. Therefore you BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 88  BIOLOGY MADE SIMPLE can conclude that if the seed doesn‘t germinate then that factor omitted must be required for germination.  The control acts as a comparison to the experiment and enables you to make a valid conclusion that the missing factor is indeed necessary for germination.  How to omit one of the factors: 1. To remove oxygen you can add alkaline pyrogallol or pyrogallic acid which absorbs oxygen. 2. To remove water don't add water to the experiment. 3. To reduce temperature place in a refrigerator. To increase temperature place in a thermostat controlled incubator.  The factors that increase the reliability of the experiment include using the same species of plant, same amount of water, same wavelength of light, repeat the investigation several times to ensure results are reliable, etc.  There must only be one independent variable (thing you change) otherwise this would negatively affect the validity of your results. 1. Experiment to prove oxygen is necessary for germination Aim: To prove that oxygen is necessary for germination. Materials: Two conical flask marked A and B, pea seeds, water, cotton wool, pyrogallic acid, test tubes. Procedure 1) Take two conical flask with a cork and mark them A and B respectively. 2) Place a wet cotton wool in each flask with some soaked Pea seeds. 3) Pyrogallic acid absorbs oxygen. Place a test tube of Pyrogallic acid in flask B in such a way that the chemical doesn‘t drop in the flask. 4) Place a test tube of plain water in flask A. Observation The seeds in flask A germinate because of presence of oxygen and seeds in flask B do not germinate because pyrogallic acid absorbs oxygen. Conclusion Oxygen is necessary for germination. BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 89  BIOLOGY MADE SIMPLE 2. Experiment to prove that suitable temperature is necessary for germination Aim: To prove that suitable temperature is necessary for germination. Materials: Two flasks marked A and B, pea seeds, water and cotton wool. Procedure 1) Take two flasks marked A and B respectively. 2) Place some pea seeds on wet cotton wool in both the beakers. 3) Keep flask A at room temperature and B in a refrigerator. NB. You may place flask A in a warm closed environment controlled by a thermostat to ensure that temperature remains stable. Observation The seeds in flask A germinate and in B do not germinate. Conclusion Suitable temperature is necessary for germination. BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 90  BIOLOGY MADE SIMPLE 3. Experiment to prove that water is necessary for germination Aim: To prove that water is necessary for germination. Apparatus: Two flasks marked A and B, pea seeds, water and cotton wool. Procedure 1) Take two flask which are marked A and B respectively. 2) In flask A place some pea seeds on a wet cotton wool and in flask B place some pea seeds on a dry cotton wool. 3) Keep the flasks at room temperature. Observation The seeds in flask A germinate and in B do not germinate. Conclusion Water is necessary for germination. BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 91  BIOLOGY MADE SIMPLE GERMINATION SUCCESS  Not all seeds germinate when planted. Basically seeds fail to germinate when one or all of the conditions necessary for germination are lacking, namely water, oxygen and favourable temperature. However, there are additional reasons why some seeds fail to germinate.  Reasons why some seeds fail to germinate include: 1. Inadequate water. Dry conditions mean the seed doesn't have enough moisture to start the germination process. 2. Lack of oxygen. Overwatering results in waterlogged soil that causes the seeds not to have enough oxygen. 3. Temperature not favourable; either very low or very high. Enzymes are denatured by very high temperatures. Enzymes are inactivated by very low temperatures. Protoplasm of seed cells is killed by very high temperatures. 4. Seeds not yet mature or still in dormancy. 5. Planting seeds too deeply causes them to use all of their stored energy before reaching the soil surface. Those planted too shallow may wash away, fail to germinate or be eaten by wildlife. 6. Cloddy and compacted soils that are high in clay will inhibit seed germination and emergence. 7. Soils that form a crust when dry may also prevent effective germination and emergence. BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 92  BIOLOGY MADE SIMPLE 8. Planting seeds in the wrong season. The growth of some plants is season-specific. Not all plant seeds can be planted throughout the year because each season has different temperatures. Successful germination of seed of different plants falls within a range of optimum temperatures. E.g. maize seeds germinate poorly in winter whereas wheat seeds germinate very well in winter. It is always best-practice to first consult the instructions on the back of the packet of seed. Seed advised for sowing in spring/summer will have a hard time emerging in autumn/winter and vice versa. 9. Loss of viability due to: a) depletion of food reserves due to prolonged storage of seeds. b) damage by pests and diseases. c) seed age. d) incorrect storage. Only seeds whose embryos are alive and healthy will be able to germinate and grow. Seeds stored for long periods usually lose their viability due to depletion of their food reserves and destruction of their embryo by pests and disease. Incorrect storage of seeds, e.g. at high heat, scorching sun and high humidity, can shorten the longevity of a seed. When buying seed from your local store always check the Best-Before date. The rate of germination and number of seeds germinating decrease as seed age. Seed past their expiration date may have degraded quality.  Germination success is measured in terms of percentage germination and is calculated using this equation: Percentage germination = x 100% Worked example: A farmer plants in a seed bed 25 rows of 50 seeds. Calculate the percentage germination if 50 seeds fail to germinate. Solution Number of seeds planted = 25 x 50 = 1250 Number of seeds which germinated = 1250 – 50 = 1200 Percentage germination = x 100% = x 100% = 96% BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 93  BIOLOGY MADE SIMPLE Disadvantages of poor germination 1. Replanting has to be done. 2. Replanting delays the crop leading to late harvesting. Late planted crops mature late and fetch poor prices on the market as compared to early planted crops. This document is intentionally incomplete. It is a SAMPLE COPY from the textbook ‘Biology Made Simple: Best ‘O’ Level Biology Revision Notes’ by G. Taruvinga. © Gladmore Taruvinga 2018 Published by Royalty Science, Inc. Harare, Zimbabwe Cell +263 772 980 253 E-mail: [email protected] Please do not plagiarize this document by removing the information in the footer and replacing it with your own. Please just do the right thing - acknowledge the author of this document. How to cite this publication: G. Taruvinga (2018). Biology Made Simple: Best ‘O’ Level Biology Revision Notes. Royalty Science, Harare, Zimbabwe. BEST ‘O’ LEVEL BIOLOGY NOTES © TARUVINGA G  O772 980 253  Page 94