DEPARTMENT OF BIOLOGY REPRODUCTION IN PLANTS

Transcription

1 DEPARTMENT OF BIOLOGY REPRODUCTION IN PLANTS BIO - Notes - Topic: Reproduction in Plants P a g e 1 27

2 1. Asexual Reproduction Is a form of reproduction resulting in the production of genetically identical offspring from one parent. The offspring represent exact copies of the parent and is referred to as a clone. There is no formation and fusion of gametes. Advantages The process is fast. Less energy is required. Large numbers of offspring are produced within a short period of time. Only one parent is involved. No gametes are needed. Offspring are identical to their parents (no variation). All characteristics of the parent are passed on to the offspring. Plants that reproduce asexually usually store large amount of food that allow rapid growth when conditions are stable. Disadvantages There is no variation which means that they are all genetically identical so if there is a change in the environment they could all die (although bacteria can survive from an antibiotic by mutation, then the mutated bacteria reproduce creating an antibiotic-resistant strain). Since a large number of offspring are produced, overcrowding usually occurs; which leads to competition among species of food, oxygen and water. If a parent has no resistance to a particular disease, none of the offspring will have resistance. BIO - Notes - Topic: Reproduction in Plants P a g e 2 27

3 Examples of asexual reproduction 1. Bacteria: Bacteria are tiny single-celled organisms. They reproduce by a process called binary fission. In binary fission, one bacterium grows and exact copy of its DNA coil which carries its genetic information. Then the bacterium completely divides witch one DNA coil in the parent and one in the daughter bacterium. Each bacterium can undergo binary fission once every 20 minutes making them able to reproduce massive numbers from one parent in very little time. 2. Fungi: fungi are multi-cellular organisms that grow long threads called hyphae on pieces of food. There are two types of hyphae, reproductive and feeding hyphae. Reproductive hyphae grow vertically above the food material. At the top of the hyphae, there is a spherical bag in which many spores are grown. This bag is called sporangium. Spores being produced in the sporangium are reproductive structures that can grow into another fungus. At some point, the sporangium will burst open dispersing the spores into the air. If a spore falls on an area of favorable conditions (food water air) it will germinate and grow into a new identical fungus. BIO - Notes - Topic: Reproduction in Plants P a g e 3 27

4 3. Potatoes: Potatoes reproduce asexually. A potato plant starts as a lateral bud (seed) under the soil. In favorable conditions, a shoot grows out from the bud vertically, which then becomes the stem of the plant above the soil. Roots also grow out of the bud downwards into the soil. The bud is now a plant. The stem then grows a part of it under the soil, which is called underground stem. Swellings start to grow from the underground stem; these swellings are called tubers which are the new potatoes. Glucose formed by photosynthesis in the leaves of the above ground stem is converted into sucrose and transported down the stem to the tubers to be stored there as starch. When the tubers are fully grown the mother plant dies and the new lateral buds form on the tubers. These buds then grow into new potato plants identical to the mother plant. BIO - Notes - Topic: Reproduction in Plants P a g e 4 27

5 Note: Asexual reproduction in plants is also called vegetative propagation. BIO - Notes - Topic: Reproduction in Plants P a g e 5 27

6 1.2 Sexual reproduction BIO - Notes - Topic: Reproduction in Plants P a g e 6 27

7 Sexual Reproduction: it is reproduction involving two parents. Formation of a new organism takes place by the fusion of gametes (fertilization). Which are produced by meiosis (each gamete has only half the number of chromosomes. The offspring produced are similar but not identical to their parents. Sexual reproduction: the process involving the fusion of haploid nuclei to form a diploid zygote and the production of genetically dissimilar offspring. Advantages: New varieties of offspring are produced due to variations which are better adapted to new habitats enabling survival of the species. Neither overcrowding nor competition among the offspring since they are few in number. New varieties can be created which may have resistance to diseases. Disadvantages: Two parents are usually needed. (though not always- some plants can self pollinate) Slow process, with production of few species. Reproduction occurs only after maturation of the reproductive organs. Sexual reproduction in plants Sexual Reproduction in Flowering Plants: The most successful group of plants is the flowering plants. When a flowering plant reproduces it is able to: Produce male and female gametes (Meiosis) Transfer the gametes between flowers (Pollination) Fuse the gametes to produce a zyogote (Fertilization) Produce seeds to protect the embryo from drying out and provide the new plant with food stores (Formation of seeds) Distribute seeds to new areas (Dispersal of seeds) In a plant, the organs which are responsible for sexual reproduction are the flowers. Sex cells are called gametes. The male gametes are produced inside the anther. And the female gametes are produced inside the ovules. Some flowers can produce both male and female gametes. They are called hermaphrodite (Bisexual). Male gametes are inside pollen grains in the anthers. Unisexual: A flower with either male or female reproductive organs. BIO - Notes - Topic: Reproduction in Plants P a g e 7 27

8 Structure of a Bisexual Flower Carpel Functional parts of a flower: 1. Calyx It is the outermost part and most often green in colour. The individual units of calyx are called the sepals. It protects the inner parts at bud stage. 2. Corolla: The individual units of corolla are called petals. They are often large and colourful to attract bees, birds, etc which are the agents of pollination. BIO - Notes - Topic: Reproduction in Plants P a g e 8 27

9 3. Stamens: It is the male reproductive part of the flower. It is made up of filament and an anther. Anther contains four pollen sacs in which pollen grains are formed. Pollen contains male sex cells (gamete) which are produced by cell division called meiosis. Filament supports the anther. 4. Carpels: (pistils) These are the female reproductive organs. A flower may have one to many carpels. Each carpel is made up of the basal ovary, middle style and the upper stigma. The stigma is a sticky structure that receives the pollen grains during pollination. The style is hollow and provides a passage for the male gametes to reach the female gametes, the eggs. The ovary is the chamber where there are many ovules. Each ovule consists of a haploid egg. After fertisation, the whole ovary becomes a fruit and the ovule becomes a seed. BIO - Notes - Topic: Reproduction in Plants P a g e 9 27

10 5. Petiole: (stalk) It supports the flower to make it easily seen by insects and to be able to withstand wind. 6. Receptacle: It is the expanded end of the flower stalk. The flower is attached to it. 7. Nectar gland: It produces sugary fluid called nectar to attract insects for pollination. BIO - Notes - Topic: Reproduction in Plants P a g e 10 27

11 Pollination: Pollination as the transfer of pollen grains from the male part of the plant (anther of stamen) to the female part of the plant (stigma). Types of pollination: 1. Self Pollination 2. Cross Pollination 1. Self Pollination: When the pollens from anthers are carried to the stigma of the same flower, or to another flower of the same plant. The pollen and the egg carry the same genes. 2. Cross Pollination: the pollen is taken to a flower on a different plant of the same species. BIO - Notes - Topic: Reproduction in Plants P a g e 11 27

12 Agents of pollination: Flowers are usually pollinated by insects, birds, mammals, water and the wind. The structural adaptations of a flower depend upon the type of pollination the plant uses. Insect Pollinated Plants Often large, brightly coloured petals. Nectaries present Short Stamen/CARPEL inside the flower Scented to attract insects. Small stigma. Large pollen grains. Rough, Sticky pollen grains. Small amount of pollen grains produced. e.g. lupin, apple etc Wind Pollinated Plants Petals absent or small and inconspicuous. No nectaries. Long Stamen/CARPEL which dangles out of the flower. Not scented Large feathery stigma. Small pollen grains. Smooth, non sticky pollen grains. Large amount of pollen grains produced. e.g. grass Wind pollinated flower. The effects of self pollination on a species: It ensures that the flower gets pollinated. The species is ensured of its survival. In this method, there will be no variation and hence no evolution or development of new variety. The plant is unable to adapt itself to the changing environment. BIO - Notes - Topic: Reproduction in Plants P a g e 12 27

13 Implications of cross pollination on a species: Pollination may or may not occur because the flowers are dependent on external agent. There is variation and hence evolution and development of new variety can occur. The plant is able to adapt to changing environmental conditions. The Growth of a pollen tube and the process of fertilization: Pollen tube: pollen grain lands on stigma and creates a tunnel down the style, through the micropyle, to the ovules. Fertilization: is a process of fusion of male and female gametes to form a zygote. Zygote: is a cell which is formed from the fusion of male and female gamete. In plants the male gamete is found in the pollen grain and the female gamete is the egg inside the ovule. Pollen Grain Each carpel has a stigma at the top, which collects pollen grains from other plants. The ovule is completely surrounded by a wall of cells to make an ovary. Some flowers have an ovary with more than one ovule. The female gamete or egg is produced by meiosis. Each egg cell is held in an egg sac within an ovule. BIO - Notes - Topic: Reproduction in Plants P a g e 13 27

14 Fertilization in a Flower Events of fertilization: If a pollen grain lands on the stigma of a flower of the same species, fertilization takes place. This starts by the stigma secreting chemicals that start what s called a pollen tube. A pollen tube is the pathway in which the pollen moves downwards to reach the ovule. If a pollen grain falls on the stigma of a flower of a different species, the stigma will not secrete these chemicals and fertilization fails. At the tip of the pollen tube, enzymes are secreted that digest the tissue of the style digging the pollen tube further down till it opens on the other end in the ovule. The male gamete enters the pollen tube and starts its journey downwards to the ovule. When the male gamete reaches the ovary, it enters the ovule through a hole in its outer shell. The male gamete, which is a haploid nucleus starts fusing with the ovule producing a diploid zygote, then develops into an embryo plant. BIO - Notes - Topic: Reproduction in Plants P a g e 14 27

15 Seeds: Seeds are of two types. 1. Endospermic seed: It contains a large endosperm at maturity. E.g. wheat and rice 2. Non endospermic seed: The endosperm is used up to make large storage organ called cotyledon. E.g. beans and peas The structure of a non endospermic seed: (bean seed) testa cotyledons plumule plumule radicle micropyle cotyledon Embryo Plant epicotyl cotyledon stalk hypocotyl radicle (a) Longitudinal (b) Diagram of seed section structure A Seed includes the following structures: 1. Hilum: A scar where the seed was attached to the pod. 2. Testa: A tough seed coat that encloses the embryo. It becomes soft and splits during germination. 3. Micropyle: A small hole in the testa through which water enters the seed. 4. Cotyledons: Store food. 5. An embryo: A miniature plant in the seed. It consists of a plumule that grows into shoot and radicle that grows into root. Fruit and seed formation: Events after Fertilization, An ovule develops into a seed. The ovary increases in size and becomes a fruit. The wall of the ovary becomes the fruit wall (the pericarp) Stamens, sepals and petals shrivel and fall out. The integument becomes the testa (seed cover) of the seed. BIO - Notes - Topic: Reproduction in Plants P a g e 15 27

16 Fruits are of two kinds: 1) Fleshy fruits (Edible): e.g. Tomato, orange, cucumber. 2) Dry fruits (non-edible): The ovary develops into dry capsules. E.g. wallflower pod, Lupins and bean pod. Fleshy Fruit -Tomato Dry fruit- Wallflower BIO - Notes - Topic: Reproduction in Plants P a g e 16 27

17 Seeds and Fruit dispersal: Dispersal: The scattering of fruit and seeds away from the parent plant is called dispersal. This allows the species of the plant to spread across the landscape by colonizing new areas. List at least 3 advantages of dispersal Methods of dispersal: 1. Wind 2. Animals 3. Water 1. Wind: Seeds carried by wind tend to be very light. Some plants have special adaptations such as presence of wing or parachute to their seeds to help them travel. Examples: sycamore, dandelion, poppy etc. Describe some of the adaptations of the following: a) Sycamore:. BIO - Notes - Topic: Reproduction in Plants P a g e 17 27

18 b) Dandelion:... c) Lupins and Beans:... BIO - Notes - Topic: Reproduction in Plants P a g e 18 27

19 2. Animals: Animals disperse seeds either by eating them and passing the seeds out undamaged or by transporting the seeds away. There are two main modifications for animal dispersal: succulent fruits and hooked fruits. Describe the features of the following: a) Succulent fruits: BIO - Notes - Topic: Reproduction in Plants P a g e 19 27

20 b) Hooked Fruits:..... BIO - Notes - Topic: Reproduction in Plants P a g e 20 27

21 By being either succulent or hooked fruit, a plant has a mutual relationship with the animal in that the animal helps the plant to spread the seed to other new areas thus ensuring the spread and survival of the plant. The animal receives nutrition from the plant by eating its fruit. BIO - Notes - Topic: Reproduction in Plants P a g e 21 27

22 3. Water: The fibrous outer husk of a coconut allows it to float on sea currents to new areas. Many tropical islands have a line of coconut palms along the coast of the beach overlooking the ocean. 2. Growth and development GROWTH AND DEVELOPMENT KEY OBJECTIVES Define growth in terms of a permanent increase in size and dry mass by an increase in cell number or cell size or both. Define development in terms of increase in complexity. Investigate and state the environmental conditions that affect germination of seeds: requirement for water and oxygen, suitable temperature Growth: It is permanent increase in size and dry mass by an increase in cell number or cell size or both. Germination: Germination is the sprouting of a seed. Or start of plant growth in a dormant seed or bud Development: Development is an increase in complexity of an organism. BIO - Notes - Topic: Reproduction in Plants P a g e 22 27

23 It is the series of changes which animal and vegetable organisms undergo in their passage from the embryonic state to maturity, from a lower to a higher state of organization. Development is from a simple organism to a complex organism. Dormancy: A stage when seeds are unable to germinate. The factors that influence dormancy are plant growth substances, light, temperature, testa etc. Dry mass in a seed decreases in the first few days of germination. Why? When germination begins the seed needs a lot of water. Seeds absorb water and swell up, finally testa splits leading to the decrease in the dry mass. The stages in the germination of a bean seed: Describe the stages of germination of a bean seed: When germination begins the seed needs a lot of water. Seeds absorb water and swell up. Testa splits and one part of the embryo, called the radicle, comes out and grows making the roots. Another part, called the plumule, turns into the shoot of the plant. Food used during germination: The main types of food stored in the cotyledons are starch and protein. The starch has to be turned into maltose by enzyme amylase or into other soluble sugar or into other soluble sugar by the specific enzyme. Proteins have to be broken into amino acid by proteases. BIO - Notes - Topic: Reproduction in Plants P a g e 23 27

24 The phloem translocates the soluble sugars and the amino acid to the growing parts of the embryo. Sugar is used in the respiration to release energy and amino acid to build up proteins for growth. Conditions needed for germination: 1) Water 2) Oxygen 3) Suitable temperature Explain each of the conditions that are needed for germination. 1. Water: Water is needed to swell the seed, burst the seed coat and allow growth. All seed contain some moisture but more water is needed for : The water makes a chemical change, i.e. help in conversion of starch to sugar and protein to amino acid. The water also causes the baby seed ( embryo ) to enlarge and split the seed coat. It activates the enzymes. Help to keep the shoot upright since it keeps the cells turgid. 2. Oxygen: The most useful form of respiration taking place in germinating seeds is aerobic respiration, requires oxygen. 3. Suitable Temperature: A seed requires suitable temperature to germinate. BIO - Notes - Topic: Reproduction in Plants P a g e 24 27

25 Experiment on the conditions for germination: 1. Is water essential for germination? 1. Label three containers with the letter A, B or C. Put some dry cotton wool in each one. 2. Place an equal number (about 10) soaked seeds on the cotton wool in each container. 3. Leave A quite dry; add water to B until the cotton wool is uniformly moist; add water to C until the seeds are completely covered with water. Make sure the seeds do not float. 4. Replace the lids on all three containers and leave them for a week in a place where they will all be at about the same temperature. 5. After a week, remove the lids, count the number of seeds which have germinated in each container and record this number. Results In B, seeds will germinate normally. In A, seeds will not germinate (lack of water) Explanation: Seeds in container B prove that water is necessary for germination. Seeds in container C show that much water may prevent germination by cutting down the oxygen supply to the seed. BIO - Notes - Topic: Reproduction in Plants P a g e 25 27

26 2. The need of oxygen for germination: Roll two moist cotton wools in a dish containing cress seeds (small seeds) A thread holds each cotton wool is attached to a bung. Label two flasks A and B. To flask A, sodium hydroxide and pyrogallic acid solutions are added. To flask B, sodium hydroxide is added. A bung closes each flask in such a way that the moist cotton wool with cress seeds doesn t touch the solution BIO - Notes - Topic: Reproduction in Plants P a g e 26 27

27 Result In flask B, seeds will germinate. In flask A, germination of seeds does not take place. Explanation Pyrogallic acid in flask A absorbs the O2 and so no germination occurs. Flask B shows O2 is needed for germination. Note: The absence of CO2 doesn t affect germination. 3. Temperature and germination: Label four containers A, B, C, and D Put wet cotton wool in all containers. Put same number of soaked seeds in all containers. All seeds are covered. Place container A in a refrigerator (at about 4 o C) Place container B in the room (About20 o C) Place container C in a warm place, such as in incubator (30 o C) Place container D in a beaker of boiling water. Result: Seeds in A may not germinate. Seeds in D will not germinate. Seeds in C at 30 o C will germinate faster than those in B (Room temperature) Explanation: At very low temperature, enzymes are inactive. Germination may not start. As the temperature increases, the rate of germination also increases at least till 40 o C. At high temperature i.e. 50 o C and above, enzymes are denatured and seeds are killed. Controlling the variables (conditions): For fair experiment, all the variables must be kept constant except the one being tested. BIO - Notes - Topic: Reproduction in Plants P a g e 27 27