Plant Reproduction.docx

Plants Sexual reproduction is reproduction involving the fertilisation of gametes. Asexual reproduction is reproduction not involving gametes. Sexual reproduction in plants involves the production of flowers (1) Functions of flower parts Sepals Enclose and protect the flower before it opens and will...

Plants Sexual reproduction is reproduction involving the fertilisation of gametes. Asexual reproduction is reproduction not involving gametes. Sexual reproduction in plants involves the production of flowers (1) Functions of flower parts Sepals Enclose and protect the flower before it opens and will sometimes close over the flower at night Petals - Most obvious part of the flower - Large, variously shaped and bright to attract pollinators - Produce scent to attract insects Stamens - Male part of the plant and consists of two parts - Anther: Produces pollen which forms male gametes - Filament: A long thin tube that holds the anther in place where it can most effectively carry out its job Carpel - Female part of the plant and consists of three parts - Stigma: Catches pollen - Style: Connects the stigma to the ovary and holds it in a suitable position to collect the pollen - Ovary: Has one or more ovules which contain the embryo sac. Egg sac contains the egg cell and two polar nuclei which will form the endosperm Receptacle - Thickened part of the pedicel from which the flower parts grow Nectary - Located at the base of the petals and produces nectar (a sweet sugary solution) Development of gametes Female - Formed in the ovule contained in the ovary from a diploid megaspore mother cell when it divides by meiosis to form four haploid cells called megaspores - Called megaspores because they are much bigger than the pollen grain - Three of these haploid cells degenerate, leaving one - The surviving megaspore divides by mitosis three times, forming a total of 8 nuclei - Three of the surviving nuclei consist of two polar nuclei and an egg cell - This collection of nuclei is called the embryo sac Male - Inside the anther are hollow tubes lined with diploid microspore mother cells - Each of these cells divide by meiosis, producing four haploid microspores. Each of these microspores then divide by mitosis to produce two nuclei - One of these is called the tube nucleus and its job is to control the growth of the pollen tube from the stigma, through the style to the ovary, and then to the micropyle of the ovule - The other nucleus is called the generative nucleus and it will divide by mitosis as it passes down the pollen tube to form male gametes - The pollen grain matures, forming a tough layer on the outside called the exine Pollination Pollination is the transfer of pollen from an anther to a stigma. There are two types of pollination: - Self-pollination: Occurs when the anther and stigma are on the same plant. This leads to less variation and a greater chance of harmful variants occurring more often. - Cross-pollination: Occurs when the anthers and stigma are on different plants. Methods of Cross-Pollination Insect Pollination Bees visit the flower to collect nectar and pollen to feed their young, and as they do so, they become covered in pollen. As they fly from flower to flower, they carry pollen with them Wind Pollination Plants like wheat and barley produce copious amounts of very light pollen which drifts away on the slightest breeze Fertilisation - This is the fusion of the male gamete with the female gamete to form a diploid zygote - Once the pollen grain lands on the stigma it produces a pollen tube, which grows down the style to the ovule - As the pollen tube grows down, the generative nucleus (n) divides by mitosis to form two sperm nuclei (male gametes) (n). The tube is guided by chemotropism and by the tube nucleus which dies when the tube enters the ovule by the micropyle - One male gamete fuses with the egg cell to form a diploid zygote, while the other male gamete fuses with the two polar nuclei to form a triploid endosperm nucleus. This is double fertilisation Seed Formation - After fertilisation, the ovule starts to swell and the ovule wall begins to toughen up because it is going to form the seed coat(testa) - The zygote forms the seed embryo - The triploid endosperm nucleus divides rapidly to form a food storage tissue - The ovule develops into the seed - The integuments (outer wall of ovule) thicken and toughen to form the testa - When the seed is fully formed, it dries out and becomes dormant Seed Structure and Formation - Embryo: Part of the seed that will develop into the new plant - Testa: A tough and hard seed coat which protects the seed before germination. - Radicle: First part of the new plant to emerge from the seed, develops into the root - Plumule: Emerges soon after the radicle, develops into the shoot - Cotyledons: These are seed leaves which are formed by some of the embryo cells. These contain food reserves which are used in the early stage of germination. Plants can either have one cotyledon (monocot) or two cotyledons (dicot). They secrete enzymes which digest endosperm food, absorb it, and pass it on to the developing embryo In monocots, such as maize and wheat, food required by the developing embryo is stored in the endosperm. These are endospermic seeds. In dicots, such as the broad bean, the food is stored in seed leaves called the cotyledons. These are non-endospermic seeds. Fruit Formation - The function of fruit is to protect the seed and allow for seed dispersal. - Varieties of seedless fruit have been developed for the consumer market e.g., watermelons. Seedless fruit is produced in one of two ways: a. By genetic selection of plants that produce very few seeds which are then propagated asexually b. Using plant hormones, to induce fruiting, without fertilisation e.g., Auxins Seed Dispersal Dispersal is the carrying of fruit or seeds as far away as possible from the parent plant to ensure survival and minimise the chance of overcrowding and competition. Animal Dispersal - Fruit containing the seed is eaten by an animal and pass through its intestine. The seed will be attacked by the animal’s digestive enzymes. Some plants need this to happen before they can germinate. For example. Tomato seeds and apple seeds Wind Dispersal - Seeds need to be light and have a large surface area to allow the wind to catch and carry them a long distance. For example, sycamore parachutes and dandelion parachutes Self-dispersal - When a pea pod dries in the sun, lines of weakness appear and the pod curls back and splits, causing the seeds to spring out. For example, Gorse Water-dispersal - Seeds contain pockets of air to keep them afloat to allow for dispersal over large distance. For example, yellow water lily Dormancy Dormancy is a period of greatly reduced growth or metabolism even conditions are suitable for germination. Advantages of Dormancy - Allows the plant to avoid adverse conditions which could damage or kill new growth - Gives the seeds more time to disperse and so reduces competition - Gives embryos time to mature before conditions improve Dormancy in Agriculture and Horticulture Breaking Dormancy - Many plants maintain dormancy by having a waterproof Testa which prevents water entering the seed. Water can be added by soaking the Testa, breaking the seed coat by abrasion, or cracking the Testa -Soaking the seeds in chemicals to wash out inhibitors - Applying plant hormones such as gibberellins to the seeds Using Dormancy - Seeds can be kept at optimum storage conditions to allow a larger percentage of the seeds to survive and germinate - Maximising the growth season for the crop by using the optimum time of year to sow the seeds Germination Germination is when the embryo or seed begins to grow. Requirements for germination - Water: Needed as a solvent to transport materials around the plant. Acts as a medium for plant hormones and enzymes to work - Oxygen: Used in aerobic respiration to provide the plant with energy to carry out its normal life functions - Suitable temperature: Needed for enzymes to work - Food: Food reserve in the seed must be digested and used to provide energy and the materials to make new plant tissues Stages in Germination - The radicle emerges from the seeds and grows downwards forming the root - The plumule grows upwards forming the shoot. The plant continues to lose weight - The leaves break the surface, and the seedling begins to photosynthesise and produce glucose to feed itself and begin to form new tissue - The plant begins to gain weight - If the seed stays below the ground it is called hypogeal germination. If the seed is carried above the ground, it is called epigeal germination Asexual Reproduction Asexual reproduction does not involve gametes; therefore the offspring are genetically identical to the parents. Cloning plants is referred to as vegetative propagation. Natural Propagation Stem Tubers e.g., potatoes - Tubers are swollen tips of underground stems - They have axillary buds that produce new shoots and roots using the stored food in the tuber - The parent tuber eventually dies away Root Tubers e.g., sweet potatoes - Swollen areas of lateral roots that store food and act as perennating organs, allowing the plant to survive from year to year - Perennating is the ability of a plant to survive from one germinating season to the next Bulbs e.g., daffodils - An underground swollen bud that can survive winter beneath soil until the following year - Have a small stem bearing a terminal bud and axillary buds enclosed in the bases of leaves swollen with food reserves - The axillary buds can grow, using some of the stored food and form foliage leaves and adventitious roots to form independent plants that become detached from the parent Plantlets e.g., strawberry plants - Miniature plants form along the edge of the leaf and fall off - Have miniature roots which grow into the ground and the new plant quickly becomes established - The strawberry plant produce modified stems called runners which grow over the surface of the ground. These runners have plantlets at the end, when these touch the ground, roots that are usually already present grow into the ground - When the plantlet is attached to the parent plant, it can receive nourishment which gives it a better chance of becoming established - When the plant is established, the runner withers and the new plant becomes independent Artificial Propagation Cuttings e.g., geranium - Part of the shoot is removed and placed in soil or water to form roots - Rooting powder may be used to promote rapid root development Layering e.g., blackberry - A cut shoot is pegged into soil and develops into a new plant - After approximately one-year, adventitious roots grow and are cut off from parent plant Grafting e.g., apples - A shoot twig (scion) is removed and taped into the root stock - Both cambium layers unite to form one plant Micropropagation e.g., bananas - Individual cells i.e., stem, root, or leaf, can be removed from a plant and then grown in tissue culture - This is useful in the production of houseplants and commercial crops