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Pollination And Fertilisation

Check us out at Pollination : Structures the diagram shows. Include the stigma, style, ovary, ovule and embryo sac (female gametophyte). Draw a line out from each structure and then print the correct label at the end of each line. Notice again that the ovule of the flowering plant is completely enclosed within the carpel. This is an important evolutionary advancement over the gymnosperms (pine) where the ovule is exposed on the ovulate cone scale surface. Next use an orange colored pencil to shade in the megagametophyte (embryo sac). The color will help you remember the megagametophyte is the haploid (1N) stage of the plant. Everything else shown in the diagram is part of the sporophyte stage of the plant . The cells of sporophyte tissue always have diploid chromosome numbers (2N). Finally, the diagram illustrates how well the megagametophyte is protected by the surrounding tissue of the sporophyte plant.Pollination occurs when pollen grains are transferred from the anther of one flower to the stigma of the same flower or to another flower's stigma of the same species. The transfer agent is wind in some species and insects, birds and bats in others. Successful pollination requires simultaneous maturation of the stamens and carpels. Show pollination by drawing two pollen grains on the stigma of your diagram. Color the pollen grains orange to show they are haploid and microgametophytes. Draw a pollen tube extending from one pollen grain down into the style. Add a tube nucleus and generative cell to the tube. Place the tube nucleus at the tip of the pollen tube. Label the pollen grain, generative cell and tube nucleus. Fertilisation :Draw a tube from the second pollen grain down through the stigma, style and ovary to near the micropyle of the ovule (see diagram). Add a tube nucleus and two sperms cells to the tube. These cells should be near the tip of the pollen tube. The sperm cells are new cells formed when the generative cell divides by mitosis. The generative cell does not always divide in the pollen tube. In some species the division occurs during or even before pollination. Color the pollen grain, pollen tube, sperm cells and tube nucleus orange to show they are haploid and part of the microgametophyte generation. Label the pollen tube, sperm cells and tube nucleus.If you already have a copy of the embryo sac diagrams continue on. If not, go back to the title page and follow the instructions for printing diagrams for you to use with this part of the tutorial. Use the top diagram for this part of the lesson. Color the megagametophyt (embryo sac) orange. Remember, the megagametophyte only consists of seven cells. Label the antipodal cells, polar nuclei of the central cell, synergid cells and the egg cell. Color the pollen tube, sperm and tube nucleus orange. These are the parts of the microgametophyte. This stage consists of only three cells. Label the sperm cells and tube nucleus.To accomplish fertilization the pollen tube continues to grow through the micropyle opening and finally penetrates the megagametophyte (embryo sac).Following penetration there is release of tube contents including both sperm cells. After being released one sperm unites with the egg forming a 2N Zygote. The second sperm fuses with the polar nuclei forming a 3N primary endosperm nucleus of the central cell. Double fertilization of the type described is unique to the angiosperms. The zygote being 2N is the first cell of a new sporophyte generation. The 3N cell is not considered to belong to either the sporophyte or gametophyte generation. before continuing the lesson. Label the endosperm nucleus of the central cell. Color this nucleus red. This color will be used to identify 3N chromosome numbers. Remember the nucleus was formed from the fusion of a sperm cell with two polar nuclei. Label the zygote. Color the zygote green. This color was usedbefore to identify the diploid chromosome numbers of sporophyte cells. The fate of the synergid and antipodal cells is uncertain but it is thought that they disintegrate and play no further role in the plant life cycle.With fertilization accomplished there is a burst of activity and growth. The endosperm cell divides rapidly producing a food storing tissue (endosperm) which may be temporary, if consumed by the developing embryo, or permanent if stored in the seed to be used at the time of germination. Similarly, the zygote divides rapidly producing a spherical shaped embryo attached to a suspensor apparatus. In time the entire ovule will be transformed into a seed. We will see later that the seed contains an embryo, a supply of stored food and is surrounded by a seed coat. Finally, and over a longer time period, the ovary becomes transformed into a fruit. The fruit is another evolutionary first in the life cycle of plants. More information about seeds and the fruit will be found in the next two flowering plant reproduction tutorials
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