Topic6：The reproductive structures of angiosperms and their life cycle

     There are two reproductive structures in angiosperm, namely the stamen or the androecium, the male part which produce the pollen and the female part which is called gynoecium or the pistil. The stamens consists of anther which housed the pollen grain, connected by filament, a tube-like structure which connected the anther to the flower structure. The pistil are made up of ovules enclosed in the ovary, the style and the stigma. The ovary is connected to the stigma by a tube-like structure called style. Stigma is the site where the pollen will be lending during pollination. The Pollen will germinate, and the male gametes will be transported into the ovary to fertilize the ovules to form the zygote and eventually will grow into embryo and upon germination the embryo will grow into new plants.

     The adult, or sporophyte, phase is the main phase of an angiosperm's life cycle . As with gymnosperms, angiosperms are heterosporous. Therefore, they generate microspores, which will produce pollen grains as the male gametophytes, and megaspores, which will form an ovule that contains female gametophytes. Inside the anthers' microsporangia, male gametophytes divide by meiosis to generate haploid microspores, which, in turn, undergo mitosis and give rise to pollen grains. Each pollen grain contains two cells: one generative cell that will divide into two sperm and a second cell that will become the pollen tube cell.

Diagram of  angiosperms' life cycle

     For the pollination process, Pollination is when pollen grains from the anther, the male portion of a flower, are transferred to the female part of the flower, known as the stigma. In order for pollination to be successful, the pollen grains transferred must be from a flower of the same species. After the pollen grains land on the stigma, it creates a pollen tube through the length of the style or stalk connecting the stigma and ovary. Once the pollen tube is complete, the pollen grain will send sperm cells from the grain down to the ovary. When the sperm cells reach the ovary and the egg cells, fertilization will occur, which will result in the formation of the seed. The seed will then be released from the parent plant and will be able to grow into a plant and continue the reproductive cycle using the method of pollinatio. Although All Flowering Plants Rely On Pollination For Reproduction, There Is Variation In How Plants Pollinate. There Are Two Types Of Pollination Called Self-Pollination And Cross-Pollination. Self-Pollination is The More Basic Type Of Pollination Because It Only Involves One Flower. This Type Of Pollination Occurs When Pollen Grains From The Anther Fall Directly Onto The Stigma Of The Same Flower. Although This Type Of Pollination Is Simple And Quick, It Does Result In A Reduction In Genetic Diversity Because The Sperm And Egg Cells Of The Same Flower Share Genetic Information. Cross-pollination is a more complex type of pollination that involves the transfer of pollen from the anther of one flower to the stigma of a different flower. This type of pollination results in an increase in genetic diversity because the different flowers are sharing and mixing their genetic information to create unique offspring.

     Coevolution is the interactive evolution of two or more species that results in a mutualistic or antagonistic relationship. When two or more different species evolve in a way that affects one another’s evolution, coevolution is taking place. This interactive type of evolution is characterized by the fact that the participant life-forms are acting as a strong selective pressure upon one another over a period of time. The coevolution of plants and animals, whether animals are considered strictly in their plant-eating role or also as pollinators, is abundantly represented in every terrestrial ecosystem throughout the world where flora has established itself. Coevolved relationships include an immense number of relationships between plants and animals, and even between plants and other plants. One of the most obvious and complex coevolutionary relationships are the mutualisms that have evolved between plants bearing fleshy fruits and vertebrate animals, which serve to disperse the seeds in these fruits. Another example are bees and flowers.