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Animation: Pine Life Cycle

by Pearson
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Gymnosperms are seed plants that have no enclosed chambers around their seeds. Most gymnosperms are conifers -- cone-bearing trees such as the pine. The male and female parts of a conifer are found in separate locations and structures on the tree. Let's review the structures important for conifer reproduction and the sequential stages of the life cycle of a typical coniferous plant such as the pine. Microspores are formed by meiosis. Each microspore develops into a pollen grain containing a male gametophyte. A pollen cone contains hundreds of microsporangia in small reproductive leaves called sporophylls. Cells in the microsporangia undergo meiosis to form haploid microspores. Each microspore then develops into a pollen grain with tiny wings. Each pollen grain is a male gametophyte enclosed within a pollen wall. Now what do the pollen grains do? Pollination occurs when the wind carries a pollen grain to the micropyle at the base of an ovulate cone scale. A pollen tube slowly begins to grow from the pollen grain into a megasporangium. This may take more than a year. While the pollen tube is growing, let's look at what happens in the megasporangium. While the pollen tube continues to grow, the megasporocyte (2n) divides into four haploid megaspores (n), only one of which survives. This haploid megaspore will produce a female gametophyte and eggs. The surviving haploid megaspore divides several times by mitosis to form the haploid female gametophyte. Two or three archegonia, each with an egg, develop within the gametophyte. What finally brings the sperm to the eggs? During the completion of the pollen tube, the sperm cells develop in the pollen tube as it breaks through the megasporangium and reaches the female gametophyte. The sperm nuclei are then released into the megasporangium. Fertilization occurs when one of the sperm nuclei enters an egg cell and unites with the egg nucleus, forming a zygote (2n). One sperm nucleus fuses with each egg, but usually only one zygote survives. The zygote then divides by mitosis to form an embryo. The embryo and the surrounding haploid gametophyte tissue (n) and diploid seed coat (2n) form a seed. The embryo is the beginning of what process? The embryo in the seed is an immature sporophyte. The seed also consists of a food supply derived from gametophyte tissue and a seed coat derived from the parent tree. The seed falls to the forest floor and germinates. The embryo becomes a seedling, and the seedling grows up into a mature diploid sporophyte (2n), completing the pine life cycle.
Gymnosperms are seed plants that have no enclosed chambers around their seeds. Most gymnosperms are conifers -- cone-bearing trees such as the pine. The male and female parts of a conifer are found in separate locations and structures on the tree. Let's review the structures important for conifer reproduction and the sequential stages of the life cycle of a typical coniferous plant such as the pine. Microspores are formed by meiosis. Each microspore develops into a pollen grain containing a male gametophyte. A pollen cone contains hundreds of microsporangia in small reproductive leaves called sporophylls. Cells in the microsporangia undergo meiosis to form haploid microspores. Each microspore then develops into a pollen grain with tiny wings. Each pollen grain is a male gametophyte enclosed within a pollen wall. Now what do the pollen grains do? Pollination occurs when the wind carries a pollen grain to the micropyle at the base of an ovulate cone scale. A pollen tube slowly begins to grow from the pollen grain into a megasporangium. This may take more than a year. While the pollen tube is growing, let's look at what happens in the megasporangium. While the pollen tube continues to grow, the megasporocyte (2n) divides into four haploid megaspores (n), only one of which survives. This haploid megaspore will produce a female gametophyte and eggs. The surviving haploid megaspore divides several times by mitosis to form the haploid female gametophyte. Two or three archegonia, each with an egg, develop within the gametophyte. What finally brings the sperm to the eggs? During the completion of the pollen tube, the sperm cells develop in the pollen tube as it breaks through the megasporangium and reaches the female gametophyte. The sperm nuclei are then released into the megasporangium. Fertilization occurs when one of the sperm nuclei enters an egg cell and unites with the egg nucleus, forming a zygote (2n). One sperm nucleus fuses with each egg, but usually only one zygote survives. The zygote then divides by mitosis to form an embryo. The embryo and the surrounding haploid gametophyte tissue (n) and diploid seed coat (2n) form a seed. The embryo is the beginning of what process? The embryo in the seed is an immature sporophyte. The seed also consists of a food supply derived from gametophyte tissue and a seed coat derived from the parent tree. The seed falls to the forest floor and germinates. The embryo becomes a seedling, and the seedling grows up into a mature diploid sporophyte (2n), completing the pine life cycle.