Plants
1. Alternation of Generations
Alternation of generations is the process by which plants alternate between 2 phases; the diploid stage and the haploid stage. In this process the plant also uses both meiosis and mitosis the reproduce their cells. Firstly in alternation of generations spores which contain a plant inside of them are released into the air. And these spores are haploid which means that they contain half of its parents chromosomes. Also, these spores are produced in the sporophyte which is diploid and once the spores are formed there are both male and female gametophyte spores. From that point the male and female haploid spores then germinate to form gametophytes. The male gametophytes contain the antheridium which is where the sperm is located and the female gametophytes contain the archegonium which is where the eggs are located. Once both gametophytes have grown enough through the process of mitosis the male then sends its sperm which is also haploid, to the eggs (haploid) in the archegonium. Next through the process of fertilization the sperm and the egg create a zygote which is diploid because it now contains both the male and female chromosomes. The newly formed zygote is also the young sporophyte which it will soon grow into. Once the sporophyte then grows and matures, through the process of meiosis the spores are formed once again and can grow into new gametophytes. However, most plants use seeds instead of spores to reproduce.
2. Characteristics and life cycles of plant classifications
A) Bryophytes: Bryophytes are among the first plants developed on the planet. They are non-vascular meaning that they don't have a water transport system or any roots. Bryophytes also have flagellated sperm and need surrounding water for fertilization. At the start of the bryophyte cycle spores which are haploid get released into the air and land. Then the spores start to germinate into a protonema where gametophytes branch off of it. The newly formed gametophyte produces gametes and is also haploid and can be either male or female. They also contain rhizoids that act like roots but are not. The male gametophyte contains the antheridium which produces sperm while the female gametophyte contains the archegonium which produces eggs. Once the gametophyte has grown through the process of mitosis the flagellated sperm swim the eggs in the archegonium where fertilization takes place. Then a zygote is created which is diploid and grows into a diploid sporophyte which branches out of the gametophyte. The newly formed sporophyte consists of a neck and capsule called the sporangium which produces the spores through meiosis. From there on the cycle repeats. In the bryophyte cycle the gametophyte is also the dominant stage.
B) Pteridophyta: Pteridophytes are among the first vascular plants on this planet and were developed as the earth's climate became dryer. What it meant to "vascular" is that there are transport tubes throughout the plant to help it survive. The pteridophytes contain xylem for water transportation from roots to shoots and phloem for photosynthesis. These plants are ferns and just like the bryophytes, they to have flagellated sperm. However, although they also use alternation of generations as their life cycle, the sporophyte is the dominant stage while the gametophyte was more fragile. When the pteridophyte life cycle begins spores are released until they land and germinate into a haploid gametophyte which is heart-shaped and contains the antheridium, archegonium and rhizoids. This gametophyte is actually called a prothallus and grows independently of the sporophyte which also contains both gametes. Then, once the swimming sperm reaches the egg in the archegonium, fertilization takes place and a diploid sporophyte embryo starts to grow. Once the sporophyte has fully developed by mitosis spores are then produced via meiosis and are located in containers called sporangia. And these containers are also grouped into many clusters called sori. These sori are located on the underside of fern fronds which look like leaves but don't be mistaken. From there the cycle repeats.
C) Gymnosperms: These are seeded vascular plants that considered to be "naked" and are dominated by the sporophyte generation. Unlike the bryophytes and the pteridophytes the gymnosperms use sexual pollen for gamete distribution and create seeds which is better for protecting and distributing the embryo. The gymnosperms also have deep roots and contain needles to help it survive in harsher climates. In this life cycle the male and female reproductive structures are cones. The males which are pollen cones and the females which are the seed cones. In the male cones pollen grains (n) are produced via meiosis and by wind, travel to the female cone. Over in the female cone meiosis takes place in the ovule thus creating 4 haploid cells. However, only 1 of those cells will grow through mitosis into the female gametophyte. Then, in that female gametophyte endosperm and an egg cell in created. Once this has fully developed pollination then occur when pollen grain creates a pollen tube directly to the egg. Then through fertilization the egg and sperm fuse creating the diploid zygote. This zygote is also surrounded by a diploid seed coat to help protect it and with in contains gametophyte tissue (n) to help feed the embryo. Finally once the scales peel back, the seeds fly through the air due to wind and then germinate into a new plant (2n).
D) Angiosperms: These are the flowering plants and are also called the flowering plants because they are covered by a fruit. Angiosperms also mostly need pollinators to transfer pollen from the stamen (male) to the carpel (female). These plants also contain 4 main organ systems; the stems, leaves, roots and flowers. The stems are responsible for many things such as growth of the plant, the leaves help exchange gasses, roots absorb water and minerals and the flowers attract pollinators. In this life cycle the male and female reproductive structures are mostly both on one flower. The males being the stamen and the females being the carpel. For the stamen it consists are the anther which produces the pollen grains via meiosis and the filament which supports the anther. Over at the carpel it consists are the stigma which is moist and sticky to capture pollen grains, the style which supports the stigma, the ovary which contains the locule and ovule, the locule which contains the ovule, and the ovule which contains the egg nuclei created through meiosis. Once the pollen sacs open and release the pollen grains pollinators then transfer it to the stigma where a pollen tube is directly created to the ovule which is known as pollination. Then through fertilization the sperm and egg fuse creating the embryo which is diploid. Then the ovule wall thickens around the embryo creating the seed and the ovary wall also hardens creating the fruit. From there the fruit containing may be eaten or transferred some place else where the new plant will grow and the cycle will repeat.