​Plants
Alternation of generations
The alternation of generations life-cycle is the basic life-cycle that plants go through. Plants spend part of their life in the sporophyte stage. Sporophytes are multicellular and diploid, meaning that they have two copies of their chromosomes. They undergo meiosis to create spores that are haploid, meaning that they have one copy of their chromosomes. The spores undergo mitosis and grow into gametophytes that are multicellular and haploid. Gametophytes create gametes by mitosis. The gametes are haploid. The male gamete fertilizes the female gamete to create the embryo. They embryo undergoes mitosis and grows into a zygote which undergoes mitosis and grows into a sporophyte. The cycle then continues. Each of the four plant groups (bryophytes, pteridophytes, gymnosperms, and angiosperms) have specific life-cycles, but they all follow along the lines of the alternation of generations.
characteristics of the four plant divisions
Bryophytes are the first plants that evolved to live on land. Bryophytes are plants like mosses, liverworts, and hornworts. They spend most of their life-cycle in the haploid gametophyte stage and do not have vascular tissue (xylem and phloem) for most of their life. Instead, bryophytes absorb water through the surface of the plant and therefore need to live in a wet environment. The lack of vascular tissue means that the plant does not have true roots or leaves. Instead, it has "fake" roots called rhizoids that are used only for anchorage. At the top of the mature gametophyte, the plant has their reproductive organs: the antheridium (male), which produces the flagellated sperm, or the archegonium (female), which produces the egg. Bryophytes are dioecious and heterosporous, meaning that the male and female reproductive organs are on different plants and their spores are different sizes. The sporophyte is completely dependent on the gametophyte as it grows out of the archegonium.
Pteridophytes are more advanced that bryophytes. Pteridophytes are plants like ferns, selaginella, and horsetail. Like the other divisions (except bryophytes), they spend most of their life in the diploid sporophyte stage. They have primitive vascular tissue, but “fake” roots called hyphae and “fake” leaves called fronds. Thus, they need to live in wet environments. Their small and fragile gametophyte (called a prothallus) is monoecious, meaning that the archegonia and antheridia develop on the same plant. They are homosporous (spores are the same size) and require water for reproduction.
Gymnosperms are conifers. They have complete vascular tissue and true roots, so they can live in colder, dryer climates than pteridophytes or bryophytes. They spend most of their life in the diploid sporophyte stage and the gametophyte is greatly reduced. The sporophyte is heterosporous and monoecious because it creates male and female gametes and the male and female reproductive structures grow on the same plant. Gymnosperms are the first of the plant divisions to use seeds.
Angiosperms are known as the ‘flowering plants’. Like the gymnosperms, they have true roots, true leaves, and complete vascular tissue. They spend most of their life in the mature sporophyte stage. Most angiosperms are vector pollinated, meaning that there is a go-between, like an insect or bee, that carries pollen to the plant. Angiosperms have four major organ systems: leaves, stems, roots, and flowers (fruit). The leaves use stomata to obtain CO2 for photosynthesis, the stems angles the leaves towards the sun, the roots absorb water and minerals, and the flower attracts pollinators. The fruit protects and provides nutrients for the seed. They are heterosporous and monoecious. The reproductive structures are the stamen (male) and the carpel (female).
Pteridophytes are more advanced that bryophytes. Pteridophytes are plants like ferns, selaginella, and horsetail. Like the other divisions (except bryophytes), they spend most of their life in the diploid sporophyte stage. They have primitive vascular tissue, but “fake” roots called hyphae and “fake” leaves called fronds. Thus, they need to live in wet environments. Their small and fragile gametophyte (called a prothallus) is monoecious, meaning that the archegonia and antheridia develop on the same plant. They are homosporous (spores are the same size) and require water for reproduction.
Gymnosperms are conifers. They have complete vascular tissue and true roots, so they can live in colder, dryer climates than pteridophytes or bryophytes. They spend most of their life in the diploid sporophyte stage and the gametophyte is greatly reduced. The sporophyte is heterosporous and monoecious because it creates male and female gametes and the male and female reproductive structures grow on the same plant. Gymnosperms are the first of the plant divisions to use seeds.
Angiosperms are known as the ‘flowering plants’. Like the gymnosperms, they have true roots, true leaves, and complete vascular tissue. They spend most of their life in the mature sporophyte stage. Most angiosperms are vector pollinated, meaning that there is a go-between, like an insect or bee, that carries pollen to the plant. Angiosperms have four major organ systems: leaves, stems, roots, and flowers (fruit). The leaves use stomata to obtain CO2 for photosynthesis, the stems angles the leaves towards the sun, the roots absorb water and minerals, and the flower attracts pollinators. The fruit protects and provides nutrients for the seed. They are heterosporous and monoecious. The reproductive structures are the stamen (male) and the carpel (female).
compare and contrast life cycles of the four plant divisions
Bryophytes
1. Gametophyte dominant (n) 2. Dioecious 3. Archegonium 4. Antheridium 5. Flagellated sperm (n)
6. Zygote
7. Sporophyte grows out of female gametophyte 8. Sporangium on the very top of the sporophyte
9.Operculum opens
10. Spores grow by mitosis into gametophytes 11. Heterosporous 12. Gametophytes have “fake” roots called rhizoids 13. Sporophytes do not have roots but have very basic vascular tissue |
Pteridophytes
1. Sporophyte dominant (n) 2. Monoecious 3. Archegonium 4. Antheridium 5. Flagellated sperm (n)
6. Zygote
7. Sporophytes not dependent on gametophyte (prothallus) 8. Cluster of sporangia on underside of frond called sorus
9. Dispersal of spores by air 10. Spores grow by mitosis into gametophytes 11. Homosporous 12. Gametophytes have “fake” roots called rhizoids 13. Sporophytes have “fake” roots called hyphae and primitive vascular tissue |
Gymnosperms
1. Sporophyte dominant (n) 2. Monoecious 3. Archegonia 4. Male cone creates pollen 5. Mother megaspore (2n) grows in megasporangium (n) and undergoes meiosis to make four haploid megaspores. The one that survives creates the megagametophyte (n) which grows two archegonia with the eggs (n) inside 6. After fertilization, the zygote is formed
7. Sporophytes not dependent on gametophyte 8. Pollen (n) (produced instead of spores) lands on female cone and grows a pollen tube down to the archegonia to fertilize the egg (n) 9. Dispersal of pollen by air 10. Embryo grows by mitosis into sporophyte 11. Heterosporous 12. Gametophytes do not have roots and are very tiny 13. Sporophytes have true roots and vascular tissue |
Angiosperms
1. Sporophyte dominant (n) 2. Monoecious 3. Carpel
4. Stamen
5. Pollen (n) grows a pollen tube from the stigma down the style to the ovary to fertilize the egg (n) 6. Zygote
7. Sporophytes not dependent on gametophyte 8. Pollen (n) lands on stigma (which is sticky) 9. Vector pollinated or wind pollinated 10. Embryo grows by mitosis into sporophyte. The rotting fruit provides nutrients for the seed 11. Heterosporous 12. Gametophytes do not have roots and are very tiny 13. Sporophytes have true roots, vascular tissue, stems, leaves, and flowers (fruit) |
levels of organization
Algae is widely considered to be the precursor to plants; green algae in particular is considered to be the ancestors of land plants we know today. Green algae comes in three different cell arrangements: unicellular, colonial, and multicellular. Unicellular green algae is the most basic as it is just one cell living independently from other cells by itself. Colonial green algae is a little more complex as it is unicellular algae living with each other. The difference between colonial and multicellular green algae is that colonial cells do not rely on each other for anything while the multicellular cells live together and dependently on one another. Bryophytes were the first plants to evolve from algae. They use spores that are dispersed through the air for reproduction, have flagellated sperm, and have a thin waxy cuticle to protect against desiccation. Pteridophytes evolved from bryophytes. These plants evolved vascular tissue. Unlike the bryophytes, they pteridophytes can move water through the plant through “transport tubes” called xylem (move water from roots to shoots) and phloem (move products of photosynthesis from shoots to roots). They only have vascular tissue in the sporophyte generation, so they evolved to spend more of their life cycle in that generation. The gametophyte is reduced. The archegonia and antheridia develop at different times to avoid self-fertilization. From the pteridophytes came the gymnosperms. Gymnosperms have vascular tissue throughout the entire plant, including the roots. These are the first plants to have true roots, use pollen instead of spores, and produce seeds to protect the embryo. Angiosperms spend the least amount of time in the gametophyte generation and also have four main organ systems. There are the leaves, stem, roots, and flowers (and fruit). The leaves perform photosynthesis. On the underside of the leaves, there are small bean shaped openings called stomata. Stomata are surrounded by guard cells. Guard cells have a flexible outer edge and a stiff inner edge. When they fill with water, the flexible outer edge swells and pulls the inner part open, creating the stomata. CO2 enters the cells when the stomata is open and is used up, along with the water, in photosynthesis. When the water is gone from the guard cells, they become less bean shaped and the stomata closes. The stem orients the leaves towards the sun, provides structural rigidity to the plant, and conducts materials up and down the plant (xylem and phloem). Roots absorb water and minerals from the soil and also anchor the plant. The flower attracts pollinators to the specialized reproductive organs. The specialized reproductive organs are the stamen (male) and the carpel (female). They can both be found on the same mature angiosperm sporophyte. Pollinators (such as bees) can pick up pollen and carry it to another flower where it can fertilize that flower’s egg. This allows for more genetic diversity. Angiosperms also evolved to have a fruit that wraps around the seed. The fruit allows the seed to be ingested by animals and transported to other locations and it also provides nutrients for the embryo when it falls off the plant and begins to rot. The evolution of vascular tissue has allowed plants to grow taller and live in harsher,drier environments. The evolution of pollen has allowed for plants to spend less time in the fragile, simple gametophyte generation. The bryophytes, which are quite simple, have evolved into the much more complex angiosperms over time.