Alternation of Generations Life Cycle
Alternation of Generations is when a plant switches between the haploid and diploid stages in their life cycle, which is a process that all plants use. When the plant is haploid it means that it has one set of unpaired chromosomes (for example eggs, sperm, and gametophytes are all haploid). However when the plant is diploid, it means that the chromosomal DNA from both the egg and sperm have combined and there is a full set of chromosomes(the embryo, sporophyte and sporangium are all diploid). The sporophyte is the multicellular diploid stage of a plant and the gametophyte is the multicellular haploid stage. The diploid sporophyte stage is dominant in pteridophytes, gymnosperm and angiosperm organisms. Bryophytes are the only plants with a dominant gametophyte stage. The sporophyte produces haploid spores(meiosis) and the gametophyte stage produces gametes. The gametophyte ususally either has a female archegonium or the male antheridium on the end. When fertilization occurs the haploid egg and sperms come together, combining their chromosomal DNA and create a diploid zygote(mitosis). To recap, Alternation of Generations is when a plant switches between the haploid (gametophyte) and diploid (sporophyte) stages of their life cycle.
Classification of Plants
a. Bryophyta- Their life cycles depend on water because they have swimming/flagellated sperm, that wouldn't be able to fertilize an egg without water. Haploid wind borne spores are used for reproduction. They don't have vascular tissue, true roots (instead have rhizoids), or a water transport system so they can only grow a couple of centimeters high. The haploid gametophyte stage is dominant. They either have a male gametophyte with an anthuridium or a female archegonium which are involved in sexual reproduction. Utilize Alternation of Generations. Some common examples of Bryophytes include mosses, liverworts and hornworts.
b. Pteridophyta- They are seedless vascular plants with conducting tissues (don't produce seeds)! Vascular tissue has the Xylem(water from roots to shoots) and the Phloem(photosynthesis products from roots to shoots). These plants can be larger because of the movement of water and nutrients. They have swimming sperm and spores for reproduction as well as fragile gametophytes. Their dominant stage is as a sporophyte and they use Alternation of Generations. Examples of Pteridophyta include; ferns, horsetail, selaginella and psilotum. c. Gymnosperm- Are seeded vascular plants. These plants evolved to survive without completely depending on water. However they still obviously utilize Alternation of Generations. Gametophyte stage has been further reduced and now mainly relies on the dominant sporophyte stage. They have also evolved sexual pollen, distributing gametes through the air or on animals and have seeds which protect the embryo(reduces risk of drought and UV Radiation). Male pollen cones and female woody cones are present. Gymnosperms are vascular, known as "naked seed"(no fruit), heterospory, and the pollen contains the male gametophyte. Reproduction doesn't need water and they have developed true roots! Their leaves have developed into needles, which you'll see on evergreen trees and shrubs. Inside the needles is resin that acts as a natural antifreeze. Common examples of Gymnosperms are ginko trees and cynads as well as other evergreen trees and shrubs. d. Angiosperm- Are "covered seed" plants that produce fruit and protects the embryo. They have a specialized reproductiive structure called a.... flower! They are vector pollinated (ex. by bees). Their four main organ systems are the roots who get nutrients from the soil/anchor, stem that transports these nutrients/gives support, leaves who carry out photosynthesis and the flower who's in charge of reproduction/attracts pollination. They are divided into two groups; the monocots and dicots. Dicots have 2 seed leaves with a netowrk of veins and include the woody plants, trees, shrubs and beans. Monocots only have 1 seed leaf with parallel veins, these include grasses, palms and lillies. |
Life Cycles Comparison
Bryophyte Life Cycle
*Alternation of Generations Used*
When a haploid spore lands on some suitable soil, it forms protonemia (mitosis). The protonemia develop rhizoids and grows into a male or female gametophyte (mitosis). This gametophyte stage is the dominant form of the life cycle. Gametophytes are haploid. A female gametophyte has an archegonium containing an egg whereas males have antheridiums containing sperm. Fertilization occurs when a haploid sperm swims down the archegonium to the haploid egg, and together they form a diploid zygote (mitosis). It's critical that there is water involved in reproduction because the sperm still needs to swim to the egg using it's flagella. The zygote then forms a diploid sporophyte with a diploid capsule/sporangium containing haploid spores. When the capsule of spores bursts open meiosis occurs and the cycle starts over again.
*Alternation of Generations Used*
When a haploid spore lands on some suitable soil, it forms protonemia (mitosis). The protonemia develop rhizoids and grows into a male or female gametophyte (mitosis). This gametophyte stage is the dominant form of the life cycle. Gametophytes are haploid. A female gametophyte has an archegonium containing an egg whereas males have antheridiums containing sperm. Fertilization occurs when a haploid sperm swims down the archegonium to the haploid egg, and together they form a diploid zygote (mitosis). It's critical that there is water involved in reproduction because the sperm still needs to swim to the egg using it's flagella. The zygote then forms a diploid sporophyte with a diploid capsule/sporangium containing haploid spores. When the capsule of spores bursts open meiosis occurs and the cycle starts over again.
Pteridophyte Life Cycle
*Alternation of Generations Used*
It all starts when spores germinate into a haploid gametophyte that eventually grows rhizoids. The mature gametophyte is called a prothallus, is flat and heart shaped. Archegonia and Antheridia form on the prothallus, however water is still needed for reproduction. When the egg and sperm meet they form a diploid zygote that grows into a diploid sporophyte. The mature sporophyte has fronds (not leaves). On the underside of the fronds are sorti which are composed of clustered sporangium. Next, by meiosis the diploid sporangium bursts, releasing haploid sperms... and the cycle restarts.
*Alternation of Generations Used*
It all starts when spores germinate into a haploid gametophyte that eventually grows rhizoids. The mature gametophyte is called a prothallus, is flat and heart shaped. Archegonia and Antheridia form on the prothallus, however water is still needed for reproduction. When the egg and sperm meet they form a diploid zygote that grows into a diploid sporophyte. The mature sporophyte has fronds (not leaves). On the underside of the fronds are sorti which are composed of clustered sporangium. Next, by meiosis the diploid sporangium bursts, releasing haploid sperms... and the cycle restarts.
Gymnosperm Life Cycle
*Alternation of Generations Used*
(In the diploid stage). The sporophyte, in this case a pine tree develops 2 types of cones/it's gametophytes. The male cone is called a pollen cone and contains a microsporangium whereas the female cone called an ovulate cone and contains an ovule. (enter the haploid stage). The microsporangium creates a microsporocyte and undergoes meiosis making microspores. The ovule has undergoes meiosis as well and creates megaspores. A pollen grain gets into the ovaries and creates a pollen tube to get to the archegonium. Sperm fertilize through the pollen tube (entering diploid stage) and create a zygote. The zygote develops into a protected embryo known as a seed which will grow into a sporophyte.
*Alternation of Generations Used*
(In the diploid stage). The sporophyte, in this case a pine tree develops 2 types of cones/it's gametophytes. The male cone is called a pollen cone and contains a microsporangium whereas the female cone called an ovulate cone and contains an ovule. (enter the haploid stage). The microsporangium creates a microsporocyte and undergoes meiosis making microspores. The ovule has undergoes meiosis as well and creates megaspores. A pollen grain gets into the ovaries and creates a pollen tube to get to the archegonium. Sperm fertilize through the pollen tube (entering diploid stage) and create a zygote. The zygote develops into a protected embryo known as a seed which will grow into a sporophyte.
Angiosperm Life Cycle
*Alternation of Generations Used*
The plant's flower has the male gametophyte in the anther and the female gametophyte in it's ovary. The anther undergoes meiosis when it releases a diploid microspore/pollen grain. The pollen grain lands on the stigma and forms a pollen tube so it can reach the egg. In the ovary a haploid megaspore is formed by meiosis and get's encased in an embryo sac and develops into an egg. During fertilization the haploid egg nucleus and haploid sperm nuclei form a diploid zygote and triploid endosperm. The seed then germinates into a sporophyte plant with a mature flower on the end. The cycle restarts.
*Alternation of Generations Used*
The plant's flower has the male gametophyte in the anther and the female gametophyte in it's ovary. The anther undergoes meiosis when it releases a diploid microspore/pollen grain. The pollen grain lands on the stigma and forms a pollen tube so it can reach the egg. In the ovary a haploid megaspore is formed by meiosis and get's encased in an embryo sac and develops into an egg. During fertilization the haploid egg nucleus and haploid sperm nuclei form a diploid zygote and triploid endosperm. The seed then germinates into a sporophyte plant with a mature flower on the end. The cycle restarts.
Organization of an Angiosperm
The female area of the flower is the carpel which contains the stigma, style and an ovary. It's the innermost part of the flower and makes female gametophytes.
Stigma: Area at the the top of the style that pollen sticks to.
Style: Skinny, stalk-like structure of the carpel.
Ovary: The structure that creates the female gametophyte and contains one or more ovules.
The stamen is known as the male part of the flower and includes the anther and filament.
Anther: Produces haploid male gametophytes.
Filament: Skinny structure that supports the anther.
The other parts of the flower are the petals and sepals.
Petals: Are brightly coloured to attract pollinators such as bees and are located above the sepals.
Sepal: Looks like a leaf and encases the flower bud before it opens as well as protects the flower when it's still developing.
Stigma: Area at the the top of the style that pollen sticks to.
Style: Skinny, stalk-like structure of the carpel.
Ovary: The structure that creates the female gametophyte and contains one or more ovules.
The stamen is known as the male part of the flower and includes the anther and filament.
Anther: Produces haploid male gametophytes.
Filament: Skinny structure that supports the anther.
The other parts of the flower are the petals and sepals.
Petals: Are brightly coloured to attract pollinators such as bees and are located above the sepals.
Sepal: Looks like a leaf and encases the flower bud before it opens as well as protects the flower when it's still developing.