The invention of the flower was a major improvement in land based reproduction. While a few angiosperms still rely on wind-based pollination for their flowers (mainly trees), most angiosperm flowers are vector pollinated. This has resulted in a parallel evolution between angiosperms and animals (mostly insects). In the Plant Kingdom parallel evolution occurs in the plant species of angiosperms between the angiosperms and the animals/insects that pollinate them. This is also an example of mutualism, as both species are benefiting from the relationship they hold.
Within an Angisperm's body there are four main principal organ systems: leaves, stems, roots and flowers, all in which play a pivotal role in the existence of angiosperms. Within these four, the flower organ's main function is to attract a pollinator. In order to do this a flower may present itself as being very beautiful looking. It might have a certain colour that attracts a certain insect or it might give off a certain smell. The flower has a modified shoot with 4 rings of modified leaves. The first level is the sepal, then comes the petals, stamens (male) and the carpel (female). The flower also has a stigma which is a structure that receives the pollen and is also the structure in which the pollen grain germinates. The style is a long, slender stalk that connects the stigma to the ovary. The stigma is at the top and the ovary is towards the bottom. The ovary is the female reproductive gland. All three of these structures (stigma, style ovary) are apart of the carpal of the leaf. The male reproductive part of a flower is called the stamen. It is composed of a long tube, called a filament and has a pollen-producing structure on the end. This oval-shaped structure is called the anther. It is very important in the process of reproduction of flowering plants, as it produces the male gametophyte, known as pollen.
The species of Angiosperms are known both as the flowering plants and the fruiting plants. The reason why this species has these features are because they have evolved this trait to better suit their reproductive needs. The flowering plants smell nice and contain pollen which can then be transferred into nector or food for the pollenators. This is what attracts the birds, bees and bats to come and extract the flower of its pollen. As for the fruiting plants, animals eat the fruit for nutrients and energy. When it's time for the seed and non-digestible food to be disposed of, the seed will now be in a new location and be able to restart the cycle. This is how fruiting plants like cherry trees, peach trees, apple trees and flowering plants like roses, calla lillies and dafadills came to be. 1) Cherry Tree 2) Rose Peach Tree
Just like Gymnosperms, the more advanced Angiosperms use pollen as their male gametophyte which contains the sperm and have the egg cells in the archegonium of the female gametophyte! The pollen grains that contain the tube cell and the generative cell are carried to the stigma of the flower by wind or animal (pollination). The tube cell forms a pollen tube down the stigma and style to the ovary. The pollen tube will enter the ovary through an opening (micropyle). The sperm will then swim (some what) down the tube and fertilize with the egg cell in the female archegonium! Through there the endosperm which surrounds the female gametophyte provides food for the embryo . The ovules develop into seeds, the ovary develops into fruit. Animals are then attracted to the fruit and go to eat it. Once the fruit has made its way through the animals body and has been disposed, the seed will be in a new location to start over the cycle, thus creating genetic diversity.
Bryophytes, Pteridophytes and Gymnosperms. Three out of the four species of plants in Kingdom Plantae all have similarities and differences amongst one another.
Some similarities between the Bryophytes/ Pteridophytes and Gymnosperms are that all of them have features like an archegonium and an antheridium. All three use both processes of mitosis and meiosis to create reproductive units. The reproductive units that are created are all wind-borne (can travel through the wind) between the three. As time went on and evolution took place more and more differences rose between these species. The Gymnosperms developed naked seeds to protect their embryo and developed pollen which contain the male sperm and gametophyte for easier reproduction, as they have eliminated the need for water. Having pollen also creates more genetic diversity amongst the species so they can survive in all biomes, as they don't have to rely on asexual reproduction. Unlike the Bryophytes, the gymnosperms continue to be vascular by having water and nutrient transport systems like phloems and xylems. Their life cycle is dominated by the sporophyte stage like the Pteridophytes, also resulting in the reduction of the gametophyte which protects the delicate egg and embryo in protective sporophyte stage. True roots are seen for the first time which allows the gymnosperms to live in dryer conditions where water is not as abundant. The leaves have now modified into needles decreasing water loss and the resins inside the needles act as a natural antifreeze, making them the dominant tree of the northern temperate zones. As time went on the gymnosperms evolved into a stronger and more land efficient plant. Gymnosperms have been around since the Meszonic era. The first evolution of seeded vascular plants came in the Paleozoic era, as there were lots of changes in the Mesozoic that made this their era. The dominant species of gymnosperms during this time were the Ginkgo and the Cycads. Now a days, they are our familiar evergreen trees and shrubs! Ginkgo: Cycads: Evergreens:
The lifecycle of a Gymnosperm is a more complicated cycle than those of the Pteridophytes and Bryophytes species! The reasoning behind that is because of the advancements and evolved features this species of plants have made. During reproduction, the megasporangium in ovule undergoes meiosis which creates four haploid megastores. Of those four only one will ensure survival. From there that megaspore will develop into a female gametophyte. Just like in Pteridophytes and Bryophytes, the female gametophyte contains the archegonium which will produce the egg cell. Meanwhile, the microsporangia in the pollen cone will undergo meiosis to create haploid microspores. These microspores will develop into pollen grains or male gametophytes. These pollen gains contain the tube cell or pollen tube. The pollen is then carried to the seed cone by wind or animal. The tube cell forms the pollen tube through the micropyle or opening to reach the egg. Two sperm cells are created; one travels to fertilize the egg while the other sperm cell will end up dying off. After, fertilization the diploid zygote develops into an embryo, the remaining gametophyte will protect it and provide food. Once the sporophyte embryo is mature enough, the mature seed cone will open up into the wind when ideal conditions are right. The seed will then go through a process of seeding and turn into a large, mature sporophyte. The cycle then repeats.
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