The pollen grain in angiosperms, as in the gymnosperms, represents most of what is left of the male gametophyte. In both, plant groups, any tube that forms to aid in fertilization is also considered part of the gametophyte.
Consider the angiosperm pollen that develops in the anther. The mother cells produces haploid microspores by meiosis which then develop into haploid pollen grains. Once the pollen has fertilized the egg cell contained within the ovule, the ovule develops into a seed, and the ovary of the enclosing carpel begins to enlarge and ripen, forming the fruit.
This is the botanical definition of a fruit, and is not the same as the popular usage of "fruit". Angiosperms have to undergo a process called pollination before they can reproduce. Angiosperms have male sex organs called stamens. On the end of the stamen is the anther. This is where pollen is made. The pollen has to be taken to the pistil or the female part of the flower. The pollen is left on the stigma at the end of the pistil. The stigma carries the pollen down a tube called the style to the ovary.
These angiosperms start with one seed-leaf. Therefore, they generate microspores, which will produce pollen grains as the male gametophytes, and megaspores, which will form an ovule that contains female gametophytes. Each pollen grain contains two cells: one generative cell that will divide into two sperm and a second cell that will become the pollen tube cell. Life cycle of angiosperms : The life cycle of an angiosperm is shown. Anthers and carpels are structures that shelter the actual gametophytes: the pollen grain and embryo sac.
Double fertilization is a process unique to angiosperms. The ovule, sheltered within the ovary of the carpel, contains the megasporangium protected by two layers of integuments and the ovary wall. Within each megasporangium, a megasporocyte undergoes meiosis, generating four megaspores: three small and one large.
Only the large megaspore survives; it produces the female gametophyte referred to as the embryo sac. The megaspore divides three times to form an eight-cell stage. Four of these cells migrate to each pole of the embryo sac; two come to the equator and will eventually fuse to form a 2n polar nucleus. The three cells away from the egg form antipodals while the two cells closest to the egg become the synergids.
When a pollen grain reaches the stigma, a pollen tube extends from the grain, grows down the style, and enters through the micropyle, an opening in the integuments of the ovule. The two sperm cells are deposited in the embryo sac. A double fertilization event then occurs. One sperm and the egg combine, forming a diploid zygote, the future embryo.
The other sperm fuses with the 2n polar nuclei, forming a triploid cell that will develop into the endosperm, which is tissue that serves as a food reserve. The zygote develops into an embryo with a radicle, or small root, and one monocot or two dicot leaf-like organs called cotyledons.
This difference in the number of embryonic leaves is the basis for the two major groups of angiosperms: the monocots and the eudicots. Seed food reserves are stored outside the embryo in the form of complex carbohydrates, lipids, or proteins. The cotyledons serve as conduits to transmit the broken-down food reserves from their storage site inside the seed to the developing embryo. The seed consists of a toughened layer of integuments forming the coat, the endosperm with food reserves, and the well-protected embryo at the center.
The fruit of the Aesculus or Horse Chestnut tree : These seeds are enclosed a protective outer covering called the seed coat, usually with some stored food. After fertilization and some growth in the angiosperm, the ripened ovule is produced. The formation of the seed completes the process of reproduction in seed plants started with the development of flowers and pollination , with the embryo developed from the zygote and the seed coat from the integuments of the ovule.
Some species of angiosperms are hermaphroditic stamens and pistils are contained on a single flower , some species are monoecious stamens and pistils occur on separate flowers, but the same plant , and some are dioecious staminate and pistillate flowers occur on separate plants. Both anatomical and environmental barriers promote cross-pollination mediated by a physical agent wind or water or an animal, such as an insect or bird.
Cross-pollination increases genetic diversity in a species. Angiosperm diversity is divided into two main groups, monocot and dicots, based primarily on the number of cotyledons they possess. Angiosperms are classified in a single phylum: the Anthophyta.
Modern angiosperms appear to be a monophyletic group, which means that they originated from a single ancestor. Flowering plants are divided into two major groups according to the structure of the cotyledons and pollen grains, among others.
Monocots include grasses and lilies while eudicots or dicots form a polyphyletic group. However, many species exhibit characteristics that belong to either group; as such, the classification of a plant as a monocot or a eudicot is not always clearly evident.
Basal angiosperms are a group of plants that are believed to have branched off before the separation into monocots and eudicots because they exhibit traits from both groups. They are categorized separately in many classification schemes. The Magnoliidae magnolia trees, laurels, and water lilies and the Piperaceae peppers belong to the basal angiosperm group. Examples of basal angiosperms : The a common spicebush belongs to the Laurales, the same family as cinnamon and bay laurel.
The fruit of b the Piper nigrum plant is black pepper, the main product that was traded along spice routes. In gymnosperms pollen is produced in microsporangiate cones male cones or pollen cones , while in angiosperms pollen is produced in the anthers part of the stamen within the flower. Each pollen grain typically consists of one to a few cells. The wall of the pollen grain consists of two layers, the exine outer wall and intine inner wall.
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