What are the female reproductive parts of the angiosperm flower?
The female reproductive parts of a flower include the stigma, the ovary, and the style. Their collective name is normally referred to as carpel. a flower may be composed of one or several pistils. For reproduction to occur, all these parts play an important role. The stigma is the female part of a flower that receives pollen grains. It normally has a sticky surface to enable pollen grain to stick and germinate on it. There are several shapes of the stigma. The different shapes enable them to be well adapted to their role in reproduction. Some of the shapes include discoid, cepillo and plumoso. The sticky nature of the stigma may be due to chemicals secreted by stigma cells, or the surface may be feathery.
The style is the female part that connects the stigma to the ovary (Bird, 2014, p. 134). It is normally a hollow tube. Angiosperms may have three different types of style. The first one may be formed where the carpels unite congenitally, which is the most seen type in angiosperms. The second one may be formed where there is post genitally unison of plicate carpels. This is mostly observed in monocots families. The third one occurs as lines between carpels.
Source: https://www.researchgate.net/figure/26862783_fig2_Figure-3-Three-different-types-of-style-of-angiosperm-gynoecia-composed-of-two-carpels
The ovary is the enlarged part of the female parts of a flower. Contained in the ovary are the ovules, which later develop into seeds after fertilization has taken place. The ovules attach themselves to the ovary walls known as the placenta. The placenta in the ovary may occur at several different positions depending on carpels arrangement of a flower. The ovary later develops into a fruit.
Explain the difference between megasporogenesis and megagametogenesis.
In megasporogenesis, the ovule develops in the placenta as a protuberance, whereby archesporium results due to differentiation of a single hypodermal cell. The formed archesporium later develops into megaspore mother cell (Batygina, 2002, p. 112), by cutting off a number of parietal cells. The formed megaspore mother cell from this development undergoes the meiosis process resulting to haploid megaspore cells that are in a linear orientation. At the same time, two integuments form from ovule base. The lower linear tetrad megaspores become enlarged to form a viable functional megaspore. The other megaspores disintegrate. This occurs in most plants.
Megagametogenesis begins with single diploid megasporocyte undergoing meiosis process. This results in four haploid cells. As three of them die, the fourth one develops into a megaspore. This megaspore enlarges allowing its nucleus to undergo mitosis process in three cycles until eight nuclei are formed. After the eight nuclei formation is complete, the normally get rearranged in such a way that they are grouped into tetrads. Each of these tetrads delivers a nucleus towards the cell center, forming polar nuclei. The remaining three cells from each tetrad result to the formation of a center egg cell consisting of two synergids. Antipodal then results from the other set of nuclei upon formation of a cell wall. The center cells later become the central cell. The above structure made up of eight nuclei forms the embryo sac. Megagametogenesis, therefore, refers to megaspore formation to an embryo sac that is known as gametophyte. This does not occur frequently in terrestrial angiosperms. Additionally, female gametophyte formation is totally endosporous. Depending on the number of megaspores that result to female gametophyte, embryo sac may be classified as being bisporic, monosporic or tetrasporic.
What is the role of the endosperm and cotyledons in embryo development and how are they formed?
During embryo development, the endosperm acts as a storage reservoir during embryo development (Bhojwani & Razdan, 1996, p. 230). During embryo development, energy is usually needed for the process. Thereby, energy comes from the stored food source. For instance, endosperms store fats and starch which are essential during embryo development. Another function of the endosperm is that it is involved in mediation for nutrients movement from mother plant towards the developing embryo. Importantly, the endosperm also acts as gene imprinting center. This enables the removal of seeds prematurely, which are generated from parents that have a genetic mismatch. Additionally, during embryo development, endosperm secretes cytokin hormone that checks on organ formation of the embryo and cellular differentiation. Cotyledon provides the developing embryo with nourishment. Also, it may grow above the ground forming a new set of leaves as the embryo develops. This enables photosynthesis to take place, that will provide more food to the developing embryo. Again, the cotyledon may grow below ground where it remains underground where it functions as a storage center for food required by the developing embryo.
Endosperm normally results when two nuclei reach embryo sac interior from the pollen grain. One of the nuclei fertilizes the egg resulting to a zygote while the other nuclei fuse with polar nuclei at embryo sac center, resulting to the primary endosperm. The cotyledons are normally formed after fertilization alongside with the endosperm where they later develop into the first embryonic leaves. As the formation of zygote leads to the ovary developing into a fruit, the ovules form the seeds, which contain the endosperm and the cotyledon.
References
Batygina, T. B. (2002). Embryology of flowering plants: Terminology and concepts. (Embryology of flowering plants.) Enfield: N.H.
Bhojwani, S. S., & Razdan, M. K. (1996). Plant tissue culture: Theory and practice. Amsterdam: Elsevier.
Bird, C. (2014). The fundamentals of horticulture: Theory and practice.
Figure 3: Three different types of style of angiosperm gynoecia - Scientific Figure on ResearchGate. (n.d.). Retrieved from https://www.researchgate.net/figure/26862783_fig2_Figure-3-Three-different-types-of-style-of-angiosperm-gynoecia-composed-of-two-carpels
