Question 1
Angiosperms can be dioecious or monoecious and they go through sexual production (Raghavan, 1999). Angiosperm flowers are considered to be hermaphroditic or androgynous because the same flower contains both female and male gametophytes. However, angiosperm flowers with only female gametophytes are incomplete and considered to be either a carpellate flower containing only female parts or a staminate flower containing only male structures.
The male reproductive structure of the angiosperms is known as andoecium. The anthers that are found in the stamens of the angiosperms contain pollen sacks, which have the microsporocytes. The cells go through a process known as meiosis to become microspores. The microspores form male gametophytes (pollen grains) after developing in the microsporangium (Raghavan, 1999). The male gametophytes will be utilized in the fertilization of female gametophytes. Through the mitosis process, each microspore forms pollen that contains a tube cell and a generative cell. The pollen grains serve as male gametophyte. During pollination, the pollen gets into contact with the stigma while the cytoplasm of the tube cell is elongated to form a pollen tube whose growth moves towards the ovary of the flower. Research indicates that the male gametes are formed through the division of generative cells. The migration of the male gametes occurs through pollen tube (Raghavan, 1999).
The pollen tube serves as the mature male gametophyte for the angiosperm flower. The pollen tubes contain three cellular nuclei: a tube cell nucleus and two sperm nuclei. The synergid cells play a crucial role in guiding the sperm’s migration to the flower’s ovary where it is used in the fertilization process. In most instances, the pollen grains have two coverings: the intine and the thick outer layer (exine). The exine contains sporopollenins that enable it to survive harsh conditions.
Question 2
Microporogenesis is a process through which microspores are formed inside pollen sacs or microsporangia (Formation of Angiosperm Gametes, n.d.). A diploid cell found in the flower’s microsporangium, which is known as pollen mother cell or microsporocyte, undergoes meiosis. Through microsporogenesis the haploid unicellular microspores are formed. The differentiation of diploid sporogenous cells occurs during the microsporogenesis. During the process, the meiocytes or pollen mother cells are divided through a process of meiosis that results in the formation of 4 haploid microspores. Finally, the micropsores develop into pollen grains.
Microgametogenesis comprises events that result in unicellular microspores’ progressive development into microgametophytes that contain gametes. Microgametogenesis is a phase that starts with the expansion of the microspore in a process associated with the formation of a large vacuole (Formation of Angiosperm Gametes, n.d.). The displacement of the microspore’s nucleus to an eccentric position accompanies the process of vacuolation. The nucleus’ position against the wall of the microspore goes through first pollen mitosis that leads to the formation of two cells (small generative cell and large vegetative cell) each with haploid nucleus. Subsequently, the generative cell is detached from the wall of the pollen grain and vegetative cell engulfs it to form a unique cell structure. Through the second pollen mitosis, the engulfed generative cell is divided to form two sperm cells that are enclosed within the cytoplasm of vegetative cells, either within the pollen tube or before the shedding of pollen (Formation of Angiosperm Gametes, n.d.).
In summary, microsporogenesis involves meiotic haploid microspores’ formation from the mother of diploid microspore while microgametogenesis involves the development of microgametophyte in pollen grains to tetrad-celled stage of the development of microgametophyte. In flowering plants, microgametogenesis occurs within the mother cell of a micropsore inside the plant’s anther.
Question 3
Pollen tubes are hollow ducts formed from pollen grains after the completion of the pollination process. A flowering plant’s sexual reproduction is dependent on the delivery of the sperms to the egg (Glover, 2008). In angiosperm flowers, pollen tubes sprout gametophytes of their stigmas. Subsequently, it grows down the style of the flower and enters into its ovary. The growth of the pollen tube occurs at the tip. The distinguishing characteristics are the rapid growth rates that lead to extended lengths. One of the fascinating aspects of the pollen biology is concerned with the way a plant produces adequate cell wall material that is capable of accommodating the rapid expansion of pollen tube as well as the regulation of the cell wall structure and deposition to accommodate the rapid changes in the direction of pollen tube’s growth.
When the pollen tubes begin to elongate, it can achieve amazing growth rates. During the process of pollen tube elongation, the sperm cells, vegetative nucleus, and pollen cytoplasm are transported within the pollen tube. It grows in the pistil’s intercellular spaces. The pollen tube is a conduit that allows for the travelling of sperm cells to the ovary to fertilize the egg (Glover, 2008). It positions its course through the flower’s transmitting tissue to reach the eggs contained in the ovary. Molecular cues in the tissues of the pistil play a crucial role in guiding the pollen tube when it delivers the sperm cells to ovary for fertilization. In some flowers, the female tissues emit signals that attract the pollen tube (Glover, 2008). During their elongation processes, the tubes show a tip-focused, sharp gradient of intercellular calcium that orients and drives their apical growth. In this case, the concentrations of micromolar calcium increase at the growing tip.
References
Formation of angiosperm gametes: Plant reproduction. Retrieved from http://www.mhhe.com/biosci/genbio/tlw3/eBridge/Chp18/18_2.pdf.
Glover, B. (2008). Understanding flowers and flowering: An integrated approach. Oxford: Oxford University Press.
Raghavan, V. (1999). Developmental biology of flowering plants. New Jersey: Pearson, Press.
