Reproduction in plants is usually achieved by fertilization or to be more precise, double fertilization. Double fertilization is a complex mechanism of fertilization which consists of the fusion of a single female gametophyte (mega-gametophyte, also referred to as the embryo sac) with two male gametes (sperm).
Double fertilization is a process that has developed in the flowering plants (or angiosperms). In general terms, the process consists of a couple of sperm cells, where the egg cell is fertilized by one sperm cell to produce the zygote and the other sperm cell fuses with the two polar nuclei or secondary nuclei that result in endosperm.
The pollen has to develop and evolve throughout the style to make way into the ovule after it is deposited on the stigma. The pollen or the microspores consist of two cells, namely, the generative cell and the pollen tube cell.
The cell of the pollen tube develops into a pollen tube via which the developing cell passes through. The evolution of the pollen tube demands oxygen, water, and specific chemical indications. As the pollen tube passes through the style to make through the embryo sac, the tissues of the style support its further development.
Throughout the procedure, in case the generative cell has not yet divided into two cells, it dissociates to produce two sperm cells. The synergids existing in the embryo sac discharge certain chemicals that provide the pathway for the pollen tube. Further, the pollen tube goes through the micropyle into the ovule sac.
Either of the two sperm cells fertilizes the egg cell to create a diploid zygote and the alternative sperm cell associates with the other two secondary nuclei (polar nuclei) to develop a triploid cell that evolves into the endosperm.
These two individual fertilization methods in angiosperms, when combined together, are referred to as double fertilization. After the completion of the fertilization mechanism, there is no room left for other sperms to make through. The ovule that has been fertilized produces the seed, while the tissues of the ovary evolve into the fruits that normally have the seeds within them.
After the fertilization process, the development of embryo starts. The zygote splits to produce two cells, namely, the upper cell or terminal cell and the lower cell or basal cell. The dissociation of the basal cell leads to the suspensor that ultimately creates a link with the maternal tissue. The suspensor offers a path for the food to be transferred to the developing embryo from the parent plant.
The terminal cell also dissociates resulting in an earth-shaped pro-embryo. In the case of eudicots or dicots, the shape of the growing embryo is similar to a heart because of the availability of the two rudimentary cotyledons. In the case of non-endospermic dicots, including Capsella bursa, the endosperm grows during the initial stage yet is then digested. In this event, the nutrition storages are transported into the two cotyledons.
The cotyledons and embryo get congested within the growing seed and are compelled to twist as they gain size. Eventually, the cotyledons and embryo occupy the seed fully, during which, the seed becomes available for dispersion. The development of embryo is kept on hold after a certain period of time and resumes only when the seed grows. The evolving seedling depends on the nutrition storages kept in the cotyledons unless the initial pair of leaves start photosynthesis.
Angiosperms are those plants that produce flowers and are the most distinct class of the terrestrial flora. The flowers create the reproductive organ of angiosperms with individual female and male reproductive parts. Each organ consists of gametes – egg cells and sperm respectively.
Pollination aids the pollen grains in reaching stigma through the style. The two sperm cells make through the ovule-synergid cell. This process leads into fertilization. Fertilization in angiosperms leads to two structures, which are: endosperm and zygote. As such, the name of the mechanism is double fertilization.
Double fertilization: This is a procedure containing 2 sperm cells. In this process, one cell combines with the cell of the egg, while the other cell combines with the two polar or maternal nuclei, which give rise to a triploid (3n) primary endosperm nucleus (PEN) and a diploid (2n) zygote.
The reason it is referred to as triple fusion is because the endosperm is a result of the combination of three haploid nuclei. Ultimately, the primary endosperm nucleus grows into the primary endosperm cell (PEC) and later into the endosperm. The zygote turns into an embryo post several dissociations of the cell.
The moment the process of fertilization is accomplished, the development of embryo begins and no further sperm cells are allowed to make through the ovary. The fertilized ovule grows into a seed and ovary tissues evolve as a fleshy fruit that encompasses the seed.
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After the process of fertilization, the zygote splits into the upper end cell and lower basal cell. The latter evolves into suspensor that aids in the transportation of food to the developing embryo. The upper terminal cell grows into pro-embryo.
The different phases involved in the development of an embryo are as follows:
During the initial phase of development, the terminal cell dissociates producing a globe-shaped pro-embryo. In addition, the basal cell also splits into a suspensor.
The developing embryo acquires a heart-like shape because of the existence of cotyledons.
The developing embryo becomes congested and starts to adjust in different shapes.
The embryo occupies all of the space of the seed.
The significance of double fertilization in angiosperms can be elaborated in the following points:
As there are only two fusions; only two products are formed.
The secondary product of fertilization – triploid primary endosperm nucleus (PEN) – evolves into a nutritive tissue known as the endosperm that is responsible in providing nutrition to the developing embryo.
The diploid condition in the life cycle is retained in the product of the fusion process. The diploid zygote evolves into an embryo that, as a consequence, grows into a new plant.
The process of double fertilization consists of the usage of both the male gametes formed by a pollen grain.
As such, the chances of poly-embryology increases and this also leads to increased possibilities of the survival of the new plant.
Double fertilization offers stimulus to the plants that is responsible for ovary developing into fruits and ovules developing into seeds.
As double fertilization is a distinctive characteristic of angiosperms, the seeds of angiosperms are highly accessible.
Double fertilization gives rise to the re-fusion of characteristics leading to the variation among the off-springs.