Fertilisation represents the process of fusion of female gamete (i.e. the ovum or egg) with the male gamete (i.e. the pollen produced in the pollen tube by pollen grains). This process was first discovered by Strassburger in 1884 in flowering plants. In angiospermic plants flowers are the main reproductive structures. In plants the male gametes are carried to females via various pollinators. fertilisation is a method of sexual reproduction that occurs after pollination and germination. Haploid parental gametes give rise to diploid zygotes. This process is carried out in the zygote, to develop further into seeds.
Microsporangia
Stamen consists of a filament and an anther. The base of the filament of is glued to the thalamus of the flower. An angiospermic anther looks bilobed where each lobe is made of two theca. A longitudinal groove moves lengthwise separating the theca. The anther is characterised by having a tetragonal structure which comprises of four microsporangia that are situated at the corners with two in each lobe. Later these microsporangia develops and forms the pollen sacs. They expand longitudinally along the entire length of an anther and are filled with pollen grains.
While anthers develop, the cells of the sporogenous tissue go through meiotic divisions to produce microspore tetrads. The formation of microspores from a pollen mother cell (PMC) through meiosis is referred to as microsporogenesis. While the anthers mature and dehydrate, the microspores separates from each other and mature into pollen grains.
Pollen grains
Pollen grain represents the first cell of a male gametophyte. These are mostly spherical with a diameter of about 25-50 micrometres. In the beginning the protoplast of pollen grain is uninucleate but during discharge, it becomes 2-3celled. Sporoderm represents the covering of the pollen grains. It comprises two layers, outer exine and inner intine. The outer layer is further made up of sporopollenin. The intine possesses a thin and elastic covering. It comes out as pollen tube from the germ pores during germination.
Ovule or Megasporangia
Ovule represents integumented megasporangium which is situated in spermatophytes which further matures into seed after fertilisation. Each ovule remains attached to placenta via means of stalk known as funicle. Hilum represents the region where the body of the ovule fuses with funicle. Therefore, hilum shows the junction among ovule and funicle. Every ovule possesses 1 or 2 protective envelopes known as integuments. Contrary to the micropylar end, is present chalaza, which showcases the basal part of the ovule. Each ovule is also comprised by a large oval cell which is known via name of embryo sac or female gametophyte.
Megasporogenesis
The process of formation of megaspores from the megaspore mother cell is known as megasporogenesis. In this ovules differentiate into a single megaspore mother cell (MMC) at the micropylar region of the nucellus. It is characterised by large cell with dense cytoplasm and a prominent nucleus. The MMC goes through meiotic division to form 4 haploid megaspores. Out of 4megaspore of a straight tetrad, mainly the upper three deteriorate and those present at the lower end expands to form functional megaspore.
Female gametophyte
The megaspore develops into the female gametophyte. It represents the first cell of female gametophyte. Monosporic development refers to the method of embryo sac formation from a single megaspore. Two nuclei are formed from the nucleus of the megaspore by dividing mitotically, both of them moves to the opposite poles, thus resulting in a 2-nucleate embryo sac. Later more two mitotic nuclear divisions results in the formation of the 4-nuclei followed by 8-nucleate stage of the embryo sac. These Mitotic divisions are free nuclear, which means that nuclear divisions are not immediately followed by cell wall formation. Two polar nuclei are formed when the nucleus from each poles comes to the centre, later fusion of these nuclei forms diploid secondary nucleus. Antipodal cells are the ones that are formed from three polar nuclei which are present at the chalazal end. The remaining last three nuclei which are situated at the micropylar end are surrounded by cytoplasm to form egg between the nuclei. The synergids are comprised of special cellular thickenings towards the micropylar tip by the name filiform apparatus. The egg cell after fertilisation forms zygote, whereas the synergids gets muddled up after fertilisation. The antipodal cells sooner or later also gets muddled. They have nutritive function. Thus, a classic angiosperm embryo sac, at maturity, forms 8-nucleate 7-celled embryo sac.
Process of fertilisation
Pollen grains germinate on the stigma of the flower after pollination to produce a pollen tube through one of the germ pores thereby, creating a pathway for pollen grains to move down the ovary.
This tube enters the ovule via the micropyle, and rupture into the embryo sac. Inside the embryo sac, the male gamete fusion with egg is seen which results in the formation of diploid zygote (2n). This zygote later expands, and results in the formation of fruit.
Various Forms of fertilisation
Fertilisation is of 5 types:
- Porogamy: in this type of fertilisation the pollen tube enters the ovule through micropyle, e.g. Lily.
- Chalazogamy: when the pollen tube enters the ovule through chalaza it is known as chalazogamy, it is seen in Juglans.
- Mesogamy: pollen tube entry through integuments refers to mesogamy, e.g. Cucurbita.
- Double fertilisation: it refers to the process of fusion of one male gamete with egg to form diploid zygote (2n), and the other male gamete fuses with two polar nuclei to form an endosperm.
- Triple fertilisation: fusion of one male gamete with egg in the embryo sac, followed by the fusion of the second male gamete with two polar nuclei, forming a triploid primary endosperm nucleus. This process represents triple fusion or vegetative fertilisation.
Pre-fertilisation
It is defined as the structures and events that take place before fertilisation. It mainly includes two methods:
- Gametogenesis: it refers to the process of formation of both male and female gametes.
- Gamete Transfer: it refers to the process of bringing together both the gametes.
Post-fertilisation
It refers to all those events that take place after the process of fertilisation. In this seed development from ovules and fruit development from ovary takes place. The events in post fertilisation comprises the following:
Endosperm development: the primary endosperm cell undergoes repeated division to form triploid endosperm tissue. The cells of this tissue contain reserve food materials which are used for nutrition of developing the embryo.
Embryogeny: it refers to the process of development of embryo that takes place at the micropylar end of embryo sac. The early stages of embryogeny are found to be similar in both monocotyledons and dicotyledons.
Importance of fertilisation
- fertilisation ensures diploid condition of organisms by the fusion of parental gametes.
- new genetic combinations in zygote is the result of fertilisation.
- Initiating fruit formation.
- It initiates embryogenesis.
Conclusion
The fertilisation is a vital process in flowering plants. It increases the chances of polyembryony and plants have better chances of survival. fertilisation process is always followed by pollination and results in seed germination. The flower represents the reproductive part of a plant, without which the plant will not be able to reproduce. It is the place where all the fertilisation activities takes place. We hope that you were able to grasp a clear concept on fertilization in plants, its types and significance.