Egg Apparatus

The three-celled egg apparatus is found in the embryo sac of a flowering plant. The three cells comprise an egg cell and two synergids. Thick finger-like projections, also known as the filiform apparatus, are reported to be present in the synergids. These synergids direct the incoming pollen grains to the ovule and are degraded once double fertilisation occurs. The filiform apparatus has an invaginated cell wall.

The egg apparatus is present towards the micropylar end and contains the egg cell, which forms a zygote after fusion with the pollen during fertilisation. The pollen tubes are reported to grow towards the egg apparatus because the synergid cells are thought to secrete a chemical that attracts the growth of the same, or we can say that pollen tubes progress chemo tropically in the ovule. The tip of the pollen tube bursts after entering the embryo sac. As a result, the male gametes are released.

Embryo sac (Diagrammatic)

Ploidy Level of Egg Apparatus

Ploidy level refers to the number of sets of homologous chromosomes in somatic cells. Each set of homologous chromosomes is denoted by n. A cell is called a monoploid if there is only 1 set. When n = 2, the cell is Diploid. Usually, when the sets are more than three, it is depicted as Polyploid. An egg apparatus contains a group of three cells present in a seven-celled embryo sac.

The 7 cells in the embryo sac are 3 antipodals (n), 2 synergids,(n) egg cell (n), and a central cell (2n). Post fertilisation, the antipodals and the synergids are reported to degenerate. Hence, the ploidy level becomes, Egg cell = 2n = Diploid and Central cell = 3n = Triploid. Consequently, the Ploidy level of the egg apparatus is (2n + 3n).

Acacia Egg Apparatus

The egg apparatus, central cell, and antipodal cells in the embryo sac of Acacia contain abundant cell organelles, large vacuoles, and stored reserves. The embryo sac at the micropylar end is usually broad and narrow at the chalazal end. The integuments are belated in appearance. The outer integument outgrows the inner one in spite of appearing after the inner one.

In Acacia Baileyana, the ovule is reported to get its full integuments after fertilisation. In this case, the micropyle is formed by the time of the first division of the fertilised egg. The synergids appear hooked and pear-shaped. They have their characteristic filiform apparatus and a prominent vacuole below the nucleus. The position of the egg cell varies; it can be either close to and below the synergids or away from the synergids.

The antipodals begin to degenerate after fertilisation. The two polar nuclei remain to stay in close proximity without being fused till fertilisation occurs. The presence of starch grains is reported in functional megaspores of Acacia.

During fertilisation, the processes like syngamy and triple fusion can occur. Two to three pollen tubes are observed in some cases in the same ovule entering the embryo sac.

Egg Apparatus of Embryo Sac

An egg apparatus is a group of three cells in the seven-celled embryo sac of a flowering plant. It comprises an egg cell and two associated cells called synergids. The egg apparatus is located at the end of the embryo sac, where there is an opening through which pollen nuclei enter the ovule. This end is also known as the micropylar end. The embryo sac contains three antipodal cells that are haploid and located near the chalazal end. The central cell is binucleate.

It is one of the most important cells of the embryo sac and helps in the formation of triploid endosperm after fertilisation. The two polar nuclei undergo fusion to form a definitive nucleus. The filiform apparatus directs the pollen tube towards the egg cell for fertilisation. These structures participate in the process of reproduction in angiosperm plants.

Interesting Facts

• The egg apparatus, which is reported to be found near the micropylar end of the seven-celled embryo sac of an angiosperm, has three cells - 1 egg cell and 2 synergids.

• The synergids direct the incoming pollen tube to the ovule.

• The egg apparatus helps in the process of double-fertilisation.

Important Questions

1. What is the function of a pollen tube?

Ans: During double fertilisation, the pollen tube is reported to deliver sperm cells to the female gametophyte. Pollen tube is formed as a result of a chemotropic mechanism between stigma and pollen grains.

2. What is double fertilisation?

Ans: Double fertilisation is a type of fertilisation that occurs in flowering plants in which one sperm nucleus combines with the egg nucleus to produce an embryo this is called syngamy. While another fuses with a central cell (formed from two polar nuclei) to generate endosperm and this process is called triple fusion.

Key Features of Egg Apparatus

• The embryo-sac structure comprises 3 antipodals (n), 2 synergids,(n) egg cell (n) and a central cell (2n).

• Double fertilisation is the process of the fusion of one male gamete with the egg cell along with the fusion of the second male gamete with the two polar nuclei. It occurs in angiosperms.

• The egg apparatus is formed as a result of the mitotic division of the megaspore.

• The filiform apparatus of the egg apparatus is structurally related to the transfer cells.