Question 1

What is double fertilisation?

Accepted Answer

Double fertilisation is a process unique to angiosperms in which two male gametes released by one pollen tube perform two separate fusions: one fuses with the egg cell (syngamy, producing the diploid 2n zygote) and the other fuses with the two polar nuclei (triple fusion, producing the triploid 3n primary endosperm nucleus).

Question 2

Why is the embryo sac called 7-celled but 8-nucleate?

Accepted Answer

A typical angiosperm embryo sac contains seven cells: one egg cell, two synergids, three antipodal cells (each with a single nucleus), and one large central cell that contains two polar nuclei. Six cells contribute six nuclei and the central cell contributes two, giving eight nuclei in total — hence 7-celled, 8-nucleate.

Question 3

What is triple fusion and what is its product?

Accepted Answer

Triple fusion is the fusion of three nuclei: one male gamete (n) with the two polar nuclei (n + n) of the central cell. Its product is the primary endosperm nucleus (PEN), which is triploid (3n) and divides to form the endosperm tissue.

Question 4

What is the ploidy of the endosperm?

Accepted Answer

The endosperm of angiosperms is triploid (3n), because it develops from the primary endosperm nucleus formed by triple fusion (n + n + n). This contrasts with the embryo, which is diploid (2n). In most plants the endosperm forms before the embryo matures.

Question 5

What is the filiform apparatus?

Accepted Answer

The filiform apparatus is a set of thickened, finger-like projections of the cell wall at the micropylar tip of the synergid cells. It secretes chemical attractants and helps guide the pollen tube into the embryo sac, directing it toward the egg apparatus.

Double Fertilisation: Two Sperms, Two Fusions

Practice

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