Topeltviljastus

Topeltviljastus is a term originating from Estonian, literally translating to "double fertilization." This biological process is characteristic of angiosperms, the flowering plants. It involves the fusion of two male gametes (sperm nuclei) from a single pollen grain with two different cells within the ovule of a plant.

The process begins when a pollen grain lands on the stigma of a flower and germinates, forming a pollen tube that grows down through the style towards the ovule. Inside the ovule, there are several cells, including the egg cell and the central cell, which typically contains two polar nuclei. As the pollen tube reaches the ovule, it releases two sperm nuclei. One sperm nucleus fuses with the egg cell to form the diploid zygote, which will develop into the embryo of the new plant. The second sperm nucleus fuses with the two polar nuclei (or the fused secondary nucleus, if they have already fused) in the central cell to form a triploid primary endosperm nucleus. This triploid nucleus then develops into the endosperm, a nutritive tissue that provides nourishment to the developing embryo.

This unique mechanism of double fertilization ensures that the nutritive tissue (endosperm) is only formed after fertilization of the egg cell has occurred, thus conserving resources. It is a defining feature of angiosperms and is essential for their reproductive success.