Question

Formation of the primary endosperm nucleus is called triple fusion. Give reason.

Answer 1

Hint: Endosperm is a kind of food product manufactured by plants for the developing embryo present within the seed. It serves as a source of nutrition for the embryo until it's dormant stage.

Complete answer
The general process of fertilisation in plants includes fusion of nuclei of male and female gametes that later leads to the formation of the zygote. In angiosperms, the process of fertilization is not limited to the zygote formation thus, includes formation of endosperm also. This endosperm formation takes place at the same time of zygote formation.
Anther, the male gamete splits into two haploid nuclei before entering into the ovule, the female reproductive part. Whereas, the ovule already has three haploid nuclei such that one is egg nuclei and the other two are polar nuclei. At the time of fertilization, one of the two male nuclei fuses with the egg nuclei to form the zygote while the other male nuclei fuses with the other two polar nuclei that leads to the formation of endosperm.
This fusion involves three haploid nuclei, thus, is called triple fusion. Therefore, we can say that the process of formation of primary endosperm is a triple fusion. However, as this whole process involves two fertilizations thus is called double fertilization. Later, the zygote develops to form a new plant individual when provided with favourable environmental conditions while endosperm nutrifies this embryo within the seed.

Note:
Every angiosperm has endosperm in its seeds. As, it takes a quite long time and specific environmental conditions for the embryo to germinate into a plant. Thus, in order to keep the dormant embryo nutrified while in its dormant stage, the tree provides its seeds with endosperm.