Process of Double Fertilisation — Core Principles

Core Principles

Double fertilisation is a unique reproductive process in flowering plants (angiosperms) involving two distinct fusion events. It begins with a pollen grain landing on the stigma and growing a pollen tube, which carries two male gametes into the embryo sac within the ovule.

The first fusion, called syngamy, occurs when one male gamete (haploid, n) fuses with the egg cell (haploid, n) to form a diploid zygote (2n), which develops into the embryo. The second fusion, known as triple fusion, involves the other male gamete (haploid, n) fusing with the diploid secondary nucleus (2n), formed by the fusion of two polar nuclei in the central cell.

This results in a triploid primary endosperm nucleus (3n), which develops into the endosperm, a nutritive tissue for the embryo. This dual fertilisation ensures the simultaneous formation of both the new plant embryo and its food supply, making it a highly efficient reproductive strategy.

Post-fertilisation, the ovule matures into a seed, and the ovary develops into a fruit.

Important Differences

vs Single Fertilisation

Aspect	This Topic	Single Fertilisation
Organisms	Angiosperms (Flowering Plants)	Most animals, lower plants (e.g., algae, fungi, bryophytes, pteridophytes)
Number of Fusions	Two distinct fusion events (syngamy and triple fusion)	One fusion event (fusion of male and female gametes)
Products of Fusion	Zygote (2n) and Primary Endosperm Nucleus (3n)	Only Zygote (2n)
Nutritive Tissue	Endosperm (3n), formed simultaneously with the embryo	Nutritive tissue (if present) is formed either before fertilisation (e.g., female gametophyte in gymnosperms) or from maternal tissues, not as a direct product of a second fertilisation event.
Ploidy of Nutritive Tissue	Triploid (3n)	Haploid (n) or Diploid (2n) depending on the organism
Evolutionary Significance	Highly evolved, ensures coordinated development of embryo and its food supply, contributing to angiosperm success.	Simpler, ancestral form of fertilisation.

Double fertilisation, characteristic of angiosperms, involves two distinct fusion events: syngamy (male gamete + egg cell $\rightarrow$ zygote) and triple fusion (second male gamete + central cell secondary nucleus $\rightarrow$ primary endosperm nucleus). This unique process simultaneously forms the diploid embryo and the triploid endosperm, a dedicated nutritive tissue. In contrast, single fertilisation, found in most animals and lower plants, involves only one fusion event between male and female gametes, producing only a diploid zygote. The nutritive tissue, if any, is formed through different mechanisms, not a second fertilisation.