Double Fertilisation — Core Principles

Core Principles

Double fertilisation is a hallmark process in flowering plants, involving two distinct fusion events within the embryo sac. It commences when a pollen grain germinates on the stigma, forming a pollen tube that carries two male gametes to the ovule.

Upon reaching the embryo sac, typically via a synergid, the pollen tube releases these gametes. The first fusion, called syngamy, involves one male gamete fusing with the egg cell to form a diploid ($2n$) zygote, which develops into the embryo.

The second fusion, known as triple fusion, sees the other male gamete uniting with the two polar nuclei (or the secondary nucleus) in the central cell, resulting in a triploid ($3n$) primary endosperm nucleus (PEN).

The PEN subsequently develops into the endosperm, a vital nutritive tissue for the growing embryo. This dual fertilisation ensures the simultaneous formation of both the new plant embryo and its dedicated food supply, a highly efficient evolutionary strategy unique to angiosperms.

Important Differences

vs Single Fertilisation (e.g., in Gymnosperms)

Aspect	This Topic	Single Fertilisation (e.g., in Gymnosperms)
Organism Group	Angiosperms (Flowering Plants)	Gymnosperms (e.g., Pines, Cycads)
Number of Fusion Events	Two (Syngamy and Triple Fusion)	One (Fusion of male gamete with egg cell)
Products of Fertilisation	Diploid Zygote and Triploid Primary Endosperm Nucleus (PEN)	Diploid Zygote
Nature of Nutritive Tissue	Endosperm (triploid, $3n$), formed *after* fertilisation	Female gametophyte tissue (haploid, $n$), formed *before* fertilisation
Timing of Nutritive Tissue Formation	Coordinated with embryo formation; only forms if egg is fertilised	Forms independently of fertilisation; resources committed even if egg is not fertilised
Efficiency of Resource Use	Highly efficient, prevents waste if fertilisation fails	Less efficient, resources may be wasted if fertilisation fails

The fundamental distinction between double fertilisation in angiosperms and single fertilisation in gymnosperms lies in the number of fusion events and the nature and origin of the nutritive tissue. Angiosperms exhibit two fusions: one forming the diploid zygote (embryo) and another forming the triploid endosperm (food source). This ensures that the food supply is only developed upon successful fertilisation, making it a highly efficient process. In contrast, gymnosperms undergo a single fertilisation to form the zygote, and their nutritive tissue is the haploid female gametophyte, which develops prior to fertilisation, potentially wasting resources if fertilisation is unsuccessful. This evolutionary difference highlights angiosperms' advanced reproductive strategy.