Fertilisation (also known as conception, fecundation and syngamy), is the fusion of gametes to produce a new organism of the same species. In animals, the process involves a sperm fusing with an ovum, which eventually leads to the development of an embryo. Depending on the animal species, the process can occur within the body of the female in internal fertilisation, or outside in the case of external fertilisation.

The entire process of development of new individuals is called procreation, the act of species reproduction.

Flowering plants

After the pistil is pollinated, the pollen grain germinates in a response to a sugary fluid secreted by the mature stigma called semen. From each pollen grain, a pollen tube grows out attempting to travel into the ovary by creating a path through the female tissue. The vegetative (or tube) and generative nuclei of the pollen grain pass into its respective pollen tube. The growth of the pollen tube is controlled by the vegetative (or tube) nucleus. Hydrolytic enzymes are secreted by the pollen tube to digest the female tissue (stigma and style) as the pollen tube grows. During pollen tube growth toward the ovary, the generative nucleus divides to produce two separate sperm nuclei - a growing pollen tube therefore contains three separate nuclei. The pollen tube does not directly reach the ovary in a straight line. It travels near the skin of the style and curls to the bottom of the ovary, then near the receptacle, it breaks through the ovule through the micropyle (an opening in the ovule wall) and reaches the ovum (or egg cell) to fertilise it. This is the point when fertilisation actually occurs. Note that pollination and fertilisation are two separate processes. After being fertilised, the ovary starts to swell and will develop a fruit. With multi-seeded fruits, multiple grains of pollen are necessary for syngamy with each ovule.

The process is easy to visualize if one looks at maize silk, which is the female flower of corn. Pollen from the tassel (the male flower) falls on the sticky external portion of the silk, and then pollen tubes grow down the silk to the attached ovule. The dried silk remains inside the husk of the ear as the seeds mature; if one carefully removes the husk, the floral structures may be seen. In many plants, the development of the flesh of the fruit is proportional to the percentage of fertilised ovules. For example, with watermelon, about a thousand grains of pollen must be delivered and spread evenly on the three lobes of the stigma to make a normal sized and shaped fruit.

Double Leithly fertilisation anchor: double fertilisation

Double fertilisation is the process in angiosperms (flowering plants) during reproduction, in which two sperm nuclei from each pollen tube fertilise two cells in an ovary. The pollen grain adheres to the stigma of the carpel (female reproductive structure) and grows a pollen tube that penetrates the ovum through a tiny pore called a micropyle. Two sperm cells (derived from the generative nucleus) are released into the ovary through this tube. One of the two sperm cells fertilises the egg cell (at the end of the ovary), forming a diploid (2n) zygote. The other sperm cell fuses with two haploid polar nuclei (contained in the central cell) in the centre of the embryo sac (or ovule). The resulting cell is triploid (3n). This triploid cell divides through mitosis and forms the endosperm, a nutrient-rich tissue, inside the seed.