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The spelling fertilisation is a British English variant. The spelling fertilization is also used, and is the official spelling in American and Canadian English.</ref> (also known as conception, fecundation and syngamy), is the fusion of gametes to produce a new organism. 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 fertilization. The entire process of development of new individuals is called procreation, the act of species reproduction. After the pistil is pollinated, the pollen grain germinates in a response to a sugary fluid secreted by the mature stigma. From each pollen grain, a pollen tube grows out that attempts to travel to 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) cytoplasm. Hydrolytic enzymes are secreted by the pollen tube that digest the female tissue as the tube grows down the stigma and style; the digested tissue is used are a nutrient source for the pollen tube as it 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.
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