llen, which can produce the male sex cells, is transported between the male and female parts of flowers in pollination. Pollination can occur between different plants, as in cross-pollination, or between flowers on the same plant or even the same flower, as in self-pollination. Pollen movement may be caused by animals, such as birds and insects, or non-living things like wind and water. The colour and structure of flowers assist in the pollination process. After pollination, the sex cells are fused together in the process of fertilisation, which is a key step in sexual reproduction. Through cellular and nuclear divisions, the resulting cell grows into a seed, which contains structures to assist in the future plant's survival and growth. At the same time, the female part of the flower forms into a fruit, and the other floral structures die. The function of fruit is to protect the seed and aid in its dispersal away from the mother plant. Seeds can be dispersed by living things, such as birds who eat the fruit and distribute the seeds when they defecate. Non-living things like wind and water can also help to disperse the seeds. Flowers first evolved between 150 and 190 million years ago, in the Jurassic. Plants with flowers replaced non-flowering plants in many ecosystems, as a result of flowers' superior reproductive effectiveness. In the study of plant classification, flowers are a key feature used to differentiate plants. For thousands of years humans have used flowers for a variety of other purposes, including: decoration, medicine, food, and perfumes. In human cultures, flowers are used symbolically and feature in art, literature, religious practices, ritual, and festivals. All aspects of flowers, including size, shape, colour, and smell, show immense diversity across flowering plants. They range in size from 0.1 mm (1⁄250 inch) to 1 metre (3.3 ft), and in this way range from highly reduced and understated, to dominating the structure of the plant. Plants with flowers dominate the majority of the world's ecosystems, and themselves range from tiny orchids and major crop plants to large tr