The evolution of the seed some 360 million years ago was an important innovation that allowed seed plants to increasingly dominate the land by freeing them from needing water for sexual reproduction. Gymnosperms are ancient seed plants that do not have flowers or fruits but instead have cones in which fertilization occurs and seeds develop. The cones are modified leaves with male cones producing pollen (male sex cells) and more complex female cones producing eggs (ovules) and seeds. The gymnosperms have four living evolutionary lineages including the cycads, ginkgos, gnetophytes and conifers. The conifers are the most diverse group of gymnosperms, with about 630 species. Some conifers have female and male cones on separate trees, but typically both cones are on the same tree, with the larger female cones on higher branches and smaller male cones on lower branches. Since conifer pollen is spread by wind, pollen from one tree can easily fertilize a different tree (cross pollination) rather than itself. Little air sacs keep each pollen grain aloft in the wind. Before developing a seed more than a year can pass between pollination and fertilization.