Unlike animals, plants are immobile and cannot seek out sexual partners for reproduction. The first plants used abiotic means to transport sperm for reproduction, utilizing water and wind. The first plants were aquatic and released sperm freely into the water to be carried by the currents. As plants moved onto land they used a thin film of water or water droplets like liverworts and ferns, in which mobile sperm swam from the male reproduction organs to the female organs. As plants became more complex and developed vascular systems enabling them to grow taller, they used alternation of generations like in ferns or the wind to move spores. In the Paleozoic era progymnosperms reproduced by using spores dispersed on the wind, 350 million years ago the seed plants evolved, including seed ferns, conifers and cordaites all were gymnosperms. Pollen grains, the male gametophyte, developed for protection of the sperm during the process of transfer from male to female parts. It is believed that insects feed on the pollen and plants evolved to use insects to actively carry pollen from one plant to the next. Seed producing plants, which include the angiosperms and the gymnosperms, have hetromorphic alternation of generations with large sporophytes containing much reduced gametophytes. Angiosperms have distinctive reproductive organs called flowers with carpels and the gametophyte is greatly reduced to a female embryo sac with as few as eight cells and the male gametophyte develop from the pollen grains. The sperm of seed plants are non motile except for two older groups of plants the Cycadophyta and the Ginkgophyta which have flagellated sperm.