A basis difference was detected in sexual reproduction of flowering and nonflowering plants. In the sexual reproduction of flowering plants, double fertilizations occur. One fertilization leads to the egg cell, while another one forms the endosperm. Then, the fertilized ovule forms seeds. Therefore, the seed is considered a reproductive organ with a complicated structure. A seed has an embryo: a multicellular structure originating from a zygote, and seed coats: a saprophytic tissue made from the maternal ovule coat. Each seed is formed via a developmental process in some sporophytic (megasporangium and integuments of the ovule) and gametophyte (megaspore and gametophyte) tissues
