The association of two opposing reproductive processes - sexual and asexual - in the course of evolution has given rise to seed multiplication. Genetic heterogeneity, based mainly on the different origins of embryos, appears to be one of the most important traits of seeds. Seeds may contain zygotic embryos, which arise by means of the sexual process (meiosis and fertilization). This mechanism is the main source of gene recombination, which determines genetic variation. However, somatic embryos developed without fertilization also may give rise to a new generation of plants. In connection with the problem of genetic heterogeneity of seeds, special attention has been paid to a new category, embryoidogeny, which seems to be a specialized form of vegetative propagation. The origins of embryos may differ, as these arise by means of monozygotic cleavage, nucellar, or integumentary embryoidogeny. Other apomictic embryos may be formed by means of gametophytic apomixis (diplospory or apospory, followed by diploid parthenogenesis or apogamety). Though these embryos are of different origins, they all are matroclinous offspring. Based on the origin of the embryos, several types of genetic heterogeneity of seeds can be distinguished, such as seeds with sexual (zygotic) embryos, seeds with hemigamous chimaerous embryos, seeds with parthenogenetic embryos, seeds with somatic embryos (embryoids), and seeds that contain sexual (zygotic) and parthenogenetic embryos and embryoids. This produces clones with different heredities in the offspring: clones of the parental organism (on the basis of gametophytic apomixis, nucellar and integumentary embryoidogeny) and clones of the new daughter organism on the basis of the zygotic embryo, including the zygote (monozygotic cleavage embryoidogeny, which is the source of monozygotic twins, triplets, etc.). Embryogenetic traits that determine the heterogeneity of the population should be taken into account together with other morphological traits of seed structure. The homeostasis of a species or population is to a large extent provided by the seed bank in the soil. Seeds stored in the soil represent an embryonal population of species.
JavaScript jest wyłączony w Twojej przeglądarce internetowej. Włącz go, a następnie odśwież stronę, aby móc w pełni z niej korzystać.