The theory of critical periods in plant ontogenesis has been elaborated from studies of integral morphogenetic processes on different levels. The periodization of the development of various reproductive structures (anther, microspore, pollen grain, ovule, megagametophyte, egg cell, zygote and embryo) has been worked out from data on morphogenesis using systemic and complex morphophysiological approaches. Critical phases, stages and periods have been revealed, for example the stage of autonomy in different flowering plants, by means of culture in vitro. The concepts of "critical period" and "critical mass" in relation to embryonal structure periodization are discussed here. Also addressed are the question of allometry and the significance of morphogenetic fields and rhythms of cell division for revealing critical periods and the management of ontogenesis. Examination of the genesis and structure of anthers and ovules in various flowering plant species has permitted us to discover general regularities in their development and the occurrence of three common critical periods: premeiotic, meiotic and postmeiotic. Embryo development in angiosperms is characterized by two common phases (proembryonal/blastomerization and embryonal/organogenesis) and five critical periods (zygote and proembryo, globular, heart-shaped, torpedo-shaped, and mature embryo). The combination of common and specific critical periods and stages determines the taxon-specific morphogenesis of reproductive structures and contributes to the plasticity and tolerance of the reproductive systems of different species of flowering plants, and of ontogenesis as a whole.
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