The creation of the science on stem cells and development of its theoretical bases is a prevalent topic today, taking into account comparative evolutionary cell biology and the cardinal problem of the developmental biology. This allows revealing correlations and studying correlative dependencies of various structures at different levels of biohierarchy. The creation of every science is impossible without the application of methodology. This book examines the system of non-traditional ideas about the nature and role of stem cells in ontogenesis, reproduction and evolution of plants. The main properties of plant stem cells have been developed, which has shown the integrity of morphogenous and reproductive processes at all stages of plant's life cycle.