Embryogenesis, Vegetative Growth, and Organogenesis
The life cycle of all seed plants can be divided into three main stages: embryogenesis, vegetative growth, and reproductive development. During embryogenesis, the single-celled zygote follows a defined pattern of cell divisions and differentiation to form the mature embryo and eventually the seedling, which contains all tissues and organs that develop into the mature plant body. In angiosperms, early phase of the embryogenesis is marked by laying down of the basic body plan of embryo, including establishment of apical—basal polarity and formation of embryonic organs (root and shoot primordial, cotyledons, etc.) and embryonic tissue layers. The embryonic layers- protoderm, ground meristem, and procambium, are the progenitors of the future epidermal, cortical, and vascular tissues, respectively. At the end of embryogenesis phase, several physiological changes occur within the embryos enabling them to sustain the period of dormancy and adverse environmental conditions. Upon the availability of factors like ambient moisture, light, temperature, etc., the seeds germinate, and with accompanying mobilization of stored reserves, the vegetative development commences, wherein tissue primordia in the seedlings grow and cells differentiate. During this period, the meristematic zones corresponding to shoot and root become active. Seedlings display a simple body plan which consists of shoot meristem, cotyledons, hypocotyl, root, and root meristem along the apical-basal axis and a concentric arrangement of tissues along the radial axis: epidermis at the periphery and subepidermal ground tissue and conductive tissue in the center. Vegetative growth is typically indeterminate, i.e., with no definite end and characterized by development of lateral organs—the leaves. As the plant undergoes the transition to reproductive development, the shoot apical meristem (SAM) changes shape, becoming an inflorescence meristem, and gives rise to flowers or flowering shoots, thus acquiring a determinate growth pattern. Plants exhibit variations in size ranging from a few mm to several hundred meters acquiring the architecture that is suited to the local environment. They grow in size throughout their lives and produce new organs like leaves, flowers, fruits, etc. The growth of plants is defined in terms of increase in number of cells as well as an increase in size of cells. It is important to note that this growth is not in terms of increase in dry mass as up to 95% of plant cells is made up of water. Plant growth is accompanied with differentiation of cells, tissues (histogenesis), and organs (organogenesis) that are specialized structures with distinct functions. Thus, growth and differentiation result in morphogenesis with the plant acquiring an overall form.