Histogenesis and Organogenesis
After the octant stage numerous cell divisions occur in various planes. The proembryo become globular or bulb-shaped. As a general rule, some tangential divisions occur in the cells of the octant so that three cells layers are differentiated - the outer dermatogen which later forms the epidermal covering, middle periblem which gives rise to the cortex of the stem and root, and inner plerome that is responsible for the vascular tissue and pith. Such a process of differentiation of cell layers is termed Histogenesis.
Organogenesis or differentiation of the initials of various organs begins in the globular proembryo in such a way that cotyledons (which form the first leaves), epiphysis (which gives rise to the stem ex) and hypophysis (that forms the root cortex and cap) are produced. Since organogenesis differs in dicots and monocots, we may discuss this aspect separately for the two groups of angiosperms with the help of suitable examples.