Final answer:
Adventitious roots are roots that develop from a stem or a leaf, rather than from another root. They are formed from certain cells in the outermost layer of the central core of root vascular tissue, which divide and become organized to form a root apical meristem. The growth of these cells pushes the branch root apical meristem out of the original root and into the soil.
Step-by-step explanation:
Roots that develop from a stem or a leaf (and not from another root) are called adventitious roots. These roots are formed as a result of certain cells in the outermost layer of the central core of root vascular tissue being stimulated to start dividing and becoming organized to form a root apical meristem. The growth of cells produced by this meristem pushes the branch root apical meristem out of the original root and into the soil, where it continues to grow.