Primary meristems are regions in a plant where active cell division occurs, leading to growth. They are found at the tips of roots and shoots, allowing the plant to elongate and develop new leaves and flowers. The three main types of primary meristems are the protoderm, ground meristem, and procambium. The protoderm gives rise to the outer protective layer of the plant, while the ground meristem produces the bulk of the plant's body, including tissues for storage and photosynthesis. The procambium develops into the vascular tissues, which transport water and nutrients throughout the plant.