Final answer:
Primary growth in plants is facilitated by the apical meristems located at the tips of stems and roots, leading to an increase in length as the plant elongates.
Step-by-step explanation:
Apical meristems. These are regions of rapidly dividing cells which lead to the elongation of the plant, allowing the roots to seek water and the shoots to reach for sunlight. This type of growth is distinctly different from secondary growth, which is associated with an increase in girth and is caused by the lateral meristems.
Apical meristems can be found at the very tip of the stem and root and they contribute to the increase in length, whereas lateral meristems, such as the vascular and cork cambia, are responsible for increasing the thickness of the plant, which is well-observed in woody plants as wood formation.