Mitosis drives plant growth at the cellular level. Individual cells replicate their contents and then split into two, causing the elongation of stems and roots and the emergence of blossoms and leaves.
Mitosis primarily occurs in regions of the plant called the meristems, according to retired Harvard University biology professor John W. Kimball's website. These rapidly growing regions are at the tips of stems and roots, where cells undergo rapid mitosis.
Meristems serve the plant in several important ways. As noted, constant mitotic activity allows plants to grow rapidly. Meristems also control differentiation, forming leaves, new stems and flowers. This special tissue, called nodes, also grows rapidly because of mitosis.
Mitosis also occurs rapidly during germination and early growth of a seed. Meristematic tissue differentiates into root and shoot meristem during formation of the seed. When the seed breaks open, these two poles begin rapid growth through mitosis, beginning with the root. According to "Molecular Biology of the Cell" by Bruce Alberts, mitosis occurs every 12 hours in some species during germination.