When stems have gained moderate height, it is important that they begin to thicken toward their bases to give added stability and support for the leaf mass. This is called secondary growth and results from cell divisions in meristems located inside, throughout the length of the stems. These lateral meristems also extend into the roots of larger plants. Secondary growth in a tree creates the slow but measurable thickening of its trunk and branches as well as the upper portions of roots that may emerge above the soil surface.
