Shoots may thicken either during their primary phase of growth or their secondary phase of growth. In most plants primary growth which occurs at the shoot apex is associated with the elongation of the shoot. However, cell division and subsequent cell growth at the shoot tip can thicken a shoot as well as lengthen it.

      Some shoots have a primary thickening meristem located to the sides of the apical dome. This primary thickening meristem is responsible for the increase in girth of many monocots that do not undergo secondary thickening. For example, the thick stem of an arborescent palm is due to a primary thickening meristem that widens the stem during the early seedling stages of the plant. Primary thickening is also common in storage shoots such as those found in cacti.

      The shoots of most plants, especially conifers and dicots, thicken during the secondary phase of growth. Secondary growth usually occurs at some distance away from the apical meristem and is the result of activity of a lateral meristem such as the vascular cambium. In most trees the vascular cambium forms a continuous ring between the outer phloem tissue and the inner xylem tissue. Most of the increase in stem thickness is due to cell divisions that add to the xylem. In seasonal climates, the spurt of growth that occurs during favorable seasons produces the rings we see in wood (secondary xylem).

      Even herbaceous plants like maize and tobacco have some degree of stem thickening. The stem of maize thickens only during the primary phase of growth. The stem of tobacco thickens during the secondary phase. When comparing the base of a Maize shoot (a monocot) with the base of a Nicotiana shoot (a dicot), one finds an obconate shape at the base of the Maize plant and normal cylindrical shape at the base of the Nicotiana plant. When the base of the Nicotiana shoot is dissected longitudinally and stained, an obconate primary form is revealed but lies within secondary tissue.