New cells must be added as a plant grows in diameter. The sources of these new cells are lateral meristems, which form along the length of a stem. Lateral meristems are similar to shoot apical meristems in their ability to produce new cells that grow and differentiate. However, lateral meristems differ from their apical counterparts in several ways. First, lateral meristems surround the stem, rather than occur at its tip. Second, because lateral meristems form only after elongation is complete, the new cells they produce grow in diameter but not in length. Finally, as a stem becomes thicker, the number of meristem cells needed to encircle the stem also increases. Thus, lateral meristems become larger over time.

Plants produce two distinct lateral meristems that together result in secondary growth. One of these, the vascular cambium (plural, cambia), is the source of new xylem and phloem. The xylem produced by secondary growth is familiar to you: It’s wood. Plants with secondary growth are often described as woody, whereas plants that lack secondary growth are described as herbaceous. Most of the mass of a large tree consists of wood, generated from the vascular cambium.

As the plant’s diameter increases, the epidermis formed during primary growth eventually ruptures. Thus, plants with secondary growth have a second lateral meristem, the cork cambium, which renews and maintains a protective outer layer. This outer layer protects the stem against herbivores, mechanical damage, desiccation, and, in some species, fire.