Figure 18-30Growing taller: primary growth.

So, as we’ve seen, primary growth makes shoots taller and roots and branches longer. It also forms new tissues such as buds and leaves.

All primary growth occurs as a result of cell division in apical meristems (FIGURE 18-30). The cells in meristems are totipotent—they have the potential to develop into any type of cell the plant is capable of producing. Growth occurs as a meristem cell divides and creates two new cells. One differentiates into a specific type of tissue cell, such as xylem, phloem, or perhaps a storage cell. The other cell remains a meristem cell. Thus, growth and differentiation can continue while the meristem remains a perpetual source of new cells. Recall that apical meristem cells are present at all root tips and shoot tips. They reside just behind a small cap of cells, at the very end of the tip of the root or shoot, that protects the meristem.

Apical meristem cells are also responsible for the production of branches as a plant grows. In the shoot, as the meristem cells divide, pushing the top of the plant higher and higher, some meristem cells are left behind at regular intervals. The meristem cells left behind are able to begin dividing at any time—usually after stimulation by hormones, which we discuss in Chapter 19. They push outward to form a branch. As the branch grows, the meristem again leaves behind some cells near each bud, allowing further branching. Some of the new cells become xylem and phloem, and the newly formed vessels join up with the central circulatory vessels within the stem.

Most timber used in construction comes from pieces of wood cut from the tree along the length of its trunk. Knots are places where branches were connected to the trunk and where their vascular tissue merged with that of the trunk. Knots become more and more deeply embedded within the trunk as secondary growth makes the trunk thicker.