Growth of Stems

How do primary growth and secondary growth occur in stems?

Plants grow in ways that are very different from how animals grow. Cows have four legs, ants have six, and spiders have eight, but roses and tomatoes don't have a set number of leaves or branches. Unlike animals, the growth of most plants isn't precisely determined. However, plant growth is still carefully controlled and regulated. Depending upon the species, plant growth follows general patterns that produce the characteristic size and shape of the adult plant.

Primary Growth The growth of new cells produced by the apical meristems of roots and stems adds length to the plant. This pattern of growth, occurring at the ends of a plant, is called primary growth. The increase in length in a plant due to primary growth from year to year is shown in Figure 23–13. Primary growth of stems is the result of elongation of cells produced in the apical meristem. It takes place in all seed plants.

Secondary Growth As a plant grows larger, the older stems and roots have more mass to support and more fluid to move through their vascular tissues. As a result, they must increase in thickness as well as in length. This increase in the thickness of stems and roots is known as secondary growth. Secondary growth is very common among dicots and nonflowering seed plants such as pines, but it is rare in monocots. This limits the girth of most monocots.

VISUAL SUMMARY

PRIMARY AND SECONDARY GROWTH

FIGURE 23–13 New cells produced by the apical meristem cause stems to grow in length (primary growth). Meanwhile, the vascular cambium increases the stem's width (secondary growth). Interpret Diagrams What kinds of tissues are formed by the vascular cambium?

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