Shoot apical meristems enable plants to grow in length and to branch. But a plant unable to grow in diameter would not become very tall before buckling under its own weight. And a plant unable to add vascular tissue could not supply its ever-increasing number of leaves with water. Growth in diameter thus serves two functions: It strengthens the stem, and it increases the capacity of the vascular system.

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Primary growth is the increase in length made possible by apical meristems. Secondary growth is an increase in diameter. It results from a new type of meristem, one that forms only after elongation is complete. The fossil record shows that the ability to grow in diameter evolved millions of years after primary growth, and it did so independently in several groups, testimony to the benefits of being able to compete for sunlight by growing tall. Today, secondary growth occurs almost exclusively in seed plants, although this was not always the case in Earth’s history.