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Why do tree ferns not thicken their trunks in the same way as conifers and hardwood trees?

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Tall giant sequoia trees with reddish bark and thick trunks rise closely together in a dense forest at Sequoia National Park, California. — Tree A stand of giant sequoia trees (*Sequoiadendron giganteum*) in Sequoia National Park, California.

Tree ferns do not thicken their trunks in the same way as conifers and hardwood trees because they lack a vascular cambium.1

In conifers and dicotyledonous trees, the vascular cambium is a layer of meristematic cells that organizes between the primary xylem and phloem. This lateral meristem is responsible for secondary growth, which increases the girth of the plant by producing secondary xylem (wood) and secondary phloem (inner bark).12

In contrast, the trunks of tree ferns are not composed of true wood and bark. Instead, they consist of rhizomes that have been modified to grow vertically, and these are embedded in a dense, supportive mantle of adventitious roots. Because tree ferns do not develop a vascular cambium, they are unable to undergo the secondary thickening process that characterizes the trunks of seed-bearing trees.31