How do cycad trunks grow thick without becoming much wider at the base?

Cycads achieve their characteristic thick, uniform trunk diameter—a condition known as pachycauly—through the activity of a primary thickening meristem (PTM) located lateral to the apical meristem.1

Unlike coniferophytes, which lack this specialized tissue and consequently have stems that are significantly narrower at the apex than at the base, the PTM in cycads produces substantial amounts of cortical parenchyma. This process allows the stem to maintain a consistent thickness from the crown to the base, creating an appearance similar to that of palms. While the stems are internally soft and fleshy with relatively little woody tissue, this primary growth mechanism ensures the trunk remains stout and uniform throughout its height.1