By hook or by crook: how and why do compound leaves stay curved during development?

From the size scale of the DNA molecule to the branches of the largest oak trees, curved and twisted forms are evident in plants and important for biological function. Rivière et al. (2020) report that during expansion of compound leaves, the leaf stem (rachis) is maintained curved. The rachis associated with a developing leaflet pair first curves, near the tip, and then straightens farther away from the tip, causing the developing leaf to maintain a hook shape that is displaced from the plant axis while the hook is also maintained a fixed distance from the leaf tip. The authors found this developmental pattern in many species of compound leaves. They characterized it in detail in leaves of the star fruit tree, Averrhoa carambola.