How did megaphyll leaves evolve from branching axes?

The evolution of megaphylls—the complex leaves found in ferns and other vascular plants—is widely attributed to the modification of branching stem systems. According to the telome theory, these leaves originated from the fusion and rearrangement of dichotomously branching axes.123

In early Devonian plants, some species possessed lateral branch systems that grew in a single plane, causing the side branches to appear flattened. The evolutionary process of "webbing" occurred when photosynthetic tissue filled in the spaces between these forking lateral branches, effectively creating a broad, laminate leaf structure.4

Unlike microphylls, which are simple outgrowths with a single unbranched vein, megaphylls are complex structures that draw significant amounts of vascular tissue from the stem. This process creates a distinct "leaf gap" in the stem's vascular cylinder (the stele), a feature that serves as a diagnostic difference between these two types of leaves. While the exact origins of vegetative organs remain a subject of scientific study, this transition from branching axes to flattened, webbed structures represents a major development in the evolution of plant morphology.142