megaphyll

Megaphylls are leaves with a highly branched network of vascular traces, in contrast to microphylls, which have a single, unbranched vein. The megaphyll condition is widespread among vascular plants and is the typical leaf type in most angiosperms and gymnosperms as well as in ferns.

Megaphylls may be simple or compound and typically exhibit reticulate venation, in which many veins form a dense network. Veins arise from a system of leaf traces and connect by interconnecting veins, creating a broad, adaptable blade that supports a large photosynthetic surface. The arrangement of veins can vary, with some megaphylls showing dichotomous vein branching in their evolutionary history.

Most euphyllophytes, including ferns and seed plants, bear megaphylls, whereas lycophytes have microphylls. The origin of megaphylls is linked to Devonian diversification and is often explained by the telome theory, which posits that leaf blades evolved from branching networks of stem tips that became webbed and fused into a broad blade.

Functionally, megaphyll leaves enable large surface area and flexible venation patterns to support photosynthesis in diverse environments. The megaphyll concept is a morphological, not a strict phylogenetic, category, and some lineages display variation between microphyll and megaphyll morphologies over evolutionary time.