megaphylls

Megaphylls are leaves characterized by a highly branched network of veins, forming a reticulate or net-like venation pattern. They are large, often broad blades, and contrast with microphylls, which have a single unbranched vein.

Origin and evolution: Megaphylls evolved in the euphyllophyte lineage during the Silurian–Devonian periods, as lateral branches became webbed together to form a single leaf blade with a complex vascular network. The evolution of megaphylls allowed greater surface area for photosynthesis and more distributed vascular supply, supporting larger plants.

Anatomy and development: Megaphyll leaves develop from a system of branching leaf traces in which multiple veins diverge and rejoin within the blade; the vascular cylinder often shows a leaf gap where the leaf trace leaves the stem. The veins form a reticulate network, sometimes with dichotomous division early in development.

Distribution: In present-day plants, megaphylls are typical of ferns (fronds) and most seed plants, including gymnosperms and angiosperms. Microphylls, by contrast, occur in lycophytes such as clubmosses, which retain small, simple leaves with a single vein.

Function: The large, veined leaves enable higher photosynthetic capacity and support the growth of large plants, though leaf size and form vary widely among groups.