Premise of research. Cycas is the earliest-diverging extant lineage in the ancient order Cycadales, well separated phyletically from the other nine extant genera. Despite the ancient status of Cycas, all extant species are no more than 12 Myr old, and many extant species have become known only in recent decades. Given this context, a broad survey of variation in Cycas leaflet structure may show structural diversity that corresponds with and informs phylogeny and biogeography.

Methodology. We investigated the leaflet anatomy of 48 Cycas species grown in a common garden to ascertain the variation of anatomical traits and compared this to available phylogenetic and geographic information to determine patterns in this variation.

Pivotal results. We find a very strict anatomical bauplan within the genus and only limited qualitative variation in such features as epidermal cell type, stomatal structure, and extent of hypodermal specialization.

Conclusions. Our findings thus sustain the well-accepted monophyly of the genus with a consistent series of synapomorphic features. The most distinctive character is the existence of pits within the outer wall of the epidermis, a feature almost unique for gymnosperms. Unlignified midrib fibers appear to be a unique synapomorphy for Australian and Papuan members of section Cycas. Encrypted stomata and epidermal cell shape confirm a close relationship between sections Asiorientales and Panzhihuaenses and suggest a distinction from section Stangerioides but do not show a clear geographic relationship. Mucilage canals are found only within section Stangerioides. No unique synapomorphy exists for section Indosinenses, but variation in epidermal cell shape appears correlated to the presence of an adaxial hypodermis in section Indosinenses.