A vascular leaf that has increased activity of either the adaxial or abaxial meristem early in development, leading to absence of the opposite surface on the mature leaf.

A unifacial leaf may be round in cross-section (terete) or it may be laminar (ensiform), in which case lamina development is in a median plane (perpendicular to the axis), rather than a transverse plane (tangent to the axis). Unifacial leaves may be bifacial at the leaf base. Many unifacial leaves develop by reduced (or absent) activity of the marginal meristems and increased activity of the adaxial meristem early in development, leading to mature leaves with only an adaxial surface (e.g., Acacia phyllodes). However, some develop by increased activity of the abaxial meristem early in development, leading to mature leaves with only an abaxial surface. Note that unifacial leaves may have two sides, as in ensiform leaves, but both sides arise from the same surface of the leaf meristem (abaxial or adaxial).