A boundary separates a physical object from its surroundings. It defines the object's shape and regulates energy flux to and from it. Visual perception of the shape (geometry) of physical objects is an abstraction. While the perceived geometry at an object's sharp interface (macro) creates a Euclidian illusion of actual shape, the notion of diffuse interfaces (micro) allows grasping the realistic form of objects. Here we formulate a dimensionless geometric entropy of plant leaves (SL) by a 2-D description of a phase-field function. SL is estimated from the leaf perimeter (P) and leaf area (A) and correlates positively with segmental fractal complexity (DΣS). Leaves with a higher P:A ratio has higher SL and morphological complexity. An increase in SL of plant leaves could be an evolutionary strategy. The results of morphological complexity presented in this paper will trigger discussion on the causal links between leaf adaptive stability/efficiency and complexity. We present SL as a derived plant trait to describe plant leaf complexity and adaptive stability. Integrating SL into other leaf physiological measures will help understand energy dynamics and information flow in ecological systems.