FIGURE 5.

Model for the evolution of the monocot leaf. Two major morphological shifts may have occurred that dramatically altered the monocot lineage of plants. First, the reduction or loss of a canonical dicot leaf blade (depicted on the left) resulted in the petiole/lower leaf zone structure (center) that then assumed the role of the primary photosynthetic structure. Second, the petiole/lower leaf zone expanded and extrapolated into a new “leaf blade” (right) while also extending the endodermis/starch sheath into the new photosynthetic structure. This event may have also conditioned the parallel venation that is now associated with monocot leaves (model simplified and extrapolated from Arber, 1918; Kaplan, 1973; Nardmann et al., 2004).