Micropeltation in Myrtaceae: a neglected subject

The majority of taxa with peltate leaves are perennial herbs native to swampy or aquatic habitats or to mesic shaded understorey habitats. These large peltate leaves are formed by a meristematic bridge at the lamina–petiole junction. However, there are also several strong-light exposed, small-leaved, xero- and scleromorphic Myrtaceae with leaf peltation which is formed without a meristem fusion/bridge. Here, abaxial laminar tissue at the insertion point of the petiole forms a basal extension, so that a weak peltation occurs. This shifts the petiole onto the adaxial laminar surface. The formation of micropeltation in Myrtaceae leads to erect leaves that are strongly appressed to the shoot axis and the entire foliate, vertical shoots appear as “green columns”, a result that is also the case in taxa with reflexed minute leaves. It seems that micropeltation achieves the same goal as leaf reflexion in small-leaved taxa—reduction of heat-load and transpiration during the hottest phases of the day by a lower light interception at midday compared to the morning and evening. Thus, physiologically micropeltation and reflexion of minute leaves seem to be the result of convergent evolution.