Convergent morphology and anatomy in the microphyllous leaves of selected heathland Myrtaceae and Asteraceae

Abstract

Plants cope with a range of environmental conditions, especially related to water relations, and have developed an array of physiological and structural solutions to maintain a functional water balance. There has been considerable recent work on physiological solutions to water deficit but little attention paid to leaf characteristics. In many species there is a change in leaf form from seedlings to adults. We examine such changes in several small-leaved species from the distantly related Asteraceae and Myrtaceae, some of which develop micropeltate or reflexed leaves as adults. All are native to dry or seasonally dry sites. Three major morphological groups were recognised as follows: (1) leaves erect, nonpeltate and scale-like (Ozothamnus hookeri), (2) leaves erect and peltate (Phaenocoma prolifera, Regelia inops), (3) Leaves reflexed (Olearia lepidophylla, Ozothamnus scutellifolius, Ozothamnus reflexifolius, Melaleuca diosmifolia). The microphyllous peltation in P. prolifera and R. inops in the absence of a meristematic fusion/bridge differs from typically peltate leaves. These small-leaved taxa occur in open, high light environments which are very different from the mesic, shaded understorey habitats of typical peltate-leaved plants. Many small-leaved species have leaves closely appressed to the stem and often with recurved margins. The erect leaves are functionally similar to reflexed leaves. Environmental filtering leads to superficially similar plant forms that may have somewhat different ontological origins. Such morphological forms are examples of convergent evolution in distantly related species but within each family are likely phylogenetically related.