Leaf dimorphism in Thuja plicata and Platycladus orientalis (thujoid Cupressaceae s. str., Coniferales): the changes in morphology and anatomy from juvenile needle leaves to mature scale leaves

Abstract

Thuja plicata and Platycladus orientalis initially produce only bifacial needle leaves. When the first lateral shoots develop, the leaf morphology and anatomy changes dramatically. Subsequently, only greatly reduced, bifacial scale leaves are developed. A new kind of “superimposed bifaciality” occurs with the change from juvenile needle leaves to mature scale leaves. Anatomical dorsiventrality affects not only the individual leaf, but also the complete plagiotropic lateral shoots of Thuja, which have a sun- and shade-exposed side. The upper light-exposed median leaves show adaxial leaf anatomy, contrary to the lower shaded median leaves showing abaxial leaf anatomy. Due to their mixed exposure, the lateral leaves show a lateral differentiation. At vertical lateral shoots of Platycladus, a predominant light-exposed side is absent. Thus, the anatomical dorsiventrality does not affect the complete shoot. Here the morphological abaxial side of a scale leaf becomes functionally and physiologically adaxial by reorientation of the palisade parenchyma and stomata. In juvenile needle leaves, the palisade parenchyma is located adaxial, with the majority of stomata being located abaxial. Conversely, in mature scale leaves, the palisade parenchyma is abaxial and the majority of stomata are adaxial.