Leaf epidermal structures of extant plants of Cunninghamia and Taiwania (Cupressaceae sensu lato) and their taxonomic application

Abstract

The leaves of Cunninghamia are characteristic of lanceolate, evergreen, coriaceous and serrulate margins. The wall of epidermal cells is clearly thickened and appears to be undulate for the pits. The long axes of most stomata of fossil specimens of Cunninghamia are sometimes perpendicular or oblique to that of leaves, which is different from those of extant species. The diagnostic features of the leaves of Taiwania are subulate, convex, evergreen, coriaceous and entire margins. Stomata occur on each surface of the leaves. The stomatal bands on the adaxial surface are more strongly developed than those on the abaxial surface of the leaves. The widest part of the stomatal band on the adaxial surface is located in the middle of the leaves. Most stomata on the abaxial surfaces are located at the base of the leaves. The distinctive oval or round appearance of the group of subsidiary cells on the inner surface of the cuticle is described under the SEM. Foliage shoots of the fossil specimens of Taiwania are commonly similar to those of extant plants. The conifer family Taxodiaceae traditionally includes nine genera, and the nine genera were suggested to be merged into the Cupressaceae sensu lato based on morphology, immunological data, chloroplast genome and karyotype analyses. The nine genera would be distinguished by compare the epidermal structures and foliage shoots, even though they commonly display similar leaf forms and phyllotaxis, and a wide range of variation within a genus. The anticlinal walls of most epidermal cells are obviously undulate in Metasequoia. Sometimes, they are slightly undulate in Taxodium and Cunninghamia, while they are mostly straight in the other genera. Linear or lanceolate leaves are found in the genera Cunninghamia, Sequoia, Metasequoia, Taxodium and Glyptostrobus. The long axes of most stomata are perpendicular or oblique to those of the leaves in Taxodium, while they are oblique in Cryptomeria. They are parallel in linear and linear-subulate leaves, and perpendicular or oblique in the scale leaves of the genus Glyptostrobus. The long axes of the stomata are mostly parallel to those of the leaves in other genera. The stomata of Metasequoia, Taxodium and Cunninghamia are similar to those of Athrotaxis, but are quiet different from those of the Athrotaxis-type in the other genera. An identification system for the nine genera was established based on the epidermal structures and foliage shoots. The epidermal structures of the extant plants provide important information on the taxonomy of fossil specimens.