Evolutionary and ecological significance of photosynthetic organs inPhyllocladus(Podocarpaceae)

Abstract

The structure of phylloclades and true leaves in Phyllocladus was investigated with emphasis on function, ecology and evolution. Only in the earliest ontogenetic stages are true, needle leaves developed. The earliest phylloclades comprise a fusion product of a shoot axis of the first order and its inserted leaves. Later, this is augmented by the expansion of axillary short shoots from the base of some of the fused leaves. Phylloclades of mature individuals are entire short shoot systems, including leaves and shoots, becoming fused with photosynthetic tissue. These phylloclades represent one of the adaptations for increasing functional photosynthetic area that are present in Podocarpaceae. Such adaptations were probably needed because Podocarpaceae are usually medium-sized trees faced with competition from the ever-expanding angiosperm forests at mid to high latitudes in the Southern Hemisphere from the Late Cretaceous, but especially during the Palaeocene-Eocene, when dense and diverse angiosperm-dominated rainforests were common. The data obtained from living species were compared to those from fossil material of Phyllocladus. Plzyllocladus with fully developed phylloclades have been in place since at least the early Cenozoic.