Morphology and anatomy of anomalous short shoots in Pinus (Pinaceae) and their evolutionary meaning

Abstract

Recent Pinus -species are evergreen and have a typical long shoot/short shoot differentiation. For angiosperms we could show that this type of shoot differentiation is linked to deciduousness (Dörken & Stützel 2009). Evergreen angiosperms with prominent shoot differentiation are derived from deciduous ancestors. The primitive evergreen condition is however characterized by the absence of a shoot differentiation. Here it is therefore analysed if the shoot differentiation in Pinus could be regarded as a reminder of a deciduous ancestry. In such a context Pinus monophylla would be functionally closest to a primitive evergreen angiosperm and represent a nearly perfect secondary adaptation to the needs of an evergreen species. Morphology and anatomy of aberrant short shoots in Pinus -species have been analysed to test this hypothesis. We suppose that the ancestor of Pinus had several needle leaves inserted spirally on a well developed short shoot axis ending in a terminal bud, as it can be found in Larix today.