This well illustrated monograph treats the correlation of deciduousness (shedding leaves at a certain season) and long-shoot/short-shoot differentiation focused on gymnospermous tree species. The vast majority of gymnosperms are evergreen and within this group deciduousness has generally been regarded as a derived feature. Comparative studies of angiospermous tree species indicate that the vegetative long-shoot/short-shoot differentiation correlates well with deciduousness. The total leaf area of an entire short-shoot equals the leaf area of lamina of a single long-shoot leaf. So the lamina of a long-shoot leaf is replaced by the total leaf area of an entire short-shoot in the following vegetation period. This simple correlation is not observed in any of the studied gymnosperms except Ginkgo. Consequently, the evolutionary pathway to the long-shoot/short-shoot differentiation in gymnospermous tree species must be different from the evolutionary traits in angiospermous trees. It is shown that some evergreen gymnosperms can be regarded as derived from deciduous ancestors while others still represent the primitive deciduous condition. This monograph consists of two parts: In the first part several gymnospermous and angiospermous tree species have been investigated morphologically, anatomically and physiologically. In the second part those data were mapped on paleobotanic and palaeogeographic data in order to test the initial hypothesis that deciduousness has in the past been more frequent among gymnospermous tree species than the recent diversity can reflect. This book is of interest for all botanists and researchers on angiospermous and gymnospermous trees.