Development of the leaf forerunner tip (Vorläuferspitze) in Curio affinities (Asteraceae: Senecioneae), a structure unique to dicots

Forerunner tip, known also as Vorläuferspitze (Germ.), is a distinct usually unifacial part of a leaf which outraces all other its definitive constituents during leaf development and plays a central role in the modern models of leaf development in monocots. It is supposed to be absent in eudicots, but recent data indicate that at least genus Curio is an exception. Unlike monocots, the forerunner tips in this genus become unifacial without activity of the adaxial meristem. The scrutinized morphological and anatomical investigation revealed that precocious differentiation and unifacial structure which are widely used in identifying the forerunner tip are necessary but insufficient criteria of the structure concerned. Two additional criteria for recognition of the forerunner tip are proposed, viz. the forerunner tip takes significant part of the upper leaf zone immediately after the completion of its development and it remains distinguishable at all subsequent stages of leaf morphogenesis.