1. Whether the leaf is simple (Lycopus, Glechoma) or compound (Fraxinus), pinnate (Lycopus, Fraxinus) or palmate (Glechoma) in its nervation or leaflet arrangement, and acropetal (Fraxinus) or basipetal (Lycopus, Glechoma) in its development, the organogenetic zone of its primordium is characterised by a continuum of meristematic activity, a submarginal optimum mitotic density, a mitotic polarity (the main cause of lobation) and a very important individual variability.