It is clear that there are different patterns of leaf venation. It is also clear that similar patterns occur in seemingly unrelated taxa and that different patterns are found in a single taxon. It seems necessary to have these patterns organized into some sort of classification system in order to determine the relationships between the various patterns. Several classifications of venation patterns have been proposed in the past. Constantin von Ettingshausen and Alois Pokorny proposed one in 1856. It was based on the relative strength of the veins and their distribution in the leaf blade. They defined seven basic venation patterns according to the number of primary veins and the number, orientation, and behavior of the secondary veins. This system has much to recommend it but there are some drawbacks. Some leaves have no discernable primary vein and most have only one primary vein. This single primary vein forms the midrib of the leaf and secondary veins of equal or slightly less strength branch from it. On the leaves considered to have more than one primary vein in the Ettingshausen and Pokory system, examination shows that those "primaries" other than the midvein actually branch from the midvein. Ettingshausen and Pokorny?s classification system does not attempt to explain the relationships between the various patterns.weiterlesen