Douglas C. Daly
The ‘sexual system’ developed by Linnaeus for classifying flowering plants was based on the number of stamens and parts of the ovary, and the classification as well as the description of flowering plants are very much based on flowers and fruits today. The fact is that leaves constitute an extremely rich and woefully underutilized source of characters for describing as well as distinguishing or linking flowering plants.
For those working in tropical forests, where at any given time the vast majority of tree individuals and species have neither flowers nor fruits, leaves represent a hitherto untapped resource for aiding tree identification. This is of extreme importance when it comes to the forest inventories that determine which and how much timber is permitted to be harvested. As I noted in a description of my work on forest management in Amazonia, we are training tree identifiers who perform the field identifications for forest inventories, and we are developing field guides for distinguishing easily-confused species, and leaf characters figure prominently in these efforts. I was co-author of the Manual of Leaf Architecture, published in 2009, which was the first complete and completely illustrated manual for full descriptions of the leaves of dicotyledonous plants. In addition to forest inventory, I and a number of my colleagues and students have been making extensive use of leaf characters in our taxonomic work; I recently completed a taxonomic revision of the Burseraceae genus Canarium in Madagascar, in which I relied heavily on leaf characters.