Leaf size of plants can be described using the terms megaphyll, macrophyll, mesophyll, microphyll, nanophyll and leptophyll (in descending order) in a classification devised in 1934 by Christen C. Raunkiær and since modified by others. Definitions vary, some referring to length and others to area. Raunkiaer's original definitions were by leaf area, and differed by a factor of nine at each stage. Some authors simplified the system to make it specific to particular climates, and have introduced extra terms including notophyll, picophyll, platyphyll and subleptophyll.