Deep phylogenetic relationships among "plants" and their implications for classification

There has been tremendous recent progress in understanding the relationship s of plants, due to two different advances, whose cumulative impact has bee n great. One advance is theoretical and methodological-a revolution in how any sort of data can be used to reconstruct phylogenies. The other is empir ical-the sudden availability of copious new data from the DNA level. This r eview briefly sets these advances in their historical context, then covers both as to their promise and problems. An important distinction between "sh allow" and "deep" phylogenetic studies is developed, and morphological and molecular data are compared as potential phylogenetic markers in that conte xt. Recent results on relationships of plants in general and green plants i n particular are then considered. Future directions for classification, par ticularly the need for rank-free taxonomy, are also discussed in light of t he rapidly improving resolution of plant relationships.