groups. Only a small proportion of the known taxa have been included in molecular studies, and with few exceptions, only data from a single gene are available. A new focus on the collection of multiple genes or even genomes is underway (Qui and Lee, 2000 ? ; Chapman and Waters, 2002 ? ). The work of Karol et al. (2001) ? , Lemieux et al. (2000) ? , and Turmel et al. (2002a ? , b ? , c ? ) have illustrated the power of resolving critical nodes with ever increasing amounts of data. In addition, C. J. O'Kelly (Bigelow Laboratory for Ocean Sciences, personal communication) and colleagues, funded through the Assembling the Tree of Life program of the National Science Foundation (USA), are collecting plastid and mitochondrial genome-level data for 30 green algae, selected to represent some of the "difficult" regions of green plant phylogeny (http://ucjeps.berkeley.edu/TreeofLife/)\\... There is optimism that, with enough data, the more difficult regions of the tree and incongruence among single-gene data sets will be resolved (see Rokas et al., 2003 ? ), although nodes representing rapid radiations may require data that are less clocklike (e.g., informative changes in morphology or genome structure that track divergence of short branches). Morphological and ultrastructural studies will still have a role in the systematics of green algae in the future. Previous studies (e.g., Sluiman, 1985 ? ; Bremer et al., 1987 ? ; Garbary et al., 1993 ? ; Mishler et al., 1994 ? ) used explicit homology