The seed plants are often divided arbitrarily into two groups: the gymnosperms and the angiosperms. The basis for this distinction is that angiosperms produce flowers, while the gymnosperms do not. This is poor form, since it defines the gymnosperms by the absence of a character, and not by any features that the organisms actually share. The gymnosperms do share a number of features, but, as should be obvious from the above cladogram, they are not more closely related to each other than to the angiosperms (Anthophyta). The features shared by gymnosperms were likely present in the early ancestors of the flowering plants as well. It should also be noted that the "progymnosperms" are represented by a box of a different color, in order to make it clear that they are not actually seed plants, but rather are included here because they are believed to be the closest relatives of the seed plants.
Systematics within the seed plants is poorly understood. Part of the problem is that most of the major groups have gone extinct, and several of the groups alive today consist primarily of plants with highly derived morphologies. The above cladogram is based largely on the work of Jim Doyle, a professor at UC Davis, and Michael Donoghue, currently at Harvard, and is therefore somewhat preliminary. It includes some questionable groupings not explicitly supported in their papers.
For more information on seed plant relationships, visit the
Spermatopsida page on the
Tree of Life.