From Stephen Mikesell:

Re: A “Big Bang” of plant evolution (Nov. 26): The Soltis’s attribution of the mystery of diversification of flowering plants to a new evolutionary trait such as a water-conducting cell, of course, follows Darwin’s own thinking:

The parallel, and, taken in large sense, simultaneous, succession of the same forms of life thorughout the world, accords well with the principle of new species having been formed by dominant species spreading widely and varying; the new species thus produced being themselves dominant, owning to their having had some advantage over their already dominant parents, as well as over other species, and again spreading, varying, and producing new forms. —Darwin, The Origin of Species

It seems to me that one should furthermore not just look at traits in themselves within the plants, but of the relationships that these new traits engender -- as all traits basically are embodiment of relationships, simutaneously external and internal to the organism, or else they would not experience selection. Maybe this is old hat, but the relationship engendered by flowering plants to highly mobile, selective pollinators would be a powerful force in the mutual radiation and differentiation of both flowering plants and pollinators.