Three tiny grooves on a pollen grain mark out most of the world's flowering plants
Sunflowers, apples, cabbages, maples and dandelions share a secret visible only under a microscope: pollen with three furrows. That trait defines the eudicots, the true dicots, a lineage holding more than 70 percent of all flowering plant species. The magnolia, surprisingly, is not one of them.
Old textbooks split flowering plants into monocots and dicots according to whether seedlings emerge with one seed leaf or two. The trouble is that dicots in the old sense do not form a single family tree branch. In 1991 the botanists James Doyle and Carol Hotton introduced the term eudicots for the large, later-evolving group that does, separating it from older, less specialised plants that also sprout two seed leaves. Magnolias and the American tulip tree fall in that older set, the magnoliids.
The telltale feature sits in the pollen wall. Eudicot grains carry three or more pores set in long furrows called colpi, running parallel to the grain's axis, which is why some botanists prefer the name tricolpates. Most other seed plants, from conifers to monocots, produce pollen with a single groove known as a sulcus. These openings let the contents escape and allow the grain to swell and shrink as moisture changes. Molecular work later confirmed the grouping and found a genetic signature too: the shared ancestor duplicated the genes that encode DELLA proteins.
Molecular clocks put the split from other plants at about 134 million years ago, or perhaps 155 to 160 million, and fossils show early eudicots already diverse around 121 million years ago. The group divides into an informal set of basal eudicots and a coherent core. A 2010 study separated that core into the small order Gunnerales and a vast clade called Pentapetalae, which further splits into lineages built around the rosids and the asterids. The APG classification systems, from 1998 to APG IV in 2016, all use the eudicot label.
Source: Eudicots
