Sometime around 140 million years ago the first flowering plant appeared. Shortly afterward, its descendants spread to most locations on Earth, displacing other plant species from the majority of ecosystems. Darwin described their success as an “abominable mystery” – one it seems we might finally have solved.
Flowering plants (angiosperms) have been known for decades to be more efficient than legacies from earlier times like ferns or conifers; probably an even bigger part of their success than their pollination alliance with insects. This efficiency, in turn, is the result of leaves better suited to transporting water and absorbing carbon dioxide than the needles of pine trees or the fronds of ferns. However, these discoveries just push the mystery back two steps. Why is it that other plant groups couldn't match the conversion of sunlight that flowering plants achieve?
Dr Kevin Simonin of San Francisco State University and Dr Adam Roddy of Yale University claim to have an answer. After comparing the genomes, cells, and leaf pore density of 400 species, they attribute efficient leaf design to the smaller genomes flowering plants possess. As noted in PLOS Biology, each cell of a living thing must contain a copy of its DNA, so a large genome prevents the formation of small cells. Since some plants have genomes almost a thousand times larger than those of other species, the effect is considerable.
These small cells, in turn, allow for specialization. If cells are too large, a plant can't fit many different sorts into a small space. Small cells allowed for the presence of numerous smaller veins and stomata with which plants transport water and exchange gas with the air. "The flowering plants are the most important group of plants on earth, and now we finally know why they have been so successful," the authors claimed in a statement.
There is another benefit to smaller cells; the higher ratio of surface area to volume for each cell makes for more efficient delivery of water and nutrients. Not all flowering plants have small genomes – the range in size is enormous – but most angiosperms have DNA content far smaller than that of the smallest non-flowering plant.
Exactly why flowering plants developed this genome-shrinking capacity, while other plants never did, remains a mystery, if not an abominable one. The authors are also curious as to how other plants, despite their disadvantages, have managed to hold onto some ecological niches. For example, conifers dominate the large forests of the Northern Hemisphere's higher latitudes. Did these species survive despite their large genomes, or were there compensating advantages hidden in all that excess DNA?