Excerpted from Garden of Marvels: How We Discovered that Flowers Have Sex, Leaves Eat Air, and Other Secrets of Plants by Ruth Kassinger, out now from William Morrow.
Why flowers, anyhow? Plants began to conquer the land more than 400 million years ago and ruled over it for more than 250 million years without producing a single blossom. Why should they have? Flowers are expensive. Sepals, petals, pigments for color, organic compounds for scent: Creating those fancy clothes and complex perfumes takes a lot of stored energy. Instead of manufacturing flowers, a plant could have used those carbohydrates to make more seeds or grow taller, both proven strategies in the competition for survival. Besides, there seems to be nothing in gymnosperms (nonflowering plants) that corresponds to flowers. Angiosperms (flowering plants) seem to have arisen out of nothing, sui generis. Nonetheless, blossoms—from the oak’s minuscule brown nubs to the green spikelets of rice to the multipetaled splendor of the rose—appear on at least 75 percent of all the world’s plant species.
The why and how of angiosperms, Darwin wrote in 1879, are “an abominable mystery.” The mystery still has not been fully solved. Part of the difficulty is that flowers have always been fragile and when they die, they fall apart into easily scattered and perishable pieces. The fossil record of early flowers is therefore exceedingly scant. In recent years, however, evolutionary botanists have come to think the living Amborella trichopoda will help solve the puzzle.
You will never find anyone who will sell you a bouquet of amborella. For one, the cream-colored, dime-sized flowers on this knee-high shrub are not much to look at, and will set no lover’s heart aflutter. If you should want to buy an amborella plant (say, you want to write about it), you’ll also be out of luck. The shrub is rare, and grows naturally only in the cloud forests of the island of New Caledonia in the South Pacific. Only a few American conservatories cultivate amborella, which is notoriously difficult to sustain and which flowers unpredictably in captivity. Nonetheless, unprepossessing and persnickety as it is, the plant has attracted a great deal of scrutiny recently. It is likely the closest living relative to the first flowering plant, and according to Harvard professor William Friedman, “a critical missing link between angiosperms and gymnosperms.”
About 140 million years ago, at the beginning of the Cretaceous era, a new type of plant evolved from one of the seed-bearing, nonflowering gymnosperms—mostly conifers—that covered much of the landscape. This first angiosperm was the founder of a new line, the Amborellales. Sometime later a sister angiosperm line emerged, the Nymphaeales, which evolved to become modern water lilies. A third line emerged. These, the Austrobaileyales, evolved rapidly and diversified profusely to become almost all of today’s 250,000 to 400,000 (depending on who’s counting) flowering species, from cucumbers to pansies to elms. The Amborellales, on the other hand, are today represented by a single species, Amborella trichopoda, the modern flowering plant least changed from its gymnosperm ancestor. It is a living relic from the age of the dinosaurs.
So, what does amborella have to say about why angiosperms evolved and conquered? Talking about angiosperms is talking about sex. Gymnosperms are dioecious. For example, individual gingko trees, which are one of the few nonconiferous gymnosperms, bear either male or female organs. Among conifers, male and female cones can be on separate individual trees or, more often, segregated on either the lower or upper branches of a single tree. (The male cones, which have pollen sacs on the upper edge of their scales, are smaller and drop soon after they release their pollen. Female cones grow larger after pollination, taking anywhere from a few months to two years before falling.) On the other hand, most angiosperms are hermaphrodites, with stamens and carpels inside the same the flower.