It is happening right now at the Arnold Arboretum – this very week. One of the most bizarre mating rituals imaginable is playing out on our female (seed-bearing) ginkgo trees. Inside the beautiful inch-in-diameter seeds, pairs of huge sperm are being produced by the pollen tubes that grew out of the pollen grains released by the male (pollen-producing) trees last spring. Now, four months later, sperm have been formed in these tubes and are swimming to the eggs contained inside the future seed. (Yes, plants, like animals, produce sperm and egg to make the next generation.) An amazing thing to contemplate this unseen world inside a ginkgo seed.
So, why would a ginkgo tree, whose seeds may be found more than a hundred feet in the air, resort to swimming sperm to find a mate? The answer is rooted in history. The ancestors of all land plants (green algae) had swimming sperm that sought out (swam to) eggs to create the next generation. A pretty good idea if you live full time in the water. Amazingly, after all of these years (475 million, to be precise, since the first green algae colonized land), sperm of ginkgo trees, cycads, ferns, horsetails, mosses, and other groups of plants still find their egg-mates by swimming, tracking the chemical signals that the conspecific (same species) egg is emitting to attract a suitor. Truly, this is a vestige of evolutionary history.
Pictured here are seeds of Ginkgo biloba ‘hayanari’ (824-83*A) from the grounds next to the Hunnewell Building. The third image is of a ginkgo sperm taken by a Japanese botanist, Tamaki Shimamura, in 1937. While a human sperm has a single “tail” (called a flagellum) that permits it to swim to an egg, a ginkgo sperm has approximately one thousand flagella! The surprising life cycle of the ginkgo tree was discovered in Japan in 1896 by a botanical research illustrator, Sakugoro Hirase, and this is a story in and of itself. You haven’t really lived until you have seen a ginkgo sperm swimming under the microscope. Trust me on this.