Returning to the exceptional seeds series, here is the fourth entry by Tamara Bonnemaison:
The species with the longest-recorded drift range in the world, Merremia discoidesperma, is the subject of today’s entry. Susan Ford Collins (aka jungle mama@Flickr) provides this gorgeous photo of a Merremia discoidesperma seed that has been colonized by tiny corals on its long drift through the ocean from Mexico or Central America to Miami Beach.
Merremia discoidesperma or Mary’s bean, is an uncommon woody liana found only in Mexico, Cuba, Guatemala, Costa Rica, and Hispaniola. Mary’s bean is little known in its plant form–its real claim to fame is for its seed. In fact, the seeds of Merremia discoidesperma were known and described for many years before the plant. In 1605, the Flemish botanist Carolus Clusius published the first drawing and description of a Mary’s bean seed, labeling it as a “stranded seed”, but the species did not receive its first binomial name until 1889, when John Donnell Smith named it Ipomoea discoidesperma. Mary’s bean seeds are a conspicuous but rare find on beaches far from their origin, and they have been kept as treasured keepsakes, passed from mother to daughter, by people who have never seen the plant. Aside from the novelty of finding a beautiful seed washed up on the beach, the seeds from the Mary’s bean plant have a hilum in the form of a cross, giving them particular religious significance for some.
Drift seeds are not common. Of all the plant species found on our diverse and wonderful planet, only around 250 are specifically adapted to drifting at sea. Mary’s bean is exceptional even within this small group, as its seeds remain buoyant for an unusually long time–up to three years. The limited distribution of the species makes it possible to track the distance that it has traveled. Mary’s bean seeds that wash into the Caribbean and Atlantic can be found as far away as the Norwegian coast, carried a distance of 9500km by the Gulf Stream. Seeds that end up in the Pacific Drainage Region may travel even further, with records of Mary’s bean seeds washing up in the Wotho Atoll in the Marshall Islands, about 11000km from their originating site. However, it is possible that seeds from other species have drifted further. For example, sea beans, or the seeds of Mucuna holtonii are likely to travel even further than Mary’s bean seeds, but because Mucuna holtonii has a more extensive distribution than Merremia discoidesperma, it is impossible to know exactly the distance traveled.
In order to enter the “drift seed club”, seeds must not only be able to float, but must be able to do so for a period of at least one month. There are a few approaches that seeds take in order to float: some have cavities within the seeds; others are made buoyant through a corky or light-weight fibrous layer; and still others are thin enough that they float on the water’s surface. In the case of Mary’s bean, a cavity at the centre of the embryo provides the buoyancy needed for the seed to cross oceans. Despite its ability to disperse seed across the world, Merremia discoidesperma can only be found in a very limited geographic area. This was a source of puzzlement to the researcher Charles R. Gunn, who speculated that insect attack keeps this species from establishing in seemingly suitable environments where its seeds routinely wash up.