From: New Scientist February 11, 2015
The coco de mer palm of the Seychelles islands is the stuff of legend.
Its seeds- the largest and heaviest in the world- were once
believed to grow on trees beneath the waves of the Indian Ocean
and to hold great healing powers. Even when it later turned out
that the palm grows on dry land, new folklore emerged. To
produce this seed, the male and female plants embrace each other
on a stormy night, or so a local story goes.
The legends may be just that, but the palm still has unique
appeal. "The coco de mer is the only charismatic plant that can
rival the giant panda or the tiger," says Steven Blackmore at the
Royal Botanical Garden in Edinburgh, UK. Now the science behind
the charismatic palm's seeds is proving to be just as fascinating.
How does a plant that grows in poor quality soil on just two
islands produce record-breaking seeds that reach half a meter in
diameter and weigh around 25 kilograms? To find out, Christopher
Kaiser-Bunbury at the Technical University of Darmstadt in Germany
and his colleagues analyzed leaf, trunk, flower, and nut samples
taken from Coco de mer plants (Lodoicea maldivica) living
on the island of Praslin. They found that the leaves have only
about one-third of the nitrogen and phosphorus concentrations seen
in the leaves of other trees and shrubs growing on the
Seychelles. Also, before old leaves are shed, the palm efficiently
withdraws most of the nutrients from them and recycles them.
Investing so little in the foliage means the palm has more
to invest in its fruit.
But that's not the only way the foliage helps fuel fruit growth.
The huge, pleated leaves are remarkably effective at funnelling
water down the trunk during rain showers. Kaiser-Bunbury and his
colleagues showed that this stream of water also picks up any
nutrient-rich detritus on the leaves - dead flowers, pollen, bird
feces, and more - and washes it down into the soil immediately
around the base of the palm. Consequently, the nitrogen and
phosphorus concentrations in the soil 20 centimeters from the
trunk were at least 50 per cent higher than in the soil 2 meters
away. Blackmore has seen first-hand how efficiently the leaves
channel water - better than some gutters on local buildings, he
says. "But to think about it in terms not just of water flow but
of nutrients was a very significant leap of thinking and adds much
to the understanding of this amazing tree," Blackmore adds. Hans
Lambers at the University of Western Australia in Crawley, who
studies the way plant species have adapted to incredibly low
phosphorus levels in soil in south-western Australia, says the
nutrient channeling leaves of coco de mer are an "entirely
different strategy."
The discovery is linked to another remarkable thing about the palm: it seems to be unique in the plant kingdom in caring for the seedlings after they germinate. Many trees have evolved seeds that travel- on the wind or in the gut of an animal- so that seedlings don't compete with parents for the same resources. Stranded on two islands and unable to float, coco de mer seeds usually don't travel very far. But the researchers found that the seedlings benefit from growing in the shadow of the parent because they have access to more nutritious soil there. "This is exactly what fascinated my colleagues and me most about Lodoicea," says Kaiser-Bunbury. "We do not know of another plant species that does this."
This still does not explain why the seeds are so large. According to one theory, we have to go back to the dying days of the dinosaurs for explanation. About 66 million years ago, the ancestral form of the palm probably relied on animals to disperse its relatively large seeds. However, it perhaps lost this mechanism when the sliver of continental crust that includes Seychelles broke away from what is now India, isolating the palm. This meant the seedlings had to adapt to growing in the gloomy shadows of their parents. Because the large seeds contained a good supply of nutrients, the seedlings were already well equipped to do so, and outcompeted most of the other tree species in the ecosystem. To this day, coco de mer palms are the dominant species in their forests. Under the unusual conditions of forests dominated by a single species, sibling competition- rather than competition between species- drove evolution, says Kaiser-Bunbury. This meant the palm gradually grew larger and larger seeds to provide seedlings with an even bigger reserve of nutrients to boost chances of surviving competition against siblings.
Kevin Burns at the Victoria University of Wellington, New Zealand, studies the way plants evolve on isolated islands, like the Seychelles, and says that the coco de mer palm seems to follow a general evolutionary pattern. "Plants tend to evolve large seeds after they colonize isolated islands, and island plant species often have much larger seeds than their mainland relatives," he says. "Big seeds generally house competitive seedlings."
The coco de mer palm hasn't yielded all of its secrets yet. Exactly how the female flowers- the largest of any palm- are pollinated remains a mystery. Blackmore suspects bees are involved, but other researchers think lizards might transfer pollen. Local legend, meanwhile, suggests that male trees actually tear themselves from the ground on stormy evenings and lock in a passionate embrace with females. It's the kind of story that adds to the allure of the palm.