The bigger the tree, the bigger the seed

Tall plants tend to produce larger sized seeds, a new study confirms, but scientists don't know why.

Dr. Angela Moles (photo: bio.mq.edu)

The work was done by Australian researcher Dr Angela Moles of Macquarie University in Sydney and colleagues, published in the journal Science.

In nature, seeds are very different in size, from dusty orchid seeds to giant double coconuts on Maldive island. Scientists are always wondering which evolutionary forces determine their size.

The giant double seed of coconut trees on Maldive island.

In one of the largest studies of this type, Moles and the international team found a representative sample of 13,000 plant species worldwide. They charted based on their knowledge of evolutionary relationships among species, to determine the largest changes in particle size over time.

Later, the team compared this change to other plant changes, such as whether they grow near the tropics, or disperse particles through the wind or by other methods. In the past, scientists still believed that such factors determine particle size.

But the team was surprised to find that the strongest directive for the size of the seed is the size of the tree: the bigger the tree, the larger its particle, and when the tree is smaller, the seeds tend to follow.

"The surprise is that there is no obvious mechanical reason that they need to do that," said co-author Professor Mark Westoby. "So in this sense, it's a new question that we need to focus on finding."

One of the strange things about seeds is that although small particles tend to exist in large quantities, that doesn't give the tree an evolutionary advantage. That's because there is a balance between size and survival, small particles are not well equipped to survive like older friends.