A rare plant called Paris japonica has a genome 50 times longer than that of humans, making it the longest genome ever recorded. The Royal Botanic Gardens, Kew, announced the discovery last week, and details appear in the September 2010 issue of the Botanical Journal of the Linnean Society.

The Paris japonica genome weighs in at 152.23 picograms (trillionths of a gram), 15 percent larger than the previously biggest known genome, that of related herb, a hybrid trillium known as Trillium × hagae. "It's so large that when stretched out it would be taller than Big Ben," said Ilia Leitch, a research scientist at Kew's Jodrell Laboratory, in a prepared statement. Human DNA would only stretch out to about two meters. The plant has 150 billion base pairs, compared with humans' 3 billion.

More than being a monumental discovery, the genome size shows the vulnerability of this already rare plant. "In plants, research has demonstrated that those with large genomes are at greater risk of extinction, are less adapted to living in polluted soils and are less able to tolerate extreme environmental conditions – all highly relevant in today's changing world," Leitch said.

Part of the problem is that large genomes take longer to reproduce, and plants with more DNA require more time to grow. According to a growing guide at Rareplants.co.uk, Paris japonica can take anywhere from two to four years to sprout above ground after planting.

Paris japonica , a canopy plant, is native to Japan, where it lives in sub-alpine regions. According to the Global Biodiversity Information Facility, it has only been observed at seven sites in the wild.

Photo by Ray Drew, courtesy of the Royal Botanic Gardens, Kew.