By Richard Black
Environment correspondent, BBC News
They have been to space and back and virtually all of them lived to tell the tale. There are more of them than there are astronauts and they are found all over the planet.
"They" are Moon Trees grown from seeds which journeyed to the Moon and back on board Apollo 14 in 1971.
Dave Williams has compiled a list of over 50 existing Moon Trees
In the early days of the space race, various living things went into orbit - most notably Laika, the Soviet Union's first dog in space which survived a few orbits in the grim metal prison of Sputnik 2.
But seeds? When I came across the story, many questions sprang to mind - whose idea was it, and why? Was there any scientific purpose? Were the seeds changed by their journey? If you see a Moon Tree, how do you know it's a Moon Tree?
I began the quest for some answers at the southernmost tip of Indiana in the tiny town of Cannelton. Cannelton Elementary School has two claims to fame - it boasts the oldest continuously-used school building in the United States, having taken its first register in 1868; and it is the place where the second instalment of the Moon Trees story, the modern rediscovery, began in 1997.
"We were doing a project at school," teacher Joan Goble told me, "and one of my girls said 'I know there's this really neat tree at our Girl Scout camp we could do a research project on'. I said 'what is it?' and she said 'it's a Moon Tree'.
The only problem being no-one knew what a Moon Tree was. Joan Goble e-mailed Nasa to find out. The message reached Dave Williams, a planetary scientist at Goddard Space Flight Centre near Washington DC who collates records of space history.
"I had no idea. I'd never heard of these Moon Trees before," Dave told me when I met him outside the Goddard Visitors' Centre. "I talked to people I worked with, and no-one had any idea about it, including people who'd been working at Nasa for 20 years."
Astronaut Stuart Roosa took around 500 seeds on the mission
Eventually Dave found out the seeds were germinated and planted as part of the US bicentennial celebrations in 1976. "They were planted all over the country but after 1976 you didn't hear much about them," he said.
Dave put the Moon Tree story on the internet and receives regular e-mails from people claiming they have one.
He has a register of around 50, including - as he related with amusement - one which had been standing at Goddard itself all along.
Soon he was in touch with the family of the story's most important figure, Stuart Roosa, the astronaut who decided to make room for some seeds on the Apollo 14 mission. He piloted the command module around the Moon.
Like many early US astronauts, Stuart Roosa got the job partly because he had demonstrated courage beyond normal bounds, both as a test pilot and as a "smoke-jumper" - someone who parachutes out of aeroplanes to fight forest fires.
"My father used to talk about getting hung up in the trees, 100-feet up on his parachute, and then having to lower himself down on a rope," explained Christopher Roosa as we stood beside his father's grave in Arlington National Cemetery.
Stuart Roosa's love of the outdoors prompted him to choose tree seeds as one ingredient of his Personal Preference Kit, the sock-sized pouch Apollo astronauts were allowed to fill with their prize possessions. The seeds were provided by the US Forestry Service - in particular, by its then Staff Director for Forest Genetics Research, Stan Krugman.
Nasa's first attempts to germinate Moon Trees failed
On the veranda of his house outside Washington, overlooking his trim garden, Stan told me of the scientific purpose behind the Moon Trees - to see whether exposure to space, in particular the harsh radiation, would have any effect on the seeds.
"I didn't think there would be much, because the seeds were dormant," he said. "I chose seeds where we knew the mother and father, and enough seeds scattered around to get a representation of different environments."
Constraints on weight and volume meant only around 500 seeds made the journey. All survived the landing - so would they grow?
"The first attempt to germinate a few of the seeds failed, so it was decided at Houston that they were all dead. They called me, and I said 'ship them all up to me'," he recalled.
At various institutions around the US, in the tender care of plant scientists, all the remaining seeds germinated and grew into saplings. Each of the Moon Trees has an earthbound twin, derived from the same set of parents. Stan Krugman and his colleagues compared those which had made the journey with their twins.
So had the Moon trip wrought any changes? "No, nothing at all," said Stan.
During the bicentennial celebrations of 1976, when the Moon Trees were a sturdy five years old, requests came from all over a proud nation for saplings - from schools, government agencies, and Girl Scout troops in places like Cannelton, Indiana.
Demand outstripped supply. Stan Krugman created second-generation saplings, the offspring of a Moon Tree and an earthly specimen, but still requests kept on coming, from abroad as well as home.
The 'rediscovery' of Moon Trees began at a Girl Scout camp in 1997
"They went all over Europe - France, Germany, Spain - we know Douglas fir, sweetgum and redwood do very well in Europe. The British Isles got half a dozen."
Unfortunately, any record of who asked for Moon Trees and who got them appears to have vanished. But, for US$32 plus postage and packing, you can now buy your very own second- or even third-generation Moon Tree - if legislation on importing seeds into the country where you live will permit.
They are marketed by the conservation group American Forests as part of its Historic Trees programme, which will also let you buy saplings derived from tulip poplars planted by George Washington in 1795, or a descendant of the honey locust tree which stood next to Abraham Lincoln as he delivered the Gettysburg Address.
"The Moon Tree is a sycamore, and it looks like a sycamore," admitted executive director Deborah Gangloff. "But the thrill of it is the connection to history, the fact that it comes from a seed that flew to the Moon and back. If you were born after 1969, you didn't see the landings on the Moon - so I think this is a way to connect people to a little bit of fairly recent history."
And here the Moon Trees story comes full circle - full orbit, if you will - back to the youngsters of Cannelton Elementary School, who, through their project, are themselves getting in touch with the era of the first lunar footsteps. Is it making them think? From some of their comments, I believe so.
"For me, it makes me think of evolution - mankind going from cavemen to geniuses all over the world," one child said. Another added it would be "really cool" to continue discovering things on other planets.
The Moon Trees was broadcast on Wednesday 20 July 2005 and can be heard for the following seven days at Radio 4's Listen again page. The producer is Gabi Fisher. If you know the location of a Moon Tree, please let us know by sending an e-mail to email@example.com