The space shuttle Atlantis was launched Monday afternoon on its final flight of the year, carrying 24 Canadian tree saplings.

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Space shuttle Atlantis lifts off at the Kennedy Space Center in Cape Canaveral, Fla., carrying Canadian experiments and equipment for the International Space Station. ((John Raoux/Associated Press))

The 11-day mission will see Atlantis take some big spare parts to the orbiting International Space Station.

Atlantis's six astronauts will unload about 13,600 kilograms of pumps, tanks, other parts and science experiments.

The shuttle's payload includes 24 willow saplings, which will be part of a Canadian experiment to help determine the role gravity plays in the formation of different kinds of wood.

The experiment, known as APEX-Cambium (Advanced Plant EXperiments on Orbit), is led by professor Rodney Savidge of the University of New Brunswick and funded by the Canadian Space Agency. The experiment will be conducted on the ISS by Canadian astronaut Robert Thirsk.

The plant growth will take place in a layer of cells called the cambium, which eventually forms the tree's wood.

The parts of a tree that aren't vertical, such as horizontal branches, grow one type of wood on one side and a different type of wood on the other. Botanists call this "reaction wood," and it's thought to be a response to gravity, although this hasn't been proven.

Using a special tool developed for the project, Thirsk will make loops in the stems of 12 trees, leaving another six to grow naturally for comparison. After 30 days, the samples will be harvested and preserved for their journey back to Earth, where Savidge will analyze the results. The space samples will also be compared with samples grown in a lab under the same conditions on Earth.

"The idea is for the only variable to be weight," Savidge said in a CSA statement.

Atlantis is the final launch of the space shuttle of the year and one of only six remaining scheduled shuttle missions.

NASA invited 100 Twitter users behind the scenes to write about their experience live on the social networking sites.

Shortly after the launch, twitterer Tim Bailey wrote,"it is impossible to explain how jarringly LOUD a launch sounds this close!!"

"That was so amazing!!! I've never seen something so bright. It was so loud!!" wrote Laura Burns. "We were able to see the solid rocket boosters separate!"

Many of the NASA tweetup attendees were overcome with emotion.

"I just cried w relief. how wonderful those astronauts must feel," wrote Twitter user "MrsELROSS."

The reaction from the news media area at the Kennedy Space Center was noticeably more enthusiastic than usual.

"I wonder what was louder: the shuttle's engines or the squeeing from the @nasatweetup tent?" Samer Farha wrote on Twitter.

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Atlantis is shown on a launch pad at the Kennedy Space Center on Nov. 15, shortly after the rotating service structure was rolled back. ((Bill Ingalls, NASA/Associated Press))

With files from The Associated Press and The Canadian Press