Arboretum

The arboretum is a botanic garden found on board several starship classes, including the Constitution and Galaxy classes. 

Although the arboretum is considered a recreation area where crewmembers can relax in a natural setting, it is also an area of scientific research. 

Aboard the USS Enterprise (NCC-1701-D), the arboretum was on Deck 17, section 21 alpha. It contained a number of different types of plant life, including Cyprion cactus and roses. There was also a small pond. 

Civilians were allowed to staff positions in the arboretum. Keiko O'Brien was head of the plant biology lab while she was aboard the ship. Children such as Clara Sutter were also allowed to help with small projects in the arboretum. 

In 2370, the Enterprise-D arboretum took the place of Lake El'nar in Lwaxana Troi's mental recreation of the death of her daughter, Kestra, witnessed telepathically by Deanna Troi. 

When the entire ship was affected by Barclay's Protomorphosis Syndrome, many of the crew were clustered in the arboretum. 

In 2369 Vedek Bareil Antos told Commander Benjamin Sisko that he prefered the sanctity of his arboretum instead of listenening to 112 Vedeks for hours. 

In 2371, Odo spent some time in an arboretum after discovering his people's homeworld. Later in that year and following his transfer to Deep Space 9, Miles O'Brien drew up plans to turn one of the station's cargo bays into an arboretum to cheer up Keiko, who missed being able to do botanical research. However, these plans were abandoned when Julian Bashir convinced Miles that an arboretum could only afford Keiko a mere hobbyist's level of research opportunities, when what she really craved was an extended field expedition. 

Background information
The botany lab viewed in "The Man Trap" was a redress of the sickbay set, with an extra doorway added to the right of the opaque cabinet.

The refit Enterprise seen in appeared to have an arboretum in the lower part of its engineering hull, as trees were seen beyond the several large viewports on each side of the hull. This appears to be the original intention of designer Andrew Probert, and is confirmed in this image and many non-canon publications such as Mr. Scott's Guide to the Enterprise.

Some blueprints have identified the two large blue windows below the main shuttlebay on the Enterprise-D model as the arboretum windows, although the set had no such windows. It is possible there is more than one arboretum on the Galaxy-class.