European aspen trees (Populus tremula) and beech trees

(Fagus sylvatica) growing on the campus of Innsbruck Univer-

sity were used for these experiments. Branches were cut from

the trees, and the cut ends were immediately submerged in a

flask of water and recut under water and then transported to

the laboratory. Care was taken to use branches with leaves that

showed no visible signs of physical or biological wounding.

Source : http://onlinelibrary.wiley.com/doi/10.1029/1999JD900144/full

(Volatile organic compounds emitted after leaf wounding: On-line analysis by proton-transfer-reaction mass spectrometry)

Hope it helps.

Thank you Saurabh