How are you, Ms. Orchidea?

Adrenaline for plants, a gaseous hormon, is ethylene (C₂H₄). It has been known for a long time, that a prescence of a box of apples makes mature a whole hanger of bananas. It was not known why.

Apple trees emit ethylene, when fruits mellow, orchids - when flowers bloom (and stop emmiting when they are impollinated), grapes - when a leaf is broken. How we know this? Thanks to lasers!

Amount of ethylene are insignificaly small - only 10 parts over a billion of air, what requires special experimental methods. One is called photo-acustics. A beam of the infrared laser (CO₂) excites vibrations of the ethylene molecule. Ethylene molecules collide with air molecules and trasmit their excitation. If the laser is "chopped" with a frequency of a few kHz, a wave of faster moving molecules (a sound of this frequency) propagates in the gas – it is enough to use a microphone and we can hear it.*.

* Obiously, the method remains rather complicate, requires stable lasers and special resonans cavities. Its operational principle is based on the fact that in the infrared range (i.e. in the energy range corresponding to vibrational excitations of molecules) the de-excitation takes place mainly by collisions with other molecules - vibrational levels are much more long-lived than electronic excited levelsv. Furthermore, the CO₂ laser supplies so many lines, that a proper fingerprint can be chosen for such complex molecules like ethylene.