Sentence:  A more likely hypothesis was that the attacked leaves were transmitting some airborne chemical signal to sound the alarm, rather like insects sending out warnings ... But this is the first time that a plant-to-plant <WSD>language</WSD> has been detected. 
Meaning of the word marked with <WSD> tags:  A body of sounds, signs and/or signals by which animals communicate, and by which plants are sometimes also thought to communicate.