Wood's place in the Wu Xing is defined less by what it is than by what it does to its neighbours. In the generative, or Sheng, cycle each phase feeds the next: Water nourishes Wood as rain feeds a root, and Wood in turn feeds Fire, burning as fuel. In the controlling, or Ke, cycle each phase restrains another: Wood controls Earth, as roots bind and exhaust soil, while Wood is itself controlled by Metal, as an axe cuts a trunk. Neither cycle is a hierarchy; each phase is at once feeder, fed, controller and controlled, and the system's use lies in tracing which relationship is out of balance.
In Chinese medicine Wood governs the Liver and its paired Gallbladder, and by extension the eyes, the tendons, and the smooth movement of blood and temper; a Liver held in excess is treated as Wood run wild, prone to the anger the phase is also said to govern. The correspondence reaches feng shui, where columnar shapes and living plants bring Wood into a room, and the calendar, through Wood's season of spring and its direction east.
Western acupuncture schools imported the five phases in the twentieth century, but the cycle logic did not always travel with the vocabulary. Some adaptations use Wood as a fixed symptom category rather than the dynamic relationship the Chinese sources describe, a flattening that practitioners trained in the classical system have criticised as losing the point of the phases entirely.
Sources Science and Civilisation in China, Volume 2: History of Scientific ThoughtJoseph Needham with Huangdi Neijing (The Yellow Emperor's Inner Classic)Traditionally attributed to Huangdi; actual authorship unknown