Wood is one of the Wu Xing, the five phases or agents of Chinese cosmology, a system distinct in structure from the four Greek elements: the five are dynamic phases in cycles of generation and control rather than static substances. Wood, mu, is associated with spring, the east, the colour green, the planet Jupiter and the emergent, upward, growing force, the green shoot pushing up, expansion and beginning. In the generating cycle wood is produced by water and gives rise to fire (wood feeds flame); in the controlling cycle wood is restrained by metal (the axe cuts wood) and itself restrains earth (roots break soil).
In Chinese medicine wood governs the liver and gall bladder, the emotion of anger, the tendon and the eye, and its character is that of vigour, planning and assertive growth. Wood's symbolism is of vitality, flexibility that is also strength, the tree that bends without breaking, and of the springtime energy of new beginnings, benevolence and the capacity to grow and extend. It is the only one of the five phases named for a living, growing thing.
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