Sapwood is the younger wood on the outside of a tree trunk, just under the bark. It looks paler and feels softer than the wood in the middle. This is the living ring that still carries water and the tree's sap up from the roots. Cut a log and you can often see it: a lighter band around a darker core. People who work with wood care about the difference because sapwood can take water and insects more easily, and it does not always last as well outdoors. The dark, older wood in the centre is heartwood — the tree's old strength, no longer carrying sap. Though actually, not every tree shows a sharp colour change, and 'softer' here means softer than the heart of that same tree, not soft like a sponge. You will meet this word in forestry, woodwork, and biology, not in everyday chat.

邊材（相對心材）