Tree radical 木 wraps around the marker stroke — together they spell out 'red-hearted tree,' a pine or cedar whose inner timber runs vermilion. From that original wood-color sense came the abstract meaning 'vermilion / bright red' and the very common surname Zhu (notably the Ming-dynasty imperial clan).
Short horizontal marker drawn through the trunk of 木 — a pointing-stroke indicator that says 'right here, inside this tree.' Pure ideographic role: it does not mean 'one,' it singles out a spot on the timber, just like the marker dot in 本 or 末.
